Touch Mars? Europa? Enceladus?

Introduction - How often have you seen this?

How often have you seen this scenario in movies, artist's renderings and science fiction? Bold and brave astronauts explore Mars, setting out from their base in pressurized rovers and spacesuits. They scale cliffs, adventure into caves, and dig deep. They search for past, and present day life. And one of them has just made a great discovery, a fossil!

Artist's impression of human astronauts exploring Mars, and discovering a fossil - credit NASA / Pat Rawlings

However there is another side to this picture. As these brave astronauts explore Mars, their spacesuits leak air. It would be easy to make a "light air-tight anthropomorphic balloon" as James Waldie puts it, but we need to be able to bend our arms and legs at the joints, and designers achieve that with flexible joints. These have tiny gaps between the moving parts, which leak small amounts of air constantly. Wherever they go, they will leave a trail of microbes.

This photograph of Alan Bean during Apollo 12 is often shared as an example of venting from his spacesuit, or perhaps, ice crystals from the ice sublimator. Actually, Kip Teague, author of the Project Apollo Archive is cited by NASA as saying that it's a smudge on the camera. It was blue when the central subject in the camera was bright and red in tint when it was dark. It was present in the photos from frames 6813 through to 6853 (this photograph is frame 6826).

The venting from the spacesuits wasn't so visible, but they did vent liters of air constantly on every EVA, as do the modern EMU units used for spacewalks on the ISS. If we use similarly gas pressurized spacesuits for Mars, then one estimate is that over 50 litres of human borne bacteria and other airborne effluent would escape through suit bearings and joints during each EVA, potentially contaminating soil, fossil and atmospheric sample" See this paper. Perhaps this can be reduced by measures such as using counterpressure spacesuits on the body pressurized in equilibrium with the Mars atmosphere with only the regions around the head pressurized to full pressure. Still, the experts say that some leakage is inevitable.

One of NASA's identified knowledge gaps for human EVA's on Mars is the amount of microbial leakage from their suits. But they say that some leakage from the spacesuits is inevitable. They also need to vent waste gases from inside the habitat from time to time. Also astronauts' bases and rovers would leak Earth microbes into the dust, every time they open an airlock. The experts say that if we have humans on Mars, then it is inevitable that we contaminate the landing site with Earth life. There is no way to design airlocks, EVA suits etc to prevent this altogether, at present.

Also, the ground they walk on is covered with a fine dust, as fine as cigarette ash, light and easily moved, even in the near vacuum winds of the Mars atmosphere. This fine dust can travel hundreds of kilometers in a few hours during the fast winds of the Mars dust storms.

Mars has frequent dust storms, every year. The earliest storms in the year start in the northern autumn, cross the equator and expand over much of the southern hemisphere in the warmer southern spring. The global dust storms happen every decade or so. These cover the southern hemisphere, then spread north to cover the entire planet, and then can last for weeks. These global dust storms block out the sun and turn day into night, and it takes months for all the thick clouds of dust to settle back out of the atmosphere.

Cover from Andy Weir's "The Martian" to illustrate the fast winds of the Martian dust storms. He uses a dust storm as a central plot point. He knew that in the thin vacuum of the Mars atmosphere, the winds, though fast, are also feeble. So that's a bit of poetic license on his part; they couldn't blow over an astronaut. Indeed you would probably not even feel them. But the winds are very fast, reaching up to sixty miles per hour in dust storms, and even faster in the tiny dust devils. They also do pick up the dust, forming dust clouds, exactly as in the illustration.

The winds couldn't pick up most of the dust as we know it on Earth, but they can do that on Mars because the dust is so fine, as fine as cigarette ash. Indeed, they pick up so much dust that the dust storms block out the sun, turning day into night, sometimes for weeks on end. Potentially they could transport desiccation resistant dormant microbes and spores over distances of hundreds of miles a day, and throughout the planet. All this time, much of the dust will be protected from the harsh UV rays of the sun.

Mars has frequent dust storms, starting with the type A storms which form in the northern autumn, travel south, cross the equator into the southern spring and then expand around much of the southern hemisphere. The year continues with the type B storms restricted to the southern polar regions, then the type C storms which are similar to type A but more variable. Every decade or so, sometimes several times in a decade, it has global dust storms. These typically start in the south, during the southern spring or summer, encircle the planet in southern latitudes then extend north across the equator and can cover much of the planet. (see page 129 of this article) They are not sure yet how this relates to the A, B and C type storms (see end of this paper).

NASA have this as one of their many knowledge gaps for human extraterrestrial missions to Mars: "Obviously, the current understanding of microbe survival in Mars dust environments remains uncertain and represents an important knowledge gap" (page 34 of this report).

Carl Sagan once remarked, that the iron oxides that make up these dust particles are perfect to shield a microbe from the sterilizing UV light of the unfiltered sun. Such a microbe, imbedded in a minute crack in a fine dust grain, could eventually fall to the surface undamaged by the harsh UV light, thousands of kilometers from its point of origin. After a human landing on Mars, billions of hardy microbial spores and desiccated microbes in other dormant states will stream out from their base and spread in the dust and winds. If there are any Mars habitats for them to find, some of them surely have a chance to get there eventually. For more on this see the section: How could this work on Mars with dust storms and a globally connected environment? (below).

Also, what is a mission with humans in harsh difficult conditions for months on end going to do about trash? The ISS would generate many tons of trash every year, if it wasn't all burnt up in our atmosphere. See Trash, on the Moon (below) . And what if someone dies when they are on the surface? Cremate the body?

So far, you might wonder if it is possible to contain our microbes somehow. They do exactly that with rover computers, which can't be totally sterilized either at present. They enclose the core box and some sensitive electronics with high efficiency filters (like HEPA filters) to keep even particles as small as microbes inside. Could we somehow do something similar for humans on Mars? Keep everything humans touch inside of giant filters to keep the microbes away from the surface of Mars? Could we perhaps somehow include filters in the design of our astronauts spacesuits, in every joint, and vent the spaceships and airlocks etc through such filters? And recycle everything with no trash or waste gases, everything is contained? It might be hugely challenging - but it is possible?

Well, so far the answer seems to be no. The experts say that they would design the missions and equipment to limit the contamination as much as they could, but that it it doesn't seem possible to keep our microbes totally within impermeable barriers when astronauts are exploring another planet. As Rick Davis put it in a press conference,

“We’re basically, if you will, big sacks of microbes. And so keeping that segregated from the Martian environment when humans get there is probably impossible.”

However, this reaches a whole new level of challenge once you start to think about the possibility of an "off nominal" mission. Suppose that instead of a safe landing, they crash on Mars killing everyone on board? After all, the space shuttles Columbia and Challenger crashed, and the landing on Mars is particularly challenging. It's far harder to land there than on Earth, and even with the best efforts to make it safe, a crash of an early crewed mission to Mars has to be fairly high in probability. I cover this in Why do spacecraft crash so easily on Mars? (below) .

Elon Musk warns that early settlers must be prepared to die in the attempt. After a crash like that, minute fragments of the astronauts bodies, food, air, water and the spacecraft itself would spread in the dust, infused with trillions of microbes, and this is likely to irreversibly contaminate Mars with Earth life. The space shuttle debris fields spanned hundreds of miles

Columbia disaster debris field in East Texas - it spans about 250 miles

With Elon Musk's supersonic retropropulsion, the rockets come in at great speed. They don't have parachutes but just use their rockets fired in reverse, assisted with atmospheric resistance, to land vertically on the surface of Mars. His plan is to use the same technology they use to lands the Falcon 9 first stages on a barge in the sea, to land on Mars. He has only done it with first stages so far, which is less of a challenge, but he seems confident that they will achieve vertical landings of the second stages as well, all the way from orbit around Earth, back to our surface. On Mars, as with his experiments with the barge landings on Earth, they can come in too hard, and topple over. But there's an additional hazard on Mars. The astronauts' spacecraft has to skim really close to the surface to get enough resistance from the atmosphere to slow down enough to land. They skim so close to the surface that they can't land on the higher mountains and the Martian highlands, because the air up there would be to thin for this technique. When he says that early settlers have to be prepared to risk their lives, he is not exaggerating. Perhaps the NASA mission might be less risky, it's hard to say, especially since they have no detailed designs yet for their landing craft, but any human landing on Mars is bound to be dangerous. When Curiosity landed on Mars, they didn't call it "Seven minutes of terror" for nothing.

However small or large the debris field is (do say if you know of a way to estimate its size for Mars), the remains would get mixed up in the dust, and perhaps it doesn't make that much difference. The Mars dust storms would surely, in time, spread the finer particles and microbe spores from it throughout Mars. For more on this, see the sections below: How could this work on Mars with dust storms and a globally connected environment? , Why do spacecraft crash so easily on Mars, and Elon Musk's fun but dangerous trip to Mars.

Could we design spacecraft in the near future so that even in a crash on Mars, they will contain all the microbes in a human occupied spacecraft? For some ideas, see Could we send humans to the Mars surface in a biologically reversible way? (below) . At any rate we don't have any realistic near future spacecraft that can do this.

If something irreversible like that happens to Mars, it impacts on not just the explorers and not just the US, and not just the twenty first century. It impacts on all nations on Earth with an interest in exploring the planet, and also our descendants, and all future civilizations in our solar system, for as long as Mars continues. If this happens, then for the entire billions of years future of Mars, nobody will ever have the same opportunity again that we have right now, to study the present day pristine planet.

Also if we do eventually introduce Earth life to Mars, it may well make a big difference when and how we do it. If we take this fast and risky approach of sending humans there as soon as possible, the entire future biology of Mars could depend on the chance event of some microbe that got there accidentally in the first crash of a human occupied spacecraft on the planet. Can this be the right way to introduce Earth life to Mars, even if that's the decision we make eventually? For more on this see Why bringing Earth life to Mars could be like making wonderful yoghurt - or bad smelling gone off milk

There are plenty of other places we can go to do our very first experiments with spreading Earth's biosphere into space. We can start on Earth itself - continue with the line of experiments for Biosphere II. Then we can go into space and build small scale habitats of cubic meters, then eventually, cubic kilometers, in originally lifeless environments such as caves on the Moon or domed greenhouses at the lunar poles. We can make our first mistakes in a reversible way, in short term projects that take only decades to complete rather than the thousands of years of a terraforming project. There are plenty of other things we can do right now. There is no urgency to take on the huge challenge of transforming an entire planet.

See these sections (below):

• Instead of terraforming Mars in a multi-millenium project, why not terraform a lunar cave in a multi-decade project?
• Best places to introduce Earth life right now - while continuing with biologically reversible exploration of Mars
• Plenty of places to experiment with sending life to other places in our solar system - Asteroid belt resources, NEO's, caves on the Moon

Why don't explorers in science fiction have these problems when exploring other worlds?

The explorers of other planets in Star Trek, "The Martian", Kim Stanley Robinson's Mars trilogy, and the many movies, books and TV series don't seem to have any of these problems when they explore other worlds. Why is that?

Perhaps it is because these stories are fiction and are the result of the authors' imaginations. They aim to entertain, after all. The result is a collaboration of authors, script writers, directors and sometimes ideas from the actors themselves. Much of it is based on a need for drama and easy story telling rather than accuracy. It's like the "Coconut effect" - modern audiences expect horses galloping over grass, or a sandy beach, to sound like two half coconuts hit together. Of course they only sound like that when galloping over hard surfaces. Well the same is true of many on-screen science and science fiction effects.

So, for instance, we expect highly radioactive materials in movies to be bathed in a sickly green light. In reality if there is any light at all from radioactivity, it's blue from Cherenkov radiation, or else orange, for some materials that get hot from the waste heat of their own radioactivity. But if a director showed radioactive materials as blue or orange, many viewers would think it was a mistake, or just get confused and not recognize what they are supposed to be. This is an example of "Technicolour science", It's like a visual shorthand that serves as an instant cue to the audience that the scene is bathed in deadly radiation.

These tropes are often things which help move the plots forward, and make the stories more visually dramatic. For instance, most science fiction fans would expect a Mars dust storm to have strong winds with dramatic effects, and Andy Weir's novel "The Martian", as we just saw in the last section runs with that idea. It starts off with a dust storm on Mars strong enough to blow over a spaceship, or an astronaut. The winds are indeed fast, but feeble, in the thin atmosphere. An astronaut would barely notice the strongest winds, standing right in them. Even its very fastest winds in the dust devils, as fast as a hundred miles per hour (162 km / hr) could barely stir an autumn leaf in the near vacuum of its "atmosphere". The discarded parachutes from our landers remain in the same position on the surface for year after year, without moving at all in any of the winds.

This is not a mistake on his part, but a bit of artistic license, as he explains 14 minutes into this interview : Triangulation 163: Andy Weir from Triangulation (MP3). He had other ways to start the novel. But those other, more scientifically accurate scenes, didn't have the drama he felt he needed for that so important first scene of a novel. He needed to grip the reader right away and get them involved in the story. Most of the novel has enough hard science to delight the most demanding space science geek, and as a result, we are ready to overlook this dust storm scene, and the idea that wind can blow away pieces of equipment, for the sake of a dramatic story.

So, some of our science fiction is based on these tropes built up purely as dramatic devices to help with plots and visual effects. These may have no connection to reality. Some is based on deliberate number fudging as poetic license for dramatic effect (e.g. Andy Weir's Mars storm). Some is based on hard science extrapolated well beyond anything we know about. But none of this vivid and entertaining story telling is based on any actual experiences of exploring other worlds.

If we look at early twentieth century science fiction projections of their future, they are a mix of the far sighted, like stories about television-like devices long before it was invented, the almost correct like H.G. Wells prediction of atomic weapons in 1911, as devices that have no more force than ordinary high explosives, continuing to explode with a half life of seventeen days, and the bizarrely dated.

When science fiction got it right - watching baseball on a television of the future, cover picture from Science and Invention magazine, July 1922. Idea used in stories by Hugo Gernsberg. such as Ralph 124c 41+: A Romance of the Year 2660, published in 1911. (The Hugo Awards for science fiction writing are named after him).

Asimov's early "hard science fiction" stories about a supercomputer Multivac (made of vacuum tubes) are especially dated now, exciting and visionary as they must have seemed at the time. He depicts it as so large, that in one story it filled Washington DC right out to the suburbs, accessed by terminals world wide. His stories reflect accurately how computing was done at the time, right through to the early 1970s, when I first learnt programming. Your programs ran on a "main frame" computer which filled an entire room, and you never saw it. Just as in his stories, the computer itself was maintained by a team of technicians. Asimov, and his readers at the time would have thought that surely, as computers get more powerful, they will get larger and larger, needing more and more technicians to keep them going. Eventually, the world's fastest super computer would be so huge, and so expensive to build and maintain that it would surely be the only such computer in the world. Indeed perhaps there wouldn't be any other computers at all, even small ones, as you could do everything so much faster by connecting to Multivac. Why build your own?

All of that was a natural and logical extrapolation to Asimov and his readers at the time. He used the best scientific understanding of his day for his stories (he was a trained scientist with a PhD in biochemistry). It was not unreasonable either, after all if it had been a story about particle accelerators, he'd have been right. To this day the largest accelerators are expensive to build and maintain and need large teams of technicians. We have only one Large Hadron Collider.

When science fiction got it wrong - back of one of the panels of ENIAC, an early computer, packed with vacuum tubes. In his early stories, Asimov imagined that by now we would have a huge supercomputer several stories high and a half a mile long built of vacuum tubes like these. In another story it is the size of Washington DC – with many interior corridors, with the one supercomputer serving the entire world, or an entire country. All the famous hard science fiction writers have had epic fails like this, as well as, sometimes, astonishingly accurate predictions.

Two of the programmers of ENIAC in a photograph from 1945-1947. Back in the early days of computers, all the programmers were women, while men were more involved in the engineering, building the computers. Left: Betty Jennings (Mrs. Bartik) Right: Frances Bilas (Mrs. Spence) setting up the ENIAC.

Asimov envisioned a future with a vast LHC sized "Multivac" computer with corridors running throughout and many technicians and programmers to keep it going. He envisioned it as such an expensive endeavor that there would probably only be one in the entire world (again like our Large Hadron Collider). It all made perfect sense at the time, to both him and his readers. Now it seems bizarrely dated.

If you wanted a portable calculator at the time, you used a slide rule. So, also, logically enough (at the time) an early story to feature "faster than light" travel, perhaps the first story on the topic, in 1938 has explorers using slide rules to navigate their faster than light spaceships. (Full text).

Arthur C. Clarke was another one of our hard science fiction writers, striving for accuracy, with a first class degree in physics and maths,. Yet, though he had many "future prediction" successes, he also had many "fails". One of the most notable is in his "Fall of Moondust" book which described a Moon covered in thick drifts of dust, deep enough to swallow a dust skimming "cruiser". It was shown to be false within a decade.

Low resolution photograph of the cover of the first edition of Arthur C. Clarke's 1961 book "A Fall of Moondust". It was published in the very early days of space exploration, just after the first mission to impact on the Moon Luna 2. He describes a surface covered in deep layers of dust which tourists explored with a dust skimming "cruiser" as shown in the book cover. One of them sinks 15 meters into the fine dust, leaving not a trace on the surface. It's carefully written hard science based on the best knowledge of his day, and exploring many ramifications of their predicament. But it described a future that was shown to probably be false, only five years later with the first soft landing of Luna 9. It remains a well written exciting "hard science fiction" story that still delights science fiction geeks to this day (though some of its attitudes to women, are dated and even offensive to us nowadays). But we now know that our Moon has only thin surface layers of dust.

Sometimes the "predictions" of our best "hard" science fiction authors are astonishingly accurate, and sometimes they fail badly. The problem is that we don't have any way to know in advance which is which.

For a fine example of an accurate prediction, Hal Clement in 1956 wrote a short story called "Dust Rag" in Astounding Science Fiction about levitating dust on the Moon, two astronauts find their faceplates get covered in dust:

“All right. There are, at a guess, protons coming from the sun. They are reaching the Moon's surface here — virtually all of them, since the Moon has a magnetic field but no atmosphere. The surface material is one of the lousiest imaginable electrical conductors, so the dust normally on the surface picks up and keeps charge. And what, dear student, happens to particles carrying like electrical charges?”

“They are repelled from each other.”

“Head of the class. And if a hundred-kilometer circle with a rim a couple of kilos high is charged all over, what happens to the dust lying on it?”

The astronauts didn't get their faceplates obscured by dust. But they did observe mysterious bands and streamers at sunrise and sunset while in orbit around the Moon. And the Surveyor spacecraft photographs of the Moon showed a not quite sharp horizon with a slight haze. These were finally explained in 2005 as due to electrostatic levitation, nearly half a century after his story. Apparently some of Hal Clement's dust gets elevated high enough to be seen from orbit around the Moon!

Few science fiction authors have tackled the theme of forward contamination of other parts of our solar system by Earth microbes, but there's one poignant sad story, again by Arthur C. Clarke, "Before Eden". This story was published in the same year as "A Fall of Moondust", in Amazing Stories, June 1961. Back then, though they knew Venus was hot, scientists thought it was still possible that Venus could have water on its surface, perhaps at the top of its mountains.

One of the covers for Arthur C. Clarke's "Before Eden" -a poignant sad story about forward contamination of Venus, published in 1961 at a time when surface life there was still a remote scientific possibility. You can hear the complete story read as an audio book here.

These adventurers are exploring a completely dry Venus, or so they think. Up to then (in the story), everyone thought Venus had no water, and was sterile of life. That was a natural thought, because the temperatures they encountered were always above the boiling point of water. But the heroes of the story are stranded near the not quite so hot South pole, and find mountainous cliffs there. On those mountains they find a dried up waterfall - and then - a lake!

“Yet for all this, it was a miracle—the first free water that men had ever found on Venus. Hutchins was already on his knees, almost in an attitude of prayer. But he was only collecting drops of the precious liquid to examine through his pocket microscope.... He sealed a test tube and placed it in his collecting bag, as tenderly as any prospector who had just found a nugget laced with gold. It might be – it probably was – nothing more than plain water. But it might also be a universe of unknown, living creatures on the first stage of their billion-year journey to intelligence....”

“...What they were watching was a dark tide, a crawling carpet, sweeping slowly but inexorably toward them over the top of the ridge. The moment of sheer, unreasoning panic lasted, mercifully, no more than a few seconds. Garfield’s first terror began to fade as soon as he recognised its cause....”

“… But whatever this tide might be, it was moving too slowly to be a real danger, unless it cut off their line of retreat. Hutchins was staring at it intently through their only pair of binoculars; he was the biologist, and he was holding his ground. No point in making a fool of myself, thought Jerry, by running like a scalded cat, if it isn’t necessary. ‘For heaven’s sake,’ he said at last, when the moving carpet was only a hundred yards away and Hutchins had not uttered a word or stirred a muscle. ‘What is it?’ Hutchins slowly unfroze, like a statue coming to life. ‘Sorry,’ he said. ‘I’d forgotten all about you. It’s a plant, of course. At least, I suppose we’d better call it that.’ ‘But it’s moving! ’ ‘Why should that surprise you? So do terrestrial plants. Ever seen speeded-up movies of ivy in action?’ ‘That still stays in one place – it doesn’t crawl all over the landscape.’ ”

“‘Then what about the plankton plants of the sea? They can swim when they have to.’ Jerry gave up; in any case, the approaching wonder had robbed him of words... ”

“... ‘Let’s see how it reacts to light,’ said Hutchins. He switched on his chest lamp, and the green auroral glow was instantly banished by the flood of pure white radiance. Until Man had come to this planet, no white light had ever shone upon the surface of Venus, even by day. As in the seas of Earth, there was only a green twilight, deepening slowly to utter darkness. The transformation was so stunning that neither man could check a cry of astonishment. Gone in a flash was the deep, sombre black of the thickpiled velvet carpet at their feet. Instead, as far as their lights carried, lay a blazing pattern of glorious, vivid reds, laced with streaks of gold. No Persian prince could ever have commanded so opulent a tapestry from his weavers, yet this was the accidental product of biological forces. Indeed, until they had switched on their floods, these superb colours had not even existed, and they would vanish once more when the alien light of Earth ceased to conjure them into being...”

“...For the first time, as they relaxed inside their tiny plastic hemisphere, the true wonder and importance of the discovery forced itself upon their minds. This world around them was no longer the same; Venus was no longer dead – it had joined Earth and Mars. For life called to life, across the gulfs of space. Everything that grew or moved upon the face of any planet was a portent, a promise that Man was not alone in this universe of blazing suns and swirling nebulae. If as yet he had found no companions with whom he could speak, that was only to be expected, for the lightyears and the ages still stretched before him, waiting to be explored. Meanwhile, he must guard and cherish the life he found, whether it be upon Earth or Mars or Venus. So Graham Hutchins, the happiest biologist in the solar system, told himself as he helped Garfield collect their refuse and seal it into a plastic disposal bag. When they deflated the tent and started on the homeward journey, there was no sign of the creature they had been examining. That was just as well; they might have been tempted to linger for more experiments, and already it was getting uncomfortably close to their deadline. No matter; in a few months they would be back with a team of assistants, far more adequately equipped and with the eyes of the world upon them. Evolution had laboured for a billion years to make this meeting possible; it could wait a little longer.”

“...For a while nothing moved in the greenly glimmering, fog-bound landscape; it was deserted by man and crimson carpet alike. Then, flowing over the wind-carved hills, the creature reappeared. Or perhaps it was another of the same strange species; no one would ever know. It flowed past the little cairn of stones where Hutchins and Garfield had buried their wastes. And then it stopped. It was not puzzled, for it had no mind. But the chemical urges that drove it relentlessly over the polar plateau were crying: Here, here! Somewhere close at hand was the most precious of all the foods it needed – phosphorous, the element without which the spark of life could never ignite...”

" ... And then it feasted, on food more concentrated than any it had ever known. It absorbed the carbohydrates and the proteins and the phosphates, the nicotine from the cigarette ends, the cellulose from the paper cups and spoons. All these it broke down and assimilated into its strange body, without difficulty and without harm. Likewise it absorbed a whole microcosm of living creatures—the bacteria and viruses which, on an older planet, had evolved into a thousand deadly strains. Though only a very few could survive in this heat and this atmosphere, they were sufficient. As the carpet crawled back to the lake, it carried contagion to all its world. Even as the Morning Star set its course for her distant home, Venus was dying. The films and photographs and specimens that Hutchins was carrying in triumph were more precious even than he knew. They were the only record that would ever exist of life’s third attempt to gain a foothold in the solar system. Beneath the clouds of Venus, the story of Creation was ended.”

How sad it would be if future explorers on Mars get glimpses of early forms of life on Mars, and then they go extinct soon after they are discovered. Or indeed, even before, maybe they are extinct before anyone finds them. It would be great to be able to say that humans on Mars will cause no problems. It's what most of us want to be true, and we love to read science fiction stories, and watch movies, based on this idea. If you say this, you are bound to be popular with space colonization enthusiasts and science fiction geeks, and your work will probably get widely shared.

But our actions on Mars will have real world consequences, and won't just lead to popular acclaim and book or movie sequels. We don't get to write the script for what happens next. We need to take a careful and thorough look at what might actually happen before we act. Let's look beyond the widely shared optimistic stories reassuring us that nothing can go wrong.

Could we get a future news story: "Debate over Moldy Mars is a Tale of Human Missteps"?

We have made so many mistakes on Earth, already. I will start this book with an example of the many things that went wrong during our attempts to preserve the Lascaux cave paintings. Could the same happen some day with Mars? Might we some day read an article in the Washington Post,or New York Times, similar to a recent one about the Lascaux caves, but this time it says: "Debate over Moldy Mars is a Tale of Human Missteps"? Like this:

The last page of my series of possible newspaper stories in a (hopefully) "alternative future" in which humans accidentally introduce Earth life to Mars, then regret what they did. For the complete story, see the section Prestige or dishonour, first footsteps on Mars (below)

The Lascaux cave painting photo is by Prof Saxx.

If so, is this something we can foresee in advance and prevent?

At least nowadays scientific news stories about Mars sometimes mention these issues. But still, it's too often brushed over quickly. almost as an afterthought. Let's take an example from the scientifically highly respected The Sky at Night late night television program in the UK (hosted for many years by Patrick Moore until his death). A recent episode, Life on Mars aired in November 2016, briefly covered the need to protect Mars from Earth life. They also talked about the impossibility of keeping Mars pristine, with humans on the surface. But they treated it as a minor matter. The discussion starts about sixteen minutes into the program. The presenter, British geneticist and broadcaster Adam Rutherford ended by saying (around twenty minutes in)

"So, that's the balance of the argument, extreme caution to protect the pristine Martian environment, versus our desire for the most important scientific discovery of all time. If it were up to me, I think the scientific benefits outweigh the contamination costs.

Maybe none of this is going to matter, in a few years time. Last month president Obama announced a human mission to Mars by the 2030s. Elon Musk wants to get there much sooner, with hundreds or even thousands of people forming permanent Martian colonies. Now, humans are messy, leave trails of cells, and DNA wherever we go. So when that happens, who is going to really care about a few bacteria?"

(The episode is no longer available to watch for free even in the UK, and sadly, there no longer seems to be any option to buy previous Sky at Night episodes since they closed the BBC store)

In other words, the idea is that our present situation is frustrating. Once we send humans there. we will no longer need to be bothered about protecting the planet, because the die will be cast. With Mars irreversibly contaminated with Earth microbes, then you get the impression that with a huge sigh of relief, at last, we can go about exploring Mars much as we explore Earth (though in spacesuits of course).

That argument may seem convincing to you. Who cares about a few bacteria when there may be far more exciting discoveries to be made there? Indeed even many scientists think this way, as this shows. If you listen to this, and you haven't read the vast literature on planetary protection, it would be easy to think "end of story" at that point . Kudos to the BBC for raising the issue at all however, as the idea of planetary protection is so often ignored completely, as soon as the discussion turns from robotic to human missions.

Another recent video raising these issues is this one from VSauce Is it okay to Touch Mars? which they did for the National Geographic series on humans to Mars. I got the idea for "OK to Touch" in the title of this book after listening to their video. This book covers some of the same issues that they cover (starting nine minutes into that video), but there is so much more to be said.

For a slightly different view, here are a couple of quotes from the planetary protection officers for NASA and the ESF, interviewed by Larua Poppick for the Smithsonian.

Gerhard Kminek, planetary protection officer for the European Space Foundation:

“If you do it badly once, that might be enough to compromise any future investigation related to life. And that’s why there is strong international consensus making sure there are no bad players around.”

And Cassie Conley, NASA's planetary protection officer:

“For certain types of Earth organisms, Mars is a gigantic dinner plate. We don’t know, but it could be that those organisms would grow much more rapidly than they would on Earth because they have this unaffected environment and everything is there for them to use.”

It all rather depends on what you expect to find on Mars, and what you think our Earth life could do there. First, a little background on planetary protection.

Planetary protection - researches by Sagan and Lederberg onwards - and Zubrin's arguments

The first scientists to write papers on planetary protection were Carl Sagan, and Joshua Lederberg, in the 1960s. Joshua Lederberg got his Nobel prize for pioneering work on microbial genetics, and started to think about these issues already in 1957, making him perhaps the first to give it serious consideration. He wrote to the eccentric mathematician, geneticist and all round brilliant scientist J. B. S. Haldane in 1959. Recalling his visit to see him on November 6th 1957, he writes:

" I recall this was the night of a lunar eclipse, and there was also some excitement about the expectation that there might be a demonstration moonshot marked by the deposit of some visible powder! It must have been around this time surely that I began to think of the scientific consequences of lunar and planetary probes. At any rate, since then, it has become very plain that planetary exploration is close enough to realization that tangible plans must be made for it, and I have been particularly exercised at the possibility that irreparable harm might be done before our biological colleagues woke up to this and attempted to exert some influence. I have in mind the quite tangible possibility of contamination by terrestrial organisms of the surfaces of Mars and Venus, unless stringent precautions are taken to sterilize any vehicles sent there...."

Letter by Joshua Lederberg, 2nd February 1959

Already by February 14th, 1960 COSPAR had considered problems of extraterrestrial contamination and decided to set up a "Working Group on Standards for Space Probe Sterilization". Here is a copy of the letter inviting Joshua Lederberg to join the group.

Their work did have some effect, as it lead to planetary protection measures for the lunar landings for Apollo. Indeed it lead to lots of internal discussions and elaborate precautions taken. However,, NASA only published the details of the precautions they planned to take on the day that Apollo 11 launched to the Moon, as CFR Title 14 Part 1211, which they did deliberately to avoid extensive public debate (such rushed through legislation would not be permitted today). They weren't even given clear legislative authority to produce such guidelines either. (See page 452 of When Biospheres Collide).

So Carl Sagan, Joshua Lederberg and others had no chance to comment on the guidelines. The Moon was never thought to be a likely location for life, even before the Apollo astronauts landed there, which is just as well, as those planetary protection measures turned out to be little more than a gesture in their effect, despite the millions of dollars spent on them, and many hours of debating time, congressional hearings and so on. (For details, see Chapter 4: Back Contamination: The Apollo Approach, in When Biospheres Collide). They did not really protect Earth at all, and are most useful for us now as examples of things that can go wrong, and what to avoid in future planetary protection measures. There were also several major lapses in their implementations of the measures they did propose (see Example of Apollo sample return - learning from our mistakes in the past below).

Luckily the Moon didn't have any life. So, although there was no way for them to know that at the time, there was no risk to Earth from the Moon, and in the forward direction, no lunar life or potential habitats, to be contaminated by Earth microbes.

Mars is different though. It's got clear evidence of rivers, lakes, even seas in the past and had all the conditions necessary for life to evolve as far as we know. Also, though it is very inhospitable now, the atmosphere is still just about thick enough for salty brines to exist, exposed to the surface atmosphere. It can even have fresh water trapped beneath thin layers of ice at the poles. So, if there was any life evolved there in the early solar system, then, it's possible, relics of that early life may still linger on (much like the Venusian life in Arthur C. Clarke's story) in favoured places. We've had numerous workshops and detailed studies since then, and they all come to the same conclusion, that this Mars life, if it exists, could potentially be vulnerable to Earth life. Also, in the other direction, all the studies have come to the conclusion that there's a probably small, but not zero, possibility that the environment of Earth could be vulnerable to life returned from Mars. So we need to take care in both directions.

Jim Rummel, senior scientist for astrobiology at NASA and former planetary protection officer for NASA, puts it like this in his foreword to Michael Meltzer's "When Biospheres Collide":

"We are bathed in Earth organisms, which makes finding our own kind of life palpably easy and detecting indigenous life on other worlds much more difficult. We are not exploring the solar system to discover life that we have brought with us from home, and we are aware that Earth organisms (read: invaders) could very well erase traces of truly extraterrestrial life."

"Likewise, we don't know what would happen if alien organisms were introduced into Earth's biosphere. Would a close relationship (and a benign one) be obvious to all, or will Martian life be so alien as to be unnoticed by both Earth organisms and human defenses? We really have no data to address these questions, and considerate scientists fear conducting these experiments without proper safeguards. After all, this is the only biosphere we currently know - and we do love it!"

That “Would a close relationship (and a benign one) be obvious to all, or will Martian life be so alien as to be unnoticed by both Earth organisms and human defenses” is the crucial sentence. If the returned life was so alien that Earth organisms defenses don’t notice it, then it would be able to overwhelm us, live inside or on us, and our bodies would mount no defenses against it. That was Joshua Lederberg’s insight originally which he expands on in a couple of papers, as we'll see later in Why we can't prove yet that Mars life is safe for Earth (below) .

There are many other ways that microbes could harm us, without attacking any Earth life directly. The harm could just be an accidental result of something they do. Here is another of my "possible future news" stories to illustrate the idea. It uses an example of how life from another planet could harm Earth given by Chris Chyba:

The photograph there is a detail from an algal bloom of Lake Eyrie in October 2011 during its worst cyanobacteria bloom for a long time. The cyanobacteria produced microcystins which is a liver toxin and can cause sudden death in cattle within hours, also often kills dogs swimming in the water and is a skin irritant for people.

The algae is not "keyed to the hosts" in any way, and it is no advantage to an algae to kill cattle or dogs. It's used as an example of one way that life from another planet could harm our biosphere. For more about this and this example, see Many microbes harmful to humans are not "keyed to their hosts" (below)

I made this “Future Possible News” story with this online spoof newspaper generator

In the wider scientific community, there are differing ideas about how much care we need to take to protect Mars, or in the reverse direction, Earth, but not that many by way of dissenting voices about whether we need to protect them at all. But one of the dissenters is famous, Robert Zubrin, a famous space engineer and leader of the Mars Society, and a keen advocate for Mars colonization. He is a formidable debater, and often takes on astrobiologists in debate. Here is one of his latest debates with Andy Spry of the Planetary Protection Office and SETI.

His arguments for saying we don't have to protect Earth are:

• That diseases are keyed to their hosts, and so Mars life would not be able to survive on, or in a human body. That if there were indigenous Mars host organisms, they'd be as distantly related to us as elm trees, and Dutch elm disease can't infect humans. Also, he argues, there's no evidence for macroscopic hosts on Mars anyway, so how can Mars microbes harm humans?
• That a ny Mars life that could survive on Earth is already here, transported on meteorites - we receive half a ton of meteorites from Mars every year, and so Earth life is already exposed to Mars life.
• That Mars life would not be able to survive on Earth because the environment is so different here.
• That to sterilize a Mars sample would be like sterilizing a dinosaur egg, a terrible loss to science.
• That every time you turn over the soil you are returning past Earth life to the present which could potentially harm us. So since we don't take precautions when we do that, we don't need to take precautions for samples returned from Mars.

His arguments for saying that we don't need to protect Mars are:

• Earth life can't compete with native life on Mars - it would be like sharks competing with lions in the Sahara, because the planets are so different
• Any Earth life that could survive on Mars is already there, transported on meteorites
• It will be easy to distinguish Earth life from Mars life by sequencing its DNA, as we can even tell which lab a strain of anthrax comes from
• Humans on the surface can explore Mars much more quickly than any robot and do things in minutes that would take years for our robots

These arguments may seem convincing at first, even knock down arguments perhaps. His followers are ready to be convinced of course, as they are so keen for humans to land on Mars and they have wanted this for decades, some of them. These arguments are widely shared and used in discussions on planetary protection. You might even wonder why scientists continue to take care to protect Mars from Earth life. But these arguments are actually easy to demolish if you come to them without that perspective.

I will do so later in this book, in detail, but before we get there, you might like to have a try at debunking them yourself.

Here are a few questions to get started.

• Ok sharks can't compete with lions. But what about using other analogies such as rabbits and wallabies, instead of sharks and lions?
  

  

  Young rabbit in Montana. Would Earth microbes on Mars be more like sharks competing with lions in Africa, or rabbits competing with wallabies in Australia?

  Also - do we have invasive microbes on Earth? Have you ever checked to find out? You may be surprised at the answer.
  
• When was the last asteroid impact on Earth large enough to send rocks all the way to Mars? It has to be big enough so that the rocks are still traveling at 25,000 miles per hour as they leave Earth, after passing all the way through our thick atmosphere. So when do you think that happened last? Again, the answer might surprise you.
  
  Also, the outside of the rocks would burn up like a meteor. So, could any photosynthetic life from Earth get to Mars in this way, given that photosynthetic life always lives close to the surface of rocks, to access the sunlight?
  
• How would you use DNA sequencing in the other direction, to prove that a previously unknown species is from Mars rather than Earth (if related in the distant past billions of years ago)? Zubrin has explained that you could use the method to recognize a microbe is from Earth, so long as it has been sequenced already. But how would you use this method in the other direction, to prove that a species is from Mars, if it doesn't match a recognized sequence?
  
  Also, what percentage of Earth microbe species have had their DNA sequenced? 10%? 1%? 0.1%? 0.01%? Or less, or more than those figures? You may be very surprised by the answer this time, if you don't know it already.
  
• He talks about advantages of humans over present day rovers. But what would it be like to explore Mars robotically in the near future with a broadband connection, and hundreds, or eventually thousands of 3D multi-gigabyte images streamed back every day? Try his comparison again, this time with astronauts in clumsy pressurized spacesuits with stiff gloves, compared to fast traveling semi-autonomous future robots, perhaps flying even, and with broadband communications back to Earth, in the mid 2020s?

For samples returned to Earth you can ask some of those same questions again, and a few additional questions:

• Does a microbe from Mars have to be keyed to its host to harm our biosphere?
• Does sterilizing a sample from Mars destroy its science value? It might seem obvious that it would, but might this depend how it is done? And do you have to sterilize the whole sample? Might there be a way to return a sterilized fragment of a sample to the Earth's surface at an early stage before you return the whole thing?
• If life is adapted to cope with life unearthed by turning over the soil, melting ice, deep mining etc, must it also be adapted to cope with life returned from another planet, based perhaps on a different biochemistry? If so, why?
• And anyway - is it true that we don't need to take precautions when ice melts or we unearth ancient microbes? This is something that researchers have paid more attention to as a result of global warming, leading to the possibility of Siberian ice melting.

If you have been persuaded by these arguments, this book may have much that will surprise you, and perhaps also, help you to understand why so many scientists say that we should continue to protect Mars. For the answers to those questions, and for detailed debunking, in the direction from Earth to Mars, see Demolishing Zubrin's arguments (below) . For the direction from Mars to Earth, see: Zubrin's arguments in: "Contamination from Mars: No Threat" (below) and Some highlights from the rebuttals of Zubrin by astrobiologists in "No Threat? No Way" (below).

Zubrin's arguments are somewhat beside the point anyway, as all the space faring nations are in agreement that we have to protect Mars, and also to protect Earth in the case of a sample returned from Earth. Both are protected by international treaty as we'll see. It doesn't seem likely that we will drop all planetary protection measures any time soon.

However there is a great deal of variety in views about what we need to do to protect Mars from Earth microbes. NASA's planetary protection office and the Planetary Society both agree that a human landing will lead to irreversible contamination of Mars by Earth life, so long as there are habitats there for them to colonize. They both base their plans on the idea that what we need to do is to keep Earth life away from sensitive regions on Mars long enough to make scientific discoveries there (hopefully). It's basically a delaying tactic. However they have different ideas about how best to do that.

NASA's approach is to land humans on Mars as soon as practical. They still plan to do the best they can to keep Earth life away from the most sensitive areas for as long as possible, to give them time to make scientific discoveries. Their approach is to set up a corral type area around the landing site which they permit to be contaminated. The humans have to remain within this "corral" which spans many square kilometers. Then they have robots which the astronauts will send further afield to study pristine areas not yet contaminated with Earth life and return samples to their base for them to look at. Once humans land on Mars, there is probably no realistic way to prevent nearby RSL's etc from being contaminated eventually (if habitable to Earth life) but they do their best to delay the event of this happening for as long as possible. I cover this in Land humans quickly - to explore nearby regions kept free of Earth life - NASA's approach (below)

The Planetary Society has a similar approach, but a different emphasis. They strongly recommend that we study Mars as much as possible from Earth and with humans in orbit around Mars, controlling robots on the surface via telepresence, in an earlier mission, before landing there. But they still think in terms of an end date, when we send humans to the surface of Mars. They usually suggest a date for humans landing on Mars in the 2030s, but Bill Nye gives an optimistic projection of humans in orbit in 2023 and on the surface in 2025 (see When will we know enough about Mars? - below). From then on it is like the NASA idea. On that timetable, the human landing would be only two years after the telerobotic visit, which doesn't give much time for a biological study of Mars. Also, if I understand their vision right, they think we should do that, no matter what we have discovered by then about life on Mars, and no matter how incomplete our understanding of the Mars surface conditions are by then. So, it is the same idea as NASA really, a delaying tactic, but with addition of an extra mission in Mars orbit first before the humans land on the surface. I cover this in Explore Mars from orbit by telepresence first - but still land on Mars towards the end of the 2030s - The Planetary Society's approach (below)

It is beginning to seem even quite likely that there are some surface habitats there that Earth life could colonise (see Habitats for life on the surface of Mars below). If that's so, they are both of the opinion that it is impossible to keep Earth life away from Mars indefinitely once we have humans on the surface. They have given up on the idea of a biologically reversible exploration of Mars once humans land there. If that's right, it would seem that we are pretty much bound to contaminate Mars irreversibly with Earth microbes, eventually, following those policies. However, both are agreed that humans should land there in the near future, probably in the 2030s, possibly even sooner.

In this book I'll suggest that we shouldn't set a date for a human landing on Mars at all, at present, but should explore it from Earth and from orbit around Mars instead. I will suggest that we should continue to explore Mars in this way, from Earth and from orbit, until we feel we have a reasonable understanding of whether there are habitats there, and what the effect would be of landing humans on the planet. Using the example of the Lascaux caves, I argue that we need to do this, I think, to have a decent chance of making wise decisions in the future, and to avoid what might turn out to be huge mistakes, like the moldy Lascaux cave paintings, written large over an entire planet.

At any rate, whatever your views on that, before we can make the right decisions for the future, it is essential that we have a clear understanding of what the issues are. We have a long way to go by way of raising awareness of these issues, and I hope to help with this book.

Fossil optimists and early life enthusiasts

As you read this book,you may be surprised to learn

• How hard some astrobiologists think it will be to find past or present day life on Mars.
• That microbes on Mars, far from being boring, may potentially be the most exciting discovery in biology for this century.
• How much value astrobiologists place on "in situ" searches by sterilized robotic explorers. Keen astrobiologists in universities and research groups world wide have already designed many miniature in situ detectors such as UREY, SOLID3 and astrobionibbler, to name a few. Many of these experts are of the view, published in several papers, that sample return is going to do much to help answer their questions unless we detect the life in situ first. They are not the ones calling for a sample return right now.
• That we haven't sent a dedicated life detection instrument to Mars since Viking. A few astrobiological instruments have been selected for actual missions (UREY and LifeMarkerChip are both examples of this), but each time this happened, as so often happens, the mission is cancelled, or the instrument gets descoped and replaced by something else. Curiosity, for instance, is designed to search for habitability, but would probably not be able to detect life biosignatures unless they are very obvious. It doesn't have any of these supersensitive life detection instruments on it and nor does its successor.
• The many ramifications that could follow if humans touch Mars.

The main reason for this difference, I think, is that many of us, without thinking about it, are "fossil optimists" as I like to put it. After all, we are used to learning about past life from fossils on Earth, so it's not too surprising that we expect the same to happen on Mars. Enthusiasts, including scientists, even search Opportunity and Curiosity photos for what they think may be fossils of past Martian life. They search for these fossils in what they know to be lake beds on Mars that have been dried up for more than three billion years. Nearly all Earth's macro fossils date from the last half billion years of our geological history. There are older macro fossils, but they are rare and consist mainly of hard to recognize stromatolites and others that are equally ambiguous. It took a lot of proof before they were accepted as life.

These optimists searching for fossils in Gale crater are hoping that Mars life had at least a two and a half billion year head start over Earth. Actually, this fossil optimism is not absurd; indeed you can come up with some interesting reasoning in favour of it. Evolution on Mars could have been accelerated greatly compared to Earth. But the case for it isn't strong either, as we will see. You can as easily argue the case in the opposite direction, that Earth life is likely to be ahead of Mars life, and that Mars life, if it exists, is quite likely to be an earlier form of life than Earth life. That could be true even of present day life on Mars. Especially when it comes to life from three billion years ago, we may well find early and primitive forms of life there that no longer survive on Earth.

Many professional astrobiologists are "early life enthusiasts". They design all their instruments to search for life similar to whatever existed on Earth over three billion years ago. Perhaps even life so early that it predates DNA. The microbes may be so small that you can't see them at all, not just with a magnifying glass, but even with the best of optical microscopes. They don't expect to find easily recognizable macrofossils. Instead they pin their hopes on the cold and dry Mars conditions, which could preserve organics for billions of years.

They are optimistic that they could find this early life eventually, if Mars did ever have life, but they expect this signal to be weak, degraded, mixed in with organics from other sources including (large quantities of organics from meteorites and comets hitting Mars), and only present in a few rare locations. That's why they think it unlikely in the extreme that samples returned from Mars right now, with our present knowledge of Mars, will be enough to solve the major questions in their field. They are likely to be as inconclusive for the search for life on Mars as the organic rich meteorites from Mars we already have in our collections.

We just don't know where to look yet, or what to return to Earth. They also expect that they will need to drill to depths of several meters to find the clues they are looking for. This is what makes it so important to do in situ searches with sensitive biosignature detectors - so sensitive that they can detect a single amino acid in a sample. Even then it may also require a fair bit of detective work before we are sure that what we've found is evidence of past life on Mars.

They also expect present day life to be microbial, and elusive, for different reasons.

Dallas Ellman fine tunes a component of the astrobionibbler. It uses ideas from the larger UREY instrument, using high temperature high pressure subcritical water as a solvent for non destructive extraction of organics. However, with advances in technology, it's now miniaturized to a "lab on a chip". During his summer internship at JPL in 2014, he helped discover and replicate the conditions the Astrobionibbler team needed to extract and detect amino acids from Martian regolith. As a result it is now so sensitive that it could detect a single amino acid in the sample.

Asrtobionibbler was developed from an earlier and larger instrument UREY. This was developed in the US and then was approved for the European Space Agency (ESA)'s ExoMars but was descoped when NASA pulled out of the partnership. It was too heavy to launch to Mars on Russian rockets.

After that, ESA approved the lighter Life Marker Chip, mass of 4.7 Kg, another very sensitive life detection instrument, which uses polyclonal antibodies to detect biosignatures, but it was later descoped. Another version of it, LDChip300 was tested in the very dry core of the Atacama desert and was able to detect a layer of microbial life at a depth of 2 meters below the surface from analysis of less than half a gram of material. This was a habitat that no-one had ever discovered before. Sadly, this also was descoped from ExoMars, so won't fly quite yet.

The target mass for astrobionibbler is 2.5 kilograms, a quarter of the mass of UREY. We have many other instruments especially designed by astrobiologists to search for life "in situ" on Mars, but so far, none have flown since the Viking landers in the 1970s.

To find out more about these tiny, and exquisitely sensitive instruments, see the section In situ instrument capabilities below. So far we haven't sent any instruments to search for life directly on Mars since the two Viking missions in the 1970s

With that background, the tiny microbes are the very thing you are looking for. They plan such sensitive searches that a single amino acid in the sample could be a major clue to the puzzle. With such sensitive searches, introducing Earth microbes and organics could be disastrous for ones hopes of finding out about life on Mars, either early life, or elusive present day life.

So what then is the role of humans in this vision? Well, to start with, the robots on Mars themselves are our outposts there, our ways of exploring Mars, our mobile eyes and hands on Mars. We can do more in situ Mars exploring with our rovers and the pace should increase once we have a stream of data coming back, and more capable and more autonomous rovers, and many of them on Mars. Enthusiasts and experts on Earth will be able to explore the surface of Mars in great detail in the comfort of their own homes and faculties. But humans have value in space too. Eventually I think we will have humans in orbit around Mars and on its moons exploring it via telepresence.That may be a step too far at present just for safety reasons, as we'll see. There's an obvious place to begin though, closer to home.

Value of humans in space - and fossil hunters on the Moon?

In my other kindle books and booklets, and my articles, I've written a fair bit about the value of space resources, and the many ways that humans can contribute in situ to exploring the solar system. I also argue strongly for the Moon as the obvious place to get started with human exploration. It's not just as a stepping stone to Mars. Planetary scientists have proposed calling the larger gravitationally rounded moons in our solar system "Moon planets" (paper here). The Moon is of great science interest, and not just for the polar ice deposits and the lava tube caves. It's even still geologically active, with recently formed small cliffs, trenches (formed as recently as 50 million years ago) and enigmatic almost crater free patches of strangely patterned terrain like the Ina depression that formed on its surface, some as recently as fifty million years ago, There have been many surprises, as I explored in the Moon science surprises section of my Case for Moon First.

But it is also of astrobiological interest too, as we'll see. It is a place of great interest in its own right, nearby, and far easier to visit than Mars. It also has little by way of planetary protection issues to deal with, as the Moon is Category II:

"… where there is only a remote chance that contamination carried by a spacecraft could jeopardize future exploration”. In this case we define “remote chance” as “the absence of niches (places where terrestrial microorganisms could proliferate) and/or a very low likelihood of transfer to those places.”

Could we find habitats for life on the Moon which might lead COSPAR to revise this guideline? Well in short, the chance remains "remote". There's ice at the poles but it's far too cold for life. Any warm ice or liquid water would sublimate into the atmosphere at tens of meters per year. There is currently no evidence at all for liquid water on or below the surface of the Moon. It's not theoretically impossible as water could be trapped below a layer of high molecular weight organics, and the Moon does have volatiles that escape from its interior, but the chance is so remote that I have not come across a single paper suggesting this as a possibility in recent times (it was suggested as a possibility in a paper by Carl Sagan before Apollo 11). For details see Life or prebiotic chemistry on the Moon (below)

At any rate, so far, there are no restrictions on lunar explorer astronauts.We just have to document clearly what we do. There might be issues with trash, and with organics spreading in the dust, but we'll get to that in a minute (see Trash, rocket exhausts and microbes on the Moon - testing ground for planetary protection measures for a human base (below).), but it's nothing compared to the issues for Mars.

The Moon can also help bring us with the biological search for early life, rather surprisingly, through remains of life that landed there in meteorites. It has extraordinarily cold conditions at the lunar poles. We might find fossils also, after a simulated impact on the Moon, fossil diatoms are still recognizable, and indeed the smallest ones are intact, complete fossils. There must be a lot of material from the Chicxulub impact on the Moon, which may also contain fragments of larger creatures such as the ubiquitous ammonites of Cretaceous seas. Perhaps the Moon will be one of the best places for fossil hunters in our solar system, outside of Earth.

Artist's impression of Cretaceous period ammonites, courtesy of Encarta. The Chicxulub impact made these creatures extinct. It hit shallow tropical seas and the ejecta could have sent fragments of Cretaceous period sea creatures such as ammonites all the way to the Moon. Fragments in the cold polar regions may even have the organics preserved.

The Moon must have meteorites from Mars too, for us to pick up, also from early Venus, from before its atmosphere became as thick as it is now. That's especially exciting as early Venus might have had oceans and might have been as habitable as early Earth and Mars. However, most of that record is probably erased even if we get to explore the surface of Venus. It was resurfaced by volcanic processes around 300 hundred million years ago. It doesn't have continental drift, and the leading explanation for its young cratering record is that its entire surface turns over from below from time to time. We have something similar on Earth, the superplumes. These are huge but very very slow motions deep below the surface, for instance, perhaps a superplume beneath the Pacific drives the volcanic activity around the Pacific "ring of fire". Venus may have had superplumes so large they resurfaced the entire planet.

Anyway for whatever the reason, Venus' surface is geologically young. Its atmosphere is so thick that no meteorites get from Venus to Earth right now, and anyway, a modern meteorite or a sample return would tell us nothing about early Venus. That leaves any Venus meteorites on the Moon as our best, and maybe only way to find out about early Venus, including any biology from those times. That's especially so if they have the organics preserved. For more on this see Search for life from Mars, Venus, or the Earth - on the Moon in Meteorites! below.

The Moon also must have collected organics from the comets and asteroids of the early solar system that bombarded Earth, so it can help give us an inventory of the organics that lead to kick starting life on Earth. For more about all this, see also Charles Cockell's paper: Astrobiology—What Can We Do on the Moon?

Lifeboats on the Moon

The Moon also is a far safer place to start our human exploration of space. The ISS has "lifeboat" spaceships attached at any time, with enough seats to take the entire crew back to Earth within a few hours in an emergency. We can have similar lifeboats on the Moon. They won't be able to get the astronauts back to Earth in hours, but they can take the entire crew back to Earth within a couple of days. That's not too much of a challenge, as they can be kept supplied at all times with fuel and food for the journey. On Mars or in Mars orbit it can be up to two years to get back in an emergency, which may be a step too far right now. You'd need something rather more substantial for a lifeboat to withstand two years of travel through interplanetary space.

The main problem there is life support. You can't test life support intended for a zero g environment on the ground, not properly. The ISS has had numerous life support issues which were only fixed due to resupply from Earth. See this list of some of them. None were immediately dangerous, and some were relatively minor but some of them would have been fatal on a timescale of months. If issues like that arose on a spacecraft like the ISS as far away as Mars many of those issues would have lead to the entire crew dying as they could never have got their spaceship back to Earth in time. The same would be true of other issues that arise over long timescales only, e.g. damage to equipment or to hull integrity from a micrometeorite - or else on a long mission, some problem with the packaging of the food for instance and you find most of your food has gone off and you don't have enough left to survive the journey back, or some problem with the water, or microbe films building up in a way that is harmful. Or a fire, or release of harmful chemicals, in either case damaging vital equipment for life support, or essential provisions.

The retired Canadian astronaut Chris Hadfield, former commander of the ISS, interviewed by New Scientist, put it like this in their article "We should live on the moon before a trip to Mars"

"I think ultimately we’ll be living on the moon for a generation before we get to Mars. If the world and the moon were threatened and the only way to preserve our species was to launch from Earth, we could go to Mars with yesterday’s technology, but we would probably kill just about everybody on the way."

"It’s as if you and I were in Paris, paddling around in the Seine in little canoes saying, 'We’ve got boats, we’ve got paddles, let’s go to Australia!' Australia? We can barely cross the English Channel. We’re sort of in that boat in space exploration right now. A journey to Mars is conceivable but it’s still a lot further away than most people think."

The Moon is not only safer, it's also a natural place to begin to develop reliable technology for multi-year missions throughout the solar system. If we can achieve that then the cost of human missions to the Moon will go down dramatically to a fraction of the normal cost. Imagine what a cost saving it would be if we could send a crew to the Moon for two years with no resupply from Earth, as if it was an interplanetary mission to Mars? We need these shake out cruises close to Earth first. The Moon or the L1 or L2 positions are ideal places to do them, close to Earth, and also of great interest in their own right. Once we have biological closed systems working on the Moon, then missions throughout our solar system that last for a decade or more could be as easy to support as ones that last for a couple of years or less. Once we have that capability, we can go to Venus, Mars and beyond, even to Mercury, the asteroids and Jupiter's Callisto, with no worry about narrow safety margins. Or for earlier missions, using ISS type life support to get to Mars or Venus, then we could do shake down cruises in the Earth Moon system to make sure everything is working well before we send the crew on their interplanetary missions, so far from help from Earth, or any possibility of aborting back to Earth.

The Moon is also a place to explore some of the less glamorous sides of space exploration.

Trash on the Moon

Even the Apollo crew left their share of trash on the Moon, though not that much as they were only there for up to three days. The ISS produces huge amounts of trash every year.

(click to watch on YouTube)

The ISS discards that much trash into our atmosphere every few months. Like the Apollo 11 footprints, any trash you leave on the surface of the Moon will still be there thousands, and probably millions of years into the future. So will we get huge trash heaps build up around any lunar base, and if so, how will the astronauts handle this on the Moon?

Buzz Aldrin standing near a leg of the lunar module - notice how many footprints they left on the Moon?

Every time crew land on the Moon and take off, they will leave the descent stage behind on the Moon, so that's trash too. Spacesuits need to be replaced after a number of EVAs, so that's trash. Also there's packaging for equipment, equipment that fails and needs to be replaced, dirty clothes and socks, the list goes on and on. The amounts of trash they'd leave would eventually be enough to fill caves or craters around the base. So what would they do with it all? It won't degrade , rust away, and mix into the landscape. Also surely they wouldn't use precious rocket fuel to send it back to Earth?

Also, how far do organics spread in the dust? You might think hardly at all, but it turns out, that fine dust particles, large enough to carry spores and other organics, can spread out levitated at a height of a meter or more above the ground by effects of UV radiation and plasma hitting the Moon's surface, through electrostatic levitation. A few spores spreading out over the surface won't do much to confuse science results kilometers away, but they could be significant if the base is close to an area of special interest, such as volatiles at the poles. Luckily there are large areas of volatiles, probably, but still, it may be a consideration, as this review paper from 2007, the authors suggest that perhaps we might need to set up "organic special regions" on the Moon that need to be kept free of organics.

The historical lunar landing sites may also need protection from contamination by Earth microbes, as "valuable and limited resource for conducting studies on the effects of humankind’s initial contact with the Moon" (quote from page 774 of this paper) They are also decades long unplanned experiments in the interplanetary cruise stage of panspermia - the ability of microbes to remain viable in the conditions of interplanetary space for transfer from one body to another.

Another planetary protection question for the Moon (in its broadest sense) is whether our landers would change the Moon's very tenuous "atmosphere" or exosphere with rocket exhausts. We have a golden opportunity right now to observe its atmosphere "as is". The rocket exhausts from Apollo added nearly half as much again to the mass of the lunar atmosphere, for a month. But this should have dissipated long ago. Amongst other things we can study the movement of water vapour in the lunar atmosphere and see where it comes from.

When the Chinese Chang'e 3 landed on the Moon on 14th December 2013, NASA's LADEE was in orbit, and, surprisingly, they found that, if it modified the lunar atmosphere at all, the changes were beyond their detection limits.

However the rocket exhausts could have significant local effects. This is a model of the effects of the Apollo 17 landing exhausts on the lunar surface near their landing site:

Figure 28 from this paper showing their modeled rocket exhaust contamination of the lunar surface from Apollo 17 superimposed over Google Moon. The contaminated area spans 522 kilometers of the lunar surface. The red range rings contain 50% and 67% of the total contamination respectively.

This modeling suggests we may need to take care about the effects of rocket exhausts from spacecraft landing in the vicinity of the lunar village, especially once the larger rockets start landing with astronauts on board. The authors of the paper looked at ways the contamination can be reduced, including pointing the engine over the horizon during braking maneuvers with the exhaust gas velocity much more than the lunar escape velocity.

I have another suggestion, to use Hoyt's Cislunar tether transport system. His lunar tether can be built with existing materials. It doesn’t require anything exotic like carbon nanotubes. Once it is in place, you would no longer need to use rockets to land on the Moon or take off from it. His orbital tether in LEO is low mass also. Once we have regular transport back and forth between the Moon and Earth, it won’t take that many flights before this is an economical way to do it. And after that, it’s essentially free as far as fuel costs go.

For details see the Exporting materials from the Moon section of my Case for Moon First.

In short, the Moon is an ideal place to study these issues before we have to deal with them further afield. Even in a mission to Phobos, then the regolith contains a valuable record of impact ejecta from Mars, right back to the early solar system when it may have had life or pre-biotic chemistry. So, it might be rather important to keep the surface of Phobos free from trash and organic contaminants during a human mission there. If so, our early experiences on the Moon could help us devise suitable precautions, ones that we know work.

This is a summary of my sections: Trash, rocket exhausts and microbes on the Moon - testing ground for planetary protection measures for a human base and Rocket exhausts, microbial spores and organics mixing with levitating lunar dust (below) which go into more details with cites.

Alternative visions for the future

Here are my two online and kindle books where I go into detail about the value and interest of the Moon and also various challenges we'd face there. The aim is to sketch out an alternative positive and exciting future for humans in space with planetary protection and the science value of our explorations there as core principles.

"MOON FIRST Why Humans on Mars Right Now Are Bad for Science", available on kindle, or Read on my website (free)

The cover of that book shows the result of replacing the black night skies of the Apollo photographs with a blue one. This helps to bring out how "Earth like" the Moon really is, in many ways that might surprise you, a theme I expand on in the book.

Case For Moon First: Gateway to Entire Solar System - Open Ended Exploration, Planetary Protection at its Heart on kindle or Read on my website (free).

This is the first book I wrote on the Case for Moon first. In a point for point comparison with Mars, I was as surprised to find that Earth often wins hands down over Mars in terms of the resources available, the ease of using them, and compatible conditions for humans to build bases. The thin carbon dioxide atmosphere of Mars is not much of a benefit, when you realize that carbon dioxide is normally a problem gas to get rid of. Plants need only a few kilograms of carbon dioxide in a habitat greenhouse atmosphere, in trace quantities, to grow. Meanwhile the lunar vacuum is actually a benefit in many ways. There's pure iron on the surface, not oxidized, mixed in the dust, nanoscale iron. This makes it as easy to melt a kilogram of dust with microwaves as to boil a kettle. The hard vacuum also has many benefits, including solar cells that are easier to make on the Moon than on Earth. The close to 24 hour day of Mars is actually a major issue because of the huge temperature swings from day to night, at day time up to well above 0 °C, and at night, for 100 days of the year, it's cold enough for dry ice to form and precipitate out of the atmosphere as the Martian frosts (mixed in with ice). Meanwhile temperatures at the lunar poles are stable, within 10 °C or so, 24/7 and year round. The lunar caves also have stable temperatures. This makes it much easier to design the base and keep the temperatures regulated. At the poles, then the sun always comes from close to the horizon, so you can track it with a vertical solar panel or mirror spinning slowly once a month on a single vertical axis. Meanwhile heat rejection is easy, with horizontal radiators around the base, that are always in shadow, radiating the heat upwards.

That's just a few of numerous comparisons where the Moon scores over Mars. For a bulleted list of some of the advantages of the Moon over Mars, see Why the Moon is best for humans right now (below)

It's also close enough to Earth for there to be at least some possibility of commercial exports such as platinum as Dennis Wingo has suggested in his books on lunar colonization. It beats Mars on almost everything. At least, it does, for as long as you are thinking of populations of up to a few tens of thousands. If the caves can be made into habitats too, then the Moon could have populations of many millions, and it may be the easiest place in our solar system to do this, with economic support from Earth from exports from the Moon, tourism, etc. Similarly, if "city dome" type habitats become practical, they could be habitats for millions.

Longer term, using resources from the asteroid belt then we could have off world populations of trillions, with a total habitable surface area a thousand times that of Mars. There is no need to colonize Mars, in either the near or more distant future, no matter how keen you are for humans to set up habitats outside of Earth. The key to all this, I think, is maintenance. If a large city sized habitat can be closed system, producing all its own food, air and water, and low maintenance, once built, perhaps hundreds of thousands of dollars per inhabitant, then it may even be easier to live there than it is on Earth, at least once you have paid off the huge costs of building the colony in the first place. If the maintenance costs are high, costing millions of dollars per inhabitant per year, then I don't see how it can work at all. (See Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths)

In those books I also argue that with our lunar adventures, we will learn what humans can and can't do in space, and how to stay healthy there. We can also learn how to be self sufficient for months and then years at a time, without resupply from Earth. If we can do that on the Moon it will reduce costs hugely. Once we've done that, it will also be much more practical and safe to send humans not just to Mars but to the Venus clouds, Mercury, asteroids and further afield. Even Jupiter's Callisto, which orbits just outside its dangerously intense radiation belt, is less than two years journey away on a fast Hohmann transfer orbit from Earth (see Sending humans to Callisto or Ganymede (below) ) . Once we know how to keep humans healthy in space for years on end, then Callisto also should be within reach of Earth.

Once it is safe to send humans to Mars orbit, we can use this to explore the surface in an immersive way. This is similar to exploring a three dimensional virtual world in a computer game, but this time the world explored is real. Telepresence like this may be a great way to explore the Moon too, so we can gain experience of this on the Moon first. This virtual way of touching Mars, especially when combined with haptic feedback, is in some ways more immediate than touching in a spacesuit, and is an exciting and adventurous alternative vision for humans in space. It is also safer, and has none of the irreversible and possibly devastating consequences for science of landing on the Mars surface directly, with all the microbes that inevitably accompany us.

I think it's best to say all that from the outset as I've found in the past that my readers sometimes see my articles as an attempt to stop humans from exploring space. Far from it, I'm a science fiction geek and long term enthusiast for humans in space since the time of Apollo. As a teenager I found those missions exciting and followed them keenly.

Humans on Mars are not the problem. The problem comes with the microbes that accompany us, in the air, in our water and food, and indeed on and in our bodies too, trillions of them, that can't be removed or we'd die. These include microbes capable of living in extreme environments, since many extremophiles retain their ability to survive in the most ordinary conditions. So, though capable of living in conditions extreme as Mars, they can also manage just fine on and in our bodies, and on the surfaces of our spaceships. Even the organics that make up our bodies, and our food, human wastes etc could be a problem in the event of a crash on Mars, as we'll see. They could confuse those astrobiological searches for elusive degraded organics, with instruments sensitive to a single amino acid in a sample.

If we explore Mars via telepresence, from orbit, we can be there in person without these possibly devastating consequences of touching Mars.

12th April 2011: Cady Coleman takes pictures of the Earth from inside the cupola.- I've "photoshopped" in Hubble's photograph of Mars from 2003 to give an impression of the view of an astronaut exploring Mars from orbit. One of the orbits suggested for exploring Mars is particularly exciting, the sun synchronous Molniya orbit. It comes in close twice a day, approaching opposite sides of Mars, always with Mars lit up by full sunshine. It's an orbit that skims close to the ice caps of Mars on the way in and out each time, much like the view in this photograph. It flies low over the surface of Mars when closest to the planet. Then as it recedes, Mars dwindles to a distant planet, and the cycle repeats twice a day. It would provide great views of Mars, continually changing, in an exciting orbit.

The approach would be the same as for the Moon. Just as the Moon is of great interest in its own right, and doesn't need to be justified as a "stepping stone" to Mars, in the same way also, a Mars orbit optimal for telepresence exploration would be a destination for humans of interest in its own right. The immediate aim would be to find out more about Mars and its moons. It doesn't need to be justified as a "stepping stone" for humans to the Mars surface. I think we need to leave the future open. Who knows what future it might lead to, if we leave our options open after that?

Should we return samples from Mars right now?

Carl Sagan, Joshua Lederberg and others didn't just warn about the planetary protection risk in the forward direction, the potentially harmful effects of Earth life on Mars. They were also deeply concerned about the potential for Mars life returned to Earth to harm us, or the biosphere of Earth. What we discover on Mars, in some of the most interesting scenarios, could be microbial ETs. Though only microbes, they could be far beyond anything we could discover through genetic modification or adding extra bases to DNA or even using a different biopolymer from DNA to carry the genetic information in a cell.

However, this also means that what we return could be unlike anything we know about or can make in a laboratory as an artificial lifeform, or even describe in detail. Even if we replace DNA with a different biopolymer in our lab experiments, the result is just a form of Earth life, with the same genetic code, translation tables, and cell machinery. Only one (very important) part of the cell would be changed. We simply don't know enough yet to try anything more original than that. Life from Mars could be far more exotic than that, with numerous details of how the cells work different in fundamental ways throughout. So is it safe to return something to Earth that could potentially have such a revolutionary biology as that (if we find it)? If so, how can we do this safely?

Joshua Lederberg put it like this:

"If Martian microorganisms ever make it here, will they be totally mystified and defeated by terrestrial metabolism, perhaps even before they challenge immune defenses? Or will they have a field day in light of our own total naivete in dealing with their “aggressins”?

from: "Paradoxes of the Host-Parasite Relationship"

Would Earth life have biological defense mechanisms to counter a threat it has never faced based on a totally unfamiliar biochemistry? Our defenses can work if it resembles Earth biochemistry, or if, by coincidence, it has effects sufficiently similar to a threat posed by an Earth organism to be recognized as such. So the defenses might fail at the first hurdle, not recognizing it as a threat.

We don't know the answer to that. If the martian microbes do "have a field day in light of our own total naivete" then in the worst case, the prospect for Earth's biosphere could be dire. The physicist Claudius Gros briefly describes the potential results of a clash of biospheres in his "Genesis project" to develop ecospheres on transiently habitable planets (see section 4.2 Biosphere compatibilities of this paper). Here, he makes an interesting additional point. Generally our biology only evolves defense mechanisms for a threat which is actually present, not just one that is a theoretical possibility.

"Key to the functioning of an immune reaction is the recognition of ‘non-self’, which is achieved in turn by the ability of the immune systems, at least on earth, to recognize certain products of microbial metabolism that are unique to microbiota. How likely is it then, that ‘non-self’ recognition will work also for alien microbes?"

"Here we presume, that general evolutionary principles hold. Namely, that biological defense mechanisms evolve only when the threat is actually present and not just a theoretical possibility. Under this assumption the outlook for two clashing complex biospheres becomes quite dire."

"In the best case scenario the microbes of one of the biospheres will eat at first through the higher multicellular organism of the other biosphere. Primitive multicellular organism may however survive the onslaught through a strategy involving rapid reproduction and adapt ion. ... "

"In the worst case scenario more or less all multicellular organism of the planet targeted for human settlement would be eradicated. The host planet would then be reduced to a microbial slush in a pre-cambrian state, with considerably prolonged recovery times. The leftovers of the terrestrial and the indigenous biospheres may coexist in the end in terms of ‘shadow biospheres’ "

Primitive multicellular organisms might evolve rapidly enough to cope, but larger ones would probably all be made extinct. In the very worst case, perhaps even primitive multicellular life with their short lifespans and rapid evolution can't adapt quickly enough.

In a situation like that, we could only survive in enclosed habitats on Earth, in bubbles of terrestrial ecology, similar to space settlement habitats.We ,might then try to restore some of Earth's habitats by a process of gradual paraterraforming in enclosed greenhouses.

With no previous experience of a clash of biospheres, then how can we assess the likelihood of this?

It's not enough to recognize the microbes as a threat, our defenses also have to be active and respond, to stop them from causing harm as well. So, that leads to a thought (not in the sources I read). If what we have is an unusual form of biochemistry, could the attempts by our defenses to annul the threat actually make things worse, for instance provide the microbes with chemicals that they find useful, as food or in other ways, instead of harming them?

Also, there is more to it than that. The Martian life doesn't have to attack us or any other terrestrial life directly. There are many ways that microbes can harm us indirectly, with examples such as botulism, ergot disease, BMAA etc, see Many microbes harmful to humans are not "keyed to their hosts" (below) . Or they could harm us just by changing the environment. For the most famous example of this, the first photosynthetic life on Earth, which may have caused an early mass extinction on Earth, didn't directly attack anaerobic life. It just produced oxygen as a byproduct which made much of the biosphere poisonous to it, including the ocean. Could life returned from Mars, Europa etc do something similar, if perhaps on a smaller scale? Could it transform parts of our ecosystem just by creating different byproducts from Earth life, and so make it inhospitable to some forms of life native to our planet? I cover an example of this sort of thing in the section Invasive diatoms in Earth inland seas, lakes and rivers (below) .

Given Mars’ history then it's a reasonable hypothesis that whatever life there is may be an early form of life based on what we have so far. But it also seems a reasonable hypothesis that we could find life there that is more highly evolved than Earth life, even if microbial. By more highly evolved, I mean here, in the complexity of its biochemistry, as measured, for instance, by the amount of non redundant nucleotides. It might still be microbial, at least in the harsh Mars surface environment, but with microbes equivalent to those that might evolve on Earth several billion years from now.

One way this could happen is if the harsh conditions early on stimulated rapid evolution. Could it have had eukaryotes, the cells with a nucleus which are the basis for most modern complex life, already there 3 billion years ago? Anyone who searches optimistically for easily recognizable fossils in Curiosity images is hoping that Mars life is at least 2.5 billion years ahead of Earth life in evolution - or at least, was that far ahead of us up to 3 billion years ago. So that raises the possibility of life on Mars that is harmful to the biosphere of Earth as a result of being more evolved than Earth life. Or, at least, life that is as far evolved as Earth life is, and with more robust adaptations than Earth life for some particular type of habitat.

A more evolved microbe from Mars might not necessarily cause problems right away, or cause problems to humans. It might not cause any problems at all in quarantine facilities or laboratory tests. For instance suppose it is better at photosynthesis, or in some other way, is a a form of microbe that takes over from some key component of our biosphere? Or suppose it is harmless to humans but harmful to some other animals or plants on Earth, in a way that's not obvious until it gets into our biosphere? Also, maybe it needs to adapt to some challenge on Earth, then causes a problem, something microbes can do rapidly because of their short life span and rapid evolution. Or it might transfer the capabilities to Earth microbes through lateral gene transfer, via gene transfer agents, or in the other direction, our Earth microbes might give the Mars microbes the capabilities they need to thrive here via GTAs.

I can imagine that perhaps in the vastness of our universe there are places where there are planets like Earth with intelligent species living on them, and other planets or moons nearby, which unlike our Moon, have indigenous life of their own. The intelligent species on these planets, when they develop space travel, are likely to bring back life by accident, as we would surely have done if there had been native life on the Moon. Perhaps in most of the cases no harm is done, or it is a minor nuisance, like problem species of plants returned from another continent. But perhaps in a few cases the life brought back may severely degrade their biosphere and make it less habitable for them. Who can say, perhaps in a very few cases, especially if done as early as Apollo when their technology isn’t very sophisticated, perhaps they go extinct as a direct result of their first mission to a nearby Moon or planet? Perhaps looking at it this way, as something that might actually happen, or have happened to other extra terrestrials, could help put it in perspective?

These concerns don't make a sample return impossible, but they greatly complicate it, in ways which the proponents of a sample return may not have fully taken account of. If we knew what we were returning, we could handle it much as we do for comet and asteroid sample returns. If we can assess what it is, and show that it is harmless (supposing it is), then the process will be easy. Or if there is a known risk, then we can work out how to deal with it. If we knew that a sample canister contains anthrax spores, as Carl Sagan once put it, we would take great care. But we'd also know what measures to take to return it safely. So similarly if we know that the Mars life is hazardous, but know what the hazard is, we can plan accordingly.

What makes it really tricky is that the plan is to return a sample long before we know what is in it, so we can't assess its capabilities prior to the return. In that situation, everything gets so much more complicated. We end up having to design a complex facility to deal with any conceivable form of microbial exobiology. How confident can we be in such a facility, when we don't yet have a single example of any non terrestrial life to base the precautions on? For instance we wouldn't know how small the life can be or what its capabilities might be. The facility ends up having to be hugely over engineered, to return less than a kilogram of material that may not even include any life at all, and even then it's hard to have total confidence in it, with the experts adding more and more requirements to the specifications with each new study, based on new scientific discoveries about possible capabilities of tiny forms of life. And then there's the ever present risk of the canister being breached in some way before it reaches the facility, through accident, or even malicious actions of some sort, or of the precautions being bypassed by forgetful human operatives.

Just the legal processes involved may involve much more than most of you would expect, if you haven't looked into it. Margaret Race did, and found out that there are with numerous domestic and international laws to be passed and probably also involving domestic laws to be passed in other nations as well before it can go ahead. Our legal situation is far more complex than it was in the 1960s, to the extent that if NASA was really serious about doing a sample return in the 2030s they probably should have started on the process already, to have a chance of completing the legal process in time. The 1960s regulations were inadequate, but anyway, they have also been repealed so can no longer be used "as is". I cover this in Legal complexities (below)

I think that when faced with the "sticker shock" of the cost of the sample return facility (most recent estimate was half a billion dollars), combined with the complexity of the legal situation, it's possible that NASA might end up either sterilizing their Mars sample, or returning it to somewhere outside of Earth. Both of these ideas are legally and practically far simpler to achieve. I suggest a good place to return it would be to above GEO. For the reasons in detail, see the section: If likely to be of greater astrobiological interest - return samples to above GEO

The cost is a not insignificant factor also, NASA is "betting the ranch" on an extremely expensive plan to return a half kilogram of samples of rock from Mars in the 2030s at a cost of millions of dollars per gram. We already have samples from Mars in the form of Mars meteorites. Many astrobiologists have written papers saying that in their professional opinion, this is a very long shot. They expect these samples to be as controversial and hard to interpret as the meteorites we already have. They say that what we need to answer many of the central questions in their field is more in situ research with our exquisitely sensitive and light weight modern life and biosignature detection instruments, and that the money set aside for this mission could be used much more productively for in situ life detection on Mars.

I cover this interesting controversy in detail. I also ask if this emphasis on returning samples so soon could end up as a huge embarrassment for NASA in the 2030s, if the mission returns with samples that are ambiguous, as the astrobiologists are predicting, and don't answer any of their questions about life on Mars.

The image here is a detail of one of the less well known close up electron microscope photographs of ALH84001, the controversial meteorite that was first announced as the potentially the first discovery of life on Mars, but later the announcement was withdrawn as premature. It remains controversial to this day, with astrobiologists arguing both sides of the case.

I made this “Future Possible News” story with this online spoof newspaper generator

I cover this in the sections:

• What if Mars has really tiny cells - like the structures in the Mars meteorite ALH84001?
• Idea of returning samples from Mars to Earth - how does it compare with an in situ search for astrobiology?

Human settlement and exploration - hugely positive or hugely negative - it all depends how it is done

Of course many of my readers will be keen on human settlement in space. Though that's not the focus of this book, I should just touch on it briefly, in a few pages, because if I don't many of you will have this as one of the top questions on your mind. I want this book to have a positive message, and to do what I can to help us to find the way to a future that is inspiring and worthwhile.

So, first I argue in my Case For Moon First: that the Moon has huge advantages over Mars, in almost all respects, as a place to send humans. For a bulleted list of some of them, see Why the Moon is best for humans right now (below)

Now, that doesn't mean that the Moon or Mars, or indeed anywhere else is a good place to colonize right now. They are just so very inhospitable. Whatever you think about the feasibility of Elon Musk and Robert Zubrin's ideas for colonizing Mars, perhaps you can see that if you are in a desert anywhere on Earth, and there is sea nearby, and a breathable atmosphere, you have resources beyond the wildest dreams of a “Mars or Moon colonist”. A desert or a patch of our sea, with breathable air what's more, would seem an absolute paradise on Mars or the Moon.

Here where I live on the West coast of Scotland we have numerous uninhabited islands that are thought to be not worth living on because they don't have any sources of fresh water. Their owners only visit them for a few days a year, and for the rest of the year, at most, they may put a few sheep on them to graze. The same must surely be true in many places. Even islands that do have fresh water are often uninhabited here too, because it is just too inconvenient to have to take a boat every time you want to go to the shops or the post office, and maybe sometimes not be able to travel at all because of storms.

So never mind uninhabited deserts, we have many uninhabited islands, just because of the inconvenience of travel, or lack of fresh water. These are places surrounded by the sea, which you could turn into fresh water with a desalination plant, and indeed, produce salt as a byproduct. Also with plenty of rain that you could collect with water catchment areas. These issues would be comparatively easy to solve with modern technology, but nobody is bothered to do it because there are far easier and less expensive ways to live on Earth. Similarly, there are vast areas of Canada, Siberia etc, far more habitable even than our deserts, which are almost totally uninhabited. Hardly anyone is interested in colonizing these places.

It's the same also if you are floating in the sea on a floating platform or boat. The resources available to you just from sea water and sunlight, the air to breathe, and perhaps a few rocks from the sea bottom, would be far far beyond anything you could expect anywhere in space outside of Earth. That's without fishing, or using the sea in any way at all, just using the sea water itself.

Idea of the Seasteading Institute to set up floating islands, gardens floating in the sea. They would be rather like space habitats in how they function. No need to fish or exploit the sea at all, but instead in a sustainable way just use the air from our atmosphere and sea water from the sea to sustain their population along with solar power from the sun. It's like a space habitat but without the need for radiation shielding or environment control and life support.

An artificial island, floating on the sea, using nothing but sea water and air to sustain itself, would be an absolute paradise compared to Mars, and far far easier to make self sustaining.

Also much of the land surface of Earth is uninhabited or barely habited.

This shows the estimated world population in 2015. The black areas here have less than one inhabitant per square kilometer, and the white areas have none. These areas are far more habitable than Mars. The seas too of course are far more habitable.

Map from the gridded population of the world, version 4, non UN adjusted, downloaded from their gallery page here. I've recoloured the <1 areas in black to make them easier to pick out.

We could also use ideas from space habitats to support ourselves in deserts, in a self sustaining way, using only the desert sands, sea water and the air. This may seem a little idealistic and perhaps bordering on fantasy to some. But compared to the plans of Mars colonization, this is easy peasy. We could easily support several times the Earth's population just from a small patch of the Pacific ocean in a sustainable way in these floating sea cities. It would require far far less by way of resources and technology than supporting a similar population on Mars.

This doesn't make off world habitats impossible. But you need some other strong motivation to be there - also some financial benefit that can't be obtained more cheaply with robots or telerobots controlled from Earth. In the near future - that suggests bases for explorers, tourists, and research scientists. It would work much like the bases in Antarctica. Longer term - if large habitats can be made so self contained that the maintenance drops almost to zero, then perhaps they can become as easy to live in as Earth. But that prospect seems a long way away at present.

I cover this in more detail towards the end of my An astronaut gardener on the Moon in Why Humans on Mars First are Bad for Science.

Mars fantasies and realities - a sobering dose of common sense

If you've only read the articles and books, and listened to Mars colonization enthusiasts, as they wax lyrical in realms of fantasy about future Mars cities and a terraformed Mars, you may not realize that there are others who are profoundly skeptical about it all, bringing a perhaps sobering dose of common sense. Paul Spudis, senior staff scientist at the Lunar and Planetary institute in Houston, and author of The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources. is particularly scathing about these ideas of a Martian colony in the near future. If you haven't come across these views before, his Delusions of a Mars Colonist may give you an interestingly different perspective from the stories that get widely publicized extolling the virtues of Mars colonization.

"So aside from the inconvenient facts that we don’t know how to safely make the voyage, how to land on the planet, what the detailed chemistry of the soil is, or if we can access potable water, whether we can then grow food locally, or how to build habitats to shield us from the numbing cold and hostile surface environment, don’t know what protection is needed due to the toxic soil chemistry, or how to generate enough electrical power to build and operate an outpost or settlement – in spite of these annoying details that make this idea prohibitive, the creation of a Mars colony within a decade is marketed to the public as if the plans had already been drawn up."

..."With flashy artwork depicting futuristic cities, sleek flying cars, and lush green fields resplendent under transparent crystal domes (in startling contrast to the red-hued surrounding desert of the martian surface) it is simply assumed that a human colony on Mars will evolve into some kind of off-Earth utopia."

"But how will these future Mars inhabitants make a living? And by that, I mean what product or service will they offer that anybody on Earth will want? If you think that the answer is autarky (complete economic isolation and self-sufficiency), then you are imagining an economy (and likely, a political state) in which North Korea is a free market, pluralistic paradise by comparison. People who migrate to Mars need more than food and shelter – they will need imports from Earth, material and intellectual products designed to enrich and refine life on the frontier. What will they have of value to trade or to sell for these imports?"

..."Much is made of the possible economic value of “information,” but it is not clear that Mars is particularly rich in factual data marketable to those back on Earth, although a martian pioneer might have desperate need of it – which would make them their own “customers” and exacerbate the economic disparity of the colony to an even greater degree."

The Mars enthusiasts' plans get particularly sketchy when they cover the economics of a Mars colony (while Moon firsters tend to cover lunar economics in great detail). There is only one short, and perhaps not very convincing chapter on this in Zubrin's Case for Mars. In this chapter, he relies on exports of intellectual property rights by the inventive Mars colonists as one of the most important ways to pay for the colony.

I have to admit to being very skeptical that a "Mars colony" could come to anything, apart from planetary protection considerations. Elon Musk's idea that you could sell your house for $200,000, buy a ticket to a "new world" on Mars, and set up home there, particularly, seems bordering on fantasy. You've sold your house on Earth - to pay for your trip - but you still need somewhere to live on Mars. Is he going to provide free houses on Mars for all his colonists? Surely not. A house on Mars would be vastly more expensive than one on Earth. He would no longer be making a profit on every colonist, but rather, an immense loss. Even Elon Musk couldn't sustain a business shipping a hundred colonists to Mars at a time while making a loss of millions of dollars per colonist.

Also, it's not much use being on Mars without an EVA spacesuit. There are two main kinds of spacesuit, the IVA suit you wear inside a spacecraft, e.g. during launch, designed to protect you if you get a loss of pressure, and the EVA suit which protects your for missions outside your spacecraft or habitat (Extra Vehicular Activities). You would need both, but the EVA suit is the most expensive of the two. It's a little hard to get hold of unit cost estimates for a spacesuit, as they are hardly consumer items yet, and you hear varying estimates on the cost. For instance, according to one rough estimate, it will set you back $2 million as the approximate cost of making a spare EMU for the ISS. That's not including the design cost. It is just the cost for someone to make it, after the design is completed. It requires about 5,000 hours of work and would take someone who had all the necessary skills about two and a half years to build, given supply of all the parts and materials needed, a long job involving many complex intricate components. That may surprise you, but a spacesuit is not an "off the shelf" item. Building one is not unlike building a spaceship. Basically it is a very small mobile spaceship with its own independent life support. It currently costs $100,000 per astronaut just to fit the airtight bladder inside their gloves to help reduce the risk of them losing their fingernails as a result of the stiffness of the gloves, and to make the gloves a bit more comfortable. So, that's half the cost of your house, and so half the cost of your trip to Mars, already blown on this airtight bladder

For some other examples, to give an idea of the total cost of a suit including design, a 1998 Washington post report says that NASA paid $10.4 million per suit for it's initial order of twelve EMU suits (that's about $15.6 million in 2017 dollars), and the Chinese EVA spacesuits are reported as costing $4.4 million each. The Apollo spacesuits cost less per person but were less capable and only needed to last for three EVA's each. Your suit also will need to be maintained and repaired, which itself is a tricky job, and it has a finite life too, the 1998 EMUs were certified for 25 space walks each before they needed to be returned to Earth for expensive overhaul. A Mars suit would need to have a longer design life than that surely. At any rate, the cost would surely be over a million dollars for your suit, and it would need to be replaced or have expensive overhauls at regular intervals.

We don't really have the technology of a durable, low maintenance deep space suit capable of doing large numbers of space walks yet, which is likely to require many new innovations. So, our reality is a fair bit away yet from the spacesuits of science fiction.

(click to watch on YouTube) .

Suitsat - a Russian Orlon suit that reached the end of its useful life, discarded as a satellite experiment. With current technology at least, your "Mars suit", as complex as a small spaceship, would probably cost over $1 million to build, would need a lot of maintenance, and after using it for a couple of dozen EVA's, it would need to be discarded and replaced by a new one, or sent back to Earth for reservicing. Is it true that Mars colonists could pay for their spacesuits, and everything else they need, through their inventions and other intellectual property, which they sell back to Earth?

Then, to survive in your habitat for any length of time, you need complex life support for that too. You need to have oxygen supplied all the time.But as well as that, you need to have carbon dioxide scrubbed all the time too,as we can't survive long if levels build up to as high as 1% of the atmosphere, which doesn't take long in a small enclosed habitat. Many other noxious gases like hydrogen sulfide and sulfur dioxide will build up in the habitat too, like "sick building syndrome" to the nth degree. You can't just open a few windows to air your house, so all those have to be scrubbed. Then you have to filter out potentially harmful build ups of microbes too.

How are you going to pay for all that technology, which also is likely to need a fair bit of servicing? Then as well as that, you need solar power or some source of power to run all the equipment. You need batteries, or nuclear power to survive dust storms that blot out the sun. Then you have to have a habitat that can hold in the atmosphere at a pressure of ten tons per square meter outwards pressure. You also need radiation shielding meters thick covering it to protect from cosmic radiation and solar storms. How much does that kind of a "house" cost to build? You can't build it on Mars, except the shielding. All the rest has to be imported from Earth. Also if it is anything like the ISS, your habitat, which is now your only home, has a finite lifetime. After a few decades you will need to import a new "house" to replace the old one which is now aged so much in the harsh space environment, surrounded by vacuum, huge temperature changes every day, that it is no longer worth repairing.

Nothing grows there. You are suddenly in the middle of a desert, with no water, maybe ice but it has to be melted to be used, a few rocks, and most difficult of all, no air to breathe. You never needed to think about how to get air to breathe when you were back on Earth, and no Earth colonist has ever had to give this even a moment of thought. Now it is your topmost priority, your main pre-occupation. Without a pressurized spacesuit you can't even go outside to repair your habitat, so the spacesuit is vital too. The average temperatures are the same as Antarctica, but it's much worse than that sounds, because the temperature swings are so extreme between day and night.

Bouvet island in the southern hemisphere, southeast side, as seen at sunrise, eight miles distant. Black and white photograph coloured by hand. Photo taken on the German Valdivia expedition. It's the most remote island in the world.

It's in the middle of nowhere. The closest large land mass is Queen Maud Land in Antarctica is 1,700 km away

Location of Bouvet island shown with a red dot, map from wikipedia.

It's wide open to colonization as it's not governed by the Antarctic treaty. It's owned by Norway. It has a land area of 49 square kilometers and 93% of it is covered by glacier. This is one of many uninhabited islands on Earth, and I chose it as the one that is closest to Mars in habitability and remoteness, given that Antarctica is off limits because of the treaty.

If Bouvet island was on Mars, the Moon or in space, it would be the most habitable place in the entire solar system apart from Earth and would seem like a paradise to space colonists. It's surrounded by liquid water, salt available in the water, masses of pure water ice in the glacier, breathable atmosphere, fully pressurized so no need for spacesuits, full Earth gravity, already protected from cosmic radiation and solar storms. Very easy access from Earth, just need to send goods on a boat, can get there in days, and, of course much faster with the modest outlet (compared to space projects) of building an airstrip there. It's much warmer than Mars too. They would probably be drawing up plans to cite a city of a million people there. It would be a far more hospitable place for Elon Musk to send his million colonists than Mars is.

You wouldn't need to fish, though it has abundant krill. You'd just set up home there, build your Mars / Moon colony type habitats, heated greenhouses to grow your food and you'd feel you were in paradise :). Yet it is uninhabited and Norway has no interest at all in colonizing it.

Mars has such wild swings in temperature between day and night that it gets so cold at night that carbon dioxide freezes out as dry ice / water ice frosts in the morning for 100 days of the two Earth year long Mars year even in the tropics. This also puts extreme thermal stress on your habitat. You get dust storms every two years which sometimes blot out the sun completely for weeks on end. If you somehow could take one of the coldest driest deserts on Earth, the Atacama desert, and elevate it to a height of 30 kilometers on Earth, you'd have the same atmospheric pressure as the lowest points on the Mars surface, that is still far more habitable than Mars (still a little oxygen in the atmosphere, more sunlight, no dust storms, easy access from Earth, ozone layer and magnetosphere to protect you somewhat), The top of Mount Everest (at 8.848 km above sea level) is far far more hospitable than Mars.

There are many more uninhabited islands that are far more habitable. Fresh water, warm climate, many are even put up for sale from time to time, so if you wanted to set up a colony you could buy one of those, and it would be a far better deal as regards habitability than Mars or the Moon, even for a city of a million people.

And how do you pay for your colony on Mars? Elon Musk's idea is that the colonists pay for all of this through inventing things.

The ISS is funded at around a billion dollars per year per astronaut for NASA’s contribution, just the operating costs (three billion dollars a year - and normally has three people in it at any time, sometimes goes up to six but rarely more than that in recent times, see how many astronauts are in space right now). The ISS itself cost around $150 billion including the cost of building it, and that's in LEO (see also my "Is the ISS the most expensive single item ever built"). We don’t yet have any example of anyone who has lived anywhere in space without immense costs like that, and it's not at all clear how they could.

Suppose Elon Musk manages an order of magnitude reduction in costs, to the extent that it costs a tenth of the current costs to LEO to get the same cargo all the way to Mars. That seems wildly optimistic to me, erring on the optimistic side. But suppose he achieves this, then it is still going to be of the order of a hundred million dollars per astronaut per year to support them and five billion dollars per astronaut to set up a habitat to last for a few decades on Mars. Yes, perhaps you can use plants to grow food, and generate your oxygen and filter and purify the air. This is not yet tested in space. Yes, it is likely to make a huge difference for long duration missions, but it only becomes economic to do so for missions of over two years. For details, see my Sending humans to Mars for flyby or orbital missions - comparison of biologically closed systems with ISS type mechanical recycling (also relevant to long duration lunar missions). Also, it does nothing to pay for spacesuits or equipment repair. The million dollars cost for the spacesuit doesn't include transport costs to get it to your habitat.

Perhaps, as he says, Mars would have a labour shortage with jobs in short supply - but what job is going to pay you an upfront cost of billions of dollars for your habitats, millions of dollars for spacesuits and other essential equipment on Mars and on going costs of perhaps (optimistically) a hundred million dollars a year per astronaut for their maintenance and repair and replacements when they wear out? What exports will Mars have that they use to pay for all those imports?

Well, Elon Musk shares Robert Zubrin's ideas that the Martian colonists in such tough situations will be so inventive they will invent a stream of inventions that transform life on Earth and earn them huge amounts of money to pay for their colony. I suppose it is understandable that he'd find this idea compelling ,considering his own inventiveness. It's based on analogies with the technological inventiveness of early settlers in the US. Again this seems bordering on fantasy to me. Surely it will be mainly the other direction, that with all their complex technology, which they will need just to survive at all, they will depend tremendously on the many discoveries we made on Earth?

Also - why doesn't this analogy work with uninhabited islands on Earth? Those are very tough situations too, you'd be short of resources and have to rely on your inventiveness to survive. To survive on Bouvet island would be a major feat, to build up a self sustaining colony using just the ice from the glacier as a source of water for a million people. Part of the problem is that your inventions would be of main value for Mars. The US colonists, they invented things that were useful to everyone world wide. But the Mars colonists will invent things that are mainly of use on Mars. And anyway the idea that the US was far more inventive than anyone else is a US narrative. I'm from the UK and we also talk about our country as the source of a flood of inventions, frequently. Here is an example.

"We're a nation of inventors, from the worldwide web to the electric vacuum cleaner - here's a rundown of our most influential innovations", intro to a list of the 50 greatest British Inventions from the UK in the Radio Times.

Putting aside national pride, which all countries have, surely for such a small country, we have indeed made many inventions here. We don't have the same narrative that it was due to a labour shortage, nor do we think of the US that way either. I'm not talking about historians here, but ordinary folk. Robert Zubrin's quote was the first I heard of this idea, which I assume from the way he put it, must be quite commonly accepted in the US. We just think that we are a nation of inventors, and leave it at that. We don't try to explain why.

At any rate if the labour shortage explanation is true of the US, surely it can't explain why we have so many inventions from the UK as we've never had a significant labour shortage here. Indeed the opposite, here technology put many skilled people out of work leading to uprisings by working people during the industrial revolution followed by military repression

The leader of the Luddites - self employed weavers who feared getting put out of work by the newly introduced weaving technology of the late eighteenth and early nineteenth century, and replaced by less skilled workers. They destroyed industrial equipment in protest. Later on agricultural workers joined in, destroying threshing machines. The UK government responded by military action against them, executions, deportation, and they made destroying industrial machinery a capital offence. The US narrative that invention was the result of a labour shortage just doesn’t work when applied to UK inventions. It was almost the opposite, inventions caused a labour shortage here, at least of skilled workers

It would take a lot by way of intellectual property and inventiveness to support a Mars colony .

Even Elon Musk with all his inventiveness and business nous would not be able to pay to support everyone in a Mars colony, and he hasn't suggested that he hopes to do so. They are on their own. Even a thousandth of the costs of the ISS (which would make it 50 million per astronaut to build, 1 million per astronaut per year maintenance) is way out of reach for all except multi millionaires. It's hard to see how anything in the near future could reduce space habitat costs to those levels or less.

Although Elon Musk doesn't suggest there is anything of commercial value on Mars worth returning to Earth, Robert Zubrin has suggested we could extract deuterium from the water there and sell it to Earth. He has a short summary here which you can read online:

"All the in-situ chemical processes required to produce the fuel, oxygen, and plastics necessary to run a Mars settlement require water electrolysis as an intermediate step. As a by product of these operations, millions, perhaps billions, of dollars worth of deuterium will be produced."

The colonists would split water to make hydrogen, so based on that, he says in his Case for Mars

"If a deuterium / hydrogen separation stage is applied to the hydrogen produced by the electrolysis operations."

But he doesn't give any details and if you try to fill in the gaps, it doesn't really pan out.

The water on Mars has only a five times enrichment over deuterium on Earth, leaving it with still only one atom in 1284 consisting of deuterium, or about 0.08%. To be useful it needs to have 99% concentration. There are many methods used to extract deuterium. Each of them requires many stages of concentration and this just saves one step of many. It also requires vast amounts of electricity to do the extraction., of the order of megawatts, and the machinery used to refine deuterium on Earth weighs thousands of tons and is the size of a skyscraper.

Heavy water plant near Arroyito, photograph by Frandres This plant produces most of the world’s deuterium, at a rate of 200 tons per year, and is powered by a nearby hydroelectric power station at Arroyito dam with a power output of 128 MW. (I'm not sure how much of that power output is used for the plant, do say if any of you know).

The equipment for extracting deuterium weighs 27,000 tons including the support structures and includes 250 heat exchangers, 240 pressure vessels, 90 gas compressors 13 reactors and 30 distillation columns. (Statistics from Arroyito Heavy Water Production Plant, Argentina)

So, I don't think deuterium extraction on Mars and export to Earth is a likely money earner for a colony. What's more, Mars has nowhere near the highest deuterium concentrations in our solar system. Venus has the highest deuterium / hydrogen ratio recorded in our solar system of 120 times Earth’s and so 24 times that on Mars in its atmosphere. Implications of the high DH ratio for the sources of water in Venus' atmosphere. Some meteorites have it 13 times more concentrated than on Earth, which makes it more than twice the Mars concentrations. Even hydrogen in the water from Venus is only around 5% deuterium, and even at those levels, it would hardly seem worth the effort to either return it to Earth or to try to enrich it in situ. Three enrichments of the deuterium 5-fold would get you to the Venus water. You have another

He also argues on page 239 that it could potentially be a great source for precious metals such as platinum, silver etc although their existence is hypothetical. It's also in his shorter statement here.

"If concentrated supplies of metals of equal or greater value than silver (such as germanium, hafnium, lanthanum, cerium, rhenium, samarium, gallium, gadolinium, gold, palladium, iridium, rubidium, platinum, rhodium, europium, and a host of others) were available on Mars, they could potentially be transported back to Earth for a substantial profit"

. Well, if we have the technology to return them from Mars we can surely return them from the Moon. The Moon may well have supplies of platinum and related metals. It has magnetic anomalies near the South Pole Aitken Basin which may be from the metal core of the 110 km diameter asteroid that impacted into the moon to form that huge crater. Paul Spudis particularly is keen on the idea of extracting platinum group metals from the Moon. For details see the Metals section in my Case for Moon First.

Elon Musk doesn't follow Robert Zubrin in this part of his thinking, as he is skeptical about space mining generally thinking it probably won't be possible to export from the asteroids -

"I'm not convinced there's a case for taking something, say, platinum, that is found in an asteroid and bringing it back to Earth."

I am optimistic about it myself, for the Moon and perhaps Near Earth Asteroids, but I can't see it working for Mars.

The same also applies to Robert Zubrin's idea that Mars would be a gateway to the asteroid belt (page 243). It's all based on the idea that we will need miners living in the asteroid belt in the near future.

"Miners operating among the asteroids will be unable to produce their necessary supplies locally. There will thus be a need to export food and other necessary goods from either Earth or Mars to the Main Belt. Mars has an overwhelming positional advantage as a location from which to conduct such trade."

It seems rather putting the cart before the horse to colonize Mars now as a way to supply asteroid miners who don't yet exist in hope that in the future Mars will be supported economically by them, and given that they may well be out competed by miners on the Moon or Near Earth asteroids.

There are other things also that would make Mars inconvenient even for miners in the asteroid belt. First, if Mars is of some interest as a pit stop to somewhere, might it not make more sense to have the way station on its outermost moon Deimos, say, to avoid dipping into its gravity well?

Also you may get a surprise if you look in detail at the minimum energy orbits from various asteroids to Mars and back. There's a useful table of "synodic periods" here - frequency of minimum energy (Hohmann) launch windows. For instance if the asteroid miners are on Vesta, this is how often you get launch windows:

• Mars to Vesta: every 3 years 10.8 months.
• Transit time 1 year, 4.2 months.

Or for Pallas

• Mars to Pallas every 3 years 2.1 months
• Transit time 1 year 6.9 months.

It's similar for Juno, Eugenia, Ceres and Pallas. You can get to the asteroid from Mars in between one year and a year and seven months. But you only have that opportunity every three years and a few months. Meanwhile you can get from Earth to any of those asteroids in at most a year and five months, with a transit time of at most a year and four months.

As for supply of food from Mars to the asteroids, well, for any particular asteroid you have an opportunity to supply them only once ever three years and more, so if you are only supplying one asteroid, you would have to stockpile it for up to three years before export, assuming you grow the crops continuously. .You could export the food every year if you had many asteroid miners to support on different asteroids, but they would still have to order their food over three years in advance of delivery.

Also, asteroid miners may well be able to grow their own food. Self sufficient habitats can be feasible all the way out to Pluto and beyond with large thin film mirrors (see Space habitats made from asteroid and comet materials get plenty of sunlight - right out to Pluto (using thin film mirrors to concentrate it) (below) /. It's not at all clear that Mars has much by way of advantages over the asteroids for growing food, especially with the Martian dust storms that block out 99% of the sunlight sometimes for weeks on end and the huge swings in temperature from day to night.

See also K. Erik Drexler's Space Development: The Case Against Mars

"To open space to settlement, we must use space for practical purposes. What could be more obvious? In the past, mining and agriculture have motivated people to pack up and settle new lands. History likewise suggests that space development will serve space science, just as mining and agriculture have stimulated geology and plant biology."

... " Mars fits in poorly. To advance space development, we need cheap resources in near-Earth space. The Moon is obvious and attractive: the velocity increment needed to escape the Moon and bring materials to near-Earth space is fairly low, and the Moon holds oxygen, rock, metals, and (perhaps) water at the poles. What is more, it can best absorb any politically-inspired mania for a planetary base, being close enough to do so at a comparatively modest cost. The asteroids are less obvious to the casual eye, but more attractive: the velocity increment needed to bring materials from suitable asteroids is lower than that of the Moon, and asteroids contain oxygen, rock, water, hydrocarbons, steel, nickel, cobalt, and precious metals."

"Mars is not even in the running. Jesco von Puttkamer of NASA, an apparent advocate of men-to-Mars admits that "... Such a program would be unlikely to provide nonterrestrial materials in the foreseeable future as a lunar base or asteroid mining program might do ..." Since hardly anyone argues otherwise, this should seal the case against Mars as a goal for the next phase of space development."

"Why, then, do some cry out for expeditions to Mars, as at the recent Case for Mars II conference in Boulder?..."

"... The martian dream also has roots in the traditional thinking of those antique times when "space" meant chiefly "space exploration." As a planet, Mars appeals to Earth-bound prejudices and habits of thought. It has an atmosphere, a tinted sky, weather, and a desert-like surface on which one can imagine building a cabin from wood miraculously found beyond the next barren hill. It still basks in the glow of its past reputation as an Earth-like planet and an abode of Martian civilizations, though this glow fails to warm its dry-ice polar caps."

"... Mars also benefits from the misconception that human needs demand whole planets (when even the smaller asteroids contain billions of tons of resources). ... The tendency to slight the near-Earth asteroids in favor of the more numerous main belt asteroids is another symptom of the big-needs misconception."

He is an early pioneer in molecular nanotechnology, and also in space colonization, active in the L5 Society, who worked with Gerard O'Neil on his space colony ideas. It may be interesting to read the rest of his Op. Ed .Space Development: The Case Against Mars

This isn't the main focus here but I thought I should say something on this topic, as you often get people who have read "Case for Mars" saying

"It will be easy to pay for Mars, look at all the deuterium it has".

Well it might seem so at first, but how do you spell that out into something that would work in practice?

I have also taken a good look, not just at Robert Zubrin's ideas but at many other suggestions for ways there could be a commercial case for Mars. I found a few ideas that perhaps might work but they were pure speculation at present.

If it had valuable gems unique to Mars and easy to mine and worth millions of dollars per gram, it might make a difference (the Moon perhaps could have its own unique gems too). Or, what about some equally valuable product of present day or past Mars life not found on Earth? You need something you can only find on Mars and not in the asteroid belt or on the Moon. It also must be something you can't synthesize easily outside the Moon - or the Mars provenance widely admired, some huge cachet for the genuine Mars article. Mars samples may seem promising but the price would rapidly go down.

To explore further, see my Is there a fortune to be made on Mars, the Moon or anywhere else in space? in my MOON FIRST - Why Humans on Mars Right Now Are Bad for Science - this section of that book was previously published in Forbes magazine as "Is There A Fortune To Be Made On Mars?"

Earth best for a backup - maybe with a small knowledge and seeds library on the Moon with caretakers

Also Elon Musk and Stephen Hawking motivate space colonization by the idea that we have to make a "backup" of Earth. There are plenty of other reasons to go into space, but this one just doesn't stack up in my view. Let me explain why.

We live in a quiet suburb of our galaxy around a long lived stable star. The Earth is at no serious risk from an asteroid impact large enough to make an adaptable omnivore such as ourselves extinct with only minimal knowledge of technology. It's true that some think that humans in sub-saharan Africa were reduced to 2,000 around 70,000 years ago. But that was in a world with Neanderthals and Denisovians in other continents, and though homo sapiens may or may not have had clothing, it was long before humans had thought of growing crops or keeping animals, which didn’t happen until 10,000 BC onwards: Neolithic Revolution .

Modern humans are the least at risk of just about any of our species of going extinct. Even after a dinosaur extinction type event, even if there were only a few percent of species left, there would be something for at least millions of humans to eat and cultivate. Perhaps we'd get reduced to eating shellfish, the staple of many early hominids. Or seeds, roots, fish, animals, birds, fruit, and nuts. We are lucky to be omnivores able to eat almost anything. Also, unlike dinosaurs we can detect and deflect asteroids, evacuate impact zones, and prepare supplies and seed banks. Indeed we have had many surveys detecting asteroids already with a great deal of success, and, we already know all the ten kilometer diameter asteroids that could potentially hit us. None come anywhere close for centuries. At present only one in 147 of the objects that fly past Earth are comets, and that makes it a 1 in 147 million chance of a 10 km comet impact per century, a probability so low we can ignore it.

There is no chance at all of Earth getting hit by the larger asteroids like the ones that made the Hellas basin on Mars, and the larger craters on our Moon, as those impacts date back to the late heavy bombardment soon after the formation of the Moon. There have been no impacts that large in the cratering record of Mars, the Moon, Earth, Mercury or what we have of the history of Venus, for well over three billion years. Apparently Jupiter protects us from the largest asteroids and comets (and models confirm this).

It also doesn't make a lot of sense to go to Mars to create a habitat that will be safe from asteroid impacts. After all, Mars gets ten times the influx of asteroids over Earth, it also has no atmosphere to protect against the smaller ones that cause fireballs on Earth but could easily destroy a habitat on Mars, and if your habitat is damaged, there is no air to breathe outside of it. If you are worried about indirect effects of asteroid impacts, it's true that an asteroid impact on Mars would have mainly local effects, but that's because the whole planet is uninhabitable without very advanced technology. What asteroid impact could so devastate Earth as to make it as uninhabitable as Mars? None in the present day solar system.

We can't predict when a star will go supernova exactly, but the only stars that can go supernova are ones that are at a particular stage in their life, and they have to be massive too, for Type II supernovae, and for type Ia it needs a white dwarf companion. Our sun can't go supernova at all, it's too light. There are no nearby supernova candidates of either type

The Type Ia are the hardest to spot and it used to be that we didn't know if there were any of those close enough to be hazardous. But now we do know, through much more detailed star surveys. The nearest Type Ia candidate is IK Pegasi which at 150 light years away.That's far too far away to harm us. It’s moving away from us and the scientists think it won’t go supernova for several million years, by which time it will be perhaps 500 light years away. It would need to be within 30 light years to be harmful. The nearest type II candidates, such as Betelgeuse, are thousands of light years away and though they would become bright stars in our sky, briefly, they are of no conceivable threat to Earth.

We can also identify gamma ray burst candidates and the only candidate that seems likely is WR104, 8,000 light years away, and we now know that it is tilted at an angle of 30° - 40° (possibly as much as 45°) which would mean it would miss. See WR 104 Won't Kill Us After All - Universe Today. This is the only “Wolf Rayat star” we know of which we are facing more or less along its axis, so we are okay for those too for at least of the order of thousands of years.

The sun will eventually get too hot for humans, but this is so far into the future, that humans could evolve once more from the smallest microscopic multicellular lifeforms before it happens. There may be technological solutions, such as to block out some of the excess sunlight with orbiting sheets of mylar or similar in space, or even techniques proposed for moving the Earth slowly outwards in its orbit. And if not, well there is plenty of time. Indeed ideas to partially "terraform Mars" now, if they worked, would only be temporary, creating atmospheres that would be stripped by solar storms over those long timescales of hundreds of millions of years. So to attempt to terraform Mars now may well make it less useful for that distant future when humans might perhaps really need Mars. Also would they be humans by then? Maybe they have different environmental requirements from us.

As for issues due to our own technology, then they are as likely to arise in space colonies, as anywhere, the most technological settlements ever envisioned. When it comes to science fiction scenarios, then it's as easy to come up with a story in which space colonists endanger Earth as one in which they help Earth. Also a global nuclear war would not make Earth uninhabitable either. It would cause massive hardship of course, but it would tip Earth into a nuclear autumn, not a nuclear winter. That's based on the revised models developed after the spreading smoke from the Kuwaiti fires during the gulf war proved that the original models predicting a nuclear winter were false. The entire southern hemisphere is a nuclear free zone, and in the northern hemisphere, most of the radiation effects are over especially after airbursts, within weeks. It does leave hot spots (especially from ground strikes) that we would need to keep clear of for longer periods of time of decades or more, but it doesn't make everywhere so radioactive as to be uninhabitable. Of course we have to avoid global nuclear war, for many reasons, but this doesn't risk making us extinct.

Climate change can't make Earth uninhabitable either. The research so far says that we don't risk Earth tipping into a hot Venus, even if we follow business as usual and burn all the extractable hydrocarbons. We would need to burn at least ten times the amount available. The next step down is the moist hot greenhouse with an average surface temperature of 57 °C too hot for human habitability, and again, it seems we don't have enough hydrocarbons to burn to get to that state. However, if we followed "business as usual", it does seem possible that we could end up with a world with the wet bulb temperatures above 35 °C over much of the Earth, leaving only the higher latitudes habitable for humans without technology. That would happen at a temperature increase of 7 - 10 °C above pre-industrial. But even that would not make us extinct.

Anyway, we aren't following "business as usual". Even with the US out of the Paris agreement, still many in the US are doing their bit to reduce greenhouse gas emissions. Outside the US, then the commitment is stronger than ever to do something about it. The pledges to the Paris agreement so far should keep temperatures within 3.4 °C by 2100. That's plenty of time for many new administrations in the US. We can move to a carbon neutral world by 2100, hopefully well before then. For more about this, with cites and quotations from papers and experts, see my Hawking Says Trump Could Tip Earth To Hot Venus Climate - Is It True? What Can Earth's Climate Tip To?

I think it does make sense to do a backup of knowledge and data, even a seed bank in space, and the Moon is the obvious place for that. It's passively cooled, ideal for a seed bank, stable low temperatures, and geologically stable. For the idea of a backup of knowledge, seed, and a small settlement of caretakers on the Moon see Backup on the Moon - seed banks, libraries, and a small colony, in my Case for Moon First book.

As for humans, the best place to backup humans is surely on Earth. We need to protect and cherish our Earth as the only place in our solar system where humans can survive without advanced technology. So, I don't see this as a good motivation for sending humans into space. Rather, it's a motivation for setting up backups on Earth, if you think this is a serious risk, and also using space technology to protect Earth and move industry from Earth into space and such like. For more about all this, with details and cites, see my: Earth is the best place for a backup. in my Case for Moon First book.

Why the Moon could have a commercial case

The Moon is a bit different from Mars. It might actually have a commercial case. Though life would be very expensive there also, as for Mars, there is more chance of an income stream to pay for it. Authors like Paul Spudis etc pay a lot of attention to the commercial side of things in their plans. The big advantage the Moon has over Mars is its nearness to Earth, making exports far easier and tourism possible. It's not quite a "day trip" to get there with current rocket technology, but you could visit it, and be back within a week. Also there are various ideas that could reduce costs of transport from the Moon to Earth hugely, which wouldn't work for Mars. It's only two days travel to get there, also, with easy access any time of the year (not just every two years). It's also far far easier to get back in an emergency, which makes it much safer for humans. It's also far easier to leave the surface than for Mars, reducing export costs. Only half of the loaded (wet) weight of the rocket has to be fuel to export materials to lunar orbit. Also there's the possibility of ice at its poles, combined with solar power available 24/7 year round as a source of abundant power. Paul Spudis and others believe it will be economic to supply this ice as water and rocket fuel to LEO, outcompeting water sent from Earth. Water is vital to humans in space and very expensive to send to orbit from Earth, at present.

So, the "Moon firsters", though they do tend to be rather optimistic at times about the commercial value of the Moon, also tend to be far more realistic than the "Mars firsters" in my experience . They are not so involved in these ideas that seem to belong more in science fiction and fantasy than in real life, of just setting up home as if you could build a log cabin on Mars and live off the land. You may be interested in my Is there a fortune to be made on Mars, the Moon or anywhere else in space? in my "MOON FIRST Why Humans on Mars Right Now Are Bad for Science" (it was also featured as an article in Forbes magazine). It compares the economic case for Mars and for the Moon.

In “We Need to Stop Talking About Space as a ‘Frontier’.” by Lisa Messeri she suggested that language helps and that perhaps we need to stop thinking about space as a "Frontier" with its unfortunate connotations of damage to the environment of North America, and the destruction of American Indian peoples and cultures.

"Comparing outer space to the frontier is so prevalent that it’s sometimes hard to remember that it is a metaphor, not an accurate portrayal of what lies beyond Earth. The commercial space industry prides itself on newness and novelty, and yet the reliance on the same old metaphor both limits the imagination of humans in space and glosses over the social and historical problems of imagining a frontier that is empty and beckoning."

..." But mobilizations of the frontier metaphor from Turner to today don’t just ignore the historical reality of war, disease, and environmental destruction. The Americanness of the frontier metaphor is also at odds with the need for international cooperation in the new era of space exploration. While the frontier might inspire Tumlinson and his fellow American baby boomers, does it have salience more broadly? As we try and move from a model of space competition to space cooperation, does the frontier, which necessarily pits “us” against “them,” undermine the peaceful expansion many imagine?"

Steven Lyle Jordan put it rather well, I thought, in his blog post: Space is not a frontier, commenting on her article - why not refer to space as our "environment" rather than our frontier?

"There is lots of room for expansion in the Environment… but absolutely no guarantee that we can, in fact, expand beyond this oasis and thrive. Most of the Environment is downright hostile to us. Intelligence might allow us to figure out a way… but the uncontrolled elements of that vast Environment may eventually doom us to non-existence anyway. Once more… we have no way to know. But there’s nothing stopping us from trying; only the incredible difficulty and unlikelihood of succeeding."

"The word “environment” embodies the knowledge of science and nature, the desire to experience it and learn what is learnable… but not to desecrate, strip-mine or destroy it for personal gain. If that’s not a noble-enough reason to explore new environments, I don’t know what is."

"This way of thinking about space probably gives us the best and most accurate image of the universe and our place in it. It will also serve us best in imagining our future activities in space: How we should treat the vast Environment; and how we should act when or if we discover others out in the Environment. (It probably wouldn’t have hurt if we’d considered Earth this way, instead of seeing it as empty spaces to exploit. Just saying.)"

So, this focus on colonization for its own sake really narrows our vision, I think. Everything we do becomes a step on the way to the aim of eventually attempting to colonize a place with freezing temperatures, frequent dust storms, water only in the form of ice, and a near vacuum for an "atmosphere". I don't think that's even going to work as a long term inspiration for space exploration, once the reality of the situation kicks in. Well that's how I see it at least.

So, I don’t see us colonizing any of these places for their own sake, any time soon. Rather there has to be some other reason to be there. The Moon is the most likely place to provide such a reason because it is so close to Earth and also has so little gravity, so with a low escape velocity. Books on the Moon settlement have many chapters about the economic value of the Moon, unlike books on Mars that skim over this in a single chapter typically with rather sketchy ideas about how it just possibly might be economically worth while if .... Also, the lunar lava tube caves could potentially give huge low maintenance enclosed spaces. If we build closed system habitats like that, eventually, perhaps they could even be as economic to live in as Earth through economies of scale and because the Moon has no weather to speak of and is tectonically very quiet. But that's a fair way into the future.

Mars could provide such a reason too, for scientific study, search for present day life or past life, and its two moons also. But contaminating Mars with Earth life could destroy much of the most interesting motivation for studying it. Its two moons don't have those same issues. so they may be a better starting point for a scientifically orientated advance human base, rather like the ones in Antarctica.

Lockheed Martin looked into Phobos and Deimos as intermediate destinations for their "Stepping Stones to Mars" and they remain destinations of great scientific interest, both in their own right, and as a base for studying Mars from orbit. Deimos also may be a valuable resource too, as it is a type of meteorite that often has water, though this is not yet confirmed for Deimos. They are tiny worlds so we also need to consider the potential of negative scientific impact of humans building a base on them, the problem of trash, and rocket exhausts as for the Moon, but hopefully that can be worked around. Perhaps we might eventually have settlements there of some sort too. I cover this in detail in I cover this in Interesting flyby and orbital missions for Mars (below) .

Anyway I argue strongly that Moon is the obvious place to start our experiments in sending humans to somewhere else other than Earth, for safety reasons and nearness to Earth too as well as all the other reasons.

We could eventually build dome cities and settlements in lava tube caves, and Stanford Torus type settlements. For those to work we need a way to build large structures that cover a lot of habitable space and are low maintenance. For instance if a lunar lava tube cave is indeed as large inside as an O'Neil cylinder, then it might be possible to turn it into a reasonably strong and maintenance free habitat. If we can also manage closed system recycling, and solve the problem of provide light in the lunar night for crops - perhaps we could have a habitable volume in space that is actually somewhat more habitable than Earth in some ways - no hurricanes, earthquakes, volcanoes, deep below the surface protected from most meteorite impacts also. I could imagine that actually working some decades or centuries into the future though there are rather a lot of "if"s there to fill in before we can get there.

Detail of lunar colony showing a greenhouse inside a base. Detail from image from NASA, 1989. This was for the Lunar Oasis proposal for a ten year program to establish a self sufficient science outpost on the Moon to act as a test bed for space settlements. The larger the habitat, the less surface area for the enclosed volume, so - perhaps you also have less maintenance to do per inhabitant. If eventually we can make habitats that are kilometers in scale, perhaps they can be so easy to maintain per inhabitant that it is as easy to live there as on Earth? Especially with the advantages of the Moon of greater tectonic stability, and no storms, volcanoes, earthquakes etc.

I cover this topic in Maintenance for habitats in free space and city domes, and Greenhouse construction - comparison of the Moon and Mars in Case for Moon First, and my An astronaut gardener on the Moon in Why Humans on Mars First are Bad for Science.

Why the Moon is best for humans right now

I'm an unashamed Moon firster :). This is based on careful arguments however and I have no hesitation saying that the Moon simply is "the best" without qualification as my personal view on this. See what you think about my reasons. Here are a few of them:

• Planetary protection of course - that's what this book is about. The risks making native Mars life extinct if it exists, or confusing its study especially after a crash of a human spaceship on Mars with its global dust storms. The Moon doesn't have this issue.
• Pinpoint landings - with Mars, because you have to aerobrake, typically you have a landing ellipse kilometers in diameter which has to be free of obstacles such as large boulders or cliffs that could lead to the spaceship crashing. You also have to avoid accidentally landing on the habitats, so probably the landing ellipse will need to avoid the habitats, meaning you have to travel kilometers to get to them after landing.
• Because there is no aerobraking, you can abort the landing on the Moon at any time. It also happens slowly enough for humans to take over control if necessary, like Neil Armstrong navigating to a safer landing site. A landing on Mars has to be automated, with many decisions being made rapidly on timescales of seconds. There would be nothing a future Neil Armstrong could do in time, if the landing went wrong on Mars.
• Easy access from Earth, any time of the year, not just once every two years
• Two days travel to get there, not six months. And you can get there and back within a week. We can have tourism on the Moon, and hard to see that happening with Mars except for the very dedicated going on a two year cruise, with nothing to do for most of the time, drifting through interplanetary space.
• Lifeboats - you can have spacecraft ready to take everyone back to Earth in two days in an emergency. With Mars it's anywhere from six months to two years to get back. We have no practical "lifeboat" that could do that.
• The delta v needed to get back to Earth is much less. This makes exports far easier from the Moon, especially as it may well have easily extracted ice at the poles to use as fuel. With the Moon there's also the possibility of constructing the Hoyt cislunar transport system, with a couple of tethers, in lunar orbit and in Earth orbit, reducing the delta v to get to and from the Moon to nothing.
• The Moon has double the sunlight of Mars making solar power easier to use.
• Sunlight 24/7 at its poles in conditions of almost constant temperature, easy heat rejection, only a few days of darkness per year when sun hides behind nearby hills.
• The lunar night is 14 Earth days long, so that's a disadvantage. However, it doesn't have the Martian dust storms, which can sometimes block out nearly all the sunlight for weeks on end. So you'd need roughly the same amount of backup battery power in both places. This is an issue that can be solved, so it's not a make or break problem. But it's as easily solved on the Moon as on Mars. Only the lunar poles avoid this issue.
• The near 24 hour day on Mars seems an advantage, as it is the same length as Earth's day (approximately) but that's where the resemblance begins and ends. The nights are extremely cold, cold enough for dry ice to form even in the tropics for 100 days of the two Earth years long Martian year, yet above zero in daytime, so with huge swings in temperature which are an engineering issue both for life support and materials used for construction. The lunar surface of course has similar issues with the temperature changes from the lunar night to day, but the lunar caves provide a stable temperature regime on the Moon. Again you can solve it on Mars too by going underground, but it's not a major advantage of Mars over the Moon.
• No dust storms - on the Moon the dust stays put apart from some movement due to electrostatic levitation. If you turn the area around your base to glass, then there is no dust at all for things like repairs to your habitat, operating equipment outside your base etc.
• CO₂ on Mars is no advantage at all for agriculture. It's a waste gas to scrub usually. If you grow enough food to eat, you produce all the CO₂ your plants need to grow, by exhaling. If you import food you have an excess. The atmosphere of a hab will have only a few kilograms total of CO₂ as it is only present in trace quantities in our atmosphere
• We have no idea what level of gravity humans need for health so you can’t say that is an advantage of Mars, at least not at present. It might also depend on the person, age, sex etc. For all we know, lunar gravity could be better for health than full g. We just don’t know yet. Or we might need artificial gravity to stay healthy on both the Moon and Mars. If so, artificial gravity might be easier to tolerate on the Moon with much less gravity acting along the spin axis.
• The low gravity on the Moon is an advantage for construction, especially if you want to build high. It also means that humans could fly inside the larger habitats as a form of recreation, by flapping wings attached to their arms.
• The Moon has a high vacuum which is an asset for industry - you can make solar panels “in situ” by vacuum deposition in a hard vacuum harder than in any solar cell manufacturing planet on Earth. What’s more, there is nanophase iron throughout the lunar soil and dust so you can convert it to glass quickly with a microwave.
• The Moon has many useful metals for construction and industry. The one thing we are unsure about are the Platinum group metals but it has been hit by many iron meteorites and there is evidence that suggests that it may have platinum too.
• The lunar lava tube caves, in its light gravity, can potentially be much larger than on Mars. They could be kilometers in diameter and we have evidence from orbit that they are over 100 kilometers long. As large inside as an O’Neil colony - and they would be places that have stable temperatures too, so a natural place to expand into.
• Both have dust hazards. But the lunar dust is a known issue, the astronauts have already experienced it. It is a potential health risk long term. The lunar dust is more spiky, not rounded by the dust storms. On the other hand, the Mars dust may well be harder to cope with, with the perchlorates, which at high levels can impair functioning of the thyroid gland, and reduce iodine uptake. More seriously, they are also likely to be decomposed by ionizing radiation into the reactive chlorites (ClO₂) and hypochlorites (ClO) which have more serious and immediate effects "such as respiratory difficulties, headaches, skin burns, loss of consciousness and vomiting" (quote from page 3 of this paper). It's also likely to be harder to keep it out of habitats and space suits because of the dust storms - you can keep an area around a lunar base largely clear of dust by turning it into glass. At any rate this is not an advantage for Mars.

I cover many of these points, and many more, in my "Case for Moon First". For instance, here is a link to the section on lunar and Mars dust.

I do think there are good reasons to have humans in orbit around Mars once we can do it safely and for less than enormous expense, for telerobotic exploration of Mars, and for exploration of its two Moons. Also there is a possible commercial case for habitats in Mars orbit or on its moons. Perhaps there is some chance of paying for them by exports of water from Deimos, if it does have reserves of water there. However, I think that's less likely to be competitive for supply to the Earth Moon system, if the Moon has easily extracted ice reserves.

Before I wrote "Case for Moon First", I had totally bought into this idea that Mars was best for humans, and the Moon was a poor second cousin for humans to live. This gets repeated so much, you come to believe it just through repetition. The news stories and articles can seem so convincing, and it also seems to make sense as Mars looks rather more Earth-like in the white balanced photographs. Also Robert Zubrin puts what at first seems a compelling case in his Case for Mars. I argued that we shouldn't send humans to the Mars surface yet for planetary protection reasons but thought I was facing an uphill struggle in conversations with those who advocate colonization as rapidly as possible, given what seemed to be obvious advantages of Mars over the Moon for humans.

However, as I wrote the book, and researched into this topic in detail, and read the books written by "Moon firsters" carefully, I realized I'd got it all wrong. The Mars colonization enthusiasts rarely try to make a direct comparison with the Moon. When you do that, i one point after the other the Moon wins just about every time in a comparison with Mars as a place for humans to live. But you have to look at it with a lunar rather than a Mars perspective. Of course solutions designed for Mars probably won't all work "as is" on the Moon. For instance you can't use hydrogen feed stock to make methane fuel on the Moon. But then, you have water ice probably at the poles, useful for fuel. You also have abundant sunlight, and much easier transport back and forth from the Earth. Also there is no need to make fuel "in situ" on the Moon, just to get back to Earth, as you would have to do on Mars. The Apollo astronauts managed this rather easily back in the 1960s. This idea of generating fuel from hydrogen feedstock in a carbon dioxide atmosphere is an ingenious Mars motivated solution to a problem you don't really have on the Moon.

It's the same for other things. The apparent advantages of Mars all seem to just melt away when you look at the Moon in its own terms.

Further into the future, if we have millions in space - can we be one of the"wise ET's"?

More generally, looking further into the future, habitats on the Moon would probably be just a first step. Suppose we do find a way to have millions living in space - I argue in my Moon First books that settlement in space has the potential to be hugely positive but it could also be hugely negative. It depends very much how it is done, and it may well turn out to be a good thing that we are likely to have comparatively few humans in space to start with.

Though I'm keen on humans in space, I'm no advocate for sending large numbers of us there as fast as possible (except as explorers and tourists). After all think what the consequences would be if we had the likes of ISIS and North Korea as space colonies? North Korea claim to have space aspirations and have put a satellite into orbit. Right now their space program is not really credible, and most consider it to be a cover for ICBM research. But in the future with improved technology world wide and millions in space, then perhaps such governments will have the ability to set up their own colonies. Or, it may not be an extremist state or group on Earth that gets into space, it could just be that in any group of millions of people you may start to get some with strange and even destructive and violent ideologies. If this happens in space then they automatically also have space technology far advanced over ICBMs. We may get many peaceful, positive, ideologies in space, but others might turn out to be as extreme as anything we have on Earth. If that happens, then how can it last for long, with the habitats in space so fragile to any violent action, even lobbing a rock at them at a few kilometers per second.

If we ever succeed in having hundreds of thousands, and millions of people in space we can't restrict space colonization to the "good guys or gals" whoever we think those would be.

Longer term the difference between positive and negative future outcomes may become even more stark, if you start to think of a "civilization" like ours spreading to fill the galaxy, with the ability to modify their own genes, make self replicating machines, cyborgs etc, and the most rash and aggressive able to spread through the galaxy most quickly. How can we stop it from turning into never ending waves of destruction in a future galaxy filled with remote cousins many times removed, with bizarre ideas and unfathomable technology approaching at close to light speed from thousands of light years away? This might be a significant and important future challenge that we have to find a way through. Perhaps all Extra Terrestrial Intelligences (ETIs) that develop space travel encounter these issues eventually.

How can we make sure that such a future is reasonably peaceful? .

Actually I'm optimistic there, especially if we are not the first extra terrestrial space capable species in our galaxy. If there have been others like us before, then I think that the Fermi paradox "where are they all" can give us hope, that at least some of them have found such a solution. Otherwise the chaos in our galaxy from battling ETIs would be plain to view. They could never go extinct, not once they are galaxy spanning, because how could any extinction event affect ETs thousands of light years away? After a galactic chaos lasting for billions of years, nowhere would be untouched.

Once an aggressively expanding civilization has reached its nearest stars, it will fill the galaxy within a million years. Also, if they are anything like us, as we are now, I mean totally fill it, in a population explosion. It would inevitably turn into a race to fill the galaxy with the fastest growing population winning the stakes, filling a thousand star systems for every one star system filled by less aggressive species. If ETs originating around another star filled the galaxy in this way, they'd have taken over Earth long ago, as they would need to use all the resources they could find to cope with their constant wars and exponentially increasing populations.

How could our Earth and solar system remain untouched? Yet they are.

Curiosity's tracks photographed from low Mars orbit by HiRISE on NASA's Mars Reconnaissance Orbiter. This instrument has a resolution of 30 cm. We have found no signs at all of any extra terrestrial tracks or footprints yet, anywhere in our solar system. Our solar system, to all appearances, is pristine. Either it's never been visited by extra terrestrials or they have a policy of erasing all trace of their presence when they go.

So, if there are any galaxy spanning civilizations in our galaxy - they have found a way through this issue. They can't be "like us" at least not as we are in our current immature twenty first century civilizations.

Signs of optimism that we can be one of the wise ETs

I think there are signs of optimism here from our own past too, if you look at the direction we are headed. Imagine if you gave the nineteenth century people present day technology. How long would the blue whales last, or the tropical jungles? How much chaos would they cause to their environment? What kinds of wars would they fight with modern weapons, including our capability for chemical and biological warfare? Theirs was a simpler time and there are many things nineteenth century humans took for granted, and regarded as acceptable behaviour, that would be unacceptable today. Their ideas and habits would also cause utter chaos if they were combined with modern technology.

Though we have stumbled a lot, we have made many good decisions, such as dealing with the problems of DDT and CFC's, human rights (a lot of progress though much still to do), preventing chemical and biological warfare (even in the almost all out conflicts of WWII neither side used the chemical weapons of WWI such as mustard gas, even though they stockpiled them and issued their civilians with gas masks). The Geneva protocol banning many forms of biological and chemical warfare came into force on 8th February 1928. I know there have been exceptions but most wars don’t use them. Imagine how different a world like ours would be, if it was inhabited by ETIs who were so aggressive and short sighted in their thinking, and so unable to negotiate that they couldn't agree to such a protocol at all? All their wars would use those kinds of weapons and probably worse ones too.

So we have learnt a lot already, and changed as a society, slow though the progress seems on a year to year basis. Our ideas and habitats are already radically different from those of the nineteenth century, and we have those in a global shared culture too, what's more. So where is this trend headed, a couple of centuries into our future?

I think, there is evidence that we may be wiser than the most reckless ETs possible. Yes reckless ETs may well destroy themselves in space wars pretty much as soon as they begin on spaceflight. Carl Sagan refers to this as "the intrinsic instability of societies devoted to an aggressive galactic imperialism".

"If every civilization that invents weapons of mass destruction must deal with comparable problems, then we have an additional principle of universal applicability. Weapons of mass destruction force upon every emerging society a behavioural discontinuity: if they are not aggressive they probably would not have developed such weapons; if they do not quickly learn how to control that aggression they rapidly self destruct. Those civilizations devoted to territoriality and aggression and violent settlement of disputes do not long survive after the development of apocalyptic weapons. Long before they are able to make any significant colonization of the Milky Way, they are gone from the galactic stage. Civilizations that do not self-destruct are pre-adapted to live with other groups in mutual respect."

He goes on to say that because we have only just reached this stage then this future scenario of mutual respect may seem unlikely because of our short term perspective. He suggests that the required changes may take a thousand years or more, for us to reach maturity as a species. From Carl Sagan's "The Solipsist approach to Extraterrestrial Intelligence",

Actually, I don't think that nuclear weapons by themselves would be enough, to destroy creatures similar to us. To do that we'd have had to use Cobalt 60 bombs, with deliberate aim to make the surface of Earth radioactive and uninhabitable, and lots of them, and even then probably a few humans would have survived. Nobody was crazy enough to do that in our civilization. It's the same also with chemical and biological weapons. It is easy to target large numbers of people, but not so easy to kill everyone on Earth! (And who would want to attempt that?)

It's the same also for natural disasters and for the other risks we pose to ourselves. Some would impact severely on us, degrade our environment, make things more difficult for billions, kill billions. There are terrible things we could do to ourselves. But if you look at them carefully, I don't think any of them are extinction risks in the near future and quite probably never. We are lucky, that as a species we are resilient, omnivores, adaptable with minimal technology, able to live anywhere from the cold of the Arctic to the dry heat of the Kalahari desert, or tropical rainforests. We are lucky also to live at a quiet phase on a planet in a quiet phase in its solar system in ia quiet suburb in our galaxy. For details see Not our "only precious window of opportunity" for space exploration (below) and Natural disasters - resilience of humans (below)

For an extra terrestrial to set back their civilization by more than a few decades, or to make themselves extinct, they have to be far more aggressive than we were. They would need to fanatically keep on using weapons of mass destruction of all sorts, when they can see that their home planet's population has been decimated and all hope is lost. Otherwise they'd keep knocking themselves back perhaps but restore their civilization within a few decades or centuries, a bit like the story of John Wyndhams "The Chrysalids". That would be just a blip on a geological timescale.

Other civilizations could go extinct just through bad luck. For instance if they arise on stars orbiting close to the central supermassive black hole at the center of our galaxy, or in dense areas of our galaxy prone to nearby supernovae - they might be destroyed by natural events. However, we are lucky to live in a quiet part of our galaxy, and we are also very resilient. .

We’ve prevented starvation with the often forgotten Green Revolution between the 1930s and the 1960s, stopped the birds' eggshells thinning scenario of Silent Spring, the thinning of the ozone layer, stopped nearly all whale hunting, done lots of work to preserve species and environments etc, developed many international agreements that stop the worst of biological and chemical warfare and been able to prevent nuclear warfare for many decades. If you compare our present world with what it could have been without all those initiatives - I think it gives room for optimism for the future too. Also, I think we’ve made an excellent start on peaceful use of space with the Outer Space Treaty.

We've also had a measure of luck, that Apollo didn't return any microbes that were harmful to us or the environment of Earth. Though it was disappointing to find the Moon uninhabited and uninhabitable, it might have been a risk to us if it was. Perhaps some other ETIs become extinct at that point. If we have wars in space, we could easily create clouds of debris around our Earth of the exploded satellites, making space travel difficult for centuries. Perhaps an aggressive immature culture coupled with space technology could eventually make itself extinct through wars with its space colonies?

Although it’s frustrating that we don’t have warp drives or even the Star Trek “Impulse drive” they use to zip around from one planet to another in a solar system, and don't yet have easy low cost ways to build habitats in space, I actually think it helps, that space is so hostile.

We have surely changed and learnt a lot already, slow though the progress seems on a year to year basis. So where is this trend headed, a couple of centuries into our future? Hopefully by the time we figure out how to live sustainably in space habitats, we will also have figured out how to do it peacefully, or reasonably so. With competition of course, but more like the Olympic Games than WWIII. Hopefully we can become more forward looking as we continue to colonize space. Perhaps the increased resources from space can help us to become more peaceful if we can handle it right.

If so we might well eventually have a chance to explore even our entire galaxy peacefully, and without harmful consequences to ourselves and other intelligent species that may exist in our galaxy. Robotically first, probably, and then ourselves too, perhaps. And if we meet ETs, the ones that still retain space technology, then they also I think would be ones that have figured out how to explore the galaxy in a similarly peaceful way.

Surely amongst the many ETIs (if we do have neighbours) there will be those that are competitive, as with the Olympic games, innovative, eccentric, genius. But we need to find a way to embrace the good sides of those qualities, in ways that work in the universe we live in without being a nuisance to ourselves and the other species we share our galaxy and universe with.

Let’s be one of the civilizations in our galaxy and universe that flowers like a beautiful flower.

(click to watch on YouTube) - Amazonian giant water lily.

For more on this, see What abut colonizing other star systems? (below)

Humans can probably help a lot by being close at hand, for in situ exploration, because of our ability for fast and accurate on the spot decision making. But we have to be careful to look at the downsides as well as the upsides of humans "on location" in the solar system. We need to understand what could go wrong, as well as right, to decide how best to plan our explorations. We need to continue to take care with our robots as well, to make sure we understand the implications and possible effects of them "on location" as well. By doing this we can make best use of both humans and robots, and preserve the science value and interest of the places we explore.

So let's get on to the book. What are the possible consequences and ramifications if humans touch Mars? Or Europa, or Enceladus, or other places in our solar system?

Sources and structure of this book

Just about everything in this field is available to read on line, so I give my sources through hyperlinks rather than footnotes. These take you straight to high quality press releases, astronomy and science news stories, detailed studies, and technical papers. Nearly all of these are open access ( I search for an open access paper whenever there is a choice). This makes it easy to click through to read more. Some of the techy papers are behind paywalls but there's nearly always a good summary of what they say that is open access, or I can link to a well written popular article about the work.

I also strongly recommend the Google Scholar button for the Chrome browser for anyone who is interested to follow through to the scholarly articles. It lets you highlight the title of any paper cited in the paper you are currently reading and jump straight to it. What's more, it also includes a link to the full article, whenever it is available as an open access paper. Many papers that are behind paywalls when you go to the journal that publishes them are also available separately as open access papers for instance as a preprint on arxiv.org, and in many other online open access depositories of papers. The Google scholar button finds these open access versions of the paper for you automatically.

I've made the sections of this book self contained as far as possible, so that you can just click through to anything that interests you and read it. This requires a small amount of repetition, of a few sentences, for instance you'll find a quick summary of the three main types of photosynthesis several times, first introduced in Surprising distant cousins (oxygen, and sulfur based, and using a similar method to the way our eyes see light). For any large sections, for instance the oxygen rich atmosphere for early Mars , or the instruments designed for situ searches on Mars, I cover the topic in detail once, and provide links to click through elsewhere.

Sometimes I cover the topic in detail several times, but each time with a different slant, so that the sections build up on each other. To take an example, I set out a theme ih Instead of terraforming Mars in a multi-millenium project, why not terraform a lunar cave in a multi-decade project? which I come back to again in Best places to introduce Earth life right now - while continuing with biologically reversible exploration of Mars - and then again, with another slant on the topic, in Plenty of places to experiment with sending life to other places in our solar system - Asteroid belt resources, NEO's, caves on the Moon

Sometimes I go into techy details, or supply calculations. I will indent these. That way, they are easy to skip, while still readily available for those who are interested in such details.

This is an op-ed, not an encyclopedia or literature survey

This book is an opinion piece, not an encyclopedia, or a literature survey. So, along with all the views of the various experts that I cite here, I won't hesitate from expressing strong opinions of my own, which I hope will stimulate discussion.

There are plenty of books and articles that promote the ideas of geologists, chemists, physicists, space engineers, and space colonization advocates, on how to search for life on Mars. I think it may be interesting to have a book that focuses more on the ideas of the astrobiologists on how to do it, as set out in their research papers on the topic. You may be surprised, maybe even shocked, at how these differ from the received wisdom of the NASA road maps and the decadal review. This is especially striking when it comes to their views on sample return and on in situ searches on Mars, as I touched on already in the introduction in Should we return samples from Mars right now? (above).

According to the astrobiologists, the NASA sample return program is a hugely expensive attempt to return samples that are likely to be as controversial and ambiguous for their discipline as the Mars meteorites they already have. They want to send life detection instruments on rovers to explore in situ. These have been in development for decades now, designed to overcome the issues that beset the Viking instruments. These have never yet been sent to Mars.

As Chris McKay put it, in this interview

If we’re going to search for life, let’s search for life. I’ve been saying this to the point of exhaustion in the Mars community. The geologists win hands down as they are entrenched in the Mars program...

...Right now, as far as I’m concerned, there is no alignment between the Mars strategy and astrobiology.

He is one of the few astrobiologists to see some value in a sample return. But he doesn't motivate it by any idea that it would settle central questions in astrobiology either. He thinks it is worth doing it for the general science community - a low cost mission to Mars to just grab a sample of dirt and return it to Earth. For his reasons: Chris McKay's view - just grab a sample of dirt as a technology demo, and return it - one day on the surface, no rover

Amongst many other differences, the astrobiologists also think it is very important to be able to drill at least several meters below the surface, ideally 10 meters. That's not a top priority for the geologists, for most of their work, it is enough to be able to remove the surface layer on a rock and scratch beneath its surface to see the underlying geology. None of the NASA missions to Mars to date have been able to drill to any depth, and Mars 2020 will be limited in this in the same way as Curiosity (though ESA's ExoMars will be able to drill 2 meters). Their only proposal for a drill is for the Insight Lander and that's a stationary lander and a purely geological mission, with a heat probe that will drill to a depth of 5 meters.

Another difference is that the geologists, space engineers, and human spaceflight advocates, tend to think of the search for life on Mars as a search for visually recognizable microfossils, or even macrofossils. Astrobiologists are acutely aware of how ambiguous and controversial these minute structures can be, after their experiences with Mars meteorites and also as a result of the many debates over attempts to find microfossils in the oldest rocks on Earth. It's the same story for the controversial putative fossils of the most ancient stromatolites on Earth. They are interested in microfossils, yes, but organic microfossils with still recognizable biosignatures, preserved in the extreme cold of the Mars surface environment.

Then as well as that, geologists tend to think that if you detect organics, perhaps with an isotope ratio that suggests it could be life, then that will be enough to decide if a sample is worth returning for the astrobiologist to analyse. Many astrobiologists with experiences of attempting to analyse the organics in Martian meteorites are far more skeptical that this would work, and think the chance that it will help resolve any of the central questions in their field is very low. Most organics on Mars will be from meteorites, comets and other abiotic sources and they can also mimic biological isotope ratios too (see Tissint meteorite - a great example of what we might get in a sample return from Mars). For them, the way ahead is to search for clear unambiguous biosignatures, or multiple simultaneous biosignatures, in situ on Mars. They say that the time do do a sample return is after you identify life on Mars, or after we have exhausted what we can do via in situ biological searches.

So far, we haven't even started on those searches (apart from Viking). And our capabilities have improved vastly since Viking with many low mass sophisticated instruments, including chips that just a decade or two back would have required equipment filling a lab, that could do amazing feats of analysis for biology if sent to Mars.

I think myself that it is high time we let the astrobiologists try out their own ideas for how to search for life, and see what happens. You wouldn't ask an astrobiologist to design a rocket, so why expect space engineers and geologists to be the ones to make all the decisions about how to search for life on Mars?

So, I make no apology for devoting a large part of this book to the views of the astrobiologists. There are plenty of books and articles presenting in great detail the views of the geologists and space colonization advocates and space engineers on how they think we should search for life on Mars. There is so little on the views of astrobiologists on this topic, outside of their specialist papers. So, I hope this may be of interest, especially to those who haven't yet heard much from the astrobiological side of this debate.

As well as writing this as an op ed to stimulate debate, I also wanted to write a book that was lively, and that gets the reader thinking for themselves. As part of that, I have included many sections with fun speculations about things we have no answers to yet. They are clearly labeled as such.

I will be delighted if this book helps to stimulate critical and open ended thinking, and wide ranging debates on the subject of planetary protection. The aim is always to stimulate thought, and never to try to convince others to take on my views, no matter how carefully and thoroughly I argue for them. Please think through the arguments for yourself and come to your own conclusions.

I'd like to share a video by Andrew Maynard, a physicist, who runs the risk innovation lab at ASU. He ran a plenary session for the Astrobiology Science Conference (AbSciCon), April 2017. A couple of quotes. From a member of the audience who didn't give her name, at 38:30 in, who put the astrobiology case rather starkly (emphasis mine):

"I wanted to say about forward contamination being this big issue for astrobiologists, is that a lot of the public desire for space exploration is to send humans and if we send humans anywhere we will contaminate the planet, because we are dead if we are sterile, so there is a huge conflict between trying to understand microbial life anywhere and human exploration in particular."

To which Andrew Maynard responds

"That is a really big value disconnect. Talk about risk. You have got the people that are really dedicated to getting humans on other bodies see people who don't like that idea because of contamination as a risk to their mission. On the other hand if you're interest is in alien life, or the evolution of life or life like systems in other systems, the idea of putting humans on another body is a risk to what you think is incredibly important. So how do we begin to find the common ground between those."

This gives an idea of the sort of issues we are facing here. Andrew Maynard referred to these issues, seventeen minutes into the talk as "Wickedly complex". He went on to say, slightly paraphrasing:

"There are no silver bullets, no easy solutions to moving forward. In the social sciences they call it a "Wicked problem". That's one without definite solutions, and what's more, as you progress to try to implement solutions, this changes the nature of the problem, so you are always chasing after a moving target."

Another good point he makes (51 minutes in):

"The compromised science represents this community, and there is a lot of consensus of what is important here, but it's also important to realize that there is some diversity and perspective here within this room as well, and it's always relevant to understand the value and validity of that diversity of opinion"

That's the spirit within which I wish to present this book. I put forward my own views for discussion, I present other views as clearly as I can understand it. I see it as especially important to recognize the value and validity of our diversity of opinions.

Here is Andrew Maynard's presentation and debate: in full, in the Plenary session on planetary protection.

(click to watch on YouTube)

There are many other videos from the conference here.

Perhaps I can highlight another section of his presentation. At the end (58 minutes in) he says (emphasis mine)

"We are all part of this society that has a stake in what we do in the solar system and beyond. So whether you agree with people or don't, or whether people understand the science or don't, or whether they understand the politics or the policy or the broader conditions and situations, everybody has got some stake, some voice in this. And so it is really important that we actually engage with different members and sectors of the public, different institutions and organizations, if progress is going to be made."

That's really what this book is about. It's an attempt to engage with the public. These are major decisions. They are going to be made in our name in the future, one way or another, and we all have a huge stake in them, whether we realize it or not. Ourselves, or our descendants are potentially going bo be impacted in major ways by the outcomes of these decisions. So by writing this book, in as fun and entertaining a way as I can manage, I hope to help bring these issues to the general public, and to help everyone to engage with them.

Attribution for the research, opinions, and fun speculations

This is the convention I follow in this book:

• For opinions and strong views, you can assume I'm presenting my own views unless I attribute them to someone else. They are for the purposes of stimulating debate and discussion, which is best done, I think, by presenting clearly labeled strong views so you have something substantial to discuss. The aim is never to try to convert others to my views, but rather to open a debate, in which I'll argue my case vigorously, but the wisdom I think comes out eventually from the debate itself and is larger than any of us.
• For ideas and suggestions, it's the other way around. Normally I'm describing other people's ideas and theories, and hopefully it's clear from the context and the links where they come from. You can assume that these ideas come from the linked to articles, unless I label them as my own. I explain the ideas as clearly as I can, but do click through to the links if you want to know in detail what the researchers said.
• For fun speculations - I clearly label these as my own ideas, or if they are anyone else's ideas, I will label them as such and attribute them..

When it comes to attribution, the sheer number of names would soon become confusing if I attributed everything inline. With a book designed for online reading, I felt that it worked better to do the attributions as linked text rather than footnotes, so I use the links to the sources to credit them. It's easy to click through to find out who did the research. So, sometimes I'll give the author, especially if it has a single author, or someone particularly notable, or I'm discussing their paper or papers at length. Other times I just link to the paper and don't give the authors here.

I want to engage the reader in the process of trying to think through innovative ideas for themselves. So I don't hesitate also to discuss new ideas not in the academic literature. Some examples include the suggestion that Mars astronauts, if we ever have any on the surface, would wear sky blue swimming goggles, in Could we see green on Mars? and the idea of Thistledown light planes with low take-off speeds for the thin atmosphere of Mars. Perhaps these ideas are mentioned somewhere but I haven't found them anywhere yet, and there are several other examples like that. I label the ideas clearly if they are speculative thoughts of this nature based on informal discussions, so there should be no difficulty recognizing when I'm referring to established research, and when I'm referring to speculative thoughts to stimulate the imagination.

I also sometimes do what on wikipedia they would call "synthesis" - develop ideas that follow fairly immediately from ideas in the literature, but in cases where I don't actually know of anyone else who drew those conclusions.

Here are three favourite examples which I use often in this book:

• Early forms of life, such as RNA world cells, as a planetary protection example of a form of life that could exist on present day Mars and could be vulnerable to Earth microbes. I use this example often, as it is the easiest way I know to see that Mars life could potentially be vulnerable to Earth microbes. The background to this speculation is that there are articles about ALH84001 speculating that the tiny cell like structures could be a form of early life. (See What if Mars has really tiny cells - like the structures in the Mars meteorite ALH84001? (below) . Then, the limitations of size workshop in 1999 worked out the smallest possible size of a microbe returned from Mars based on speculations about early life on Earth with fewer genes, and without DNA, based on ideas of early Earth RNA world cells. Finally, the shadow biosphere hypothesis suggests the possibility of RNA world life on present day Earth co-existing with DNA (see RNA world and the shadow biosphere (below) ), from which it doesn't seem a huge extrapolation to suggest such a "shadow biosphere" on present day Mars, indeed, perhaps without the DNA life, or very little DNA life.
  
  It's not much of an extrapolation from all that to suggest that present day life on Mars could be an early form of life that is now extinct on Earth, but I haven't yet been able to find a cite to anyone else who uses this example in planetary protection discussions as a way to show how vulnerable modern Mars life could potentially be to introduced modern Earth life. It's my favourite example because you can see so quickly that life like this could be vulnerable to Earth microbes.
  
• Example of novel forms of photosynthesis on Mars, or of Mars life without photosynthesis at all, as planetary protection examples. It's a natural corollary from Charles Cockell's paper The Interplanetary Exchange of Photosynthesis. He shows that it's especially hard for photosynthetic life to be transferred between the planets, because it is easily damaged by shock, and also, tends to be on the outside of rocks, so would be ablated as the rocks pass through Earth's atmosphere in either direction. Also on Earth we have at least two independently evolved forms of photosynthesis as the haloarchaea use proton pumps, not related in any way at all to chlorophyll based carbon fixation or the sulfur bacteria. So, it seems not impossible that Mars could have evolved its own form of photosynthesis, or else, not evolved photosynthesis at all.
  
  However, I don't know of anyone using this example in planetary protection discussions, that life returned from Mars could perhaps have a novel form of photosynthesis, never evolved here, that could perhaps out compete photosynthetic Earth life in some environments, that it could be more efficient in some ways. I also don't know of anyone using it in the forward direction either, with the idea that Earth life introduced to Mars could introduce photosynthesis for the first time, giving it a competitive advantage over Mars life.
  
• Idea that present day Mars life could have evolved further than Earth life. It seems a natural follow up from the suggestion that Mars could have had multicellular life already three billion years ago. Also it connects to the speculations that Earth life could have evolved either more slowly, or more quickly, than life does on average in our galaxy. However, I don't know of anyone who actually applies either of these ideas to present day Mars specifically,especially to planetary protection discussions, to speculate that what we find there might have evolved further than life on Earth in some directions, or indeed, overall. See Life that has evolved further on Mars (below) and What are the effects of these frequent ups and downs in habitability? (below)

They are natural extrapolations from the literature, so surely someone else has used them before in planetary protection discussions, or so it seems to me. But so far, I haven't yet found a clear example for any of these. Whenever I say something like that, as a synthesis, I will cite the sources that I have used, and I will certainly cite the literature if I know of anyone else who said the same thing. Do say if you know of good cites for any of these, so I can mention and discuss them, thanks!

Some sections of this book are entirely speculation based on just a few sources, such as Would photosynthetic life on Mars be green - or could it be other colours such as red, purple, orange, yellow, brown or black? - I don't know of any articles that speculate on this, but felt it was an interesting topic, relevant to the question of how easily an astronaut could spot photosynthetic life on Mars visually. I based this section on a study of possible colours of photosynthetic life on exoplanets, and various ideas about how the colour of vegetation evolved on Earth and colours of various forms of photosynthetic life here, including the brown seaweeds, and the purplish pink haloarchaea. And then I took off from there in a section I label clearly as synthesis and speculation, for the reader's enjoyment. I do something similar also in the section Could oxygen generating photosynthetic life set up an "anti Gaia" feedback on Mars? I do this in a few other sections also, clearly labeled. I hope these sections may be fun for the reader, and of interest, and stimulate discussion, and who knows, maybe new ideas.

Again do say if you know of any published papers on these topics. And more generally, if you spot any mistake in any of this, however small, please be sure to let me know. Thanks!

About me

Perhaps some of you might like to know a little about me, and how I came to write this book? Also you might find it helpful to know where I'm coming from. My background is that I'm trained as a mathematician, with a good first class degree in maths, from York university, and I have had a long term interest in science, space exploration, astrobiology etc, which dates back to at least since I was a young teenager, before the astronauts first landed on the Moon in the late 1960s. I was inspired to an interest in astronomy since a young child by Patrick Moore amongst others. Though I was already keen on astronomy, loved books about stars, galaxies and planets. I can't remember where the interest came from originally - seems like it is something that always fascinated me.

I also have an M.Hum - on paper it's a masters degree, but the content of the masters was a second undergraduate degree in philosophy, which I completed in two years of study (skipping the first year). I then did postgraduate research for several years at Wolfson College Oxford, with Robin Gandy as my supervisor, in the foundations of mathematics and particularly, the logic of maths (mathematical logic), and the philosophy of maths. That's when I learnt how to do rigorous research into the literature, to check sources, and follow up the sources cited in those sources too. I was keen to follow them all the way through to the original research to check what they say too, and I'd find that even peer reviewed academic sources sometimes get details wrong when they report other people's research.

I think if anything, that is my "trademark approach" as it were, it's to follow through to the sources, and the sources for the sources, and read them carefully, perhaps more so than most do. I also have always been interested in thoroughly understanding the maths, science and physics, at a fundamental level,much more interested in understanding how things work than in memorizing rules and methods. That also was the nature of my research at postgraduate level - rather than ordinary maths research, which I could have done easily, it was research into the foundations of maths, what makes maths "tick" at a fundamental level. I had a special interest in work on developing new axiom systems - and the philosophical motivations for them. That's always been the sort of thing that interests me most of all, getting right down to the basics, to the simplest concepts behind anything.

I've also developed a special interest in planetary protection which I've read up on extensively over the last several years. I've engaged in many discussions about these issues with others expert in exobiology, space engineering etc. I am not a trained in biology or astrobiology, or in space engineering either. But perhaps that has its advantages too? This is such a vast field that nobody could be expert in everything.

The positive side to all this is that I have no personal investment in any of the research, or the planetary protection measures currently being used, or in future planetary protection measures. I am not affiliated to any organization, or university. I don't have any particular research interests to promote or astrobiological life detection instruments that I favour because of my research. Nor do I have a research interest in some particular hypothesis for the origins of life, or anything like that. I am not a spokesperson for the NASA planetary protection office, or the planetary society, or the Mars society or any of these other organizations that have various established priorities and programs and views in the topic area. I don't need to worry that I'm going to embarrass my establishment or team by presenting outspoken views on the topic that diverge from their own established policies and approaches.

All this perhaps gives me an opportunity to come at it from a new perspective, and to say challenging things.The maths has trained me in logical reasoning, analysis, and looking for underlying themes. The philosophical training helps with dealing with the fuzziness of planetary protection issues, and situations with no single right answer, with many valid opinions that can't be proved or disproved by scientific experiments. It's also helped me to learn to present other people's views "as is" without adding personal prejudices to my summary of what they said. My blogging and answers on quora have helped me develop the literary skills to present the material clearly, and in a fun way, to the non specialist general public, using helpful analogies. And my many online debates on the topic have given me experience in the views of the Mars colonization advocates, and to understand some of where they are coming from. I also have the background of a keen interest in space exploration and humans in space myself, from my own side, both from the human exploration programs we've had so far and from a keen interest in science fiction. The Mars colonization enthusiasts are saying things that I myself would have said just a decade or two ago, before I became aware of the many ramifications of the planetary protection issues. Back then, though I was not a space colonization activist (we are not so strong on colonization here in the UK as in the US), their approach certainly had my sympathies, and if the topic came up, then back then, I'd have been cheering the Mars colonization initiatives on with the best of them. I'd have had no idea there could be any planetary protection issues with it.

I was invited to give a presentation on planetary protection to the Icy Moons symposium held in one of the Oxford colleges during their summer break, as a blogger on space exploration and astrobiology. I have written many op-ed type planetary protection posts for my blog on Science20, along with many other topics, often answer questions on this topic on Quora, and have been invited as guest to David Livingston's "The Space Show" several times, usually to talk about planetary protection issues.

Contents

see also: Main sections (above)

• Blurb
• Preface
• Earth or the Moon as a backup - not Mars
• Plan for this book
• Introduction - How often have you seen this?
• Why don't explorers in science fiction have these problems when exploring other worlds?
• Planetary protection - researches by Sagan and Lederberg onwards - and Zubrin's arguments
• Could we get a future news story: "Debate over Moldy Mars is a Tale of Human Missteps"?
• Raising awareness - fossil optimists and early life enthusiasts
• Value of humans in space - and fossil hunters on the Moon?
• Trash, on the Moon
• Lifeboats on the Moon
• Alternative visions for the future
• Should we return samples from Mars right now?
• Human settlement and exploration - hugely positive or hugely negative - it all depends how it is done
  • Mars fantasies and realities - a sobering dose of common sense
  • Earth best for a backup - maybe with a small knowledge and seeds library on the Moon with caretakers
  • Why the Moon could have a commercial case
  • Why the Moon is best for humans right now
  • Further into the future, if we have millions in space - can we be one of the "wise ET's"?
  • Signs of optimism, that we can be one of the wise ETs
• Sources and structure of this book
• This is an op-ed, not an encyclopedia or literature survey
• Attribution for the research, opinions, and fun speculations
• About me.
• Contents
• Touching Mars
• Microbial ethics and biorespect
• Planetary Protection, Outer Space Treaty and protecting the science value of Mars
• Searching for fossils on Mars in the popular imagination
• Oxygen rich atmosphere for early Mars
• Mars geological periods - Noachian, Hesperian and Amazonian
• Fossil optimists and early life enthusiasts
• Tougher conditions for life on Mars - sometimes - and at other times much easier
• Greenhouse gases on early Mars
• Oceans that are only liquid part time as Mars' tilt and orbital eccentricity change
• What are the effects of these frequent ups and downs in habitability?
• What about thick deposits of life on Mars, like our oil rich shales?
• Even multicellular fossils would be hard to find
• How would we recognize fossils on Mars?
• "Blueberries" - Mars equivalent of Moqui marbles - are they signs of past life on Mars?
• Need to search for biosignatures if there are no easy to identify macrofossils
• Searching for present day life on Mars in the popular imagination
• Why it's likely to be hard to search for present day life on Mars
• What are Zubrin's arguments?
• Demolishing Zubrin's arguments
• Sharks in the savannah or rabbits in Australia?
• How do we use a DNA sequencer to prove that a microbe is from Mars? Can you guess the percentage of microbial species sequenced?
• Comparison of spacesuits on Mars with shirt sleeves exploration from Mars orbit and Earth - with a hundred new multigigabyte 3D landscapes streamed back each day
• Could we see green on Mars?
• Would photosynthetic life on Mars be green - or could it be other colours such as red, purple, orange, yellow, brown or black?
• Why is it that, decades after the Viking landers, we still know so little about life on Mars?
• What about Zubrin's meteorites argument?
• Summary
• Microbe stowaways on meteorites traveling from Earth to Mars
• Case study - can photosynthetic life be transferred from Earth to Mars or vice versa?
• More background on whether photosynthetic life could be transferred from Earth to Mars - other photosynthetic species, shock, and spall zone
• General case of transfer of life from Earth to Mars
• Microbe stowaways in a human occupied ship
• What if Mars has really tiny cells - like the structures in the Mars meteorite ALH84001?
• RNA world and the shadow biosphere
• How small can a living cell be?
• Half of the pages of the book of evolution have been torn out
• Could we peek into these missing pages of the book of evolution on Mars?
• Life on Mars dancing to a different tune
• Distant cousins with last common ancestor from a planet around another star
• How life could transfer between sibling stars in a stellar nursery by "weak transfer" of meteorites
• Infecting an entire star cluster with life from planets from an earlier, perhaps billions of years older star
• Surprising distant cousins
• How could our cousins differ? Skins of rust, freezer life, different amino acids translation table, ... .
• What else could have come before modern life? Alphabet soup of "XNA", Ostwald ripening organic crystals, "naked genes", or almost alive "autopoetic" cells
• Something amazing to discover - but hard to find
• Organics created on Mars by non life processes
• Preservation of past organics
• Organics from meteorites on Mars may boost a molecule over its mirror image, mimicking biosignatures (chirality)
• Joyce's ribozyme which joins together strands of opposite "handedness" - could early life be chirality indifferent - ambidextrous?
• For a clear signal of past life
• Are there any "magic minerals" to preserve microbes from early Mars?
• Follow the nitrogen, dig deep and look for biosignatures
• NASA's plan for safe zones - based on finding Mars life easily
• How could this work on Mars with dust storms and a globally connected environment?
• Why do spacecraft crash so easily on Mars?
• Supersonic retropropulsion - or huge parachutes
• Elon Musk's fun but dangerous trip to Mars
• Methane plumes on Mars and the possibility of water deep below the surface in its hydrosphere<
• Porous basalt
• Two ways Curiosity's methane spikes could be generated in the shallow subsurface (centimeters deep at most)
• Subsurface ice in equatorial regions
• Ice covered lakes habitable for thousands of years after large impacts
• Ice covered lakes from volcanic activity
• Possibility of geological hot spots in present day Mars
• Life in ice towers hiding volcanic vents
• Habitability of the Mars surface (top few centimeters)
• Temperature limits for Earth life
• Salinity limits, and more generally, water activity limits for Earth life
• Nilton Renno's droplets that form where salt touches ice - why did he call a droplet of salty water on Mars "a swimming pool for a bacteria"?
• Ionizing radiation
• Ultraviolet radiation
• Perchlorates on Mars
• Perchlorates irradiated with UV light to create sterilizing chlorites and hypochlorites
• Perchlorates as a way to scavenge water from the atmosphere
• Mixing perchlorates with other salts lets them take up water from drier air than either separately
• After taking up water, perchlorates retain it for a long time even when very dry, or very cold
• So deliquescing water is liquid in a wider range of conditions than you'd expect
• Challenges for life in these liquid layers of deliquescing salts
• Microbes could find tiny oases of water inside micropores in salt pillars
• Sources of nitrogen - essential for life on Mars
• Other requirements for life on Mars: major and trace elements, organics, and pH (alkaline or acid)
• Habitats for life on the surface of Mars - warm seasonal flows - now known as Recurring Slope Lineae or RSLs
• How did they form?
• Details of what they found
• Why its convincing even though they didn't spot flowing water
• Perhaps a hundred thousand metric tons of flowing water per year in the Valles Marineres
• Are the RSLs (Recurring Slope Lineae) habitable?
• Southern hemisphere flow-like features - these may involve fresh water!
• Creates potential for flowing fresh liquid water on Mars!
• Interfacial liquid layers model
• Could Mars have patches of fresh water in summer, throughout the polar regions, at a depth of six centimeters below optically pure ice?
• Dark streaks in Russell Crater, 55 degrees South
• Northern hemisphere flow-like features - not likely to be habitable
• Sun warmed dust grains embedded in ice
• Could Mars life in the polar ice exploit thin film monolayers in the ice, wrapped around individual microbes?
• Other surface and near subsurface habitats for life on Mars
• Plausible microbial metabolisms for present day Mars - what else apart from photosynthesis?
• Earth life able to contaminate Mars habitats
• Vulnerable to photosynthetic life
• Analogy of ET microbes on Mars with microbes from a planet orbiting another star
• What we could learn - some examples - Early life or proto life - Life originated from Mars - Distantly related life - life not based on DNA style double helixes - life with capabilities Earth life hasn't developed yet
• Life that has evolved further on Mars
• Uninhabited habitats
• Uninhabited habitats on an inhabited Mars - potential habitats that life can't reach or hasn't reached yet
• Uninhabitable liquid water on Mars
• Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells - habitable to Mars life and not to Earth life
• Early life not so versatile as present day life - habitable to modern Earth life and not to Mars life
• Early pre-archaea type life on Mars could be extremely localised
• Life evolving from scratch many times - multiple genesis - and could Mars have multiple "shadow biospheres", perhaps some extremely localized?
• Idea that we have contaminated Mars too much already, so there is no point in protecting it
• Could we have contaminated Mars already?
• What if we have contaminated Mars?
• James Lovelock's argument for a lifeless Mars
• Original argument
• Atmosphere out of equilibrium without life
• James Lovelock's life detection method
• Viking results
• Hydrothermal vents
• Cold seeps
• Mars hydrosphere
• Snowball or slushball Earth
• Snowball Mars
• What would happen to life on present day Mars
• Mars can't get out of its current "dry snowball" phase through biology.
• Underground caves
• Life in the rocks themselves
• Life that can survive in tiny microhabitats - could we spot it from orbit through its effect on the atmosphere or otherwise?
• How much oxygen would surface photosynthetic life produce on Mars?
• How much oxygen would life produce in rare habitats like the warm seasonal flows?
• Searching for surface life from orbit with ESA's Trace Gas Orbiter
• What if the most habitable spots on the Mars surface are similar to the driest parts of the McMurdo dry valleys?
• Microbes that are almost dormant - but not quite
• Summary of ways present day Mars could have life - with its atmosphere close to equilibrium
• Why Lovelock's method wouldn't have detected life on Earth for most of its history!
• Idea of returning samples from Mars to Earth - how does it compare with an in situ search for astrobiology?
• 2007 report favoured a sample return - what has changed since then?
• Impossibility of sampling everything - example of the clay layers of Mawrth Valles
• Will the samples be out of date already when they are returned?
• Tissint meteorite - a great example of what we might get in a sample return from Mars
• Chris McKay's view - just grab a sample of dirt as a technology demo, and return it - one day on the surface, no rover - or SCIM - sample a dust storm in a flyby
• Astrobiologists advocating strongly for an in situ search on Mars first
• Decadal review white paper by Bada et al - sample return of "pristine" Mars meteorites is not worth the price of millions of dollars per gram - we need to search in situ with parts per trillion sensitivity
• Mars samples would cost as much per gram as the world's most expensive diamonds
• Difficulty of keeping returned sample free of contamination from Earth - Mars 2020 will have a permitted 1 part per billion of Earth originated biosignatures
• Paper by David Paige: "Mars Exploration Strategies: Forget About Sample Return!"
• Paper: "New priorities in the Robotic Exploration of Mars: The Case for In Situ Search for Extant Life"
• "Priority 1: In situ search for extant life"
• "Priority 2: In situ search for extinct life"
• "Priority 3: Sample return"
• Decadal summing up doesn't discuss this issue
• Will the sample return happen this time?
• Why we can't prove yet that Mars life is safe for Earth
• Biosphere, crops, sea, and environment of Earth
• Example of Apollo sample return - learning from our mistakes in the past
• Why Carl Sagan and others thought that planetary protection was necessary for Apollo 11
• Were we just lucky? Do extraterrestrials sometimes become extinct after their first Apollo style mission to their nearest Moon or planet?
• Zubrin's arguments in: "Contamination from Mars: No Threat"
• Some highlights from the rebuttals of Zubrin by astrobiologists in "No Threat? No Way"
• Zubrin's meteorite argument for safety of a sample return
• Can life get into the ejecta from an asteroid impact on Mars
• Do we know if Mars life has caused extinctions on Earth? - Example of Great Oxidation Event
• But we unearth ancient life all the time
• Why Mars life would be expected to survive the warmth, and oxidative stress of Earth
• Many microbes harmful to humans are not "keyed to their hosts"
• Why sterilizing a sample from Mars is not like sterilizing a dinosaur egg
• What can we do about the possibility of life in the Mars 2020 cached sample - or Chris McKay's"Grab a sample of Dust" or the SCIM sample from a dust storm? - Sterilize!
• Rhythms from Martian sands - what if Viking detected life already in 1976?
• How the experiment works
• Detects life "doing something" - no need for the microbes to reproduce - and not confused by non life organics
• Why there was controversy back in the 1970s
• Joseph Miller's analysis of rhythms in the Viking data
• Ice and water in equatorial regions on Mars
• Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere
• Equatorial frosts - and a source of water for the deliquescing salts
• Life in the Atacama desert without rain
• Desert varnish
• Transgressing sand dunes bioreactor
• Liquid brines beneath the surface of sand dunes at night - beneath the sand that Curiosity drives over
• How the future might unfold if Viking did find life in the 1970s
• A Mars sample return would return rocks interesting for geology and would be useful as a technology demo - but would it help the search for life?
• If the motivation is astrobiological, listen to the astrobiologists
• Could Mars 2020 pivot to support astrobiology?
• Why did the decadal survey choose a sample return?
• "Safe on Mars" - could a sample return tell us if Mars is safe for astronauts?
• A sample return can only tell us that there are some patches on some rocks on Mars which are safe for humans
• Sample return needs to take account of the possibility of life in the sample even if it is a low probability
• Risk assessment survey of microbiologists
• Is this true: "We cannot take even a small risk with a billion lives"? The bold, cautious and intermediate
• Bold
• Cautious
• Can't resolve this
• Nick Bostrom's existential risk
• Example calculation: a 1 in 10 billion probability of permanent degradation of Earth could harm an expected one million future people
• Worst case, almost certainly not an extinction event - rather - debilitating to habitability of Earth
• Not our "only precious window of opportunity" for space exploration
• World war III
• AI Apocalypse scenarios
• Natural disasters - resilience of humans
• Large impacts
• What about a comet as large as Hale Bopp (60 km in diameter)?
• Supernovae, gamma ray bursts and passing star
• Supervolcano
• Global warming and climate change
• Risks from self replicators
• Summary - humans are hardy
• Need for public debate - decision for all humanity
• Legal liability in case of damages
• How can we protect Earth during a sample return mission?
• Safe return of unsterilized sample
• Easier part, sample return to surface in a container
• Harder part - human error
• Hardest part - what do you do next once you have a sample on the Earth?
• Legal complexities
• Natural contamination standard - great for asteroids and comets but doesn't work for Mars
• This is based on zero data about extraterrestrial biology - so doesn't that mean we can ignore the risk?
• Drake Equation style approach to Mars sample return risk probabilities - could risk of harmful degradation of our biosphere be as high as 1 in 1,000?
• Zero data - anything from 0% to 100% risk of sample return degrading the habitability of Earth
• Risks from Mars life if it is at a similar level of development to us
• Niggly minor issues like easily spoilt food in freezers
• The knotty problem of human quarantine - and what about exposure of humans during a robotic sample return?
• Returning samples from Mars - what if we know already that it is of astrobiological interest
• Legal requirements in detail
• If it is just an astrobiology "technology demo" - we could sterilize the entire sample returned to Earth
• Subsequent sample returns
• If likely to be of greater astrobiological interest - return samples to above GEO
• Or return samples to the Moon
• Search for past life from Mars, Venus, or the Earth - on the Moon, in meteorites!
• Search for past life from Earth on the Moon
• Search for past life from Venus on the Moon
• Search for past life from Mars - on the Moon
• What about native lunar life?
• Search for past life from Mars - on Phobos
• How martian surface material can get to Phobos
• Other things we could find out from the mission to Phobos
• Exploration of Phobos
• Planetary protection for samples returned from Phobos
• Search for early life on Ceres
• Myth of automatic terraforming, or easy terraforming
• Could oxygen generating photosynthetic life set up an "anti Gaia" feedback on Mars?
• Closing off future options - hard enough to roll back introduction of feral camels to Australia, never mind of microbes to Mars
• Why bringing Earth life to Mars could be like making wonderful yoghurt - or bad smelling gone off milk
• Myth of automatic terraforming and the strong Gaia hypothesis
• Why "Strong Gaia" should give Mars a methane / hydrogen / carbon dioxide / hydrogen sulfide / sulfur dioxide atmosphere - not an oxygen rich one
• Instead of terraforming Mars in a multi-millenium project, why not terraform a lunar cave in a multi-decade project?
• Searching for a non confrontational way ahead
• Value of a non confrontational approach
• Moving your house to avoid a pond for great crested newts
  Examples of invasive Earth life
• Invasive diatoms in Earth inland seas, lakes and rivers
• Risks of invasive microbes in lake Vostok and Antarctica
• What if there seems to be no alternative? - we must have Mars!
• Ponds and flightless parrots again - back to the Moon
• How many years are needed to do a biological survey of Mars?
• Speeding the search up with miniature robots - what could we do if we had funding for a preliminary exobiological survey of the whole of Mars from Earth?
• A rapid survey of Mars does seem possible - however none yet planned to guide our decisions as soon as the 2020s or 2030s
• Clash and confrontation?
• This approach doesn't mean that humans can never land on Mars ever
• What about humans on Mars later on?
• When will we know enough about Mars?
• Two futures - humans landing on Mars or a habitable Mars surface with humans in orbit only
• First scenario - uninhabitable to Earth life and humans (perhaps) land quickly
• Second scenario - habitats on or near the surface - exciting for astrobiology and humans remain in orbit for now
• More scenarios
• Only one Mars - no warp drive yet
• Precautionary principle and super positive outcomes
• Benefits to humanity from astrobiology
  • New dimension to biology
  • New biological materials
  • Genetic firewall
  • Industrial applications of astrobiology already- enzymes from extremophiles
  • Example enzymes from cold adapted extremophiles used in industry today - detergents, for fruit juice, bread, textiles, cosmetics, cement, research
  • Enzymes from heat adapted extremophiles used in industry today - manufacture of syrups from starch, baking, brewing, paper making industry
• Searching for life on Europa and Enceladus
• Hubble's confirmation of probable geyser activity on Europa
• Hubble's confirmation of a repeating plume
• How can we find out more?
• Why a Europa lander is likely to draw a blank - it looks so smooth and those brown lines seem biologically interesting, but what is it really like?
• Enceladus geysers as the low hanging fruit right now - with other interesting targets in the Saturn system - could Dione also have a subsurface ocean?
• Keeping safe - sample geysers
• So then what about landing on Europa?
• Rugged unknown surface of Europa - ice knives, crevasses, and upturned icebergs
• Ice knives on Europa
• Upturned icebergs - for regions like Thera Macula - amongst the most interesting regions on Europa
• Possibility of liquid water close to the surface or breaking through
• We don't yet know how to sterilize a spacecraft to Europa 100%
• Analogy of a Ming vase - what is an acceptable level of risk for making all the life in Europa's ocean extinct?
• Where will it land? Crumbling and tumbling ice blocks - or smooth ice moving as slowly as continental drift?
• Design change suggestions that could make a lander close to 100% sterile
• RTGs instead of batteries
• Other radioisotopes - what about Curium 242
• Post launch sterilization
• Deep cleaning with supercritical CO₂and CO₂ snow
• Would a Europa lander be likely to spot life?
• Icy world oceans exploration sequence - orbiter - geyser flythrough - lander - ice mole - submarine
• Using microscopes to search for Europan life
• Should we prioritize dual purpose geochemical / biochemical instruments over dedicated biology instruments?
• Requirement that life detection experiments should also provide valuable non biological information
• Strengths and weaknesses of the "lego principle" approach
• Cholesterol - an example of a molecule with a unique stereoisomer signature, one out of 256 possibilities
• How diagnostic is the carbon 12 to carbon 13 ratio?
• Mixtures with other organics from comets, meteorites, and produced locally on Europa - and degraded organics - it could do a nice study of tholins but could it find life?
• Abiotic organics that mimic biosignatures
• Degraded organics from life
• Would we be able to detect life using sub-millimeter precision photography - or could it be confused with abiotic organics?
• How easy is it for Europan life to reach the surface?
• Cell counts in the most inhospitable environments on Earth - and application to astrobiology
• Why top down mission planning may not be the best way to organize a scientific search for life in the Europan ocean
• Sample returns from Europa
• Could there be life which uses the surface ice of Europa or Enceladus itself as a habitat?
• Could a Europa lander help find geysers for a future geyser fly through mission?
• Does Europa have thousands of smaller geysers only ten kilometers high?
• How this could change the prospects for a Europa lander
• What we could lose - not just us - but all countries on Earth, all our descendants, and all future civilizations and intelligent species that arise in our solar system
• If life in a Europan ocean can have the intelligence level of squids, basking sharks, octopuses and cuttlefish - what about a non technological civilization?
• Suggestion: geyser sampling in late 2020s, then 100% sterile lander in early 2030s
• Mission to Enceladus geysers - and perhaps identical mission to Europa
• Planetary protection friendly version - artificial geyser from a "dumb impactor" - Bernd Dachwald’s idea
• Europa Ice Clipper (not to be confused with Europa Clipper)
• Redesign of spacecraft before launch - leave the launch to one or two years after the multiple flyby (Europa Clipper) gets to Europa
• Sampling geysers - particle sizes
  • Studying dust and ice by analysing the plasma created during impact of particles - method used by Cassini
  • Capturing the ice and dust particles more gently, in an aerogel - for sample return, or in situ analysis
  • In situ analysis of particles in an aerogel - the Enceladus Amino Acid Sampler
  • Suggestion - in situ analysis of the particles within a transparent silica aerogel using, e.g. Raman microspectroscopy
  • Suggestion - for low relative velocities, capture directly into water, brine, ice, or foamed up water or ice
• Why Enceladus is so interesting
• Mystery of the Enceladus heat sources
• Evidence for a global ocean for Enceladus
• Is Enceladus' ocean temporary or long term?
• What we know about the geysers so far - and prospects for life
• What could we find in the Europa or Enceladus oceans?
• Enceladus orbiter proposal
• Would we use solar power or radioisotopes for missions to Saturn's moon Enceladus?
• Suggestion - hibernation with solar power to permit Juno sized solar panels out to the Saturn system and perhaps even further, right out to Neptune?
• Each target as "our only chance" to find something unique
• Ice moles or submarines for exploring Enceladus or Europa
• Safe and easy landings for Europa - “ice breaking” instead of “aerobraking”
• Chipsats for Europa - could they be 100% sterilized?
• Two dimensional planetary surface landers
• Origami rover
• Can we achieve 100% sterile electronics for an Europa, Enceladus, Ceres, or Mars lander?
• High temperature sterilization
• Multiple methods at once - e.g. 200 °C or higher to sterilize, CO₂ snow to remove organics
• A rather more far out idea - 3d printer on Europa plus raw materials for some of the components
• If we can’t achieve 100% sterile landers for Europa
• Why not just send Earth life there?
• Plenty of places to experiment with sending life to other places in our solar system - Asteroid belt resources, NEO's, caves on the Moon
• Space habitats made from asteroid and comet materials get plenty of sunlight - right out to Pluto (using thin film mirrors to concentrate it)
• Sending humans to Callisto or Ganymede
• Why there's probably no need for planetary protection for humans to Callisto, even with a subsurface ocean
• Interleaved "Ocean Worlds Exploration" to Europa, Enceladus, and Titan - and Ceres?
• In situ instrument capabilities (sections)
• Rapid non destructive preliminary sampling
• Detection of trace levels of organics and of chirality
• Search for life directly by checking for metabolic reactions
• Electron microscope
• Optical microscopy and Raman microscopy
• Atomic force microscopy
• Using an optical microscope to watch microbes swimming
• Using an optical microscope to watch fluorescing microbes
• Near field scanning optical microscopy
• Beyond the diffraction limit with expansion microscopy (physically expand a cell to make it easier to study) and other forms of superresolution microscopy
• Direct search for DNA
• Could Europa or Enceladus have DNA based life?
• What if we get ambiguous results from Europa just as we did for Viking?
• See also (for search for life in icy moon oceans)
• Other places in the solar system where potentially life could be found - Titan, Triton, Io, etc
• Planetary Protection classifications
• Category I
• Category II
• Provisional Category II
• Category III / IV
• Category V
• Life in liquid ("supercritical") CO₂
• Life or prebiotic chemistry in comets
• Temporary water on comets doesn't mean life
• Wickramasinghe's theory
• Crust of "sunburnt organics"
• Life or prebiotic chemistry on the Moon
• Life in atmospheres of gas giants
• Supercritical liquid hydrogen layers in gas giant atmospheres
• Life in liquid nitrogen - Triton
• Life in liquid nitrogen on Pluto
• Life in a Pluto subsurface ocean?
• Other subsurface oceans - Ariel, Triton, Eris, Sedna etc
• Life in the oceans of ethane and methane on Titan
• Titan as potentially the easiest place for humans to live outside Earth
• Life in neon seas of Kuiper Belt Objects and beyond
• Life in liquid hydrogen or liquid helium as far away as the Oort Cloud
• Rogue planets and very distant planets - liquid water or liquid hydrogen surface oceans
• Sulfur based life on Io
• What about life in lava flows on Venus?
• What would silicon based life forms living in molten magma think about water based life?
• Life in the clouds of Venus
• Possibility of Earth life able to survive in the Venusian clouds
• Possibilities of indigenous life in the Venusian clouds
• The main questions still unanswered for planetary protection for the Venus clouds
• Does it matter if life from the Venus clouds gets established on Earth
• XNA based life in the Venus clouds
• Venus cloud life might give the best chance for XNA in our solar system
• COSPAR study of the Venus atmosphere didn't consider XNA or gene transfer agents
• Hazards of XNA from Venus
• Impossibility of containing XNA at sufficient probability levels
• Need to send robotic explorers like VAMP to the Venus clouds to study in situ first
• Need to treat the Venus atmosphere as a restricted category III destination for COSPAR
• Humans in habitats at the Venus cloud tops
• Travel to other stars
• Robotic spacecraft to the stars
• Humans to the star
• Planetary protection for other stars and exoplanets
• Galaxy protection - what about colonizing other star systems?
• Could the solution to Fermi's paradox be galaxy protection as an extension of planetary protection?
  • First idea - galaxy protection as a motivation for ETIs and ourselves
  • Second idea - no need to colonize planets or search for habitable zones - we could set up colonies almost anywhere in habitats around almost any kind of star
  • Third idea - the trap of exponential growth - and the need for galaxy planning of some sort for sustainable growth into an uncolonized galaxy
• The problem with exponential population growth into an uncolonized galaxy
• What about populations that plateau or crash?
• Boom and bust - a nightmare future
• Slower growth, for instance cube or square law growth rates
• Could humans achieve slower than exponential growth like this?
• So what might a sustainability solution look like?
• Optimism - especially if we are not first
• Any expansionist interstellar ET must have minimal impact on the galaxy - or else - we are the first such, at least in our galaxy
• Will we be an invasive weed, or a flower in the galaxy?
• What should our objective be for humans to Mars?
• Telerobotics as a fast way for humans to explore Mars from orbit
• Much safer
• Mobility for humans and robots
• Robotic and telerobotic drilling versus human drilling
• No need to suit up for an EVA when exploring by telepresence
• Need for adventurous experiments in life support and artificial gravity in LEO first
• Astronauts don't get nauseous when spun rapidly in zero g - so could a device as simple as a spinning hammock be all that is needed to keep us healthy in space?
• Spaceship tethered to final stage and then spun up - experiment first proposed for a Voskhod in 1965-6 but never flown - could this solve all the zero g health problems?
• Starting point - using 18 liters (so 18 kg) of spirulina algae per astronaut to supply all their oxygen - and some food
• How long does an expedition have to be to make it worth growing crops in space - or indeed - any recycling at all, even of the ISS type?
• Biological closed systems research in LEO
• Trash on the Moon - testing ground for planetary protection measures for a human base
• Rocket exhausts, microbial spores and organics mixing with levitating lunar dust
• Protection of the historical Apollo landing sites
• Imagining a telepresence mission in the HERRO Molniya type orbit
• Small planes and entomopters etc
• Thistledown light planes with low take-off speeds for the thin atmosphere of Mars
• Exploring caves, steep slopes and other inaccessible places
• Safe ways to get humans to Mars orbit or its moons to avoid any risk of crashes on the surface
• Interesting flyby and orbital missions for Mars
• Advantages of Deimos
• Advantages of Phobos
• Mining Phobos and Deimos
• Lack of gravity on the Martian moons
• Free flying habitats
• Planetary protection issues for Phobos and Deimos
• More ideas for these early orbital or flyby missions
• Artificial real time
• Telerobotics with humans in orbit compared to robots controlled from Earth
• Need for new comparison studies of the various ways of exploring Mars
• Compared with Mars surface missions
• Cost savings compared with surface missions
• Telepresence would be immersive, not using primitive mouse and keyboard control, and needs to be compared to humans in a pressurized spacesuit
• Summary of advantages of telerobotic exploration
• Humans on the surface could start a new geological era on Mars - if they introduce Earth life irreversibly.
• Land humans quickly - to explore nearby regions kept free of Earth life - NASA's approach
• Humvee journey through the Arctic - contamination study
• Explore Mars from orbit by telepresence first - but still land on Mars towards the end of the 2030s - The Planetary Society's approach
• Prestige or dishonour, first footsteps on Mars
• Inspiration value of telerobots on Mars
• Biologically reversible exploration of Mars
• Best places to introduce Earth life right now - while continuing with biologically reversible exploration of Mars
• Could we send humans to the Mars surface in a biologically reversible way? - Humans in a metal sphere?
• Plants on Pristine Mars
• Biologically reversible exploration gives us a breathing space and leaves our future options open
• What if the decision is to keep Mars biologically pristine for ever?
• Collective sense organs for humankind on Mars
• My Moon First books
• Facebook groups - OK to Touch? and Case for Moon First
• See also
• Kindle bookshelf - for my author's page
• Change log
• 6th January, 7th January, 8th January, 14th January, 18th January, 20th January, 22nd January, 23rd January, 24th January, 25th January, 26th January, 28th January, 30th January , 31st January, 1st February , 2nd February , 3rd February , 4th February, 5th February, 10th February, 15th February, 8th March, 10th March, 11th March , 13th March, 19th March , 22nd March, 24th April , 6th July, 15th July, 26th July , 1st August. 4th August, 14th August, 18th August, 28th August, 29th August, 30th August.
  2018: 818th February

see also: Main sections (above)

Touching Mars

We love to touch things. If you put a sculpture in an art gallery and say "please touch", you can guarantee it, that both children and adults will do so. So it's natural that we want to touch Mars too, and other planets, if we can.

However, there are plenty of things we can't touch on Earth. It’s not just that you might want to touch a Van Gogh painting, say, to feel the texture of the paint. Not just to touch sculptures and works of art in art galleries. The Lascaux cave paintings for one,

Photograph of the Lascaux paintings by Prof Saxx.

Many of us would love to touch these paintings, as the original painters did, and feel the texture of the rock they are painted over. But not only are we not permitted to touch them - we have to take care even about going into caves like these at all.

The warmth, humidity and carbon dioxide from our breath have taken their toll. Fungi and black mold are attacking the ancient cave paintings.

The purple markings in this photograph show some of the damage we've caused, not directly, but through our breath and in other ways, unintentionally.

The cave was found by four children, out with their dog in the 1940s after a tree blew down exposing a hole in the ground. It was opened to the public immediately after WWII, when the owners of the land, the La Rochefoucauld family, enlarged the entrance, added steps and replaced the sediment that covered the cave floor with concrete. This venture was wildly successful, with 1,500 visitors a day, but the humidity, carbon dioxide and warmth of all the visitors took their toll.

This lead to microbes, fungi and black mold colonizing the cave. They eventually closed down the cave and made a copy of it for the visitors, known as Lascaux II, recreated using the same techniques and pigments, as best they could. Only specialists can visit the original now, but it is already too late to restore it back completely to its original condition.

Scientists have often made things worse as we try to fix them, with one more misstep after another. For instance, after a white fungus spread over the floor and up the walls, the scientists took great care to photograph every single painting in detail, to keep track of the damage. It seemed an eminently sensible thing to do.

What they didn't realize is that the bright lights they needed for their photographs were damaging the cave paintings, by encouraging the growth of a black mold. This is now a major issue there with black spots spreading over the paintings.

For details see the Washington Post article: Debate Over Moldy Cave Art Is a Tale of Human Missteps.

In a recent conference, climatologists said that it is possible to restore the original environmental conditions of the cave. But the microbiologists said that it is not possible to restore the pre year 2000 microbial conditions. They say that the only way forward is to just accept that we can't do anything about the new species of microbes we've brought there. Instead, we have to try to find a new equilibrium. Trying to destroy the new invasive microbes will only make things worse.

If we can’t restore the original microbial conditions inside the Lascaux cave - how can we hope to restore the original microbial conditions on Mars after introducing Earth life there? Clearly we can’t.

We need to know it is what we want to do, or, just as for the Lascaux caves, the next generation will not be able to explore Mars as it is today, and there may be numerous treasures there, biological treasures, that we may put out of reach for them.

Will we some day see a similar headline?

"Debate over Moldy Mars is a Tale of Human Missteps?"

Enthusiasts who are keen for humans to land on the Mars surface as soon as possible tend to brush these concerns aside.

"We are going to Mars, that's what humans do. We always push beyond frontiers, whatever they are and wherever they are ".

You ask them, "What about planetary protection from Earth life? They say (I'm paraphrasing):

"Oh, that will get sorted out, the scientists will find a way. We will go there in the 2020s or 2030s."

"We care about protecting Mars and will do whatever they ask us to do. But we must not be stopped or delayed. The scientists just have to find a way to make it work for us. They have to find a way to protect Mars, while at the same time permitting humans to land on the surface."

The idea that scientists might ask them to delay their landing, or not to land on Mars at all is something they may dismiss or even find outrageous, as I've found in many conversations. Yet there are places on Earth where humans can't go. We can't go into the Lascaux caves without great care. When new cave paintings or etchings are discovered nowadays, the cave is immediately closed off to the general public and only a few scientists can visit.

New cave etchings in the Iberian peninsular, as much as 14,500 years old. They were discovered in May 2016, and immediately closed off to the general public to preserve them. They will use technology instead to give us the best view of them possible without directly visiting them.

Then there are other places on Earth where humans can't go at all. Even if you desperately want to visit lake Vostok in Antarctica, kilometers below the surface of the ice, you can't go there. Even if that is the one place you most want to visit; even if you hoped to do it for your entire life; you wouldn't be allowed to go there. Even if you are a billionaire, even if you raised your fortune just to go there, and fund the expedition entirely yourself, none of that is sufficient. You still would not be permitted to go down in a sub and explore it looking for hydrothermal vents and whatever unusual lifeforms live there. If you did that, you'd introduce surface life to the lake. This would confuse the scientific study of a body of water that has been cut off from the surface, perhaps for millions of years. The scientists themselves would dearly love to mount such an expedition to explore this lake, but they haven't yet found a way to do it that preserves its science value and interest, in the way they would like to.

Microbial ethics and biorespect

So, could we harm Mars, as much as we did the paintings in the Lascaux cave, or perhaps more so, just by visiting it? The debate about this often centers around ideas of "microbial rights" and microbial ethics. Of course, nobody would say we have to take account of the rights of an individual microbe. But if we discover life on Mars, in whatever form, does that form of life, that species, not perhaps have the right to evolve undisturbed by interference from humans? Might we even decide to restore early Mars conditions, to help the life there to evolve undisturbed by us?

Arguing along lines like that, some will say that microbial life on another planet deserves a "biorespect" from us independent of whether we can actually make use of it, and independent of whether we find it of value to ourselves. The astrobiologist Charles Cockell has written extensively about this. For instance see what he says about it in "A Microbial Ethics Point of View". Also his Planetary protection—A microbial ethics approach. He says that what matters is whether our actions make the microbes extinct. So contamination may be fine so long as it doesn't lead to extinctions. He writes (these quotes are from Planetary protection—A microbial ethics approach)

"Microorganisms have specific metabolic requirements and quite narrow physiological ranges of pH, temperature, chemical tolerances, etc., which are unlikely to be met on a planetary scale, thus disrupt ecosystems on global scales. More likely, even if a microorganism can grow, its effects will be local.

... The ethical approach suggests that contamination is acceptable. The shedding of microorganisms from human habitats and space suits, microorganisms which themselves originate from unsterilizable humans, is acceptable provided that they do not destroy the integrity of indigenous microbial communities."

He mentions that if there is a globally connected system, of similar habitats then the situation may be different. He uses Europa as an example:

"One potential and important exception might be the Jovian moon, Europa. This moon has a subsurface ocean. The invasion of a globally connected subsurface ocean by an organism that can grow in the oceanic conditions might stand a chance of causing planetary-scale contamination. In the case of Europa, more information is needed on the extent of homogeneity of the ocean conditions to assess the potential environmental impact of contamination."

He goes on to draw a difference between biorespect and planetary protection, by outlining a situation where we might deliberately introduce new microbes in order to protect native life.

"Consider a hypothetical community of Martian microorganisms in an oasis underground. They are localised and running short of nitrogen compounds in the soil. Soon they will go extinct. Planetary protection guidelines would counsel against contamination. However, suppose we have access to another microorganism that can fix nitrogen from the Martian atmosphere. If injected into the oasis in their trillions these microorganisms will provide nitrogen to the indigenous organisms and the latter will not go extinct.

"Furthermore, suppose we can show that the non-indigenous organisms will have no detrimental effect on other organisms or processes. A respect for microbial life might allow us to introduce these organisms. We do not have to do this, but if we respect microorganisms and believe that they have instrumental and intrinsic value, then we could do it as a manifestation of a respect for them."

These papers are from 2005. We now are in the situation where we have found clear (though indirect) evidence of present day liquid water on the Mars surface in the form of transient brines.We don't know yet if they are habitable as they could be too salty or too cold for life, or both, but many scientists think that there may be habitats for life on the surface of Mars. We also have some species that could potentially survive in these habitats just about anywhere on Mars such as the polyextremophile chroococcidiopsis. Also microbes could be transferred in the dust during global dust storms.This makes the situation on Mars rather more like Europa, than like Mars as most scientists envisioned it in before 2008. We are now in a situation where life introduced at one point on Mars may well be irreversible, and can potentially have global effects, whether this happens quickly or over a longer period of decades or longer. This doesn't mean that introduced Earth life will necessarily have a global effect on Mars or adversely impact on Mars microbes if it does. But we need to be very sure, when so much may depend on the outcome.

Also, what if what we have on Mars is some very early form of life, made extinct on Earth by DNA based life? It could be widespread and occurs as many different species, in different habitats,perfectly adapted to Mars over hundreds of millions and billions of years. But the same surely was the case for early life on Earth before DNA, that it was perfectly adapted to Earth, probably in numerous species too. If that is what we have on Mars, is it not possible that modern Earth life could make all those species on Mars extinct quickly, just as it made early pre-DNA or maybe even pre-RNA life extinct on Earth?

I think there is much of interest in this approach of biorespect. And it's an interesting idea, that once we find out more about Mars, and depending what we find, perhaps we may consider doing things such as introducing new lifeforms to protect the native Mars life and do actions to optimize Mars for the micromartians. Though of course this has to be done with a lot of care, even more so than with species introductions on Earth.

Biorespect is not universally accepted by everyone. Robert Zubrin has an argument against the need for biorespect for Mars which he presented in the "Making of" episode 0 of season 1 of the National Geographic series Mars.

"I would say that we have not only the right, but the obligation, to go and establish ourselves on Mars. We are the creatures with all of our flaws that the Earth's biosphere has evolved to allow itself to reach out and establish itself on additional worlds. And we will take this nearly dead world and we will create a fully living world there. And so there'll be new species of birds and fish and plants. And it will be magnificent. No-one will be able to look on it and not feel prouder to be human."

Do you find that convincing? After all we can build habitats in the lava tube caves of the Moon, or Stanford Torus type habitats in space using materials from the asteroid belt. There are plenty of places in space where we can set up habitats with Earth life in them. The smaller asteroids, and the Moon also, are not just "nearly dead" but completely dead. We can fill those with all sorts of creatures and they could be magnificent. Or just let them evolve on Earth. We don't need to go to Mars to create conditions where new forms of Earth life can evolve.

Also, our Earth's biosphere has no plan or foresight, as there is no being there who decided to evolve humans in order to get into space. So we can't ask our biosphere why exactly it evolved us and what we are expected to do in space. It also has no way of getting into space without us. We provide a way of getting into space, yes. But we also at the same time provide our biosphere with the intelligence, foresight, and deep scientific and ethical understanding to guide that exploration. We are the Earth's biosphere's guiding intelligences in space, and that may be one of our main roles. It's us who have this responsibility, and we can't delegate it to any other creatures in our biosphere at present. We are the ones who have to work out what to do with this capability, and what its value is. For instance one of the main reasons for going into space may be to protect Earth from hazards (such as asteroids), or to find resources for use on Earth, or to increase our understanding of ourselves, and of science, biology and the universe, or indeed, as a place for adventure and recreation. We may find many reasons for being in space, indeed have already found many benefits, already, through our satellites in Earth orbit.

It's not automatic that what anything humans find inspiring and want to do is going to work, and is going to be harmless to ourselves or to others. That's like saying that there is no need to protect the Kakapos, flightless parrots in New Zealand because the dogs and cats we introduce that kill them will be magnificent. I think part of the reason for this tension is the idea that Mars is the only possible destination for humans in space. But we have the Moon right on our doorstep, far more interesting than previously expected, and a natural first destination for humans in space. Also, the solar system is vast with many other places of great interest where we can visit, and even set up home, without risk of contaminating them. See Searching for a non confrontational way ahead (below) .

But whatever ones views on these ideas of biorespect, our current motivation for protecting planets and moons in our solar system from Earth life is much more practical.

Planetary Protection, Outer Space Treaty and protecting the science value of Mars

The current policies are not based for biorespect, but rather on the wish to protect the science value of Mars, Europa and so on for us. The legal basis is this phrase in the Outer Space Treaty

"Article IX: ... States Parties to the Treaty shall pursue studies of outer space, including the Moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination and also adverse changes in the environment of the Earth resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose."

All space faring states have signed it along with all those with space faring aspirations. Nearly all have taken the additional step of ratifying it (formally indicating its consent to be bound by the treaty, a process that varies according to the country but for most democracies involves passing a bill in parliament). The only states with space faring aspirations who haven't ratified it yet are the United Arab Emirates, Syria and North Korea. It's ratified by 104 states so far in total.

There's no sign that anyone wants to evade these provisions, and indeed even those who haven't ratified the treaty are keen to abide by the provisions. Cassie Conley said recently on the Space Show that she was approached by the UAE who have ideas for a robotic mission to Mars, asking for advice to make sure they comply with the planetary protection provisions of the OST. Also, it already has the status of customary international law because of the consistent and widespread support of its fundamental tenets, and because it is based on a 1963 declaration that was adopted by consensus in the UN National Assembly. This means that it is binding on all states, even those who have neither signed nor ratified it. See page 220 of this paper.

The central phrase here is "harmful contamination". All of our planetary protection policies are based on interpretations of that phrase. The currently widely accepted customary interpretation is that

“any contamination which would result in harm to a state’s experiments or programs is to be avoided”.

This is interpreted in detail by COSPAR, a group of scientists that meet internationally, every two years. The current COSPAR policy is based on this policy statement:

“Although the existence of life elsewhere in the solar system may be unlikely, the conduct of scientific investigations of possible extraterrestrial life forms, precursors, and remnants must not be jeopardized. In addition, the Earth must be protected from the potential hazard posed by extraterrestrial matter carried by a spacecraft returning from another planet or other extraterrestrial sources. Therefore, for certain space-mission/target-planet combinations, controls on organic and biological contamination carried by spacecraft shall be imposed in accordance with directives implementing this policy.”

We may be on the point of making the greatest discovery in biology, perhaps since discovery of evolution, and the helical structure of DNA. It just makes sense not to make this hard for ourselves, or even impossible, by introducing Earth microbes first, to confuse the search.

So, why would microbes confuse the search for life on Mars? For the search for past life, surely we just go and look for fossils, which will be easy to spot? Also, won't present day Mars life be easy to distinguish from Earth life too?

Let's start with the fossils. I get to present day life later in Searching for present day life on Mars in the popular imagination.  I'll go into Zubrin's arguments in detail in What are Zubrin's arguments? But we need some background first to set the scene. So first, let's look a bit closer at this idea that we can find life by searching for fossils on Mars.

Searching for fossils on Mars in the popular imagination

In the popular imagination, this is probably how most would think we would search for life on Mars. Pick up rocks, crack them open, and find fossils.

Painting "20/20 vision" by Pat Rawlings, courtesy of NASA illustrates search for life on Mars. I've used a detail from this painting for the cover of this book.

Original caption: "Did life ever exist on Mars? If so, the best evidence may be fossils preserved in the rocks. Geologists and biologists will one day explore Mars, piecing together the history of the planet and perhaps its ancient life".

After all that is how fossils of earlier lifeforms were first found on Earth. For instance, here is Mary Anning - the Victorian fossil hunter who is described in the popular tongue twister

"She sells sea shells on the sea shore"

Sketch of Mary Anning by De la Beche, gathering fossils. Her hammer is made of wood clad in iron. It's displayed in Lyme Regis’s Philpot Museum. Details from page 78 of this World Heritage assessment of the Dorset and East Devon fossil beds.

More about her in this video:

(click to watch on YouTube)

She used to dig up fossils of ammonites and the squid like belemnites and sell them in her fossil shop at Lyme Regis. And indeed, if we found something like this, the search would probably be over, after all, could anything take this form except through life processes :)

:

Fossil ammonite from Lyme Regis museum, photo by Kimtextor.

Or if we saw this, well what else could it be but a past lifeform?

Pen and ink drawing of a Plesiosaur by Mary Anning, from 1824. This and more photos and video on the BBC Mary Anning famous people site for children. She also found a two meters long (6.5 foot) skull of an ichthyosaur, and fossil dinosaur faeces and pterodactyls, amongst some of her most notable discoveries there.


Mary Anning's ichthyosaur


One of her pterodactyls

There would be no question about what we had found if we found something like this on Mars

But how likely is it that we find fossils like this on Mars? It was only as habitable as Earth for the first few hundred million years. After that it got more and more hostile for life over much of its surface. So did it ever develop plants or creatures large enough for us to see as fossils?

Oxygen rich atmosphere for early Mars

Well there is at least one thing in favour of this suggestion that Mars could have fossils for us to discover there. Mars may well have had an oxygen rich atmosphere early on, over three billion years ago, long before Earth did. On Earth this happened gradually over hundreds of millions of years with the main spike starting a billion years ago. The Gale Crater deposits are between 3.3 and 3.8 billion years old. While exploring them, Curiosity found manganese oxides. These can only form in highly oxygenated water.

The dark patch on the rock in this photograph, cleared of dust, is made up of manganese oxide. It filled a fracture and was resistant to erosion. The three pale circular dots in a row in the bottom left enlargement are drill holes made by Curiosity to analyse the material. Manganese oxide can only form in highly oxygenated water. So, the early Mars water would have had plenty of oxygen for ammonites and indeed fish and pleisorus. Such oxygen rich water isn't so very astonishing, as after all, the reason Mars is red is because the iron on the planet rusted long ago - but it was unexpected even so.

We don't know how the oxygen got there. It could be the result of ancient Mars microbes, if they developed photosynthesis very early on. After all that is how some of the manganese deposits formed on Earth. But Mars has another way to make large quantities of oxygen over geological timescales. It has no protection from solar storms, because it has almost no magnetic field, not any more anyway. It probably did have one originally but all remains of it now are some patches of magnetic rock. Without protection from a magnetic field, the solar storms can split water vapour in its upper atmosphere. The lighter hydrogen escapes into space making its atmosphere more and more oxygen rich. See How a weird Mars rock may be solid proof of an ancient oxygen atmosphere

Magnetic map of Mars courtesy of NASA / University of California, Berkeley. Earth's magnetic field is more than 40 times stronger than Mars' field. Mars must once have had an internal dynamo like ours to support its magnetic field but it shut down long ago and these patches are the remnants of its ancient magnetism.

With no magnetic field, it has no protection from solar storms and so water vapour in its upper atmosphere gets split into oxygen and hydrogen and the hydrogen escapes. This may be how it developed an oxygen rich atmosphere early on.

Earth has another way to make manganese deposits too. According to one theory, the vast manganese deposits of the Kalahari in Africa, which form 80% of the economic reserves in the world, may have been the result of UV interactions with ice forming hydrogen peroxide during snowball Earth. Hydrogen peroxide freezes just one degree below ordinary water ice. The result is that repeated freezes and meltings can collect and concentrate it in snowball Earth conditions, and that concentrated hydrogen peroxide could lead to manganese oxides forming and precipitating out of the water.

So, though you might think at first that this evidence for oxygen on Mars is pretty good evidence for past life, it's not at all as conclusive as you might think. All we can say is that if Mars did have photosynthetic microbes, they might be responsible for these oxygen rich waters. But they could easily be the result of non life processes.

However, whatever the source of the oxygen, it did make the water very habitable for multicellular life and fish if there were any such around at that time. On Earth the Great Oxygenation Event seems to have triggered evolution of multicellular life, by giving access to a very rich source of energy for their metabolisms. So could it have done the same on Mars, at a much earlier stage?

Even before the Cambrian explosion, the increasing oxygen levels may have encouraged evolution of cells with a nucleus (eukaryotes) which later became the basis for complex multicellular life here on Earth.

This paper for instance suggests that the last common ancestor of the eukaryotes may have lived between 1.855 and 1.677 billion years ago. That's at a time when the oceans were only moderately oxygenated. Most of the varieties (clades) of eukaryotes diverged before 1 billion years ago, probably before 1.2 billion years ago. But the huge diversity we have today within those clades only started 800 million years ago when the oceans started to change to their modern chemical state.

So could the oxygen on Mars have triggered the same thing, or something similar, to happen, two or more billion years earlier than on Earth? Well the oxygen rich oceans are promising, but what about the other conditions at the time?

Mars geological periods - Noachian, Hesperian and Amazonian

There are three main periods of Mars geology, the Noachian period with a thick atmosphere and seas, the Hesperian with extensive flooding and a brief appearance of a second sea, and the Amazonian which continues to the present day with localized flooding, with the atmosphere getting thinner and thinner, and the surface cold and very dry. In a little more detail:

• The early Noachian and pre-Noachian periods - the entire northern hemisphere was covered in an extensive sea (though possibly often ice covered). The early Noachian, is also when the large basins such as the Hellas basin formed. Many huge asteroids hit Mars. The earlier pre-Noachian is when Mars itself formed, as well as the northern lowlands. Pretty much the entire northern hemisphere is lower in altitude than the mountainous southern hemisphere,which is why the sea filled the northern hemisphere.
• The Hesperian period of volcanic eruptions and extensive flooding. It also had a second sea, briefly, three billion years ago. This is also when most of the great rift valley of the Valles Marineres formed. It had the beginnings of continental drift, but after part of the surface pulled apart, forming the Valles Marineres, it stopped.
• The Amazonian period (which continues through to today),- no seas, but occasional massive localized flooding. Mars has had three billion years of cold dry conditions with occasional massive localized flooding with outflow channels. These show that the flooding rushed to fill craters and then soon broke through their walls and drained away in dramatic fast moving flows of water. The climate is quite variable depending on the tilt of Mars, the eccentricity of its orbit, volcanism, and local effects of impacts. At times the atmosphere is thick enough for pure water to be liquid. Right now, with the tilt nearly vertical, almost the same angle as for Earth coincidentally, there is no chance for large areas of water or rivers. The ice everywhere on the surface is either already at boiling point as soon as it melts, or close to it. But this will change in the future as the tilt of the planet's axis changes, also the eccentricity of its orbit.
  
  If you want to find out more, Emily Lakdawalla has a good overview of these periods, written in 2013. It's reasonably up to date still. The main difference since then is that we now have lots more evidence for its early seas, including clear deltas and a shoreline all the way around the northern lowlands, so this is generally accepted as something that happened.

Curiosity is exploring deposits in Gale Crater that formed during the Hesperian period over three billion years ago. Mars may have had plenty of oxygen back then, but what about higher lifeforms complex and large enough to form fossils?

Simulated oblique view of Gale Crater as it would have looked three billion years ago. This is where Curiosity is exploring right now. Though it was very habitable in the past, that was long before there were any easy to spot fossils on Earth.

Fossil optimists and early life enthusiasts

Did Mars have life capable of forming easily recognizable fossils so early on? There are quite a few obstacles in the way.

• Did life ever evolve on Mars at all? We don't know.
• Was it ever abundant? It's quite possible that it evolved around hydrothermal vents, say, and never developed photosynthesis or any other way to spread any further.
• Did it ever develop to macroscopic life? Most fossils from Earth which are large enough for us to see date back to the last half billion years, out of over four billion years of evolution. The earlier fossils of stromatolites and microbial mats are similar to the effects of non life processes and would be hard to prove to be life on Mars.
• If it did develop macroscopic life, did it do it so early, more than two and a half billion years before it did so on Earth? The oxygen rich conditions would seem to make it possible that it did. But that's a long way from proving that it actually happened.

So, is it possible that multicellular life got off to a much faster start on Mars than on Earth? Well, yes and no, you can argue both ways. Life may have had a much tougher time on Mars, but does that slow down or speed up evolution? What was the climate like on early Mars?

Tougher conditions for life on Mars - sometimes - and at other times much easier

You'd expect Mars to be colder because it's further from the Sun, and gets about half the light of Earth. And on the whole it was, but the question is more complex than that. Its orbit is much more variable than Earth's, varying hugely through the influence of the other planets. Currently its orbit is close to circular and it is cold all the year round. When its orbit is at its most eccentric, it gets moderately warm every two Earth years when it is closest to the Sun.

It would still be very cold and it's somewhat a mystery, how it had liquid water at all in the early solar system. However, there is plenty of evidence that it did, especially with the discovery of features such as deltas feeding into the ancient oceans.

Confirmed ancient delta on Mars, left ,compared with delta on Earth to the right. This is amongst the strongest evidence that Mars had an ocean in the Northern Hemisphere. We can trace a shoreline all the way around the Northern Lowlands, with rivers and deltas flowing into it. How it managed to have an ocean, is still something of a mystery as it would seem to be too far from the sun to be warm enough for this, even with a thick atmosphere. (paper)

If we could somehow magically transfer Earth's atmosphere to Mars, it would still not be nearly warm enough to keep an ocean, or lakes liquid. Yet we know that early Mars did have oceans and lakes. Even a pure carbon dioxide atmosphere with several times the atmospheric pressure of Earth wouldn't be able to keep Mars nearly warm enough for large areas of liquid water. The average surface temperature would be still be around -40 °C (-40 °F) (see figure 2 in this paper).

Meanwhile, the lake in Gale Crater seems to have been liquid at a time with at most a few tens of millibars of carbon dioxide. That's even more of a challenge to explain, yet Gale crater doesn't show any sign of features you'd expect from an ice covered lake such as ice wedges. It seems to have been not only liquid, but warm enough to be ice free most of the time. How could that be?

Ice wedges in Sprengisandur, Iceland - the ice penetrates deep into the ground in these ice wedges as a result of repeated melting and refreezing. There are no signs of any ice features in Gale Crater so it probably never froze over for long. So how did it stay liquid?

Greenhouse gases on early Mars

Carbon dioxide just isn't warming enough, so what if Mars had much stronger greenhouse gases than carbon dioxide? There are quite a few of these, including sulfur dioxide, hydrogen sulfide, methane, and ammonia. Surprisingly, even hydrogen, which is usually not a greenhouse gas at all, can be warming when it's mixed with a heavier gas such as nitrogen, through "collision induced absorption".

Techy aside about how greenhouse gases work, and how the symmetrical molecules of hydrogen and even nitrogen can be greenhouse gases through "collision induced absorption":

Most greenhouse gases like sulfur dioxide and water vapour have molecules which are asymmetrical. This lets them interact with electromagnetic radiation through a permanent "dipole moment", a charge separation as a result of their asymmetry, in just the right way to trap photons in the far infrared.

Carbon dioxide is symmetrical, but it can also bend and stretch in a way that makes it sometimes asymmetrical which is how it is able to absorb photons. This light then causes it to change how fast the entire molecule is spinning - and those transitions happen in the far infrared, ideal for trapping heat. (techy details here)

Hydrogen, methane and nitrogen are not only symmetrical molecules, they also have such stable symmetrical structures that they can't bend or stretch either, not in a gas consisting all of the same type of molecule. So there is no way for them to create an asymmetry. Indeed methane on its own is a slightly "anti-greenhouse" gas because it absorbs incoming light in the near infrared. (see section 3 of this paper) while it is transparent in the far infrared so lets the heat out. So you'd think that these can't possibly be greenhouse gases, and indeed they can't when they are on their own, in a gas consisting of a single type of molecule

However, when a heavier molecule such as nitrogen hits a hydrogen molecule then it distorts it momentarily in a way that makes it able to absorb light over quite a broad part of the spectrum, and so it can absorb heat also, more easily. Technically it does it by giving it a "dipole moment", an uneven charge distribution. The hydrogen in Titan's atmosphere keeps it warmer than it would be otherwise through this process. Titan is especially interesting because it has both a "greenhouse effect" and an "anti greenhouse effect" - because of its smog layer which reflects heat away.

You can also get a similar effect when two nitrogen atoms collide and stick together momentarily to make a temporary "super molecule" which can be asymmetrical and absorb light. This nitrogen collisions processes actually has a significant warming effect in the far infrared for Earth, Titan and early Mars.

Carl Sagan suggested a mix including hydrogen or ammonia as a way to warm up early Mars or Earth in a letter to Nature in 1977. However both of those suggestions have drawbacks. Hydrogen is lost rapidly. Ammonia gets decomposed by UV light, and we don't know of a way that Mars could have made large enough quantities of ammonia to keep it warm.

Volcanoes can produce sulfur dioxide, which is a greenhouse gas, and a study of the sulfur content of our Mars meteorites suggests that early Mars might sometimes have had enough sulfur dioxide to keep it warm. Mars has had many episodes of volcanic activity in the past. So the idea is that though it would normally be far too cold for liquid water, it would warm up from time to time after those episodes. This sulfur dioxide approach is still a strong contender, but it's no longer the only way that early Mars could be warm enough for liquid water.

Mars would also produce hydrogen sulfide, but this is much less effective as a greenhouse gas, with a third of the temperature change for the same partial pressure, Also, sulfur dioxide's effect is amplified by water vapour, especially in a dense atmosphere, while water vapour actually reduces the greenhouse effect for hydrogen sulfide, see table 3 of this paper).

One new idea is to have a thick carbon dioxide atmosphere mixed with a small amount of both hydrogen and methane. Collisions of carbon dioxide with the methane and hydrogen jostle the molecules, temporarily changing their state in a way that makes them more absorbing of some frequencies of light. This has a much greater warming effect than any of these three gases separately. Research (published 24th January 2017) shows that when you add in these effects, the greenhouse effect can be strong enough for liquid water on early Mars. This graph shows how the collisions help fill in the gap in the carbon dioxide absorption spectrum.

Here the grey line shows how carbon dioxide traps sunlight. It's got a window in the region shown in grey in the infrared, which lets heat out and cools the planet. The red and blue lines show the optical depth for collisions of carbon dioxide with methane and hydrogen which are both strong in the gap. The dotted lines show the effects of collisions with nitrogen. Visible light extends from around wavenumbers 14,000 to 25,000. So this figure shows a region in the far infrared. This is figure 1 from their paper.

The authors of the paper found that adding just 3% of hydrogen and 3% of methane to a carbon dioxide atmosphere raises the surface temperature by a rather dramatic 40 °C. That's enough to reach a temperature of zero degrees centigrade, averaged over the Mars surface, which means many regions will have temperatures above zero. For details see their Figure 2 and discussion. This makes it more than enough to permit liquid water in the form of lakes and seas, given local variations in climate.

It's an interesting proposal, but there are quite a few problems with this model. See the discussion section of their paper which I'll summarize.

First, their model can't work for present day Mars as its surface is highly oxidizing today. The methane and hydrogen would soon be removed from the atmosphere. It could work for the early Mars surface, but for that to work, its surface would need to be very different, reducing, rather than oxidizing. (A reducing atmosphere is one with methane or hydrogen etc which removes oxygen and reactive oxidized materials). That's reasonable enough. Atmospheres don't have to be oxidizing, with Titan as an example of a moon that still has high levels of methane, in a nitrogen atmosphere.

However, even with an early Mars that's compatible with their model, with a reducing surface, and reducing atmosphere, you still need a continuous source of hydrogen to keep the atmosphere hydrogen rich. Volcanoes produce carbon dioxide as the main gas, both on present day Earth and on Mars. However, that can change, as it depends on whether the mantle is oxidising or reducing. So, one way the early Mars could have a hydrogen rich atmosphere s if the volcanoes produced hydrogen instead of carbon dioxide.

However, hydrogen is not warming by itself, it needs the carbon dioxide in the atmosphere to collide with. So how do you get the carbon dioxide into the atmosphere, to collide with the hydrogen? It is hard to get volcanoes that produce both carbon dioxide and hydrogen in large quantities at the same time. If the mantle was reducing enough to outgas hydrogen then it would tend to retain carbon in the melt, and so wouldn't produce carbon dioxide in any quantity.

So that then becomes the central question with this model. How can you get enough hydrogen into a carbon dioxide rich atmosphere to act as a greenhouse gas, with these high percentages of 3% each of hydrogen and methane?

One way is through serpentization. Even with a non reducing mantle, with volcanoes producing carbon dioxide as they do on Earth - you can get hydrogen from serpentization. That's the reaction of the rock olivine with water to produce hydrogen, and it is something that happens on Earth locally in hydrothermal vents. The problem is that on Earth this happens only over small parts of its surface. However, if 5% of the Mars surface was rich enough in olivine for serpentization it might create enough hydrogen for as long as it stayed like that. That would work, but that's a very large amount of serpentization.

The hydrogen and methane could also be created during huge meteorite impacts, through the heating of the atmosphere and reactions caused by the impact itself. Of all their ideas about how it could happen, perhaps this impact generated hydrogen has most in its favour. Mars had numerous really huge impacts in the early solar system at just the same time that it had its oceans and lakes. If this is right, then the picture is one of episodes of warmth after and during a time of massive impacts, rather than a continuously warm climate in early Mars, similarly to the sulfur dioxide and the volcanic eruptions idea.

So, in short either sulfur dioxide or hydrogen and methane could warm up early Mars.But we don't have proof yet that either of these things happened. If Mars did have strong greenhouse gases like that, then in both cases they are likely to have been temporary. They might have kept it warm enough for liquid water for short periods of time, perhaps after volcanic activity (for the sulfur dioxide) or large meteorite impacts (for the hydrogen and methane), or maybe both were factors.

Another idea is that Mars had only a "part time liquid" sea in every two year orbital cycle, when Mars was closest to the Sun. This does away with the need for super powerful greenhouse gases like sulfur dioxide, hydrogen and methane in a nitrogen atmosphere, and would let it have liquid seas mainly when its orbit was at its most eccentric. The details of its climate would also depend on its tilt too, which would change which hemisphere gets warmest when Mars is close to the Sun, and by how much. The tilt varies a lot - Earth's hardly at all. And Earth's orbit stays close to circular, for billions of years, while Mars' orbit constantly changes in eccentricity too.

This is a matter of ongoing research. Since the early seas always formed in the northern hemisphere, because most of the low lying land is there - the best times for liquid water seas might be when Mars is closest to the sun during its northern summer. Mars' axis precesses, just like Earth's axis, sometimes with the northern hemisphere tilted towards the sun when it is closest to the sun and sometimes with the southern hemisphere tilted towards the sun, so the northern oceans would be liquid only at times when the northern hemisphere is tilted to the sun.

The tilt of its axis varies hugely too, sometimes almost vertical, sometimes so tilted over that it is coldest at its equator instead of its poles. This turns out to be probably one of the biggest effects on its habitability in the models.

Variations in the tilt of Mars' axis. At present it is tilted by 25 degrees, similar to Earth. But with no stabilizing Moon, its tilt varies wildly. Sometimes it tilts so far that its equator is colder than its poles as shown at top right. Other times it is almost vertical. When it is almost vertical, then ice migrates to its poles creating large ice sheets that trap it there, and it probably never gets warm enough for the ice to melt. The best time for liquid water - and for global oceans in the past - is probably when the equatorial regions are coldest, because that drives ice and snow to lower latitudes where they are more likely to melt.

So, to find out when Mars is most habitable, we need to look at how the tilt of its axis varies. The tilt of the axis of Mars is actually chaotic in the mathematical sense of "chaos theory". This means you can't predict it exactly over long timescales. This also means we can't retrodict - work out what it must have been in the past based only what we know about Mars' orbit and spin axis in the present. Perhaps we may get ground data to sort out the past history, but meanwhile, we have no way to retrodict precisely. Instead we have to try out different possible past histories and compare possibilities to see what sorts of things could have happened in Mars' past.

This next graph may look complex, but it isn't really. The black, red and green lines here show three different possible pasts from different runs of the model. They could have gone on to calculate many more possibilities. It starts with the present day on the left, running to the past as you follow the graph to the right.

At present, Mars' axis is tilted by 25 degrees. When the tilt is at least 40 degrees then it may get warm enough for water to stay liquid. Early Mars with a thicker atmosphere could have had liquid seas at those times, the times when the graph goes above the blue horizontal line. Figure from page 4 of this report.

They also needed to take account of the eccentricity of its orbit, as it needs to be reasonably eccentric to have liquid water. When they took account the eccentricity of its orbit as well, they got this

 

These show three equally likely possible pasts for Mars. The blue peaks show availability of liquid water. The black line shows the atmospheric pressure, and the red line shows the variation in the tilt.

These show three equally likely possible pasts for Mars. The blue peaks show availability of liquid water. The black line shows the atmospheric pressure, and the red line shows the variation in the tilt.

During the times shown with liquid water in these diagrams, Mars doesn’t have liquid water on its surface all year round. It would still be frozen with no liquid water, and so largely dry, every two years, when furthest from the sun.

They count Mars as continuously habitable if it has liquid water for at least part of every Mars year (two Earth years). In their simulations, the longest continuous reasonably habitable period was 60 thousand Earth years.

Study of sediments on present day Mars back up these conclusions of temporarily more habitable Mars

Sedimentary layers in in an unnamed crater in Arabia Terra, Mars.

Caltech researchers studying these layers in a 3D stereographic projection found evidence of variation in climate with each layer formed over a period of about 100,000 years when conditions were favourable for forming them. Though they can't say in detail how they formed, there's clear evidence that they formed due to variation in the climate of Mars which would also correspond to variations in habitability.

These ideas suggest early Mars could have changed in habitability frequently, sometimes more habitable, and sometimes less habitable, in two year periods because of its orbital period, and also over longer timescales because of varying tilt and eccentricity. It might have been almost completely dry for most of the time, alternating with periods of a few tens of thousands of years with liquid water every two years. Impacts also might have made a big difference to habitability, both by creating liquid water and destroying life. Especially in the very early solar system, when Mars was most habitable, it would have had many large impacts.

Mars is still changing in habitability frequently. It's had 6 to 20 glaciations in the last 800 million years Colgate Planetary Geologist Publishes Groundbreaking Analysis of Mysterious Martian Glaciers

In the last five million years it has gone through forty ice ages, on average once every 125,000 years, with its ice sheets melting away and moving to its equator, then back to its poles.

Zhurong reveals recent aqueous activities in Utopia Planitia,

This also leads one to wonder - how unusual is Earth, which has been continuously very habitable for billions of years? Our Moon may have a lot to do with it. Mars might be more representative of what we will find with habitable exoplanets than Earth is. Though an investigation into early Venus shows that it had a much more stable axial tilt than Mars, similar to Earth's so that gives another approach which means you might not have to have a stabilizing Moon. Still, it's possible that Mars is the most normal case for an exoplanet. If so, this is just one of many ways in which we may have just been lucky which is the thesis of Mark Waltham's "Lucky Planet"

Anyway whatever the situation for exoplanets, it seems likely that Mars was very different from Earth with these many brief periods of oceans alternating with much drier colder conditions. The greenhouse gas models also suggest pulses of warmth and liquid water depending on amounts of volcanic activity, and so frequent changes of habitability. So, whether you rely on axial tilt and changing eccentricity, or greenhouse gases, or both, it seems that the same picture emerges, of a planet with constant changes in habitability.

What are the effects of these frequent ups and downs in habitability?

So Mars may have had long periods of time with permanently frozen oceans and other times when the oceans melt every two years. It might have had more liquid water if it had greenhouse gases to assist with the warming. But still it would still have a lot of climate variability with liquid water only after major asteroid impacts or episodes of volcanic activity.

What would all those changes in habitability do to evolution? That's very different from anything that happened on Earth, apart perhaps from the time of the Snowball Earth hypothesis, 650 million years ago, so we don't have much to go on by way of an analogy.

And what also about the solar storms and cosmic radiation? Again we don't have those on Earth, which is protected by its thick atmosphere and the Earth's magnetic field. Also what about the frequent meteorite impacts? Mars is closer to the asteroid belt and still gets about ten times the number of asteroid impacts for the same area as we do. Also, it had many more large impacts than Earth in the very early solar system at the times of its oceans, which is the very time when it had an oxygen rich atmosphere and was most habitable for multicellular life. These impacts would boil the oceans and melt the rock, at least locally (and the earliest impacts, perhaps globally) so if evolution did get started, what did that do to the life that was evolving there?

I think everyone would agree that Mars was a tougher place for life to evolve. But what would it do to the pace of evolution? That's much harder to answer, with only the Earth as an example to base all our reasoning on.

It might have accelerated evolution, especially after it stopped getting impacts large enough to boil its oceans, with life continually faced with new challenges to overcome. Some think that the Cambrian Explosion was a result of a previous snowball Earth, so that might back up the idea that it accelerated evolution.

On the other hand these variations in habitability might have kept knocking evolution back so that it never evolved far, keeping life on Mars at an early stage. The evolution would have to be accelerated hugely to have multicellular life there already three billion years ago, and especially with it perhaps having only brief periods of tens of thousands of years of liquid oceans at a time. If you are optimistic about macro fossils on Mars, however, you could go with the hypothesis of hugely accelerated evolution on Mars, accelerated further by its oxygen rich oceans, to back up your hopes.

If evolution on Mars proceeded independently of Earth evolution, it would be a great surprise if it has reached exactly the same stage of evolution as life on Earth. However it's rather amazing how large the differences are between these different views on the possibilities for past and present day life on Mars. If you are a fossil optimist, and expect to find fossils in the Hesperian age deposits on Mars such as Gale Crater, easily recognizable as fish, or plants or similar - that means that you think that Mars had its equivalent of the Cambrian explosion more than three billion years ago. The Cambrian explosion is a short period of a few tens of millions of years when life diversified hugely, including some with unusual shapes like opabinia.

Opabinia - if Mars evolved creatures as advanced as this already in the Hesperian period, it's evolution would be about two and a half billion years ahead of Earth evolution. We had many "experiments" with creatures that seem quite bizarre to us now, and if you are a fossil optimist you might wonder if ancient Mars had creatures as diverse as this too.

Also we are used to the idea that photosynthesis lead to multicellular life. But multicellular animal life on Earth only evolved in the conditions created by photosynthesis, with the photosynthetic life mainly important because it created an oxygen atmosphere. The animal life didn't evolve from the photosynthetic life itself. So could Mars have had multicellular animal life before it evolved photosynthesis? Might it even have had multicellular life early on, yet never developed photosynthesis at all? Is our idea that photosynthesis comes first just a bias that results from living on a planet with a strong magnetic field, so that photosynthesis is the only way to create an oxygen rich sea? Or is photosynthesis one of those things that is bound to happen fairly early on?

There was a huge diversity of strange creatures back then on Earth. Only a few survived and continued to evolve into present day life. If Mars had a head start over us for the prokaryotes of two and a half billion years. this might lead one to wonder, what happened next after that on Mars? What will Earth life be like two and a half billion years from now? And, what would happen to Earth life, after two and half billion years of evolution in Mars like conditions starting from now? If Mars life continued to have an accelerated evolution, through the equivalent of our pre-cambrian period, and then beyond, it might be the equivalent of many billions of years ahead of us by now, as compared to Earth's slower pace of evolution, evolving very rapidly in the brief moments of greater habitability.

But you can argue just as convincingly for Mars life to be at a much earlier stage of evolution. With so many setbacks, it might have proceeded much more slowly. Perhaps early life evolved multiple times then went extinct and had to start again from scratch. If it was unable to develop photosynthesis, and hadn't yet developed highly resistant dormant states or spores, perhaps you could imagine even multicellular life evolving only around a few hydrothermal vents in an earlier hospitable ocean, and then being made extinct by an asteroid impact or the vents stopping, or the sea freezing over so that no more oxygen could reach the water. Perhaps Mars had multiple genesis of life, with completely different forms of biochemistry that evolved one after another, or even simultaneously in geographically separated parts of the ocean or surface.

Indeed with all those obstacles to evolution, you can also argue that Mars could still be at a pre-biotic stage with cell like structures that resemble Earth life with some of the qualities of life, but perhaps they don't yet reproduce exactly. Or what we might find there in the most habitable conditions might be self replicating chemicals like the "RNA ocean" hypothesis, with no cell walls at all. Both of these of course would be very vulnerable to introduced Earth life.

Or perhaps life on Mars evolved in pace with Earth life, more or less, so that it had early forms of life in the Noachian and the Hesperian periods, similar to Earth life, and evolved to later forms of life roughly in step with us. You can argue convincingly for this possibility too. Perhaps evolution has a steady pace that it follows almost irrespective of what happens, so long as there are habitats sufficient for life to evolve in. It may have got off to a slightly earlier start than us, as the Earth - Moon system formed quite late, but then that was followed by many ocean boiling impacts on Mars which may have leveled the playing field.

If so then we might, just possibly, find fossil microbial mats, stromatolites and acritarch's in the Hesperian deposits. Stromatolites are boulder like structures that are made from algae if left to grow undisturbed for long periods of time in the sea. Though made of single cells, they combine together to form these larger structures which then form fossils.

Modern stromatolites in Shark Bay, Western Australia - if Mars had stromatolites in the Hesperian era then its evolution was similar to Earth life as the earliest possible stromatolites date back to 3.7 billion years ago on Earth, 1-2 cm high putative stromatolites found in Greenland.

However ancient stromatolites are hard to identify conclusively. If Mars does have stromatolites, there might be much debate and further research before they are accepted as such

Then Acritarch's are a general term for ancient microscopic patches of organics which seem to be associated with algae but nobody is quite sure what they are. More generally, this refers to any ancient organics that we don't properly understand.

Acritarch - these organic microfossils are also very ancient , date back to between 1.4 and 3.2 billion years on Earth. The name was coined by Evitt in 1963 and means "of uncertain origin" and the term is used for any microscopic organic fossils that can't be assigned to any other classification. They may be associated with green algae, some kind of a cyst or resting state. Since nobody is sure what they are then they are classified by their structure instead. For instance as prismatic, spindle shaped, egg shaped, spiky like a thorn bush, etc. See also wikipedia article on Acritarch.

If Mars evolution reached a similar stage to Earth evolution then we might find similar organic microfossils on Mars. If so, there might be a lot of debate about what they are. We could expect similar announcements to these about Mars: "Organic-walled microfossils in 3.2-billion-year-old shallow-marine siliciclastic deposits" or "Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia" followed by much discussion of what they were, and indeed, about whether they were life or not. That's often a matter of considerable debate for early Earth putative microfossils. Are they the products of life or not? It would be even more a matter of debate on Mars, because Mars has many impacts by meteorites with organics in them, and it has many ways to create organics by inorganic processes on the planet itself.

If Mars life evolved to creatures with a skeleton, such as fish, then they should be easy to spot. However, if it is a soft bodied creature such as is preserved in the Burgess shales, before the creatures with backbones evolved, then that's far more of a challenge. It may be possible if it finds clay of the right type. This study suggests that rocks rich in berthierine, are ideal for preserving soft bodied fossils. This is a mineral that forms in tropical settings with the sediments rich in iron.

So, in short, if there are easily recognizable macrofossils in Hesperian deposits on Mars, like Gale crater, then evolution there has to have been at least two and a half billion years ahead of Earth life when the multicellular life first evolved. If it reached an equivalent stage of evolution to Earth life, neither ahead nor behind us, we might find the equivalent of those ambiguous acritarch and stromatolite fossils from 3 billion years ago on Earth.

If evolution on Mars evolved much later than on Earth, with many setbacks, which is also a distinct possibility, then we might find very early life there, so early that even stromatolites and acritarch fossils might be unlikely. This is an especially interesting possibility, because we know so little about early life on Earth, with nothing that's survived to help fill in the big gap between pre-biotic chemistry and modern life. On the plus side the stable geology of Mars without continental drift, and the extremely cold conditions there, may make early life easier to study, even without clear large fossils. This early life might also still survive on Mars to the present day. If so, that would put present day evolution on Mars three to four billion years behind Earth, and this would be very exciting for astrobiologists, as a way to peer into the processes of a planet in early stages of evolution.

So, evolution on Mars could be anywhere between three to four billion years behind Earth, and billions of years ahead of us. It could also have never happened at all, so all we find is pre-biotic chemistry, another very interesting and intriguing possibility.

I don't think we can distinguish between these and many other possibilities on the basis of what we know so far about Mars. And even that much is based on a lot of speculation and assumptions about a similarity between evolution on Earth and Mars. We only know of evolution on Earth, so we have no idea even if the pace of evolution here was typical for the universe as a whole. Perhaps life got off to an unusually rapid start on Earth, or perhaps evolution here was unusually slow compared to other analogous planets in our galaxy. We can speculate endlessly, but without at least a bit more data it's hard to draw any definite conclusions quite yet.

What about thick deposits of life on Mars, like our oil rich shales?

Could Mars have meters thick layers of ancient life in some form or another? Could it have organic deposits like our oil rich shales, or the equivalent of chalk, thick deposits made up entirely of shells? Well, if it did evolve those forms of life, it did also have enough time for this to happen. It probably had hundreds of millions of years of relatively stable conditions, in the very early solar system, and continued to have seas and lakes (probably intermittently) for over a billion years. That would be plenty of time to build a thick deposit of oil shale in ideal conditions. The whole of the 5.5 km high Mount Sharp consists of sediments (for the lower layers) and wind blown deposits.

If we found something like this, even without the multicellular life fossils, in deep meters thick beds of organics, our task would probably be easy, with plenty of organics to analyse to see if it was formed through life or non life processes:

Fossils in Ordovician oil shale (kukersite), northern Estonia (Ordovician period)

However we haven't found anything like this yet. Maybe conditions on Mars were never favourable for creating thick deposits of organics (caused by life or otherwise). Or, it could be that they were washed out by the later floods, and what's left was destroyed by surface conditions. Or maybe Mars still has deposits like this, many meters below the surface beyond the reach of the cosmic radiation, Any surface deposits of organics, even meters thick, would soon be degraded to just water vapour and other gases by the cosmic radiation over these very long billions of years timescales, which breaks the bonds in complex organics. So we'd only spot them if they were unearthed in the recent geological past, perhaps by crater impacts.

At any rate if those deposits exist, we don't know where to look for them yet. There is no sign of them from orbital observations, and our rovers haven't spotted anything like this yet either.

Even multicellular fossils would be hard to find

Even with accelerated evolution, if Mars had birds, and fish, with its equivalent of a Cambrian explosion in the early solar system, the chances are that we wouldn't have found any signs of them yet.

This picture shows Archaeopteryx. It was hard to find. They had to search through tons of quarry material to find a few thin flakes with Archaeopteryx preserved.

You could send a rover to Earth and set it to explore rock formations in our desert regions, at the slow pace of our current generation of Mars rovers, for decades, and it might never spot a single fossil, depending where you send it. Or it might find a layer of chalk or similar with hundreds of them, and discover them right away.

How would we recognize fossils on Mars?

The other problem is that we don't know what to look for on Mars. If we are lucky enough to find a fossil archaeopteryx or a fish it would be obvious. Even a fossil multicellular plant. But for billions of years,as we've just seen, the only macro fossils on Earth were microbial mats, stromatolites, and the acritarchs. So, what if we find these?

These are now known to be early stromatolites from 3.4 billion years ago. But it took a lot of work and evidence, particularly the evidence of organics caught up in the material of the stromatolite fossil itself, before they were accepted as such. The later stromatolites were easier to identify but these very early ones were particularly challenging.

There are many formations on Earth that look for all the world as if they were some fossil lifeform, such as this.

Baryte Rose from Cleveland County, Oklahoma, photograph by Rob Lavinsky

If Curiosity found this on Mars, I'm sure many people would be convinced it was a fossil. But no. It's a "Desert rose" - a crystal like structure that can form in desert conditions. Enthusiasts have found many strange shapes on Mars that they think may be fossils. For some remarkably compelling examples, see for instance “Mars Fossils, Pseudofossils or Problematica?”, by Canadian scientist Michael Davidson.

So, what's the answer, how do we cope with this conundrum? We have to use the Knoll criteria to evaluate them. It's not enough that they look like fossils:

"The Knoll criterion is that anything being put forward as a fossil must not only look like something that was once alive -- it must also not look like anything that can be made by non-biological means.”

Oliver Morton, author of Mapping Mars: Science, Imagination, and the Birth of a World

This criterion is named after Andrew Knoll, author of “Life on a Young Planet" a book about past Earth life, who is on the Curiosity mission science team. We will be very lucky indeed if we find a lifeform on Mars that we can conclusively identify as living just by its physical shape. Even if it turns out that the planet had stromatolites, or even multicellular fish and birds, in the past, the problem is finding them. We are more likely to find something like this - these are potential fossil signs of past life found on Curiosity photographs by geobiologist Nora Noffke

To her expert eye these look like trace fossils of microbial mats. But another geobiologist Dawn Sumner thinks they are just the result of normal erosion processes. See Follow Up - Signs of Ancient Life in Mars Photos?

To add to the difficulties, Mars has radically different geological conditions from Earth in many ways, which could lead to many structures forming on Mars that would be impossible on Earth:

• Perchlorates, chlorates, sulfates and hydrogen peroxide instead of chloride, and sulfides. The Mars surface chemistry is highly oxygenated and reactive
• Dry ice geysers and avalanches caused by dry ice - we don't have any geological formations on Earth to compare with them.
  Even though temperatures in Antarctica and other places get cold enough for dry ice, the partial pressure is far too low for it to freeze out (about 0.04%), indeed even at a temperature of -86 °C, dry ice will sublime rapidly in an Earth atmosphere. The depositional temperature for dry ice on Earth under 1 bar of pressure is −140 °C (−220 °F) [14]. The lowest temperature reached in Antarctica is −94.7 °C (−138.5 °F) recorded in 2010 by satellite
• Low gravity which lets the wind sculpt shapes that are impossible on Earth because they are unstable here.
• Near vacuum atmosphere, and in present day Mars the fastest winds are only barely strong enough to move an autumn leaf. This again permits many strange structures to form that would be unstable on Earth, because the wind would just blow them away before they can form
• If there is no life there, then geological processes may step in to form structures that only form through life processes on Earth (the "blueberries" are an example, see next section).
• Even very small meteorites make their way to the surface. No continental drift, but lots of impact gardening. This turns the surface to a "soil" or regolith to depths of several meters, which is something we have no analogue of on Earth.
• Much finer dust than we have on Earth, similar in consistency to cigarette ash
• Much larger temperature variations from day to night

Mars is such a different world, with such different geological processes, that it won't be surprising at all if we find unusual hard to identify geological formations on the Mars surface. So, no, it's not very likely that an astronaut could pick up a fossil on Mars and identify it as such, if it is just an ancient stromatolite or fossil microbial mat. They might suspect that it is formed by life, but proving that would be another matter altogether, and they might easily be mislead and think it is life originated, by analogy with similar structures on Earth, when it is not.

"Blueberries" - Mars equivalent of Moqui marbles - are they signs of past life on Mars?

Opportunity's blueberries, to take another example, are made of iron oxides. That may not sound very life like. But on Earth similar nodules form in the presence of life.

Blueberries - photographed by the Opportunity Rover of the "berry bowl" in Eagle crater, near to Opportunity's landing site on Mars. This is how you would see it with naked eye, approximately true colour. But the Mars light is a reddish gray because of all the dust in the air. When it is colour balanced the berries look somewhat blue. These so called "blueberries" are made up of hematite and so must have formed in water. Similar

Here is a colour balanced example which brings out their blue colour:

Cluster of hematite rich spherules (blueberries") photographed by Opportunity at its Eagle Crater landing site - image from exhibition in the Smithsonian institute in 2014.

They are very similar in both structure and composition to the Moqui marbles

Interior of a Moqui marble from the Navaho desert to show its structure.

Moqui marbles which are left behind as sandstone erodes, in the Navaho desert. Photo by Brenda Belter, University of Utah

They originally formed in wet conditions and then got embedded in the sandstone, then eroded out of it. The Mars blueberries seem to have been formed in a similar way. Though there are many mysteries still to be unraveled about how exactly the Moqui marbles formed, with competing ideas.

So anyway what makes this interesting is that on Earth there is some evidence that microbial life played an active role in the formation of the metallic oxide outer coatings of the marbles - the evidence is based on looking at structures that resembled micro-organisms in the marbles, and on the carbon 13 to carbon 12 ratio (living beings preferentially take up a bit more of the lighter isotope).

So, is it possible that the blueberries on Mars were made in a similar way, by past life? Well it's rather circumstantial evidence, and the jury is out on that. We can't say that they must have been formed in the same way just because they look so similar - as Mars is so different from Earth.

Need to search for biosignatures if there are no easy to identify macrofossils

So, if there is life on Mars, and it doesn't form fossils or the fossils are ambiguous, how will we find it and recognize it? Well one way is to look for biosignatures. After all, that's how the ancient stromatolites on Earth were eventually proven to be fossils rather than geological formations. Once we prove that some particular type of formation is the result of life then we may be able to identify them by their shape too. For instance if we manage to use biosignatures to show that the blueberries are the result of life processes, they will be easy to find after that. For all we know, our rovers may be driving past fossils of ancient Mars life every day, such as biomats, concretions caused by life, stromatolites, etc, but we just can't prove that they are fossils yet. In the future, we may be able to confidently identify many of them as fossils. Astrobiologists, when devising life detection experiments for Mars, focus almost exclusively on various biosignatures. For instance, the chirality of the amino acids - whether it uses the molecule in one form, or its mirror image

That sounds straightforward enough, but it's not as easy as it sounds. The past life on Mars may have been destroyed long ago except in a few favoured patches. These may have only a few trace amounts of organics left from the past mixed with organics from meteorites, volcanoes, and various non life processes on Mars, degraded by cosmic radiation and reactive chemicals from the surface, washed out by floods, and easily masked by just the minutest traces of present day Earth life contamination. We may have to dig deep and drill in many spots to find this faint signal which might need a lot of detective work to sort it out. For more on this see What if Mars has really tiny cells - like the structures in the Mars meteorite ALH84001? (below) .

With the exobiologists keen to detect even a single molecule biosignature from past life, how can that work if samples of past life get contaminated by modern Earth life?

Searching for present day life on Mars in the popular imagination

In the popular imagination, at least to judge by TV and movies, life on Mars will be easy to find as soon as we send humans there. For instance, as dramatized in the National Geographic Mars TV series, the astronauts need to take no precautions to avoid contamination of Mars with Earth life. They even bury dead members of their crew in the Mars soil. In season 1, episode 6, Crossroads, they drive up to a potential habitat that they think could have present day Mars life and walk right up to the habitat to search for life there (they say that it is an RSL so a patch of dark streaks caused by flowing thin films of salty water just below the surface, though in the movie it just looks like a small outcrop amongst sand dunes).

The astrobiologist Marta Kamen, then walks along the outcrop with her two companions inspecting it visually. She picks up a rock and looks at it and turns to her companions, sighs, "not what I'm looking for", and they walk on dejected until suddenly she spots a small patch of reddish soil in a crack in the rock which she picks up with a pair of tweezers:

.

Somehow she and her companion astronauts immediately know that this is what they are searching for.

She returns it to their lab for inspection, where she sees a network of interconnected purple strands.

She recognizes it as native Mars life by feeding it a nutrient with a pipette and noticing that it moves slightly in response.

Can it be? Dawning realization

Yes, there is life on Mars!

Though that's obviously a dramatic simplification, it's pretty much how Robert Zubrin thinks it would happen, the founder of the Mars Society, space engineer, and author of Case for Mars. He thinks that Mars life would not be harmed in any way by introduced Earth life and would be easy to spot, and easy to distinguish from Earth life if it is present. He thinks that introduced Earth life would not harm the search for life on Mars in any way, and that human astronauts on the Mars surface would greatly speed up the search for life there. So, in this story, the National Geographic series presents in a simplified dramatized form how the search for Mars life would go according to Zubrin's ideas.

Why it's likely to be hard to search for present day life on Mars

What's the problem with this? I will go into this in a lot more detail later, but let's just introduce some of the main difficulties with searching for preset day life on Mars.

First, if it is anything like life in similarly harsh areas of Earth, then it's likely to be microbial. Or at most, lichens perhaps, huddled in cracks in rocks for protection from UV light. That seems likely to be the case even if life on Mars did get off to a great start, with rapid evolution and multicellular life already there in the oxygen rich atmosphere three billions years ago. The conditions now are very harsh for present day multicellular life. If we can judge Mars by comparison with Earth then the closest we have to such harsh conditions are the very dry (hyperarid) core of the Atacama desert and the McMurdo dry valleys in Antarctica. In the most cold and dry conditions on our planet, even with our rich variety of multicellular life and the oxygen rich thick atmosphere, all we find are small populations of microbes, in thin films, which live very slowly, and for multicellular life, at most some lichens.

Also, in such harsh conditions, even photosynthesizing microbes tend to be hidden from view below the surface of rocks, beneath the soil surface and in salt pillars. It's also likely to be patchy with some places that have life and others apparently identical right next door that don't.

Researchers in Beacon Valley in Antarctica, one of the most Mars like regions of Earth. There's no snow or ice, kept dry by fast winds blowing off the Antarctic plateau, and there is little moisture, just some limited melting around the edges of the valley and thin films of brine around permafrost structures.

In cold dry areas like this, typically there are no visible signs of life. It's hidden in the soil, beneath the surface of rocks, inside salt pillars. Some places have life and other identical patches nearby don't. This type of terrain corresponds very roughly to the most habitable regions of Mars. It's only a partial analogue. Yet there is life here, very slowly metabolizing, in small quantities in scattered locations.

So, present day life is going to be hard to spot by eye on Mars. Unless, that is, it is obviously novel, say a purple lichen that you can be pretty much certain never got to Mars on our spacecraft.

Lichen P. chlorophanum on a Mars analog substrate for the DLR Mars simulation experiments. - colour adjusted to a dark purple. If we saw a lichen or other multicellular lifeform on Mars, especially if coloured in some unusual non Earthly colour, it would be convincing evidence of native Mars life which we could spot visibly. At least, it would be, in early stages of human exploration (if we send humans to the Mars surface). In a Mars that's been settled by humans for some time, perhaps an unusually coloured lichen could be an adaptation of introduced Earth life, maybe partly through gene exchange with native Mars life.

Generally, multicellular life on Mars, large enough to see visually would be easier to spot, and it would be easier to show that it is from Mars, if it is. Also we'd be less likely to have accidentally introduced multicellular life than accidentally introduced microbes, in early stages of Mars exploration, if we do send humans to the surface. However most of the suggestions for searches for life on Mars focus on microbes. If these microbes are mixed in with the dust or in the rocks, thinly distributed, and perhaps are reddish in colour, similar to the Mars surface, or a darker black in colour - how would we spot them visually?

Also, if we go to the most habitable areas, we'd be lucky to find a total area of a few square meters of sparse, slowly metabolizing life, in an exploration region of several square kilometers. If there are liquid brines, say, some parts of the RSLs may be more habitable than others. Some may be too salty, too cold, have harmful reactive chemicals in them. Some of the dark streaks may have life and some not. Or we might find a few lucky spores in the dust, but perhaps have to examine a fair bit of dust to find them. So you are talking here about searching for something in sparse populations and hidden just beneath the surface, quite possibly dark in colour, or so spread out that it hardly has any colour at all or is hidden from view.

Robert Zubrin argues forcefully that Earth life can't harm the scientific exploration of Mars in any way at all, either the search for past or present day life. It would be so wonderful if that was true. It would make things so much easier, as there would then be no need to do anything special to protect Mars. We could just explore it looking for unusual life forms, past and present, much as we do on Earth. But is that really the situation? We need to be sure here, since so much will depend on getting this right.

I'll go into how we would search for life on Mars in some more detail in a moment, but first, let's look at Zubrin's arguments.

What are Zubrin's arguments?

The enthusiasts who want to send humans to Mars right away, as soon as we have the capability to do it, tend to brush off all of these concerns, and say either

"No need to worry, Mars life will be identical to Earth life so it doesn't matter what we bring there."

Or they may say:

"No need to worry, it will be easy to tell the difference between Mars and Earth life, so it doesn't matter what we bring there."

And then

"No need to worry, Earth life can't survive on Mars or vice versa. It's like sharks trying to survive in the African Savannah."

They get these arguments from the Mars colonization enthusiast and space engineer Robert Zubrin, author of Case for Mars, and head of the Mars Society. He will often bring up all three of those arguments in the same talk, as different reasons why we don't need to worry about introducing Earth microbes to Mars. His audience of fellow human spaceflight enthusiasts find these arguments very persuasive and clap him enthusiastically.

I think that perhaps they feel he has covered all bases. Either Mars is so inhospitable to Earth life that it's like sharks surviving in the savannah; or it is so similar that Earth life not only would fit right in, but has already got there on meteorites; or if neither of those apply then Earth life would be as easy to distinguish as anthrax by genetically sequencing it. But actually, as we'll see, those are just three of numerous possibilities and indeed they are all rather unlikely ones at that.

He also talks about the advantages of human astronauts over robotic rovers on the surface, citing as an example, a fossil discovery that he and a team of others made in Arizona in a Mars exploration simulation. He comments on how they were able to find petrified wood and a fossil bone fragment within two days. Here is a quote from his log book a the time of the discovery:

"There is a lesson in all of this for those who think that robots represent a superior way of exploring Mars. With a human crew on this site, impaired by all the impedimentia of spacesuit simulators with the cloudy visors, backpacks, thick gloves and clumsy boots, our crew found petrified wood and a fossil bone fragment within two days. But to do it we had to travel substantial distances, and climb up and down steep hills from which we could take views and map out new plans. We had to search the sites we visited, processing the equivalent of millions of high-resolution photographs with our eyes for subtle clues. We had to dig. We had to break open rocks and take samples back to the station for detailed analysis. In short, we had to do a ton of things that are vastly beyond the capabilities of robotic rovers.

"Sojourner landed on Mars and explored 12 rocks in 2 months. Today we explored thousands. If a robot had been landed at the position of our hab, it would have spent months examining a few uninteresting rocks in the immediate vicinity of the station. It would never have found the fossils."

This turned out to be an interesting discovery, a new place to search for fossil dinosaurs: Scott Williams of the Burpee Museum of Natural History rates it as one of the nations best places to search for Jurassic era fossils. That might seem rather convincing, if human astronauts could find something like that so quickly, would that make it far faster than using robots? If you find his arguments convincing, that's not surprising at all. After all, we've seen human exploration like this dramatized in countless movies and works of fiction. It's also how we are used to exploring Earth itself. It's just what you would expect.

But, as I said in the introduction, nobody has actually explored a planet like Mars before, or indeed any other planet. It's something completely new and out of our experience altogether. Do these Earth analogies actually work for Mars? Zubrin is such an accomplished debater that even when he has a one on one debate with a planetary protection expert, as he does frequently at Mars Society conferences, he is generally seen as winning the debate by his target audience. They come away remembering all of his vivid points, and none of the responses to them by his distinguished planetary protection expert debating partners.

I covered this in the introduction, under Planetary protection - researches by Sagan and Lederberg onwards - and Zubrin's arguments. So here are his main arguments in summary::

• Earth life can't compete with native life on Mars - it would be like sharks competing with lions in the Sahara, because the planets are so different
• Any Earth life that could survive on Mars is already there, transported on meteorites
• It will be easy to distinguish Earth life from Mars life by sequencing its DNA, as we can even tell which lab a strain of anthrax comes from
• Humans on the surface can explore Mars much more quickly than any robot and do things in minutes that would take years for our robots

Could you see any flaws in them? Now is the time to look at that in detail. If you are one of those convinced by them, perhaps this may give some pause for thought?

Demolishing Zubrin's arguments

It may seem so simple after listening to him, Why does anyone continue to think about planetary protection at all? There would be no need to write the rest of this book, or for scientists to continue to research into planetary protection. But sadly, as you'll see, the arguments are easy to demolish.

Sharks in the savannah or rabbits in Australia?

• Yes, Earth life on Mars could be like sharks in the savannah. Or it could be like rabbits or cane toads in Australia. Or like invasive plants like Kudzu or Himalayan Balsam

The Mala or shaggy haired wallaby, considered as creation ancestors for the Anangu Aboriginal people - are in competition with the introduced rabbit.

Would Earth microbes on Mars be more like sharks competing with lions, or rabbits competing with wallabies? We can't decide this by using colourful analogies.

We have many examples on Earth of native species that have gone extinct due to rabbits, rats, cats, cane toads etc. The wallaby is perfectly adapted to Australia, and the rabbit isn't; it's a generalist. However, the rabbit happens to be better at living in Australia than the marsupials that evolved there originally. And this is not just an issue for higher animals.

It's not just multicellular life that is affected though. Much to the surprise of microbiologists, you get invasive microbes on Earth as well, especially in fresh water ( fresh water lakes are often isolated from each other). The Great Lakes in the US have over 180 species of invasive microbes, and New Zealand is trying to eradicate an invasive diatom accidentally introduced to it from the northern hemisphere, probably on damp recreational diving gear. It is causing problems in its lakes.

For more analogies, see the section Examples of invasive Earth life (below), and for details about invasive microbes, see single cell diatoms, and concerns about invasive microbes in Antarctica,Invasive diatoms in Earth inland seas, lakes and rivers

The same could happen for Earth microbes competing on Mars. They don't need to be adapted to attack Mars microbes. They just need to be able to

• Outcompete them, like the rabbits in Australia,
• Smother them, like invasive plants, block out the sunlight or deny the Mars life access to essential nutrients
• Change the habitats there in ways that make them no longer so habitable to the native Mars life like the invasive diatoms in New Zealand cold fresh water lakes.

The same could also happen the other way around for Mars life returned to Earth. The. Mars life could be like the invasive rabbits and Earth life could be like the wallabies. More about this in Safe return of an unsterilized sample below.

As Cassie Conley, NASA planetary protection officer, put it in a recent interview with Space.com,

"It's unfortunate so many people don't seem to understand that transferring potentially biohazardous material between Mars and Earth could be problematic for life on both planets. There are lots of biohazards on Earth … Do we really want to bring them to Mars indiscriminately?"

Yes, in some places in our solar system, this analogy of sharks in the savannah works just fine, and Earth life would have no chance, like a fish out of water. Titan is like that, with temperatures of -180 °C, in liquid ethane and methane (see Life in the oceans of ethane and methane on Titan - below). Earth microbes need temperatures above- 20 °C to complete their life cycle, with some metabolic activity down to -26 °C and perhaps lower. So, surely, this -180 °C of the Titan oceans is way too low to be habitable for Earth life. The environment is also different in many other ways, particularly, that it has non polar ethane and methane in place of liquid water, but the low temperatures by themselves are enough reason to be confident that Earth life can't survive there. In the other direction, we can't really prove that Titan originated life could not survive on Earth, but it would need to be extraordinarily versatile to have a chance of survival in such a different environment. If neither can live in the other's habitat, that would indeed be like the sharks and lions in Zubrin's analogy.

Mars could have habitats like that too. Many of the potential habitats on Mars may be either too cold or too salty for Earth life, or both. It's possible that the only native Mars life survives at extremely low temperatures, down to -80 °C, laced with antifreeze mixtures of perchlorates and other salts, and mixed with hydrogen peroxide. All Earth life has water mixed with sea salt inside their cells, but in such different conditions, the Mars life could have water mixed with perchlorate salts and hydrogen peroxide inside them. Such life probably wouldn't be able to survive on Earth, and Earth life couldn't survive in the habitats that it favours either. See. Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells (below).

There's at least one other potential "Sharks in the Savannah" type habitat on Mars. It could also have Life in liquid ("supercritical") CO₂ at depths of 100 meters below the surface. Again, Earth life introduced by astronauts would have no chance of surviving there, and unless the life that lives there is extraordinarily versatile it couldn't survive on the Earth's surface. So again, compared with Earth life, any life in such habitats might well be like Zubrin's lions and sharks.

But there are many potential surface habitats that would be habitable to Earth life if they exist. Indeed, Earth life has rather a lot in its favour when it comes to surviving on Mars, mainly because we have somewhat Mars like conditions on Earth, especially in the McMurdo dry valleys in Antarctica and in the Atacama desert. Zubrin actually recognizes this himself in his other argument where he says that microbes transferred on meteorites could survive on Mars. In this meteorite argument, he even suggests that Mars will have all the same identical species in the same conditions. In other words not at all like sharks competing with lions as almost none of the multicellular species that share a habitat with sharks will be able to survive in the savannah. Some of the advantages of Earth life for survival on Mars include

• Earth life comes from an oxygen rich atmosphere, so is pre-adapted to deal with oxidative stress. The surface of Mars is highly oxygenated with perchlorates, and hydrogen peroxide. But because of our oxygen rich atmosphere, this should be no problem for some hardy microbes. We even have microbes that use perchlorates as food.
• Some Earth microbes are well adapted to UV light. That's especially so if they are adapted to deserts and also survive in ordinary conditions. For instance Chroococcidiopsis which has a wide distribution can handle the Mars levels of UV just fine in conditions of semi-shade. And many of the potential habitats are beneath the surface of rocks or protected from UV light by dust or in other ways protected from UV.
• Earth microbes have hardy spores that could probably survive for long periods of time on Mars until they find a habitat.
• Some Earth microbes have mechanisms to repair their DNA . They evolved this probably to deal with desiccation, but they also turn out to be effective methods to repair damage from cosmic radiation and solar storms.
• Although Earth microbes can't reproduce below -20 °C, many of them are not destroyed or harmed by much lower temperatures. They can survive very low temperatures no problem by going into a glassy state. Mars life might be limited in the same way and have the same coping strategy.

There are many suggested habitats for Earth life to survive in on Mars, including the potential for photosynthetic life to survive almost anywhere using just the humidity which reaches 100% at night on Mars due to the huge temperature swings from day to night, and often leads to morning frosts even in equatorial regions. See Habitats for life on the surface of Mars (below).

We simply don't know for sure what the situation is on Mars, until we have a chance to study the potential Mars habitats close up and find out what is in them and what the conditions are there. Once we do that, then yes, we will know for sure if Earth life can survive in them, and if there is life there, and if it has the same limitations and adaptations as Earth life or different ones.

If Earth life can survive on Mars then the correct higher animals analogy to use for Earth life on Mars is rabbits competing with wallabies or any other similar example of species that can survive in the same habitats. Which of course could also include non invasive species that co-exist happily. Then the question is, which are the rabbits and which are the wallabies in the comparison, or are they just happily co-existing?

We can look at this from the point of view of the evolutionary status of Mars life. As we saw, the life might well be at a much earlier stage in evolution than Earth life since it has had such an interrupted difficult past for evolution. Or it might be equally evolved, or more evolved than Earth life, or more evolved in some directions. So some of the possibilities are:

• If it is some feeble RNA world life for instance, it might just be a walk over for Earth life with perhaps just about every single Earth microbe that could survive there becoming an invasive species on Mars, as in Arthur C. Clarke's Venus story in the introduction (in the section: Why don't explorers in science fiction have these problems when exploring other worlds? (above).
• If the life on Mars is at a similar level of development to Earth life, then some Earth life might be invasive on Mars, and some Mars life might be invasive in the other direction back to Earth, in a situation analogous to invasive species of higher animals and microbes transported between continents on Earth.
• Mars life could be so highly evolved that it is almost universally invasive and defeats Earth life. In that case, though it might be safe to send astronauts to Mars as far as Mars is concerned, with no risk of forward contamination, it might not be so safe for the astronauts to return to Earth.
• The Sharks and the Savannah analogy might be correct, and there are no habitats for present day Earth life on Mars. Perhaps Earth life can't survive there, and there is Mars life but it can't survive here either. That is still possible at present.
• Or his last possibility might be correct, that Mars and Earth life survive in the same habitats but play nicely together.

We can't settle any of this by using vivid analogies. The only way to find out is to learn more about Mars. Vivid analogies may well help convey our understanding, once we do know what the situation is there. Zubrin's "Sharks and the Savannah analogy could be a great way to explain the situation to the general public, if that is indeed what Mars is like. However, we don't have that understanding yet.

(N.B. I think this evolutionary way of looking at the planetary protection issue for present day Mars may be my own idea. It would seem to follow logically from things the researchers say. If Mars had early forms of life early on, as was suggested, for instance, for ALH84001, and there was no significant evolution since then, and it survived to the present, it would follow that it still has an early form of life there now. The suggestion of a "shadow biosphere" of RNA world life on Earth also suggests this idea of a co-existing present day early form of life, which if it is possible even on present day Earth, surely is a possibility on present day Mars, that early RNA life survives to the present day. Then in the other direction, if it had multicellular life already three billion years ago, then it must have been far ahead of us in evolutionary terms. But I'm not sure if anyone puts it quite in these terms, asking whether present day Mars life is evolutionarily ahead or behind Earth life or at the same stage. It seems an obvious way to look at it once you set it out like this, so surely someone else has done it, but I just don't happen to know of any papers that put it quite like this. It just kind of dropped out, when working on this book, as a neat way of organizing the material, but I can't remember reading it anywhere. Do say if you know of someone who presents it like this, so I can cite them, and discuss their ideas, thanks.).

How do we use a DNA sequencer to prove that a microbe is from Mars? Can you guess the percentage of microbial species sequenced?

• Yes a well known microbe like anthrax on Mars would be easy to detect. So in some special cases like that we can prove that a microbe is from Earth. A more likely species to find on Mars could be chroococcidiopsis which is a well studied microbe on Earth. If that got to Mars from Earth we could recognize it by its DNA sequence.
  
  But how do we use DNA sequencing in the other direction to prove that a microbe is from Mars? When you try to flesh this out, it gets hard to see how it could work.

If we find life on Mars, and have a portable DNA sequencer and it matches Earth life, great, we know it was introduced by us in our spacecraft. That is, unless of course it was already there, brought there on a meteorite which is a whole other dish of worms to deal with, but we will come to that later in the meteorite argument section.

But suppose we have already introduced Earth life to Mars, perhaps accidentally, and then we find clear proof of life in one of the habitats there. We find biosignatures, or even see the microbes swimming. And then suppose you find DNA and it doesn't match a known Earth species - would that count as a proof of native Mars life?

In the introduction, I suggested you try to guess what percentage of Earth microbe species have had their DNA sequenced? 10%? 1%? 0.1%? 0.01%? Well to a microbiologist, the most striking thing is the amount that we don't know about Earth life. Hardly any Earth microbes have been sequenced, and even fewer have been cultivated.

This is the problem of what microbiologists call "Microbial dark matter" by analogy with dark matter in astronomy. It's a rather close analogy. These "dark matter" microbes probably account for most of the Earth's entire biomass and biodiversity, but we know nothing yet about their most basic metabolic or ecological properties (paraphrasing from this paper). Of an estimated one trillion species, only ten million have been identified and catalogued. Of those only about 100,000 have classified sequences, and only 10,000 have ever been grown in the lab.

That makes it only 0.00001% of all microbial species on Earth that have been sequenced to date. Then, of that 0.00001%,, 90% can't be cultivated in the lab, and are the result of sequencing a single isolated cell using new techniques which reached maturity in the last three years. See Largest ever analysis of microbial data (May 2016).

It's even more striking if you look at bacterial phyla. That's a very "broad brush" classification. Humans belong to the phyla of chordates, which is, one step above creatures with a backbone, Of the 89 bacterial phyla known, half don't have a single cultivated species as yet. So that's many entire phyla which have none of their species cultivated. We just have gene sequences. We know almost nothing about these microbes, and what their capabilities are, as we have no way to cultivate them and study them to see what they do. That's the situation for half of the bacterial phyla discovered to date.

In any spaceship occupied by humans, sent to Mars, then just as for any other habitat containing Earth microbes, then nearly all the species on board will not have been sequenced, and of the few that have been, most won't have been cultivated. Even spacecraft assembled in clean rooms have numerous microbe species that are not sequenced, and their properties are not well known. These include archaea, though they seem to be under represented, and it's not clear if they are viable. Most of them are bacteria, eukaryotes, and surprisingly, also many viruses, all of which are still viable. But a human occupied spacecraft can't be sterilized in this way, so would have vast numbers of species of unidentified microbes of all types.

Also, with so few species sequenced, then it would be no surprise if those 89 known phyla are just the tip of the iceberg. The number of phyla still to be discovered may be as many as 1,500. If that is correct, we only know a bit short of 6% of the bacterial phyla. As for the ones we can cultivate and study in the laboratory, that would mean we only have example cultivable species from 3% of the phyla. That doesn't mean that those 3% are thoroughly understood at all. Many may have only one or a few cultivable species. It just means that for three percent of the phyla we have at least one known species that we can culture and study.

With this background, you can see that it would be easy to discover not just a new species on Mars but even an entire new phylum, and have no idea whether that entire phylum is indigenous, or got there on our spaceships. There would be no way to use gene sequencing to resolve this, not right away. We'd then have to do a massive search on Earth to see if we can locate the same phylum here. If we can't find it, then that would just make it inconclusive, whether it is a hard to find phylum that got to Mars, or is indigenous.

So, if a wide range of species of Earth life was introduced accidentally to a Mars habitat, for instance after a human spaceship crashes on Mars, then typically only the tiniest fraction of a percent of the species there could be recognized as definitely coming from Earth. For the rest, you'd just have to say you don't know where they came from, even if nearly all or all of them actually came from Earth originally. It would be the same situation indeed if nearly all of them are indigenous Mars life. We might never find out, or the debate might continue for decades before we get some resolution of the question.

So in short, if Mars life has common ancestors with us, even as long ago as over three billion years ago, there may well be no way to recognize it as Mars life after contamination from Earth. You'd have to prove that a particular species or phyla couldn't have come from Earth. But how would you do that?

Photomicrograph of the anthrax microbe, Bacillus anthracis using Gram-stain technique. This is one of 100,000 species of microbes that have been genetically sequenced out of possibly a trillion species to be discovered, and its phyla is one of the 89 known phyla of bacteria out of perhaps 1,500 yet to be discovered.

Robert Zubrin is fond of using anthrax as an example in his talks. Yes if we found a microbe like this, we'd be able to tell that it is from Earth. But after contaminating a Mars habitat with Earth life, for instance, after a crash of a human occupied spaceship on Mars, we can expect to be unsure about the origins of 99.99999% of the species that actually originally came from Earth. So genetic tests can't tell us for sure which lifeforms on Mars originated there and which came from Earth, after contamination by Earth microbes.

Also many of the exquisitely sensitive tests that astrobiologists want to send to Mars, able to find the faintest of biosignatures or to detect just a few microbes metabolizing, would be just useless once there are Earth microbes widely spread over the Mars surface. Whenever you detect a biosignature, you'd assume it was from Earth. Even if the sample had abundant Mars life in it, you'd detect biosignatures, true, amino acids, maybe even more complex chemicals like chlorophyll, but that would not prove that it was indigenous life.

The only way to identify life as from Mars, after the Earth life contamination of a human occupied spaceship crashing there, or irreversibly introducing Earth life to the planet, would be to use gene sequencers, unless it has unusual biochemistry.

Then on top of that, there's an additional complication. If Mars life has a common ancestor, even billions of years ago, it will exchange gene fragments with Earth life very readily via gene transfer agents. This is an ancient mechanism that permits gene transfer between totally unrelated bacterial phyla, and in the right conditions, for instance in the sea, so also perhaps in salty habitats on Mars, this can happen overnight, within hours. So after introducing Earth life to Mars, as a result of all this horizontal gene transfer, you are likely to end up with a hodge podge of Earth microbes that incorporate Mars gene sequences, and Mars microbes that incorporate Earth gene sequences. Imagine how hard it would be to disentangle all that and work out which of the fragmentary gene sequences in these hodge podge microbes are from Earth originally, and which from Mars in this situation? And how much harder will that be, when, in addition, most of the microbes, whether from Earth or Mars, can't be matched with any known Earth microbe gene sequence.

Also, even if Mars has no DNA in it, the introduced Earth life could cause major complications, making it hard to distinguish. For instance suppose we find what seems to be early RNA based life and we are only able to extract RNA from the cells - this result would probably be highly controversial. It could just be a mistake that for some reason the instrument failed to identify the DNA. Even if that seemed extraordinarily unlikely, you'd still probably get many people arguing that that must be what happened. This would become especially hard to establish, if you had DNA from other Earth microbes in the habitat mixed up with it.

So if Mars and Earth life have a common ancestor at any time since the origin of DNA, then the DNA sequencing test would probably be useless as far as identifying native Mars microbes. If there is no common ancestor, or the ancestor predates DNA based life, then it would make it far harder to identify Mars life, if the only way we can do it is by proving that it doesn't have DNA. It would be far harder to find life and prove it is not from Earth than to just identify it as life by the biosignatures, especially if it has a lot in common with Earth microbes, such as RNA, and similar cell walls and other cell components.

In short, it's hard to see how introducing Earth microbes to Mars could avoid hugely complicating in situ searches for present day native Mars life, even if it doesn't make it extinct.

What do you think? Do give this careful thought, as we need to be very sure here. It could easily be case of gambling with what could be the most important discovery in biology and astrobiology of this century, or even, the most important discovery in biology perhaps of all of human history to date (maybe with the exception of Darwin's original theory of evolution).

Comparison of spacesuits on Mars with shirt sleeves exploration from Mars orbit and Earth - with a hundred new multigigabyte 3D landscapes streamed back each day

• Advantages of human astronauts for fossil hunting on Mars - first - would they be fossil hunting in the first place? This all depends on Mars life having easy to recognize fossils. As we've seen, even early stromatolites are likely to be tiny (one or two cm in size), hard to recognize, and controversial once found.
  
  If the search is for organics, then how would the astronauts distinguish between organics from meteorites and organics from early life, just by strolling over the landscape and looking at it by eye and breaking open rocks.
  
  And anyway - in practice the NASA plans for Mars astronauts are to restrict them to a contamination zone around their base. They wouldn't roam widely. They would leave that for the robots.

The astronauts could spot things like the blueberries easily- but how much use is that if they can't tell if they are the result of life processes or not? Also, our rovers can find them anyway. It would work for dinosaur bones, petrified wood, and other macrofossils, which they can recognize to be life just by their visual shape - but how likely are those on Mars? When it comes to the search for organics, robots have the huge advantage that they can be sterilized and search in situ.

With NASA's plans, humans would have to explore within a zone permitted for humans, and can only explore regions that could potentially have present day life telerobotically. They have to wait for the robots to return with the samples for them to analyse. They would have to stay well away from any sensitive areas with present day life or organics exposed to the surface.

In this scenario, seems to me, that it might be quite a challenge to keep the samples from being contaminated when the robots return them to the human base. After all that's a major issue for Mars 2020 even with all the facilities we have on Earth. See Difficulty of keeping returned sample free of contamination from Earth - Mars 2020 will have a permitted 1 part per billion of Earth originated biosignatures (below) . How much harder will it be to do this for multiple expeditions by rovers away from a human occupied habitat on Mars? Yet, they would have to do better than that, to have clear incontrovertible results, especially if the signals are faint, as most experts expect them to be. How could they keep them sterile before they send them out, and to keep the samples clean even of slight traces of amino acids, when they recover them too - could they do that?

And if they can do that once, how do they resterilize the robots to better than Viking standards to send them back out again to look for a new sample to return? For more about NASA's plans,see NASA's plan for safe zones - based on finding Mars life easily (below) .

It might be better to just send the robots to Mars independently, to do in situ searches, sterilized back on Earth. Although we haven't had any robotic in situ searches for life on Mars since Viking, we can get an idea of how the search could be done robotically from the in situ tests of LDChip300 (part of the SOLID project). This is an organic biosignature detector designed by astrobiologists for in situ searches on Mars. It uses 300 different antibodies - which together can be used for exquisitely sensitive tests for organic biosignatures. They tested it in the very dry "hyper arid" core of the Atacama desert, drilling into the extremely salty "hypersaline" subsurface. From in situ analysis of just half a gram of a sample, it found a previously undiscovered microbial habitat two meters below the surface. Humans had explored the desert for decades and never found it.

Microbes in salt crystals two meters below the ground in the Atacama Desert, discovered using LDChip300, part of the SOLID program. SOLID is one of several extremely sensitive instruments, low mass, and low power, which astrobiologists hope one day can be sent to Mars for in situ search for life. Image credit Parro et al./CAB/SINC. For many more such instruments, see In situ instrument capabilities below

Future rovers would also be more robust and capable than Sojournor, Spirit, Opportunity or even Curiosity. Curiosity has made many improvements in autonomy over its predecessors, and ESA's next rover, ExoMars has made many more with its SAFER approach for autonomous driving tested in 2013 in the Atacama desert. (Video of the tests speeded up 16 times here). They have also worked on ways that it can make intelligent decisions for itself about what to photograph - something that Curiosity already does with its photographs of dust devils (for instance), extending that to other things like photographing interesting details from rocks without being asked to do it first.

But this is just a start. With better communications they could also be far more mobile. Even in the 1970s the lunakhod traveled as far as Opportunity did in a decade, in a few months. Most of that difference in speed is not so much because of the light speed delay, as because of the low bandwidth to Mars. At present the Curiosity team typically exchange data with Mars once every Martian sol, for about eight minutes, and it transmits around 200 - 250 megabytes during that window of opportunity. With that 24 hour turnaround time, they could drive it nearly as quickly if it was studying a Kuiper belt object many times the distance to Neptune.

Before we send humans to Mars, whether in orbit or on the surface, we would need almost continuous broadband communications back to Earth. NASA plans a new relay satellite for the 2020s which will increase the bandwidth to 800 gigabytes of information a day, a dramatic difference.

That by itself would make a huge difference to the pace of exploration, even with robots controlled from Earth. We'd be able to stream back millions of images, or hours of realtime HD video, every day. Back on Earth, these would be reconstructed to make a 3D landscape that we can then explore at leisure. We could even fly over the virtual landscape, and don't have to trudge around in clumsy spacesuits. Also anyone on Earth could explore it and use their expertise to look for things.

Here is a four gigapixel image made by Curiosity over a period of 14 days from sols 136 to 149. I show a series of snapshots, gradually zooming in on a single point in the image. The first in this sequence is itself a detail in a 360 degree image.:

Click here to go to the scene itself and try zooming in yourself


By the mid 2020s, our rovers will have enough bandwidth to "phone home" with two hundred of these full 360 degree four gigapixel landscapes every day.

An image that currently takes 14 days to return from Mars could be returned every four minutes throughout the twelve hour day. Or they could stream them back in binocular 3D, in which case they could stream back one hundred of these multi-gigapixel landscapes each day (one every seven and a half minutes).

Curiosity was going to have binocular 3D 15:1 zoom capabilities for its Mastcam - but that was descoped to save money, and instead it has two cameras, one fixed in the zoomed in position, and one permanently zoomed out. Mars 2020 will have a binocular zoom with its Mastcam Z though with only a 3.6: 1 zoom, giving 3-4 cm details 100 meters away. Surely future rovers will continue to have more impressive 3D zoom capabilities.

These future rovers will surely have zooms right in to microscopic detail. There are several examples on the GIGAmacro website which is a commercial system for producing automated images of this sort. This shows a zoom in on one of their images of a geological core specimen using their automated gigapixel macro images.

First zoom in:

Zoomed in to

Then to

You can try it out here with other examples on their website GIGAmacro. That's a 1.44 gigapixel image so by the 2020s we can return 555 of those a day, or in binocular 3D, 277 of these a day.

For more examples see this zoom in on a museum specimen of a Splendid-necked Dung Beetle one of several in a microsculpture exhibit by Levon Biss. More about this project here. Also for some wonderful multigigabyte photographs of microbiomes, for instance of fungi and lichens on branches, where you can zoom in to almost microscopic detail in Mathew Cicanese's Gigamicrobiome project.

With images streamed back like that, you'd be able to not just explore the surface of Mars, but also, to zoom in on any rocks close to the rover as if you were able to examine them with a geological hand lens.

Once that satellite is in operation, then every day we will get more data back than we got for the entire 500 Gb for the New Horizons flyby of the Pluto system. It would also mean we get the same amount of data that we currently will get from Curiosity after eight years and ten months, streamed back, every single day! Think how much more they could do with 800 gigs a day and with communication back and forth every eight minutes throughout the day, when Mars is closest to Earth? With these improvements, and more autonomy for the rovers, a Mars rover could travel tens of kilometers per day or even in an hour, like the lunar rover, streaming back high frame rate binocular HD video of everything it sees, wherever it goes.

Also, there's a third possibility which can combine the best of both approaches, of astronauts and of robots. We can have astronauts in orbit around Mars. They could explore it via telepresence with binocular vision, haptic feedback so that they can feel the rocks on the surface and pick them up easily, white balanced to make the landscape easier to read, and as before, stream everything they see back to Earth, where, as for the robotic missions, we can explore everything they found at our leisure and find things that they missed.

This shows how researchers on Earth or in Mars orbit could explore a Mars environment with virtual Avatars and even talk to each other within the virtual environment.

With broadband streaming of data back from Mars we could build up these environments rapidly, many times a day. Indeed with continuous HD binocular video streaming back from Mar,s we could have continuously expanding 3D VR landscapes developing in real time as the rover travels over the surface, and image the landscape in 3D to hand lens resolution around the rover wherever it goes. The same idea could also be used for teleoperation and telepresence exploration from orbit above Mars. This could build on techniques currently in use for exploring the ocean depths, for telesurgery and for computer games. See Almost Like Being There.

You wouldn't be able to actually turn over the rocks yourself. But you'd be able to roam throughout the landscape, much like a geologist on Earth, and without the encumbrance of a spacesuit. You could then mark points of interest and get those future highly mobile landers to drive, or even fly up to the site of interest, photograph it closer up, drill, or turn over the rocks.

So - how much of an advantage is it to be able to physically turn over the rocks yourself, to drill into them rather than schedule a drilling for the future, and to be able to conduct the experiments in real time? Well, though we wouldn't expect to find easily identifiable fossils, still, there's lots that a human could do to speed things up if there "in situ". However those advantages might not be as great as you'd think, if you compare the present day exploration of Mars with humans there.

Remember, that you have response times of eight minutes, between you seeing an image, and you telling the rover to do something in response to something it saw eight minutes earlier. That's a whole lot better than the current delay time of 24 hours. The rovers also would be able to travel at great speed, kilometers per hour. We just don't have any experience of exploring Mars, or even the Moon, in conditions like that, which will be common place probably by the mid 2020s.

Then there are various other ways to speed things up, especially, the idea of "artificial real time". Finally though, there's the possibility of telerobotic exploration of Mars from Mars orbit. Hardly anyone ever tries to compare in situ exploration with humans on the surface with humans exploring Mars from orbit via telepresence. And when it comes to drilling, often presented as a major advantage of humans " on site", it turns out that in the near vacuum of the Mars atmosphere in clumsy spacesuits, that drilling is probably best done by automated moles, especially since you also want to keep the drillings clear of any possibility of contamination by Earth microbes, again.

More on all this later in the section What should our objective be for humans to Mars? including the subsections What should our objective be for humans to Mars? and Telerobotics as a fast way for humans to explore Mars from orbit. Other sections devoted to this discussion include:

• Imagining a telepresence mission in the HERRO Molniya type orbit
• Artificial real time
• Telerobotics with humans in orbit compared to robots controlled from Earth
• Need for new comparison studies of the various ways of exploring Mars
• Compared with Mars surface missions

Meanwhile, for now, let's look at one other topic. You might think that a human being can spot photosynthetic life easily on Mars because on Earth typically you crack open a rock or salt in a desert and you see a green stain which is a sign of photosynthetic life here. Is this not a huge advantage of humans, that an astrobiologist on Mars could wander around the surface just cracking open rocks looking for life visually? Anyway, would that work on Mars? Let's just look briefly at how easy it would be to see green on Mars, and what colour Mars life might be.

First there's the question of whether you could see the colour green at all. That's not as much of a problem as one might think at first. But then there's the question of what colour life would be on Mars. Would it be easy to spot like the (often) green photosynthetic life on Earth?

Could we see green on Mars?

Green is a rare colour on Mars. Even purple is. So, you might think it would be easy to spot on Mars. You might have to break open a rock, but after that it would be easy to see. Here for instance is what green photosynthetic life looks like in the core of the Atacama desert.

Figure 2 from a paper on distribution of scytonemin, a UV protecting pigment, in cyanobacteria in the Atacama desert. Cyanobacteria typically are green, as are many (but not all) photosynthetic lifeforms on Earth. You'd think this would be easy to spot on Mars.

However, would Mars life be as easy to spot as this? There's one thing to look into briefly first. Colours such as green are likely to be hard for the human eye to spot on Mars. The problem is that light on Mars has a permanent reddish brown cast to it, because most of the blue light is filtered out of the sunlight by dust. Even if there were green rocks there, you wouldn't notice them, probably The pictures we see are all white balanced to help geologists on Earth recognize rocks. This for instance is what the Curiosity Mars dial looked like on Earth.

Curiosity Mars dial as photographed on Earth before attaching it to Curiosity.

This is what the same dial looks like on Mars. It hasn't faded. It's the same colours as before, but the reddish brown Mars light dulls out most of the variation of colours.

Curiosity's "Mars dial" colour calibration target as photographed on sol 69. Raw unprocessed image. This gives a rough idea of what it would look like to human eyes on Mars with the natural ambient light there.

The green patch there is barely distinguishable from the gold and even the bright red. To human eyes everything in the Mars landscape, however brightly coloured, will seem to have subtle shades of brownish gray.

The amount of blue light removed isn't that much actually; it's only reduced to between 42% and 62% of the original according to one study which was based on taking photographs of the calibration target (the "Mars dial" above). Even the red light is absorbed to between 60% and 80%. However, that imbalance is enough to change the balance of the colours significantly to human eyes.

This shows the amount of light transmitted through the atmosphere for global sunlight from 400 nm to 1000 nm as measured using photographs of the Mars Exploration Rovers calibration target (compared with its pre-flight calibrated reflectance). This was at a time when the sky was reasonably clear of dust, though not at its clearest (in a scale according to which the optical depth 1 is more or less opaque, Opportunity had an optical depth about 0.94 for Opportunity and Spirit had a depth of 0.93 - in winter when the air is much clearer Spirit's optical depth goes down to 0.2). Shows the linear visible spectrum superimposed.

Here is the original image of the cyanobacteria in the Atacama desert adjusted with the red reduced to 80%, green and blue to 60%, and then increased in brightness (corresponding to the way our eyes auto balance the brightness of images). It's very approximate, just to give a rough idea of how the colours would change with Mars illumination.

That's why nearly all the images shared with the public here are white balanced - colour adjusted so that the brightest colour in the image is white. That's also presumably why the National Geographic Mars TV series had so many bright bright colours in them, on the suits, buildings, and the landscape much brighter and colourful than it would actually be. If they had shown the colours realistically as they would appear to human eyes, the scenes would have been dim and hard to make out.

So a visual search on Mars based on colour would be hard to do without enhanced vision. But this may not be too hard to fix. An easy way to deal with it would be to tint their spacesuit visors cyan (close to sky blue) to let through more blue and green and block out some of the red light. The landscape would be a bit darker but they could pick out colours more easily because with reduced red, the green and blue colours in the landscape would be more obvious.

Cyan is the colour of white sand seen through shallow water because the water filters out red, leaving green and blue. Astronauts with cyan tinted goggles would see the Mars landscape with much of the red filtered out, in a similar way which could make it easier for them to see colours such as green and blue in the landscape.

Am I the first person to suggest that astronauts on Mars would have cyan, or perhaps "sky blue" coloured spacesuit visors :). I expect they would have cyan tinted habitat windows too, if we do ever have astronauts on Mars. Or even simpler, they could just wear sky blue goggles inside the suit and inside their habitat to colour correct the landscape to Earth vision.

Something like this might be a vital piece of equipment for future astronauts on Mars if we do ever send humans to the surface, worn inside their spacesuits and inside the habitats to make the landscape look much bluer and so more like an Earth landscape. Or alternatively, they have all their spacesuit visors and habitat windows tinted "sky blue" or cyan. Image public domain from Pixabay.

So, this is a nuisance but it's not a big deal as we can just use cyan coloured spacesuit visors to fix it. This would make the landscape a bit darker, but especially if searching with the sun high above the horizon in the tropics then you should be able to see colours fine. When searching in low light conditions, these tinted spectacles would dim the light further, so then, digitally enhanced vision would help. When searching with robotic spacecraft or telerobotically then we can use digital enhancement automatically.

So, if the photosynthetic life on Mars is indeed green, it might be easy to spot so long as you wear sky blue tinted glasses. Then indeed, you could split rocks open to find it, just as on Earth.

What colour would photosynthetic life be though, on Mars?

Would photosynthetic life on Mars be green - or could it be other colours such as red, purple, orange, yellow, brown or black?

What colour would Mars life be? H.G. Wells in War of the Worlds speculates that it could be a vivid blood-red.

"Apparently the vegetable kingdom in Mars, instead of having green for a dominant colour, is of a vivid blood-red tint. At any rate, the seeds which the Martians (intentionally or accidentally) brought with them gave rise in all cases to red-coloured growths."

Of course we now know that Mars doesn't have vegetation like Earth, but it could still have photosynthetic lichens and microbes. What colour would they be? I can find almost nothing specifically on this question, in the literature, no articles on the possible colours of photosynthetic life on Mars. The closest I could find was some material on the possible colours of photosynthetic life on exoplanets with low light levels. So I invite you to a section of synthesis and speculation.

Actually, we'll see, H.G. Wells' red or rust coloured does have some advantages on Mars, but there are many other possible colours too. It's actually a bit of a puzzle why Earth photosynthetic life is green. After all the green colour of Chlorophyll means that it absorbs mainly blue and red light, It rejects light in the strongest part of the solar spectrum. Why aren't leaves dark red, or dark purple, or indeed black?

Perhaps it is all just a historical accident. Leaves might be green because they get plenty of light, need to reflect some light away to resist dehydration, and there is no advantage in changing to black or purple. Or perhaps early life was indeed purple, absorbing green, like the modern haloarchaea, and photosynthetic green microbes originally evolved to take advantage of the light that the purple microbes rejected? That's the "Purple Earth Hypothesis" (abstract here). Or perhaps green is reasonably optimal anyway, once you take account of other effects?

At any rate, whatever the reason why so much Earth vegetation is green, yes, green photosynthetic life does work in Mars simulation experiments, for instance in the DLR lichen experiments (see Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere (below) ). So Mars could potentially have green photosynthetic life. But this is hardly the most efficient way to make use of the spectrum on Earth, and it would be even less efficient on Mars with half the light levels of Earth, and with the dust filtering out much of the light towards the blue end of the spectrum.

Solar radiation for direct light at the top of the Earth's atmosphere, and at sea level. Shows the linear visible spectrum superimposed.As you can see, we get more green light than any other frequency, yet for some reason, most of our vegetation is green. Would Mars photosynthetic life be green too? The sunlight at the top of its atmosphere would be identical, though half the intensity.

Well actually, even on Earth we have many photosynthetic microbes that are are red, purple, or pink.

Lake Hillier in Western Australia, a "pink lake". It's pink partly because of the purple haloarchaea, and partly because of red carotene accumulating in a green algae dunaliella salina.

For instance Mars life could be like the purple haloarchaea which are able to survive in very salty conditions, and use Bacteriorhodopsin and Halorhodopsin for photosynthesis which resemble the pigment rhodopsin that we use for vision. Bacteriorhodopsin is a purple and absorbs green light most efficiently.

Lake Hillier is also pink because of ordinary green algae which have made carotenes, the same pigments that make a carrot red. These pigments absorb UV, violet and blue light, while scattering red and orange light. They dissipate some of this light as heat so protecting other organics such as proteins and membranes from the damaging effects of UV light, which would be useful of course on Mars. However, they also transmit some of the energy they receive to chlorophyll.

Pigments like carotene that can transfer energy to chlorophyll like this are known as "antenna pigments" and they do it by dipole to dipole coupling (the process is called Förster resonance energy transfer). There are many other antenna pigments which photosynthetic life on Earth uses, and so Mars life could probably use them too, or something similar.

For instance, Mars life could have the equivalents of the yellow pigments Chlorophyll b and xanthophylls (which colour egg yolks and autumn leaves yellow) which have similar roles. Another example is Lycopene which makes tomatoes red. Cyanobacteria and red algae also have phycocyanin and allophycocyanin which absorb orange light, and a red pigment phycoerythrin which absorbs green light. Any of these could be useful on Mars

Deep dark red of algae in the crater lake of Mount Simba volcano at a height of 5,900 meters in the Altiplano, Chile. The microbes have developed special pigments to cope with extreme levels of UV. A few years ago the researchers measure what remains to this day the highest levels of UV measured in the world. Image credit SETI Institute/ NAI High Lakes Project

Life uses lots of different pigments to capture light in many areas of the spectrum. This next diagram shows how it works. For instance, Chlorophyll a can only absorb a narrow band of light in the red part of the spectrum (688) and Chlorophyll b in the blue part of the spectrum. But as you can see, there are other antenna pigments that help it take advantage of other parts of the spectrum, and Mars life could have these pigments, or similar ones, coloured purple, orange, pink etc in colour.

Part of Figure 1 from this study of the colour of life on Earth and exoplanets.

This graph doesn't show the purple "retinal pigments" such as bacteriorhodopsin, just chlorophyll and antenna pigments that work with it.

However, the most efficient way to do photosynthesis is to capture all the light that lands on a plant. And actually, photosynthetic life that evolved in low light conditions might need to be much more efficient at absorbing light than it is on Earth. Jack O’Malley-James of the university of St Andrews, Scotland, has suggested that life which evolved around red dwarf stars, especially binary star systems, could be dark in colour, or black, because it would receive far less visible light than Earth life does, so would need to make use of as much of it as possible. We do have black plants on Earth though they are rare.

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Examples of Earth vegetation with black flowers or leaves. When levels of light are low, then plants may become dark in colour or black to use as much of the spectrum as possible.

Possibilities for plant life under low light levels around other stars - perhaps in low light conditions the vegetation would be black.

We have dark coloured microbes too, as well as the black plants, and dark coloured seaweeds too. Some red algae and brown algae are nearly black, and grow in depths around 270 meters where the light is less than 1% of surface light. Life on Mars might experience not dissimilar conditions. That's especially so, since any photosynthetic life on Mars is likely to be hidden in cracks or beneath thin layers of dust or underneath the crust of rocks in order to shelter from UV light, so it might well be evolved to gather as much light as possible in dark conditions like that. The red iron oxides of the Mars surface are especially good at filtering out UV light so photosynthetic life might well use it as protection from sunlight, so that takes us back to H.G. Wells' red plants idea. Perhaps a rusty red could be useful to protect against UV.

This likely semi-shade habit, huddling in cracks for protection from UV, cuts down the available light more. Dust storms then would cut it down even further. So Mars:

• Already gets half the visible light of Earth life because of distance
• Blue is reduced further because of the dust in the air
• Photosynthetic life likely to huddle beneath the crust of rocks or in the dust in semi-shade for protection from UV.
• Photosynthetic life on Mars would receive far less during dust storms, less even than for a red dwarf star, and the storms can last for weeks.

The next few diagrams show how Mars life will have to cope with low levels of light during dust storms:

This shows photographs taken by Opportunity during a dust storm from sols 1205 to 1236 (one month). Each horizon view has been compressed horizontally (but not vertically). By the end of this period it reached a visual optical depth tau 4.7 which means that 99% of the sunlight was blocked. However that is for direct light.

Of course the dust will also scatter a lot of light, and if you include ambient as well as direct light then the figures are not quite so extreme. Here are a couple of graphs to show the distinction between light levels from ambient light combined with direct light, and the light levels from direct light alone, during a Martian dust storm.

This shows theoretical prediction of the combined direct and ambient light for different optical depths on Mars. Here optical depth is a number which is defined in such a way that, the larger it is, the more the light is blocked out. Tau 4.7 corresponds to around 99% of direct light blocked, which can happen during a dust storm on Mars. However, much of that light is scattered so adding to the amount of ambient light.

The curves in this graph show the amount of the combined direct and ambient light. The numbers 10, 20, 30 etc show the angle of the sun from the zenith - so with the sun at 90 degrees from the Zenith it's horizon skimming with very little light, and if there is an dust it goes through thick layers of dust, with almost no illumination.

So for instance, to find out how much light you get on the surface at the height of a typical dust storm go to around 4.7 in this diagram. Of the original 600 watts you still get around 200 watts when the sun is vertically overhead, at its zenith, even during a dust storm. However, you get it greatly reduced when the sun is closer to the horizon, as you'd expect, almost to nothing.

This shows the irradiance for direct sunlight. Very much less. During a Mars dust storm there is almost no direct sunlight with the sun at any angle from the zenith, even if it is directly overhead. Graphs from this 1999 paper. Our ideas of Mars have changed a bit since then but not enough to make these significantly out of date I think, at least for our purposes here. If you know of more recent graphs do say.

So, during a dust storm there is almost no direct sunlight. The amount of indirect sunlight is cut to a third even when the sun is directly overhead, at the tropics, and much more at other times of day even in equatorial regions (and much more so at higher latitudes). So, this is just my own suggestion here. Might some Martian life have very dark photosynthetic life in order to take advantage of as much light as possible during dust storms? Kelp for instance and other forms of seaweed that are adapted to the lower light levels in the sea are often brown in colour, to absorb as much sunlight as possible.

Giant kelp is brown to absorb as much sunlight as possible. It's coloured brown because of the accessory"antenna" pigment Fucoxanthin which absorbs light in the blue-green to yellow-green part of the spectrum. Some photosynthetic microbes are dark in colour too. In the conditions on Mars with dimmer light and the dust storms, might photosynthetic life on Mars be dark like this to use as much of the sunlight as possible for photosynthesis.

On the other hand, unless it is well sheltered by dust, or within rocks, it would also need to reflect away UV light for UV protection and may need to prevent desiccation. Perhaps another likely colour could be purple?

"Plants would appear darker under much dimmer, redder stars that emit more infrared than visual wavelengths of light but the color could vary widely"

Colour adjusted photograph by Tim Pyle of Caltech to illustrate possibilities for vegetation around other stars. Red dwarf stars particularly would have much more light in the red and infrared and photosynthetic life could have evolved to take advantage of it.

Or indeed, if it used iron oxides somehow for protection from UV light, it might be rust red in colour. Or perhaps it absorbs red light too, for the maximum amount of photosynthesis during dust storms, but it rejects blue, which is only present on Mars in small quantities, making it a dark blue in colour when seen with our sky blue tinted glasses (from the last section)?

So from those examples, it's hard to know what colour photosynthetic life on Mars could be. It might well not be green. It could easily be various shades of red, yellow, pink, purple, dark blue, or indeed almost black, amongst other possibilities.

What about non photosynthetic life. Actually, we don't know if Mars does have photosynthesis. If it doesn't, then we will be looking for life depending on chemosynthesis, in salty brines perhaps, just below the surface. Also, unless photosynthesis is common everywhere on the surface of Mars, those might well be the first lifeforms we find, protected from the surface UV by a cm or so of dust, and relying on chemistry rather than photosynthesis as their source of energy. If so, even if it doesn't photosynthesize, it can be damaged by UV if it is ever exposed to the sunlight. So, again, it's likely to use the red coloured surface rusty iron oxides for UV protection, or if it uses carotene for UV protection, it's again likely to be reddish in colour. By way of example, one of our most ionizing radiation resistant microbes, not a photosynthetic lifeform, Radiodurans ranges in colour from red to pink. That's because it uses carotene for UV resistance. It is thought to have got its ionizing radiation resistance incidentally as a desert species from desiccation resistance (which has similar DNA damaging effects). Radiodurans requires oxygen (it's an obligate anaerobe), so it's not a candidate microbe for present day Mars (except perhaps as part of a community with photosynthetic life to provide oxygen for the microhabitat?). But it's an example to show that UV resistant non photosynthetic life could easily be reddish in colour on Mars. Life doesn't have to be in the form of algae or lichens to take advantage of red colouration for protection from UV.

So, even non photosynthetic UV resistant life might well be red or purple on Mars. This is one thing the National Geographic sequence got right. It seems quite possible that Mars could have purple lifeforms, or red, or pink, though it could also be many other colours such as black, dark blue, orange, yellow, etc as well as green.

Rust coloured or red life would be hard to spot amongst the iron oxides even with filtered vision. And dark or black life, hidden beneath a layer of dust, or in the shadow of a crack, might be almost impossible to see by eye. Green photosynthetic life is still possible however, and would be easy to spot with sky blue tinted visors.

You couldn't split rocks like that in the VR landscapes streamed back to Earth. Though you could also do this from orbit via haptic feedback. You could also mark a bunch of rocks you want the rovers to split, from Earth, leave it to them to split them all, then go back and look at them in VR from Earth as it splits them, and when closest to Earth, you could mark a bunch of rocks for splitting, and then start to see the result of your instruction as soon as eight minutes later. It's not as streamlined and as easy as it would be if you were there in situ, but you could learn to work around the issues and make efficient use of what you can do. However, to confirm that it is life, you'd have to analyse it and search for organics and other biosignatures.

Why is it that, decades after the Viking landers, we still know so little about life on Mars?

We can also get some insight into the difficulties of searching for life on Mars with humans from our experiences with the Viking spacecraft, which helps bring another perspective to Robert Zubrin's arguments. You might wonder why we know so little about Mars biologically, even after several decades. Is it because we haven't sent humans there yet? Not really.

First, it's hard to send landers to look at the best places on Mars for present day life because of the planetary protection issues. Curiosity is just a few kilometers away from some dark streaks on Mount Sharp, which may be possible habitats for present day life there. It would be wonderful to go close up to check them out, but it can't go up to look at them because it isn't sufficiently sterilized. It may be able to take photos of them from a few kilometers away.

The fictional astronauts in the Mars National Geographic TV series can just walk straight up to a possibly habitable Recurring Slope Lineae (RSL) in their spacesuits, and pick out a sample of life with tweezers. But in real life, Curiosity can't approach them, although sterilized to have less than 300,000 spores over its entire surface on leaving Earth, and with far fewer spores than that surely by now after the journey to Mars and the UV radiation on Mars itself. Even with such a sterile rover, the risk of introducing Earth life to the RSL is thought to be unacceptably high if it approached them.

What's more, even the Viking sterilization levels weren't designed for a rover to actually contact habitable liquid water directly. Just a few viable spores might be enough if it actually touches the water and those spores happen to be pre-adapted to be able to survive in those conditions. I'd actually argue that we should aim for 100% sterilization, when exploring potentially habitable liquid water in our solar system. Though we can't do it quite yet, I think such levels are not impossible to achieve in the near future. We need to create conditions that a microbe can't survive, but our rover can, and with modern technology, that's actually within the bounds of feasibility, indeed there's been a serious proposal already for a 100% sterile lander for Europa. I discuss this in detail later in this book, see Can we achieve 100% sterile electronics for an Europa, Enceladus, Ceres, or Mars lander? and Design change suggestions that could make a lander close to 100% sterile.

We've known about the RSLs since August 2011. But we haven't yet sent a rover to look at them. Nor do we have any plans to do so in the near future either. It's at least partly because of these planetary protection issues, how do we sterilize them sufficiently? The Viking methods can't be used with modern spacecraft because it would damage the sensitive electronics, put delicate instruments out of alignment, etc. So we have to use new methods which are still being developed though some have reached maturity already. They are also on very steep slopes. Far steeper than in the National Geographic movie, 1 in 2 slopes, or about 33 degrees. This doesn't mean that we have to send humans to explore them. Indeed, humans are more vulnerable than robots. An astronaut could easily stumble, fall, and perhaps die, on a 1 in 2 slope. It would seem much steeper to someone trying to walk up it. It would also be a slow and hard slog in cumbersome spacesuits. Or they could rappel down from above - but robots can rappel as easily as humans.

Here is one way it could be done, with a rover which drives to the top of a cliff with RSLs and then lowers another rover over the edge using a cable which then can explore the RSLs.

AXEL Rover - Mars Cliffbots - this is one way a rover could explore the RSLs, to rappel down the steep 33 degree slope, much like a human astronaut would do.

There are many other ideas for ways we can explore Mars telerobotically, to access hard to reach places such as cliffs, caves etc and to expand the search radius of our rovers. We can also use these approaches robotically from Earth. Most of these ideas have never been tried out, though there is an idea to add a helicopter to Mars 2020. For more on this see Small planes and entomopters etc. See also the Origami rover which is capable of climbing a 45 degree slope.

So, with that background, how can it be right to send a human expedition with telerobotically operated rovers that set off from a human base on Mars to explore the RSLs? I don't get it. Why not do it from orbit?

Then, as well as that, even if a rover was sufficiently sterilized and sent right up to a potential habitat, rappelled over the cliff and down to the RSL to look at it, it still might be difficult, even very difficult, to spot any life there. At least, our only attempt to search for life to date, in the 1970s with Viking suggested that it is likely to be difficult to do. If you crack open a rock on Mars, or scrape away some dust, and the life is rust red, or dark brown say,, mixed in, in small quantities, with rust red or gray soil, how would you recognize it as life? How could you tell that it is not, for instance, organics brought there on meteorites? Which might also be the same colour? You need some way to detect it as life, in situ, it would seem.

This is not a problem we face on Earth. If you find organics, it's almost certainly from life. Not necessarily alive of course. It may be from present day life but decomposed long dead. Ii may be from past life, long ago, but it's a clear sign of life in most circumstances, a clue that this is an interesting place to do a biological investigation. The only likely chance of finding organics from meteorites is if you actually analyse a meteorite fragment, because any other organics that reaches the surface is quickly scavenged by Earth life. Even if you analyse meteorite fragments, you have to get to it quickly after its fall, or find it in some environment with hardly any life, like Antarctica, to have a chance of finding unmodified meteorite or comet organics in it. Generally you can just forget about meteorite organics. Unless you already know it's a meteorite, if you find organics, it's from life, present day, or from the distant past.

But that's not the case on Mars, not at all. Curiosity has detected organics on Mars, but it is thought to have come from meteorites. Similarly, in the future, if Mars 2020 detects organics, or indeed ExoMars or any other future expeditions to Mars, the first assumption will always be that it comes from meteorites or comets, unless you have a clear indication that it doesn't. It's the other way around from Earth, where you can assume organics come from life originally unless you have clear indication that it's from some other source.

That's because Mars has a constant rain of meteorites that hit the surface bringing fairly large quantities of organics to the planet, and meanwhile, if it does have present day life there, it's not likely to produce much by way of organics, and any past organics, unless buried deep below the surface, is likely to have been destroyed by cosmic radiation over timescales of hundreds of millions or billions of years.

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Detail from the drilling which provided Curiosity's first detection of organics. The organics seem to come from meteorites. Past and present day evidence of life on Mars is likely to be like this, soil or mud that has organics in it. Organics doesn't normally mean life on Mars, as many meteorites have organics in them and the main puzzle for Mars is to understand why it has so little organics. It should have a lot more and there is some process actively destroying it.

If there was no degradation of the organics, Mars should have 60 ppm of organics from organics deposited into the regolith, averaged over its entire surface to a depth of a hundred meters (see page 10 of this paper). So, if we find organics there, the first guess is that it is organics from meteorites that has escaped destruction from the surface processes that clearly destroy both organics from life and from meteorites and comets. So, to tell if there is life there, it doesn't do at all to just detect organics. That's likely to mean nothing. We need to use life detection instruments. Even then it needs care as there are potential biosignatures such as a chirality imbalance, or nitrogenous organics, or complex sugars, that can be mimicked by organics from meteorites. For more about this, see the section Organics created on Mars by non life processes (below)

Now, a few people actually think that the two Viking landers may have found life on Mars already in the 1970s. If this is so, it might be easy to find present day life. We might just need to send sensitive life detection instruments to check the sand dunes in similar locations to the Viking landers.

So did it find life? Well, even with all the extra information we have today after several more rovers, landers and orbital missions, we still don't know enough to rule it out (or confirm it). We can't say, totally for sure, whether Viking found life or not in the 1970s. Indeed some new research has actually, if anything, increased the chance that Viking actually did discover life in the 1970s. What makes it so tricky to tell is that Levin's labeled release experiment on Viking was extraordinarily sensitive for its day. It remains by far the most sensitive experiment we have ever sent to Mars, indeed the only one we've sent that had a chance of detecting life in such small quantities. Technology has moved on of course, and we could easily send more sensitive life detection experiments to Mars, or send the equivalent of the Viking labeled release as a much smaller payload. But all the instruments since then have focused on geology and habitability rather than a direct search for life.

The Viking labeled release was ahead of its time in sensitivity, as it was able to detect microbial respiration from just a few cells, even if they couldn't reproduce in the culture. It just detected whether any of the radioactive carbon in the organic "food" gets exhaled as a gas. Our detectors can be very sensitive to radioactivity, so it doesn't take much by way of microbial respiration to generate a signal. His experiment also assumed very little about Mars life. It depends on the Mars life being able to survive in the culture medium and not be poisoned by it, but there is no need for it to reproduce and grow. So it doesn't have to be "cultivable". We don't need to know how to create conditions for it to reproduce. Just not kill it, and create conditions where it can take up food and respire.

So, the idea was that even if Mars life has a different biochemistry from Earth life, even if it doesn't use DNA or proteins, still, if it works anything like Earth life, it is likely to exhale carbon in some gaseous form when it metabolizes organics. If it does that, that's all that's needed for his experiment. In preliminary tests before the mission, published in 1976, the experiment detected life in all the samples from Earth except for a few samples from Antarctica that might have been sterile. So, if there is any life like that and it starts to metabolize the organics into gases containing carbon, and is not killed by the culture medium, then Viking's labeled release would detect it.

This sensitive experiment did seem to detect respiration, just as it was designed to do, but it could have been confused by the chemistry of the Mars soil. The main reason for skepticism about this possible "detection of life" is that Viking didn't find any organics. However, as the scientists discussed the results, they found out that the organics detection experiments on Viking also may well have been confused by the same chemistry that confused the labeled release - what we now know to be perchlorates and other super-oxides in the soil. Also the Viking organics instruments were not as sensitive as the labeled release, and would not have been able to pick up the small levels of organics you find in cold dry deserts on Earth that we know contain life.

Meanwhile, later researchers have also found ways that the complex chemistry of Mars that we now know about, with the perchlorates, could have mimicked some at least of the characteristics of life, enough perhaps to have fooled the labeled release experiment.

So the jury is out on this. For more on this see Why there was controversy back in the 1970s.

The Viking landers did find extraordinarily harsh conditions in the equatorial regions where they landed, so harsh that life there may seem to be between unlikely and impossible. At the time that was another reason for skepticism about the labeled release results. However, it's not such a strong reason now as there are a few ideas now for ways that life could be possible there, including:

• Transgressing sand dunes bioreactor (see below)
• Equatorial frosts - and a source of water for the deliquescing salts
• Desert varnish
• Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere
• Microbes could find tiny oases of water inside micropores in salt pillars
• Liquid brines beneath the surface of sand dunes at night - beneath the sand that Curiosity drives over - that one was reported as uninhabitable but Nilton Renno was not so sure, biofilms might make it habitable

Also, Viking could have found dormant spores in the dust and sand, which then revived. It didn't have to sample a habitat directly to get a positive signal. The spores would just need to wake up and start metabolizing. So, in one way or another, it's not impossible that it did find life.

Some of the 1970s labeled release results are puzzling for life. However other results are just as puzzling for the non life explanations. Also, there's been some renewed interest after another researcher re-analysed the old Viking data and spotted patterns characteristics of circadian rhythms (metabolic cycles) significantly offset from the temperature variations, by two hours. This is especially hard to achieve through complex chemistry without life, which could only explain a 20 minute delay.

Patterns characteristic of circadian rhythms in the Viking labeled release data. The interesting thing is that they are significantly offset from the temperature variations, which to an expert on circadian rhythms who spotted this, strongly suggested life rather than non life processes. More on this in the section Rhythms from Martian sands - what if Viking detected life? (below) and following

With all these ambiguities in the results, and many still unresolved puzzles, we have no way to answer this question definitively at present. Probably most scientists who specialize on Mars would still say that they don't think Viking found life, though if not, it surely did find some form of unusual and rather complex chemistry in the Martian soil not yet fully understood in every detail. A few think that it is possible that it did find life. You can't say at present that those who think it found life are definitely wrong in those views.

This example of Viking helps highlight how hard it is likely to be to search for life on Mars. But our experiences with Martian meteorites show that it is equally hard to search for life using samples returned from Mars. It just is a tricky thing to do, which seems likely to need great care and multiple lines of investigation.

The obvious thing to do for Viking was to send a follow up mission to find out what happened, and settle the question. Not to settle the question of whether Mars has life of course, as this is only one of many possibilities for life there, but to settle the question of whether Viking discovered life in the 1970s. But the problem with this is that you need to get your instrument approved for launch to Mars when competing with many other instruments with other objectives. There's a rather simple update of the Viking experiment which has the potential to solve this question quickly, in a positive way, actually confirm life on Mars. It could also rule out many possibilities if it came out blank.

Many organic molecules come in two mirror image forms, such chemicals are called "chiral". Modern Earth life can only eat one of them - for much the same reason that all DNA spirals the same way. The mirror image chemical is inedible. Astrobiologists think that even unrelated life on another planet is likely to work in a similar way.

So the natural follow up experiment is to feed the sample with amino acid "food" of only one type, then try the mirror image chemical in its place and see if there's a difference. If whatever is in the Martian soil can only respond to one of these chemicals, then what Viking discovered surely must be life, or something else approaching the complexity of life. While if it is just chemistry it should make no difference which of the two mirror image chemicals you feed it. This is the "chiral labeled release experiment".

A chiral labeled release would not be totally conclusive as there are some ideas for biochemistries that are chirality indifferent, especially for early life and prebiotic "almost life" (see Joyce's ribozyme and "autopoetic" cells (below) ). Conceivably the Viking labeled release could have detected some non chiral form of life or an interesting chirality indifferent life precursor. So, it can't disprove the detection of life. However, if Viking did detect life, the chiral labeled release has a good chance of a positive result. And if there is no chiral dependent response, the chance that Viking detected life gets significantly reduced because non chiral life would seem quite unusual.

So, this is the obvious next follow up for the Viking results. If this was an experiment on Earth then it would have been totally non controversial. They would have done a chiral labeled release right away. Depending on what it found this could confirm that Viking found life, or, it could give more information to constrain non biological explanations. Or indeed it could still be inconclusive, in the remote chance that perhaps what Viking found was chirality indifferent biochemistry. But that's how science goes. Depending on what it found, you'd then work on more follow up experiments to address the remaining questions and ambiguities, until you understand the situation well enough to announce a result, either way. If Viking did discover life, it's possible that this simple experiment could have confirmed it decades ago.

However, sending instruments to Mars is of course immensely expensive, and something that only NASA has been able to do successfully since Viking. The main problem is that NASA changed its emphasis after Viking and stopped looking for past or present day life on Mars, instead focusing on past habitability. It's a matter of emphasis, more than an absolute rule, but the outcome is that the chiral labeled release was an excellent fit for its Viking old remit to search for life directly, but it isn't such a good fit for NASA's new remit to search for evidence of past habitability. Specialized life detection instruments get beaten by other experiments that focus on their main goal. The only way to do life detection with a decent chance of approval would be to send an experiment that can do both - search for habitability, and also do biosignature detection as well. However, sadly, the chiral labeled release is specialized for life detection. It's not going to help with the search for past habitability.

As a result, none of NASA's rovers since then have been equipped with instruments anything like as sensitive as the labeled release experiment, either for organic biosignatures in such low concentrations or to find signs of metabolism. We have many such instruments now, and some were actually approved for a launch and then descoped. But we just have never flown them in space. Russia does not have the same policy as NASA, and was interested to send Levin's chiral labeled release experiment to Mars in 1996, but it was descoped and then the launch failed. So for now, nobody can say for sure whether Viking discovered life on Mars or not. Also NASA was interested to collaborate with ESA to send UREY to Mars to search for biosignatures, and UREY was actually developed in the US. So it's not like they have no interest at all in this. But they pulled out of this deal, leaving ESA to find another partner, Russia, for ExoMars. This then reduced the payload capacity for ExoMars leading to UREY being descoped.

I go into this in much more detail in the sections Rhythms from Martian sands - what if Viking detected life? (below) and following.

The main point right now though is that this experience with Viking shows how hard it can be to search for life on Mars. It is easy to get ambiguous results, as Viking did. Then when you do, with transport to Mars so expensive, it's hard to do the obvious follow up experiments. If Viking did find life, perhaps we confirm this with the first follow up mission to search for it directly. But if not, well we are going to want to do similar life detection experiments in many other locations on Mars until we find what we are looking for (if it is there to be found). So, to get back to Zubrin's arguments, imagine trying these sensitive Viking experiments with humans there as well who contaminate everything around them with Earth life. Now imagine that you get similarly ambiguous results, which is surely rather likely in your first searches for life there...

Contamination by Earth life would make the types of experiments that flew on Viking far harder to operate, and perhaps impossible. If we do ever send the chiral labeled release experiment to Mars, it is so sensitive that a single Earth microbial spore that gets into the nutrient and finds it to its liking, would be plenty enough to confuse the results and make it no longer useful. There are many other modern experiments that are as sensitive as this, that we could fly in the near future, and some that are much more sensitive to contamination. We have experiments sensitive enough to pick up not just a few microbes, but just a single molecule of a biosignature in a sample.

Microchannels like this can be used in a "lab on a chip" to move minute quantities of liquid about for analysis. The Astrobionibbler is an end to end instrument design which drills into the Martian soil, takes a sample, mixes it with water at around 180 °C kept liquid at high pressure. The water at this temperature can dissolve out organics because it turns into a non polar solvent. The organics are then labeled with fluorescent dyes. The resulting system is so sensitive it can detect a single amino acid in a gram of sample, and the whole system, including its sample collection, weighs only 2.5 kilograms.

With instruments like this we can increase the sensitivity for in situ searches for organics hugely over Viking. With the chiral labeled release we can also send exquisitely sensitive metabolism tester to Mars which would be able to detect life so long as it metabolizes (doesn't have to grow at all) even slowly and so long as it has a preference for one amino acid over its mirror image, as most life probably does according to astrobiologists.

These experiments are exquisitely sensitive, designed for the extremely challenging conditions we now know exist on Mars, and would be confused by even the smallest traces of life from Earth.

Viking may or may not have found life. However, whatever the situation about that, astrobiologists now think that our experiences with it serve as a useful precedent. We are likely to face similar problems wherever it is that we search for present day life on Mars. In such cold, harsh conditions, any life is likely to be sparse, slow growing, and require exquisitely sensitive instruments to detect it. Just a few organic molecules from Earth life could confuse the biosignature detection, and a few Earth microbe spores would throw them off completely.

Any discovery of life there that's native to Mars will be such a major discovery. Extraordinary discoveries require extraordinary evidence. If there is only one chance in a million that somehow a microbe spore or dormant state from Earth has got into the Mars sample, then there will be doubters who will say "perhaps it is just Earth life". Just the possibility of such contamination, whether it happened or not, would be enough to cast the whole thing in doubt and greatly complicate the process of confirming life on Mars, even if the Earth life doesn't make it extinct.

In short, modern life may be scarce and hard to find and we may need sensitive instruments to find it, and to try a wide variety of different ways of testing for the presence of life there. With such harsh and unfamiliar conditions and such sensitive measurements, a few stray spores from Earth might well be all that was needed to hide the signal altogether, even if Earth spores don't reproduce there.

I agree, Zubrin's arguments may seem persuasive at first, especially if you are keen for humans to touch Mars. But once you reflect on those points, they may not seem quite so compelling. For more on this see also How a human spaceship could bring microbes to Mars - Zubrin's arguments examined in my MOON FIRST Why Humans on Mars Right Now Are Bad for Science. So anyway I leave that as something to reflect on.

Meanwhile, let's turn to the last, and for many, the most persuasive of Zubrin's arguments.

What about Zubrin's meteorites argument?

This may seem one of the most convincing arguments of them all at first. If life gets to Mars from Earth on meteorites anyway, what does it matter if we take it there in our spaceships? So I'd better go into this in some detail, as again, some readers will think that there is no need to read any further if I don't answer this.

Summary

This one takes a while to answer properly. Can life be transferred to Mars from Earth anyway? And if so, could so many species be transferred that Mars and Earth have essentially the same ecosystems? And if the two ecosystems did turn out to be essentially the same, would that then mean that there is no reason to take any care about whether we introduce Earth microbes to Mars?

I'll look at whether life can be transferred to Mars from Earth in the next few sections. But first, supposing Robert Zubrin was right here, and we did find that Mars life is nearly identical to Earth life. That by itself would be a hugely surprising discovery to many, and it would actually make our job of protecting Mars from Earth life harder, if anything. As Cassie Conley (NASA's planetary protection officer), said about this argument, quoted by National Geographic:

“It becomes more difficult and more important to prevent Earth contamination if Mars life is related to Earth life. If we're totally different, then it's easy to tell the difference. If we're related to each other and we want to study Mars life, then we really need to make sure that we don't bring Earth life with us.”

She is an astrobiologist and microbiologist and the current planetary protection officer for NASA.

If Mars life is related, it would give us a wonderful opportunity to learn about how easily meteorites can introduce life to another planet, and about what happens to life which is transposed to a planet as different as Mars is from Earth. For instance, does it evolve in a different direction, under the very different conditions on Mars. Or is Robert Zubrin right, does it just keep identical in lock step with evolution on Earth, to such an extent that you can't tell which planet you are on from studying the microbes? If so, why and how does that happen?

Alberto Fairén and Dirk Schulze-Makuch put forward this meteorite argument in some detail, in "The Over Protection of Mars". published in Nature Geoscience in June 2013. Many humans to Mars enthusiasts will have heard of this paper, which is popular amongst them of course, because of its message that we don't need to take any precautions. Naturally enough, a paper so optimistic that humans on Mars would lead to no planetary protection issues is going to get shared far more widely than one that suggests that there are potential planetary protection issues.

This is an example of our natural "Confirmation bias". Though it's been known throughout history, this term was first coined by the English psychologist Peter Watson on the basis of an experiment in 1960, where he found participants often made up hypotheses and then ignored data they were presented with that went against their hypothesis. For details see the history section of the wikipedia article, which is good on this topic. As Wikipedia puts it, it's

"a tendency to search for, interpret, favor, and recall information in a way that confirms one's preexisting beliefs or hypotheses."

As a result many of you may not know of its rebuttal. This is so seldom shared, that hardly anyone knows about it. This follow up article, "Appropriate Protection of Mars", was published in Nature Geoscience just one month later, in July 2013, by the current and previous planetary protection officers Catherine Conley and John Rummel. The two papers are summarized in The Overprotection of Mars? published in NASA's online astrobiology magazine, and also in Overprotection may be hampering hunt for Mars life in New Scientist.

One of the things that might surprise you, if you have been convinced by Robert Zubrin's arguments, or by this "Over protection of Mars" paper, is how hard it can be for a microbe to transfer between planets.

It has to withstand the tremendous shock of ejection after the impact, and the heat caused by rushing through Earth's atmosphere at kilometers per second (it has to do this in both the forward and the backward direction). Then once it is in interplanetary space, the fragment of rock is spinning in the extreme cold and vacuum of interplanetary space, with hot sterilizing UV rich sunlight bathing the sunlit side whenever the sun spins into view, and it has to withstand the penetrating cosmic radiation of interplanetary space, and solar storms. It hasn't got a rocket engine, and can't steer towards Mars or Earth. Most rocks will take millions of years, and many flybys of Earth, Mars, and possibly other planets before they hit anything. Many eventually are ejected from the solar system, or hit the Sun or Jupiter, and a few of the rocks that are ejected from a major impact on Earth will eventually hit planets and moons even further afield, to Saturn or beyond. See Could Europa or Enceladus have DNA based life? for details.

Out of all those rocks, perhaps around a fifth of a percent will get to Mars eventually, though that does amount to hundreds of thousands of fragments of rock. Most of those take millions of years and arrive there thoroughly sterilized by cosmic radiation to depths of meters. The fastest ones get to Mars after about a century. That is still a long time for a microbe to spend exposed to cosmic radiation, in interplanetary space. When your rock gets to its destination, it then has to withstand the shock of impact, and then any life on it has to find somewhere to live on the new planet, and it has to be pre-adapted to survive there.

Indeed, when researchers started to publish papers saying that it might be possible for some hardy microbes to get transferred between planets after a meteorite impact, this came as quite a surprise to specialists. The most likely time for microbes to get from Earth to Mars is in the early solar system, over 3.75 billion years ago, soon after formation of the Moon. It was challenging even then. Most microbes could not survive the shock of being ejected from Earth at velocities high enough for the rocks to punch all the way through our atmosphere and exit it at escape velocity. It's easiest with the very largest impacts from the early solar system, as after a huge impact, some of the rock hit by the impactor that are shocked less than most on the way into space, and also the very largest asteroids, hundreds of kilometers in diameter, will punch a hole in the atmosphere making it easier for some of the debris to get into space. We don't get impactors that large any more. The larger the impactor, the larger these regions of less than average shock, and the less the shock in them. But these regions of rock with minimal shock are also well below the surface, meters down, so not so likely to have abundant life in them. Through repetition, the general public I think have forgotten quite how difficult this journey is. We are so familiar with the idea now, that we forget the formidable obstacles that are still in the way of a microbe traveling between planets on a small interplanetary rock.

By far the easier direction for microbe transfer is from Mars to Earth, because the shock of ejection is less, but that direction also is still a huge challenge for most lifeforms. Also (just as for the Earth to Mars direction), the least shocked rock, and most likely to get into space as well, comes from some meters below the surface of Mars. The most habitable regions on Mars are probably close to the surface, in the top few centimeters, or else, too deep underground to be reached by these impacts. Microbes in those types of habitat couldn't get into a meteorite headed for Earth. Habitats also may be rare on present day Mars. It's true that some species seem to be capable of surviving the transition. However, most species can't survive this either, or is handicapped in this respect. Most photosynthetic life, to take a significant example, does not seem to be robust enough to get from Mars to Earth easily (it might just about be possible at the lower end of the range of shock), and it's even harder for it to get from Earth to Mars (almost impossible). I cover this in detail in Case study - can photosynthetic life be transferred from Earth to Mars or vice versa? .

With this background it would be an extraordinary discovery to find that Mars has identical life to Earth. If that happened, we would need to re-evaluate all this research which seems to say clearly that many species couldn't get there. We would surely want to keep Mars free of contamination from Earth life until we found out more about how something so amazing happened. And the bottom line is that, though we have lots of theory, we have zero empirical data on this topic. We don't yet have a single example of a species that has been transferred from one planet to another by meteorite impact.

Let's look at this argument in more detail.

Microbe stowaways on meteorites traveling from Earth to Mars

So, yes, it's true, meteorites get from Earth to Mars. On average tons of them must get there every century. But that's an average over timescales of billions of years. The numbers fluctuate hugely. We get meteorites here on Earth from the Moon, from Mars, possibly from Mercury, but so far we haven't found a single confirmed "Earth meteorite" originating from our own planet, in modern times. That is to say, we haven't found any that actually left Earth, and then came back here again some time later. This suggests that Earth has already cleared its orbit of all the "Earth meteorites". So, probably, similarly, there are no meteorites from Earth arriving on Mars right now either.

This is not too surprising as impacts large enough to create such "Earth meteorites" are rare. That is apart from tektites, which are small rocks millimeters to centimeters in scale, that just head off into space in suborbital trajectories and immediately return to Earth.

Two splash form tektites, dumbbell and teardrop. Image credit Brocken Inaglory. Most large meteorites will send fragments of rock into space on suborbital trajectories, but they immediately return to Earth as tektites, and have no chance of hitting Mars.

The impactor that created Meteor crater in Arizona wasn't anything like large enough to send ejecta all the way through the atmosphere with escape velocity.

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Meteor crater Arizona, slightly over 1 kilometer in diameter, result of an impact by a meteorite around 50 meters across. This was nowhere near large enough to send material through our atmosphere all the way to Mars.

You need a rather huge impact to do that, like the Chicxulub meteorite impact 66 million years ago. Earth clears its orbit of asteroids over a period of twenty million years. So all the material ejected by that impact is probably gone by now. With that background, it's not such a surprise that we have no "Earth meteorites" hitting Earth right now. Probably we'll find many of them on the Moon, but they are long gone from interplanetary space.

The last meteorite to get from Earth to Mars may have got there over forty million years ago. Then, even if there are rocks from Chicxulub that have somehow survived to the present, still traveling through interplanetary space, that have not yet hit any planet, the solar storms and cosmic radiation would sterilize a meteorite that spent twenty million years in space to a depth of several meters. The best time for a meteorite to get to Mars is in the first century after the impact on Earth, as that's when the first ejecta gets there, A small amount can get there even sooner, perhaps as soon as ten years after the impact.

So, yes, the simulations suggest that many meteorites got to Mars even as soon as one century after the Chicxulub impact on Earth.

Artist's impression by Don Davis of the Chicxulub meteorite impact into a warm tropical ocean. A huge impact like this could send debris all the way to Mars through our thick atmosphere, and the first rocks would get there even in the first century after the impact on Earth. However they would leave Earth's atmosphere at escape velocity, so any photosynthetic life exposed to the surface as well as the surface of the rock itself would ablate away. The shock to send such rocks all the way to Mars is also huge.

Though some life may have transferred to Mars in this way, it's likely to be especially hard for photosynthetic life. For all forms of life it may have been easiest in the early solar system over three billion years ago, with huge impactors tens of kilometers through to hundreds of kilometers in diameter that created regions of lower acceleration in the crust, so less shock and also formed holes in the atmosphere so that the rocks could escape through a near vacuum to outer space.

Remarkably, experiments suggest that some very hardy extremophiles could survive this travel time of a century in the vacuum, cold, and solar radiation of space. But then look at the obstacles in the way of a microbe before it can get to Mars by this route. First, to get to Mars, it has to be able to:

• Withstand the heat of ejection from Earth and impact on Mars. It has to travel through the thick Earth's atmosphere so quickly that it is still traveling at over 11 kilometers per second as it exits our atmosphere, so the outside would get very hot indeed and ablate away.
• Withstand the shocks (extreme acceleration) of ejection from Earth, also (not so much) of impact on Mars.
• Survive deep inside a rock - because anything on the surface would be ablated away during the passage through Earth's atmosphere, and also be sterilized by the external heat, and then also probably by the hard UV radiation in space.
• Withstand the hard vacuum of space.
• Withstand the extreme cold of space - so after the heat and shock of he ejection, it has to survive freezing well below the freezing point of water (so it has to vitrify, not freeze, as freezing causes ice crystals to damage the cell).
• Survive the cosmic radiation and the solar storms of the journey to Mars. These break apart the molecules of DNA and RNA and other organics in the cell.

But that's just the beginning. It then has to find its way to a habitat on Mars and then survive there.

• It has to find its way to a habitat. Remember that to have survived so far, it is deep inside a rock. So it's not likely to be dispersed in the dust storms. Most of Mars is very cold, and dry, so though there may be places it could live there, it has to find them. How does it do that from inside a rock?
• Then, it has to be pre-adapted. This is the same problem as Zubrin's sharks surviving in the Savannah, or is it like a rabbit in Australia? Maybe it could evolve to live on Mars, and microbes can evolve quickly, but it won't have that opportunity unless it can survive right away when it lands there. In the case of Mars this probably means it has to be pre-adapted to tolerate perchlorates (which are pervasive in the dust, and highly oxidizing) as well as as perhaps some amounts of its dissociation products (from UV and ionizing radiation) chlorites, hypochlorites. It probably also has to be pre-adapted to tolerate the hydrogen peroxide. Yet it also has to be able to survive with hardly any free oxygen (must be an anaerobe).
  
  If it is photosynthetic life, then it also has to tolerate high levels of UV light in the unfiltered sunlight it needs to use to photosynthesize, though it may be able to use a covering layer of dust to help filter out UV. It also has to be able to cope with the near vacuum atmosphere if it is going to survive exposed to the surface air like the DLR lichens and cyanobacteria. Or else, it has to cope with the sulfates, chlorates and perchlorates in soda lake type conditions, if it is able to find its way somehow into a salty surface brine still exposed to sunlight.
• If there is native Mars life, then the life from Earth has to compete with this too. It has to be either better adapted than Mars life, or at least, it has to be as well adapted, right from the moment it arrives on Mars.

Many microbes that could survive on Mars just couldn't survive the rigours of the journey there, or wouldn't have much chance of finding a suitable habitat once there, or would not be pre-adapted to the habitat once it is found. As for whether they could compete with Mars life right off the bat when they get there, we have no way to know.

It's remarkable that scientists think that there may be microbes that could survive all this. It may well have happened. But if so, it might not have happened for billions of years. The easiest time for it may be in the period soon after formation of our Moon during the "late heavy bombardment". However, was Earth life back then hardy enough to be transferred to Mars on meteorites (or vice versa)?

So, it might never have happened, or it might have happened billions of years ago, or it might have happened more recently. The most recent chance of this happening was 66 million years ago. So, that's one thing we can know pretty much for sure. Any Earth life that got to Mars must have evolved independently on Mars for at least that long. In conditions of greatly increased solar storms and cosmic radiation, with those perchlorates, chlorites, hypochlorites, hydrogen peroxide, almost no ordinary salt, shortages of nitrogen, night time temperatures cold enough for dry ice to precipitate from the atmosphere for 100 days a year in the tropics, an "atmosphere" of almost pure carbon dioxide, and close to a laboratory vacuum. With half the light levels of Earth, those reddish brown skies, and the global dust storms.

Microbes evolve rapidly, most of them with shorter generation times than multicellular life, they make frequent use of horizontal gene transfer, and they have greater tolerance for errors. So, is it not possible, or even likely, that microbes transferred to Mars so long ago evolved in a different way from Earth life over the last 66 million years? But this is for the ones that can get there in a meteorite. What about the ones that can't?

Case study - can photosynthetic life be transferred from Earth to Mars or vice versa?

Surely this must be one of the more dramatic ways that a microbe could transform a planet - to introduce oxygen producing photosynthetic life for the first time. This could transform the composition of the atmosphere and of liquid water throughout the planet, as well as make huge differences in the greenhouse warming effect of a carbon dioxide atmosphere, so it can change global temperatures as well. Oxygen may well have made many species of microbe extinct leading to possibly the first of the great extinctions here on Earth. So, could photosynthetic life be identical on Earth and Mars?

Chroococcidiopsis is our top candidate for photosynthetic life that might be able to survive on Mars. This is a green algae that ticks nearly all the boxes for meteorite transfer. It can withstand high levels of cosmic radiation and solar storms,and is able to repair its own DNA in real time when damaged. It is also UV resistant, and it may even be able to survive on the surface of Mars almost anywhere. To do this it only needs partial shade or a thin covering of dust or dead microbes. It can do this so long as there is liquid water available. In Mars simulation experiments, it can also manage this feat using the night time 100% relative humidity with no liquid water at all.

It's an ancient microbe, probably one of the ones responsible for bringing oxygen to Earth originally, a polyextremophile that can withstand many extreme environments. It also does just fine in ordinary conditions too. You can find varieties of this species anywhere from Antarctic cliff faces through to the tropics, in fresh water, extremely salty water, the most arid conditions where life survives on our planet, in acidic or alkaline conditions, freezing cold, or extremely hot conditions. It can handle just about everything. It's one of the most versatile microbes we know of, with a huge range of different biochemical pathways it can use to deal with different situations it might encounter. Although it's called a species, it reproduces asexually (and can also share gene fragments even with completely unrelated life forms via gene transfer) and there are many varieties of it. For instance desert varieties have especially high resistance to UV light, and lose some of that resistance when brought into the laboratory.

So it could, potentially, have a huge effect on Mars if it is not there already. However, unlike many microbes, it can't use a more hardy spore or dormant resting state to resist the shock of ejection from Earth because it doesn't have that capability as far as is known. It has to survive as an ordinary cell. So, it's not as resistant to the shocks of ejection as some other candidate microbes.The sharing can go both ways, and if chroococcidiopsis evolved on Mars originally, it could survive ejection from Mars, though only at the lower end of the scale. Lichens can survive the shock of ejection rather more easily and so may be better candidates to get from Mars to Earth on a meteorite, and perhaps even from Earth to Mars.

Techy detail: the range of shock experienced by the Martian meteorites we have, when they left Mars, was 5 - 50 GPa (billion Pascals). Chroococcidiopsis can survive up to 10 GPa. The microbe Bacillus subtilis and the lichen Xanthoria elegans survived up to 45 GPa See this paper.

The shock would probably be greater for an ejection able to send photosynthetic life from Earth to Mars.

So, would chroococcidiopsis get there on meteorites? Is it already there perhaps? Or might it perhaps have come to Earth from Mars? Or must any photosynthetic life on Mars be native to the planet?

The UK astrobiologist Charles Cockell looked into these interesting questions in his paper The Interplanetary Exchange of Photosynthesis. (You can use the Google Scholar button to read the paper in full). He didn't just look at single cell life. He also looked at whether lichens or any other multicellular photosynthetic life could survive transfer from Mars to Earth or vice versa. This section is a summary of a few of his findings in this paper. He found out that it is possible, but difficult and rather unlikely, except perhaps in the early solar system.

As with the other researchers, he found that the easiest direction is from Mars to Earth. He found that the toughest part of the journey from Mars to Earth was actually the entry into our atmosphere, which happens at a minimum re-entry speed of over 11 kilometers per second. Typically 10% - 20% of the radius of a hand sized rock ablates away. The rock also has to be larger than 20 centimeters in diameter to avoid heating up to 100°C all the way through to the center (which would sterilize it of photosynthetic life). Since photosynthetic microbes normally grow at most a few millimeters below the surface of a rock, it's clear already that it's going to be tricky for it to survive entry into our atmosphere.

He did an experiment in which he inculcated some Chroococcidiopsis into an artificial gneiss rock, at a depth where it could be expected to grow naturally. Then he fixed the rock into a heat shield of a re-entry capsule launched by a Soyuz rocket. None of the microbes survived re-entry, nor did any of their biomolecules either.

Charles Cockell's experiment - the circle shows where they attached a sample of gneiss with chroococcidiopsis below the surface at a depth where it could still photosynthesize in natural conditions. None of it survived re-entry, not even biomolecules.

It could survive re-entry to Earth if deep within the rock, for sure. The interior of ALH84001 never got hotter than 40°C during entry into our atmosphere. But how does the photosynthetic life get deep into the rock in the first place? It can flourish in cracks, if light filters in through them - but that also would give cracks that channel hot gases into the interior of the rock. Cracks like that would also be places where the rocks are quite likely to break apart during ejection from Mars or re-entry to Earth.

Also, chroococcidiopsis is rather susceptible to shock of ejection from Mars. It's killed at only 10 GPa. Typically ejection from Mars requires 5 - 55 GPa, based on analysis of the Mars meteorites. So that suggests that it can just survive ejection from Mars at the lower end of impact shock levels. Lichens manage somewhat better here. But that's not much use if they can't survive entry into the Earth's atmosphere when they get here.

In the other direction, from Earth to Mars, with escape velocity 11.2 km / sec, so more than twice the escape velocity of Mars of 5.02 km / sec, the higher shock levels would make it very hard for Chroococcidiopsis to survive ejection.

You can work out scenarios by which photosynthetic life could get from Earth to Mars. For instance, make the original impact into an ocean, and then the photosynthetic life gets forced into rock as a result of the impact and that rock, now impregnated with photosynthetic life to some depth, now gets ejected into space.

You still have the problem of the shock of ejection from Earth. So perhaps you assume that it is some other form of photosynthetic life that is more resistant to shock than chroococcidiopsis, such as lichens perhaps. Also some photosynthetic algae are more resistant to this than Chroococcidiopsis.

If somehow the microbes got into space still surviving, buried deep below the surface of a rock, then they have to get to Mars. Some rocks get there as soon as ten years after ejection from Mars. But most take between a hundred thousand and ten million years to get there. They can survive the low temperatures and vacuum of space and the UV radiation at least for 1.5 years, and probably much longer, tested in experiments flown on the exterior of the ISS. Also, UV light is not so damaging as you might think, as it can be protected by a thin layer of a few mm.

But the cosmic radiation is more serious. This sterilizes the surface to a depth of 2 cm within 100,000 years by breaking up the nucleic acids . That's below the maximum depth you'd expect to find photosynthetic life in normal circumstances, even in fine cracks. So nearly all the rocks that get to Mars from Earth will be totally sterile by the time they get there. Then it has to get out of its rock when it gets to Mars.

Charles Cockell's concludes in this section of his paper that it might not be impossible, but it would need a rather extraordinary combination of events:

"Thus, the planetary exchange of photosynthesis might not be impossible, but quite specific physical situations and/or evolutionary innovations are required to create conditions where a photosynthetic organism happens to be buried deep within a rock during ejection to survive atmospheric transit."

His final conclusion of the paper is that photosynthetic life has the potential to make dramatic changes to a planet, but that this transfer of photosynthetic life is less likely than for heterotrophs (which use organic carbon) or chemotrophs (which use chemical reactions as a source of energy and synthesize all their organics from carbon dioxide, living in places such as hydrothermal vents).

More background on whether photosynthetic life could be transferred from Earth to Mars - other photosynthetic species, shock, and spall zone

I thought I'd look more into the background here, so I've had a look in the literature for more about this question of whether some other form of photosynthetic life could get from Earth to Mars in unusual physical conditions as he described, forced deep into a rock somehow, and with unusual organisms. First, could there be other forms of photosynthetic life that could survive the shock of ejection more easily than chroococcidiopsis? In one paper, samples of a marine photosynthetic algae nannochloropsis oculata frozen in ice were able to survive 6.93 km / sec impacts into water with approximate shock pressure of 40 GPa. (It's not a candidate for present day Mars surface life as far as I know though.)

Also, what is the minimum shock of ejection for the larger impacts needed for the Earth to Mars direction, for a really huge impact like the Chicxulub one? When the Martian meteorites were discovered, researchers were surprised to find that they were so lightly shocked. But this is something that is now well understood, first explained in 1984. The low levels of shock arises from interaction between the shock wave moving away from the forming crater and a reflected shock wave moving backwards. The shock moving back is 180 degrees out of phase so the two shock waves cancel, creating a lightly shocked "spall" zone where the two interact. The spall zone depth is proportional to the radius of the impactor, so a large impactor would have a thicker spall zone (summarized in section 2 of this paper, original paper here). But I can't find any figures for the minimum level of shock expected for a Chicxulub type impact in its spall zone. It seems likely to be a hard to model scenario. Does anyone reading this know of good information on this?

At any rate, whether it can happen in unusual conditions or with especially hardly photosynthetic life, it seems quite likely that photosynthetic life never got from Mars to Earth or vice versa. If it did happen, then it is most likely to have happened in the very early solar system when there were many more impactors and many were also much larger, so that the ejection shock could be more gentle.

If Mars never developed photosynthesis, and never evolved it, then any photosynthetic life introduced on our spacecraft could potentially have a major impact. It could make some species of native Mars life extinct (if they exist) by competing with it for resources or shading it, or by creating chemical byproducts that make the habitats inhospitable to other lifeforms already there. Of course it could also serve as food for the right kinds of organisms there. Either way, it would be a major change in the microbial ecology and the ecology of micro habitats of Mars. If it was was able to survive on the surface just using night time humidity, as in the DLR experiments, and spread over large areas of Mars, perhaps it could also reduce the carbon dioxide levels in the atmosphere slightly too, cooling down the planet more than it is already. Or if not now, it could have that effect in the future when Mars' climate warms up as it does from time to time in chaotic fashion. It could also act to counteract attempts to warm up the planet by artificial means if we ever attempted that, by removing more carbon dioxide from the planet, reducing its greenhouse effect, as the planet gets warmer. You might be interested in my speculations in the section Could oxygen generating photosynthetic life set up an "anti Gaia" feedback on Mars? below.

Photosynthetic life flourished on Earth at just the right time to cool it down as the sun got hotter, but Mars doesn't need cooling down at present. One way or another, it might not be the most brilliant of ideas to introduce photosynthetic life to Mars, if it doesn't have it yet, until we have a clear idea of what the effect may be.

What about other microbes?

General case of transfer of life from Earth to Mars

Most of the papers that study this topic focus on viable life transferred from Mars to Earth. They are generally agreed that this is theoretically possible, though we can't yet prove that it has ever happened. But what about the other direction from Earth to Mars. That direction is central to Zubrin's meteorite argument. This direction is rarely studied, and we get much more cautious statements in the literature in the papers that do mention it.

The big problem with transfer of life from Earth to Mars is the shock of ejection because the material has to leave Earth's surface at very high velocities - not just escape velocity. It has to leave the surface at such a speed that it is still traveling with the escape velocity of 11.2 km / second when it leaves the Earth's atmosphere. The smaller fragments especially have to be traveling much faster than that when they leave the Earth's surface. After all, in the other direction, the debris from meteorites typically hits the atmosphere at many kilometers per second, but slows down to terminal velocity, measured in meters per second, before it hits the ground. Imagine how big a rock has to be, and how fast it has to be traveling, to survive ejection from the Earth's surface and still have some of it left to exit the atmosphere at 11.2 kilometers per second!

So the surface of any rock that gets from Earth to Mars will get boiled away, just as for meteorites re-entering our atmosphere and even more so, at least, so long as it has to punch through our atmosphere on the way there. But even more significantly, it's going to experience high levels of shock of ejection, to achieve an even higher initial velocity than 11.2 km / sec, which will damage cells throughout the rock. The best chance for still viable life to get from Earth to Mars is in the early solar system during impacts that were so violent that they blow away part of the Earth's atmosphere, so clearing a gap for the ejecta to travel through on its way into space, without the resistance of the Earth's dense atmosphere. Large impacts also have a larger "spall zone" where the expanding shock wave from the impact meets its reflection (below the surface of the rock) - for details see

We don't get impacts that large any more, so the last chance for something like this to happen is probably more than 3.75 billion years ago. That leaves us with a big question. Was such early life already robust enough back then to survive interplanetary transfer?

More recent impacts would indeed send material all the way to Mars. The Chicxulub impactor did just that, 66 million years ago. It would surely have transferred organics, and fossils too. The main question is whether any of the life in that debris was still viable after the shock of ejection from Earth and transit through the Earth's atmosphere. There, there is no way to be certain, but the authors of papers that I studied are often skeptical that it happened in such geologically recent times. Here is a quote from the conclusion of a 2007 paper.

“'Lithopanspermia' also includes a potential transfer of microorganisms in the opposite direction, i.e., from Earth to Mars. A direct transfer scenario is severely limited because very high ejection velocities in the solid state are required to escape the Earth's gravity field and to pass its dense atmosphere. Favorable transfer conditions may be only achieved by very large impact events, which blow out at least part of the atmosphere. Such impact events happened frequently during the 'early heavy bombardment phase',”

From Experimental evidence for the potential impact ejection of viable microorganisms from Mars and Mars-like planets (2007)

A planetoid plows into the primordial Earth" - artist's impression by Don Davis for NASA. An asteroid this big would melt the land surface, boil the seas, and sterilize the Earth to some depth. However since it blows away part of the Earth's atmosphere, it gives a gentler ride for the debris that does get into space. Larger impacts can also accelerate the debris to escape velocity over a longer period of time, with less of a shock. Huge impacts like this may have given life it's best opportunity to get from Earth to Mars.

These impacts only happened on early Earth, soon after formation of the Moon. After sterilizing Earth to some depth, they might have reseeded Earth and Mars and maybe other places in the solar system. So if life got off to an early start and was already hardy enough to survive these encounters, Earth and Mars could easily have a common origin for life. Perhaps early Venus, Ceres and other places could also share life with us, or indeed be the source of this life.

We have only limited ground data on the ages of the lunar craters, from Apollo, as they sampled only a small part of its surface, most of it affected by the debris from Mare Imbrium. As a result, most of the earliest cratering history timetable for the Moon, the timescale for the very largest craters, is informed guess work. According to some recent research, such as this paper, perhaps those dates may need to be revised. It's possible that the very largest impacts on the Moon may all date back to as long ago as 4.35 billion years ago or earlier, soon after the formation of the Moon. They think there were no ocean boiling and Earth sterilizing impacts after that. However, there probably were many later large impacts that, though not large enough to boil away Earth's oceans or melt its crust, covered large areas with melts (and could have boiled away the surface of the oceans). See this paper.

There have been no impacts anything like that for nearly four billion years - Jupiter protects us by breaking up larger comets, diverting them to hit the sun or itself, or ejecting them from the solar system. The asteroids in between us and Jupiter are easily large enough, but they are in stable orbits, and have been for billions of years, and should continue in them for at least hundreds of millions of years into the future (there's a very small fraction of a percent per billion years of Vesta hitting Ceres in the distant future). This is confirmed by the impact crater history on Mars, the Moon, Mercury, what we have of the history of Earth, and the recent few hundred million years,, all that's available, since the last global resurfacing volcanic event, on Venus.

This suggests that for microbes generally, the situation may be similar to the situation for the less shock resistant photosynthetic life. There was a brief window of opportunity for a few hundred million years, during which it may have been somewhat easier for life to be transferred from Earth to Mars (if already hardy enough to survive the journey) during the period of bombardment of Earth by huge impactors that blew away great holes in our atmosphere, and also would have had large "spall zone" where the shock wave meets its reflection so reducing the shock of ejection for the microbes (see More background on whether photosynthetic life could be transferred from Earth to Mars - other photosynthetic species, shock, and spall zone (above). Since then, with the smaller Chicxulub type impacts, the situation is much more uncertain and depended on unusual events.

In short, the transfer from Earth to Mars may have happened billions of years ago. Perhaps it could have happened since then. It's also possible that it never happened.

There is another very interesting possibility here. Though the microbes themselves might not be able to survive transfer from Earth to Mars, their genetic information might be able to get there, as fragments of DNA or RNA. If so, perhaps that genetic information could be taken up by Martian microbes. Perhaps we could find indigenous Martian microbes that incorporate genetic sequences that did come from Earth more recently, even as recently as 66 million years ago - even though the microbes themselves did not. This could make us more closely related genetically, through horizontal gene transfer, than you'd expect otherwise, though with our last common ancestor billions of years ago. For more on this, see this paper: Microbial Survival Mechanisms and the Interplanetary Transfer of Life Through Space.

The bottom line here is that so far we have no confirmed case of life transferred to another planet (panspermia) to base all these ideas on. It is just theory. The direction from Earth to Mars is especially challenging. If there is any life transferred to Mars on meteorites, then surely most life on Earth wouldn't be able to do it. It would be a great surprise to find life on Mars that was identical or almost identical in all respects to Earth life. This is a topic of great interest to astrobiologists. There are many papers on the topic discussing it in great detail, exploring many possibilities, but there are no definite conclusions as yet.

Microbe stowaways in a human occupied ship

Meanwhile microbes on a human occupied ship don't have to survive any of these rigours of a journey to Mars after a giant impact.

• They get a comfortable ride inside a human occupied spacecraft, in a rapid journey, protected from UV light, and extreme cold or heat, with hardly any ionizing radiation (compared to meteorite transfer), and protected from ablative heating from the Earth's atmosphere and the Mars atmosphere by an aeroshell and heat shield.
• Once they get to Mars then they can slowly adapt to Mars conditions in microhabitats outside the human base. For instance every time humans open an airlock then air from inside, along with moisture, flakes of human skin, shed hair, lint from their clothing, dust from any agricultural activities, and other debris, and numerous spores will be dispersed out into the Mars landscape. This will also happen whenever they use spacesuits, as these are designed to leak small amounts of air constantly through the joints for mobility. All of this will help to create perfect intermediate conditions for microbes to slowly adapt to the surface conditions. If it is possible for them to adapt they have close to optimal conditions in which to do so.
• The microbes that leak from spacesuits and airlocks can feed on the dead remains of their predecessors, flakes of skin, and other debris.
• Vast numbers, trillions of these microbes will leak from the habitats, in thousands of species. This makes it easier for rare events to happen. It only needs a few of those to be pre-adapted or to adapt quickly to Mars, and you now have a potentially invasive species on Mars.
• These microbes are dispersed in the atmosphere right away. They don't have to get out from the interior of rocks or from hidden cracks in the surface of a rover that has been sterilized on Earth and bathed in UV light. Many of them are already suspended in the air inside the habitat, or are on the skin or hair of the astronauts, and they can just float out onto the Mars surface when the airlock is opened - as spores in the air , or on flakes of skin and hair.
• The UV light is easily shielded, in the shadow of a rock or spacecraft, or by a thin layer of dust, The spores can fall into shadows without getting exposed to sunlight at all, especially at night, and will be protected from the UV light during daytime as well if the airlock entrance is in the shade of the habitat, or if they leak from the shaded side of an astronaut's spacesuit. They can also get caught up in the dust and spread throughout Mars in the dust storms, which can get so dark that any microbes in the dust would be shielded almost completely from the sterilizing UV light of the sun. The iron oxides are great for shielding from UV light. A spore could get into a crack in a grain of dust for yet more shielding.
• Not just individual microbes, but numerous species, that can work together as communities and biofilms. Often microbes can't do much on their own but really thrive in communities of many species together, especially the "uncultivable microbes". Many microbes are uncultivable because they depend on other species to survive. A microbe like that is not likely to survive on Mars. What's more, Mars has (according to various estimates) 15 to 20 biocidal factors the microbes have to overcome. Also they need to be primary producers too, to have a chance of survival, unless there is other life on Mars for them to eat. Most individual microbes, except in the case of a few very hardy polyextremophile primary producers, maybe chroococcidiopsis, don't have a chance on their own. But in a community, they can survive together, with some microbes making up for the deficiencies of others, forming a biofilm or mat which also creates a microhabitat that they can all survive in. So once you have the possibility of introducing a whole community to Mars in one go, as you do with the tens of thousands of species on a human occupied spacecraft, then the issues become much more serious. Those communities and biofilms can overcome multiple hazardous conditions that they would be unable to deal with individually. This is a point Andrew Schuerger makes in a brief comment 47 minutes into this discussion at AbSciCon 2017 day 1. - he is known for his papers on biocidal factors on Mars.

The stowaways in the human occupied ships have a far easier time than their panspermia cousins hidden inside rocks for a century on the journey to Mars in the cold vacuum and ionizing radiation conditions of interplanetary space. They also have a far easier time than the few hardy microbe spores that may get there on the sterilized robotic explorers.

So, in summary, Zubrin's arguments don't have a lot of force to them, impressive though they may seem when you first encounter them. Even the meteorites argument, which may seem so conclusive at first, has much less force to it, as you work through it in detail.

If you listen to the debates he stages for the Mars society, you may get the impression that he is debating on equal terms with the astrobiologists. However, in actuality, his arguments are not taken seriously by most astrobiologists, and especially, they are not taken seriously by those who are involved in working on planetary protection as their specialist topic. He may seem to have won all of these staged debates, at least to the satisfaction of many of the Mars colonization enthusiasts. However you need to bear in mind that they are, naturally enough, disposed to like his message and cheer him on. This is a case of a confirmation bias I think. He is also a very skilled public debater. Few astrobiologists would agree that he has won these debates.

If you are super keen for someone to "touch Mars", your next obvious question might be

"Is the planet worth protecting in its present state? Does it matter if we contaminate it with Earth life?"

So now, let's look at what many astrobiologists think we might find on Mars. Not fossils, but tiny micro-organisms. Perhaps too small to see with an optical microscope. Why is that so interesting? Do we really need to protect them from contamination by Earth life? Or, as was suggested in the Sky at Night Mars special program, should we just heave a great sigh of relief, when we finally do contaminate Mars irreversibly, that we no longer need to think about such issues? (See Could we get a future news story: "Debate over Moldy Mars is a Tale of Human Missteps"? above)

What if Mars has tiny cells - like the structures in the Mars meteorite ALH84001?

Some of you may remember president president Clinton's comments on NASA's announcement of the possible discovery of past martian life in a meteorite in 1996. This was the famous ALH84001 (originally found in 1984 but only recognized as a Mars meteorite a decade later). Perhaps you may remember the anticlimax afterwards when the scientists investigated further, and were not able to prove that it was definitely life?

What you may not realize is that it hasn't been disproved either. We need strong incontrovertible proof to claim discovery of life from Mars. The whole thing ended inconclusively, and the jury is just out on what it is at present, with some scientists arguing in both directions. Maybe in the future as we find out more about Mars, we will discover that these were traces of ancient Mars life all along. Or maybe the abiological explanation will get proven conclusively. So far though, we can't say.

(click to watch on YouTube)

Many astrobiologists think that potential microfossils and traces of organics, like those in ALH84001, give us a much more likely model for what we may find in the search for life on Mars, than fossils you could spot by eye, or with a lens, or even an optical microscope. They also think traces of organics like this are much more likely than massive deposits of organics like the shale oil deposits. The life in this meteorite, if that is what it was, was so small it could only be seen with an electron microscope, and though there were possible biosignatures, they were rather elusive and controversial, mainly because most of the carbon in the meteorite was Earth based contamination.

Here are some of their images from the original press release:

The structures in these photos are between 20 and 100 nm across, well below the resolution of a diffraction limited optical microscope of 200 nm.

If it is life, then the supposed cells seem too small to include all the cell machinery of modern life. This discovery lead to a 1999 workshop to try to figure out if living cells could be so small. And the answer was yes. Although modern DNA based Earth microbes life simply can't be this small and still include all the machinery it needs to function and to reproduce, modern cells must have evolved from earlier simpler forms of life. The relevant section of the workshop concluded that this early life could be as small as tens of nanometers in scale, far beyond the optical resolution limit of 200 nm, and still have all the cell machinery needed to reproduce, especially if it is based on an RNA world type cell, with no DNA and no proteins.

So, to the ordinary person, not an astrobiologist, and especially if you are keen to "touch Mars" or at least if you are keen for someone else to do that - perhaps your thought at this point is something like this:

"Well what's the big deal. Just a few microbes, so small you can't see them in a microscope? This will only interest a few microbiologists.

"Why should anyone else care if we mess up their chances of finding this life. It is so uninteresting that it shouldn't stop humans from doing what we want to do, land boots on Mars and touch Mars."

Well, if you look at it like that, it might not seem that interesting. But if you look at it another way, if Mars life does turn out to be like the structures in ALH84001, there's something much more interesting about this than another obscure microbe that happens to be smaller than any others found to date. Something that would be fascinating and exciting to everyone with an interest in science or biology, I think.

RNA world and the shadow biosphere

To understand how exciting and interesting this discovery would be, first you need to know how similar all modern life is. It might seem that modern life is diverse already - the fish, fungi, trees, birds, animals, starfish, octopuses. Surely most of the variety is in the higher lifeforms like that? Adding a few microbes too small to see, hardly seems likely to add to that diversity.

However underlying all that life on Earth is an almost identical structure. If you look deep inside the cells of every living creature on Earth, seaweeds, plants, amoebae, microbes of all sorts, birds, animals, they all look pretty much the same at that level.

This is not an actual video of the interior of a cell, but scientific art, that depicts it as accurately as possible. Exactly the same amazingly complex process is going on in each and every cell of your body, and what's more, every cell of all Earth life, including microbes - and at roughly the speed of this visualization in every cell too.

(click to watch on YouTube)

What's more, it's not even just a similar process. All Earth life uses the same language here, the same basic components that are used to make proteins, DNA and RNA, and the same translation tables.

It's as if none of us have ever heard anything spoken, or seen anything written except English (substitute your favourite language here, it's just an analogy). We know that it must have evolved from some earlier language or languages, and possibly also as a result of a merge of several previous languages. Yet (in this analogy), all historical records, all inscriptions, everything we have is in modern English, with no evidence even of Shakespearean English or middle English. All we have is the modern language, and only one dialect of it.

In that situation, imagine what an intense interest there would be if someone found an inscription written in another language, or in an earlier form of English. An early form of life on Mars would be easily as revolutionary for biology as that discovery would be for language studies.

What's more, there's even more to this than a new "language of life" discovered by a species that so far is monoglot in its understanding of biology. The interior of every Earth cell is the same or similar in many other ways too, not just the language used in the translation tables and encoding. For instance, consider carotenoids - these are the pigments that make carrots, peppers and poppies red, yolks of eggs yellow, flamingos and shrimps pink, and autumn leaves red or orange. Carrots, poppies, fungi and trees can make the carotene for themselves. This substance is used not just to make them colourful, but to protect chlorophyll and to convert blue and green light (wavelengths in the range 450 to 570 nm) into light at the right frequency for chlorophyll to use. So it's an important part of photosynthesis.

Most animals and insects can't make this substance. Flamingos, birds etc get their carotene by eating plants. But the plants, fungi etc all use the same identical biochemical pathway to produce it. And as it turns out, in a surprising discovery, some red pea aphids can make their own carotene, which they use just to turn themselves red. Astonishingly they got this ability by horizontal gene transfer from a fungus, of all things.

Pea aphids get red coloration by producing their own carotene. The horizontal gene transfer from a fungus didn't transfer carotene, but rather, the instructions for making it. Credit: Zina Deretsky, National Science Foundation

For details of how they made this discovery, see First case of animals making their own carotene and for techy background on carotene and the biochemical pathway by which it is made in cells, see Carotenoid Biosynthesis in Arabidopsis: A Colorful Pathway.

This only transferred the DNA instructions for making carotene, which got incorporated into the DNA in the cells of the aphid. Not only was the aphid's cell machinery able to read these instructions and work with them. It's much more striking than that. The aphid makes carotene using a complex biochemical pathway that is identical in both the fungus and the aphid. This shows how similar the cells are, even though a fungus looks and behaves very differently from an aphid. All life on Earth is fascinatingly similar at a cellular level.

This horizontal gene transfer is an ancient mechanism and works between organisms that had their last common ancestor back in the early solar system. It might even work with modern Mars life, so long as it uses DNA, and we are distant cousins. It could do that, even if our last common ancestor lived billions of years ago. In one experiment 47% of the microbes in a sample of sea water left overnight with a GTA conferring antibiotic resistance had taken it up by the next day. These include microbes in totally unrelated phyla with last common ancestors dating back to early Earth, so they could be as unrelated as Earth microbes, and microbes from Mars that diverged from Earth life billions of years ago. See also, Horizontal gene transfer in microbes much more frequent than previously thought

All present day life is like this. Amazingly complex, yet they all are complex in the same way internally, over nearly all of their cell machinery. Also, the biochemistry in our cells is so complex, there's a limit into how small a cell you can fit it all into. If we found very early cells on Mars, so early that they don't use this same language and don't have the same interior biology inside the cell, this would be a remarkable, groundbreaking discovery.

How small can a living cell be?

This was the question asked in the Size Limits of Very Small Microorganisms (1999). If you are talking about modern life, then even the smallest cells, the ultramicrobacteria as they are so called, have to be quite large. Every cell has DNA for inheritance, which is unzipped and converted to messenger RNA, and then to proteins, always using the same translation table to convert the RNA chain, three bases at a time, into amino acids. But the main limiting factor in all this is not so much the complex DNA to RNA conversion - but rather, the ribosome which does this translation from RNA to proteins. It is a rather huge molecule made up of a mix of proteins and RNA. One well studied ultramicrobacterium, S. alaskensis, manages just fine with only 200 ribosomes though it can contain up to a maximum of 2000 ribosomes. The smallest spherical cell you can fit all the ribosomes into is about 250± 50 nm in diameter.

One of the main questions for this workshop was whether early life could be smaller than this estimated 200 nm in diameter. They identified several ways that fossils of life could be smaller than this:

• Shrink after death
• Fragments of larger organisms
• Pathogens or symbionts which depend on a host - e.g. viruses
• Live in consortia of smaller cells unable to survive independently on their own
• Based on biological systems different from the ones we understand (such as the RNA World cells)

Let's look closer at the last of these, in the context of this discussion of RNA life. Early life simply couldn't have started like modern life, no way! The whole thing is far too complex to form spontaneously from non living chemicals. One of the striking things about modern life is how it is so interlocked, that almost everything depends on everything else. How could such a thing spontaneously arise from non living chemicals in one go?

As Cairns-Smith put it in his "Seven Clues to the Origin of Life" (which approaches the problem of the origin of life like a detective puzzle modeled after Sherlock Holmes novels):

"Subsystems are highly INTERLOCKED within the universal system. For example, proteins are needed to make catalysts, yet catalysts are needed to make proteins. Nucleic acids are needed to make proteins, yet proteins are needed to make nucleic acids. Proteins and lipids are needed to make membranes, yet membranes are needed to provide protection for all the chemical processes going on in a cell. It goes on and on. The manufacturing procedures for key small molecules are highly interdependent: again and again this has to be made before that can be made - but that had to be there already. The whole is presupposed by all the parts. The interlocking is tight and critical. At the centre everything depends on everything"

(page 39 of Seven Clues to the Origin of Life)

This also explains why modern life is so conservative at centre, so identical that a GTA from a fungus can be transferred into an aphid and the genetic sequence can continue to produce carotene as easily as it did in the fungus. Maybe there were different ways that life biochemistry could have been organized, but if so, once it is set in place like this, almost nothing can be changed in the overall architecture of modern life. If you made a major change to any part of the architecture, with such strong interdependence, the rest would stop working and you'd just have a non living cell.

So what came before this modern life? First, early life surely didn't have those huge complex ribosomes with their mix of RNA fragments with proteins. They are so large and elaborate that they must be a late development. It probably didn't have DNA either, as that's quite fragile if it isn't held together inside a living cell with a lot of support for it. Also it surely didn't have two biopolymers either, DNA and messenger RNA, as the whole process of separating out the two helixes, translating it to mRNA then to proteins is just too complex to envision that suddenly coming into place in one go from non living chemicals.

Perhaps early life had only RNA (or some other biopolymer). This is the so called "RNA world hypothesis". The huge ribosomes were a mystery until the discovery of the far smaller ribozymes made up of fragments of RNA alone, which gave renewed vigour to the hypothesis. These could do the translations and other cell operations that need enzymes. It may not have needed proteins at all. The interior of the cell may have consisted largely of RNA in different forms, both as strands for the genetic material and reproduction, and also, cut and diced together to make these ribozymes.

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This is the key to the RNA world hypothesis - a ribozyme . This particular example is the "hammerhead ribozyme", made up of fragments of RNA, stitched together without any use of protein chains, to make the enzyme. This was a surprising discovery. This reinvigorated the idea of an RNA world with tiny cells and only needing RNA without DNA. The cells would have no need for proteins or amino acids and they would not need all the translation machinery to convert DNA into messenger RNA. Their interior would consist largely of RNA strands as well as these ribozymes. As a result the cells could be far simpler than modern DNA based life. This is one suggestion for an intermediate stage between the earlier organics and modern life.

Early life based on those ideas could have had cells as small as 50 nm across. Stephen Benner and others have suggested that we might be able to find RNA world organisms still here today on Earth, undetected because they have no DNA or proteins and have ribozymes instead of ribosomes. That's the Shadow Biosphere hypothesis. This idea was quite popular a while back. It was one possible way to explain nanobes, structures that visually resemble life, but are far too small to be DNA based life:

from "New life form may be a great find of the century" (1999) The nanobes discovered on Earth are mysterious. Nobody knows if they are life, non life, or something in between.

The idea was that these tiny structures could be a form of life that we miss because all of our tests target DNA based life. What if these were RNA world cells, and we just don't spot them because they only use RNA and don't have proteins or many of the materials that make up the larger cells we are used to? We might have a second "shadow biosphere" living amongst us unrecognized, to this day.

So far nobody has been able to prove that this shadow biosphere exists still on Earth, either now, or in the past. But even if it doesn't exist on Earth, and any traces from the past have long been erased here, what about Mars? Could Mars have had an RNA world shadow biosphere in the past, and if so, could remnants of it still survive to this day? Or indeed, could Mars perhaps have had only RNA world life, right through to the present?

That's actually one of the ideas for the structures in ALH84001, that they might be these RNA world cells. This was originally suggested by the fourth panel in Size Limits of Very Small Microorganisms (1999) - which was convened shortly after the martian life announcements. Now scientists have found alternative ways to explain these structures, including the magnetite, and organics. Instead of being formed by life, they could be the result of rather unusual conditions on the Mars surface. This means we can't use the meteorite to prove that Mars had life in the past. But their research hasn't disproved it either, and the jury is still out on whether the structures in ALH84001 were the result of life or not. In "Towards a Theory of Life" in the book "Frontiers of Astrobiology" (2012, CUP) by Steven A. Benner (notable as the first person to synthesize a gene) and Paul Davies, the authors talk about RNA world cells as a possible explanation of the structures.:

"The most frequently cited arguments against McKay's cell-like structures as the remnants of life compared their size to the size of the ribosome, the molecular machine used by terran life to make proteins. The ribosome is approximately 25 nanometers across. This means that the "cells" in Alan Hills 84001 can hold only about four ribosomes - too few ... for a viable organism.

"Why should proteins be universally necessary components of life? Could it be that Martian life has no proteins?

... Life forms in the putative RNA world (by definition) survived without encoded proteins and the ribosomes needed to assemble them. ... If those structures represent a trace of an ancient RNA world on Mars, they would not need to be large enough to accommodate ribosomes. The shapes in meteorite ALH84001 just might be fossil organisms from a Martian "RNA world".

Though we can't seem to prove the case either way for ALH84001, astrobiologists have learnt a lot from it about ways to search for life on Mars. The structures and organics in this meteorite give us the closest we've ever got to something that could be extraterrestrial life, in a real world situation, and actually accessible to study in our laboratories. The challenges they faced analysing it may well be good training for the challenges we will face interpreting whatever we find on Mars. As Harry McSween put it in an early paper in 1997

"this controversy continues to help define strategies and sharpen tools that will be required for a Mars exploration program focused on the search for life."

So, anyway, surely the life we have on Earth didn't arise all in one go. Perhaps it arose as RNA world cells or perhaps as something else but it must have had a precursor of some sort. It would teach us so much, to find some evidence, anything at all, to help fill in this huge gap in our understanding of the evolution of life.

There is another way to see that we must be missing a huge amount of knowledge about early life.

Half of the pages of the book of evolution have been torn out

This was an idea of the entomologist and ecologist Alexei Sharov, and the mathematician and theoretical biologist Richard Gordon, to plot the increase of complexity of DNA against the time of origin of the lifeform. They found a way to ignore junk and duplicated DNA, so that they could count only what is essential to its genes. They found that as life increases in complexity, it follows a near straight line on this plot, through many different changes of structure of organism, from the prokaryotes, to the eukaryotes with nuclei, worms, fish, and mammals. It's a log plot so the straight line means that it always takes about the same amount of time for the complexity to double.

It's similar to the idea of Moore's law, that the number of transistors doubles roughly every two years. If you draw a line back through the graph you could estimate when the computer was invented.

In this diagram, the number of transistors in a computer chip increased a million fold in 40 years from 1971 to 2011, with a more or less steady growth, doubling every two years, so about a thousand fold every 20 years. From this you'd estimate, extrapolating back, that the computer was invented 22 years before the start of this graph (2*log(2300)/log(2) ) or some time after 1949 (assuming the first computer had more than one transistor or vacuum tube).

That's actually more like the date of the first transistor, invented in 1947. Computers were invented a little earlier, the first electronic computer, the Z3 in 1941 already had 2,000 relays, though of course not nearly as powerful as the same number of transistors.

So - extrapolating back too far can be a bit risky. It's suggestive rather than a proof, but it's still a useful way of looking at things.

So anyway, they traced the timeline back for the complexity of our genes, expecting it to cross the zero line at the time of origin of life, and found to their surprise that the zero line is nearly ten billion years ago. That's over twice the history of the Earth.

This diagram shows the complexity of the DNA as measured using the number of functional non redundant nucleotides. This is a better measure of the genetic complexity than the total length of its DNA. Some microbes have more DNA than a human being - much of that used for other purposes instead of genetic coding. This is the so called C Value Enigma. Measuring the DNA by functional non redundant nucleotides deals with that issue.

The graph is adapted from figure 1 of this paper which also explains in detail how it was derived.

Notice that the prokaryotes; the simplest primitive cell structures we know; are well over half way in complexity between the potential earliest forms of life and ourselves. Here, eukaryotes are cells with a nucleus to store the DNA, and prokaryotes don't have a separate nucleus. Mammals have around 3.2 billion base pairs or 3.2× 10⁹ The smallest prokaryote base pair has 500,000 base pairs (for Nanoarchaeum equitans and Mycoplasma genitalium) or 5 × 10⁵.

So, there are two ways to take this. If you take it on face value, perhaps evolution started before the beginnings of our solar system. The original paper just touches on this briefly and mentions a rather minority view idea that our solar system formed from the remnants of an exploding parent star after a supernova, which might have had life on its planets already. Actually there are many other ways it could happen. One rather likely way it could happen is that it could get transferred from sibling stars in the birth nebula of our solar system, and those stars may in turn have been infected by a passing star and planet born in a much earlier nebula. However, that's a "distant cousins" case so I'll cover this a bit later under Distant cousins with last common ancestor from a planet around another star

The other way to take it is that perhaps evolution was far more rapid in its early stages. The straight line may just show the characteristic slope for DNA based evolution. Perhaps evolution was far more rapid in the RNA world or whatever happened before. For instance, just a thought, perhaps early copying was far more error prone, and also less affected by errors too, leading to faster evolution?

Of course it could also go the other way. Maybe the pace of evolution was been slower in the early stages, like the rate of evolution of computer complexity during the time when computers were made of valves and relays, developing far slower than the later transistor based computers under the processes of technological innovations by their human inventors. There is no way to know for sure as we have no idea how rapidly or slowly pre-DNA based life evolved.

Could we peek into these missing pages of the book of evolution on Mars?

Whether this means that Earth life originated in our solar system or predates it, the graph brings out another interesting point. Let's just duplicate the graph again for reference

With the prokaryotes more than half way across in the graph from left to right, indeed, almost two thirds of the way across, you'd expect at least as many stages of evolution to get from non living chemistry to the most primitive known cells as were needed to get from them to modern mammals. The only difference is that if it got off to a rapid start, it went through those stages more quickly. That might suggest that in the left half of this graph, we are missing steps as radical as the step to cells with a nucleus, multicellular life, creatures with a backbone, warm blooded animals and mammals.

We can look at this another way too. We have a wide variety of things in the modern cell that must have arisen somehow. Astrobiologists have so many questions they don't know the answer to. They have plenty of ideas but we don't know which of them if any corresponds to the way it actually happened

I'll look at some of this in a little more detail in the next section Life on Mars dancing to a different tune, meanwhile here are a few questions to get you started.

• How did early life develop the two biopolymers system of RNA and DNA?
• How did early life replicate? Did it just grow until it got so large it split, or did it have some more sophisticated method like modern life?
• How did the sophisticated error correcting system for DNA replication evolve?
• How did the translation table by which RNA is converted into proteins evolve? There is much that seems arbitrary in this translation table by which triplets of RNA base pairs get converted to amino acids to make proteins, including many duplicate redundant entries. Why did it happen like this? Were there experiments with other translation tables that failed?
• How did life come to use one particular set of amino acids, and no other ones? There are thousands of possible choices.
• How did cell walls evolve? Has life always had cell walls? Might early cells have had walls that were incomplete or porous or non existent?
• What about the internal structure if a cell, the cytoskeleton of microtubules and actins, how did that arise?
• How did cells develop mobility and how did the first motile early cells move?
• How was the metabolism of early cells powered - what was the source of energy and how was this supplied to the cell?

And in what order did those various things arise? Surely not all in one go.

If we find early precursors to Earth life, it can't possibly work in the same way as modern life because it will be missing most of our cells' complexity, and the internal machinery to make the genetic code work. The genetic instructions to make carotene aren't going to work in some early cell, with only a few thousand distinct chemicals. With so little by way of internal biochemistry, its probably not even able to recognize DNA as an informational biopolymer. Probably it will just ignore it. And surely the first cells didn't have millions of distinct complex chemicals interacting simultaneously as in a modern cell. There is no way that could that arise in one go. So, how many distinct chemicals were there in the first cells, and how did they interact?

You might think, why not just make an RNA world cell in the laboratory, following the "blueprint" that astrobiologists have sketched out, and see if we can get it to work? That wouldn't be proof but it could give us some ideas of how early evolution worked, and answer many questions. Well at present anyway, that idea is a non starter. Astrobiologists don't have anything resembling a complete blueprint for an RNA world cell. They just have a sketch. They don't know how to combine RNA, ribozymes etc to make something that actually works, although by analogy with DNA based life, they think that an RNA based lifeform seems very plausible.

Even with modern life, though we know how it works in great detail, we can't make a modern cell from scratch from non living chemicals. We can tweak modern cells, and tinker with them. We can even make a novel complete gene sequence and insert it into a cell, replacing its existing sequence. We can add an extra base pair to the genetic code that has never occurred in nature. But those are all relatively minor tweaks compared to the complexity of building an entire living cell from scratch. We can't do that. It's just too complex, and interconnected, with all the parts needing to work together right away as soon as it is assembled. The simplest modern living cell is way way beyond anything we could make from scratch with inorganic chemistry. The only way we have to create a living cell at present is to replicate an existing one and tinker with it.

Similarly, astrobiologists can't make a complete blueprint for an RNA world cell, and if someone provided us with one somehow, say an extra terrestrial sends us a complete description of an RNA world cell, how would we put it together to make a living cell? We wouldn't know how to begin, not unless we found a way to tinker with a modern DNA world cell and convert it into an RNA world cell by stripping it down somehow. But we are nowhere near that stage. All they have is a rough sketch of a way that perhaps various components might be able to come together to make an RNA world cell.

Also, our experiments in randomly combining chemicals in conditions to replicate early Earth can only get us a tiny way along the path towards evolution. We can't simulate an entire ocean left to evolve for millions of years. None of our experiments of this nature have come anywhere close to forming living cells. So, there isn't really much we can do to explore these ideas of early life, either by trying to work from the present backwards, or from the early chemicals forward. The only way we can make progress is to actually find it, or find other forms of life that may shed light on what is possible.

I hope that makes it a bit easier to see why it would be the most amazing discovery in biology as well as astrobiology, and a fantastic advance in our understanding, to actually discover such a cell for real, and to find out how it works. So - that is one thing we might be able to find on Mars. If we found something like this, it would be revolutionary. It would be epic.

Also, it's a not unreasonable thing to hope for. We definitely have a possibility of finding out about early evolution of life on Mars because the conditions there should preserve early organics in near to pristine fashion,when the conditions are exactly right. If we are very lucky, then like the shadow biosphere idea, this life might actually still survive there to the present day. If so it could be very vulnerable indeed to whatever made it extinct on Earth.

It would just be so very sad to lose the opportunity to study life that's dancing to a different tune from Earth life. Especially if we lost the opportunity through an accident, through not knowing that it was there for us to find, until it is too late to do anything to protect it. Let's take a slightly closer look at how different it could be from modern Earth life.

Life on Mars dancing to a different tune

Suppose we found early life on Mars, too early to use DNA, or life on Mars that is as evolved as Earth life, but with a separate genesis. We'd have a different dance to compare with the dance followed by all Earth life.

It would have its own internal structures and biology. Here are some examples of things you find in modern Earth microbes, which it might have its own versions of - or maybe not have some of them at all in simpler forms of life.

RNA polymerase used to decode DNA to mRNA, present in all living cells. Does Mars life have DNA? If not, does it have two biopolymers like this, its equivalent of our DNA and RNA, or only one? What are its two biopolymers? What does it use to decode them into each other? Does it do error correction and if so how? Can it repair its DNA or equivalent if it is damaged?

Golgi apparatus - essential organelle in most Eukaryotes (cells with a nucleus) - which acts a bit like a post office. Amongst other things it wraps up items (such as proteins) and sends them out to different parts of the cell. Do any microbes on Mars have a separate nucleus for their genetic material? Do they encapsulate proteins and enzymes like this? Do they have an analogue of the Golgi apparatus, and if so how does it work?

(click to watch on YouTube)

Ribosome translating mRNA into a protein

Does life on Mars use proteins? Early RNA world life might not. If it does, are they encoded in mRNA or some similar molecule like this? Does it use the same 20 or so amino acids to make up the proteins as Earth life does, or different ones? One study found 4,000 biologically plausible amino acids that Earth life could have used. Does it use the same translation table to map nucleotide triples to amino acids?

Amino acids codon table - shows how a triplet of bases gets translated into an amino acid, used to build up proteins. All life uses the same table here, and it seems somewhat ad hoc and arbitrary. It has a lot of redundancy and could code up to 64 different amino acids s that list of twenty amino acids in some way optimal or is it just a historical accident, and other life does it differently? Might life based on a different biochemistry which originated independently from Earth life have a different codon table? Might it use more, or fewer amino acids? Or not use amino acids at all?

Microtubules, strands that stretch through cells. They are a bit like the "skeleton" of a cell, they are used to help cells move, and to keep its structure. They don't do this by themselves but along with actin filaments and intermediate filaments are part of the cytoskeleton.

Do Mars microbes have a cytoskeleton? If so what is it made of, and does it resemble the microtubules and actin filaments? How does cells use it to move around?

Then there are the cell walls, and lysosomes (in eukaryotes) which contain enzymes which break down larger molecules and structures that are no longer useful so they can be built up again into new material. and many other components of the cell.

These processes in Earth life are immensely intricate and complex. One analogy that I've heard is that if you are a cell microbiologist studying the interior of a cell, the whole thing is so complex and unique, it can feel as if you are studying an entire ecosystem. So, let's use that as an analogy to understand why even an ET microbe could be so interesting.

Imagine that you have been brought up in the African savannah - with its grasses, trees, elephants and antelopes. You've never seen a marsh or a forest, or a beach. All your life you've lived in a hut in the African Savannah, and never traveled more than a few miles from your hut, and that's the only thing you've ever known. In this analogy the savannah is like the interior of a cell on Earth, not just one cell, the interior of any cell from any living organism or microbe on our planet.

View of Ngorongoro from Inside the Crater

Then one day someone takes you to the sea shore, with its fish, shellfish, seaweeds, and sea anemones, and perhaps they take you on a dive to see a coral reef.

A Blue Starfish (Linckia laevigata) resting on hard Acropora coral. Great Barrier Reef. Photo by Richard Ling The interior of an ET microbe could differ from the interior of an Earth microbe, by as much as a coral reef differs from the African savannah.

Think how much that would expand your horizons! This gives an idea of what it would be like to find a microbe on Mars with a different biochemistry from Earth life. As boring as it might seem from the outside, just one small microbe looking for all the world like many others, perhaps much smaller - inside it could be as different as this.

Even an early RNA world cell would be so different from modern DNA based life, that it would be like a different ecosystem inside as well.

So hopefully this can help you see why the astrobiologists got so excited by ALH84001, and why it would still be so exciting to find a new form of life on Mars. What they are looking for is not just another boring microbe that happens to be smaller than anything we have on Earth. In the best case, in the case of what I like to call a "super positive outcome" then it could be the most amazing discovery you could imagine, revolutionary for biology, medicine, agriculture, nanotechnology,... There is no way to know how far reaching the implications could be.

Distant cousins with last common ancestor from a planet around another star

The other main possibility is that we find our distant cousins on Mars, DNA based life, following the same tune as Earth life, but evolved separately for tens of millions, or more likely billions of years. One way this could happen is if life originated on Mars, Earth, Venus, Ceres, or somewhere else in our solar system and then spread through the solar system via asteroid impacts. We've already covered the Earth to Mars and Mars to Earth case in the answer to Zubrin's arguments, under What about Zubrin's meteorites argument? and General case of transfer of life from Earth to Mars. It's far easier in the direction of Mars to Earth, but can happen both ways, and is most likely in the early solar system soon after the formation of our Moon. It's the same idea for transfer from Venus, Ceres etc.

Another way we could be distant cousins is if life originated in a a planetary embryo, or protoplanet, one of the hundreds of smaller objects that combined together to make the larger planets. Perhaps it originated in the parent body for some of the carbonaceous chondrites, or perhaps in objects that no longer exist, because they were destroyed when they hit Earth, some time after the formation of the Moon. One of the big issues here is how these small bodies could have held onto enough water for life to form, also the question of where the water came from - planetary embryos that formed at the same distance as Earth should be "dry" with no water, but ones that migrated inwards from further out could be wet, or indeed, even entirely made of ice. For an extensive discussion of this possibility, and some of the ways that it could happen. see From Protoplanets to Protolife: The Emergence and Maintenance of Life

However, there's yet another way it could happen. Both Earth and Mars, and other places in our solar system, could be seeded by life from an earlier star, far older than our one. I've already touched on this in Half of the pages of the book of evolution have been torn out, but let's look at it more closely. The authors of the original paper relied on a very minority view that our solar system formed from remains of a supernova explosion from a star system with life bearing planets. But there's a much easier way this can happen which has been explored in several papers.

First, we could be seeded by life around sibling stars in the Sun's stellar nursery soon after it was "born".

How life could transfer between sibling stars in a stellar nursery by "weak transfer" of meteorites

Normally it's almost impossible for life to get from one star to another as they are so far apart. The problem was that after being ejected from another solar system, the rocks would pass through our solar system with high velocities, higher than the solar system escape velocity. Entering our solar system at perhaps six kilometers per second, due to the relative velocities of the stars, there would be almost no chance of capture.

However, recently Edward Belbruno and Amaya Moro-Martín reexamined this situation using Belbruno's new idea of "weak transfer", and also the idea of transfer while the stars are still in their birth nebula. This is the idea that when you have gravitational tugs from several different bodies pulling in different directions at once, it can sometimes be possible for an object to get into a very weakly bound orbit around another one, in a situation where this would normally be impossible. Belbruno used this to rescue the Japanese lunar probe Hiten in 1991. The probe itself was lost, and its mother ship was stranded in an orbit around Earth without enough fuel to get it into orbit around the Moon by conventional means. He found a way of saving the situation using his new theoretical idea of weak transfer.

He's got an interesting approach. He's an expressionist artist, and he solves these problems using art, to inspire his mind to new approaches, including this idea that saved the Japanese spacecraft in 1991.

(click to watch on YouTube)

His art is here.

Anyway, using his weak transfer ideas, they found that our Sun could have exchanged life bearing rocks at least a hundred trillion times with its nearest neighbour in the cluster. Here they assume that the stars move at relative velocities as low as one kilometer per second as is normal in a young star forming region.

Shows how transfer of life was possible between two stars in the same cluster soon after the stars are born, with relative velocities between the stars of only of order one kilometer per second. It happens through the weak transfer, using the two weak stability regions shown, caused by the two stars themselves, other stars in the cluster and giant planets around both stars.

Our solar system would have had perhaps 700 million years to do this before it got dispersed from the new cluster. It had liquid water already long before the end of that window of opportunity.

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Table showing window of opportunity for panspermia in the early solar system - sharing life with its sibling stars. Image by Amaya Moro Martin

Close up of a star forming region in a new version of the famous "Pillars of Creation" photograph by Hubble. Stars are forming right now, especially in the denser "pillars".

It's a tiny part of a vast star forming region. Most of the dust and gas has been blown away as a result of stellar winds from nearby stars, and the whole area is dense with baby stars. This is a zoom in view of a new HD image in visible light from Hubble released in January 2015. Zoomable and HD versions here.

This infrared picture lets us see through the dust and clouds, and you can see how the Pillars of Creation are embedded within a cluster of numerous baby stars, juts forming. High res and zoomable views here.

And this region is just one part of the much larger Eagle nebula.

Photograph of the Eagle Nebula taken with the European Southern Observatory. I've outlined the position of the "Pillars of Creation". This entire region is a star forming nebula, dense with baby stars packed close together.

If life has evolved around just one of those stars in the Eagle nebula, many of them will infect each other by meteorite transfer. It's easy for that to happen right now, because the stars are closely packed together and also not moving fast relative to each other. Typical relative velocities in a young star forming cluster like this are only one kilometer per second.

Within a few million years the dust and gas will be gone. The stars are orbiting the center of our galaxy in independent orbits, with nothing to hold them together as a cluster. Within 700 million years typically they have already spread out so far through our galaxy that it's no longer possible for them to infect each other with life. However, those 700 million years are plenty long enough so that it's possible that nearly all these stars will have life on them, before they disperse, even if just a few or only one of them has life at present.

For more about the Pillars of Creation, see The pillars of creation - a glimpse into how stars are born by Tanya Hill, and this Space.com video about it.

If this is right, then that leads me to a suggestion (my own idea) that one good place to look for life might perhaps be on any planets around HD162826 as it's thought to be a sibling of our Sun, because its composition is similar. We must have thousands of siblings, but most are so scattered through the galaxy that they are probably tens of thousands of light years away. This is the only one identified so far and it is only 110 light years away. If it had the same birth nebula - could it have been seeded by life? Could it have life that's a distant cousin of Earth life and DNA based, with the same amino acids etc as Earth life?

So anyway now, based on that, perhaps we can begin to see how our solar system could also be infected by life from a much older star than our own.

Infecting an entire star cluster with life from planets from an earlier, perhaps billions of years older star

Edward Belbruno and Amaya Moro-Martín's ideas of course just let us exchange life with our siblings, so it wouldn't explain how it could have started ten billion years ago. However, the authors of another paper on this topic take this further, with a suggestion that life could be transferred from one cluster to another more easily than from star to star. With so much exchange of life between stars within a cluster, it would just need a single wandering star from a previous life bearing cluster to pass through a new star forming region, to infect all the stars in the new cluster. The stars from the previous cluster would soon spread throughout our galaxy and it just needs one of them to pass through a new star forming cluster to infect it.

This earlier star of course doesn't need to be a yellow dwarf like our Sun. If this is right, Earth life could have evolved around a different kind of star, maybe an orange dwarf or even a red dwarf.

Artist's impression of the five planet system around the nearby orange dwarf star, Tau Ceti, destination for interstellar travel in so many science fiction stories. Credit: J. Pinfield for the RoPACS network at the University of Hertfordshire

As it turns out, Tau Ceti does have planets, five in all so far discovered. Most seem likely to be uninhabitable, but one of its planets is well within the habitable zone (planet f) and it probably moved into this zone in the last billion years. Tau Ceti is an orange dwarf star. That's a more common type of star than our Sun, and research so far suggests they may be very habitable stars. See this recent survey of the literature: The Habitability of Planets Orbiting M-dwarf Stars

It could also have originated around a red dwarf star. This is by far the most abundant type of star in our galaxy, and it's also a very long lived type of star, lifetimes up to trillions of years and they have habitable regions that the planets could remain in for billions of years.

Artist's impression of the newly discovered planet orbiting our closest stellar neighbour, the red dwarf Proxima Centauri. New ideas suggest that planets huddling close to the smaller red dwarf stars could also be habitable, even though they would be tidally locked with one side always facing to the parent star. This is by far the most abundant type of star in our galaxy. Our nearest star Proxima Centauri has an approximately Earth sized planet, Proxima Centauri b, which is on the edge of its habitable zone and if conditions were right, with a magnetic field to protect its atmosphere, may have been habitable for six billion years. However other research suggests that planets, huddled so close to their parent star, also face more intense UV and X-Ray emissions than our Sun, that may denude the surface of oxygen and water within a few million years.

Whether it was a red dwarf, an orange dwarf, a yellow dwarf like our Sun or some other star, only one of these stars is needed to seed the entire star forming region where our sun was born. Of course a star forming region is a far larger area to target than a single star, and if it originated in a nebula that was itself seeded with life, then there would be thousands of these life bearing stars around, and only one of those needs to pass through the next star forming region, for the life to propagate.

In the same way also, in the future, our Sun, or one of our Sun's siblings, such as HD162826, might infect stars in some future not yet born cluster, and so the process continues. In that way even if evolution of life is very difficult, it could still spread through much of a galaxy from a single origin. It would evolve further and further each time. The authors conclude:

" These results suggest that a young cluster is more likely to capture life from outside than to give rise to life spontaneously. Once seeded, the cluster provides an effective amplification mechanism to infect other members."

So that does seem a distinct possibility. At least, it's perhaps not as implausible as you might think when you first encounter the idea. If that's the situation, nearly all life in our solar system would be related. It would also mean that even the earliest life in our solar system would be hardy and indeed pre-selected as life that can be transferred via meteorites easily. Although many microbes on Earth can't be transferred by meteorite, this would suggest that the first life to arrive in our solar system had this capability already.

However it wouldn't be multicellular life, or even eukaryotes, because those came later on Earth. Our last common ancestor could also predate the prokaryotes, and be somewhat simpler than any form of life we know. Also it might or might not have photosynthesis which seems to have been a later development on Earth.

This scenario doesn't rule out the possibility of places in our solar system where we can study life evolving from scratch. A good candidate for that might be the ocean of Enceladus as according to some ideas, its ocean may be young, only a billion years old. If that's so, it could have started with organics but no viable life left, and any traces long decayed. Or anywhere else where ancient life was sterilized, leaving a situation with organics, and the possibility of life, but no life had reached it yet. I wonder if on this hypothesis, there could even be places on ancient Mars where life tried other experiments in a "shadow biosphere", or pre-biotic chemistry that almost lead to life?

However, it does give another way that Mars life could be a distant cousin of us.

Surprising distant cousins

So, there are many ways we could find distant cousins on Mars. Our common origin could be within our solar system, or perhaps our common ancestors came from another star in our birth nebula, or they might even have arrived on Earth with billions of years of evolution behind them already, transferred from a planet orbiting a star that long predated our sun. If that is what happened, we could find life on Mars that is closely related to Earth life, but yet different. Distant cousins with a common ancestor billions of years ago.

So what might those cousins be like? Well we do have some surprising distant cousins on Earth already which may give us a few clues to get started. As an example, most creatures on Earth with shells, including the microscopic forams, use calcium carbonate. It's the same for creatures with bones, like ourselves, we use calcium carbonate too.

But the tiny microscopic diatoms use silica (found in nature as quartz) to make their shells. Basically their cell walls are made out of a type of glass. Would we have guessed that you could have microscopic creatures with "glass" shells if these creatures did not exist? They are unique - no other Earth lifeform has silicon skeletons or silicon shells. Yet it's also a very successful adaptation for them, as they are numerous with many different species, amongst the most common type of phytoplankton (microscopic single cell "plants").

(click to watch on YouTube)

And then, insects and crustaceans have yet another type of "skeletal material" (as an exoskeleton), to structure their bodies, chitin, which is an organic product, a derivative of glucose.

So could life on Mars use something different yet again? Perhaps they might use the iron oxides in some way, as they are so ubiquitous on Mars and would be useful for protection from UV light?

For another unique ability, most photosynthetic life works by splitting water to make oxygen, taking up carbon dioxide

The basic equation for photosynthesis is 6CO₂ + 12 H₂O → C₆H₁₂O₆ + 6O₂ + 6 H₂O where the oxygen atoms in bold are the same ones on both sides of the equation. Note that photosynthesis doesn't split the oxygen from the carbon dioxide, but rather, from the water. See Plants don't convert CO₂into O₂, and Notes on lamission.edu

Some photosynthetic life works by splitting sulfides to sulfur. These can even photosynthesize in the dim glow of a hydrothermal vent, a discovery from 2005. In all these cases, the photosynthesis is eventually used to power a "proton pump" to move hydrogen ions across a membrane and this is used with assistance of the enzyme ATP synthase to form ATP which powers the cells (and other bioenergetic processes)

Most remarkably of all, some microbes are able to do bypass all of this chemistry and use light directly to move the hydrogen ions (protons) across a membrane out of the cell, with no byproducts such as sulfur or oxygen. These are the  halobacteria (or haloarchaea) which use Bacteriorhodopsin and Halorhodopsin for photosynthesis. It's similar to the method the rod cells in our eyes use to detect light which make them sensitive to very low levels of light (they use the related pigment rhodopsin). These pigments are most sensitive to green light, and reject blue and red light, and so are purple in colour.

This shows the salt ponds of San Francisco. According to the Earth scientists who maintain the Earth Story, the area of pinkish red (with a somewhat purple hue) here is coloured by the haloarchaea.

The green at top right is from green algae. The orange pool at top left is that colour because of brine shrimp. But the pinkish red (with a purple hue) comes from haloarchaea which use light to generate energy directly without any chemical byproducts, much in the same way our eyes see light.

Many red algae are red for other reasons. For instance the red algae that make the Red Sea red are that colour because of phycoerythrin, a red pigment that is involved in the process of normal chlorophyll based photosynthesis.

These haloarchaea also occur in "Pink Lake" in Western Australia, but that's red partly because of the haloarchaea, and partly because of carotene accumulating in a green algae dunaliella salina, another red pigment that is involved in normal photosynthesis.

Here is how it works:

Shows how halobacterium salinarum gets energy from sunlight using bacteriorhodopsin - similar to pigments we use for vision, as a source of energy, instead of chlorophyll.

The light deforms the molecule which then acts directly as a proton pump which pumps hydrogen ions out of the cell. This proton gradient is then used, for instance, to synthesize ATP. It does this without generating oxygen or capturing carbon dioxide. Ordinary photosynthesis also works as a proton pump, but only indirectly, after carbon fixation and release of oxygen.

Again, if we didn't have examples to show it is possible - who would guess that there could be photosynthetic life that gets its energy in this way? So, even if the life on Mars is a distant cousin of Earth life, it could easily have some unique capability like that which surprises us.

How could our cousins differ? Skins of rust, freezer life, different amino acids translation table, ...

Let's just let our imaginations roam free for a moment, and list a few of the ways that our distant Mars cousins could be surprisingly different. Once again I invite you to a section based on synthesis and speculation. These are just my own ideas, not based on any sources as I don't know of anyone in the literature who has tried listing ways that our Mars DNA based cousins might differ from us. I took the list in Life on Mars dancing to a different tune (above) of the internal structures and biology for Earth life, and asked, "what might it be like if those are varied?". Then I added in a few other ideas that seemed natural extrapolations, such as skins of rust, and freezer life.

Do say if you know of a paper that covers this topic! Also what other possibilities can you think of?

• Skins of rust - Mars is the "red planet" after all. And iron oxides are great at blocking out UV light. What if there are microbes there that are like our diatoms but they have cell walls that incorporate iron oxides, making them rust red? Walls that are completely opaque to ultraviolet light? I base this on Carl Sagan's observation that the iron oxides on Mars are perfect for keeping out UV light, for cells imbedded in the dust. What if, as a result of evolution, instead of just hiding in cracks in the dust, they actually take up the dust into their own skins?
• Its own version of microbes with a nucleus. If Mars had multicellular life three billion years ago - what if it developed its own version of eukaryotes which are the basis of all complex Earth multicellular life? If so, it would have developed its own version of the Golgi apparatus, lysosomes etc.
• Unique cytoskeleton not based on microtubules or actin. Suppose it split off at a very early stage from Earth life, perhaps it has DNA just like Earth life, but doesn't use microtubules or actin at all, but has other ways of structuring its interior and of moving.
• More or different amino acids in its translation table. The translation table that translates sequences of three base codons to amino acids seems somewhat arbitrary. What if Mars life has found a way to incorporate more amino acids into it, which are never used by Earth life? Or the other way around, what if Earth life uses amino acids that Mars life ignores? Or what if we just have a different set altogether with some overlap, which might perhaps give us clues to the original set of amino acids used by our last common ancestor?
• Evolves more rapidly and more tolerant of errors in its DNA. Earth life doesn't reproduce DNA exactly, and indeed, perhaps it's adapted to tolerate a level of error to permit evolution to happen. So what if Mars life has a higher tolerance of error than Earth life in its DNA? That could be an advantage on Mars. It would be able to tolerate higher levels of ionizing radiation, and also be able to evolve much more rapidly when faced with a novel situation through its frequent changes of climate.
  
  That might be worth doing on Mars despite the downside of having more errors, and the higher proportion of cells that just don't work after replication. As we saw, in Tougher conditions for life on Mars - sometimes - and at other times much easier (above), its climate is much more variable than Earth's, with its orbit continually changing between circular and very elliptical, with ice sometimes at the equator instead of the poles, right through to the present. Also, even today, it has temporary short lived habitats after asteroid impacts or volcanic eruptions creating huge lakes that only last for a thousand years. In the early solar system it had frequent huge ocean sterilizing impacts as well as ones that caused massive flooding and created deep basins like Hellas basin filled with water. Its climate probably varying frequently between habitable and ice ball in the early solar system. With all those frequent changes, you can imagine a premium on microbial life that is able to evolve rapidly to face new situations.
• Ionization resistance way above Earth life. The most resistant Earth life is able to survive in a dormant state for up to hundreds of thousands of years in the ionizing radiation of the Mars surface (and longer if it can wake up briefly to repair its DNA). It seems unlikely that anything could survive in dormant state for billions of years on or close to the surface as that is enough time to break up all its organics into water vapour and other gases - but what if it can survive for millions, or tens of millions of years of dormancy? This might fit in naturally with a higher tolerance of error. After all, Earth life did not evolve in the presence of ionizing radiation, it just has that capability as a byproduct of desiccation resistance. So, how radioresistant would life be that actually evolved in conditions that specifically favoured higher ionizing radiation resistance, evolving for billions of years? If it could remain dormant for millions of years near the surface, then revive, that could be very useful given the way that Mars' climate changes so frequently on the millions of years timescale.
• Can resist desiccation better than Earth life - the most ionizing resistant Earth life seems to have evolved its capability as a way to resist desiccation Well the ionizing radiation resistance on Mars might have the same effect in reverse. It could be more desiccation resistant on Earth than the most hardy Earth life. Add in iron oxide skins and it may be able to survive UV radiation too in spore form. Also desiccation resistance could be a major advantage and evolution driver in the thin atmosphere on Mars. That might well favour evolution of highly desiccation resistant cells, more resistant than any Earth microbes.
• Takes in water from dry air- perhaps using perchlorates. Mars life might actually make its own perchlorates. It could run the processes that some Earth life uses to eat perchlorates in reverse, given a source of energy. Perchlorates are great at taking up water from the atmosphere even when it is quite dry. So maybe the Mars life pumps itself full of perchlorates, in dry conditions, and uses that to extract water vapour from the atmosphere? Or expels the perchlorates and creates a little patch of perchlorates salts around itself? Converts normal salts into perchlorates which then take water out of the atmosphere so creating a tiny pool of perchlorate rich brine around it. Perhaps it actually creates Nilton Renno's ice / salt interface "microbe swimming pools" by converting chlorides, hypochlorates, and chlorates into perchlorates in the proximity of ice, so melting the ice. Or uses other ways to extract water in very dry conditions so that it can continue metabolizing and reproducing. Maybe it adjusts the mix of chlorides and perchlorates to make the habitat around it optimal for its metabolism.
• Freezer life - this seems an obvious adaptation for Mars life -that it can reproduce below the -20 °C that our freezers operate at. This seems a likely adaptation for Mars life. If it is possible at all for life to survive and reproduce way down to temperatures of -80 °C, then surely Mars life has found a way. Our Earth life hasn't yet found a way to adapt to be able to reproduce inside our freezers below -20 °C but we haven't had freezers for that long and there are almost no natural habitats on Earth at temperatures much below that.

I expect you can think of many more ideas. Though as a caution, these attempts to "foresee the future" almost always fail abysmally. Whatever we find there might well surprise us, and not be anything any of us have thought of in advance. Still, it's the best we can do and perhaps it can give a flavour of how much Mars life could differ from Earth life, even if it is a distant cousin of us.

What else could have come before modern life? Alphabet soup of "XNA", Ostwald ripening organic crystals, "naked genes", or almost alive "autopoetic" cells

Let's now return to the idea that what we find on Mars could be an early form of life, whether related to Earth life or independently evolved. Then there are many other options apart from the RNA world cell idea, popular though that idea is. Astrobiologists have often suggested that life might be able to use some other helical structure, neither DNA nor RNA. Their ideas include a  PNA world which has a different backbone from DNA or TNA world, or a molecule that's a hodge-podge mixing different backbones in the same molecule with non heritable variations in backbone structure. They have also suggested a whole "alphabet soup" of other possible precursors such as HNA, PNA, TNA or GNA - Hextose, Peptide, Therose or Glycol NA. They call all of these XNA by convention, though actually many of them are not nucleic acids.

PNA

So what were the earliest cells like? What are the simplest possible cells? And what were their precursors? Even early RNA world or TNA world cells or similar may not get us right back to the origins of life. There still seem to be many gaps to fill in, even to get to early cells like that from non living chemicals.

There are various approaches to this. One is the idea of autopoesis, that a minimal cell might be able to reproduce in a less rigorous, approximate fashion, by having a simple structure, a vesicle that takes in material from outside the cell wall.

Diagram of an autopoetic cell, from "Chemical Approaches to Synthetic Biology: From Vesicles Self-Reproduction to Semi-Synthetic Minimal Cells" There, L is the cell boundary, lipids in case of Earth life. P and Q are the basic ingredients of cell growth and W, Z the waste materials.

E is the genetic and metabolic network, which converts the ingredients into the cell wall, as well as the internal components of the cell, creating waste products that leave the cell.

The idea is that , inside the cell, there is a network that turns these precursors into the cell wall itself, as well as using them to regenerate itself, and expels waste products. The vesicle as it gets larger splits to replicate, or alternatively, it creates a daughter cell inside which then leaves the cell. In these primitive protocells, this is regulated, for instance, by the surface area to volume ratio.

This could happen without any DNA or RNA to regulate it. This process happens with some fatty acid vesicles for instance. Some researchers are working with Butschli droplets, a complex mixture of oils and other chemicals such as detergents, that behave rather like cells to explore such ideas.

(click to watch on YouTube)

These are either droplets of oils in water or of water in oil. These are not likely precursors for us, of course, but are examples that work like protocells which let us explore artificial life scenarios in a different medium.

Here is Martin Hanczyc talking about protocells.

(click to watch on YouTube)

Researchers exploring this analogy include Rachel Armstrong and Martin Hanczyc.

This is just one of many ideas. There's a biological survey of some ideas for the earliest cells and their precursors here: The Origins of Cellular life. The next couple of paragraphs summarize some of its ideas.

More generally, there are two general basic approaches for this very early biology and pre-biotic chemistry. Either metabolism first or replication first. The protocells can "reproduce" in a way but imperfectly, just grow and then split. So metabolism comes first for those. We might find "almost alive" protocells that don't replicate exactly and stretch the boundary of our definition of life.

Perhaps protocells used naturally porous cell walls, made of fatty acids. New protocells might form spontaneously as new membranes assemble themselves around genetic material in the solution. Another way it could happen is that clay attracts RNA, and so perhaps a clay particle already covered in RNA also stimulates a cell wall to form around it, leaving the original clay particle, still covered with the RNA crust, trapped inside the newly formed cell wall. Another way protocells could form is inside microscopic channels within the rocks that precipitate around hydrothermal vents. The thermal gradients and the thin channels could concentrate the nucleotides and the larger nucleic acids. Then fatty acids could also be concentrated until they form vesicles at the bottom of the capillaries, with DNA inside, so leading to spontaneous self assembly of protocell like structures with DNA inside at the ends of the capillaries where they meet open sea, and they then could split off.

All in all, making protocells isn't that difficult and they can do that already in experiments. So - surely Mars must have had protocells and probably still does, even if it doesn't have life. The hard bit of the puzzle to fill in is what happens next, and how these cells come to replicate exactly or almost exactly. Getting DNA to form isn't anything like enough as you have to find a way for the two strands to separate and then replicate with the same gene sequence as before, then re-assemble as two identical double helixes. Perhaps the early life DNA just unfolded through variations in temperature. Perhaps in these early cells, DNA separates at higher temperatures, replicates, then reassembles into a double helix at lower temperatures. There are many such ideas. Perhaps we may find the answer on Mars.

Another thing we could find if we can trace back early enough in the origins of life on Mars, is that it formed huge crystals of organics all of one symmetry. When you have a mix of crystals of different sizes, the smaller ones often dissolve and then build up into larger ones, because larger crystals are more energetically favourable.The organics can change symmetry as they do so flipping to the mirror image to accrete onto the large crystal (solution phase racemization). Eventually you end up with one big crystal. This is one theory for how modern life got its chirality - the preference for one form of a molecule over its mirror image. The process is known as "Ostwald ripening" and it could take all the amino acids of one chirality out of a solution into one big crystal, amplifying a tiny signal of a slight excess of one version of an amino acid over its mirror image.

Eventually this ends up with one big crystal. This is one theory for how modern life got its chirality - the preference for one form of a molecule over its mirror image. The process is known as "Ostwald ripening" and it could take all the amino acids of one chirality out of a solution into one big crystal, amplifying a tiny signal of a slight excess of one version of an amino acid over its mirror image (Blackmond, 2010. The origin of biological homochirality : figure 7).

(click to watch on YouTube)

Salt crystals - notice how some grow larger at the expense of others. Most of the action happens from about 40 seconds into the video onwards.

If this happened on Mars, who knows, maybe at some point we find traces of these very early pre-biotic Ostwald crystals there.

Or the precursors of life could consist of chemicals that replicate, with no metabolism or cell wall. One possibility, for the RNA world hypothesis, is some kind of a mineral substrate. The chemist Leslie Orgel in his 2004 paper "Prebiotic Chemistry and the Origin of the RNA World" writes:

"A scenario that I personally find attractive is one in which the very first replicators were 'naked genes' adsorbed on the surface of mineral particles, and in which impermeable membrane caps were 'invented' by the genetic system as it became metabolically competent. Escape from the mineral surface, enabled by the development of a closed spherical membrane would occur at a relatively late stage in evolution"

So we might find something like these ideas. Far more complex than any of the things we can make in the experiments in our laboratories, perhaps with thousands of different chemicals interacting, on their way towards becoming life, but not yet what we'd recognize as life as such. Perhaps not replicating exactly for instance. "Almost alive" precursors for life.

And there are many other ideas.

Mars has had such a long period of cold conditions ideal for preserving organics. So, of all the places in our solar system, it may give us one of the best opportunities to trace out some of the steps of evolution of early life. If it is difficult for life to evolve, and Mars never developed life, it would be fascinating to find out what it did develop. Did some of these possible precursors of life develop there? If so I imagine there'd be a lot of interest in trying to find out why they didn't evolve further, all the way to life. It would also give us insight into what happens on the many planets in our galaxy that are Earth or Mars like if they don't evolve as far as life.

If Mars never developed life at all, at any time in the past, does it perhaps have autopoetic cells, or Ostwald crystals, or"naked genes" adsorbed on mineral particles, there today in favoured spots on Mars?

Something amazing to discover - but hard to find

If any of that is waiting for us to discover there, in the past or the present, most astrobiologists don't expect it to be an easy thing to find. Some of the things that make it so hard to know if the ALH84001 meteorite has traces of life or not is that

• Many of the organics could be produced by non life processes, especially the Polycyclic Aromatic Hydrocarbons (PAH's) in this particular case
• Non life processes could also produce the magnetite crystals found in the sample, which originally seemed so characteristic of life.
• The carbonate globules could also be produced by non life processes
• The possible life structures are so small that most can only be seen with an electron microscope. Could they be artifacts of the process by which the samples were prepared?
• Nearly all the organic carbon in ALH84001 is known to be terrestrial contamination.

We will have the same, or similar problems on Mars if we study similar samples there. But at least we can deal with the last of these issues. If we can keep Mars samples free of Earth life, we can deal with the contamination problem. But that's not the only problem there.

Organics created on Mars by non life processes

Some of the organics made through natural processes on Mars might mimic biosignatures. They might even be chiral, occur in only one of two mirror image forms. Also any biosignatures we do find are likely to be mixed up with organics created by non life processes making the signal weaker and harder to detect, especially since past life is likely to be damaged and degraded.

Some of the non life organics on Mars could be

• Created in volcanoes - A study of the organics in meteorites from Mars showed that some of them, at least, were created by volcanic processes. This process is something that also happens in volcanoes on Earth.
• Created by lightning, as in the Miller-Urey experiment, from basic ingredients of water, carbon dioxide, ammonia etc
• Created in serpentization reactions in hydrothermal vents.
• Other abiotic synthesis
• Delivered to Mars in the form of meteorites and comets. In this case the organics formed in the solar nebula or were already present in the nebula when it condensed.
• Always on Mars since it formed, already present in the planetary embryos that came together to form Mars.

If there are any organics from ancient life, they will need to be well preserved for us to detect them. It will be hard enough to disentangle real native Mars life biosignatures from these Mars originated biosignature mimics. Once again, the last thing we should do is to confuse it further by adding Earth microbe biosignatures into the mix as well.

Organics from meteorites on Mars may boost a molecule over its mirror image, mimicking biosignatures (chirality)

Many meteorites, and comets, are rich in organics. So could these mimic biosignatures from Mars life? The Australian Murchison meteorite is a famous example.

Fragment of the Murchison meteorite, and particles extracted from it in the test tube. The meteorite was a witnessed fall, collected soon after it landed in Australia, which means it is fresh with little by way of contamination by terrestrial organics. It has many organics in it. It includes rare amino acids such as Isovaline:

Isovaline, a rare amino acid found in the Murchison meteorite.

This helps confirm that the organics in it are of extraterrestrial origin as this amino acid is not involved in Earth life. Incidentally, it may be of value for treatment of acute and chronic pain.

You might not realize this from the enthusiastic news stories when Curiosity finally discovered organics, but the biggest surprise was that it didn't find them sooner. All those organics "raining in" on Mars from space from comets and carbonaceous chrondite meteorites have to go somewhere. For instance organics found by Curiosity in the Yellowknife bay were consistent with presence of 300 to 1,200 parts per million (ppm) organic carbon from meteorites. They actually expect them to be of meteoritic origin.

A 1990 paper predicted that between 2% and 27% of the Martian soil would be contributed by meteorites, of which 1% to 10% typically is organics. According to calculations, if there was no degradation of the organics, Mars should have 60 ppm of organics from organics deposited into the regolith, averaged over its entire surface to a depth of a hundred meters (see page 10 of this paper)..Also the meteorites from Mars that we have on Earth have lots of organics in them, roughly consistent with that estimate (they come from a few meters below the surface of Mars). So the main puzzle on Mars is actually, what happened to all those meteoritic organics? They seem to be gone from the surface at least (we don't yet have any data about organics centimeters or meters below the surface). The puzzle here is much more a question of how they were destroyed than why some of them are still there. This has been a big puzzle, ever since the time of Viking?

One possibility is that it was under-estimated because the organics get destroyed by perchlorates when the samples are heated up in the ovens typically used to analyse the samples on Mars. The other possibility is that there are processes actively removing the organics from the surface. Probably both of these are factors.

So, when we find non degraded organics on Mars in the future, there's a good chance it is meteorite organics, like the organics already found by Curiosity. So how do we distinguish organics from life, from the organics from meteorites? One way to search for past life is to look for a chiral excess, i.e. the preference of a molecule over its mirror image. But organics in the meteorites we analyse on Earth often have chiral excesses already. Some of this is surely due to contamination from Earth life, but we find these excesses in witnessed falls, where the meteorites are picked up (often in a desert where they are easy to spot) soon after they fell to Earth - or in meteorites from places like Antarctica that have probably not been exposed to much by way of Earth originated organics. Also, sometimes they have an excess in amino acids that are rarely used by Earth life or not at al. Also, sometimes the excess is a chiral imbalance in the opposite direction from Earth life.

In the case of the Murchison meteorite this imbalance is subtle and controversial,. In other meteorites, however, much larger excesses have been detected. In this 2006 analysis of the EET92042 and GRA95229 meteorites from Antarctica, they had chiral excesses ranging from 31.6 to 50.5%.

GRA95229 - another chrondite, collected in Antarctica, had chiral excesses of +31.6‰ for a-AIB to +50.5‰ for the (non terrestrial) amino acid isovaline, while the EET92042 meteorite ranged from +31.8‰ for glycine to +49.9‰ for L-alanine. These excesses seem to be extraterrestrial and not due to contamination by Earth life.

These meteorites are certainly not pristine. They are altered by water, at least. But they come from Antarctica, collected from the ice, so are less likely to be contaminated by organics. Also, the mix of amino acids seems non terrestrial which is another line of evidence to suggest that they may not be a result of contamination. Also GRA95229 has 2.5 times greater levels of organics than are typical in Antarctica. All of this suggests that these excesses may be extraterrestrial. For a more recent review of this, see the Chemistry Society Review article: Understanding prebiotic chemistry through analysis of extraterrestrial amino acids and nucleobases in meteorites.

Also a recent study of sugars in the Murchison meteorite, along with others, found evidence of quite major excesses of sugars in one form rather than the other, excesses of up to 55% for thereonic acid, a four carbon sugar acid. The excess for one of the 5 carbon sugar acids was even more dramatic, it had an excess of up to 82%. It was the same even for biologically rare sugar acids, which the researchers see as evidence that these excesses are not due to contamination from Earth life.

These meteorites have been unchanged since they formed in the early solar nebula. So how did they get this excess (assuming these results are correct)? There are various theories of processes that would do the trick, but nobody is entirely sure yet, and most of the ideas would lead to a rather subtle excess. See this 2000 paper Circular Polarization and the Origin of Biomolecular Homochirality. For a more recent idea which could lead to a large excess, see this 2012 study.

"The researchers propose that, in the solar system’s early days, heating as a result of radioactivity could have melted ice trapped deep inside asteroids. Liquid water then dissolved already present amino acids, which crystallized into mostly left-handed groupings."

Whatever the reason, if this chiral imbalance of organics from meteorites is real, it complicates the search for life on Mars.

Now, it's not a hopeless lost cause, if the meteorite organics do have these huge excesses. If we have a well preserved specimen of billions of years old past Mars life we should be able to figure it out so long as the original organics had only one form of each amino acid. Totally monochiral amino acids would be easy to distinguish from an excess or imbalance in that amino acid.

It's not as easy as you'd think, because if you just leave organics for billions of years even in very cold conditions, occasionally an amino acid will "flip" into its mirror image form. This happens much more quickly in warm conditions so if the sample ever got warmed up, in that long history, for instance if it was sometimes near the surface or if it lay at the bottom of a warm pond for some time, it would lose this chiral signal very quickly, and we would have no way to tell that it ever had a chiral excess.

But if we search long enough and carefully, then we may well find samples there that preserve traces of this chiral signature even after billions of years, in samples that have spent all that time in cold dry conditions. We'd still expect only a small excess, since even in cold dry conditions, much of the chiral signature would get lost. However, different amino acids flip at different rate, so we could look at what the excess is for several different amino acids and then work back to find out when they were originally all the same chirality. If we do that and get a sensible date for the original monochiral sample, and it ends up with the same calculated original date for all the acids in the sample, this would be good evidence that it may have originally had all its amino acids in just one chiral form. That would then be a strong indication that it was once a form of life.

This is part of the detective work astrobiologists would have to do when studying ancient organics on Mars. They could also try to detect life through its preference for lighter isotopes of carbon or sulfur, and there are various other biosignatures they can search for.

Then in addition to all that, the organics could be mixed with a signal from later life, and also of course, with with a later influx of chiral meteorite organics. That would complicate things again of course. But it's possible to deal with that also, by looking at the distribution of the amino acids and at the isotope ratios. It should be solvable, but all that certainly complicates the picture.

So, in short, it's likely to require a lot of careful detective work to work that all out. How much comes from meteorites, how much from degraded organics from life, how much from later life, and how can we accurately figure out if the life originally had a preference for one molecule over its mirror image?

However, it could be even more complex than that. What if some (or all) of our samples on Mars are so early, that they take us back to a time before "homochirality"? The origin of this preference of life for one chemical over its mirror image is still very much a mystery. See a recent paper from Chemistry World, October 2015 on The Origin of Homochirality for some current ideas about this.

The reason many biologists think that the amino acids would all have the same chirality, even if the life is unrelated to Earth life, or very early life, is that if you have some of the mirror image amino acids in a cell, it blocks and poisons the process that joins the other amino acids together to make a longer chain. This is known as "enantiomeric cross inhibition". This leads to a big puzzle though, how then could life evolve in the early pre-biotic "chemical soup". The Urey Miller experiments with lightning and organics produced equal numbers of both types of amino acid. And the excess from meteorites even in the most remarkable cases is nothing like 100% of just one of the two types of amino acid. There are various ideas about how this might have happened.

But what if that wasn't even necessary, at least not at the beginning? We should look briefly at this very challenging possibility for early life. This would be the worst case for exobiologists trying to find out about early Mars life. Though if it started off chirality indifferent and later evolved chirality, then that would make things easier for the astrobiologists. If Mars life somehow stayed chirality indifferent right through to the present, then chirality just would not be a useful biosignature to test for, and we'd need to look into other ways to test for biosignatures.

Joyce's ribozyme which joins together strands of opposite "handedness" - could early life be chirality indifferent - ambidextrous?

This is an intriguing discovery by Gerald Joyce that suggests that perhaps very early RNA world life was ambidextrous. He found a ribozyme using right handed RNA that could join together left handed RNA strands. It could replicate its mirror image also. It couldn't replicate itself, but its mirror image (enantiomer) could in turn replicate its original self. In this way - though it can't directly deal with both handedness of life in one go, it can make a copy of its mirror self, which can deal with the opposite handedness, and its mirror self can copy the original, so together with its mirror image, it can handle everything you throw at it, whatever the handedness, as well as replicate itself in both handedness too. Techy details in their paper in Nature here. Discussion of its implications here.

He did it by a test tube "chemical evolution" gradually building up more and more complex ribozymes, selecting for the ones that can join together the opposite sense of RNA strands. In a few months, after only 16 rounds of this evolution, he finally created an 83 nucleotide ribozyme that works with the opposite sense of RNA. So - though it doesn't fill in all the gaps in the puzzle, it suggests the possibility that early life could have been ambidextrous. So what if early life on Mars is like that, so that there is no chiral imbalance to detect? Equal amounts of all the essential molecules and their mirror images, each happily replicating the other through ribozymes?

Well there are two other main ways to find a possible signature of life from looking at the amino acids. See the section on amino acid target selection in this paper (about sensitive electrophoresis to examine liquid samples). One way to spot it is if there is a large excess of an amino acid that is never formed naturally, only through biotic processes. Another way is to look at the relative abundance of the more complex amino acids compared with the simplest one, glycine. Abiotic samples normally have a much higher abundance of the simpler and easiest to synthesize molecules. So if you saw an abundance of more complex amino acids, that would suggest it is a result of life rather than abiotic chemistry.

In short we don't know what we will find there, and we may have to do a lot of detective work. There are many other kinds of organics we can look for, but amino acids are a likely early starting point. If the biosignatures are very faint and hard to detect, then contamination by Earth microbes, even dead ones, and even in tiny quantities, could be a major nuisance or make the whole thing impossible to sort out, as happened for ALH84001.

Then another thing that makes it even harder is that we don't know where to look for evidence of past life on Mars. It may take a while before we learn which conditions on Mars are best for preserving ancient organics, and then when we do, perhaps they preserve those organics only sometimes. Maybe we find suggestive but ambiguous signals first and then only gradually home in on the best places to find life there. So, let's have a look at this next.

Preservation of past organics

Mars is a great place for preservation of organics in some ways, because it is so cold. One of the things that makes it hard to find past life on Earth is that warm organics gradually either break apart (DNA) or as the molecules jostle around in the warm conditions, they spontaneously swap over into their mirror image forms.

Just as DNA only spirals one way, the other chemicals used by life such as amino acids occur in just one of two possible mirror image forms. But in warm conditions, then molecules can spontaneously flip into their mirror image forms.

So, first, , let's look at the plus points, some of the things that may make it easier to find well preserved billions of years old organics on Mars than on Earth.

• The Mars surface is very cold. Even just centimeters below the surface, it may be cold enough so that some of the amino acids and other organics haven't yet swapped into their mirror image forms, even billions of years later.
• Also with no continental drift, much of the Mars surface has remained undisturbed for billions of years.
• Most of its surface is hardly changed since the formation of the planet, apart of course from the craters from larger meteorite impacts, the regolith gardening by smaller meteorites, and the layers of sediments added during later flooding events.
• Mars has very little by way of weathering compared to Earth. Lots of dust sediments and some localized flooding, that's all.
• Either no present day life, or not much present day life to confuse the signal. If it does have life on the surface, it probably can't survive more than a few centimeters below the surface, because it is so cold. That is, except in geological hot spots (if they exist) and deep in the subsurface hydrosphere. Dig a few meters below the surface, the best place to look for past life anyway (protected from cosmic radiation) and there's probably no present day life to confuse the signal.
• Has large areas of clays and salts - these are ideal for preserving organics
• Large areas of the surface are thought to have had water in the past, including almost the entire Northern hemisphere. What's more the water seems likely to have been habitable, at times, moderate in temperatures, not too salty, and neither too acid nor too alkali.

However there are other things that make preservation of those ancient organics from any past life harder.

• Mars was most habitable billions of years ago. This is a long timescale for preservation of organics, during which Mars lost most of its water, had many floods, and changed its inclination, orbital eccentricity, atmospheric density, and climate many times. That's a lot of changes in the Mars climate for the sample to last through. It just needs to warm up for a geologically short period of time to lose its chiral signature, for instance.
  
• We don't yet know which parts of Mars had life in the past. For instance, what if the only life occurred around hydrothermal vents, and it never developed as far as photosynthesis? Then we may need to search ancient hydrothermal vents to find it. There are many other ideas about where life might have started. What if it remained in the place where it first evolved and never got any further?
  
• The cosmic radiation degrades surface organics. That's because the Mars atmosphere gives little protection from cosmic radiation. Every 650 million years you get a 1,000 fold reduction in the numbers of small organic molecules such as amino acids on the surface because of cosmic radiation. It degrades the organics to water vapour, methane, carbon dioxide etc, leaving nothing but volatiles. Since it's an exponential process, that's a million fold reduction every 1.3 billion years. It becomes a trillion fold reduction after 2.6 billion years and so on.
  
  This is the main reason that astrobiologists say we may have to dig deep to find life that has escaped this process. Astrobiologists recommend that we search for early life at a depth of ten meters ideally. ExoMars will be able to drill two meters which is enough so that it has a chance of finding evidence of past life. But though that's a necessary condition, it's not by itself enough for the present day sample to be deep down. It must also have been buried quickly in the past and stayed buried well beyond the reach of cosmic radiation ever since. We could also find organics that were unearthed recently as a result of an impact, or weathering by the wind, but it would have to be fresh, recent unearthing, for it to have a chance to preserve the organics.
  
• Mixed in with organics from meteorites, volcanoes etc (already covered in Organics created on Mars by non life processes). To make things more confusing, the meteorites often have a chiral signature, which would need to be disentangled.
  
• Life is most likely in places that had water in the past. These are the very places where warmth, flooding, consumption of the organics by other lifeforms, and other forms of degradation can happen.
  
• Later episodes of flooding
  
  Artist's impression of Gale crater as it might have looked during one of its flooding episodes (by Kevin Gill). For more on this: Curiosity Rover Data Indicates Gale Crater Mountain Used to be a Lake
  
  Of course, floods like this may make it briefly habitable, but they can also wash out earlier deposits. Especially as the later floods on Mars were often rapid flash floods, so probably not giving much time for new deposits of life to form.
  
  (click to watch on YouTube)
  
• Chemical degradation of near surface materials by the perchlorates, hydrogen peroxide etc. We know that there must be processes actively removing organics because it should have reasonably large quantities of organics from meteorites and comets and instead it only has small amounts.

So, how should we search to try to find a sample that has survived for billions of years?

For a clear signal of past life

For a clear signal, for past life, your sample somehow has to be deposited originally, and then avoid all those things that could destroy or degrade it. It needs to be:

1. In a place where there was life in the past, for instance in the remains of ancient hydrothermal vents, or salt lake deposits, or wherever it is that life was back then. Perhaps it was almost everywhere in the ancient seas if it had already developed hardy dormant states, and photosynthesis. If not, maybe it is in only a few special locations, so we will need to learn where to look for it.
2. Form in a place where dead microbes were accumulating, perhaps the bed of a lake. Or the detritus from a flash flood.
3. Preserved quickly (dried out, caught in clays or salt, or the microbes rapidly entombed in fast forming rocks like chert)
4. Plunged rapidly into freezing conditions, because in warmer conditions, the organics flip easily into their mirror images, which makes it harder to distinguish life from non life.
5. Buried quickly, ideally within a few tens of millions of years, to a depth of several meters for protection from the cosmic radiation degradation.

Then once it is safely buried like that, it has to survive for billions of years and then be returned to the surface. So it

1. Wasn't washed out with later floods, or altered or destroyed by the perchlorates and other chemicals, or returned to the surface temporarily for more than brief time periods.
2. Wasn't mixed with other sources of organics, or if it was, in a way that is easy to disentangle
3. Was returned to the surface rapidly (perhaps as a result of a meteorite strike or weathering of the rock by the Martian winds), and did this in the very recent geological past. Or else, your rover needs to be able to drill deep, or search in caves protected from the surface cosmic radiation.

To find out more about all this, see John Grotzinger's Habitability, Taphonomy, and Curiosity's Hunt for Organic Carbon as a starting point:

When you put it like that, it may begin to seem almost a hopeless task. But on the plus side, Mars is a huge and varied planet, with its surface area the same as the land area of the Earth. There are plenty of places to look for this life on Mars. It is also geologically diverse, with many varied geological features and terrains.

Amongst other things, we can search for this life in ancient deltas, shore lines, salt beds, and preserved hydrothermal vents. Also, Mars must have many caves made through the passage of water, and vast layers of sediment (though these caves are hard to spot from orbit, far harder to spot than the lava tube cave entrances we know about already). The whole of Mount Sharp in Gale Crater, which Curiosity is exploring, is a sediment deposit built up over hundreds of millions of years. Surely somewhere amongst all this geology, in all these layers of sediments, we will find the ideal conditions leading to preservation of past life? We also have a great chance to find optimal conditions for present day life somewhere in all this varied terrain too.

Are there any "magic minerals" to preserve microbes from early Mars?

The downside of this vast search area is that we don't know where to look yet. On Earth one key to discoveries of early life was the realization that gunflint chert is a "magic mineral" that preserves traces of early life in exquisite microscopic detail.

Galaxiopsis, one of the fossil microbes found in gunflint chert, which has turned out to be a "magic mineral" for search for evidence of early biology on Earth. These fossils are so exceptionally well preserved that scientists can even detect organics from microbes as old as 1.88 billion years ago. The organics are degraded of course but they could still detect functional groups (which attach to the hydrocarbon chains and play an important role in life processes) such as amides and hydroxyl, and other oxygen based functional groups, and attempt to compare them with modern cyanobacteria and micro algae. See discussion section of this paper.

However, though these newer fossils are uncontroversial, the older Apex Chert alleged microfossils from 3.46 billion years old are much more controversial with the possibility that the traces of organics associated with them may be a later contamination. Papers continue to be published arguing both ways. I'm not sure if it has been resolved yet.

What are the "magic minerals" for the search for life on Mars, in the very different conditions that prevail there? Where are the best places to look? We don't know yet. We are making a great start with Curiosity. We will find out more with future missions like Curiosity's successor and Exomars. But there are likely to be many more steps still to go through before we know where is the best place to look, and for that matter, what to look for. See John Grotzinger's Habitability, Taphonomy, and Curiosity's Hunt for Organic Carbon again for more details.

So in short, we can't expect to just land on Mars, go to a likely spot and find a sample of past life on Mars. We would be extraordinarily lucky if that happened. We may have to search long and hard. And we may have to search for just faint traces of a long degraded signal. That means we may be looking for just a few amino acids in the sample.

Once again, if we get any Earth life on Mars, and it contaminates the samples, it will confuse this search.

Then, we don't know what we are looking for, yet. It may be unknown biology. It could be based on XNA (like DNA but with a different backbone) or it could be something else not DNA at all. What we are searching for is:

• Likely to be single cell micro-organisms
• We don't know what it looks like
• We don't know what chemical signatures to look for
• It may only form microfossils, which are notoriously hard to identify as life or non life.
• May easily be so small that you can't see the cells even in a good optical microscope, especially if it is early life

Follow the nitrogen, dig deep and look for biosignatures

So far NASA has been using a "follow the water" strategy. But in some ways it has been too successful. There's abundant evidence of water on Mars past and present. Shannon in his thesis "Elemental analysis as a first step towards "following the nitrogen" on Mars" uses the example of the Leprechaun in a traditional Celtic story.

Leprechaun - by SatyrTN

Here is the story as told on the Discovering Ireland website:

"In one tale, a young farmer captures a Leprechaun and forces him to hand over his gold. The Leprechaun says that the gold is hidden beneath a tree in the woods and shows him which one it is. The farmer ties his red scarf around the tree and after making the Leprechaun promise not to remove the scarf he heads to his farm to get a shovel. But when the farmer returns he finds that the Leprechaun has tied a red scarf around every tree in the woods."

Here is a longer version of the same story.

It's like that with water on Mars. To start with the scientist were excited with every new discovery, and said "Wow this spot had water in the past". Now much of the map of Mars is dotted with the red scarves showing the location of probable past habitable water on Mars. There are so many of these "red scarves" there now, that they don't really give us much focus in the search for life there.

The best way to search for early life, as far as we can tell at present, is to search for organics. However, that also is not nearly specific enough. As we've just seen, the organics are easily confused with organics from non life processes and from space. Eight astrobiologists looked into this in a white paper which they submitted to the most recent decadal review: Seeking Signs Of Life On Mars: In Situ Investigations As Prerequisites To A Sample Return Mission

One of the main conclusions of the white paper was that we should look more specifically for nitrogen rich organics on Mars. These nitrogenous organics are likely to be rare because Mars has few sources of nitrogen. This is important because nitrogen is central to the functioning of biology as we know it. Nitrogen bonds are easily broken and re-attached, which is important in biology, so this may be more than just a prejudice from our experience of Earth life. Even if life on Mars is very different from Earth life, perhaps using different amino acids for instance (see Alien life could use an endless array of building blocks) and perhaps use PNA or some other form of XNA (Xeno nucleic acid) with a different backbone from DNA, still it is likely to use nitrogen if it resembles Earth life. Curiosity recently found evidence of nitrates on Mars, also fatty acids, but it hasn't found these nitrogenous organics which the astrobiologists suggest we look for.

Once we find these compounds, that's not enough, as you also get nitrogenous organics from comets and meteorites and natural processes. We would then need to search for biosignatures to distinguish the ones from life processes from the others. We also need to be able to drill below the surface (as ExoMars will be able to do) to the maximum depth possible to find life less damaged by ionizing radiation.

Their main points are:

• Need for increasing mobility, and precision landing, supported by orbital observations, to access the many and varied habitable environments including subsurface, layered sediments, gullies and ice sheets.
• The "follow the water" strategy should now be followed by a "follow the nitrogen" phase combined with a search for biosignatures.
• The biosignature search can use exquisitely sensitive in situ electrophoresis techniques to identify and characterize and find the chirality of amines, nucleobases, polycyclics and other essential organic molecules.
• This search should include drilling to the greatest depth possible for the best chance of success for detecting biosignatures of past life on Mars
• They recommend that we should do a sample return only after we either identify clear biosignatures on Mars, or have exhausted all other possibilities by in situ research

If we follow this program, then our top priority right now should be to "follow the nitrogen" and to send instruments to Mars of exquisite sensitivity to look for traces of past life in situ. For details of instruments we could send there, see In situ instrument capabilities below

The astrobiologists couldn't be clearer in their recommendations for an in situ search first,. Several other groups of astrobiologist have published papers arguing this point very strongly in exactly the same way. In situ is the way to go right now, not sample return, not until we find unambiguous biosignatures, or have exhausted our tools for searching for life in situ. But for some reason, NASA continues to see a sample return as a top priority for the search for life. They continue to take this as their main near future goal, even though hardly any astrobiologists think this is a priority, and most think it is actually a distraction and diverting of funds away from what should be our top astrobiological priorities on Mars.

When commenting on ideas to return a sample from Mars at an early stage, the astrobiologist tend to say (paraphrasing their papers)

"Right, that would be nice, and it will be good to have that for later on. But right now it is more of a technology demo, and a geology mission. If we do it now, it is likely to be little more than an expensive way to add extra samples to our collection of controversial Martian meteorites. Without a way to intelligently select samples with evidence of past life, on Mars itself, it's just not worth the price tag of millions of dollars per gram for a sample return. It would be much better value to use the funding for in situ instruments searching for biosignatures directly on Mars".

Hopefully what I said here is enough to give some background to help understand this point of view. I go into this in much more detail, discussing several papers on the subject, in my section Astrobiologists arguing strongly for an in situ search on Mars first (below) .

NASA's plan for safe zones - based on finding Mars life easily

If we knew where to look, and what to look for, then we could just land on Mars, dig up a well preserved sample of ancient life, and then that answers the question of whether there was life on Mars. Then we find a present day habitat, and find present day life and that answers the question of whether there is present day life there. Enthusiasts for humans on Mars seem to imagine it happening like that, pretty quickly.

If you find life as quickly as that, and supposing you are content in just making the discovery, and you are not so much worried about what happens later as Earth life spreads to Mars habitats - then it's a matter of landing somewhere, making sure the humans don't contaminate too much of Mars too quickly, and then sending out robotic scouts to bring back materials for the astronauts to analyse, first on Mars, and then, back on Earth.

That's NASA's current plan - an exploration zone, with the human occupied field station in the center, and teleoperated rovers heading off for in situ study around the perimeter, and returning samples to the center. Meanwhile the astronauts do their own survey missions within the area that they have set aside as "cleared for human exposure".

To them, this seems like a good compromise. It lets humans "touch Mars" but they do their best to limit the effects of the contamination by Earth microbes and organics by restricting human movement geographically on Mars. Here is one example from a paper in 2010, with the human exploration zone shown close to an area of special interest which humans can't visit directly.

Illustration from Mission to Mars: The Integration of Planetary Protection Requirements and Medical Support . This paper is from 2010, so before the discovery of the RSL's, first reported in Science in 2011, and at that time the gullies shown were thought to be amongst the most likely to have present day life.

These particular features are still a matter for much discussion, but are no longer top candidates for present day habitats on Mars. One suggestion is that they formed from water originally, perhaps just from a small amount of water mixed in with dust. They are still active today with occasional new features forming, and this seems to be the result of subliming carbon dioxide mobilizing the dust, rather like a pyroclastic flow. It's a complex situation, see this discussion by Tanya Harrison in Astronomy magazine. See also this announcement from NASA which she comments on.

A modern version of this diagram would probably show the Recurring Slope Lineae (warm seasonal flows (see below) ) in place of the dry gullies, but it's much the same idea. See also Mars colony will have to wait, says NASA scientists

The "Safe Zone - cleared for human exposure" shown on this sketch is a region set aside for the astronauts to roam, where you don't mind Earth contamination. The idea is that the human exploration zone is contaminated with Earth microbes and this is just accepted as a necessary part of human exploration of Mars, but only clean rovers are permitted to travel to the habitats that potentially could host surface life on Mars. They bring samples back to the human base for analysis, or the rovers are used to study the regions beyond the zone remotely.

That could work fine on the Moon. Indeed, perhaps that could be a good place to test it out first? If humans don't travel too far from their base, they will preserve pristine lunar surfaces just a few kilometers away, untouched by human footprints, wastes or garbage. So long as the rovers can also be sterilized sufficiently in a human base, which may be quite a big "if", they could be used in just this way to do clean studies of, say, the volatiles at the lunar poles. The rovers could travel from a human base into the pristine craters just kilometers away and collect samples and return them to the base for analysis, then be sterilized, and go out again to repeat the process.

Even on the Moon there will be some issues of contamination immediately around the base. For instance the Apollo astronauts left a fair bit of organics behind when they left the Moon. It is enough to confuse analysis of samples if you are looking for just traces of organics from the cosmic wind impacting on the Moon. The Apollo samples were recently re-analysed and the composition of amino acids does suggest some extraterrestrial sources, However, that analysis was a tricky one due to contaminants from Earth in the form of rocket fuel, organics taken to the Moon by the astronauts, and organics introduced while handling them on Earth.

This suggests we need to take care to avoid this sort of thing in the future as we explore the Moon. Then there is some transport of the contamination further afield, even on the Moon, by electrostatic levitation of dust, but it's a rather minimal effect. There are no winds and there is no way for water to flow either. Also, how easy would it be to sterilize the rovers at the base? It would probably depend on the design. For instance if the exterior can withstand high temperatures, you could just heat sterilize them and then send them out again.

There are many details to be filled in, however, it seems reasonable to be optimistic that with a bit of care we should be able to keep the Moon surface relatively free of contamination except for the region immediately around a human base. Even this is quite challenging and the Moon may be a good place to learn about how effective such methods can be in practice.

But when it comes to Mars, it begins to seem far more of a formidable challenge. First, it's not just a risk of contaminating ice, but of liquid water where microbes could reproduce (not a risk for the Moon). Could the astronauts really sterilize the rover to Viking levels of cleanliness or better every time it returns to the base with a sample, and then send it out again to study an RSL or similar habitat? And could they keep the returned samples clean while handling them in their base as they analyse them to try to find out what is in them? Especially when they need to do very sensitive analyses to detect even single molecule biosignatures.

Or would they sterilize the rover only once, on Earth and then use it on Mars only for a single mission to the RSL where it would collect a large number of samples in one go and return it to the base, then use it for missions that don't require such cleanliness later on? Or how would it actually work out?

It may seem easy to do on paper, but when it gets to actual practice, are they going to be able to keep the Earth microbes away from an RSL even for the duration of their visit, with rovers driving back and forth between the human base and the possible habitats for present day life? Perhaps this is something we can find out on the Moon first, where it will be equally important to keep the organics from the base away from the study region, especially when searching for organics in the polar ices. It will be far easier to do on the Moon, so we can get preliminary ideas there first about whether it is possible in an easier situation.

However, there is an additional major complication with Mars of course

How could this work on Mars with dust storms and a globally connected environment?

If it does work on the Moon, how can this work at all on Mars with the Martian dust storms? The main problem here is that microbes can form hardy spores, and on Earth these can survive for long periods of time, hundreds of thousands of year. In rare cases, they can survive for millions of years of dormancy. Though the Mars grains of dust are so fine, they are plenty large enough for a microbe to hide in a crack in the dust, and so be protected from the UV radiation. And any microbes that get into a shadow are sheltered completely from the UV. Even in equatorial regions, some areas under rocks will be permanently shadowed from UV light.

And then you get these:

This is a Martian dust devil - they race across the surface of Mars picking up fine dust and would also pick up any microbes imbedded in the dust. The microbes would be protected from UV radiation by the iron oxides in the dust.

HiRISE image from Mars Reconnaissance orbiter, of a dust devil in a late-spring afternoon in the Amazonis Planitia region of northern Mars. The image spans a width of about 644 meters.

The strongest winds on Mars, though fast, would barely move an autumn leaf, because the air is so thin. But the dust is also so fine on Mars, as fine as cigarette ash, and easily lifted by these feeble winds. Also, it's made of iron oxides, which would help to shelter any spores imbedded in cracks in the dust, from UV light.

Then from time to time dust storms will cover the entire planet. A microbe imbedded in a typical dust particle transported around Mars during one of these thick dust storms would be much more shielded from UV than it would be in normal conditions.

Global Mars dust storm from 2001 Mars has local storms every two years, and from time to time it has larger global storms. The first global storm recorded is from 1873: the other ones reported were in 1909, 1924, 1956, 1971, 1973, 1975, 1977 (2 storms), 1982, and more recently in 1994, 2001 and 2007. So we get a global dust storm roughly every decade or so, though sometimes several per decade (five storms in the 1970s)..

This relates to an observation Carl Sagan made in an old paper "Contamination of Mars", back in 1967.

"The prominent dust storms and high wind velocities previously referred to imply that aerial transport of contaminants will occur on Mars. While it is probably true that a single unshielded terrestrial microorganism on the Martian surface ... would rapidly be enervated and killed by the ultraviolet flux, ... The Martian surface material certainly contains a substantial fraction of ferric oxides, which are extremely strongly absorbing in the near ultraviolet. ... A terrestrial microorganism imbedded in such a particle can be shielded from ultraviolet light and still be transported about the planet."

He continues:

"A single terrestrial microorganism reproducing as slowly as once a month on Mars would, in the absence of other ecological limitations, result in less than a decade in a microbial population of the Martian soil comparable to that of the Earth's. This is an example of heuristic interest only, but it does indicate that the errors in problems of planetary contamination may be extremely serious."

Of course we know much more about Mars than they did back then. But the situation is still the same, the dusts do indeed contain large amounts of iron oxides. We have also found out that some microbes are far more UV hardy than realized in the 1960s. Some especially hardy strains of bacillus can survive many hours of Mars surface conditions unshielded from UV by dust or other shields. In one experiment in 2010, one of their strains survived four hours of Martian conditions and in one case 28 hours of Mars surface conditions, in both cases, unshielded from UV light, in simulated Mars winter conditions. This is far longer than the few seconds to minutes that researchers used to think was the maximum for Mars.

This shows survival of spores in Mars daytime summer conditions (left) and winter conditions (right) exposed to the full UV flux of the Mars sunlight. As you see a significant percentage of the most resistant strain B pumilus DSMZ 27 survived for the entire 90 minutes shown on the table. In later experiments they found that one of the strains could survive for at least 4 hours, and in one case 28 hours of simulated Mars surface UV flux in winter conditions

Their paper is summarized in this article in Universe Today: Bacteria Could Survive in Martian Soil.

The dust storms and high wind velocities are the same as in the 1960s. The dust does contain perchlorates, which they didn't know back then, but microbes can survive exposure to perchlorates at the low temperatures on Mars.

The dust particles in dust storms range from less than a micron to 50 microns in diameter. Endospores are from 0.25 microns upwards. Some experiments suggest, that Earth microbe spores could survive at least twelve hours of being blown over the surface within a Martian dust storm. See also Survivability of Microbes in Mars Wind Blown Dust Environment. They could also be transported at night during a dust storm, when there's no UV light, yet still dust suspended in the atmosphere.

The largest global dust storms occur only every 30 years or so. With wind speeds of 10 to 30 meters per second (22 to 67 miles per hour) average for the faster winds during a dust storm, the dust could travel 240 to 720 miles every twelve hours, and some of the dust rises to many kilometers in the atmosphere, and it takes months before all the dust settles. If the dust particles, and any spores embedded within them, end up in a shadow at the end of that, they will then be protected from UV radiation until the next time they get transported by the winds. The NASA save zones idea suggest that the human habitat may be positioned close to a special region - one that could potentially have habitats for present day life. If so, these figures suggest that they might get to a vulnerable region near the base, in a dust storm in much less than twelve hours. That would seem to suggest that the microbes could get to nearby habitats perhaps quite early on, perhaps even during the first human mission to the Mars surface, if there is a dust storm to carry them in that direction. After all the contamination zone around the human base would have large numbers of spores in it, also additional protection for the spores in the form of flakes of skin, hair etc.

I've tried to find experiments simulating transport of microbial spores in Martian dust storms. But I can't find much. There are plenty of experiments to show that microbes can survive covering by a thin layer of dust on the surface. I suppose it is understandably hard to simulate conditions in a dust storm accurately.

The best I can find is a recent experiment from 2016, "Assessment of the Forward Contamination Risk of Mars by Clean Room Isolates from Space-Craft Assembly Facilities through Aeolian Transport - a Model Study" which uses Staphylococcus xylosus, a microbe that is commonly found on the skin of humans. It's an aerobe so not likely to survive on Mars, but it could introduce contamination by biomass and genetic material. Their experiment simulated Mars conditions, but with quartz dust instead of the iron oxides, and only for a few minutes. They found that the vacuum conditions had an effect on survival, but the sub zero conditions and transport in the Mars winds had little effect, One of their main focuses was to simulate electrical charge effects in the suspended dust, but they only simulated that for twenty minutes. The experiment itself wasn't very conclusive, but in their conclusion they combined their data with previous results on survival of microbes when shielded by dust on Mars, and said that they thought that microbes removed from a spacecraft surface by impact of dust would not be killed so long as they were not exposed to UV radiation for long periods of time.

There isn't any other more complete experiment mentioned in their list of citations at the end, so assuming they did a reasonably thorough literature survey, perhaps that means that a more complete experiment has just never been done. Their paper has no citations yet in Google Scholar (which would be a way to find more recent experiments of that nature). The only other paper I've found so far is this one from 1970 which found that UV light shining on simulated Martian dust storms did not sterilize the spores, but it's just an abstract ,and it doesn't go into much detail, and of course we knew much less about Mars conditions back then:

"A chamber was constructed to create simulated Martian dust storms and thereby study the survival of airborne micro-organisms while exposed to the rigors of the Martian environment, including ultraviolet irradiation. Representative types of sporeforming and non-sporeforming bacteria present in spacecraft assembly areas and indigenous to humans were studied. It was found that daily ultraviolet irradiation of 2 to 9 X 10(7) erg cm-2 was not sufficient to sterilize the dust clouds. The soil particles protected the organisms from ultraviolet irradiation since the numbers of survivors from irradiated environments were similar to those from unirradiated environments. Pending further data of the Martian environment, the contamination and dissemination of Mars with terrestrial micro-organisms is still a distinct possibility."

Do say if you know of any other experiments to test survival of microbe spores in Mars dust storms, thanks!

Given the challenge of keeping the samples clean of Earth life, and the difficulty of finding nanoscale fossils and traces of degraded organics mixed in with the organics from meteorites, comets and non life processes on Mars, how can this approach keep Mars pristine for long enough to complete the search for past life? Never mind the search for present day life which I'll cover later. Also, do we not have some responsibility to keep Mars free of Earth life for rather more than the duration of our own first missions there? Do we not have a responsibility for future generations, or indeed even ourselves in future decades when we come back again, to learn more about Mars and any life there may be on Mars, long after the first human landings on Mars? We may have much more sophisticated ways to study Mars in the future. There may be, experiments we will want to do, to answer questions that we don't even have the understanding to ask yet, but it will be too late if it is already irreversibly contaminated by Earth life introduced there in the early twenty first century.

So far we have only considered microbes that escape from air locks, and from spacesuit joints and such like - and any wastes intentionally released onto the surface. Those are all ways we could contaminate the surface in the course of a "nominal" human mission to the surface where all goes as planned. That's hard enough to cope with.

But all those issues pale into insignificance when you consider what happens if a human occupied spacecraft crashes on Mars.

Why do spacecraft crash so easily on Mars?

The basic problem is that Mars gravity is twice lunar gravity. To get a first rough idea of the issue, the Mars escape velocity is 5.03 km / sec, and for the Moon it is only 2.38 km / sec. The delta v to low lunar orbit is under 2 km / sec for the Moon, for example Apollo 14 ascent stage trajectory had a total delta v of 1.845 km / sec (6053.4 fps, close to optimal). By comparison, for an optimal ascent trajectory, it's around 4.2 km / sec to get to a low Mars orbit from Mars.

So, very roughly speaking, the delta v is around double for Mars compared to the Moon, to get down to the surface, if you use rocket propulsion all the way, or to ascend back to orbit. The difference in the amount of fuel needed isn't that huge, since even a 4.2 km / sec delta v is still fairly small, for decent thrust rocket motors, with an ISP of say 302 seconds (exhaust velocity 2.96 km / sec).

How ISP relates to exhaust velocity - techy aside. The fuel efficiency of a rocket is measured using the "specific impulse". This is often abbreviated as I_sp or just written as ISP.

This is the ratio of the change in momentum to the amount of fuel used up to achieve that change. If it's a conventional rocket and you measure the fuel used in units of mass such as the kilogram, and the rocket is flying in a vacuum, then the ISP is just the exhaust velocity. When flying in an atmosphere, it's the effective exhaust velocity.

But often it's given in units of seconds. That's for convenience as you then have the same number for the specific impulse no matter what units you use for the length measurements (meters or feet). The specific impulse in seconds gives the ratio of the change of momentum to the weight of the fuel changed. Here the weight is measured as the force acting downwards on the fuel, in pounds force, or newtons. When you divide a momentum (mass times velocity) by a force (mass times acceleration), it turns out that the result is a number measured in seconds.

Anyway the main thing you need to know is that you get from the specific impulse in seconds to the exhaust velocity in meters per second by multiplying by the standard gravity of 9.807. That's because one kilogram exerts a force (weight) of 9.807 newtons in standard gravity (the Earth's gravity varies slightly depending where you are, reduced in equatorial regions because of the Earth's spin, counteracting it, also less as you get higher and varying depending on whether or not you are above a gravity anomaly, so they use a standardized gravity). Similarly, if you want the exhaust velocity in feet per second, multiply the ISP in seconds by 32.175.

You can try this rocket equation calculator here to get an idea of how much difference it makes to the fuel needed to increase the delta v from 1.845 to 4.2 with a specific impulse of (say) 302 seconds. However, you also need to take account of the need to discard the first stage for the ascent to orbit as well, and the maximum fuel fraction for a first stage. When you do that, it turns out that the ascent from the Moon, and return to Earth, is far easier than it is from Mars. This paper makes some comparisons between a Mars sample return to orbit around Mars, with a Moon sample return all the way back to Earth in terms of payload ratios. Although the Mars sample return only has to get to orbit, and then is picked up by another spacecraft, in their plan, it still is much harder to do than the sample return from the Moon all the way to Earth.

Anyway our focus here is on the descent to the Mars surface. A spacecraft landing astronauts on Mars would need to have a capacity of many tons. All the spacecraft landing on Mars so far, have used aerobraking, instead of relying on rocket propulsion all the way. It's likely to continue like this for the foreseeable near future,, as otherwise they would have to carry a lot of fuel, especially if the plan is to be able to abort back to orbit during a failed descent. This is what makes it so hazardous to land on Mars, especially since the atmosphere also contributes to make it more of a one way process. The Apollo astronauts could fly as close as they liked to the surface. Apollo 11 had enough fuel to delay landing on the Moon while Neil Armstrong looked for a better place to land. They also had the ability to abort back to the lunar orbit and return to Earth at any stage, if a problem arose during the landing sequence. The only external danger was from the lunar mountains. Also the landing sequence is slow enough so that a human can pilot a spacecraft to a landing on the Moon by hand, as Neil Armstrong did with Apollo 11. There would be no chance of doing any of that on Mars with present day technology.

On Mars, once you start the landing sequence, and you hit the atmosphere, you are committed. There is now no way to abort back to orbit again, unless you carry huge amounts of fuel with you. You are streaking through the atmosphere at kilometers per second. Everything after that has to work in a perfect sequence with timings accurate to fractions of a second, with critical steps in the timeline passing by faster than a human being could assess the situation and react. Also, a landing on Mars is far more complex than a landing on the Moon or indeed anywhere else in the inner solar system. It should be no surprise when spaceships crash on Mars. Indeed, it's rather remarkable that we've had as many successful landings there as we have. Only the Americans have achieved totally successful landings on Mars to date, and they also have had their share of failures too.

All landings on Mars so far started with an aeroshell and aerobraking to slow down in the upper atmosphere. Next comes the parachute, because it would just take so much fuel to do all the rest of the slowing down using rockets. But a parachute can't slow you down enough for a landing, because the atmosphere is so thin. So then you have to find a way to slow your spacecraft down even further, from those hundreds of miles an hour to a slow enough speed for a soft landing. This shows the sequence for the Schiaperelli entry, as it was supposed to happen.

See Schiaperelli: the ExoMars Entry, Descent and Landing Demonstrator Module

So that’s why you have the retro propulsion stage for many landers on Mars. But you have to take care because if you do retropropulsion when the parachute is still attached you will get the lander tangled up in the parachute. So you have to release the parachute first before you fire the rockets. The moment of parachute release is very important, to get that right. Schiaperelli released its parachute too early, because of bad data from its inertial measurement unit, which was the start of its problems. The other main method used to date is the one used by Opportunity, and Spirit, to use air bags, followed by bounces on the surface. But nobody has suggested we use air bags for a human landing.

Now even after that, you still are not quite home and dry. The problem is that unlike a landing on the Moon you have no precise control over where you land. Instead you have a landing ellipse. This is the one for Schiaperelli, 100 km by 15 km

There is almost no chance of steering your landing craft during the landing, except possibly in the last few meters. Up to then, it is dependent on whatever the atmospheric conditions are as you land. The Mars atmosphere is very thin, a near vacuum, but it also varies hugely in density between day and night and there are lots of variations depending on altitude, temperature etc, also dependent on the dust content and dust storms, and it is hard to predict exactly. There’s also always some small amount of uncertainty in the speed and position of the spacecraft as it enters the atmosphere, and all of that adds up to contribute to the overall uncertainty shown by the landing ellipse.

The size of the ellipse depends on the spacecraft and the technology used. Curiosity had a smaller landing ellipse of 20 kilometers by 7 kilometers. That's why it could land in Gale Crater - which would be too small for ExoMars to land safely. ExoMars couldn't land there, as it would risk hitting the central mountain or the crater walls. But even Curiosity had nothing like the precision we had for lunar landings back in the 1960s.

Neil Armstrong could decide exactly where to set down the lunar rover, and if necessary just fly a bit further to find a good spot. With modern technology we should soon be able to do pinpoint landings on the Moon. It is not likely to be a major problem for the ESA village, to land the astronauts near to their habitats. On Mars you have to be able to land safely wherever you happen to be in that huge landing ellipse. After that, you have to find your way to your habitat somehow, if you have a previously landed habitat on Mars.

Either that, or you take a risk that if you hit a boulder, that’s the end of the mission. Viking 1 landed not far from a boulder which would have certainly ended its mission if it had landed on it

Photo by Viking 1 showing "Big Joe", a boulder two meters in diameter close to the landing site in western Chryse Planitia. It could not have survived a landing on this boulder.

The landing site was chosen, after much deliberation, by a combination of radar and photographic observations from orbit. They couldn't see surface features this small from orbit, and the radar observations were confusing. We can take photographs from orbit now down to 30 cms resolution (with HiRISE), but we still can't control where exactly our spacecraft land on Mars.

Mission planners deal with the issue of boulders as best they can by choosing regions on Mars that are very flat. Ideally you want to have hundreds of square kilometers with no boulders or steep slopes in all directions. That’s why Curiosity had to drive for so long before it got to Mount Sharp. It wasn’t safe to land it any closer because it would risk landing on a big boulder or on a steep slope. Curiosity could never have survived landing on a huge boulder like the one in the Viking 1 photograph. Similarly, unless we achieve pinpoint landings first, of course, astronauts would have to drive several kilometers from the landing site, to reach any previously emplaced habitats on Mars (you don't want to risk the spaceship landing on a habitat).

Of course, it's possible that we develop the ability to do pinpoint landings on Mars, but we don't have that quite yet. It would require some way to steer the spacecraft as it descends through the atmosphere.

Supersonic retropropulsion - or huge parachutes

If we ever land humans on Mars we'd need to be able to land much larger payloads than any attempted to date. There are two ideas of ways to simplify this process. The first is supersonic retropropulsion. That’s what Elon Musk plans to do for SpaceX. It's safer in some ways, and it does permit a much heavier payload, but in other ways it is riskier.

Conceptually it is about as simple as you can get, and the simpler, the less there is to go wrong. The rocket doesn’t need to have an aeroshell or parachute or anything. It just decelerates. Though in practice it may well have a heat shield as well.

Early artist’s impression of supersonic retropropulsion

It slows down by coming in very very close to the surface in the thicker atmosphere at huge speeds. Its rockets switch on when it is still traveling at supersonic speeds. It skims across the surface below the height of the higher mountains on Mars. Indeed it has to come in so low, that if landing in the Valles Marineres, a big rift valley in the Martian highlands, it would have to skim down between the walls of the canyon. All this time the rocket is firing and it is also affected by the air resistance of the atmosphere, so it slows down for both reasons at once, the retropropulsion and the air resistance. Finally, it comes to a vertical landing on the surface. It may have a heatshield and backshell but no parachute.

Entry, descent, and landing sequence, figure 4 from recent 2016 paper by Humphrey Price, Robert Manning, and Evgeniy Sklyanskiy

SpaceX has actually done something very like this on Earth. Their barge landings of the first stage used supersonic retropropulsion, in the very early stages when the first stage slowed down from 70 km down to 40 km, at just the right altitude to stand in for the tenuous Mars atmosphere.

What’s more, they can achieve a pinpoint landing as well, as they have demonstrated several times - when it works. Perhaps that means pinpoint landings on Mars will be possible after all, once this technology matures. So it can certainly be done, but it is rather risky and tricky to do on Mars with the very thin atmosphere and its atmosphere far more variable in density than Earth’s.

The other way to do it is to use an absolutely enormous parachute. If the parachute is big enough, you can have a conventional landing just as for Earth. Simply use an aeroshell, and then parachute down, and it will slow you down enough so you get a soft landing.

The problem is deploying those parachutes and making sure they work. You can work it out with computer models, test tiny parachutes etc. But at some point you have to test it with real parachutes. The parachutes they use so far were tested by firing rockets in suborbital trajectories in the upper atmosphere, because the Earth's upper atmosphere is similar in density to Mars'. This required many expensive tests. To make even larger supersonic parachutes will require a new set of these very expensive rocket tests. NASA are working on this with their Low-Density Supersonic Decelerator.

What I've presented here is just a first rough idea of how it all works. For more details of these ways of landing on Mars with supersonic retropropulsion or large supersonic parachutes etc, hear Robert Manning talk about it here Mon, 03/28/2016 - 14:00 Elon Musk's idea is to use supersonic retropropulsion, and to use data from his experiences returning first stages and later second stages of his rockets on Earth to help develop the design.

Artist's impression of red dragon doing supersonic retropropulsion over Mars, image SpaceX

Elon Musk recently announced that he is canceling the Red Dragon - he says referring to the idea of a retropropulsion landing on Earth

"The reason we decided not to pursue that heavily is that it would have taken a tremendous amount of effort to qualify that for safety for crew transport. There was a time when I thought the Dragon approach to landing on Mars, where you've got a base heat shield and side mounted thrusters, would be the right way to land on Mars. But now I'm pretty confident that is not the right way."

His Dragon 2 will now land with parachutes on water. Then he said in a tweet:

"Plan is to do powered landings on Mars for sure, but with a vastly bigger ship"

So he still has the idea of supersonic retropropulsion eventually, but on a much larger scale. However it would be slightly smaller than his ambitious Interplanetary Transport System with its ability to carry 100 people in one go.

Elon Musk's fun but dangerous trip to Mars

With this background, it's no wonder that Elon Musk said in his talk to the International Astronautical Congress that the mission to Mars carries a high chance of death for the first would be colonists. See Elon Musk envisions 'fun' but dangerous trips to Mars

"I think the first trips to Mars are going to be really, very dangerous. The risk of fatality will be high. There is just no way around it," Musk said. "It would basically be, 'Are you prepared to die?' Then if that's ok, then you are a candidate for going." (emphasis mine)

He isn't talking about dangerous as in a scary haunted house or a fairground ride, where it's scary but you know that you are in safe hands. It's not at all the idea that the equipment is inspected and though it seems dangerous, you won't actually be hurt by it. He is talking about dangerous as in something that is far more dangerous than base jumping. You could easily be killed by it for real.

So, yes, for sure, he may find many people willing to sign on for such a ride. But what would the consequences be for Mars?

It will surely take a while to perfect this technology. Even if, say, he has four successful previous unmanned missions, this doesn't prove it is safe. With a 50/50 chance of success for each mission, you can get four successes in a row with a 6.35% probability. So four successes would not show at all conclusively even that it is 50% reliable. Other ideas such as enormous parachutes far larger than any tested to date also have similar issues.

So, if we accept that there is a high risk of a crash, how can you be sure you won't get this sort of thing happening?

Debris from Columbia - broken into tiny pieces by the crash. If something like this happened on Mars, with the debris spread over the surface and dust and small debris and organic materials from the crash carried throughout Mars eventually in the global dust storms - that would be the end of any chance of planetary protection of Mars from Earth life.

The debris field for Space Shuttle Columbia, with a debris track around 350 miles long, and about fifty to a hundred miles wide (depending on whether you measure to the most distant debris). An accident, especially if it happened early during the supersonic retropropulsion entry to the Mars atmosphere, could scatter debris over a large area of Mars.

With this background, how can we land humans there, without a significant risk of a crash? As for the space shuttle, this would mean dead bodies, broken up into minute fragments, food, air, and water spread over the surface of Mars and mixed in with the dust. It could then spread anywhere on the planet. This would have an immediate impact on science studies throughout the region of the debris field. Your first assumption, if you found biosignatures anywhere near the crash site would be that they came from Earth. That could be immediately devastating for science, especially if the humans crash happens close to somewhere biologically interesting on Mars, which they might well if the plan is to situate the human base close to a special region such as the RSLs. Or any other place where they hope to search for life on Mars, past, or present.

However, it gets worse than that. Because Mars is a connected system through its dust storms, as we saw, the crash site would be a source for life itself to spread throughout the planet. I think most would agree that if there are Earth microbes able to adapt to live on Mars, and any habitats there for it to inhabit, a crash of a human occupied mission on Mars would mean essentially the end of all planetary protection of the planet.

This section includes material from my articles:

• Does Elon Musk's Plan Violate The Outer Space Treaty - Planetary Protection For Mars After Human Crashes
• Why Spacecraft Crash on Mars

Elon Musk, though he is so in favour of sending humans to Mars as quickly as possible, does care about the science impact of introducing Earth microbes to Mars. Here he answers a question on this topic, in the 2015 AGU conference in San Francisco, 30 minutes into this video:

Q. "I am Jim Crawl from Arizona State University. I was listening to doctor Chris McKay, another advocate of humans to Mars, and he was talking about when we do go to Mars and if we find life either currrently there or extinct, we should consider removing human presence so that we can allow that other life to thrive. I was wondering what your thoughts on that were. "

A. "Well it really doesn't seem like there is any life on Mars, on the surface at least, we are not seeing any sign of that. If we do find some sign of it, then for sure we need to understand what it is and try to ensure that we don't extinguish it, that's important. But I think the reality is that there isn't any life on the surface of Mars. There may be microbial life deep underground, where it is shielded from radiation and the cold. So that's a possibility but in that case I think anything we do on the surface is really not going to have a big impact on the subterranean life.".

So, it's clear (as I'd expect actually), he does think it is important we don't extinguish any native Mars life. But he thinks there isn't any present day life on the surface. Many of you reading this may be of the same view.

But is that right? Up until around 2008, and possibly for a year or two later,, many scientists would argue that the surface of Mars is sterile, and that if there is any life on Mars it is deep underground and not connected to the surface in any way. With that background, it seemed reasonable to suppose that anything humans did on the surface wouldn't matter, as Elon Musk suggests in that interview. But that's no longer the situation. Indeed even Robert Zubrin now says that he thinks life on Mars is likely. So, let's take a look at this, what has changed?

I'll come to the surface habitats in a bit - if you want to jump ahead, it's the section Habitats for life on the surface of Mars - warm seasonal flows. (see also Habitability of the Mars surface (top few centimeters). But let's first start with the life below the surface. As Elon Musk said, scientists have thought for a long time that the subsurface may be one of the best places to look for life. Is it as cut off from the surface as scientists used to think?

Methane plumes on Mars and the possibility of water deep below the surface in its hydrosphere

The Mars crust, like Earth's, gets warmer as you go deeper down. It might have a hydrosphere, a layer of liquid water rapped below thick layers of rock and ice. There's probably ice at great depths, even in equatorial regions, and below it, water kilometers below the surface even in the equatorial regions. This water would be trapped beneath many layers of rock and ice, and so could stay liquid, warmed up by the geothermal heat of the interior of Mars. So, even before the Phoenix observations in 2008, which lead astrobiologists to re-evaluate the possibility of surface habitats, astrobiologists thought that there could be life deep below the surface.So then, a natural question, "What about hot spots closer to the surface?" Despite many searches, we haven't yet found any sign of current volcanic action or hot spots.

However , there are signs of geologically recent volcanic eruptions in the Olympus Mons caldera, and other volcanic features that formed as recently as a few million years ago. The phoenix lander also found evidence from isotope ratios that some of the carbon dioxide in the atmosphere came from volcanoes in the recent past. So there could easily be geothermal hot spots still there, closer to the surface. So far, our searches have turned up some slightly puzzling infrared anomalies but no clear signs of hot spots. However, localized hot spots would be easy to miss. Our orbiters can only measure the temperature of the top few millimeters of the surface, so we have to work out the subsurface temperatures through modeling. Perhaps there is liquid water in caves not that far below the surface, or just damp rock, the moisture trapped in liquid form by overlying layers of rock and ice, and kept warm by geological heating, yet sufficiently insulated from the surface that we haven't detected them yet from orbit?

All this was rather abstract theory until the methane plume observations in 2009 by astronomers using NASA's infrared telescope facility, and the Keck telescope, both at Mauna Kea, Hawaii. They were puzzling, as the methane seemed to disappear from the atmosphere so rapidly that it was hard to work out a physical process that could fit the observations. Also these were delicate measurements and needed to be confirmed.

Now, Curiosity seems to have confirmed these observations, though its results continue to be puzzling because they appear and disappear over such short timescales. Perhaps that means they form somewhere close to Curiosity's location. It's also possible that the methane is contamination from Curiosity itself, but so far, that seems unlikely. Hopefully ESA's Trace Gas Orbiter will help clear up some of these mysteries once it starts its science mission. It has to circularize its orbit first, and the current plan is to start its science mission in early 2018. It is by far the most sensitive instrument of its type ever sent to Mars. It's sensitive to up to an amazing 10 parts per trillion to many different chemicals in the Mars atmosphere, including methane. It may help answer many of our questions about the methane, and may detect other interesting trace gases as well.

So where does the methane come from, if these signals are genuine? Well there are various ideas but most suggest a connection between the surface and the subsurface. The methane plumes on Mars could be results of

• Past inorganic processes such as serpentization (reaction of olivine with water at high temperatures - olivine is an ingredient of many rocks including basalt). Other sources include inorganic processes in the atmosphere, volcanoes, or it could be that it was already present on Mars when it formed. In all these cases, it would be locked in clathrates for maybe up to billions of years, and then released
• Products of past life, again locked in clathrates
• Present day deep subsurface life using the hydrogen from serpentization as an energy source
• Present day deep subsurface inorganic processes

We may have spotted methane on Mars. If so this figure from NASA / JPL shows some possible sources. One possibility is that early Mars had large amounts of methane in its atmosphere which helped keep it warm, and its been trapped in the methane clathrates for billions of years, now released.

Whether it is the product of present day life or not, these plumes may show a connection between the surface and a habitable region below. But a connection can go both ways, especially if, for instance, the methane plumes are accompanied by seeps of liquid water that reach to the surface or to caves or cracks or other features open to the surface. What happens if Earth life gets into this habitable region after a human crash or landing on Mars? It could be contaminated by methanogens that generate more methane, or methanotrophs that eat them, confusing the scientific study of what caused the plumes. And if there is Mars life down there, the Earth life could confuse the search for the Martian microbes, or compete with them, maybe even make them extinct (especially if it is some vulnerable early form of life)..

This is not the only way that the surface could be connected to the deep subsurface. One of the theories for the warm seasonal flows (see below), or Recurring Slope Lineae is that they might be the result of water from deep below getting to the surface in regions of geological hot spots. Again this means it could be possible to contaminate the subsurface, and maybe even the entire hydrosphere, if it is connected to the surface via the RSL's.

Cassie Conley, astrobiologist and NASA's planetary protection officer, made an interesting observation, that this could also contaminate subsurface aquifers with microbes that are known to create calcite when exposed to water with CO₂ dissolved in it. Later explorers might find subsurface aquifers converted to cement as a result of contamination by Earth microbes. See Going to Mars Could Mess Up the Hunt for Alien Life (National Geographic).

Porous basalt

If Mars does have a deep hydrosphere and it's inhabited with Mars life, it may live in porous (vesicular) basalt, a rock that's ideal for life to live in. 90% of the rocks in the vicinity of Viking 2's landing consisted of porous rocks like this, riddled with holes. If there's some source of hydrogen, for instance, it could be very habitable. Basalt has has the chemical elements needed to support a million cells per gram (limiting factor is phosphorus) and there are likely to be perchlorates, nitrates and sulfates from the surface to increase its fertility for life. If it is brought to the surface, then this might be a good place to look for traces of life on Mars, past or present.

See this paper.

A - sample of vesicular basalt from Earth (Columbia River Basalt).
B - from Mars, false colour photo by Spirit in Gusev crater.
C - lunar basalt collected by Apollo 16.
D - Ordovician basalts with the holes (vesicles) filled with carbonates.

Two ways Curiosity's methane spikes could be generated in the shallow subsurface (centimeters deep at most)

Curiosity's methane spikes could also come from the shallow subsurface, rather than the deep subsurface. There are two ways it could happen, as described in a paper from 2016. Both rely on the low temperature brines that Curiosity detected indirectly, in the sand dunes it drives over. See Liquid brines beneath the surface of sand dunes at night (below). These brines are just centimeters below the surface. Both theories require this water to be very abundant.

First, the regolith could take up methane when dry - and release it when wet. That would work, because methane can't dissolve in water, so wetting the rock, somewhat paradoxically, forces the methane into the atmosphere.

Techy detail. The reason methane can't dissolve in water is because unlike ammonia, it can't do hydrogen bonding with water. Also it's able to force its way between the hydrogen bonds that join water molecules to each other in liquid water).

The main problem with this theory is that the regolith might not able to take up enough methane quickly enough. Its adsorption energy is around 18 kJ/mol for a Mars soil analogue JSC-Mars-1. That's just an analogue, of course and not the real soil, which might have rather different properties. But for this theory to work, it needs an adsorption energy of 36 kJ/mol. That's rather high, higher than activated carbons and similar to that of synthetic nano-porous titanium silicate, an artificial material which can separate methane from nitrogen.

Another way it can happen is that methanogens living in the water would make it themselves by taking in carbon dioxide from the atmosphere. The main difficulty with this model is that it doesn't explain how the methane suddenly vanishes from the atmosphere with a residence time of a few days. But it could be that Curiosity was no longer downwind of the plume, so that it wasn't dissipated quickly. It just changed position, or Curiosity moved out of it.

So, they concluded that neither of these theories can be ruled out quite yet. The third theory they looked at is methane from the deep subsurface, the one we already mentioned, and there were no major issues with it.

How else could Mars have deep subsurface water with some connection to the surface?

Subsurface ice in equatorial regions

Mars could also have subsurface ice from past more habitable times. There have been several papers suggesting ice in the equatorial regions below the surface. But they are none of them definitive, though interesting. But let's just take a look at them. First here is a map of some of the features on Mars that may be associated with subsurface ice.:

Shows the position of Medusae Fossae, Utopia Planitia, the mountains Arsia Mons, and Olympus Mons and the rift valley Valles Marineres on Google Mars. All of those have evidence of ice deposits beneath the surface, but not confirmed.

One of the most promising places to find subsurface ice in equatorial regions is Hellas Basin, the deepest point on Mars, which has many features that suggest ice. It may even have liquid water occasionally in the depths of the basin - as the lowest point on the Mars surface, the atmospheric pressure is higher there, and even fresh liquid water could form though it would be close to its boiling point and evaporate quickly. Salty brines would be more stable there and longer lasting than anywhere else on Mars. It's often obscured by dust and clouds.

The right hand image shows a photo of three craters in Hellas Basin. The left hand image shows what ground penetrating radar suggests maybe lies just below the surface, covered in an insulating layer of debris. This is a discovery from 2008.

In another more controversial measurement, ground penetrating radar showed what may perhaps be ice below the surface, in the Medusae Fossae Formation - it's either miles deep layers of equatorial dirty ice or volcanic ash all the way down. If it's ice then it was probably deposited there when Mars' axis was tilted so far that it had equatorial ice sheets, as happens occasionally - unlike Earth, it's axial tilt varies chaotically on long time periods (as discussed in Oceans that are only liquid part time as Mars' tilt and orbital eccentricity change (above) ) .

The vertical distance below the surface here shows the radar time delay, roughly corresponding to depth below the surface. Do you see how there is a faint extra line of white in the middle of the picture showing a secondary reflection layer in the radar image? That's the region with probable ice deposits in equatorial Mars.

See also Water on Mars May Have Piled Up as Ice Near Equator and for the technical paper Radar Sounding of the Medusae Fossae Formation Mars: Equatorial Ice or Dry, Low-Density Deposits? (abstract). Later research confirms these observations, strongly suggesting that this indicates ice in some form.

This is not at all conclusive however. It might be volcanic ash as this paper suggests.

Another set of observations suggested ice at the base of craters in the Sinus Sabeus region. It's strong evidence that this area had ice in the past, when Mars' axis was tilted more than it is now, but they thought there was a possibility that some ice might still be there today, buried and hidden from view, concluding (from the paper):

"It is unclear from available data whether any relict ice is currently present at these locations, although estimates for fill thickness are noteworthy. The equatorial setting suggests that if present, this ice is likely buried by a thick, insulating debris layer or a near-surface layer of reduced permeability."

Another paper suggested there may be still be large amounts of ice buried beneath the surface in the Valles Marineres region. Then there could be ice on the flanks of Arsia Mons area and of Olympus Mons. More recently, the Mars Reconnaissance Orbiter found a massive sheet of ice in the Utopia Planitia area but that's a rather higher latitude, not in the equatorial regions.

So, in short, there is a fair bit of indirect evidence that now suggests that the subsurface ice may be more widespread than just in the higher latitudes. If that is so, it is hard to tell how much there is. The idea that there is ice and possibly even occasionally liquid water in Hellas basin is perhaps the surest bet of all these ideas. After all it's one of the best locations for stable ice covered in debris in equatorial regions, because of the higher atmospheric pressure, and even water could survive for a while there, though close to its boiling point.

Ice covered lakes habitable for thousands of years after large impacts

When comet Siding Spring was discovered in 2013, before they knew its trajectory well, there was a small chance that it could hit Mars. Calculations showed it could create a crater of many kilometers in diameter and perhaps a couple of kilometers deep. If a comet like that hit the martian polar regions or higher latitudes, away from the equator, it would create a temporary lake, which life could survive in.

Artist's impression of Mars as seen from comet Siding Spring approaching the planet on 9th October 2014. It missed, by less than half the distance to our Moon. But sometimes comets will hit the Mars ice caps or higher latitudes. If that happens, it will create lakes and hydrothermal systems that last for thousands of years.

These lakes can last for a surprisingly long time, insulated by the ice and heated from below by the rock. The models suggest that large craters of 100 - 200 km in diameter in the early solar system would have made lakes that stayed liquid for as long as one to ten million years. This happens even in cold conditions, so it is not limited to early Mars. A present day comet a few kilometers in diameter could form a crater 30 - 50 km in diameter and an underground hydrothermal system that remains liquid for thousands of years. The lake is kept heated by the melted rock from the initial impact in hydrothermal systems fed by water from deep underground.

Also, there's another way to keep water liquid. Any ice deep enough below the surface, only 100 meters deep, can actually stay liquid indefinitely if covered by an insulating layer of gravel. There'd be enough heat from below, just from the heat of Mars itself and enough insulation above from the gravel, to keep the water permanently liquid. See section 2.2.3 of Niton Renno's article. This is also one theory for the Martian "dry gullies" that they formed through liquid water suddenly flowing out of a subsurface aquifer like this. This was the most popular theory for them at one point, though there are other explanations for them now.

It's much harder to keep water liquid below ice, since rock is much more insulating than ice. It's especially hard for water to form below an ice sheet. If the ice cap was four to six kilometers deep, then you'd expect the base of it to be liquid water, melted from below just through the heat of Mars itself. Though Mars does have ice at both poles, its ice sheets aren't quite as deep as that. But it could still have liquid water at the base of its ice sheets, if there's localized geothermal heating from below.

Also, if a lake formed, originally by geothermal melting or a meteorite impact, it's much easier to keep the lake liquid than it was to melt the water in the first place. In one model, then if a lake forms at a depth of over 600 meters below the ice (originally open to the surface) then it can remain liquid indefinitely from the heat flux from below, even without local geothermal heating.

We'd be able to detect this water using ground penetrating radar because of the high radar contrast between water and ice or rock. MARSIS, the ground penetrating radar on ESA's Mars Express is our best instrument for the job. After several searches, it hasn't found anything yet. See page 191 of this paper. Their resolution isn't that great, however, around a kilometer.

From the searches done to date, we can say with reasonable certainty that Mars doesn't seem to have an equivalent of our Lake Vostok (250 km by 50 km by 0.43 km deep) beneath its ice caps at present. It could however still have small subglacial lakes of up to a kilometer or so in diameter. They were looking for water liquid through geothermal heating, but their search would surely have found impact lakes too.

So, Mars doesn't seem to have any large lakes created from impacts just now. Nor does it have any major lakes formed through geothermal activity below glaciers or ice caps, though it could have smaller lakes.

However, we do have good evidence of past subglacial lakes on Mars, caused by volcanic activity. One of these is rather recent on the geological timescale, backing up the research suggesting that such lakes could form even today.

Ice covered lakes from volcanic activity

There is evidence that volcanism formed several lakes 210 million years ago on one of the flanks of Arsia Mons. That's relatively recent in geological terms, so recent that there seems no reason why it couldn't happen again today. It probably formed two lakes with around 40 cubic kilometers of water each, and a third one of 20 cubic kilometers of water. They probably stayed liquid for hundreds, or even thousands of years.

By comparison, Loch Ness, the largest fresh water lake in the UK has a volume of 7.5 cubic kilometers. The smallest of these three lakes would be similar in volume to the combined volume of the seven largest lakes in the UK: Loch Ness, Lough Neagh, Loch Lomond Loch Morar, Loch Tay, Loewr Lough Erne, and Low Awe. So, though it's nothing compared to the largest fresh water lakes in the world, which have volumes of tens of thousands of cubic kilometers, you are talking about a seriously large volume of habitable water here all the same.

This image shows scratch lines on the flanks of Arsia Mons which must have been made by a glacier. The lead author Kathleen Scanlon, at the time a doctoral student at Brown university, also found evidence of "pillow-like lava". That means, rounded globules of rock which are signs of lava melting under water, or beneath a glacier at high pressure. This type of lava is quite common on the sea floor on Earth.

If Mars has present day life capable of living in such habitats, then there must be a good chance that it proliferated when these lakes formed. I wonder if perhaps there might still be signs of it on the flanks of Arsia Mons today? There might even be ice still preserved from those times as remnants of glaciers long buried as this paper suggests.

Possibility of geological hot spots in present day Mars

There is clear evidence that Mars is not yet geologically inactive

• Crater counts in the Tharsis region showing some regions are as young as tens of millions of years old (detailed paper here)..
• Small scale volcanic features associated with some of the volcanoes on Mars including eruptions on Olympus Mons as recently as two million years ago. There's also evidence of ice near its summit, perhaps even present today, covered by dust. So as recently as a few million years ago, Olympus Mons had ice and hot spots in close proximity, which strongly suggests it had liquid water as well.
• Phoenix's carbon and oxygen isotope measurements of the Mars atmosphere, suggesting release of carbon dioxide to the atmosphere in the recent geological past. That probably is a sign of volcanic activity. It also detected evidence of exchange with liquid water which must have had connections with the surface too.

Mars might even have present day volcanism, releasing gases to the atmosphere if not lava. NOMAD on ExoMars' trace gas orbiter will be looking for geological as well as biologically relevant molecules in the atmosphere. It's targeting carbon, oxygen, sulfur and nitrogen atoms and carbon-hydrogen bonds.

Probably Mars also has magma plumes deep underground, at least, to explain this recent volcanism. Also, given that there has been activity on Olympus Mons as recently as two million years ago, it seems very unlikely that all activity has stopped permanently. We should see more again in the future, millions of years into the future at least. The main question is, is there any activity right now.

Mars Global Surveyor (using TES) and Mars Odyssey (using THEMIS) have searched most of the surface of Mars for hot spots. So far they haven't found any. If Mars does have these geological hot spots, they could melt the subsurface ice. The water would need to trapped under overlying deposits to keep it liquid. Perhaps the water could then come to the surface from time to time. If so that could explain why some of the Martian hillsides have warm seasonal flows (see below), and others, apparently identical, don't. Perhaps the ones with RSLs have geological hot spots beneath them - though that's just one hypothesis for the warm seasonal flows. We will look into this in more detail below.

It's not too surprising that these hot spots are hard to spot from orbit as we can't measure the heat below the surface directly. We can only measure the heat radiated from the surface. That also leads to a rather intriguing possibility that the hot spots could be hidden from view by ice.

Life in ice towers hiding volcanic vents

So, this is another suggestion, that we could find habitats on Mars inside ice fumaroles. It's a nice idea, and perhaps ice fumaroles do form on Mars from time to time. So far we haven't found any on present day Mars. But it may well be worth keeping a look out for them, as it would be a very interesting habitat if we find one, or one of them starts to form, around a volcanic vent on Mars. If Mars does have any volcanic vents which vent water rich gases through a fumarole, they are likely to form ice towers like this, as happens in Antarctica.

Let's look at the idea in some more detail. This photo shows an ice fumarole - an ice tower that forms around a vent of volcanic gases in the extremely cold conditions right near the top of Mount Erebus in Antarctica.

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One of the numerous Ice Fumaroles near the summit of Mount Erebus in Antarctica. If these also occur on Mars, they could provide a habitat for life, and would be extremely hard to spot from orbit due to the low external temperatures. Image credit Mount Erebus Volcano Observatory

For more photos of ice fumaroles see "Ice Towers and Caves of Mount Erebus",

They were originally discovered by the Antarctic explorer Shackleton during his 1908 Nimrod expedition, when he and a few others set out to climb Mount Erebus.

Photograph from Shackleton's Mount Erebus expedition with a fumarole in the background

He described them like this.

"The ice fumaroles are specially remarkable. About fifty of these were visible to us on the track which we followed to and from the crater, and doubtless there were numbers that we did not see. These unique ice-mounds have resulted from the condensation of vapour around the orifices of the fumaroles. It is only under conditions of very low temperature that such structures could exist. No structures like them are known in any other part of the world."

Ice caves form below the fumaroles, and these are especially interesting as a habitat for life.

Entrance to Warren Cave on Mount Erebus. Credit Brian Hasebe. Volcanically heated, the temperatures inside their three study sites were 32, 52 and 64 degrees Fahrenheit (2,11 and 18 degrees Celsius), far warmer than the surroundings.

These ice caves on Erebus are of especial interest for astrobiology, as analogues for habitats outside of Earth, because they are so biologically isolated. Most surface caves are influenced by human activities, or by organics from the surface brought in by animals (e.g. bats) or ground water. These caves at Erebus. are high altitude, yet accessible for study. There is almost no chance of them being affected by photosynthetic based organics, or of animals in a food chain based on photosynthetic life. Also there is no overlying soil to wash down into them.

As described in this paper, these ice towers eventually collapse and then rebuild themselves, but though temporary features, they persist for decades. The air inside has 80% to 100% humidity, and up to 3% CO₂, and some CO and H₂, but almost no CH₄ or H₂S. Many of the caves are completely dark, so can't support photosynthesis. Organics can only come from the atmosphere, or from ice algae that grow on the surface in summer, which may eventually find their way into the caves through burial and melting. As a result most micro-organisms there are chemolithoautotrophic i.e. microbes that get all of their energy from chemical reactions with the rocks. They don't depend on any other lifeforms to survive. They survive using CO₂ fixation and some may use CO oxidization for their metabolism. The main types of microbe found there are Chloroflexi and Acidobacteria.

This makes them very interesting as an analogue for Mars habitats. If Mars is currently geologically active, then in such cold conditions, it may well have ice fumaroles around its vents, and if so they would be only a few degrees higher in temperature than the surrounding landscape and hard to spot from orbit. We haven't found these yet. The closest we have got so far is that the silica deposits in Home Plate which Spirit found, might have been formed by ancient fumaroles on Mars, (not necessarily ice fumaroles) though they could also have been formed by hot springs or geysers.

This article Martian Hot Spots in NASA's Astrobiology magazine presents Hoffman's ideas. He explains that ice fumaroles on Mars could be up to 30 meters tall in its lower gravity and 10 to 30 meters in diameter, circular or oval in shape. So, potentially these things could grow to be huge on Mars, as high as a nine story high skyscraper, and potentially some of them could be as wide as they are high.

He suggests searching for them on Mars from orbit, and he wondered if some temperature anomalies in Hellas Basin could be ice fumaroles. They wouldn't need to be in polar regions because the fumaroles themselves would bring large quantities of water vapour to the surface to keep replenishing the ice towers as they sublime away in the thing Mars atmosphere. They might be quite easy to spot as white circles or ovals, probably in permanently shadowed regions, and they would be slightly warmer than their surroundings. This shows one of his candidates.

Daytime infrared from Odyssey IR

Anomalous warmth in infrared at night as well on all nine infrared bands, so not a chemical signature.

That candidate is in Hellas Planitia and is from 2003. Despite a search of high resolution visual images they were unable to find anything visual corresponding to them, they were only visible in infrared. But it shows the sort of thing they would be looking for. Lots of small dots around 10-30 meters in diameter each, clustered around a potential fracture. For details see their paper.

The idea is that just as on Earth, volcanic action could bring water vapour and other gases from below. The water vapour, as in Antarctica, would freeze out to form these ice towers. If these environments do occur on Mars, they would provide a warm environment, high water vapor saturation, and some UV shielding. The ones we have on Earth don't have significant amounts of liquid water. However, as they have close to 100% humidity inside, that doesn't matter. They sustain microbial communities of oligotrophs, i.e. micro-organisms that survive in environments that are very poor in nutrients. The same could be true of Mars.

Though we haven't found ice fumaroles on Mars yet, we have found recently formed rootless cones, which are the results of explosive contact of lava with water or ice. This shows that ice (or water) and lava were in close proximity as recently as around ten million years ago.

This shows rootless cones on Mars (to the left) and in Iceland. They are the locations of small explosions of steam, when lava surges over the surface over water or ice. These rootless cones on Mars formed around ten million years ago which shows that Mars has had ice and lava in close proximity very recently. They range in diameter from 20 meters to 300 meters.

So, could there be other ways that volcanic processes on Mars produce habitats by interacting with ice, such as the ice fumaroles? From this 2007 paper:

Hoffman and Kyle suggested the ice towers of Mt. Erebus as analogues of biological refuges on Mars. They combined the idea of still existing near surface ice deposits with the assumption that there is still some localized volcanic activity on Mars today.

There are several examples from Mars that show a direct interaction between lava and ice in the geological history of Mars. The most obvious cases are the rootless cones seen in the northern lowlands. HRSC images show direct and violent interaction in the relatively recent geological history, for example at the scarps of Olympus Mons. Mars today is in relatively dormant phase, and any interactions which might be occurring today are presumably on a much less dynamic scale. Nevertheless, they may be driving local hydrothermal systems. Studying the geothermal processes in the first few tens to hundreds of meters below the surface of Mars today might thus uncover a wide variety of new habitats where biological activity may survive on this cold and dry planet.

For more about this topic see Volcano-Ice Interaction as a Microbial Habitat on Earth and Mars. These ice fumaroles would be of great interest, but of course, being open to the surface, would easily be contaminated by Earth life from surface explorers or brought in to them through dust from the Martian storms.

So far we've been looking at habitats deep below the surface of Mars, though perhaps connected to the surface. But what about habitats on the surface itself? They would make planetary protection even more of an issue, so it's important to look at the possibility. First we need to look at the question, is surface life possible there at all. Just a decade ago, most scientists (with the exception of Gilbert Levin) would have answered with a resounding "No". But that's all changed.

Habitability of the Mars surface (top few centimeters)

First, a short summary of the latest conclusions about the habitability of the Mars surface and near surface.

?The first thing Earth life needs is liquid water, and water that's not too cold or too salty. It can't survive in very acidic or very alkaline conditions either - but those limits are quite extreme and not likely on Mars. The UV radiation is shielded by a thin layer of dust or a shadow. The ionizing radiation is well within the tolerance limits for life as we will see. Perchlorates are not a problem, indeed are a food source for many microbes and are now considered to be a habitability factor for Mars.

It doesn't need organics for photosynthetic life, as photosynthetic microbes can create them from the atmosphere and water. It's short of organics for other types of life (heterotrophs) but there are sources for instance from organics. It does need nitrogen, which is likely to be the element most in short supply on Mars. But there are some nitrates on the surface.

Anywhere where Earth life can survive is a potential habitat for Mars life. In the other direction though, Mars life may have adapted to tolerate colder or saltier conditions than Earth life.

So, let's look at all of this a bit more closely and see which factors limit habitability of the Mars surface, and which factors we can ignore as not limiting in any way.

Temperature limits for Earth life

I'll summarize the 2014 paper A New Analysis of Mars ‘‘Special Regions’’ The "special regions" by definition are places where present day life may be possible on Mars. Spacecraft have to be especially thoroughly sterilized to enter them, so it's important to study them carefully. They did a survey of the literature to date, and these are their conclusions:

• The usually stated limit for cell division is - 20 °C with it confirmed down to - 18 °C but genome replication (which probably means cell division) occurs down to -20 °C, which is the temperature for our freezers
  
  However, it is hard to study this because cell division proceeds so slowly at such low temperatures, with a doubling time as long as 50 days already at - 18 °C, and it gets longer, the colder the temperatures. It's especially hard to study this because it's so cold, making the working conditions tricky. Very slow cell division may be possible down to - 32 °C or lower, but there are no experiments to prove this yet.

In more detail: experiments with yeast show doubling at - 18 °C, so it's confirmed at that temperature. One study showed genome replication in permafrost at - 20 °C which is "highly suggestive of cell division". Another experiment show ammonia oxidation at - 32 °C sustained for 300 days, the duration of the experiment. Since cell division would be so very slow at those temperatures, then, so far, it's impossible to be sure whether this is just maintenance metabolism, or whether it actually did support very slow cell division.

Some substances can help microbes be active at lower temperatures, which could reduce the limits even further. These are known as chaotropic agents, and include ethanol, urea, butanol etc. They work by disrupting the hydrogen bonding of water molecules with each other. There are many chemicals on Mars which could act as chaotropic agents and so reduce the minimum temperatures for cell division, including MgCl₂, CaCl₂, FeCl₃, FeCl₂, FeCl, LiCl, chlorate, and perchlorate salts

Typically, these chaotropic agents reduce the lowest temperatures for cell division by 10 °C or 20 °C for many microbe species. However the authors couldn't find any experiments testing these agents at the very low temperatures that would be needed to reduce the lowest temperature limit for cell division for life.

Salinity limits, and more generally, water activity limits for Earth life

Salinity is measured as "water activity" which is a measure of how available the water is to Earth life, which can also be used to measure other solutions (e.g. honey). Salty water has a lower partial vapor pressure of water which means that there is less water in the atmosphere above salt water than above fresh water in equilibrium conditions. Water activity is defined simply as the ratio of the partial water vapour pressure of the solution divided by the partial water pressure of pure water. So, fresh water has water activity 1.

Here the limit is much clearer. There is no evidence at all of cell division or metabolic activity in solutions with a water activity level below 0.6.

In more detail: Honey has a low water activity level of 0.6. That's why honey doesn't spoil - you don't need to keep honey in a fridge, because its water activity level is so low that though microbes would find plenty to eat, and though there is plenty of water there in the honey, the water is not available to the microbes because of the low water activity level.

The record at the moment is 0.605 is for a fungus Xeromyces bisporus which was discovered in 1968 in a study of spoilage in prunes. It can divide at these low water activity levels, so can germinate, but needs higher levels of water to create fungal spores through asexual sporulation and even more for sexual sporulation. Most microbes can't handle a water activity level below 0.755. However, there have been a number of other reports of microbes that can manage lower levels similar to the bisporus fungus. In a recent 2014 survey paper of the literature on the subject "Multiplication of microbes below 0.690 water activity: implications for terrestrial and extraterrestrial life", the authors came to the conclusion that the best consensus at present is that the lowest level of water activity needed for cell division is about 0.605, and that some halophiles (salt living microbes) are able to tolerate such low levels.

They remark on the difference between the situation for water activity and the situation for temperatures, that there's this sharp cut off for water activity, but much better evidence of microbes able to tolerate temperatures below the usually cited -20°C.

Nilton Renno's droplets that form where salt touches ice. Why did he call a salty droplet on Mars "a swimming pool for a bacteria"?

This is perhaps one of the most striking discoveries in recent years because of its implications for habitability of Mars. Nilton Renno found that liquid water can form very quickly on salt / ice interfaces. Within a few tens of minutes in Mars simulation experiments.

Erik Fischer, doctoral student at University of Michigan, sets up a Mars Atmospheric Chamber on June 18, 2014. These experiments showed that tiny "swimming pools for bacteria" can form readily on Mars wherever there is ice and salt in contact.

This is striking as it could open large areas of Mars up as potential sites for microhabitats that life could exploit. The professor says

"If we have ice, and then the salt on top of the ice, in a few tens of minutes liquid water forms. Our measurements clearly indicate that. And it's really a proof that liquid water forms at the conditions of the Phoenix landing site when this salt is in contact with the ice.

"Based on the results of our experiment, we expect this soft ice that can liquefy perhaps a few days per year, perhaps a few hours a day, almost anywhere on Mars. So going from mid latitudes all the way to the polar regions.

" This is a small amount of liquid water. But for a bacteria, that would be a huge swimming pool - a little droplet of water is a huge amount of water for a bacteria. So, a small amount of water is enough for you to be able to create conditions for Mars to be habitable today'. And we believe this is possible in the shallow subsurface, and even the surface of the Mars polar region for a few hours per day during the spring."

(transcript from 1:48 onwards)

(click to watch on YouTube)

That's Nilton Renno, who lead the team of researchers. See also Martian salts must touch ice to make liquid water, study shows . He is a mainstream researcher in the field - a distinguished professor of atmospheric, oceanic and space sciences at Michigan University. For instance, amongst many honours, he received the 2013 NASA Group Achievement Award as member of the Curiosity Rover " for exceptional achievement defining the REMS scientific goals and requirements, developing the instrument suite and investigation, and operating REMS successfully on Mars" and has written many papers on topics such as possible habitats on the present day Mars surface.

His announcement sparked headlines in many papers such as:

• 'Is there life on Mars?': Water can and does exist on the planet says new research
• ‘Swimming pool for bacteria’: There could be life on Mars today - new study
• Does ice mean life on Mars?

This helps to explain how the droplets of liquid formed on the legs of Phoenix lander. At least, that's the leading hypothesis, that these are droplets. They look and behave like droplets, including coalescing and falling off. But sadly, Phoenix had no way to analyse them to prove what they were.

Possible droplets on the legs of the Phoenix lander - they appeared to merge and sometimes fall off. In this example the rightmost of the two droplets - coloured green in this black and white image just to pick them out - grows apparently taking up the water from its companion to the left.

If you remember news stories and press releases from a decade ago, you might wonder what has changed. Until around 2008, when Phoenix took those photos, most scientists believed that the surface of Mars was uninhabitable. But now you can hear them talk enthusiastically about the possibilities for life there. Hear for instance Nilton Renno talking about a discovery of a new way of creating habitats with liquid water on Mars on the interface between salts and ice

So, you might wonder what has changed, if you have read the many articles on this subject from about six years ago which seemed to show that present day life on Mars is impossible. For instance, Encyclopedia Britannica now says

"It could be argued that the best strategy is to look for fossil remains from the early period in Mars’s history when conditions were more Earth-like. But the Martian meteorite debate and disagreements about early terrestrial life point to the difficulty of finding compelling evidence of microbial fossil life. Alternatively, it could be argued that the best strategy is to look for present-day life in niches, such as warm volcanic regions or the intermittent flows of what may be briny water, in the hope that life, if it ever started on Mars, would survive where conditions were hospitable.""

From The question of life on Mars (Encyclopedia Britannica)

So it might help to explain why we no longer think that this rules out indigenous life on the Mars surface. The planetary protection report A New Analysis of Mars ‘‘Special Regions’’ covers UV radiation, and ionizing radiation. Their findings in brief are that ionizing radiation from solar storms and galactic cosmic radiation have negligible effect on habitability. UV is severely limiting but easily blocked by a thin layer of dust. But let's look at this in more detail.

Ionizing radiation

Before 2008 - the idea was that if there are any spores on or near the surface, they must have been dormant for a long time, ever since the last time Mars had a slightly thicker atmosphere. They would have to have been dormant for millions of years. This seemed to make perfect sense since the Mars atmosphere is so thin that any water on the surface would either be above boiling point already or close to it as soon as it melts. Even salty water would be close to boiling point. So then they worked out that the levels of ionizing radiation from cosmic radiation in the surface layers could easily destroy even the most radioresistant microbes in a million years. So (they deduced), any remaining viable life would have to be well below the surface.

It is not easy to simulate Mars condition in experiments on Earth, so it was quite a surprise to researchers when Phoenix found evidence for liquid drops forming in Mars surface conditions. They then did the experiments to try to duplicate the Phoenix observations, and found that liquid water is possible in those conditions after all, and what's more, if you balance the conditions carefully, the liquid doesn't have to be too salty for life to use it. The researchers now had to redo all their calculations.

There are some places in our solar system that are made totally uninhabitable because of radiation. The surface of Europa is laced with sterilizing radiation. Jupiter has such strong ionizing radiation that humans would only last hours in vicinity of Europa before they died of radiation poisoning. Even highly radioresistant microbes wouldn't last long on the surface of Europa

But the surface of Mars - though it gets far more radiation than the surface of Earth - gets roughly the same amount of radiation every year as the interior of the ISS.

The surface of Mars, with its thin atmosphere to shield from radiation.

Receives about the same amount of cosmic radiation per year as the interior of the ISS (above the Earth's atmosphere but shielded by our ionosphere )

Curiosity measured radiation equivalent to an estimated 76 mGy per year at the surface - or 0.076 Grays per year. Humans would find such hazardous long term - increases your risk of getting cancer. Also, there is not much to do to protect from it except stay buried under meters of radiation shielding. Spacesuits and rovers would be no protection.

But microbes don't get cancer. Their DNA gets damaged, yes, but the most radioresistant microbes have remarkable abilities to heal the damage to their own DNA, in real time, with no need even to reproduce. They have structures that keep the DNA fragments in proximity to each other when they are damaged. When they are able to metabolize again, their cell machinery can join those fragments together to repair the DNA. They can do this even in semi dormancy. They can wake up for a few hours, repair their DNA damage for the last few thousand years, and then go back to "sleep" again.

Take Chroococcidiopsis for example, one of the microbes we have on Earth best able to survive in Mars surface conditions. Experimenters have found that it can repair, 2.5 kGy of damage within 3 hours given the opportunity to wake up for a few hours and metabolize. Here a kGy is a thousand Grays, and a mGy is a thousandth of a gray. So 2.5 kGy corresponds to 2500/0.076 or over 32,000 years of radiation on the surface of Mars. So if it wakes up for a few hours every year, it will have no trouble at all keeping its DNA repaired. And - that's nowhere near a lethal dose. If able to wake up for 24 hours, it can repair 64,000 years worth of damage.

The most remarkable thing about it is that these are microbes from Earth that have never encountered such high levels of cosmic radiation - at least as far as we know. Perhaps this ability is a side effect of its evolution of mechanisms to stop the DNA from getting broken up when it dries out (resistance to desiccation)? At least that's the most usual explanation. Perhaps Mars microbes, evolved in conditions of high levels of radiation may be far more radioresistant even than this.

The most radioresistant microbe currently known is Thermococcus gammatolerans, which lives in conditions just about as as sheltered from ionizing radiation as you can imagine, in hydrothermal vents.

Thermococcus gammatolerans - an obligate aerobe from hydrothermal vents, the most radioresistant organism known, able to withstand 30 kGy of gamma radiation, and still reproduce. That's about 400 thousand years (30,000/0.076) worth of surface radiation on Mars at the radiation levels detected by Curiosity during the current solar maximum - possibly it could survive surface radiation for longer than that when you include periods of solar minimum.

It's not a likely candidate for the Mars surface as it lives at the bottom of the sea and requires high temperatures. But - its radioresistance seems to be a side effect, it certainly doesn't encounter cosmic radiation down there. So a Mars microbe, adapted through billions of years of evolution to the ionizing conditions on the surface of Mars may well be as radioresistant as this, or more so.

The findings on ionizing radiation in the 2014 report: A New Analysis of Mars ‘‘Special Regions’’ were (Finding 3-8):

• From MSL RAD measurements, ionizing radiation from GCRs at Mars is so low as to be negligible. Intermittent SPEs can increase the atmospheric ionization down to ground level and increase the total dose, but these events are sporadic and last at most a few (2–5) days. These facts are not used to distinguish Special Regions on Mars.

Here a SPE is a Solar Proton Event (solar storm), and a GCR is a Galactic Cosmic Ray. So why then was it sterilizing for organisms dormant for millions of years, when it has negligible effect on active life?

Ionizing radiation is exponential in its effects. So though it has negligible effect on timescales of decades, it is devastating on timescales of hundreds of thousands of years.

Example, suppose that a dormant population is halved in ten thousand years by ionizing radiation (within the range of possibility for hardy ionization resistant life). That would have negligible effect on life able to reproduce every year.

Yet if a population of such microbes is kept dormant for 500,000 years you would have only 1 in 2⁵⁰ or around 1 in 10¹⁵ left, or one cell remaining out of a quadrillion.

After a million years only one viable cell will remain out of an original population of a nonillion (10³⁰).

So ionizing radiation on Mars is devastating for life that remains dormant for millions of years but has no noticeable effect on life that can either replicate or "wake up" for a few hours to repair its own DNA as many ionization resistant microbes can do.

So that's how it works with ionizing radiation. Dormant populations can be completely sterilized over millions of years. Yet the radiation causes no problems at all, so long as they can revive enough to repair their DNA at least once every few millennia, which only takes a few hours. Life in these Mars habitats, if they exist, could revive every year. So ionizing radiation is not a limiting factor any more, for these new ideas about possible present day surface habitats on Mars.

Ultraviolet radiation

The levels of UV on the surface of Mars are also very high and would destroy most microbes within seconds. So scientists used to think that there was almost no chance of any life able to live exposed to the UV on Mars. However, first of all UV light is easily shielded. It differs in that way from cosmic radiation, which goes through meters of rock without noticing it. UV light is like ordinary light - it can be blocked by just about anything that casts a shadow. A mm or so of soil will block it. Also if a microbe is in the shadow of a rock, or pebble, it is shielded. Even if it is in a tiny microscopic crevice in a grain of Martian dust, it is shielded, especially since the Martian dust contains iron oxide, which is rather effective at shielding out UV light. Also in a microbial mat, microbes can be shielded by the cells of their dead siblings.

Then, even a cell can shield itself from UV light using pigments. It turns out that some microbes can last a lot longer than a few seconds exposed to UV light. Indeed, it also turns out that our pal Chroococcidiopsis has adapted to shield itself from UV. Unlike radiation, we get significant amounts of UV light in cold deserts and high mountains. Though they are nowhere near the Mars levels, there is enough UV so that cyanobacteria have evolved some protection from it This UV resistance is so good, that when the German aerospace company DLR (sort of their equivalent of NASA) researched into this, they found that our friend Chroococcidiopsis, could survive partial shade in conditions on the surface of Mars. And to their surprise, it doesn't just survive - in their Mars simulation chambers, they found that these microbes could metabolize and photosynthesize, slowly, on the simulated Mars surface. And it wasn't just microbes. - they found that some lichens, also have developed UV resistance, with various specialized pigments to block it out.

These are still early stage experiments - but it looks promising that you might get photosynthesizing lifeforms actually on the surface of Mars using the sun's light for energy. Either in partial shade, or shaded by a thin layer of dust - or protected by transparent rock such as quartz or gypsum (which has been found on Mars). In the Atacama desert, in regions of high UV light, enough to sterilize the surfaces of rocks, then 1 mm of gypsum is enough to protect from UV light. So, UV light also is a challenge, but some life will have no problems with it.

The findings in the 2014 report: A New Analysis of Mars ‘‘Special Regions’’ were (Finding 3-7):

• The martian UV radiation environment is rapidly lethal to unshielded microbes but can be attenuated by global dust storms and shielded completely by < 1 mm of regolith or by other organisms.

After ionizing radiation and UV light, the next thing you are likely to say is "What about the perchlorates, don't they make the Mars surface uninhabitable even to microbes?"

Perchlorates on Mars

This is another thing you often hear, that Mars surface life is impossible because of the perchlorates. Now, for humans, the perchlorates are quite nasty. The Mars dust has these chemicals in it, at levels 10,000 times higher than on Earth. This is harmful to humans as perchlorates prevent us from absorbing iodine. We need iodine for our thyroid glands, which regulate our metabolism. They may also have other effects on us, see this chemical might make Mars more dangerous. Also, to make things worse, the ionizing radiation may decompose these perchlorates into the reactive chlorites (ClO₂) and hypochlorites (ClO) which have more serious and immediate effects on us

"Ionizing radiation can decompose small quantities ofClO4-into other Cl-oxyanions, such as ClO₂-and ClO-(Quinnet al. 2013), which are much more reactive and can be the cause of other health concerns such as respiratory difficulties, headaches, skin burns, loss of consciousness and vomiting "

(quote from page 3 of this paper). For more on this, see Perchlorate on Mars: A chemical hazard and a resource for humans.

So, it's nasty stuff for humans for sure. But microbes don't have thyroid glands and don't get headaches or lose consciousness or vomit. Also perchlorates are more potent at the warmer temperatures of a human body. As it turns out some microbes actually eat perchlorates. It is nutritious food for them. So it doesn't by any means rule out microbial life on Mars. Indeed it figures on both sides of the ledger as it were, as a habitability limiting factor for some microbes, and a source of food for others.

Even when Phoenix first discovered perchlorates on Mars, scientists were clear that they are not a problem for native Mars life, for instance in this article in Scientific American: NASA Says Perchlorate Does Not Rule Out Life on Mars - Unexpected chemical in Martian soil is a food source for some Earthly microbes. Before the announcement there were rumours that they had proven that the Mars surface was uninhabitable forcing them to make an early statement to quell those rumours.

Cassie Conley, current planetary protection officer for Mars, put it like this (as reported in the NASA Astrobiology magazine)

"The salts known as perchlorates that lower the freezing temperature of water at the RSL's, keeping it liquid, can be consumed by some Earth microbes. “The environment on Mars potentially is basically one giant dinner plate for Earth organisms,” Dr. Conley said."

Perchlorates irradiated with UV light to create sterilizing chlorites and hypochlorites

This is an experiment reported in Nature in 2017 as "Perchlorates on Mars enhance the bactericidal effects of UV light" by Jennifer Wadsworth and Charles Cockell (her supervisor for her research). It got widely reported with headlines such as Mars covered in toxic chemicals that can wipe out living organisms, tests reveal . As is often the case, the experiment wasn't as dramatic as those headlines suggest, but it was interesting, well worth mentioning in a section here. It suggested that the UV light irradiating brines on Mars could produce toxic products of perchlorates - the same chlorites and hypochlorites mentioned in the previous section (Perchlorates on Mars), and that these can be bactericidal.

The experimenters studied the effect of UV light (not cosmic radiation as discussed in the last section) on the perchlorates. Also they looked at its effects on perchlorates in solution - as they might be, for instance, in the RSLs and other possible liquid brine layers. They weren't testing the effects of UV light on dry dust.

What they found is that though the perchlorates are not normally bactericidal, this changes when they are irradiated with UV light simulating the UV flux on Mars. Of course UV light by itself is also bactericidal.

Some details of the experiments: They tested Bacillus Subtilis, a microbe which is a common spacecraft contaminant. They mixed the cells in with a nutrient solution they call M9 which contained magnesium sulfate, glucose and calcium chloride dissolved in distilled water. They then added perchlorates, or hydrogen peroxide, or iron oxides, and mixtures of all three, and irradiated it with UVC monochromatic light, and in other experiments, with polychromatic light for a more realistic UV flux similar to that for Mars. They also repeated the experiments with silica disks soaked in the medium (after addition of the perchlorates) which they used as an analogue for microbes beneath the surface of rocks on Mars.

They found that vegetative cells (not spores) of Bacillus Subtilis were completely sterilized (no viable cells left) within 60 seconds when irradiated by UV at 25 °C. When they added perchlorates, they were sterilized much more quickly, within 30 seconds. When they used the silica disks (which provide some protection from the UV radiation), then 60 seconds of irradiation lead to a 9.1 fold drop in viability after 60 seconds compared with a 2 fold drop without the perchlorates. Those experiments were all done in room temperature conditions, of 25 °C. They then tried colder temperatures of 4 °C and found that the samples were completely sterilized after three minutes, with or without the perchlorates, but the sample with the perchlorates had many fewer viable cells after two minutes (11.4 times fewer).

All of this demonstrated that the perchlorates enhanced the bactericidal effects of the UV light. So how did it do it? They found evidence of hypochlorite and chlorite in the absorption spectrum of the UV irradiated sample, and conjectured that this might be what caused the bactericidal properties. They found that adding hematite (a form of iron oxide) on its own increased viability (as you'd expect, as it shields from UV) but that hydrogen peroxide (which is also present on Mars) decreased viability further - and adding all three decreased viability most of all.

Many of the news story write-ups of this research said that this meant the surface of present day Mars os uninhabitable, and that we will have to search at least three feet below the surface to find life. I'm not sure where that three feet figure came from as it isn't mentioned in the paper as far as I can see. Three millimeters would be plenty to block out UV light. The primary author of the paper, Jennifer Wadsworth, was more circumspect, when interviewed, as reported in the Smithsonian magazine:

"It's also possible that hypothetical Martian bacteria could be much tougher than the common Bacillus subtilis. On Earth, researchers have found all types of extremophile organisms with the ability to survive under intense heat and pressure, in the presence of acid, without water and even inside rocks. “Life can survive very extreme environments,” Wadsworth tells Fecth. “The bacterial model we tested wasn’t an extremophile so it’s not out of the question that hardier life forms would find a way to survive.”

If I can make a few more observations

• This is for vegetative cells in a liquid water media. So it wouldn't necessarily impact on spores embedded in the dust - spores are protected by a coat and are far more resistant to oxidizing agents, bactericidal agents chlorites, hypochlorites etc than vegetative cells. They are also more UV resistant. Some spores can withstand many hours of surface UV radiation on Mars, including one strain still viable after 28 hours of direct UV radiation in Mars simulation surface conditions. See How could this work on Mars with dust storms and a globally connected environment? (above).
  
• It's also for liquid media irradiated with UV light. Many of the Mars brine habitats would be a few millimeters below the surface, inhabited by chemolithoautotrophic microbes. These get energy from the rocks and chemicals in the dust and regolith, and don't need sunlight at all. Some of them would use the perchlorates as a source of food, and they would not need to be irradiated by the UV light.
  
  Even if the UV light did create reactive chemicals, say, at the head of the RSLs, would these reactive chlorites and hypochlorites persist, as the thin film of brine flow down the forty five degree slopes beneath the surface of the Martian soil? Or would they react with other constituents of the brine at an early stage? A few millimeters of dust would be enough to shield out most or all of the UV light.
  
• Indeed, since UV light is sterilizing even without the perchlorates, then photosynthetic life would need to either filter it out by huddling in semi shade or sheltered by the iron oxide dust - or incorporate specialist UV absorbing pigments such as are used by many microbes and lichens on Earth adapted to cope with high levels of UV
  .
• Also UV light of course can be blocked almost completely by a shadow of a boulder, hill, in an overhang, crevice or cave, or indeed in the shadow of a spacecraft (leaving only a small amount of ambient light that reflects into the shadow).
  
  In the DLR experiments, then the lichens survived in semi shade. In Antarctica, lichens huddle in cracks and photosynthetic microbes live beneath the surface of the rocks for protection from UV light.
  
• The UV light could also be blocked by gypsum, as happens in the Atacama desert. As we saw in the Ultraviolet radiation (above), 1 mm of gypsum is enough to protect from UV light in the Atacama desert. Somewhat more would be needed on Mars of course.
  
• Also - what effect would this UV radiation of perchlorate solutions have, if any, on the lichens and cyanobacteria that take their water directly from the humidity of the air? They don't need to be in contact with perchlorates as they don't depend on perchlorates for water.
  
• Also even the 4 °C experiment involves temperatures far higher than are normally considered for Mars habitats for microbes. Often the discussion centers over whether the habitats would be warmer than the -20 °C that's usually thought to be the minimum temperature for cell division. Such high temperatures as 4 °C or higher could only be attained in the very surface layers, and by the time a brine reaches those temperatures in the thin atmosphere of Mars, then, depending on its composition, most often, the liquid in the brines has dried out completely.
  
• In their experiment they were careful not to add enough of the B. Subtilis to form a biofilm - as this could protect against the UV. Microbes on Mars could form biofilms which could protect the rest of the colony from both the UV and also the reactive chemicals formed from splitting the perchlorates, much as the coat of a spore protects microbes in spore form. The microbes on the surface, exposed directly to UV, would be killed but the dead microbes protect the ones beneath them.
  
• Then, also, as Wadsworth said, there are many microbes that are much more UV hardy than B. Subtilis - for instance the ones that withstand hours of UV radiation in semi shade in the Mars simulation conditions, taking in moisture from the atmosphere and photosynthesizing (see Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere (below) ).
  
• Microbes form communities in biofilms - the biofilms are more hardy than any individual species can be. Andrew Schuerger makes this point in a brief comment 47 minutes into this discussion at AbSciCon 2017 day 1. - he is noted for his research on biocidal factors on Mars, and he remarks that with 15 - 20 recognized biocidal factors on Mars, it's easier for a community of microbes to overcome them than individual microbes.
  
• Also there are many other salts on Mars such as the sulfates, that can create brine habitats when in contact with ice, or deliquesce, and take water out of the atmosphere.
  
• Some of the suggested microhabitats, as we will see later, can potentially have fresh liquid water at 0 °C, with no need for salts at all to keep it liquid, trapped beneath a solid state greenhouse of transparent blue ice (see Southern hemisphere flow-like features - these may involve fresh water! (below) . Also, the concentrations of perchlorates and the mix of other salts will vary across the surface of Mars.
  
• One possibility for Mars is life based on hydrogen peroxide and perchlorates within the cell in place of the chlorides used by all Earth life. Life like that based on a hydrogen peroxide biochemistry is not going to be affected by hydrogen peroxide clearly - and may well be able to cope with other reactive chemicals that modern Earth life can't tolerate. See. Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells (below).

So, those are a few things to think about.

It's interesting research for sure. It suggests that there are more challenges for life on Mars than we thought before the experiment. But I think it's going too far to say it means the surface of Mars has been proven to be inhospitable to Earth or native Mars life, or that any life has to be at least several feet below the surface. I think it's just another example of interesting research that got rather overhyped by the media, as happens so often in this topic area.

Let's now look at how perchlorates can help with the formation of liquid brines on Mars.

Perchlorates as a way to scavenge water from the atmosphere

The perchlorates are not just useful as an energy source for food. The salts lying on the Mars surface also extract water from the atmosphere and could be the basis for microhabitats for life. Many salts will take in water from a humid atmosphere, with no need for rain or surface water flow. But perchlorates are amongst the best at doing this.

(One of the slides for the NASA press conference Water flowing on present day Mars)

The Mars atmosphere reaches 100% relative humidity at night because it gets so very cold at night. Warm air holds more moisture than cold air (which is why clothes on a clothes line can dry) .So, though Mars has hardly any water vapour in its atmosphere, these huge night time drops in temperature raise the humidity so high that the thin atmosphere gets saturated at night, and Mars often has frosts even in the extremely dry equatorial regions of Mars, as the Viking landers found out.

Ice on Mars- Utopia Planitia, photo taken by Viking Lander 2 at its Utopia Planitia landing site on May 18, 1979,

These frosts formed every morning for about 100 days a year at the Viking location. Scientists believe dust particles in the atmosphere pick up bits of solid water. That combination is not heavy enough to settle to the ground on its own. However, carbon dioxide, which makes up 95 percent of the Martian atmosphere, then freezes and adheres to the particles and they become heavy enough to sink. Warmed by the Sun, the surface evaporates the carbon dioxide and returns it to the atmosphere, leaving behind the water and dust.

The ice seen in this picture, is extremely thin, perhaps no more than one-thousandth of an inch thick. These frosts form due to the 100% night time humidity, which may also make it possible for perchlorate salt mixtures to capture humidity from the atmosphere at night, through to the early morning. This process could occur almost anywhere on Mars where suitable mixtures of salts exist.

Perchlorate salts on the surface would take this humidity out of the atmosphere and form damp patches of liquid water at certain times of day. The best times to find this water depend on the site but typically it's around midnight and early morning.

So could this water be habitable?

(One of the slides for the NASA press conference Water flowing on present day Mars)

Well the temperature range is fine. Perchlorates actually can permit liquid brines on Mars at higher temperatures than for fresh water, though they also permit water at temperatures far too low for Earth life. A pool of perchlorate brine below 24 °C would be below boiling point even in the thin Martian atmosphere, but it would dry out pretty quickly, like a puddle on a hot sunny day on Earth. So don't expect puddles of hot salty water on Mars, but it's a little easier for warmer perchlorate rich water to be present there in daytime than fresh water (which is already boiling when it melts over much of Mars, and in low lying places like Hellas Basin is close to boiling point in the thin atmosphere)..

The problem is that in practice, perchlorate based brines on Mars are likely to be very cold, or very salty, or quite likely, both. The next couple of sections are a little techy, so if you want to skip them, just jump to "So deliquescing water is liquid in a wider range of conditions than you'd expect".

Mixing perchlorates with other salts lets them take up water from drier air than either separately

The ability of the salts to form liquid brines on Mars is improved hugely by the process of eutectic mixtures. The name comes from the Greek "ευ" (eu = easy) and "Τήξις" (tecsis = melting). If you have a mixture of two salts, for example, a mixture of chloride with perchlorate, then the mixture stays liquid at a lower temperature than each of the salts separately. The melting temperature is the "eutectic point". This phenomenon is related to the way antifreeze works, and the reason why salt keeps roads free from ice, even though the melting point of salt is far higher than that of water. See also Freezing-point depression.

The same thing happens with humidity too, in which case it is called a eutonic mixture, or a eutonic solution (when it has taken up enough water vapour to become liquid), and the relative humidity at which this happens is the eutonic point. A mix of salts is able to take up water from drier air (lower relative humidity) than either of the salts separately. Interestingly, it doesn't matter much what the actual percentages of the two salts are, so long as there is some of both in the mixture.

This diagram shows how it works - for a fictitious mixture A and B.

Here the graph shows the mixture of A and B along the bottom with 100% A to left and 100% B to right. The ERH or Deliquescing Relative Humidity is the humidity at which some of the mixture of the salts starts to take up water from the atmosphere. Below that, everything is solid. Above that point, if you have the optimal mix of A and B then the whole thing goes liquid. If not, you get a mix of liquid (L) with the solid, and then eventually as you increase humidity then the whole thing goes liquid. As you see, the perfect mix of salts means that it can all go liquid at a much lower humidity than for either salt separately.

You can also see from the diagram that even if you don't have the perfect mix, so long as you have some of each salt, then it will start to take up water as soon as it reaches the much lower Eutonic Relative Humidity (ERH), or "eutonic point". Depending on the mix of salts, then at the ERH you get either a mixture of liquid and salt as A + L or L+ B (L = Liquid). Either way you will get some liquid right away at the ERH.

The upper curved line shows the Deliquescing Relative Humidity (DRH) at which all of the mixture goes liquid. The point at which this happens depends on the mix of A and B.

So as the humidity is increased, for a given A / B mixture, first the lower horizontal line is reached, at which point some of the mixture of salts becomes liquid. This is known as the "eutonic relative humidity" - the point at which any mixture will start to take up some water vapour.

As humidity is raised further, more and more of the mixture becomes liquid. Eventually the upper, curved line is reached - and at that point, the entire mixture will be in its liquid phase.

Because of this eutonic mixture effect, then even if you only add a tiny amount of perchlorates to the less deliquescent chlorides, this is enough to reduce the minimum relative humidity needed to deliquesce hugely, right down to the eutonic relative humidity for the mixture. This is not only lower than the deliquescence relative humidity of the chlorides, it is also lower than the deliquescence relative humidity for the perchlorates as well.

You can also get similar eutonic mixtures of three or more different types of salts, which typically have even lower ERH than any of the mixtures of two salts. Salts on Mars could have a mixture of perchlorates, chlorates, sulfates, and chlorides and perhaps nitrates also if present, along with cations of sodium, potassium, calcium, and magnesium. So there are many possibilities to consider here.

Similarly if the axis is temperature - then as the temperature is raised, first part of the mixture will go liquid, at a temperature corresponding to the optimal mixture of the salts, and then when the upper curved line is reached, the entire mixture will be liquid.

After taking up water, perchlorates retain it for a long time even when very dry, or very cold

There's another effect that makes things even better for microbes on Mars. It's much harder for salt mixtures to lose water, than it is for them to take it up. So, as the air it gets more humid, the salt mixtures start to form liquid solutions at the eutonic point. But as it gets drier, stay liquid even when the humidity is reduced well below the eutonic point (this is known as delayed efflorescence). Similarly for eutectic freezing, the salty brine solutions can be supercooled well below the temperature at which they would normally freeze, staying liquid for a fair while below the eutectic point.

You get a eutectic also for freezing of a single salt in solution. If you have a mixture of salt and water then different mixtures will freeze at different temperatures. The eutectic is the optimal mix of water and salt with the lowest freezing temperature. As you freeze a mixture, then no matter what the original concentration, some of it will remain liquid down to the freezing point of the eutectic mixture. Then, as you freeze further below that temperature, you may find that the salt continues to remain liquid. The reason for this is that for a salt to come out of solution through nucleation, it has to form a new interface between the crystal surface and the liquid, which requires energy. Once the nucleation starts, then crystallization is rapid, but the nucleation can be delayed often for many hours.

Here is a table of some salts likely to be found on Mars, showing the eutectic temperature for each one (with the molar concentration for the optimal eutectic concentration in brackets) and the amount of supercooling below that temperature that they found with experiments (adapted from table 2 of The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to Mars- omitted some of the columns).

4

Salt system	Eutectic (°C)	Amount of supercooling below eutectic (°C)
MgSO4	-3.6°C (1.72 m)	15.5
MgCl2	-33°C (2.84 m)	13.8
NaCl	-21.3°C (5.17 m)	6.3
NaClO4	-34.3°C (9.2 m)	11.5

As the salt / liquid solution cools in Mars simulation conditions, then the results can be complicated, because for instance MgSO₄ has a eutectic of -3.6 °C but it releases heat in an exothermic reaction when it crystallizes. This keeps it liquid for longer than you'd expect. In their experiments, it remained liquid for twelve hours as it gradually cooled below the eutectic temperature before eventually it froze at 15.5 degrees below the eutectic temperature. In simulated Mars conditions you also have to take account of the effect of soil mixed in with the salts. Surprisingly, when you work with the Mars analogue soil, instead of the solution, this does not reduce the supercooling and can in some cases permit more supercooling. (see The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to Mars and "Formation of aqueous solutions on Mars via deliquescence of chloride–perchlorate binary mixtures).

With some of the salt solutions, depending on chemical composition, then the supercooling produces a glassy state instead of crystallization, and this could help to protect supercooled microbes from damage.

So deliquescing water is liquid in a wider range of conditions than you'd expect

The combination of all these effects means that mixtures of salts, including perchlorates in the mixture, can be liquid at lower temperatures than any of the salts separately, and also take up water from the atmosphere at lower relative humidity, and once liquid, can remain liquid for longer than you would predict if you didn't take account of these effects. Then in addition to all that, if there are micropores in the salt deposits, any life within them could also take advantage of an internal relative humidity higher than the external humidity of the atmosphere, and so have access to water for even longer.

On Mars the relative humidity of the atmosphere goes through extremes. It reaches 100% humidity every night in the extreme cold, even in equatorial regions. In the daytime the relative humidity becomes much less, and becomes very dry indeed, approaching 0%, and any exposed salts would lose their liquid. Then at night, the surface temperatures of Mars, even in the equatorial regions, drop to tens of degrees below freezing every night. So, the surface temperatures of the top few also change enormously from day to night (more stable but lower temperatures are encountered deeper below the surface). But because of these other effects these liquid layers, may resist efflorescence and remain liquid longer than you'd expect as the air dries out in the daytime, and also stay liquid longer than you'd expect through supercooling as the temperatures plummet at night.

The result is that you could have layers of liquid, on Mars, and especially if they are some way below the surface 1 or 2 cms, then liquid brines can form and be stable for hours every day. So this discovery of perchlorates on Mars has major implications for presence of liquid, and so habitability.

Challenges for life in these liquid layers of deliquescing salts

Mars has so many salts and perchlorates that the optimal mixes are bound to be present in places, so these deliquescing liquid layers must form. They are probably very thin layers but these would be plenty thick enough for a microbe to live in. The main focus of much of the research is on whether there are mixtures of salts able to deliquesce on Mars, that at the same time are also warm enough and not too salty for life. Many of these potential habitats would be far too cold or too salty, but it seems that in optimal conditions, with the right mixture of salts, at the right depth below the surface, these might just possibly form habitats for salt tolerant microbes - the haloarchaea. They would have to be perchlorate tolerant, and ideally, able to use it as a source of energy The perchlorates could act as a substitute for oxygen, as an "electron acceptor" on Mars in the anaerobic (low oxygen) environment.

One interesting thing about these deliquescing salts is that since they take up water vapour directly from the atmosphere, they can give a way to form liquid water where there is no ice present on the surface such as the arid equatorial regions of Mars. They could also could form in very dry conditions inside salt pillars, perhaps one of the most interesting possibilities of all.

Microbes could find tiny oases of water inside micropores in salt pillars

This is a way for life to access water when the atmosphere is far too dry for normal deliquescence, so at relative humidities way below those in the laboratory experiments. This section is based on  several papers that studied life in salt pillars in the Atacama desert, and suggested that the way the life gets water there may be of interest to Mars. Here are some of them:

• "Novel water source for endolithic life in the hyperarid core of the Atacama Desert"
• "Microbial colonization of Ca-sulfate crusts in the hyperarid core of the Atacama Desert: implications for the search for life on Mars"
• "The key to life on Mars may well be found in Chile".

In experimental studies of salt pillars in the Atacama desert, microbes are able to access liquid through spontaneous capillary condensation, at relative humidifies far lower than the deliquescence point of salt (NaCl) of 75%.

'The Atacama desert hosts the closest analogue of what a real, live Martian might be like', in its salt rock formations in the hyper-arid core of the Atacama desert, where microbes were found, living inside the salt, and getting their water entirely by deliquescence. Perhaps the same process might work on Mars. See Paul Davies' Blog The key to life on Mars may well be found in Chile. Cassie Conley also talks about these as a possible habitat for life on Mars.

These micro-organisms can survive because of numerous micro-pores in the salt, less than 0.1 micrometers in diameter in the salt. Theoretically, this reduces the limit for trapping water from the atmosphere down to relative humidities as low as 50-55% instead of the usual limit of 75%. Also once the water is trapped, it is retained for a long time, as the air gets even drier, right down to an extremely low relative humidity of 20%.. The authors suggest this seems a likely niche for Martian life to exploit. See Novel water source for endolithic life in the hyperarid core of the Atacama Desert

In year round studies of the pillars, they found that the external atmosphere reached a minimum relative humidity of 2.90%, maximum 74.2% and average of 34.75% (see table 2 of their paper). So the external atmosphere never quite reached the 75% level where salt deliquesces naturally, but inside the salt pillars it was able to capture the water in these micropores, easily, and retain it as well. The values for the interior of the salt pillars were: minimum 2.20%, maximum 86.1, average 54.74%.

The researchers, Wierzchos et al, did detailed studies with scanning electron microscopes. At 75% relative humidity then brine was abundant inside the salt pillars. As the humidity was reduced, even at 30% relative humidity for the atmosphere outside of the pillar, the clumps of cyanobacteria in the micropores shrunk due to water loss, but still there were small pockets of brine in the salt pillars. In "Novel water source for endolithic life in the hyperarid core of the Atacama Desert" the authors write:

"Endolithic communities inside halite pinnacles in the Atacama Desert take advantage of the moist conditions that are created by the halite substrate in the absence of rain, fog or dew. The tendency of the halite to condense and retain liquid water is enhanced by the presence of a nano-porous phase with a smooth surface skin, which covers large crystals and fills the larger pore spaces inside the pinnacles... Endolithic microbial communities were observed as intimately associated with this hypothetical nano-porous phase. While halite endoliths must still be adapted to stress conditions inside the pinnacles (i.e. low water activity due to high salinity), these observations show that hygroscopic salts such as halite become oasis for life in extremely dry environments, when all other survival strategies fail.

Our findings have implications for the habitability of extremely dry environments, as they suggest that salts with properties similar to halite could be the preferred habitat for life close to the dry limit on Earth and elsewhere. It is particularly tempting to speculate that the chloride-bearing evaporites recently identified on Mars may have been the last, and therefore most recently inhabited, substrate as this planet transitioned from relatively wet to extremely dry conditions"

Microbes also inhabit Gypsum deposits (CaSO₄.2H₂O), however Gypsum doesn't deliquesce. The micropores can still enhance the humidity of the atmosphere. Researchers found that the regions of the desert that had microbial colonies within the gypsum correlated with regions with over 60% relative humidity for a significant part of the year. They also found that the microbes were able to imbibe water whenever the humidity of the atmosphere increased above 60% and gradually became desiccated when it was below that figure.

For more recent work, see for instance Microbial colonization of halite from the hyper-arid Atacama Desert studied by Raman spectroscopy

This shows images of salt crystals from the hyper-arid core of the Atacama desert. The dots show regions that were analysed with Raman spectroscopy and found to contain life.

In short, it's a major challenge for life to find water on Mars, but it may be possible, and there are many potential methods that life could exploit to find it. We will come across several other possibilities later on. We won't know for sure whether life on Mars actually can use these various sources of water in practice until we have a much better understanding of the Mars surface conditions.

There is one other major thing that could make Mars, or parts of Mars, uninhabitable. That's the very low level of nitrogen in its atmosphere. It does have some, but the partial pressure is only 0.2 mbar. Earth's atmosphere has 781 mbar.

Sources of nitrogen - essential for life on Mars

Nitrogen is important for life, on Earth at least. It's what lets DNA zip up and unzip and it's also what holds together the helical structures of proteins. In more detail, when hydrogen is attached to nitrogen, it forms weak bonds with other elements like oxygen,

What about life on Mars though? Well, these bonds are central to biology as we know it because the bonds are so easily broken, which is so important, for instance, to let the DNA zip and unzip. Hydrogen can't make these special bonds when it is attached to carbon, instead of nitrogen. So even if life on Mars is very different from Earth life, perhaps using different amino acids, or with a different backbone from DNA, still it is likely to use nitrogen as a way to create these weak hydrogen bonds. Indeed, the astrobiologists think that anything that even remotely resembles Earth life would be likely to use nitrogen. Here is how it is used in DNA

How Nitrogen mediated hydrogen bonds are used in DNA

And here is how it is used to hold proteins together

Diagram of helical structure of a protein, showing how the nitrogen mediated hydrogen bonds hold it together

For more, see Searching for Organics in a Nibble of Soil. Diagrams credit J. Bada.

So, if this is right that nitrogen would be essential to life on Mars, how can it find the nitrogen it needs to survive?

First, there is that 0.2 mbar of nitrogen in the atmosphere. This may be too little for life to fix the nitrogen. Or is it? Let's look at this first, because if it was possible, you could have nitrogen fixing microbes anywhere on Mars. Some Earth microbes can actually make do with astonishingly low levels of nitrogen. I can't find much recent research about this for some reason. The main paper is from 1989, Biological nitrogen fixation under primordial Martian partial pressures of dinitrogen. The authors grew a couple of species of microbes often used for nitrogen fixation experiments, Azotobacter vinelandii and Azomonas agilis under conditions of low nitrogen levels, though at full Earth atmospheric pressure. They found that they could continue to fixate nitrogen down to 5 mbar. They found no evidence of fixation below 1 mbar.

However that's just a study of two species. A natural follow up would be to look at some of the extremophiles that might be able to survive on Mars. Could any of these, chroococcidiopsis (say) do nitrogen fixation at these very low pressures? The authors of a more recent paper in 2006 propose doing this experiment with Antarctic cyanobacteria, but as far as I can see, though they did some preliminary research, they haven't actually tried it yet in Mars simulation chambers. It does seem a bit of a stretch. Perhaps the answer would indeed be "No". Maybe it was only possible in the past when nitrogen concentrations were a bit higher. It's quite a long way to go, from a limit of somewhere between 1 and 5 mbar all the way down to 0.2 mbar. Yet, it would make present day Mars far more easily habitable if the answer was "Yes". If anyone here knows of any more research on the limits of low pressure nitrogen fixation for extremophiles, do say.

Luckily, there are many other ways to fix nitrogen on Mars, especially in the early solar system, including

• By volcanic processes and lightning.
• By chemical reactions. A recent (2012) paper suggests reaction of FeS with NO might have fixed nitrogen in the primordial Martian atmosphere.
• By meteorite impacts. The very early Mars had far more nitrogen than now, between 200 and 600 mbars of nitrogen (present day Earth has 780 mbars). That's enough so to make it likely that impacts on Mars fixed large quantities of nitrogen in the past.

The impacts there are especially promising. One estimate suggests that 80 to 150 mbars of Nitrogen was fixed by giant meteorite impacts on early Mars. If so, then much of those nitrates would get washed into the northern seas. Assuming about 100 mbar was fixed in this way globally, the authors suggest that Mars could still have a layer of nitrates a hundred meters deep, with 10% sodium nitrate by mass. The likely place to find this is in the northern lowlands. See Nitrates on Mars: Evidence from the 15/14N isotopic ratio

We haven't drilled deep below the surface yet. So the situation on Mars could easily be like the Atacama desert, that it does indeed have rich deposits of nitrates buried just a few meters below the surface. That estimate of a layer a hundred meter deep covering the Northern lowlands of Mars below the surface would give life on Mars vast amounts of nitrates to use. The main problem would be how to get it to the surface,as it would be below the permanently frozen permafrost layers. But perhaps it could get churned up by the wind in moving dust dunes. It could also get unearthed by meteorite impacts ("meteorite gardening"), and so become useful to surface life.

Anyway that's just one of several solutions. There are other ways for Mars to get fixed nitrogen. It can be delivered on meteorites, since some carbonaceous meteorites are rich in nitrogen. Another possible way for the life to get nitrates is through nitrogen fixation in interstatial thin films of fresh water. These films can form at well below the point where ice would normally freeze. They are really thin, less than a micron in thickness. For details see this paper:An active nitrogen cycle on Mars sufficient to support a subsurface biosphere. The author finds that these films can fix between 5 and 54 trillion molecules per square centimeter per second. That's 1.577 * 10^24 molecules per square meter per year, or about 2.6 moles per square meter per year, which is plenty enough to be biologically useful.

Life on Mars doesn't need much by way of nitrates. For instance in Antarctica, the main source of nitrates is from nitric acid created partly in the stratosphere as well as in photochemical reactions in the lower atmosphere. This is a slow process but it's enough to support small populations of microbes. The amounts available are tiny, around  1 to 2 micro moles per square meter per year (measured using snow pits). That's around 0.6 to 1.2 milligrams per square meter per year. That's far less than, for instance, the estimated quantity from the nitrogen fixation in interstatial thin films.

Though much here is still unknown, we do know that there are at least some biologically useful nitrates on Mars, as Curiosity discovered them in 2015. More details. Also the Martian meteorites have fixed nitrogen in them. But it's probably patchy and in short supply on the surface. For an overview with more about this issue and possible ways that life on Mars could access nitrates, see page 189 of Charles Cockell's "Trajectories of habitability". He also looks into many other requirements for habitability of past and present day Mars life.

This issue of the distribution of fixed nitrogen on Mars remains one of the most important unknowns for both past and present day habitability. Astrobiologists have recommended that NASA follow up its current "Follow the water" strategy with a "Follow the nitrogen". I cover this in Follow the nitrogen, dig deep and look for biosignatures (below) .

So, in short, there mightn't be much by way of nitrates on Mars, especially on its surface, but we know there is some, and there may well be enough to support small populations of microbes, as for the McMurdo dry valleys in Antarctica. There are a few ways it could be fixed even today, it could also get there on meteorites and comets. Also Mars may well have thick subsurface deposits of nitrates, and if so, perhaps other processes bring them to the surface from time to time.

Other requirements for life on Mars: major and trace elements, organics, and pH (alkaline or acid)

Charles Cockell looks at the other elements that life needs. First, most of the elements needed for life are no problem on Mars. The igneous rocks such as basalt are a rich source for trace elements as well major elements used by life (such as calcium). These are abundant on Mars, and have all the same major and trace elements as they do on Earth. There are no obvious omissions. There's plenty of sulfur there. Phosphates are also common. Hydrogen and oxygen are no problem. Hydrogen also is not a problem, as it can come from water, through chemical reactions like serpentization or can be produced by interactions with radioactive elements common in the crust.

What about organics? Many microbes need organics as food, because they can't make their own (the heterotrophs). Carbon dioxide fixing photosynthetic life can make organics, just from water, and air, but there can't be much by way of photosynthesis of that sort in the surface conditions, because we'd notice the effect of the oxygen it produces on the atmosphere if there was (I look at this in How much oxygen would surface photosynthetic life produce on Mars? below).

However even if Mars has no photosynthesis, organics shouldn't be much of a problem for life, as organic compounds continually rain down in meteorites. The reactive chemistry removes them quickly, but there should still be some organics available for life to use. Also, there's carbon in the basalt, which could have reacted with hydrogen to make organic compounds for the heterotrophs.

Most of the Mars surface is slightly alkaline. It may have acidic environments due to weathering of sulfates but these would be well within the tolerance limits of Earth life. See the section on pH, page 191 of Cockell's Trajectories of Martian Habitability. Finally, there are various chemical gradients that life can exploit as a source of energy (electron acceptors and electron donors). For details see his paper "Trajectories of Martian Habitability"

So in short, there is nothing else that should be a problem for life, on Mars, apart from the need for water, and nitrogen, which we've already covered. So, anyway, all that's just theory so far. Is there anywhere on the Mars surface where we can hope to find life on Mars?

Well as it turns out, yes there are quite a few suggestions for places to look, and some of them seem rather promising. We've already mentioned Nilton Renno's droplets that form where salt touches ice. (above) which may form rapidly, anywhere where there is ice on the surface in contact with salts. Now let's look at a few more of these suggestions.

Habitats for life on the surface of Mars - warm seasonal flows - now known as Recurring Slope Lineae or RSLs

When we got our first look at Mars in detail, it hardly seemed to change at all, apart from the dust storms. There were many signs of activity in the past -evidence of river beds, even seas, huge craters, and the volcanoes and the Valles Marineres rift valley. But since then, nothing.

However, when we started to put high resolution cameras in orbit, looking down on the surface, scientists started to notice many small scale seasonal changes. Most of these are caused by the wind blowing the dust, or by dry ice. The scientists found a few candidates for features that might be caused by liquid water, especially the dry gullies, which were interesting for a while, but they all had alternative explanations and didn't prove the case.

However, back in 2011, NASA announced discovery of the Warm Seasonal Flows. This discovery was a lucky break, made by a young Nepalese American student Lujendra Ojha (Luju for short), who rather remarkably made this startling discovery while working on an undergraduate thesis, under the guidance of Alfred McEwan. He first spotted them in 2010, with the results published in a paper by McEwan in Science.

“When I first saw them, I had no idea what it was. I just thought it was a streak made by dust or something similar. It was a lucky accident”, - Lujendra Ojha

They appear in spring, gradually extend during the summer, and broaden out and fade in autumn. And they always appear on sun facing slopes when the temperatures rise above 0 °C. Right from the beginning, the scientists were only able to come up with hypotheses that involved liquid water in some form. The temperatures are far too high for dry ice (which also would be a surprising thing to find in equatorial regions). The other likely explanation is that they could be wind formed features. There are many temporary wind formed changes on Mars, and you can get dark streaks rather like this from avalanches of dust after a strong wind. However, the RSL's are not associated with winds and the seasonal changes, with the streaks gradually extending, broadening and fading through the season, again don't match what you'd expect from winds.

(click to watch on YouTube)

This made them hard to explain as anything else except features caused by liquid brines in some form. We haven't detected water yet, but it's now pretty much confirmed that they are caused by liquid brines, indirectly through detection of changes of hydration levels in salts, in a paper published on 28th September 2015 along with a press conference.

Unusually, these can form even close to the equator, These locations have no surface ice at all, at any time of the year (except for the ephemeral morning frosts photographed first by Viking). The RSLs extend downwards from bedrock outcrops, with many streaks extending from the same outcrop, as you can see in the video above. They only form on very steep slopes. Typically the slope is 33 degrees (about 1 in 1.54), on a concave slope, with the runoff a slightly gentler 27 degrees, or about 1 in 2 (online angle to slope calculator). For details see section 5.2 of this paper.

For some reason they only form on some steep slopes and not on others. Nobody knows why yet. These examples are from the slopes in Newton crater. High resolution version and techy details here.

These dark streaks are not damp patches. Rather, it's some other effect on the surface due to the brines flowing beneath. They are very hard to study from orbit because they can only take the highest resolution photos during local afternoon, the worst time to detect the water. That's due to the orbit of the spacecraft, which is optimized to let it take close up photographs always at the same local time of day - more on that in a moment.

Warm seasonal flows on Mars. This is in Palikir crater, which is 41 degrees North. They have also been spotted in regions close to the equator.

There is some chemical process going on here as the streaks show traces of ferric and ferrous iron. So far nobody has detected water in them, but that might be because they are so thin, well beyond the resolution of their spectroscopic observations, and any water may be in small quantities.

They may also be caused by water which then dries up, or the water may be easier to spot in the morning, perhaps the flows happen in the morning depending on melting frost - the spectroscopic observations were made in the afternoon. See Are These Water Flows On Mars? Quite Possibly, New Observations Reveal

Even before the spectroscopic observations, our only explanations for these streaks involved liquid water in some form. But the explanations involving water aren't easy either. It's easy enough to explain warm seasonal flows for one year, but they keep coming back every year in the same spot. This means that something must replenish all the water that flows down the slope over geological timescales.

(for details, see Seasonal Flows on Warm Martian Slopes).

So, first things first, why they are so confident this means flowing water?

Details of what they found

Although Mars is currently surely the most studied planet, outside of Earth, even more thoroughly scrutinized than our Moon, still it has nothing like the constellations of satellites continually observing Earth. They did the measurements using a spectroscopic mapping instrument called CRISM.

CRISM is one of the instruments on Mars Reconnaissance Orbiter. It lets us do spectroscopic mapping of Mars.

We can't expect too much of CRISM. Our best optical telescope in orbit around Mars. HiRISE has wonderfully fine resolution. It is able to spots the streaks at widths of 5 meters widest, right down to 30 centimeters - its optical resolution limit. It benefits from Mars's thin atmosphere. This let's us spot fine details on the surface rather more easily than on Earth, and do it with comparatively small satellites also.

CRISM however has a best resolution of 18 meters per pixel. So there is no way you can use it to distinguish the composition of the streaks from the composition of the slopes around them, and they didn't try. Also, it can only look at the streaks at around 3 pm Mars time. That is, unfortunately, the time of day when they are likely to be at their driest.

We'd dearly like to be able to observe them in the mornings, which is probably when they are actually flowing. Perhaps we might even get the spectral signature of water in that case. But sadly it's not possible. MRO is in a slowly precessing sun synchronous orbit inclined at 93 degrees (orbital period 1 hr 52 minutes). Each time it crosses the Mars equator on the sunny side, South to North, the time is 3.00 pm, in the local solar time on the surface, and that is true all the year round. (See page 8 of Mars Reconnaissance Orbiter Communications).

What they did was to focus it on the streaks at times of the year when the streaks were very broad. At those times - it saw hydrated salts. At other times of the year it didn't see them.

Why its convincing even though they didn't spot flowing water

They did not actually see the water itself. But the only models for these RSLs involve liquid water flowing down the streaks. It is hard to see how else the streaks could grow in spring, and then fade away in autumn. The RSLs form at temperatures from -23 to 0 °C - dry ice evaporates at -78.5 °. So there is no way it can be anything to do with dry ice. Also there's no association with winds or dust storms.

So, we are in this situation where the only models that make much sense involve liquid water flowing. Flowing water would hydrate the salts. And we spot hydrated salts. It seems very convincing evidence, even though they haven't spotted flowing water directly.

How did they form?

The hydrated perchlorate salts don't need to be hydrated by flowing water. On Mars they can also take up water from the atmosphere, the process of deliquescence. Like this, another of the slides in the presentation:

This is one of the three main hypotheses for the formation of the streaks. However they were careful to say, they haven't proved that the perchlorates cause the streaks.

So, the perchlorates could be already hydrated, picked up from the soil by the flowing water as it flows down the slope. From the shape of the streaks, and the way they spread, they seem to come from a source at the top of the slopes. That source could be liquid from deliquescing salts. However, there are two other possibilities. The three main hypotheses are:

• The deliquescing salts. It's a bit of a challenge to get huge quantities of water forming in this way enough to feed the slopes, but it seems to be possible.
• Ancient reservoirs at the top of the streaks - this is hard to explain though as some of the streaks start from near the peaks of mountains, and the reservoirs would soon be exhausted. Still, it hasn't been ruled out, the reservoirs could be filled when the Mars atmosphere was thicker, so more precipitation - and it's orbit is continually changing in eccentricity and its axis changing and there is evidence of flowing water in gullies as recently as 500,000 years ago.
• They come from deep aquifers, perhaps even connecting to the subsurface hydrosphere kilometers below. To be liquid below the surface, you would have to have geothermal heating. Some of the models here have flows of water feeding those aquifers in turn from very deep down, the kilometers deep hydrosphere where Mars is warm enough, and pressures are high enough for liquid water - there may be a liquid layer on Mars at great depth (this depends on how much water is left from Early Mars - if a lot is left there should be an extensive deep hydrosphere, which could in turn feed aquifers closer to the surface, if not, then perhaps there is no water down there or very little).

So - they all have their good points, and all are also challenging. This discovery doesn't settle the question. The discovery of hydrated perchlorates makes the deliquescing salts more plausible, but it hasn't disproved any of the other ideas, not yet. Also another possibility is that different RSLs have different sources of water.

Indeed, another point is that though it would seem reasonable to suppose that the hydrated salts occur in the visibly dark patches, since they observed them only when the dark patches were very broad, this isn't proven yet. They can't actually prove that, since the resolution of CRISM isn't good enough to prove that the salts are in the dark streaks rather than the ground around them.

What we have so far is a correlation rather than an established causal connection. Surely there is some connection between the hydrated salts and the RSLs. Maybe one causes the other or both have a common cause.

The total picture isn't at all clear yet, with nothing conclusively proven about how the RSLs form. However when you put these new observations of hydrated salts together with everything else, it is a convincing case for flowing water.

Perhaps a hundred thousand metric tons of flowing water per year in the Valles Marineres

The amounts of water are quite large. Alfred McEwan, project leader for HiRISE, said at the end of the press conference that they made a rough estimate, for the amount of water in the streaks only in the Valles Marineres region (see 54:44 into this video).

(click to watch on YouTube)

They came up with an estimate of at least a hundred thousand metric tons of water flowing throughout the Valles Marineres region. They assumed only 5% water in the solution, and a thickness of only 10 mm which is around what you need for the material to flow at all.

That may sound a lot, and it is for Mars, but it's not much by Earth standards. Suppose you have a stream, flowing at one meter per second, a slowish walking speed. Make it a tiny stream, say average 25 cm deep and 2 meters wide - deep enough so you need to get your wellies on to cross it. That would make the cross sectional area around half a square meter, so even a small stream like that would have 100,000 cubic meters of water flow past in just over two days (200,000 seconds). As much water as flows through all the RSLs in the entire Valles Marineres region for a year will flow through your little stream in two days.

So it's not a lot by terrestrial standards. But for a microbe, or small colonies of microbes, it's a lot of water. So that leads to the question, could it be habitable?

Are the RSLs (Recurring Slope Lineae) habitable?

This of course is the big question on everyone's minds - could the RSLs be habitable?. The salts can be liquid right down to -40 °C or depending on the mixture, down to -60 °C or lower. The RSLs start to form when the surface temperature is between -23 °C to 0 °C. But that doesn't mean the water is at those temperatures. It could easily be the case that the surface heats up, and triggers release or deliquescence of waters deeper down, just centimeters below the surface, which flows at far lower temperatures. Mars has a very steep temperature gradient in the top couple of cms of soil. Even when it is 20 °C at midday on the surface, then a couple of centimeters down, you have reached the permafrost layer, and you'd hardly notice anything has happened. Or the water could be warm enough, but too salty for life.

You can read the paper in Nature if you follow up the link from the BBC story here, as Nature have an arrangement by which they make their papers available to anyone to read for major news stories like this, so long as you get to their site from one of the big name journalist news outlets.

In the conclusion they say

"These results strongly support the hypothesis that seasonal warm slopes are forming liquid water on contemporary Mars. The spectral identification of perchlorate in association with RSL, also suggests that the water is briny rather than pure. Terrestrially, in the hyper-arid core of the Atacama Desert, deliquescence of hygroscopic salts offers the only known refuge for active microbial communities and halophylic prokaryotes. If RSL are indeed formed as a result of deliquescence of perchlorate salts, they might provide transiently wet conditions near surface on Mars, although the water activity in perchlorate solutions may be too low to support known terrestrial life. The detection described here warrants further astrobiological characterization and exploration of these unique regions on Mars. This enhanced evidence for water flow also provides new clues as to the nature of the current Martian hydrological cycle"

So - these observations don't settle anything about the long standing question of whether the RSLs are habitable. If the water comes from deliquescing salts, it depends on the salt mixture and its temperature, and how salty it is. When they refer to "water activity" there, they mean, how salty it is. If very salty the water is not available even to microbes adapted to salty conditions - there is just too little water for them to be able to take it up even though the solution is liquid.

On the other hand, it could easily be habitable. We just don't know at this stage. Though the RSLs get most of the publicity at present, researchers have found some other seasonal features on Mars that behave rather like the RSLs, growing in the spring, then fading away in autumn. As we'll see in the next section, some of them may actually involve fresh water!

Southern hemisphere flow-like features - these may involve fresh water!

There are two types of these flow-like features. For a technical overview of them, see the Dune Dark Spots section in Nilton Renno's survey paper. These ones in the southern hemisphere which form in Richardson crater are particularly promising because all the current models involve liquid water in some form and what's more, in the models, these features start off as fresh water trapped under ice.

These often get confused with the rather similar looking Northern hemisphere flow-like features (see below) - which are far too cold to be habitable in the models,at least to Earth life, and may not involve water at all. The two Martian ice caps are rather different. The northern cap is low lying, mainly ice, with a thin layer of dry ice that disappears in summer. The flow like features in the northern hemisphere form at 12.5 degrees from the pole at surface temperatures of about -90°C, which is low enough for dry ice to be stable on the surface. Their models involve either extremely cold salty brines or dry ice and sand. These features are far too cold to be habitable to Earth life and may not even involve liquid water It's important not to confuse these two very different phenomena. They are easily confused because they are so similar in appearance, and because both are referred to as "flow like features".

The more interesting ones, for habitability, are in the south. The southern ice cap consists mainly of dry ice. It is colder, and higher up (at a higher altitude). It stretches as far as forty degrees from the pole in winter (so spanning over 4,700 km), but it reduces to just 300 km across in summer, Richardson's crater is 17.4 degrees from the south pole (that's over 1,000 km).

So though the features resemble each other in appearance, the conditions in which they form are very different and not directly comparable. The southern hemisphere features from at much higher surface temperatures than the northern hemisphere features, and they appear late in spring, after the rapid disappearance of a vast and thick layer of dry ice that covered the entire southern polar region, and beyond. In the summer then surface temperatures at Richardson crater can actually get above the melting point of ice at times in daytime, as measured by the Thermal Emission Spectrometer on Mars Global Surveyor. (See figure 3 of this paper)..

This map shows where the crater is. It is close to the south pole - this is an elevation map showing the location of Richardson crater in Google Mars, and I’ve trimmed it down to the southern hemisphere. You can see Olympus Mons as the obvious large mountain just right of middle, and Hellas Basin as the big depression middle left. Richardson crater is about half way between them and much further south.

Here is a close up - see all those ripples of sand dunes on the crater floor?

Link to this location on Google Mars

Well it’s not the ripples themselves that are of special interest, Mars is covered in many sand dune fields like that planet wide. What interests us are some tiny dark spots that form on them which you can see if you look really closely from orbit.

And, would you ever guess? Although it's one of the colder places on Mars, there's a possible habitat for life there in late spring? It is due to the "solid state greenhouse effect" which causes fresh water at 0°C to form below clear ice in Antarctica at a depth of up to a meter, even when surface conditions are bitterly cold.

The Warm Seasonal Flows often hit the news (probable salty brines on sun facing slopes). But for some reason, the flow-like features in Richardson crater are only ever mentioned in papers by researchers who specialize in the study of possible habitats for life on Mars. I first learnt about them in the survey of potential habitats on Mars by Nilton Renno, who is an expert in surface conditions on Mars (amongst other things, he now runs the Curiosity weather station on Mars). You can read his survey paper here, Water and Brines on Mars: Current Evidence and Implications for MSL. The models I want to summarize here are described in his section 3.1.2 Dune Dark Spots and Flow-like Features under the sub heading "South Polar Region". But it's in techy language so let's unpack it and explain what it means. I will also go back to the papers he cited, and some later papers on the topic.

In the case of Richardson's crater, both models involve liquid water in some form, and also potentially habitable liquid water. One of the two main models involves relatively thick layers of fresh water below optically clear water ice, up to tens of centimeters thick, and so is very promising for microhabitats. The other model involves microscopically thin layers of fresh water that join together to make a larger stream and pick up salts on the way out. That's very promising too. So let's now look at these two ideas in detail.

First, early in the year, you get dry ice geysers - which we can’t image directly, but see the dark patches that form as a result and are pretty sure this is what happens:

Geysers which erupt through thick sheets of dry ice on Mars. Clear dry ice acts as a solid version of the greenhouse effect, to warm layers at the bottom of the sheet. It is also insulating so helps keep the layers warm overnight. Dry ice of course at those pressures can't form a liquid, so it turns to a gas and then explosively erupts as a geyser. At least that's the generally accepted model to explain why dark spots suddenly form on the surface of sheets of dry ice near the poles in early spring on Mars.

So that would be cool enough, to be able to observe them, video them and study them close up. I hope the rover would be equipped with the capability to take real time video. These geysers are widely known and many scientists would tell you how great it would be to look at them up close, and see them actually erupt.

But most exciting is what happens later in the year, when it is getting too warm for the thick layers of dry ice needed for geysers. These layers of dry ice vanish rather quickly in spring. You would think that the dark spots that you get in the aftermath of the geysers would just sit there on the surface and gradually fade away ready to repeat the cycle next year. But no. Something very strange happens. Dark fingers being to form and creep down the surface as in this animation. Very quickly too (for Mars). I haven't been able to find a video for this, as the papers just use a sequence of stills, so I combined together some of the images myself into an animation to show the idea:

(click to watch on YouTube)

Flow-like features on Dunes in Richardson Crater, Mars. - detail. This flow moves approximately 39 meters in 26 days between the last two frames in the sequence

I made this animated gif using HiView: the image viewer for the HiRISE database and the images

ESP_011640_1080 : 19 January 2009, 4:14 PM Mars local (sol 396)
ESP_011706_1080 : 24 January 2009, 4.22 pm Mars local (sol 401)
ESP_011772_1080 : (29 January 2009, 4:28 PM Mars local(sol 406)
ESP_011917_1080 : 10 February 2009, 4:21 PM Mars local (sol 418)
ESP_012273_1080 : 09 March 2009, 4:10 PM Mars local (sol 444).

You can use the viewer to explore the terrain yourself - this is just one detail. The dunescape here is covered in numerous dark spots like this, most of which have flow like features that extend during the spring in this way. For more details see the wikimedia commons description.

BTW I found it hard to align these images exactly. I cut them out from the raw data, and aligned them by eye - unlike the RSL's there aren’t any widely shared images of them, and the figures in published papers weren't really suitable for this sort of thing. I’ve done my best to register them with each other but I couldn’t figure out a way to do it automatically.The problem seemed to be that the images are centered on different spots on the terrain. This means that there is no correct registration that puts each frame entirely in sync with the next one. So that’s why you may see some alignment shifts from one image to the next. It’s the best I can do - perhaps there is some way of remapping them to get a more exact alignment. However this was enough to give a general idea of what they are like.

All the likely models for these features, to date, involve some form of water. Alternatives that one might try to use to model them might include a second ejection of material by the dry ice geyser, or dust deposition, but researchers think these are unlikely to produce the observed effects.

So, these southern hemisphere flow like features seem very promising. That’s not as surprising as you might think. The same thing happens in Antarctica - if you have clear ice, then you get a layer of pure water half a meter below the ice. The thing is any water on Mars exposed to the surface would evaporate quickly, so quickly that there would be none left. If ice melts there, it turns directly to water vapour because the atmosphere is a laboratory vacuum, it’s so thin. But - water beneath a layer of transparent ice - that’s a different matter. The water is trapped by the ice so stays liquid. And what’s more, if they model it assuming clear ice like the ice in Antarctica they find that the ice there gets enough heat from the sun in the day to keep it liquid through the night to the next day so the layer can actually grow from one day to the next (ice is an excellent insulator). Also the Mars atmosphere is so thin that it doesn't matter at all that the air above the ice is very cold in these regions. The atmosphere is a near vacuum and works as a great insulator. Better in some ways than Antarctica.

Möhlmann's model is pretty clear (abstract here). If Mars has transparent ice like the ice in Antarctica, then it should have layers of liquid fresh water 5 - 10 cm below the surface and a couple of cm in vertical thickness in late spring to summer in this region. His model doesn't involve salt at all, so the water would be fresh water. The only question here is whether clear ice forms on Mars in Mars conditions and whether the ice is sufficiently insulating. We can’t tell that really from models, the only way is to go there and find out for ourselves.

Blue wall of an Iceberg on Jökulsárlón, Iceland. On the Earth, Blue ice like this forms as a result of air bubbles squeezed out of glacier ice. This has the right optical and thermal properties to act as a solid state greenhouse, trapping a layer of liquid water that forms 0.1 to 1 meters below the surface. In Möhlmann's model, if ice with similar optical and thermal properties forms on Mars, it could form a layer of liquid water centimeters to decimeters thick, which would form 5 - 10 cm below the surface.

In his model, first the ice forms a translucent layer - then as summer approaches, the solid state greenhouse effect raises the temperature of a layer below the surface to 0°C, so melting it. This is a process familiar on the Earth for instance in Antarctica. On Earth, in similar conditions, the surface ice remains frozen, but a layer of liquid water forms from 0.1 to 1 meters below the surface. It forms preferentially in "blue ice".

On Mars, in his model, the melting layer is 5 to 10 cm below the surface. The liquid water layer starts off millimeters thick in their model, and can develop to be centimeters thick as the season progresses. The effect of the warming is cumulative over successive sols. Once formed, the liquid layer can persist overnight. Subsurface liquid water layers like this can form with surface temperatures as low as -56°C.

Creates potential for flowing fresh liquid water on Mars!

That's for fresh water. The liquid layer below the surface is warmed by the solid state greenhouse effect to 0°C even when the surface temperature is as low as -56°C. The same thing happens in Antarctica, that you get fresh liquid water forming below the surface when the surface temperatures are far too low for liquid water. It's because ice traps heat in much the same way that the CO₂ on our atmosphere does, and then the ice and snow is also is very insulating (the reason the Inuit build igloos), so keeps the heat in. That's why the layer forms up to a meter below the surface in Antarctica and why it would form 5 to 10 cm below the surface on Mars, so that the solid state greenhouse effect can warm the subsurface to a much higher temperature than the surface and so that there is enough ice to insulate it to keep it warm.

Inuit village, Ecoengineering, near Frobisher Bay on Baffin Island in the mid-19th century - ice and snow are very insulating.

In the model, then the ice below the surface is first warmed up in the daytime sunshine, due to a greenhouse effect, the infrared radiation is trapped in the ice in much the same way that carbon dioxide traps heat to keep Earth warm. Then because the ice is so insulating, the heat is retained overnight, and the water remains liquid to the next day. To start with it would be only millimeters thick but over several days, gets to thicknesses of centimeters.

This should happen on Mars so long as it has ice with similar properties to Antarctic clear ice.

If there is a layer of gravel or stone at just the right depth, the rock absorbs the infrared heat and that can speed up the process. In that case, a liquid layer can form within a single sol, and can evolve over several sols to be as much as several tens of centimeters in thickness. That is a huge amount of liquid water for the Mars surface.

In their model it starts as fresh water, insulated from the surface conditions by the overlaying ice layers. This fresh water of course can't flow across the surface of Mars in the near vacuum conditions, as it would either freeze back to ice, or evaporate into the atmosphere. But the idea is that as it spreads out, it then mixes with any salts also brought up by the geyser to produce salty brines which would then remain liquid at the much lower temperatures on the surface and flow beyond the edges to form the extending dark edges of the flow-like features.

Later in the year, pressure can build up and cause formation of mini water geysers which may possibly explain the "white collars" that form around the flow-like features towards the end of the season - in their model this is the result of liquid water erupting in mini water geysers and then freezing as white pure water ice

This provides:

• A way for fresh water to be present on Mars at 0 °C, and to stay liquid under pressure, insulated from the surface conditions.
• 5 to 10 cm below the surface, trapped by the ice above it
• Depending on conditions, the liquid layer is at least centimeters in thickness, and could be tens of centimeters in thickness.
• Initially of fresh water, at around 0°C.

If salt grains are present in the ice, then this gives conditions for brines to form, which would increase the melt volume and the duration of the melting. The brines then flow down the slope and extend the dark patch formed by the debris from the Geyser, so creating the extensions of the flow-like features.

They mention a couple of caveats for their model, because the surface conditions on Mars at these locations is unknown. First it requires conditions for bare and optically transparent ice fields on Mars translucent to depths of several centimeters, and it's an open question whether this can happen, but there is nothing to rule it out either. Then, the other open question is whether their assumption of low thermal conductivity of the ice, preventing escape of the heat to the surface, is valid on Mars.

The process works with blue ice on Earth - but we can't say yet what forms the ice actually takes in these Martian conditions. The authors don't go into any detail about this, but ordinary ice can take different forms even in near vacuum conditions. As an example of this, the ice at the poles of the Moon could be "fluffy ice"

"We do not know the physical characteristics of this ice—solid, dense ice, or “fairy castle”—snow-like ice would have similar radar properties. [then they give evidence that suggests fluffy ice is a possibility there] "
(page 13 of Evidence for water ice on the moon: Results for anomalous polar)

That's the main unknown in their model, whether the ice is blue ice like Antarctic ice, or takes some other form. The ice should at least be in the same hexagonal structure crystalline phase as ice is on Earth - Mars is close to the triple point in this ice phase diagram

Phase diagram by Cmglee, wikipedia. Ice outside of Earth can be in many different phases. For instance in the outer solar system it is often so cold that it is in the very hard orthorhombic phase, where it behaves more like rock than what we think of as ice. However ice on Mars is likely to be in the Ih phase similar to Earth life. The Mars surface is close to the triple point of solid / liquid / vapour in this diagram.

So, the ice is likely to be of the same type as the blue ice in Antarctica. Not likely to have bubbles of air in it. But it could still take a different forms. The model shows that Mars should have layers of liquid water ten to twenty centimeters below the surface if there are any areas of clear blue ice as in Antarctica.

This solid state greenhouse effect process favours sun facing slopes (equator facing). Also, somewhat paradoxically, it favours higher latitudes, close to the poles, over lower latitudes, because it needs conditions where surface ice can form on Mars to thicknesses of tens of centimeters. (The examples at Richardson crater are at latitude -72°, longitude 179.4°, so only 18° from the south pole. There is no in situ data yet for these locations, of course, to test the hypothesis. Though some of the predictions for their model could be confirmed by satellite observations.

Interfacial liquid layers model

Another model for these southern hemisphere features involves ULI water (Undercooled Liquid Interfacial water) which forms as a thin layer over surfaces and can melt at well below the usual melting point of ice. In Möhlmann's sandwich model, then the interfacial water layer forms on the surfaces of solar heated grains in the ice, which then flows together down the slope. Calculations of downward flow of water shows that several litres a day of water could be supplied to the seepage flows in this way.

The idea then is that this ULI water would be the water source for liquid brines which then flow down the surface, mixing with dust, to form the features. That would still be interesting as you end up having flowing liquid water on Mars, several litres a day what’s more. Here is a paper from 2016 describing the idea.

See also Möhlmann's paper The three types of liquid water in the surface of present Mars

Those are the only two models so far. So it does seem very likely that there is liquid water here, and even with the interfacial liquid layers, the water starts off as fresh water beneath the ice, or possibly salty (in either model) if there are salt grains in the ice for the water to pick up. Either way the features start out as a flow of fresh water trapped beneath a layer of ice. This is one of the least publicized types of habitat on Mars, seldom mentioned outside the specialist literature. Yet in some ways it's one of the most interesting, if it exists, because of the potential for fresh water at 0 °C.

This liquid water is hard to observe because the features are so small, beyond the resolution of CRISM. However, analysis of the larger spots, at around the spring equinox, produced a signal that just possibly could be liquid water, where the ice is in contact with the dark material of the dune spots.

" In the gray ring area the water ice 631 surrounds darker surface, where liquid interfacial water layer or brine (Möhlmann 2004, 632 2009, 2010) may form. We found no firm evidence for the presence of liquid water in near-IR 633 spectra, although linear unmixing results show that the data are not inconsistent with a 634 possible slight contribution (a few %) of liquid water in the dark core unit." page 26 of this paper.

Could Mars have patches of fresh water in summer, throughout the polar regions, at a depth of six centimeters below optically pure ice?

Möhlmann has also suggested that this could be a more widespread phenomenon in the Mars ice caps, as for Antarctica. Liquid water could form at a depth of around 6.3 cm wherever there is optically clear ice on Mars in snow / ice packs, just as it does in Antarctica. In summer, it could form layers from centimeters to tens of centimeters in thickness.

Results of Mohmann's modeling of the solid state greenhouse effect in clear ice on Mars. The plateaus show temperatures that get above the melting point of water regularly every Martian sol, at depths of about 6.3 cms. L here is 11.4 cm. Ice at this level will melt periodically, and especially in summer can stay liquid overnight, leading to subsurface liquid water in layers of from cms to tens of cms in thickness. This should happen on Mars not just in the flow-like Features of Richardson crater, but also, anywhere where there is optically clear ice.

In another paper he writes "This liquid water can form in sufficient amounts to be relevant for macroscopic physical (rheology, erosion), for chemical, and eventually also for biological processes. "

His models seem clear enough. The air temperature hardly matters, because the Mars air is so thing it's a near vacuum, insulating the ice, like a thermos flask. The only unknown here is whether Mars does have optically clear ice like this, which is common on Earth in cold conditions like this in Antarctica.

Dark streaks in Russell Crater, 55 degrees South

These streaks form in the southern hemisphere, but in a different crater, Russell crater, 55 degrees from the south pole (compared to 17.4 degrees for Richardson crater). They are visually rather similar to the flow like features. Again one needs to be careful not to confuse them with the Richardson features as they seem to behave rather differently when studied in detail. These are also different features from the better known constant width linear grooves on Russell crater which formed some time between a century and a few thousand years ago (two suggestions are that they may have been formed originally by slurry flows involving liquid water, or they may be features cut out by blocks of dry ice rolling down the slope). They occur higher up the slope than the grooves.

This shows the location of the dark flows they studied for the paper relative to the better known grooves

Detail of HiRISE image PSP_002548_1225. Location of the the zoom in for the next image shown in blue.

Zoom in on previous detail of the HiRISE image PSP_002548_1225 which you can explore with HiView (drag and drop the J2 link into the viewer) - showing a late stage in formation of the dark flow features in Russell crater.

Notice how the features flow in between the wind formed ridges in the dunes, and also cross each other's tracks, and split and braid (unlike the Richardson crater features), all suggestive of avalanche type features. They also found evidence that these features extend episodically and very rapidly, at speeds of 2-4 meters per second. They could work out the speed, because they were able to pass up and over features in the terrain up to 1 to 2 meters in height, which is only possible for very rapidly moving flows. The evidence is reasonably conclusive that these are dust avalanche features. This image is a zoom in on the location of their figure 4d. For the complete sequence, see figure 4 in this paper.

These are braided, divide, recombine and cross each other's tracks. They flow down the slopes channeled by wind formed ridges in the dunes, and most distinctive of all, they are able to rush up over small features of up to two meters high and down the other side.

These seem to be dry features associated with defrosting and small dust avalanches as they are episodic, moving rapidly at speeds of 2-4 meters per second like an avalanche. The authors call them "dark flows". For details see this paper.

Northern hemisphere flow-like features - not likely to be habitable

Note that there are rather similar looking flow-like features in the Northern hemisphere, but these typically form at much colder temperatures for some reason, around -90°C - the two hemispheres on Mars have a very different climate. These are sometimes confused with the Richardson features which may be partly why those features get so little attention?

(click to watch on YouTube)

Flow-like features in the Northern polar dunes . These are thought to form at much lower temperatures. Some of the models for these also involve liquid water but there are other hypotheses as well, some of them involving dust and ice slipping down the cliff faces. This is another animation I made by hand cutting out the images from the raw data, and I was unable to do exact alignment throughout the image, due to the changing angles at which the photos were taken from orbit.

I created this animation myself by combining the following HiRise images from NASA/JPL/University of Arizona, all taken in 2008 - PSP_007468_2575 - 29 February (2.03 PM), sol 80, PSP_007758_2575 - 22 March (2:06 PM) , sol 101 and PSP_007903_2575 - 3rd April (2:07 PM), sol 113. For more details see the wikimedia commons description.

The northern hemisphere has shorter warmer winters (due to Mars’s eccentric orbit), and a lower elevation, but the flow-like features there form at times when the surface temperatures are lower than in Richardson crater. There are several different mechanisms for the northern hemisphere flow-like features, not all the models for those involve liquid water, and the ones that do involve very cold water. So the Richardson crater ones are the surest bet, seems to me, for a habitable flow-like feature.

Note that some authors have come to the conclusion that all these features may have more in common than they seem to at first sight, and may be formed in a similar way. This is a paper from 2012 which studies the Northern hemisphere features. They acknowledge the papers requiring liquid water in their models for the southern hemisphere features, but point to the similarities in appearance, and the sequence of events in all the cases, although the timing is different in detail.

They think that perhaps the features they studied in their work, the features in Russell crater, and the features in Richardson crater may all form in a similar way, in which case they might all be the result of avalanche like dry flows, which is a likely explanation for the northern hemisphere features, and is pretty much conclusively the explanation of the features in Russell crater. (To read the paper in its entirety you can use a google scholar search for "Observations of the northern seasonal polar cap on Mars III: CRISM/HiRISE observations of spring sublimation" to download the pdf from academia.edu, or use the google scholar button for chrome).

Sun warmed dust grains embedded in ice

This is another suggested habitat for life in the Mars higher latitudes based on processes that happen in the Antarctic ice. Dust grains in the ice often produce tiny melt ponds around them in the heat of the summer sunshine. The dust grains absorb the heat (preferentially over the ice), and so heat up and melt the surrounding ice. Then this heat gets trapped because of the insulating effect of the solid state greenhouse effect, because ice traps heat radiation, so forming tiny melt ponds of a few millimeters thickness or more. This could happen on Mars too, so is another possible habitat with fresh water.

It's just a few millimeters of fresh water, but that could be significant on Mars. Another example of this process, then meteorites in Antarctica are often found associated with gypsum and other evaporates - minerals that can only form in the presence of liquid water and must have formed after they fell in Antarctica. Sometimes the researchers find capillary water, or thin films of water, and sometimes they even find evidence of a rather large meltwater pond which formed around the meteorite, or find the meteorites in depressions filled with refrozen ice.

A similar process could be at work in the Martian icecaps too. This process could melt the ice for a few hours per day in the warmest days of summer, and melt a few mms of ice around each grain. Indeed, if I can venture a speculation of my own, perhaps just as in Antarctica, there could be larger melt ponds around meteorites embedded in the ice too - as Mars must have many meteorites embedded in the polar ice sheets.

This could explain another puzzle. Particles of gypsum (the same material that is used to make plaster of paris) have been detected, first in the Olympia Undae dune fields that circle the northern polar ice cap of Mars, See this paper for details. Later on, they were detected in all areas where hydrated minerals have been detected, including sedimentary veneers over the North polar cap, dune fields within the polar ice cap, and the entire Circumpolar Dune Field. There's strong evidence that the gypsum originates from the interior of the ice cap. See this paper for details. Gypsum is a soft mineral that must have been formed close to where it has been discovered (or it would get eroded away by the winds) and as an evaporite mineral, it needs liquid water to form. Opportunity later found veins of gypsum in the equatorial regions, in 2011, a clear sign of flowing water on ancient Mars. But these polar deposits are more of a mystery because they are found in the dust dunes on Mars, so must be produced locally, but where?.

Losiak, et al, modeled tiny micron scale dust grains of basalt (2-2 microns in diameter) exposed to full sunlight on the surface of the ice on the warmest days in summer, on the Northern polar ice cap. They found that these tiny dust grains were large enough to provide for five hours of melting which could melt six millimeters of ice below the grain. They say that with pressures close to the triple point, on windless days, you should get a significant amount of melting. They speculate that this might possibly explain the deposits of gypsum in the polar regions. Could it have formed in a similar way to the gypsum that sometimes forms around Antarctic meteorites?

Möhlmann did a similar calculation. This time he was looking at the possibility of liquid water forming inside snow on Mars. The snow would be exposed to the vacuum, but as the ice melted it would plug all the pores in the snow and eventually form a solid crust of ice on the snow, and so protect it from further evaporation. It would trap the heat as well and so encourage melting. This could happen anywhere between a few centimeters depth down to ten meters below the surface.

Could Mars life in the polar ice exploit thin films of ULI water in the ice, wrapped around individual microbes?

This is an interesting suggestion by Möhlmann in an article in Cryobiology magazine, that life may be able to make use of thin film monolayers of the " ULI water" (Undercooled Liquid Interfacial water) wrapped around a microbe, even in tiny nanometer scale layers of liquid water only two monolayers thick.

"In view of Mars it should be mentioned, that there is water ice in the permanent polar caps. At mid- and low-latitudes, ice can form, at least temporarily, via adsorption and freezing in the soil. There, the adsorbed and frozen water overtakes the role of ice, as described above. So, ULI-water can be expected to, at least temporarily, exist also in martian mid- and low-latitudinal subsurface soil. A similar environment can be expected to exist in isolation heated parts of icy bodies in the asteroidal belt, and analogously in the internally heated icy moons of Jupiter and Saturn. It is thus a current and challenging question if ULI-water can act as supporting life in environments with temperatures clearly below 0 °C by delivering that water, which is necessary for metabolic processes, and by permitting transport processes of nutrients and waste. It is the aim of this paper to demonstrate the potential importance of ULI water in view of the possible biological relevance of nanometric undercooled liquid interfacial water."

He cites research suggesting life can remain active in the presence of just two monolayers of water wrapped around a microbe.If there is just a small thermal gradient in the ice, of one degree centigrade per meter, then enough liquid water will form to fill a micrometer sized microbe once a month. Enough will form to fill it once a day if there is a locally steeper gradient of one degree centigrade per 10 cm. This can lead to a constant transport of fresh water to bring fresh nutrients to the microbe, and to remove wastes. The main question is whether this is a sufficient flow of water to sustain life. For more details of this intriguing idea, see his article.

Other surface and near subsurface habitats for life on Mars

Nilton Renno's "swimming pool for a bacteria" on salt / ice interfaces, the RSLs, the Richardson flow-like features and these potential microhabitats within the polar ice, covered so far are just a few of many possible microhabitats suggested on the Mars surface. I cover some other possibilities below, see

• Transgressing sand dunes bioreactor (see below)
• Equatorial frosts - and a source of water for the deliquescing salts
• Desert varnish
• Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere
• Liquid brines beneath the surface of sand dunes at night - beneath the sand that Curiosity drives over - that one was reported as uninhabitable but Nilton Renno was not so sure, biofilms might make it habitable.

However, let's take a break now, and try to speculate about what forms of life could live in these microhabitats, if they exist. Also are any of them accessible to Earth life?

Plausible microbial metabolisms for present day Mars - what else apart from photosynthesis?

One way to examine the possibility for life on Mars is to look at the Redox pathways that the life could use as a source of energy. This involves a pairing of an electron donor and an electron acceptor. For details see Electron transport chain, and Microbial metabolism.

Here is a table of some of the available donors and acceptors in Mars conditions, table from this paper: Plausible Microbial Metabolisms for Mars (added CO₂, also added available to NO₃^- and removed "not shown to support life" from ClO₄^- because we now know that there are microbes that metabolize perchlorates).

electron donors, any of:	electron acceptors, any of:
FeSO2+: available in Fe-rich silicates	Fe3+: available in numerous alteration minerals
S : suggested at Gusev Crater	SO42- available in salts
H2: available in subsurface?	O2: partial pressure too low
CO: available in atmosphere	NO3-: available
H2O available for oxygen photosynthesis	ClO4-: available and abundant
organics: meteoritic likely to be present at surface	CO2: in the atmosphere
organics: endogenous available in subsurface	CO2: in the atmosphere

A candidate metabolism would use one of the electron donors in the first column paired with one of the electron acceptors on the right as a source of energy. (The final dash on left hand side is there just because the list of electron donors is shorter than the list of electron acceptors).

See also the presentations in: Redox Potentials for Martian Life. Cockell gives a long list of examples of potential redox couples on page 190, table I of his Trajectories of Martian Habitability.

Note, you might think that there can't possibly be any photosynthesis on Mars right now, because there isn't much oxygen in the Mars atmosphere, only 0.13%. If Mars was an exoplanet, we'd look at it from a distance, analyse its atmospheric composition by looking at the spectrum of a single pixel, and might well say "No oxygen, so probably no photosynthesis". However in conditions on Mars similar to the Antarctic dry valleys and if that corresponds to a few localized optimal places for life on Mars, it wouldn't produce much oxygen, in a tiny seasonal and long term signal easily masked by 0.13% of the atmosphere. See How much oxygen would surface photosynthetic life produce on Mars? (below) . Also we have forms of photosynthetic life on Earth that don't produce oxygen, such as the haloarchaea, so the same could be true on Mars.

Earth life able to contaminate Mars habitats

None of this would matter if there was something that made Mars so different from Earth that no Earth life could survive there. For instance, temperatures on Titan are well below the temperatures for Earth life and the only water is thought to be in the form of solid rock, while the fluid is ethane or methane. There are no issues contaminating Titan unless it has cryovolcanism - volcanoes with liquid water as lava.

And it's true, no humans, animals, birds, or insects could survive on Mars, and most plants couldn't either. But some lichens and microbes from Earth have been shown to be able to survive in Mars simulation chambers and other experiments to simulate Mars-like conditions on Earth. So these could potentially survive there, contaminate them and make it difficult or impossible to study them to find out what was there originally.

This all depends on whether or not the habitats exist, which is not known yet. They could all be too cold or too salty for Earth life. But if they do exist, here is a list of lifeforms that might potentially be able to "emigrate" from Earth to Mars. For the cites for this section, see my Candidate lifeforms for Mars in my Places on Mars to Look for Microbes, Lichens, ...:

• Chroococcidiopsis - UV and radioresistant, and can form a single species ecosystem. It needs no other forms of life, and only requires CO₂, sunlight and trace elements to survive.
• Halobacteria - UV and radioresistant, photosynthetic (using hydrogen directly - proton pumps, doesn't generate oxygen or sulfur), can form single species ecosystems, and highly salt tolerant. Some are tolerant of perchlorates and even use them as an energy source, examples include Haloferax mediterranei, Haloferax denitrificans, Haloferax gibbonsii, Haloarcula marismortui, and Haloarcula vallismortis
• Some species of Carnobacterium extracted from permafrost layers on Earth which are able to grow in Mars simulation chambers in conditions of low atmospheric pressure, low temperature and CO₂ dominated atmosphere as for Mars.
• Geobacter metallireducens - it uses Fe(III) as the sole electron acceptor, and can use organic compounds, molecular hydrogen, or elemental sulfur as the electron donor.
• Alkalilimnicola ehrlichii MLHE-1 (Euryarchaeota) - able to use CO in Mars simulation conditions, in salty brine with low water potentials (−19 MPa), in temperature within range for the RSL, oxygen free with nitrate, and unaffected by magnesium perchlorate and low atmospheric pressure (10 mbar). Another candidate, Halorubrum str. BV (Proteobacteria) could use the CO with a water potential of −39.6 MPa
• black molds The microcolonial fungi, Cryomyces antarcticus (an extremophile fungi, one of several from Antarctic dry deserts) and Knufia perforans, adapted and recovered metabolic activity during exposure to a simulated Mars environment for 7 days using only night time humidity of the air; no chemical signs of stress.
• black yeast Exophiala jeanselmei, also adapted and recovered metabolic activity during exposure to a simulated Mars environment for 7 days using only night time humidity of the air; no chemical signs of stress.
• Methanogens such as Methanosarcina barkeri - only require CO₂, hydrogen and trace elements. The hydrogen could come from geothermal sources, volcanic action or action of water on basalt.
• Lichens such as Xanthoria elegans, Pleopsidium chlorophanum, and Circinaria gyrosa - some of these are able to metabolize and photosynthesize slowly in Mars simulation chambers using just the night time humidity, and have been shown to be able to survive Mars surface conditions such as the UV in Mars simulation experiments.

Most of these candidates, apart from the lichens, are single cell microbes (or microbial films). The closest Mars analogue habitats on Earth such as the hyper arid core of the Atacama desert are inhabited by microbes, with no multicellular life. So even if multicellular life evolved on Mars, it seems that most life on Mars is likely to be microbial, especially on the surface

For more about the value of Mars for biology and implications of sending humans there, see

• Does Elon Musk's Plan Violate The Outer Space Treaty - Planetary Protection For Mars After Human Crashes
• Wait, Let's Not Rush To Be Multiplanetary Or Interstellar - A Comment On Elon Musk's Vision

For more about the flow-like features habitat, and many other possible habitats on Mars, the lifeforms that could live there, Mars analogue habitats and other topics, see my:

Places on Mars to look for Microbes, Lichens, ... Salty Seeps, Melt Water Under Clear Polar Ice, Ice Fumaroles, Dune Bioreactors, ...: Where early Mars lifeforms could survive to the present day,

It’s also available to read online for free at Places on Mars to Look for Microbes, Lichens, ... and the section on the Richardson flow-like features is here: Flow-like features (notice I put the Richardson flow-like features on the cover - for me, this is the most exciting feature of all on Mars for exobiology)

I also recently got the material there accepted on wikipedia. They rejected it when I first submitted it in 2011, saying that, though well written, it was more suitable for an essay or a journal article than wikipedia. However, they accepted it when I tried again earlier this year. I added it as two separate articles this time, as it was a bit long for a single article there. See Modern Mars habitability and Present day Mars habitability analogue environments on Earth

Vulnerable to photosynthetic life

In the section above: Case study - can photosynthetic life be transferred from Earth to Mars or vice versa? I looked at Charles Cockell's study where he showed that it's quite possible that photosynthetic life has never got to Mars via meteorites.

Many of the Earth lifeforms that could survive on Mars are photosynthetic Chroococcidiopsis , Halobacteria the black molds such as Cryomyces antarcticus and Knufia perforans, and lichens such as Xanthoria elegans, Pleopsidium chlorophanum, and Circinaria gyrosa. So what would those do to native Mars life if they got there somehow as a result of planetary contamination, and Mars never had any photosynthetic life before? Chroococcidiopsis particularly is a polyextremophile, seems pre-adapted to survive in the Mars conditions, is even able to survive in Mars simulation chambers without any liquid water, taking up the humidity from the atmosphere, has efficient protection from UV radiation, can repair damage from desiccation and ionizing radiation rapidly within hours, and is widespread in many different habitats on Earth.

Analogy of ET microbes on Mars with microbes from a planet orbiting another star

Imagine if you could learn about life on a planet or in the ocean of an icy moon around another star? Not just investigate the planet as a faint single pixel speck of light. But actually send a rover there to study it in situ?

Even if all you found was extraterrestrial microbes or lichens, imagine how exciting that discovery would be? Well Mars, Europa and Enceladus may be like exoplanets and exomoons in our own solar system, they may be as interesting as that. They could easily be as interesting to biology as an exoplanet or exomoon is to geology. When, or if, astrobiologists discover the first biology from another planet, this will be just as revolutionary, indeed more so, as the first ever discovery of an exoplanet. We could easily find something that overturns understanding of biology in a similar way to the way the discovery of hot Jupiters overturned our understanding of the formation of planets. Indeed, it would be in some ways as revolutionary as the first discovery that the planets in the night sky are other worlds like our Earth. We don't know what we will find out until we study other forms of biology close up.

Microbes on Mars, in the more interesting case, would be so different from us, they'd be more like a microbial version of ET than like a tiger. See Will We Meet ET Microbes On Mars? Why We Should Care Deeply About Them - Like Tigers

What we could learn - some examples - Early life or proto life - Life originated from Mars - Distantly related life - life not based on DNA style double helixes - life with capabilities Earth life hasn't developed yet

What sort of things might we hope to discover on Mars? This is another "synthesis" section. What might we find on Mars? Surprisingly, I can find hardly anything by way of detailed speculation about this - as usual do say if you know of a good paper to cite here. So I've based this section on various ideas for early life and alternative forms of biochemistry, and especially on any ideas that seem particularly relevant to Mars. I've also added a couple of speculative ideas of my own, clearly labeled (the idea of life based on replicating sheets instead of replicating strands is one of them).

Here are a few ideas.

• Early life or protolife, to fill in the huge gap between the organics and cell-like structures that turn up in laboratory experiments, and the immense complexity of modern life. We might find the much simpler RNA world cell with no proteins, or ribosomes, or DNA. Instead of the huge ribosome it could use RNA sliced into pieces and recombined to make a ribozyme, that tinier distant cousin of the ribosome. For more on this see RNA world and the shadow biosphere (above). Or, we might find the so called autopoetic cells that replicate just by producing daughter cells with a similar mix of chemicals when they get too large. They might have no genetic code yet to regulate the process, or they might be able to regulate it, but only achieve very approximate, imperfect reproduction, leading to questions about whether they count as life or not. For many other ideas of what we could find, see What else could have come before modern life? Alphabet soup of "XNA", Ostwald ripening organic crystals, "naked genes", or almost alive "autopoetic" cells (above)
  
• Evidence that life originated on Mars. DNA and RNA are both particularly fragile, and DNA especially is rather hard to form naturally. It needs the environment of the cell or special conditions to keep it stable. RNA is not quite so fragile and is more stable when it is very cold. Also, the ribose in its backbone is stabilized by the presence of special chemicals, borates. As it happens, these are found on Mars in useful quantities.
  
  

  The white crystals here are Ulexite, (the yellow ones are calcite). Ulexite is also known as "TV rock" because it is a natural optical fibre.
  
  This is one of the minerals that include borates - compounds involving complex chains of boron and oxygen atoms terminated by hydrogen atoms. These minerals can help stabilize ribose in the backbone of RNA, and so may have helped with early stages of evolution of life. Boron is very rare in extraterrestrial materials, and quite rare on Earth, and dissolves in hot water. However it is found in in the Martian meteorite MIZ 09030 in similar concentrations to Earth clays, and Curiosity found it in Gale Crater.

  Steve Benner presented an interesting theory in 2013. He thinks that the borates prevented early organics from turning into tars, instead forming carbohydrates. He thinks that molybdenum in the form of molybdate played a role too, as it helps catalyse the formation of ribose, the "R" in "RNA". Both of these are common on Earth now, but being soluble in water, would have dissolved in our early oceans. Early Earth may have been an almost entirely ocean world, with far less dry land than today. Meanwhile Mars being drier could have created better conditions for this process and so might be favoured for the origins of life.
  
  If that's right, then maybe we can get evidence of this process of evolution of RNA based life on Mars with borates and molybdate at at some point, as we trace the evolution there back and back. For more on this idea, with many cites to the literature, see also Romulus Scorei's Is Boron a Prebiotic Element? A Mini-review of the Essentiality of Boron for the Appearance of Life on Earth
  
  For other approaches suggesting that life originated on Mars:
  
  • Mars, Panspermia and the origin of Life: Where did it all being? by Joseph. Kirschvink and Benjamin Weiss. They argue that though both Earth and Mars had strongly reducing conditions in the early solar system, only Mars had strongly oxidizing conditions early on. Indeed oxygen arose on Earth at around the time that it disappeared on Mars (see their figure 2). Early Mars also probably had a more reducing crust than Earth. This gave early Mars more opportunity for niches where life could exploit large chemical gradients between oxidizing and reducing conditions. Also, the early Mars atmosphere would have had ozone to shelter life from UV, which was not present on early Earth.
    
    Finally, they argue from the inferred biochemistry of our last common ancestor suggests it arose in conditions with large redox chemical gradients, already able to exploit oxygen, nitrate, sulfate and sulfur. They say that later massive lateral gene transfer can be ruled out, at least for oxygen, because most of the genes responsible are found in rRNA (ribosomal RNA) and the sequences in rRNA tend to be conserved within species and not transferred horizontally so much (see page 9). So, it looks as if our last common ancestor of all modern Earth life may have arisen in an environment where it could exploit oxygen. If that's true, it seems to rather strongly favour Mars as the most likely place for it to evolve.
    
    
    Figure 1 from their paper. Here, the black dashed lines in the bottom figure show various possibilities for oxygen levels for Earth's atmosphere, and the red dashed line shows a possible scenario for Mars. The top figure shows the origin of various microbes - see especially the origin of the cyanobacteria, not long before the first oxygen spike in the Earth's atmosphere. (The process of oxygenation of Earth's atmosphere may have begun as early as three billion years ago).
    
    Mars had more oxygen in its atmosphere early on, and so a greater redox potential. It was also a more likely place for an organism to evolve able to exploit oxygen, which our last common ancestor may have been able to do. They conclude:
    
    "At face value, all of these lines of evidence suggest that, compared to early Earth, early Mars might have had a greater supply of biologically useable energy and was perhaps, by implication, a better place for the origin of life. And so we salute you, all you descendants of space-traveling microbes from the Red Planet!"
    
  • Paul Davies, see Was Mars the cradle of life?,
    
    "Mars is the most Earth-like of our neighbouring planets and enjoyed a number of advantages during the early history of the solar system. Though a freeze-dried desert today, Mars was warm and wet before about 3.6 billion years ago . Being a smaller planet, it cooled more quickly, making it suitable for life sooner than Earth. Gene sequencing indicates that the oldest and deepest branches of the tree of life are occupied by hyperthermophilic archaea and bacteria , hinting that the earliest life forms dwelt deep beneath the oceans near volcanic vents, or even kilometres underground in the crust itself. "
    
    "The deep subsurface zone remains populated on Earth today, and probably offers the most promising location on Mars for finding any extant life. It would have become cool enough for hyperthermophilic microbial life on Mars perhaps as long ago as 4.5 billion years, when the Earth’s crust was still sizzling. Ensconced in this Hadean niche, shielded by a kilometre of two or rock, Martian life could have withstood the ferocious early bombardment that afflicted Mars just as it did Earth."
    
    He has an interesting calculation based on this. He assumes that Mars was favourable for the emergence of life before Earth and had a longer window of opportunity - as he suggests there. He also assumes that life was an unlikely event (if life starts very easily, then it would develop quickly on both Earth and Mars, so is likely to have made a fresh second genesis on Earth). Finally he assumes that the length of time needed for life to transfer from Mars to Earth in the early solar system via meteorite transfer after a large impact is a million years, and the window of opportunity during which both Mars and Earth had conditions ideal for life, so that transfer was easy, was a hundred million years.
    
    Based on that, he finds that Mars is hugely favoured over Earth as an origin for life. This is especially so if the origin of life required a succession of several different improbable steps to happen. It is also especially likely to originate on Mars if early life is fragile and goes extinct easily. The calculation is in Does Life’s Rapid Appearance Imply a Martian Origin?
    
    Techy note: He works out that p_M/p_E = 10^0.8nm(NE/PM)^mn
    
    p_M is the probability of life originating on Mars, p_M/
    p_E is the probability of it originating on Earth,
    n is the number of successive improbable steps needed for life to evolve,
    m is the number of genesis attempts before a robust form of life is evolved
    τ_M is the average length of time you'd expect it to take for each of those improbable steps of evolution necessary for life in early Mars surface conditions.
    τ_E is the average length of time you'd expect it to take for each of those improbable steps of evolution necessary for life in early Earth surface conditions.
    10^0.8 = 6.3 is his estimate for how much longer life had available to evolve on Mars than on Earth. For this, he assumes that life was present on Earth 100 million years after it became habitable, and that, by then, Mars had been habitable for around 600 million years.
    
    For those who want to read his calculation in detail, note, that for the deduction from equation (3) to (4) he is using a mathematical trick that some readers may not be familiar with. By the McLauren series expansion of the exponential function - you can express e^x = 1 + x + x/2! + ... If x is very small (positive or negative, then this simplifies to 1 + x.
    
    He calculates that the chance of an interstellar origin is very small. However that part of his calculation doesn't take into account of the possibility of life around another star seeding another star in our sun's stellar birth cloud, see Distant cousins with last common ancestor from a planet around another star (above)
    
• Unrelated or very distantly related life, perhaps based on some form of XNA (Xeno Nucleic Acid) instead of DNA. This would be one of the most amazing discoveries we could make. It would lift biology into a new dimension, by showing that life can exist based on completely different biochemical principles from DNA based life.
  
  It's a reasonable hypothesis, because DNA is very fragile indeed, and RNA is fragile also. Perhaps what that's telling us is that life started as one of the many alternatives to DNA? Some think we may have started with a PNA world for instance, as it is far more robust than RNA and forms more easily.
  
  Other ideas for a more robust early life include TNA world, or a molecule that's a hodgepodge mixing different backbones in the same molecule with non heritable variations in backbone structure (or a whole alphabet soup" of other possible precursors such as HNA, PNA, TNA or GNA - Hextose, Peptide, Therose or Glycol NA). XNA based life can also be used to create enzymes - one of the necessary requirements for life as we know it.
  
  Perhaps we find one of these forms of life on Mars and assume that it is unrelated life at first - but later we find it is an ancient predecessor to Earth life that still remains on Mars barely changed since those early times. Or perhaps it has no connection to Earth at all and we discover that life originated separately on Mars. Or perhaps Earth life originated as an early form of life on Mars, yes, but it hasn't just stayed "as is" but has evolved and elaborated hugely in some different direction on Mars, into a different modern form of life, just as complex as our DNA and mRNA based life, but organized on different principles altogether. Perhaps life on Earth and Mars both began as PNA, for instance, with a common ancestor but on Mars it evolved in a different direction, for instance a more elaborate form of PNA based life, or to something else. Maybe we are such distant cousins that the Mars life is not DNA or RNA based at all.
  
• Life with mirror helixes, triple helixes quadruple helixes, or side by side molecules that don't wind round each other. The first idea here is life based on an exact mirror of DNA. To get that to work, all the molecules in the cells would be reflected as if the whole thing was just reflected in a mirror. As far as we know aa mirror image cell like that would function perfectly well in its mirror image world. That's mirror image life, a trope in countless science fiction stories - but it is one of the more sensible ones, as the idea is perfectly feasible for extra terrestrial life, as far as we know.
  
  The idea that we could flip someone or even a single cell into their mirror image is pure science fiction at present, and way beyond anything we know about. But idea of life that evolves independently using mirror image chemistry seems to be eminently sensible and perfectly possible as far as we know.
  
  However there is also a form of ordinary DNA here on Earth that spirals in the opposite direction in a rather zig-zaggy way. It's called "Z DNA. It is the right most of these three models.
  
  
  

  A, B and Z DNA - image by Richard Wheeler . The one in the middle, B type is "normal DNA". The A type, on the left, is wider, with a shallower pitch on the outside, and occurs in dehydrated samples but may also occur in cells in pairings of RNA with DNA. The Z DNA, on the right, spirals in the opposite direction from ordinary DNA in a rather zig-zaggy way - it does this when it is methylated (has the methyl group CH₃ added) and can be stabilized in this form by using special Z-DNA-binding proteins and may play a role in transcription.

  Could life be based on Z DNA?
  
  Then what about a helical structure with three strands instead of two? Well, Earth life does that as well in some situations
  
  
  
  

  Triple helix - which forms in certain circumstances with Earth microbes and is very stable. The third strand rests in the large gap of the 2 strand DNA. It may be part of a larger molecule which is entangled with the DNA as shown - the dashed lines are meant to show connections to the rest of the molecule.

  Before the double helix structure of DNA was discovered, triple DNA was one of the hypotheses they were looking for. They found it was difficult to get it to fit the theory and observations. But now we do have an actual form of triple DNA that we know is stable. Could ET life use a triple helix for some reason?
  
  Or could ET life even use a quadruple helix? Earth life uses those sometimes, it sometimes, forms four strand helixes, a recent discovery!
  
  

  Cross section of a DNA quadruplex which can create four strand DNA. Forms very slowly but once formed is reasonably stable.

  If life on Earth can use DNA in those different forms in special situations - is it possible that extra terrestrial life could use something like one of these as its "normal" form of DNA?
  
  Or what about Rodley's "Side by side DNA"? This is just a hypothesis. There is no evidence at all that Earth life ever uses it, but if it doesn't exist here, perhaps ET life could use it? It spirals one way for a while, then changes direction and spirals the other way then back again in such a way that the two strands aren't actually twisted around each other. This could help them to separate more easily, which may perhaps be an advantage for replication.
  
  
  (click to watch on YouTube)
  
  
• Life that is based on completely novel principles, for instance replicating sheets instead of replicating strands. Are we being blinkered in some way because all we know of is life with double helix DNA and single helix RNA (along with a few variants like the triple helix DNA mentioned above)?
  
  Suppose it is much more radically different than that? To help stimulate a few ideas here, suppose for instance that ET life uses a sheet-like two dimensional structure, planar rather than linear, and replication happens by a second layer forming on top of the original sheet? This is one of my own "fun speculations" to stimulate the imagination and get you thinking a bit "out of the box" perhaps.
  
  This is related to Graham Cairns-Smith's Clay Hypothesis for the origins of life. He outlines it in his controversial 1985 book "Seven Clues to the Origin of Life". Clays consist of alternating layers of alumina and silicate. His example is the common clay montmorillonite Like all clays it is made up of alternating sheets of alumina and silica. The alumina sheets consist of edge to edge connected octahedra and the silica layers fill in the gaps with tetrahedra.
  
  
  
  It's formula is (Na, Ca)_0.33 (Al, Mg)₂ (Si₄O₁₀) (OH)₂·nH₂O. There the comma in (Na,Ca) means sodium or calcium can substitute for each other - they are not shown in this diagram but are attached to the outside layers of oxygen - the layers have a net negative charge which is balanced by the positively charged sodium or calcium ions attached to them. Similarly the (Al, Mg) shows that either Aluminium, or Magnesium can substitute for each other in the Alumina layer. Other atoms not shown in this formula can also substitute in the Alumina layer, such as iron and phosphorus.
  
  The layers are separated by water molecules and when you add a lot of water they can slip relative to each other which is what makes clay slippy.
  
  Anyway the idea is that defects, variations in the choices of ions, and irregularities in one layer somehow propagate through from the layers below to the weakly bonded layers on top of them. Some might be better at replicating than others, breed more true by this stacking method - and the ions in the layers could speed up catalysis of various reactions including polymerization of RNA.
  
  The main problem with this model is that nobody has ever got anything like this to work in the laboratory - breeding true from one layer to another in a consistent way and doing something useful as a result. So, you just have to wave your hands and say "perhaps after millions of years of evolution, Nature will find a way". So though it does seem very promising as an idea, with a lot about it that is in common with the way that DNA works, it is hard to test it as we have nothing that works like this at present. Sadly, we can't leave oceans to evolve for millions of years to see what happens.
  
  Graham Cairns-Smith's idea is that it would be a very early stage of life, with no cell walls, just these replicating sheets of clay, which would transition to RNA and DNA chemistry and eventually lead to us. As just said, we don't have good evidence for it at all.
  
  But as a purely speculative idea, for fun, just to stimulate ideas, what if we run with it? What if ET life similarly started off as essentially two dimensional sheets of clay just a few atoms thick, separated by water only loosely bound together and they replicated in such a way that later forming layers duplicated the structure of the layer below? And then suppose they went a step further than Graham Cairns-Smith suggested, and incorporated those sheets inside their cells and used them to replicate? Eventually, could they perhaps build up a whole complex biochemistry, as complex as our helix based life, but based on using 2D sheets for replication instead of linear or helical strands? As with the "side by side DNA" it would have the advantage that the 2D biopolymer doesn't have to uncoil to replicate - and what's more, it could hold a high density of information in a small piece of material.
  
  Techy aside for sci. fi. geeks, this is not quite the same as Greg Egan's fun but very speculative science fiction story "Wang's Carpets" based on the mathematical idea of "wang tiles" - in his short story the lifeforms themselves are entirely two dimensional, evolving patterns of tiles within larger two dimensional sheets, exploiting the complexity of the patterns you can make with a few simple shapes. These can form in a "Wang carpets", with patterns so complex to define that questions about them can be mathematically undecidable. Here though, it's just a 2D sheet of "instructions" standing in for the one dimensional sheet of instructions we know as a strand of DNA, within a larger 3D microbe otherwise similar to Earth life.
  
  Or could the genetic instructions even be preserved somehow in a 3D informational polymer? Is there any approach that avoids the need to uncoil to read it? We can do this mechanically through laser scanning, in prototypes for future memory devices, so the idea is not so far fetched as to be totally impossible.
  
  This is just fun speculation at present. But suppose that you are an ET biologist and your life uses 2D sheets to replicate or even, 3D informational polymers scanned in some way without altering its structure. Would you not find the idea of a helical structure that has to uncoil and unzip to replicate implausible and unlikely too?
  
  Could anything like that be possible? What do you think?
  
• Life that has evolved further than Earth life. Mars has had such very harsh conditions in the early solar system, alternating between a frozen "snowball Earth" type ice phase and more habitable phases. It's also been subject to strong ionizing radiation, extremes of cold, and near vacuum atmosphere. Some think that we have multicellular life on Earth as a result of a snowball Earth phase. If that's true, you could make a case for Mars life to be more highly evolved than Earth life - more complex, with more robust cells, extra non redundant nucleotides, additional amino acids, anything that it can exploit to add to the complexity and make it more capable than Earth life, maybe even with totally novel capabilities never explored here.
  
  Present day Mars probably only has microbes, or perhaps lichens, at least if it is fair to make a comparison with similarly harsh environments on Earth. Microbes and lichens seem to be better adapted to such conditions than complex multicellular life. But the harsh environment may mean it evolved further on Mars than on Earth. Microbes yes, but microbes with the equivalent of several billion years of evolution advantage over us. Could it be vastly more sophisticated than Earth life, even if it is just a single cell lifeform?
  
  The harsh conditions on early Mars could mean it didn't get as far and is an early form of life. It's hard to say in advance which way this would go. See also See Life that has evolved further on Mars (below) and What are the effects of these frequent ups and downs in habitability? (above)
  
• Life with a capability Earth life doesn't have at all, e.g. a new form of photosynthesis
  
  We have three ways of doing photosynthesis on Earth - broadly speaking:
  
  • Green sulfur bacteria, which use light to convert sulfides to sulfur, which is then often oxidized to sulfur dioxide
    
  • Normal photosynthesis which splits water to make oxygen, also taking up carbon dioxide in the process. (basic equation 6CO₂ + 12 H₂O → C₆H₁₂O₆ + 6O₂ + 6 H₂O where the oxygen atoms in bold are the same ones on both sides of the equation - see Plants don't convert CO₂into O₂, and Notes on lamission.edu)
    
  • The photosynthesis of the haloarchaea which works similarly to the receptors at the back of our eyes, based on a "proton pump" which moves hydrogen ions across a membrane out of the cell using bacteriorhodopsin similar to the rhodopsin in our eyes, with no byproducts such as sulfur or oxygen, just creates energy directly from the proton gradient. For more on this see Surprising distant cousins.
    
  ET microbes might well use some fourth form of photosynthesis that has never been explored on Earth.
  
• Life that is better adapted to some conditions than Earth life e.g. better able to resist ionizing radiation, or perchlorates, able to use hydrogen peroxide, better more efficient metabolism, more efficient photosynthesis.
  
  As an example, C4 photosynthesis as used by maize is more efficient than C3 in conditions of drought, high temperatures and limitations of nitrogen, and is a later development over C3 photosynthesis, though it requires more energy so not so good in cold conditions. But it's not the most efficient possible. It still relies on the enzyme RuBisCO which helps catalyse conversion of CO₂ into glucose which is not the fastest of enzymes.
  
  Scientists have found a way to increase the efficiency of this carbon dioxide fixation step 20 fold, - though only in a test-tube, not in an actual microbe. They combined enzymes from the human body and gut bacteria, plants, microbes that live in the sea and microbes from the surface of plants, in total they used 17 different enzymes from 9 different organisms, which they combined into a new 11 step cycle. They worked out and synthesized the gene sequence to reconstitute this pathway in a test-tube. More details here.
  
  Nature never discovered this cycle on Earth. What about on Mars? Might Mars life have found this pathway, or if not this one, some other novel CO₂ fixation cycle? The scientists didn't invent it from scratch, but rather obtained it by serendipity, combining enzymes that already exist in nature. What else might nature be able to do, given hundreds of millions of years of evolution in a planet spanning sea punctuated by numerous impacts and changes of climate?
  
• Life similar to Earth life in most respects - this is Zubrin's idea - and it would raise many questions. How has it evolved in such a different environment, since last transfer from Earth, surely at least tens of millions of years ago. How did it get there? We can test the theory of panspermia, find out in practice how easy it is for life to be transferred to another planet. Or test ideas of convergent evolution on microbes that have evolved on different planets.
  
• Some think that all life has to be essentially similar to DNA in almost all respects, that DNA life is optimal. If so, we might find something that closely resembles Earth life, but independently evolved. Based on DNA and RNA. With a similar number of amino acids. With many of the amino acids the same as with Earth life. It's hard to imagine it could be identical in all respects, but it might be so similar that the differences require specialist studies to discern them. There's a short survey with cites to various authors presenting these views in section 4 of this paper. This incidentally gives another way that life from Mars could look so similar to Earth life that it's not easy to tell it apart. It might even use DNA, mRNA, and similar or even the same nucleotides, and proteins, again similar or even most of the same amino acids, if the ones used by Earth life are optimal in some sense. Depending how optimal Earth life is, we might discover the result of a convergent evolution of many characteristics of Earth life, yet be independently evolved.
  
• Uninhabited habitats - no life but with organics, and all the ingredients for life. This may seem boring at first sight. But it could also be wide reaching in the consequences. It would tell us a lot about how hard it is for it to evolve on a planet, and about the paths it follows on the way to life. If not life itself, there has to be some complex organic chemistry going on, and cell like structures surely form, as that happens even in short term laboratory experiments. So how far did it get and what exactly happens on a world similar to Earth in many ways (especially in the early solar system), but without life?
  
  Also, on Earth it's impossible to study uninhabited habitats, except for a very short time after a volcanic eruption. Life appears rapidly on any uninhabited habitat here. On Mars, we might have the opportunity to study uninhabited habitats on a planet that hasn't been inhabited for billions of years. This could help us to understand exoplanets and the origin of life and maybe find out that life is harder to evolve than we thought. It can also help to disentangle effects of life and non life processes on Earth.
  
  Indeed, if no life ever evolved on Mars, that would suggest that evolution of life is hard and often doesn't happen. If so, Mars could be an example of one of the most common types of planet in our galaxy, and the only one of its type close at hand for us to study.
  
• Some major unexpected discovery that nobody currently is likely to predict. We need a catch all wild card here as there's no hope of being exhaustive surely :). Not when all our experience is based on trying to generalize from a single example of biochemistry based on a single origin for life.

If you think of anything I've left out here, do say!

All the possibilities here are of exceptional interest for biology. If there are habitats for life at all on Mars, whether inhabited or uninhabited, then biologists world wide will want to study them as they are now, and the results in the best case could be revolutionary for biology. Let's look a bit closer at some of these ideas.

Life that has evolved further on Mars

This is another of my speculative synthesis sections.

If all that survives are microbes, perhaps Mars life evolved further than Earth life. Its life may have novel capabilities our life doesn't have. What will Earth microbes develop in the future after another several billion years of evolution? The answer to that question could tell us about possibilities for present day Mars life.

• More efficient photosynthesis.
• More efficient metabolism
• Better adapted to sudden changes of temperature - any life on the surface has to cope with huge day / night swings
• More resistant to drying out, UV radiation, ionizing radiation.
• Able to repair itself more quickly,
• More robust and longer lasting spores and resting states.
• More resistant to oxidizing chemicals like hydrogen peroxide.

If Mars ever developed life as robust and varied as DNA based life on Earth, or even more so, then it is probably still there. Yet the conditions are so harsh that some of the habitats could be uninhabited. If life is rare on Mars and there are few spores in the dust, then it might take quite a while to find it. Indeed you could imagine a situation where some of the RSL's on Mars are inhabited, some uninhabited, and different RSL's are even inhabited by different lifeforms.

So a search even for life on Mars that has evolved further than on Earth could be elusive. And of course it could be more advanced in some ways and less advanced in others. E.g. it could be better at photosynthesis than Earth life, but less efficient metabolism, or vice versa.

See also What are the effects of these frequent ups and downs in habitability? (above)

Uninhabited habitats

This is something that Charles Cockell has explored in a series of articles. His latest is Trajectories of Martian Habitability. One thing that greatly complicates the search for life on Mars is the possibility of uninhabited habitats. On Earth, if you find a habitat with all the conditions that life needs to survive, you expect to find life also. The only uninhabited habitats are new ones, such as recently cooled lava flows, or artificially created habitats such as petri dishes, or occasionally in very extreme conditions such as patches in the McMurdo valleys (as mentioned in the quote above)..

On Mars though some or all of the present day habitats may well be uninhabited. That is to say, habitats with all the conditions for life to arise, even with organics, but there is no life there. Perhaps life never evolved, or it evolved but became extinct.

So then there are three states for Mars at any given time in its history (the abbreviations U, H and L are my own):

1. Uninhabitable - doesn't have the conditions for life (U)
2. Habitable but uninhabited. Has habitats but they are all uninhabited (H)
3. Has at least some habitats with life (L)

As Mars evolved, initially when it first formed in the early solar system, it was too hot for life, and so was uninhabitable. Then there are various trajectories it could follow after that, starting from the early Mars, during which it passes through one or more of those three states. In his paper "Trajectories of Martian Habitability" he identifies six main possible trajectories.

Let's use the abbreviation U for uninhabitable, H for habitable but uninhabited (either permanently or transiently) and L if there are at least some habitats with life. Then his possibilities are:

• "Trajectory 1. Mars is and was always uninhabitable."
  (U)
• "Trajectory 2. Uninhabited Mars, has hosted uninhabited habitats transiently or continuously during its history."
  (H)
• "Trajectory 3. Uninhabited Mars, was habitable and possessed uninhabited habitats but is now uninhabitable."
  (H → U)
• "Trajectory 4. Mars is and was inhabited."
  (L)
• "Trajectory 5. Mars was inhabited, life became extinct, but uninhabited habitats remain on Mars."
  (L → H)
• "Trajectory 6. Mars was inhabited, life became extinct, and the planet became uninhabitable."
  (L → U)

He also suggests other more complex trajectories. For instance that it starts with uninhabited habitats and the life evolves there at a much later date (H → L) . Or perhaps, it is seeded from Earth at a later date. He also suggests trajectories where life on Mars becomes extinct, and then reoriginates on Mars or is transferred to Mars from Earth (L → H → L) . Or even, a logical possibility but seems unlikely, that it was for some reason uninhabitable in the early Noachian and became habitable later (U→ H).

In his paper he discusses ways that this could be tested with observations. For instance, if you find that promising environments with water in present day and past Mars lacked some fundamental requirement for all known life, or if we found that the conditions have always outside the range of physical and chemical tolerances of all known organisms, then that could be evidence for trajectory 1 (U). If you find it was habitable for life in the past, and also in the present, but find no evidence of life, past or present, that's evidence for trajectory 2 (H), and so on.

In his Uninhabited habitats on Mars he points out that if Mars does have uninhabited habitats, these would be a useful control to investigate the role of biology in planetary scale biological processes on Earth.

Uninhabited habitats on an inhabited Mars - potential habitats that life can't reach or hasn't reached yet

Cockell also mentions the possibility that a large habitat may form on an inhabited Mars, but no life from Mars gets into it. He gives the example of a habitat created by a meteorite impact surrounded by an inhabited surface. In his example the life can't get to it from above because of UV. Here is his example:

Cockell's Figure 6

We now know about some very UV hardy life able to survive hours of exposure to Martian UV, and given that the iron rich Mars dust could protect spores from UV, and the dust-storms, I wonder if a better example might be an impact into an ice sheet?

For this idea, see Ice covered lakes habitable for thousands of years after large impacts and Ice covered lakes from volcanic activity (above) See also Ice covered lakes habitable for thousands of years after large impacts and Ice covered lakes from volcanic activity (above).

If liquid water forms but rapidly freezes over, it could trap a system of liquid water and the hot rocks and hydrothermal system below the ice before any spores get a chance to get into it from the surface. If so, and if there are no spores in the ice already and if the newly formed hydrothermal system does not connect to the deep hydrosphere, perhaps the lake could stay liquid for decades or a thousand years, or as long as it lasts, with no life in it. Perhaps that might happen most easily for a small impact creating a smaller shorter lived lake - so that it freezes over more quickly and is not so deep.

Another suggestion - even when the habitat is open to the surface and can be colonized - it might just take a very long time for life to colonize a new habitat in the harsh conditions on the surface of Mars. Not just years or decades. Perhaps it takes hundreds of thousands of years or millions of years for life to colonize a newly formed habitat on Mars?

If that's the case, then some of the RSLs for instance, could be like that in habitable, but uninhabited lake or crater. Some may be uninhabited, some may be habited and some may be in the process of being gradually colonized perhaps from one side to the other right now.

In Antarctica then layers of microbes on Antarctic rocks take from a thousand to ten thousand years to completely colonize a thin layer on the surface of a rock before the rock flakes off and the process starts again. So we have something not unlike that in Antarctica.

Processes like that could be even slower on Mars. So - how long would it take for a microbe to colonize a new habitat - also given that the habitats may be hundreds of kilometers apart and quite small scale and only viable seasonally - and given that the original population is similarly slow growing and may not produce many spores per year to be carried in the dust storms? (Individual RSLs are far closer together, but many of the outcrops and steep slopes that host RSLs are far away from each other). It might go more easily if some of the regions are colonized from below, e.g. RSLs colonized from the deep hydrosphere.

Also - if Mars life never developed photosynthesis, that would make it much harder for life to colonize it. In that case, many habitats that we might consider habitable for Earth life might be uninhabitable for Mars life, because they require photosynthesis for life to thrive there. See Case study - can photosynthetic life be transferred from Earth to Mars or vice versa? above.

Uninhabitable liquid water on Mars

There's another complication for water on Mars that we rarely encounter on Earth. Some of the water might not be habitable at all. You find life on Earth almost everywhere where you find water, or even water vapour from the atmosphere. That includes salt lakes, concentrated sulfuric acid, permafrost, inside the ice of glaciers, and places like the Atacama deserts and the McMurdo dry valleys. But you could get liquid water on Mars which is even more inhospitable for life than any of these.

We do have some natural uninhabited habitats on Earth. A nice example is honey. Though it's got plenty of moisture, the water activity level is too low and it also has anti-microbial properties. No life can colonize it; though spores can survive there in dormant form.

About the only other place where we have uninhabitable permanent liquid water on Earth may be the extremely salty Don Juan pond in Antarctica. This small pond, 100 meters by 300 meters, and 10 cm deep, is of great interest for studying the limits of habitability for present day life on Mars. Research using a time lapse camera shows that it is partly fed by deliquescing salts revealing dark tracks that resemble the Recurring Slope Lineae on Mars. The salts absorb water by deliquescence only, at times of high humidity. This then flows down the slope as salty brines. These then mix with snow melt, which then in turn feeds the lake. The deliquescing salts which start this process may be related to the processes that form the RSLs on Mars.

Though microbes have been cultivated from this pond, they have not been shown to be able to reproduce in the salty conditions present in the lake, and it is possible that they only got there through being washed in by the rare occasions of snow melt feeding the lake.

The tiny Don Juan pond in Antarctica, 100 meters by 300 meters, and 10 cm deep. This pond is about as salty as it could possibly be, with the CaCl₂ levels approaching saturation at 60% w/v. It's so salty it stays liquid all the year round, at temperatures ranging from 0°C to -40°C. It has a eutectic of -51.8°C so is believed to be liquid all the year round. The water activity level measured is an exceptionally low 0.3 - 0.6. Though the temperature range is fine for life, it may be too salty for life to reproduce there. Microbes have been found, but they could only grow in less salty conditions. It might be that microbes sometimes can grow there when the water activity level is occasionally raised through influxes of water, and then die. Or they might be washed in from the surroundings.

So far there is no evidence that microbes can actually grow there. It's of great interest to scientists studying the water activity limits of habitability for astrobiology.^] There is some doubt about whether it is completely uninhabited. But if it is, it might be the only natural body of water on the Earth of any size which doesn't have any form of indigenous life.

The interesting thing here is that though an uninhabited pond like this is so rare here on Earth that we have only one possible natural example, water like this could be the norm on Mars. Perhaps much of its liquid water is just not available for life to use. Reasons could include, too much by way of salts like Don Juan Pond (including too many chlorates, and sulfates), too much acid, or lacking essential trace elements and nitrogen. Mars also gets so cold that the potential habitats there could also just be too cold for life.

Conditions were better in the past, even in recent times when the Mars atmosphere was a bit thicker on occasion. But as we saw above, on present day Mars, even when water is not already at boiling point, it is so close to it that only salty brines could be stable in habitats exposed to the surface,- and these may be too salty for life to use. There's a narrow habitability zone between water that is salty enough to remain liquid, and water that is so salty that life can make no use of it. Since 2008 scientists have been saying that it may be possible for Mars to have habitable water. But we haven't been able to study any of these potential habitats close up yet, and we just don't know if any of them are habitable. If it does have habitable water, it may have many uninhabitable patches of liquid brines on Mars for each habitable patch.

Also habitats that seem similar may actually form in different ways. What if all three of the main hypotheses for RSL's describe different ways they form? Some due to hot spots leading to liquid water from the deep hydrosphere reaching the surface by repeated sublimation and refreezing. Some due to ancient ice from times when Mars was so tilted in its axis that it had equatorial ice sheets. Some due to salts that deliquesce in the night time humidity of the very cold though thin air on Mars.

Then some of them could be inhabited by different forms of life, and others uninhabited. Maybe even some of the streaks in a particular group of RSL's are inhabitable and others are uninhabitable, depending on the mixtures of the salts that feed them. So, on a Mars that has surface life, you could get

• Uninhabitable RSLs, water is just too cold or too salty for life
• Otherwise habitable RSLs, but they are lacking in some essential ingredient for life. Not enough nitrogen, or organics, or life is prevented by some other limiting factor
• Otherwise habitable, apart from presence of a biocidal agent (such as the chlorites and hypochlorites in the section Perchlorates irradiated with UV light to create sterilizing chlorites and hypochlorites above)
• Habitable but uninhabited RSLs which life hasn't got to yet.
• Inhabited RSLs

We could get all that variety even in what superficially seems to be the same kind of habitat from orbital photos.

There might also be a difference in habitability for Mars and Earth life. Any of these habitats could be habitable to Earth life but uninhabitable to Mars life, or vice versa. For instance if Mars life hasn't developed photosynthesis, there may be many habitats that are only habitable for Earth life. Or, as we'll see in the next section, depending on the biochemistry of the cells, there may be many "habitats" that are only habitable for Mars life.

Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells - habitable to Mars life and not to Earth life

So far, the idea is to look for habitats on Mars that would be suitable for at least some forms of Earth life, on the basis that if Earth life could live there, potentially Mars life could as well. As we saw already in the section Perchlorates on Mars (above), perchlorates were first found on Mars they seemed a major issue for life. But now they seem more like an advantage. They take in water from the atmosphere so could help create local habitats and as well as that, many microbes are able to metabolize perchlorate. So nowadays they are thought to make Mars more habitable for microbial life, not less so.

However what if, more radically, the microbes used perchlorates mixed with water as a solvent in their cells? I.e. they don't just eat it, or tolerate an environment of perchlorates, they actually have perchlorates inside their cells? They would take the place of the sodium. potassium and magnesium chloride salts that we have inside our cells. That's Dirk Schulze-Makuch and J. M. Houtkooper's suggestion in A Perchlorate Strategy for Extreme Xerophilic Life on Mars?

Also what if it has hydrogen peroxide inside its cells as well? Just as life on Earth evolved in conditions of salty water and the interior of our cells resemble those ancient seas, perhaps life on Mars could have evolved in conditions of perchlorates and hydrogen peroxide mixed in the water, and developed cells with those chemicals inside them.

If life can cope with perchlorates internally like that, then it has many advantages.

• It's a great antifreeze so they can stay active at lower temperatures, right down to -70 °C for magnesium perchlorate. That would be very useful on Mars.
• It deliquesces, takes in moisture from the atmosphere. If the perchlorates are inside the cells body it may be able to take up water directly from the atmosphere into its body at night
• Being adapted to highly oxidizing compounds could help it also deal with DNA damage from radiation

For more on this: A Perchlorate Strategy for Extreme Xerophilic Life on Mars?

Also, if Mars life can use perchlorates, what then about the hydrogen peroxide idea? The same authors presented this interesting hypothesis that some life on Mars could use a mixture of water and hydrogen peroxide as a solvent inside their cells instead of water. This is not as strange as it seems even Earth life uses hydrogen peroxide.

The Bombardier beetle uses a 25% mixture of hydrogen peroxide as one of the two fluids it mixes to make a steam explosion to fire at a predator. Mammalian cells produce it to catalyse many processes. There doesn't seem to be a fundamental issue with using hydrogen peroxide inside every cell, it's just that Earth life doesn't need it. Selection on Mars might well have favoured it strongly. They say:

"There does not appear to exist a basic reason why H₂O₂ could not be used by biology. On Earth, utilizing H₂O₂ in the intracellular fluid has little advantage with regard to temperature and availability of oxygen and water, thus the majority of Earth organisms never developed extensive adaptation mechanisms. On Mars, on the other hand, directional selection may have favored organisms which developed on an early warm and wet Mars to adapt to the progressive cooling and desiccation of Mars"

A Possible Biogenic Origin for Hydrogen Peroxide on Mars: The Viking Results Reinterpreted

Earth life almost never needs to be able to reproduce below -20 °C, at least, not before the invention of the modern freezer. For life on Mars, this would be a major advantage.

Hydrogen peroxide has many advantages for Mars life too:

• Lower freezing point - for a mixture of water and hydrogen peroxide, it's -56.5 °C. But they can also supercool sometimes forming glasses, right down to liquid air temperatures.
• Source for oxygen
• A mixture of water and hydrogen peroxide can also take in water from the atmosphere

It does have its downsides. Hydrogen peroxide is easily decomposed by UV light, so it would need UV protection. Hydrogen peroxide also decomposes spontaneously, so the cells would need some way to stabilize it. The cells would die if exposed to too much water or water vapour, so attempts to culture them for testing might easily kill them. Also if you were to heat them to analyse them in an oven, the cells would auto-oxidize because of the reactive hydrogen peroxide, leaving only gases and very little by way of solid residue and almost no organic content.

This idea of hydrogen peroxide based life was originally suggested as a possible explanation of some of the Viking observations. See: A Possible Biogenic Origin for Hydrogen Peroxide on Mars: The Viking Results Reinterpreted

Life on Mars could combine both those strategies of the hydrogen peroxide and the perchlorates. Life based on these principles would find colder and drier conditions optimal for it, so we might need to look for it in places that would be thought uninhabitable for Earth life. For instance we never get perchlorate rich liquid water at temperatures of -70 °C on Earth, so there is no reason for Earth life to be adapted to use it. Also perchlorates and hydrogen peroxides are rare on Earth, and are common on Mars. The combination of very cold water laced with perchlorates and hydrogen peroxide may be a form of liquid water that is very common on Mars. If so, and if life can adapt to life in it at all - well this may even be the most common "habitat" for Mars life. Perhaps it has both forms of life, with ordinary salt inside like Earth life, and with the perchlorates and hydrogen peroxide, the first adapted to warmer conditions (evolved first in the early oceans on Mars) and the second adapted to the colder conditions of the very cold perchlorate brines. Or perhaps - what if Mars only has the perchlorate based life, and the few patches of fresh or somewhat fresher fresher water are as uninhabitable for Mars life as Don Juan pond is for Earth life?

If this is possible, it might even be that most habitats on Mars are habitable to Mars life and uninhabitable to Earth life.

Since the perchlorate life might be very fragile in warmer moister environments - then we might also have habitats on Mars that are habitable for Earth life but uninhabitable for Mars life, perhaps with an overlap that both can occupy.

There's one other habitat for life on Mars that may exist there, though it is hard to be sure, and where we'd never think to look if we base our strategy on looking for places for known Earth life. That's in liquid ("supercritical") CO₂ - which probably exists on Mars at depths. of 100 meters if geothermal processes make the local temperatures above 31.1°C at that depth (with pressures above 73.8 atmospheres). Interestingly, that habitat might exist on Earth as well below the ocean floors.

This habitat would indeed be like Robert Zubrin's Sharks in the savannah. Again it would be uninhabitable for ordinary Earth life - though we might have lifeforms we haven't discovered yet here on Earth living in these habitats deep below the sea floor. At any rate if we do have them - you'd expect they probably wouldn't be able to get to Mars on our spaceships. More on that below.

There are several other ways that we could have habitats on Mars that Earth life could colonize but Mars life can't.

Early life not so versatile as present day life - habitable to modern Earth life and not to Mars life

This is most likely for early Mars. You might think that uninhabited habitats would be rare in the early Noachian, so long as life evolved on Mars at all. It had oceans covering much of the planet, and organics delivered from comets and meteorites. Unless its water was extraordinarily acid, alkaline, or salty, then once life arose, surely it would have spread almost anywhere?

However, if you start thinking in terms of early life, even before the evolution of the first archaea on Earth, the early Noachian may not seem so hospitable after all. For one thing, it might have taken a while before life developed hardy resting states and microbial spores. Without that, it would be confined only to habitable regions and couldn't spread from one to another easily.

So then - what would those habitable regions be, before it has hardy spores and resting states? Well, first, once again, nobody knows when photosynthesis first evolved on the Earth. Perhaps it was present almost from the beginning, but maybe it developed rather later. In an early Mars without photosynthesis, life would be confined to places where it could take advantage of chemical energy.

Perhaps it lived in hydrothermal vents, but there are many other ideas for abiogenesis (origins of life). Some think that life could have evolved in icy conditions, where melting and refreezing ice concentrates organics (eutectic freezing). Or it might have evolved on a clay substrate in a hydrogel which experimenters found can be used as a "cell free" medium for protein production from DNA, amino acids, enzymes and some components of cellular machinery. Or perhaps it evolved on pumice rafts.

(click to watch on YouTube)

One theory of the origin of life (amongst many) is that it might have started in pumice rafts like these. If this is what actually happened on Mars, and if it took it some millions of years to evolve to the stage where it could colonize harsher conditions, then we might have to search for pumice rafts to find evidence of the earliest life on Mars.

This is one out of dozens of suggestions for the origins of life. The hydrothermal vent hypothesis is perhaps the most popular but there are many others.

Another theory is that it originally evolved kilometers deep underground, rather than on the surface.

Wherever life started on Mars, the big question then is - how long did it take to spread to other habitats of the same type, and how long did it take to diversify to other habitats? Or did it ever?

What if it is still localized to its original habitats? That could lead to a planet that is very habitable to Earth life, even a planet with seas like early Mars - but with almost no life there. Only small colonies around hydrothermal vents say. Or in pumice rafts. Or wherever they are able to survive. If this is right, it would of course have huge implications on our search for past life on Mars.

Early pre-archaea type life on Mars could be extremely localised

This is another speculative section, where I invite you to think over possibilities, and present ideas of my own here to stimulate thought. I don't know of any paper that looks into this in depth. Do say if you know of anything.

So, if we search for early Noachian and pre-Noachian period life, we may find the primitive pre-archaean cells only in the hydrothermal vents. Other apparently equally habitable areas could be devoid of life. Also, even the hydrothermal vents of the Noachian period might not all have life in them. Assuming early life hasn't yet developed hardy resting states or spores ,then - it might just fall apart when it is washed away from the habitat where it originally formed. It might die because it is too cold, or too warm, or doesn't have the right chemicals in the water around it to use as "food". It would be hard for such primitive life to transfer from one vent to another. Perhaps we find that the earliest life on Mars is localized to a single hydrothermal vent. You could also envision a scenario where life evolves around a single vent there, survives maybe for as long as that vent lasts, but becomes extinct when the vent ceases to function, because it hasn't yet developed a way to transfer from one vent to another. Then, if that's possible, then different vents might have life, or protobionts, that developed independently through different pathways. There are exposed remains of hydrothermal vents on Mars, so is that where we need to go? And which one should we explore?

The first cells also might not have reproduced like modern cells with their complex transcription methods and error correction. Or, if they could reproduce in the modern sense, yet it might be with many errors and changes, and not reproduced as exactly as present day cells. Early life might have got going in fits and starts, with the first cells easily going extinct.

So another possibility there is that the remains of one attempt at life provided the raw materials for the next attempt until finally it succeeded long term. We might find long gaps with no life, then a build up of organics from life, but then conditions turn unfavourable, upset the delicate balance that let that fragile early life thrive, and it goes extinct. But not before it leaves behind layers of dead organics. So then, that life is then the basis for the next lifeform to evolve and so on. These early protobionts might not have had any informational coding molecules at all. Just split apart and the daughter cells inherit similar chemistry and chemistry of the cell wall particularly to their parent.

So, then, to take this a step further, what if Mars never developed life as robust as modern Earth life? Then life may have evolved, for instance in the hydrothermal vents, then gone extinct when the vent was no longer active. Then perhaps it evolved again from scratch around another vent. Perhaps evolution happened in slow motion until eventually, millions of years later, more robust forms developed that could survive the end of activity of the birth place hydrothermal vent. If Mars life never developed photosynthesis or hardy resting states, then the search for past life there may be elusive.

The same could be true for present day life as well. We might find that it is only present in a few locations even if there are many places that modern Earth microbes could survive. Imagine if life on Mars is like that, not photosynthetic life, only occurs in particular habitats, perhaps only in particular RSL's, and is an early life less developed than DNA life, perhaps even only recently re-evolved. It could be so vulnerable to introduced Earth microbes, and also, hard to find. Then the events of Arthur C. Clarke's "Venus story" of the introduction could happen so easily. (See Why don't explorers in science fiction have these problems when exploring other worlds? )

Indeed, it could be worse than that - though we might never know what happened in the very worst case. It could easily be extinct before we can find it. We might never know it was ever there. Perhaps eventually someone might suspect what happened and then you'd probably get endless debates about whether there was early life on Mars just before the humans got there, that just never gets resolved, like speculations about the behaviour and life habits of Dodos.

The main problem is we have no timescale for this. Of course life must start somewhere, or perhaps several places at around the same time; but how long does it take for it to develop a robust reproduction system, to develop the ability to colonize many different habitats, and to spread from these starting points to cover a planet? It might have needed millions or hundreds of millions of years in stable conditions such as hydrothermal vents for primitive pre-archaea to evolve to the complexity of a modern cell. Or maybe all this is possible within a million years or less, even just a few millennia? Or very fast - at some lucky moment, everything came together in some situation we can't yet duplicate in a laboratory and it rapidly evolved to a recognizable lifeform with exact replication in years, days, even hours??

Nobody knows. We can't yet create these conditions in a laboratory, have no near future prospect of doing so either, and have no evidence at all yet from early Earth.

Life evolving from scratch many times - multiple genesis - and could Mars have multiple "shadow biospheres", perhaps some extremely localized?

This is another of these fun speculative sections. We are used to the idea of a single genesis of life, over four billion years ago, with all present life derived from it. But is that typical of a planet with life on it?

After all with the shadow biosphere hypothesis, if that turns out to be true, Earth will have two distinct forms of life living here at the same time. Distinct in the sense that they don't use the same genetic code or use the same structures at a cellular level. If the life on Mars was less robust, no hardy spores, no photosynthesis, evolving around multiple vents (say) then it could have evolved in many different ways. There are at least some things it could have done differently. Different amino acids choice, different translation table, some subtle or some very obvious differences.

So - what if Mars has a "shadow biosphere" - and not just one, but multiple simultaneous different biochemistries? Maybe including Earth life as well, the few microbes that got there on a meteorite maybe are able to co-exist with whatever is there already. For instance, Mars could have a form of life that uses perchlorates and hydrogen peroxide inside to live at very low temperatures, and that could be the dominant life there. Then it could have Earth life like a shadow biosphere living in habitats that are warmer and wetter. Then it could have other shadow biospheres as well.

ere on Earth though many microbes are found widespread throughout the world, fresh water lakes have distinct populations of diatoms, see Invasive diatoms in Earth inland seas, lakes and rivers (below) . Mars though globally connected through its dust storms and in another way possibly through geothermal hot spots and the deep biosphere, may easily have many biologically very isolated regions. It just depends how easily the local life is able to spread in the dust storms.

Some of the life there may be able to do that while others may not. Microbes on Earth can spread anywhere through the wind. But we still have diatoms in fresh water lakes that can only be transferred from place to place on materials damp with fresh water, such as the invasive diatom that got to New Zealand on wet diving gear. So in the same way,there could be species as localized to regions of Mars as the diatoms in our freshwater lakes.

For instance perhaps there is a small shadow biosphere of relic lifeforms from billions of years ago that only lives in Hellas basin - and another one only on Olympus Mons where there may be ice, and possibly geothermal heating below the surface, and another only in Richardson crater, and another only in the RSLs along the Valles Marineres.

Hellas Basin as an example one of the many unique features on Mars, the deepest point on Mars on the Mars surface. If Mars life never developed photosynthesis and some of it is unable to spread easily in the dust storms - or for some reason requires conditions only available in particular places - then Mars could have life that is there and nowhere else. Life that is able to use the moisture in the atmosphere at night would find this one of the easiest places to live on Mars because the atmospheric pressure is highest.

Valles Marineres has the densest population of RSLs on the planet. Each blue dot here marks a site with many Recurring Slope Lineae (RSLs). The RSLs here are also associated with downward slumping of the slopes which is adding to a picture in which perhaps significant amounts of water are involved in the RSLs. Nobody knows why the Valles Marineres has so many of these. It might be due to the clouds and low level haze often seen in this area - or perhaps deliquescing salts, but if so why do they form preferentially here? Or could it be due to subsurface aquifers, but if so,how did it form? It's easy to see a newly formed crater puncturing an aquifer, but again, why here?

As we know so little about what causes the RSLs, except that they involve liquid brines in some way, different areas of RSLs might have different mechanisms. Perhaps the RSLs in Valles Marineres have unique conditions that favour a particular relic form of life. Even different RSLs in the same area could have different formation mechanisms or be biologically different.

Mars has many permanent features that could provide locally distinct conditions like this.

Or maybe they co-exist in the same habitat. There doesn't seem any reason, on the face of it, that all Earth life has to be related through a common ancestor. It's just the way it is here, and we've got used to it. But for all we know, this may be rare. Maybe most planets have a variety of biochemistries, with no common ancestors, or only very early past common ancestors?

After all, we have the fungi, plants, animals, and other branches of the tree of life, that did have a last common ancestor, but long ago. Is there anything to stop us having a similar "tree of life" but with multiple trunks and no common ancestor to them all? Is Earth perhaps even unusual in this respect?

So, we could have a very complex situation. To expand on Charles Cockell's three divisions of uninhabitable, uninhabited, and inhabited - at any stage in the history of Mars, it might be

• Uninhabitable
• Has habitats but they are all uninhabited.
• Inhabited but only in special locations, such as hydrothermal vents, or rock pools on ocean margins, or deep underground. Perhaps even each of these could be locations for a different genesis for life or a multiple genesis in the same place, different forms each time.
• Inhabited almost everywhere, so that you can find traces of early Martian life in any habitat with conditions suitable for modern life, so long as it was buried and then rediscovered in a way suitable to preserve it.
• Inhabited by a mix of forms of life with a different genesis. May have multiple "shadow biospheres". If there is Earth life, the life that has got there so far could even be a shadow biosphere compared to the dominant Mars life. Other shadow biospheres could have RNA world cells, PNA life, maybe even mirror life, etc. Some biospheres could be localized to particular types of habitat or geographical regions. Perhaps some of these could be localized to a single geographical region on Mars like some of the relic life we have on Earth.

Then any of those separate shadow biospheres could go extinct, and maybe from time to time new ones evolve too, perhaps in separate uninhabited habitats such as the isolated lakes and craters (depending how rapidly life can evolve)?

There is no way to decide between these various scenarios on theoretical grounds. The only thing we can do is to search, everywhere we can think of, and see if we can find it. We might finally find the first traces of early life on Mars in some unexpected place nobody predicted.

Mars could also have extremely localized "relic species" . As an analogy, look what happened with the "dawn redwood":

Fossil leaf of Metasequoia

Metasequoia, or the "Dawn Redwood" was originally only known from fossil leaves like this, described in 1941, and scientists thought it was extinct. That was until Zhan Wang found a small stand of a new species of tree in Moudao in China in 1944 (then called Modaoxi). He didn't know what it was.

After the end of the World War II, in 1946. the eminent Chinese botanist Wan Chun Cheng and Hu Hsen Hsu went back to this location, and described it as a new living species of metasequoia. If it weren't for that one small relic stand of metasequoia in China, we wouldn't have any living species of metasequoia left in the world today.

Now it is often grown as an ornamental tree - here is one of them in San Jose State University campus.

So, perhaps Mars similarly might have some precious relic species of some unusual form of life, microbial or even multicellular, that is still hanging on there as a relic species but is extremely localized. Perhaps to a single patch of RSLs, or it's only in Richardson crater, or only in the Hellas basin etc. Again we have no way to know about such a thing until we explore it thoroughly. It's also the sort of thing that could easily go extinct before we find it, especially if it is a relic species of microbe, of unusual biochemistry.

But will we get the opportunity? Perhaps it is already too late to find present day life on Mars? Maybe the signal from present day life is also already confused beyond recognition by life we have already introduced?

Idea that we have contaminated Mars too much already, so there is no point in protecting it

This is another argument that enthusiasts for humans on the Mars surface often bring up. The idea is that Mars is already contaminated with Earth life. It's a sad loss for astrobiology, and a total loss. We have lost all opportunity to study a pristine Mars already. There may have been life there in the 1960s but if so it is now extinct or has merged with Earth life to the extent that we can't tell it apart. But it's too late to do anything about it, and we might as well press ahead and send humans there and study what we have now, such as it is.

That would be an overwhelmingly sad message for astrobiologists. But a message of encouragement and hope to some space colonization enthusiasts.

Well, there is some truth in the background to this. It's surely true that there is Earth life there already from our spaceships. But our planetary protection measures take this into account and they did take care. Carl Sagan's aim has often been misunderstood I think. He and the other early planetary protection scientists never aimed for a certainty of keeping Mars free of Earth life as there was no way they could have achieved that, at least, not with the technology of their day. It was either not to go there at all, or to set an acceptable probability of contaminating Mars. So his idea never was to be 100% sure we can't contaminate it.

He actually proposed a 1 in 10,000 chance of contaminating Mars per mission and a 1 in 1000 chance of contaminating it during the exploration period. Those were the actual numbers used for the Viking missions.

Of course ideally we would like a 100% sure chance that we won't contaminate it. But we can't do that at present. I think personally that we should aim for 100% or as close as makes no difference, for Europa and Enceladus by sampling the plumes rather than landing, and eventually finding a way to 100% sterilization. That's not impossible for near future technology. I think we should also aim for 100% sterilization for anything that contacts a liquid water habitat on Mars. The Viking spacecraft were not sterilized sufficiently for direct contact with a liquid water on Mars, but this is something we could achieve in the near future. See Can we achieve 100% sterile electronics for a Europa, Enceladus, Ceres or Mars lander?

But for Mars the die is cast. However, if we have managed to achieve Carl Sagan's goal, we can be at least 99.9% certain that it is not yet contaminated.

The decision to stop sterilizing to Viking level also is sometimes presented as if the scientists just threw in the towel and gave up on protecting the planet properly. But far from it. This was a carefully thought out decision based on the Viking discovery that the conditions on Mars are so harsh that in their opinion they roughly corresponded to the heat sterilization stage of preparation for the Viking lander. So after Viking they did all the sterilization for the Viking lander except for the final heat sterilization phase on the understanding that the Mars surface would do an equivalent level of sterilization for us.

Critics say that they stopped protecting Mars after Viking, but that's not true or was not the intention at least. We still have planetary protection officers and regular biannual meetings of COSPAR to protect Mars and the rest of the solar system. They continue to protect Mars with the same idea as Carl Sagan had originally, that the chance of contaminating Mars per mission is tiny, somewhere around 1 in 10,000 (probably less).

The Viking lander being prepared for dry heat sterilization for purposes of planetary protection. Since then this stage of planetary protection has been omitted, based on the reasoning that the harsh Viking surface roughly corresponds to the heat sterilization stage.

The reason for the change in sterilization policy is that before Viking they didn't know quite how hostile conditions were there. After Viking, these measures are still needed if the spacecraft contacts regions in Mars that could be habitable for life- the"Special regions".

They did give up on the use of probabilities pioneered by Carl Sagan et al, because of the impossibility of doing a calculation to find an approximate probability to life contaminating Mars when conditions on Mars are so hostile to life. However the basic objective hasn't changed. The aim is still to have a tiny chance of contamination, of the order of 1 in 1000 during the "exploration phase" of perhaps 57 ground missions an 30 orbiters (Carl Sagan's figures). Yes, we've had crashes on Mars, notably of the Mars Climate Orbiter which was not sterilized to that bioburden of 300,000 normally required for a Mars lander. However it also didn't have many microbes, not like a human occupied spaceship, and was not designed to withstand re-entry into the Mars atmosphere either, with no heat shield, so probably disintegrated, and was sterilized pretty much during the fiery heat of the re-entry and the crash itself.

The current guideline, for Curiosity and for all other missions to the surface (apart for those that search for present day life or contact the special regions) is to reduce the bioburden to 300,000 bacterial spores on any surface from which the spores could get into the Martian environment. Any heat tolerant components are heat sterilized to 114 °C. Sensitive electronics such as the core box of the rover including the computer, are sealed and vented through high-efficiency filters to keep any microbes inside.

That is a level of protection we can do with rovers and landers. It is totally impossible to achieve anything like that once you have humans on board. So, the scientists say they think it probably has worked, and you can't expect them to say this for a certainty as the aim never was certainty. Let's look at that a bit closer. Has it (probably) worked?

Could we have contaminated Mars already?

Mars has turned out to be a bit more hospitable than we thought when they first relaxed these provisions after Viking. So that raises the prospect - what if it is already contaminated? I think the Phoenix lander is one of the most likely to have contaminated Mars, or alternatively the Mars Polar Lander because it crashed in polar regions. The Mars Climate Orbiter also is worth considering too, because it wasn't sterilized for a landing on Mars, although it probably disintegrated pretty much (but the Mars atmosphere is rather thin). The early Russian probes that landed on Mars also may need consideration because the Russians were a bit vague about how much they sterilized them, and what they did exactly to protect Mars.

After all, Phoenix observed what seemed to be droplets of liquid salty water on its legs, possibly Nilton Renno's droplets forming on salt / ice interfaces. These fell off the spacecraft onto the ground (or at least suddenly vanished from the leg).

Possible droplets on the legs of the Phoenix lander

Also Phoenix got crushed by the advancing dry ice in winter, as was expected for its location. It was never designed to last the winter as they knew it would get sheets of dry ice form on top of it and break it up.

Phoenix lander crushed by frost - layers of dry ice forming on the solar panel in winter snapped one of them off. It was not expected to last the winter. The right hand image shows it in 2010, two years after the image on the left which shows it after landing, in 2008.

(NASA, 2010. Phoenix Mars Lander Does Not Phone Home, New Image Shows Damage)

If any of our landers have contaminated Mars, I'd have thought Phoenix was a likely candidate. As usual it was sterilized to high standards, but before Phoenix nobody realized there was any possibility of liquid there. Most of those micro habitats are probably either too salty or too cold for life but are there any that Earth life could survive in? And are there any of those close to where Phoenix lies on the Mars surface? We just don't know. Experiments show that it is possible to achieve habitability but it depends on the particular mix of salts.

Jim Young (left) and Jack Farmerie (right) from Lockheed Martin, working on the Phoenix lander science deck under clean room conditions to protect Mars, following planetary protection guidelines. Credit: NASA /JPL/UA/Lockheed Martin.

However nobody back then knew that liquid water could form on the surface in those regions.

The entire polar regions of Mars are now declared a "Special Region" and all modern landers there will need Viking level sterilization for any part of the spacecraft that could potentially contact a microhabitat. Has Phoenix contaminated Mars already? The consensus seems to be that it probably hasn't, but its site would be an ideal one to visit to check how effective our measures to date have been.

I think myself that a priority mission for planetary protection is to send a lander to investigate one of these sites in situ. If Phoenix, say has started to contaminate Mars we might find a small enclave of life around the lander. I think that it is high time that we actually had a mission to the surface to test to see how effective our planetary protection measures are. Phoenix is also a good candidate to study because we know exactly where it is, and have a lot of ground data from it too from before it got crushed by the advancing dry ice.

A mission like that could be dual purpose, first to search for life habitats, past and present life signs etc, in what is now thought of as a "special region" that could have present day life. So it would land some distance away from Phoenix, equipped with in situ life detection instruments. Then it would travel up to the crashed lander, photograph it, and analyse the remains and test for liquid water droplets and for signs of life. It could also examine the spacecraft itself for viable life there to test how sterilizing the Mars conditions actually are and whether they do correspond to the dry sterilization stage of Viking.

A similar mission could study the Mars polar lander crash site. That is, if we can find it. The "crash site" identified in 2005 turned out to be just natural features- the "parachute" was a sunlit hill.

They are both going to be interesting sites to study on the ground. Of course we need these follow up missions to be well sterilized themselves. So maybe we aren't quite ready for them yet, but we need to work on that anyway if we are going to do in situ present day life searches.

What if we have contaminated Mars?

First, if there is Earth life there already, brought on our landers - the last thing we should do is to introduce new life. For instance if it has been contaminated by a photosynthesizing green algae, perhaps chroococcidiopsis, well perhaps that plays nicely with much of the native Mars life. Even if what we have there includes a vulnerable RNA world shadow biosphere that has been made extinct on Earth, well whatever is there is obviously well adjusted to oxygen, including the perchlorates and hydrogen peroxides. A little extra oxygen from green algae is not likely to bother it. The green algae as primary producers may even be a source of food, creating new organics from just sunlight, CO₂ and trace elements. In this scenario, the green algae could also be harmful of course, it produces various biochemical byproducts, some of which may be toxic to the native life there, and so it may even have made some vulnerable Mars life extinct already, but there may be many lifeforms there not affected or even benefited.

So if something like this has happened already, this doesn't mean that it is okay therefore to introduce all the microbe species that would get there after a crash of a human occupied spacecraft on Mars. That's like saying

"Oh dear, we've introduced rabbits to this island! That's s the end of any attempt to protect it from invasive life, so you might as well introduce rats, cane toads, goats, cats etc, etc, go ahead, any invasive species is okay now."

Who would say that? There may be many things on the island that are vulnerable to rats, cats etc that are not harmed by rabbits.

Or it's like, if your garden or field is overrun by kudzu, the answer is to say

"Okay, this place is overrun by kudzu now, so let's have Japanese knotweed, let's have Himalayan Balsam, let's have every single invasive species that ever causes problems as obviously it's all over now."

Again, no gardener or farmer would ever do that. Instead they'd minimize the effects of the kudzu as much as you can, try to clear areas and eradicate it where possible, and do whatever they can to prevent the other species from invading. They'd be even more alert for other invasive species after that.

Kudzu, an invasive species, on trees in Atlanta Georgia.

By analogy with the situation of that farmer or gardener, if we find that Phoenix has introduced life to Mars, or any of our other landers or the spacecraft has crashed there - then the first priority would be to see if we can limit or reverse the damage. The life would be slow growing in such harsh conditions as prevail on Mars. If we are lucky, perhaps it hasn't yet spread more than centimeters beyond the crash site or lander.

Perhaps we could sterilize it with ionizing radiation or similar. We could take a high intensity gamma radiation emitter to Mars, shielded of course, and unshield it and use that to sterilize the immediate vicinity around the lander. It might be a useful piece of equipment to carry with us on that mission I proposed to look at the Phoenix lander close up. Who knows, maybe it is not too late and we can sterilize and reverse the contamination completely. And if not, we manage it as much as we can, slow it down as much as we can, to at least give us a chance of studying other habitats before the invasive species gets to them, maybe even protect them somehow, and we would make sure we don't introduce any other invasive microbes to Mars.

This is just being sensible, and keeping our options open for the future. If we have introduced the equivalent of rabbits to Mars - well let's be careful not to introduce the rats and cane toads as well, until we find out what is going on there.

However you might bring up one more argument (not one of Robert Zubrin's this time) - surely there is no life on Mars, however habitable it may seem to be in these microhabitats, or deep down, because its atmosphere is more or less in equilibrium? Does that not mean we have nothing to worry about here?

This argument is of special historical interest as it was one of the main reasons that Mars was thought to be free of life, at least on the surface, for many decades. Viking found sterile seeming conditions (apart from the labeled release experiment) and James Lovelock predicted that it would be lifeless from studying its atmosphere, and Viking seemed to confirm his prediction. Putting all that together, it seemed a reasonable conclusion that present day Mars is sterile.

Has everything changed now. If so, how and why did that happen? Let's look at this argument.

James Lovelock's argument for a lifeless Mars

This idea goes back to 1967. James Lovelock, originator of the Gaia hypothesis, found a way to use a planetary atmosphere to detect life. He suggested that we look for:

• Simultaneous presence of pairs of gases like oxygen and methane that react together.
• Gases such as oxygen above levels expected from abiotic processes.

As far as we can see, Mars atmosphere seems to be close to equilibrium, and has only a small amount of oxygen. So when Viking I and II landed there in 1976, and found a barren desert-like surface, this confirmed the prediction from these atmospheric measurements. It seemed natural to conclude that there is no life on Mars. As James Lovelock puts it in retrospect (see Some historical comments on this page):

"We found an astonishing difference between the two atmospheres. Mars was close to chemical equilibrium and dominated by carbon dioxide, but the Earth was in a state of deep chemical disequilibrium. In our atmosphere carbon dioxide is a mere trace gas. The coexistence of abundant oxygen with methane and other reactive gases, are conditions that would be impossible on a lifeless planet. Even the abundant nitrogen and water are difficult to explain by geochemistry. No such anomalies are present in the atmospheres of Mars or Venus, and their existence in the Earth's atmosphere signals the presence of living organisms at the surface. Sadly, we concluded that Mars is lifeless now, although it may once have had life."

However, just the year after Viking, in 1977, scientists trawling the ocean depths found life in the hydrothermal vents, which could not be detected by his methods. Since then we've found other ways life can get hidden from these observations of the atmosphere. Indeed, as we will see, perhaps Earth is the unusual case here, rather than Mars, with its biosphere that could be detected indirectly just by studying its atmosphere, from a distance.

His argument is no longer thought of as being a clear cut way of finding life around another planet, with a problem of many false negatives and false positives, as we'll see. Yet it still remains one of our best tools for searching for life on distant exoplanets around other stars. Also, the idea behind it, applied at much more sensitive levels than he envisioned would be necessary, is one of our tools for searching for hard to detect subsurface life in our solar system too (for instance it's essentially his idea that we use when we search for a methane signal on Mars).

This section is synthesis again. I can't find a paper or other source that puts it quite like this, but I think it's an interesting parallel development - how our ideas about Lovelock's argument have evolved at the same time as we have developed new understanding about the possibility of life on Mars, even with the atmosphere in equilibrium, first deep below the surface, and then even on the surface. Also, I think it's a nice way to approach the subject. Why are so many scientists upbeat about life on Mars now? What has changed since the extreme pessimism many scientists had about any present day life there in the immediate aftermath the results from Viking in the 1970s? I think it is a lot to do with this change of ideas about how to interpret Lovelock's argument, and increasing awareness of the possibilities of false negatives.

The calculations in the section below How much oxygen would surface photosynthetic life produce on Mars? are my own as I can't find a paper that calculates this.

Original argument

For his original paper see Life detection by atmospheric analysis (1967). When he wrote this, we had no observations from the Mars surface. The only close up information he had was from the Mariner 4 flyby.

Our best image of Mars when James Lovelock's paper was published - this was taken by Mariner 4 in 1964.

But he didn't need any observations from close up. Its atmospheric composition is something we can measure from Earth, by clever disentangling of doppler shifts - a technique pioneered in the Lick Observatory observatory in 1926,

Lick observatory in 1900. First conclusive proof that there is almost no oxygen on Mars was a paper from this observatory in 1926

They showed that the amount of oxygen was at most  two thirds of the levels at the top of Mount Everest (which is 8.848 km above sea level) . More sensitive measurements since then, including measurements on Mars, show that it's atmosphere is 0.146% oxygen, and since the atmosphere is only 1% of Earth's, that means, it's around 0.015% of Earth's oxygen pressure.

You would need to be at a height of 30 kilometers on Earth, to have the same atmospheric pressure as the lowest points on the Mars surface. But the oxygen levels are much lower even than the levels at 30 kilometers altitude on Earth.

Atmosphere out of equilibrium without life

James Lovelock's argument is not about oxygen alone, because this can be created abiotically. After all, Mars is red, covered in iron oxides. Where did all the oxygen come from, to turn it red? There are no iron oxides to speak of on the Moon, although it has iron mixed in with the dust everywhere. So you need some kind of special processes to form them. So, Is this evidence of past photosynthetic life on Mars?

Earth, which has abundant iron oxides in the ancient "banded iron formations", so how did these form?

The banded iron formations, of iron oxides. We still don't know how they formed with several competing theories.

Banded iron formation at the Fortescue Falls, photograph by Graeme Churchard

The best known theory for these formations is that the early Earth oceans had iron dissolved into them, which then precipitated out as iron oxides when the first oxygen producing photosynthetic life evolved. (see Role of Microorganisms in Banded Iron Formations by Inga Köhler et al.).

However there are other ideas. They could come from photo-oxidation of the iron through UV light hitting water in the top layer of the ocean (see UV-Photooxidation Model - page 9), because there was no UV blocking ozone layer. An alternative theory by Desmond Lascelles is that the iron came from black smoker hydrothermal vents - he argues that iron would not have been able to dissolve into the early oceans in significant quantities, as it is only soluble in acid conditions (more details here).

So even for the Earth, we don't know for sure that they were created through biological processes. Those explanations could also work on Mars, for instance, here is a suggestion that the iron oxides could have been created through photo-oxidation by UV in shallow seas in early Mars.

However, we have yet another possible abiotic explanation, on early Mars, this time, one that wouldn't work on Earth. It ties in with another mystery on Mars. We have evidence of early seas on Mars, but there is not nearly enough ice there now to fill those seas, if it was all melted. So what happened to all the water? It could have sunk deep below the surface or formed hydrated minerals. However, one other possibility is that Mars lost all that water from its upper atmosphere. It lost its magnetic field long ago, and so has almost no protection from solar storms and solar winds. The high energy solar particles can split the hydrogen from the oxygen in water, and then the hydrogen could escape, leaving an oxygen rich atmosphere. So, in this scenario, the oceans on Mars lose water vapour to the atmosphere, the atmosphere loses hydrogen to space, oxygen builds up in the atmosphere and this reacts with the iron to form the iron oxides which turn Mars red.

Bernard Wood suggested this, in a paper published in Nature (2013). You don't need much oxygen in the atmosphere at any one time to turn Mars red, not the 20% of Earth's atmosphere, so all this might happen more easily than you'd expect.

This shows Gusev crater. Bernard Wood came to his conclusions after studying rocks from Gusev crater, amongst other evidence. He showed that the rocks came from a more oxygen rich environment than the subsurface rocks sampled by the Mars meteorites in our meteorite collections. He suggests that the Mars atmosphere was "warm, wet and rusty" billions of years before Earth. Curiosity later made observations of manganese oxide, leading to similar conclusions, see Oxygen rich atmosphere for early Mars (above).

However, this does not mean that early Mars had photosynthetic life (though it could) because the oxygen can be created by non biological processes, such as splitting water into hydrogen and oxygen, and then the hydrogen escapes from the upper atmosphere.

Jupiter's moon Europa has a similar story going on - though for different reasons. Jupiter's intense radiation splits hydrogen directly from the ice in its icy surface, leaving it oxygen rich. Eventually enough of this may find its way to the under ice ocean keeping it oxygen rich.

See Europa's churn leads to oxygen burn

So, though an oxygen rich atmosphere or evidence of oxygen dissolved in the sea may be suggestive of life, it's not at all conclusive as there are many other ways it could form.

James Lovelock's life detection method

James Lovelock was well aware of non biological ways of creating an oxygen rich atmosphere. So, his life detection method is based instead on a search for methane and oxygen simultaneously, or any other similar trace gases that would react with each other quickly and be removed from the atmosphere if it weren't for the presence of life.

"The biological significance of an atmospheric mixture lies in the relative concentrations of a variety of constituents and not wholly in the presence or absence of any single one of them. Such a mixture is biologically significant if it represents a departure from a predictable abiological steady state. The strongest evidence is the simultaneous presence of two gases, like methane and oxygen, capable of undergoing . irreversible reaction; even with gases which can be expected to occur under abiological conditions, departures from the abundances to be anticipated will be of biological significance."

He also suggested that we can search for life in a different way by looking at isotope ratios. Living organisms preferentially makes use of lighter isotopes, which is a distinctive feature of life.

Viking results

Most of the experiments on Viking, didn't detect any life on Mars. One experiment gave ambiguous results, and some scientists think that perhaps Viking did detect life on Mars. We haven't sent any missions since then with greater sensitivity to check this - for more about this, see Rhythms From Martian Sands - What Did Our Viking Landers Find in 1976? Astonishingly, We Don't Know . Another possibility is that Mars has life that self destructs when exposed to the conditions of the Viking instruments, see Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells. Also the Viking landers landed in what we now know to be probably amongst the most inhospitable regions of Mars for life.

Many scientists felt that this confirmed the idea that Mars is a completely lifeless world. However, the Viking landers just sampled a couple of locations on Mars. The results were inconclusive because of the confusing surface chemistry. Even back then they knew that Mars was geologically varied planet. So why were the scientists so ready to conclude that the surface was sterile from these ambiguous results?

Well part of it at least was that it confirmed James Lovelock's prediction from observations of just the atmosphere. They also had the evidence of an atmosphere so thin that fresh liquid water would not be stable for long, and the ionizing radiation was severely limiting for dormant surface life over millions of years timescales.

Yet, even back then, some scientists, Gilbert Levin particularly, thought that Viking possibly did detect life. They thought that life could exploit the frosts that form on Mars in the mornings even in the equatorial regions, as they evaporate. They didn't have the modern idea of deliquescing salts, but it still seemed possible that somehow life could exploit it. For more about these frosts, see Equatorial frosts - and a source of water for the deliquescing salts (below)

There is no doubt that Lovelock's paper was an important contribution to the field - and it went on to become the foundation of his influential and important "Gaia" theory. However almost immediately after Viking, scientists began to find ways that life could evade detection by James Lovelock's method.

Hydrothermal vents

This is a discovery from 1977, just a year after the Viking lander on Mars, and ten years after Lovelock's paper, with the first paper on the subject published in September 1977. Scientists discovered the hydrothermal vent communities around the black and white "smokers" on the sea beds. They form at great depths, where the Earth's plates are pulled apart along the mid ocean ridges. These were able to survive with almost no communication with the surface.

The higher lifeforms there did use oxygen dissolved in the water. Some of the microbes however didn't rely on anything except chemical energy and did not need oxygen.

Hydrothermal vent

Tubeworms deep under the ocean living on the hydrothermal vent

Complex flow of chemicals for a hydrothermal vent. See Deep sea vents (Microbe wiki)

Cold seeps

This was just the first of many such discoveries. Now we also have cold seeps, where microbes use methane and hydrogen sulfide as an energy source

See Cold Seep Communities

The interesting thing here is - that a planet could have communities like this at the depth of oceans - and they would have no effect at all on the atmosphere. Even the chemical byproducts just get dissolved in the ocean and do not return to the atmosphere.

They put the ocean out of equilibrium locally but would not be detected as life by his method, in an ocean world with no other life on it. This lead to the idea of life on Mars not connected to the surface, deep down below the surface.

So then, though the scientists of the 1970s (apart from Levin), for the most part had given up on the idea of surface life on Mars, the search was on for ways it could survive below the surface.

Mars hydrosphere

Though Mars doesn't have oceans with hydrothermal vents, it might have life in its hydrosphere. Unlike Earth, Mars has a cryosphere over its entire surface - a deep layer of ice, which is well below the surface in equatorial regions, which remains at temperatures below freezing point of water. But deep down, the temperature rises, and a few kilometers below the surface - it gets warm enough - and also pressures high enough - so that there may well be a layer of liquid water. This could be as much as a a hundred meters thick, possibly more. This could be an ideal habitat for microbial life, and it would be as disconnected from the surface as Earth's hydrothermal vents, so Lovelock's atmospheric measurements would miss it. We also have deep subsurface life on Earth, for instance in the Mponeng Gold mine in South Africa, indeed wherever we drill, deep below the surface on Earth we find life. These are thought to be good analogues for the deep subsurface on Mars, see section 3.2.2. of this paper.

Pole to pole cross section of Mars showing the cryosphere - permanently cold enough for ice - and below it, the saline groundwater. This may well be the most habitable region of Mars - on Earth similar habitats deep below the surface have abundant life - but may not be much exchange with the surface.

Mars probably also has hot spots nearer the surface - as it has been geologically active in the recent geological past (though no confirmation yet of any present day hot spots and geological activity). If so - if the hot spot is beneath a "trapping layer" much like an oil field - it could provide another habitat with liquid water and almost no communication with the surface.

Snowball or slushball Earth

Then a bit later we discovered that a habitable planet can have stages in its history when it is almost devoid of life that can interact with its atmosphere, yet life still goes on, in its oceans. Earth has gone through phases in the past when it was covered in ice, or almost completely so. This is the Snowball - or Slushball Earth

Slushball Earth

Perhaps Earth had patches of open ocean in the tropical regions, or perhaps it was completely covered in ice.

Possible Snowball Earth models from the Snowball Earth teaching slides.

So for millions of years - the entire surface of Earth -except possibly a small region of pack ice near the equator - was completely covered in ice. It was as barren as Antarctica - but without the penguins of course. There was life there - but so little - that if an extra terrestrial had landed a Viking probe on a random spot on Earth, chances are they would not have spotted a thing.

Once a planet is covered in ice - then it becomes highly reflective, it doesn't absorb as much heat from the sun - so it is pretty hard to get out of it. Earth got out of this phase because of continental drift, then eventually limestone that deposited in the oceans during its warm phase gets subducted beneath the continents, and then returns to the atmosphere in volcanoes. With no life to turn that back into organics, and no oceans for the CO₂ to dissolve into - then the atmosphere got richer and richer in CO₂ Finally the ice over the oceans began to melt, and life started to flourish on the surface again. It was always there, below the ice, in the oceans, maybe in some of the warmer spots where the ice was melted, and now it has its chance to return.

Snowball Mars

This then raises the interesting idea - what if the same thing happened on Mars? Well perhaps it did. Perhaps we see the results.

As we've seen, Mars is dry right now, or at least, it has no streams or lakes or ponds, but it might have shallow ponds occasionally as the atmosphere gets thicker when its axial tilt changes. At other times, it might survive as dormant spores, perhaps buried deep below the surface in caves or underground, protected from cosmic radiation.

However, we have pretty much conclusive evidence that Mars had a shallow ocean covering much of its northern hemisphere in the first few hundred million years of the solar system (and briefly again a billion years later). At the very beginning, Mars, Earth and Venus could have been almost identical with dense CO₂ rich atmospheres of tens of atmospheres of pressure. They were "planetary pressure cookers" - the high pressure atmospheres raised the boiling point of water permitting all three planets to have global oceans of water well above boiling point. These atmospheres were also nitrogen rich - and the seas were rich in nitrates formed during asteroid impacts in the "Late heavy bombardment" - and they probably also had organics delivered by comets.

Mars is further out from the sun of course, and had no Moon forming impact to sterilize it. Also, it may have formed more quickly than Earth did. So life may have begun there earlier than on Earth. It's surface cooled down more quickly however over time periods of hundreds of millions of years, it lost its atmosphere, perhaps partly to space, and partly into the ground - and eventually, it entered the snowball phase far sooner than Earth.

Once it became "snowball Mars" then - Mars has no continental drift. It has some volcanic activity, but nothing like Earth with its numerous volcanoes returning carbon dioxide to the atmosphere. So unlike Earth which went through many snowball phases and survived - Mars just got colder and colder. There were temporary respites - floods perhaps due to giant impacts - also its axial tilt and orbit changes far more than the Earth's so that also led it's oceans to melt from time to time. But before long it went into a permanent snowball phase.

So, we can think of present day Mars as being "Snowball Mars" except - that eventually - the ice disappeared as well - either lost to space - or underground - we don't know. The result is a planet that is as cold or colder than Snowball Earth ever was - but without any ice so dry as well.

What would happen to life on present day Mars

So - let's suppose for purposes of discussion that Mars did have life in the early solar system? What would happen to it as a result of this snowball phase? Well - to start with for sure, it would be like Earth in its Snowball Earth phase. There is life there - but it's hard to detect and mainly underground. What little there was left on the surface - would have no detectable effect at all on the atmosphere.

It would continue to have oxygen at high levels in its atmosphere for a while, but with much less life on the surface because of the cold conditions, levels would fall down to whatever levels can be maintained by abiotic processes and the small amounts of life that still remain.

If we were to spot an exoplanet like that around another star - then again - as for Snowball Earth - we would have no way of knowing that there is life there. And if you landed Curiosity on it - you would not expect to find anything. Eventually - you might think - with the surface completely frozen, and no liquid water - it would be totally underground - so - this seems, on the face of it - to feed into the idea that Mars must be totally lifeless on the surface - and only have life deep underground.

However, various discoveries in Antarctica and in the permafrost layer have changed this picture

Mars can't get out of its current "dry snowball" phase through biology.

There is no way that life on Mars could turn it back into a planet like Earth. It's too cold, too far from the sun, too dry etc. Earth was only able to escape its snowball phase because of continental drift, which Mars doesn't have. Oxygen generating photosynthesis cools the planet down which for Mars makes it less habitable, not more so. Perhaps life could warm Mars up by generating methane and other potent greenhouse gases biologically. But it doesn't seem to have done that, not in vast quantities, not in modern times.

Indeed, if anything, perhaps photosynthetic life from Earth, if transported to Mars, would be a form of "Anti Gaia" making it less habitable. I discuss this in Could oxygen generating photosynthetic life set up an "anti Gaia" feedback on Mars? (below) .

However - though it couldn't keep Mars habitable - life on Mars could have continued to survive, by "going slow" as the planet became less and less habitable, exploiting tiny niches or using very slow metabolisms more on that in a minute. Or by going deep below the surface.

Underground caves

One way that life could continue on Mars is in underground caves that have almost no communication with the surface. Such as microbes in the Lechugulla cave, isolated from the surface for millions of years.

The caves were carved out by sulfuric acid - which recently was drained exposing the giant crystals.

Life in the rocks themselves

Earth also has life kilometers underground within the rocks themselves.

the "Worm from Hell" from 1.3 kms deep, about half a mm long, lives on microbes, need minimal oxygen.

So - perhaps similarly Mars could have life - but only in caves or deep underground, in some way not connected to the surface.

Life that can survive in tiny microhabitats - could we spot it from orbit through its effect on the atmosphere or otherwise?

Then the next thing that happened, quite recently, in 2008 onwards, now we have had many discoveries of possible habitats for life on or near the surface. But this time they are microhabitats. Tiny millimeters deep thin films of liquid brines, micropores and such as the warm seasonal flows (see above) and sometimes just droplets of brine, still a "swimming pool for a bacteria" with Nilton Renno's discovery of a new way for droplets to form rapidly where salts and ice meet,. So, what effect would something like this have on the Mars atmosphere? Would this life be detectable from orbit? Might we even be able to just see it from orbit?

If plant life is actually on the surface, covering parts of the planet and photosynthesizing, we may be able to spot the chlorophyll itself through its characteristic "red edge"

Figure 1 from this paper, showing the "red edge" of normal plant life on Earth. The graph shows the reflection spectrum from a deciduous leaf. The visible spectrum is from 0.39 to 0.7 microns. Blue light is to left (shorter wavelength) and red is to right. Chlorophyll absorbs strongly to either side of the "Chlorophyll bump" in the green. But above 0.8 microns, in the infrared, there is much less absorption and the graph shoots up abruptly. This is the "red edge" and you get this with most chlorophyll based photosynthetic life.

In the early twentieth century, one of the big mysteries of Mars was its "wave of darkening" that often happens in the Martian spring. Mars gradually darkens, from pole to equator, changing from dull coloured to darkish greenish hues within a few weeks of the polar caps disappearing. Was this due to vegetation nourished by the melting polar snow in spring? Astronomers took spectra of Mars during the darkening events and searched for the chlorophyll hump and the red edge. But they didn't find any evidence of vegetation. Millman in 1939 was able to rule out vegetation (at least Earth-like vegetation) with his 72 inches telescope. Later on they found out that the darkening was due to wind blown dust. In 1993, Carl Sagan tried observing Earth from Galileo during a couple of flybys of Earth on its way to Jupiter and he was able to detect vegetation on Earth through its "red edge". For more details see the history section "Red edge observations in the solar system" in Vegetation’s Red Edge: A Possible Spectroscopic Biosignature of Extraterrestrial Plants

This could be a way of detecting life around other planets. It doesn't work for Mars though as it doesn't have significant amounts of vegetation, if it has any. Mars may have lichens, but the "red edge" is weak or sloping in lichens. In that paper they also found that the one example of purple bacteria they looked at didn't have a red edge either and these are a very likely organism for Mars. See Spectral Signatures of Photosynthesis. I. Review of Earth Organisms. They concluded there that extra terrestrial plant life might not necessarily have a "red edge" but it might still be noticeable as a result of pigmenting the surface in unusual ways as seen from a distance.

So anyway - although it's a nice idea, probably we won't be able to detect vegetation on Mars. First, with the very strong UV levels there, most photosynthetic life is probably huddled inside rocks. Even lichens would be in partial shade and huddled into cracks in the rocks. I wonder if they could even be pigmented with the iron oxides in some way to help shield them from UV? If so maybe they'd even be rust coloured with a rather similar spectral signature to the dust. At any rate they are not pigmented in some unusual and obviously non geological fashion, or we'd have spotted them from orbit by now. Photosynthetic microbes also, such as the cyanobacteria are probably huddled inside the rocks, just below the surface, or beneath a thin layer of dust on the surface, or in partial shade again.

But what about the oxygen produced by life on Mars? Could that be detectable? Or any other surface life byproducts?

Most of the attention seems to be on detecting gases from subsurface life, and especially the methane of course. But by James Lovelock's arguments, if there is some surface life, even in an extreme "snowball" Mars as it seems to be (apart from lack of ice), might we still have a chance of detecting it? Maybe even if it is very sparse as we do have very sensitive instruments, such as the ExoMars Trace Gas Orbiter due to start work later in 2017?

If anyone knows of a detailed research paper on this, please say and I'll update this section accordingly. The closest I've found to date is Charles Cockell's Cryptic photosynthesis – extrasolar planetary oxygen without a surface biological signature . But that's about detecting exoplanets at a similar stage to early Earth, and assuming that it had little by way of land vegetation, except cyanobacteria and the like hidden within rocks, and with the sea photosynthesis mainly at lower depths within the sea, not right on the surface. It's interesting material, and I've used a figure from it in my calculations (below), but it's not directly applicable to present day Mars, as that's much less habitable than this.

So, until we can find a study of this to cite, let's just have a try at a first rough estimate. I'm not going to try any detailed analysis. But let's look at something quite crude, the amount of oxygen the lifeforms produce. I know that oxygen is not a sign of life by itself. But the amount of oxygen produced by photosynthetic lifeforms can give us a rough first idea of how much of a difference microbes can make to the atmosphere in these arid sparse microhabitats. Also if they produce enough oxygen, this would be noticeable. Also, perhaps it could lead to limits on the amount of photosynthetic oxygen producing life there is in the surface habitats?

This is only relevant of course if the surface life is photosynthetic, and moreover, based on oxygen producing photosynthesis. The "proton pump" based photosynthesis of the haloarchaea would have no effect on the atmosphere at all.

So, if Mars has oxygen producing photosynthetic life, on it surface, could we detect it from orbit? Assuming conditions similar to the Antarctic dry valleys perhaps?

How much oxygen would surface photosynthetic life produce on Mars?

This section is my own attempt at a calculation - I can't find a published paper that does this. As usual, if you know of one, do say. Meanwhile, let's have a go at an estimate. So, first, let's suppose that the entire Mars surface supports oxygen producing life just beneath the surface of rocks in similar conditions to the Antarctic dry valleys. Charles Cockell estimates their oxygen production as 2.2 grams per square meter per year (see page 7 of his paper). That's 2.2 tons per square kilometer per year. In the process it fixes 0.606 tons of carbon per square kilometer per year.

For the next step we need to know the "residence time" - how long the oxygen remains in the atmosphere. I can't find figures for Mars - does anyone know? Meanwhile let's just use the residence time for oxygen for Earth, to show how the calculation would work. On Earth, atmospheric oxygen has a residence time of 4,500 years. So, if oxygen lasts as long in the Mars atmosphere as it does on Earth, that takes the total amount of oxygen in the atmosphere up to 4500 *2.2 tons, or around 9,900 tons per square kilometer That is, of course, if the entire surface was as habitable as the Antarctic dry valleys.

How does that compare with the mass of the Mars atmosphere? Using the average atmospheric pressure of 7 millibars, gravity 38% of Earth's, so 10 tons per square meter requires 26 tons per square meter on Mars, so 7 millibars requires about 0.182 tons per square meter, or about 182,000 tons per square kilometer. So our 9,900 tons per square kilometer would contribute about 5.44% by weight of oxygen to the Mars atmosphere. As it turns out, Mars does have oxygen already, but not as much as that. Curiosity measured 0.145% of oxygen in the Mars atmosphere (mole fraction).

So, we would notice an effect on the Mars atmosphere if the entire surface was inhabited by oxygen producing photosynthetic life below the surface of the rocks, and it had a residence time of the order of thousands of years. However that's assuming that all the organics from the life just accumulates in the atmosphere for those 4,500 years (or whatever the residence time is), leaving 2.7 kilograms of carbon fixed per square meter of the Mars surface. The carbon has to be removed permanently because if it is returned to the atmosphere and recombined with some of the trace amounts of oxygen to make carbon dioxide then there would be no net effect.

However we already know that there is some process that actively removes organics from the Mars surface, probably the highly reactive surface chemistry. If it wasn't for that, we'd expect far more by way of organics from meteorites and comet dust. So - presumably the same process will actively destroy organics from life, even if there are no secondary consumers to eat them and respirate. So we might have a situation where life is constantly taking carbon from the atmosphere and fixing it into organics, but the reactive surface chemistry is as rapidly returning it to the atmosphere as carbon dioxide.

If that's so, we could miss the signal from the oxygen, because it is mopped up as quickly as the life produces it. The life takes carbon from the atmosphere as organics, releasing oxygen. But the reactive chemistry rapidly returns it to the atmosphere as carbon dioxide, consuming oxygen.

But let's suppose that the oxygen generating photosynthesis is fixing carbon constantly like this with a residence time of thousands of years, and most of it just stays there once fixed. Then there's another way we could have missed it so far.

How much oxygen would life produce in rare habitats like the warm seasonal flows?

Even the most optimistic probably don't expect the entire surface of Mars, or even most of it to have photosynthetic life. And of course it might not have developed photosynthesis at all. But running with this for now, suppose that it has photosynthesis, but it is very rare. Suppose that it only occurs in the warm seasonal flows (see above),? Each diamond on this map shows one of the sites

Recurring Slope Lineae. Sixteen sites in total marked, over the entire surface of Mars. See Water seems to flow freely on Mars - and this figure is from their detailed paper

So there aren't many of them. And the habitats themselves consist of just thin streaks on the slopes like this



Suppose, for example, that we had a total of ten square kilometers surface area of these dark streaks on Mars - that's in all the RSL's throughout the planet, and counting just the streaks themselves, not the regions with streaks. That's a plausible figure.

Estimating area of the RSLs: the streaks are very narrow. The widest are around 5 meters wide. They are up to 100 meters long. So in an area of 100 streaks, the streaks themselves would occupy up to 50,000 square meters. Suppose there were two hundred sites with RSL's on Mars, probably an over estimate, then that would be around 10 square kilometers in total.

I expect that's an over estimate - and some of the RSLs might be uninhabited, and if temperatures are very low then some may have minimal life barely surviving - but - this is just to give a very rough idea of what's involved here, and how hard they may be to detect, so best to over estimate.

Suppose that they really do involve films of liquid water, as they seem to be - and suppose these films are roughly as habitable as the Antarctic dry valleys and they are all inhabited by cryptoendoliths. And suppose these are all photosynthetic lifeforms. Then that would be less than 22 tons of oxygen production over the entire Mars surface, per year (using Cockell's 2.2 tons per square kilometer per year).

If we now suppose that the organics are fixed for long periods of time, assuming a residence time of 4,500 years (for now) with those 2.7 kilograms of organics fixed per square meter - then it's a total of 99,000 tons of oxygen in the atmosphere kept there by this photosynthetic life. Let's round up to 100,000 tons.

The total mass of the atmosphere is 2.5 x 10¹³ tons. So 100,000 tons would be 1 part in 250 million. Or 0.0000004%. That's the amount of oxygen that's kept in the atmosphere by the RSLs after thousands of years of production, with these assumptions, not the yearly input. Many of the other proposed habitats for Mars are also rare, such as the flow-like features. And others may be common - but consist of tiny droplets, like the ones that form in the Mars simulation experiments by Niton Renno's team.

So we are talking about a rather tiny signal here. Can we hope to spot it form orbit?

Searching for surface life from orbit with ESA's Trace Gas Orbiter

. NOMAD on ExoMars' trace gas orbiter will be looking for geological as well as biologically relevant molecules in the atmosphere, starting its science mission early 2018 (it is currently using atmospheric braking to get into its science orbit as of writing this in 2017). It's targeting carbon, oxygen, sulfur and nitrogen atoms and carbon-hydrogen bonds.

This instrument is amazingly sensitive to up to 10 parts per trillion to many different chemicals. Trace gases at a level of 0.000000001% of the atmosphere. So, out of the 182,000 tons per square kilometer of the Mars atmosphere, TGO could detect a signal of as little as 1.82 grams (call it 2 grams) of these atmospheric gases. Rather surprising...

Using our figure of 2.2 tons per year per square kilometer of oxygen output from the cryptoendoliths - it could detect even the effect on the entire Mars atmosphere of a single year of output of a few hundred square kilometers of cryptoendoliths (a hundred square kilometers would contribute 1 part in 2.5 x 10^13/220).

But better than that, it can also detect local concentrations and pinpoint the location. If significant amounts of a gas are produced locally, or seasonally, and it takes a while for it to disperse, it might notice this. The only problem here is that the oxygen signal would be masked by the oxygen in the atmosphere of 0.145%. It would be a challenge to notice even the effect of 4,500 years of production of the RSLs as 0.0000004% added on top of 0.145%.

So, sadly it doesn't seem that the oxygen signal will be noticeable. But in the case of a methane signal, then, though we'd have to go through the calculations again using typical productivity of cryptoendolith methanogens - it does seem that we are talking here about a signal that in optimal circumstances might be easily detectable from orbit by an instrument as sensitive as NOMAD on TGO. That's one of the many hypotheses for the methane spikes, that they could be produced by methanogens in these shallow subsurface films, see the section Ways that the methane spikes could come from close to the surface (centimeters deep at most) above.

So these habitats may exist and if so, any life there would be hard to spot. We wouldn't have spotted it yet, because it would have minimal effects on the atmosphere unless it is abundant and present over much of the surface of Mars. But with instruments as sensitive as TGO, perhaps we have a chance of spotting it if it produces distinctive gases like methane. It would be hard to spot oxygen, unless it has major local spikes in oxygen production.

So, the upshot is that the atmosphere doesn't rule out surface life yet. If we are very lucky perhaps TGO will detect imbalances in the atmosphere which will tell us where to look on the surface for the first signs of life there. It could of course detect deep subsurface life, but one of the methane hypotheses does involve surface life (Two ways Curiosity's methane spikes could be generated in the shallow subsurface (centimeters deep at most (above).).

What if the most habitable spots on the Mars surface are similar to the driest parts of the McMurdo dry valleys?

Our calculation so far may be a bit optimistic for TGO. We've been assuming conditions similar to typical areas of the McMurdo dry valleys. But Mars is so inhospitable that the most habitable regions could be more comparable to the higher altitude drier parts of those valleys. If that's so, the life is likely to be way beyond the limits of detectability even for TGO.

As an example, the University Valley at a height of 1650 to 1800 meters above sea level. One study looked at the harshest area in the entire valley as far as they could guess, a shadowed region where the soil has few thaw hours, and the ice comes from water vapour diffusion into the soil rather than liquid water. Though it is so very inhospitable, there is life here too. Perhaps this is more like what we might find on Mars in the most optimal habitats there.

In this study of this driest most inhospitable part of University Valley they found no detectable microbial activity although there were microbes present in the soil. The conditions were not unlike Mars, apart from the atmosphere, UV etc. It accumulates soil at a rate of about a micron per year.

• Nitrogen ranges from undetectable to 0.09%
• Microbial mass by direct cell counts between 1,400 and 5,700 per gram of soil - the Antarctic peninsular has about a hundred times as many cells per gram.
• Carbon: 0.01% to 0.05%
• Soil average temperature -23.5 °C to -26.5 °C and average daily temperatures always below 0 °C, maximum temperatures rise above 0 °C for less than 80 hours a year, and even then only briefly with the average temperature for each hour always below 0 °C. Probably experienced similar conditions for at least 150,000 years.

Many of their samples had microbes in dormant or damaged states, needing liquid enrichment steps followed by three to five months of incubation at 5 °C before colonies appeared on the agar plates. They came to the conclusion that most of the life there never experienced conditions suitable to metabolize at all, throughout the year. They may be just life that got into the soil but is not growing there. Or if it is growing, it is doing so, so slowly that they couldn't detect it. They did however find that "viable microbial communities exist within the porous sandstone rock" from the valley walls.

They remark that these conditions of a totally dry permafrost are not unlike the conditions Phoenix found on Mars.

"Dry permafrost as observed in University Valley is rare on Earth, likely only occurring in the McMurdo Dry Valleys, but is commonplace in the northern polar regions of Mars at the Phoenix landing site . Thus, our results have implications for our understanding of the cold limits of life in terrestrial environments, with potential implications for habitability models of Mars near surface permafrost and other icy worlds."

So perhaps we might get situations rather similar to that. Habitats that are so inhospitable that there is life, but most of it is dead or dormant throughout the habitat with the exception of a few spots, where microbes flicker into life and metabolize a bit from time to time, similar to the marginally habitable patches in the porous sandstone boulders in University Valley.

Something like that sounds like it might be pretty hard to detect from orbit. Not square kilometers of metabolizing life, but tiny patches here and there in favoured niches in the region. With most of the life there at any time dead, damaged, or dormant. But on Mars we also have the complication of the surface chemistry removing organics rather rapidly.

Or were their cells really dead, damaged or dormant in these experiments? Could it be that they were just metabolizing very very slowly? They give that as a possibility in the paper. There are some microbes that do that. That also would be a form of life that would have no detectable effects on the atmosphere even for exquisitely sensitive measurements.Life that is perhaps a thousand times less productive than most microbes.

Microbes that are almost dormant - but not quite

Some deep sea microbes have such a slow metabolic rate that they take 1,000 years to divide once, They metabolize at a rate of only a millionth, or a ten millionth of a gram of carbon per hour for every gram of carbon in the cells (typical rates are between a tenth and a thousandth of a gram of carbon per hour). This obviously would have minimal effect on the atmosphere. Some metabolize even more slowly. Yet even with these slow metabolic rates they can still actively repair their DNA to counteract the effects of ionizing radiation.

Mars polar regions - one of the places where slowly metabolizing microbes may be able to survive, similar to P. cryohalolentis, an ordinary microbe, not especially radiation hardy compared to radiodurans, but still with the capability to repair up to 10 DNA base pairs a year.

One species, P. cryohalolentis, was found still alive in 600,000 year old deposits in permafrost and Antarctic sea ice. This species couldn't survive the ionizing radiation damage for that long even on Earth. So, the researchers believe it did this by metabolizing extremely slowly throughout the time period, using an ability to slowly repair DNA damage. In their tests, it was able to synthesize an average of 90 base pairs of DNA per day at -15 °C ( 5 ° F) for 400 days. This makes it a good candidate for a Mars microbe. The researchers said that it might also have reproduced occasionally during those 600,000 years. They had no way to tell whether it did. However it had the capability to survive simply by using a slow metabolism with DNA repair. See Even Ordinary Microbes May Survive Radiation on Mars.

A later study of a microbe Psychrobacter arcticus showed by direct measurement after artificially damaging its DNA, that it could repair 7 - 10 double strand breaks a year at -15 °C. Bacteria like this one are able to repair complete breaks in their own DNA by seeking out identical strands of DNA that are not yet damaged and splicing copies of them to bridge the gaps. This is known as homologous recombination. Eukaryotes (cells with a nucleus) can do it too, using a slightly different process, and even viruses can do this.

On Earth each cell in a population would experience one double strand break every 14,500 years based on the typical ionizing radiation levels of 2 mGy a year for Arctic permafrost. They'd expect 37% of the population to survive 9 double strand breaks, which without repair would take 126,500 years to accumulate. Its repair is very very slow process at - 15 °C. After damaging 16 double base pairs in this way, they found that it took 155 days of incubation at -15 °C before it started the repair process, while the colony forming units had a period of no growth at this temperature for 350 days. It must take a lot of patience to do these experiments!

On Mars, then they calculated it would take 305 years to accumulate 9 double strand breaks, but this paper is from December 2013 and it assumed an ionizing radiation dose for Mars of 830 mGy per year. That's from before Curiosity's measurement of the surface radiation levels as dramatically lower than previously expected, only 76 mGy per year. Using that rate, it works out as nine double strand breaks every 3,300 years on Mars. It hardly makes much difference though, this ability to repair ten base pairs a year is well within what is needed on Mars.

Life like that would have almost no effect at all on the atmosphere.

It's also possible that we find viable but completely dormant life on Mars that has been dormant since the last time the Mars atmospheric pressure was high enough for liquid fresh water to occur on the surface in small quantities. If the dormant life has been reanimated as recently as 450,000 years ago, which is possible, then our rovers on Mars could find viable life that can be reanimated at a depth of only one meter below the surface, according to an estimate in the paper that published the Curiosity ionizing radiation measurements. See page 8 of this paper.

So, if we are lucky, we find Mars life from orbit. But if TGO doesn't spot anything, and we don't find any unusual spectral signatures that we think needs close investigation in case it is life, I don't think we can say too much about whether or not there is present day life on the Mars surface in these micro habitats.

It could either be in very fragmented, tiny habitats, in just a few patches on Mars with most of it dead or dormant at any one time. Or, it could be that it is living life in the very slow lane, respiring, but so slowly that it has no noticeable effect on the atmosphere even for instruments as sensitive as the NOMAD instrument on TGO. Or it could be dormant, a meter below the surface, and was last reanimated 450,000 years ago.

Summary of ways present day Mars could have life - with its atmosphere close to equilibrium

To summarize, there are many ways that Mars could have life right up to the present day, with the atmosphere still in equilibrium or close to equilibrium

• Deep underground, for instance in the deep hydrosphere
• In caves with little or no connection to the surface
• Sparse populations in any of the many proposed microhabitats on or near the surface right now. (see "How much oxygen would surface photosynthetic life produce on Mars? " above).
• Almost dormant microbes, which can be close to the surface, slowly metabolizing for thousands of years, and able to repair their own DNA
• Totally dormant microbes one meter or more below the surface.
• Life that flourishes every few million years and is currently dormant, or almost dormant, or not present on the surface at all. This is quite possible on Mars with its large swings in habitability. It's like the way poppies and other flowers bloom in a desert after the first rain for years, but on a longer timescale. It could be dormant life close to the surface, revives and colonizes the surface from time to time, when conditions are more hospitable. Or else life that is living in a subsurface habitat right now, and spreads to the surface when conditions are right.

It could also have had life in the past, that is now extinct (this was always a possibility)

So James Lovelock's ideas - though useful and informative, don't give us conclusive proof that there is no life on present day Mars. They do suggest that if Mars does have extant life, this life may be sparse and may well be hard to discover, as there isn't enough to put the atmosphere out of equilibrium in an obvious easy to observe way.

Why Lovelock's method wouldn't have detected life on Earth for most of its history!

Nobody would expect to detect life around another star in the subsurface oceans of icy moons like Europa, or on or inside cold dry planets like Mars. However, you might be surprised to hear that this remote life detection may not even work for ocean bearing habitable exoplanets resembling Earth, and not just for planets in the equivalent of the snowball Earth phase. I've based much of this section on this paper.

The problem is that for much of Earth's history, most of the biological activity happened in the ocean and the ocean sediments. For over three billion years the entire land area of Earth was barren desert and rock, except perhaps for a few lichens and bacteria, and algae mats along the sea shore. In particular, most of the methane in the oceans is produced in the ocean sediments, and is almost totally decoupled from the atmosphere, so you wouldn't expect to see much sign of it from a distance.

Artist’s concept of an early Earth. With any biology probably in the sea or at the sea margins, mainly decoupled from the atmosphere, and with non biological processes going on in the atmosphere - could we detect it from a distance by looking at the fingerprints of oxygen / ozone / methane spectra from its atmosphere? The answer seems to be, no, not with the James Webb telescope or similar. This would rule out any other planet with life history resembling Earth for nearly all of its history. Even for the phase with terrestrial life, detecting the fainter signal of methane and disentangling it from non biological sources is so hard that it could probably only be done on a statistical basis, that in a sample of planets there’s evidence that at least one has life but you don’t know which one.

So is there any way we could detect evidence of biology in the oceans of a planet like Earth around another star, before the land was colonized by life? Well sometimes there was an excess of oxygen in our atmosphere, but the amount of oxygen present is hard to estimate. This oxygen would be easily detectable after the land was colonized, for the last 500 million years or so. There might have been another chance to discover it, during a possible oxygen "overshoot" from about 2.2 billion years ago to 2 billion years ago. Apart from that, however, although there was oxygen in the oceans, the amounts in the atmosphere could have been too low to detect with the likes of the James Webb telescope (oxygen is hard to detect when there is less than 1% in the atmosphere). They could detect ozone at parts per million, but that also would be undetectable at the lower range of possible oxygen levels.

At other times, there was an excess of methane in our atmosphere (from about 3.8 billion years go to 2.5 billion years ago). This would be easier to detect from a distance. However the best way to spot the signature of life would be through large seasonal variations in methane, and this is unlikely without a terrestrial biosphere. So, it would be hard to recognize this as life, as seen from a distance.

It's also hard to have both methane and oxygen in the atmosphere together in detectable quantities, for an ocean planet without a terrestrial biosphere. Ozone might be easier to detect.

So, in short, oxygen is harder to spot from a distance than methane, and it also would be in low concentrations for much of Earth's history. Methane is easier to detect but it would be hard to spot also for much of Earth's history as most of the methane would be produced and consumed in the ocean sediments, and would be present in the atmosphere at times when there was little or no oxygen. There could be non biological explanations of low levels of methane in a reducing atmosphere without oxygen, and until the planet begins to develop a terrestrial biosphere, it would be unlikely to have noticeable seasonal fluctuations in methane. Even with modern Earth, the oxygen levels would be easy to spot, but Lovelock's simultaneous presence of methane and oxygen remains a challenge to detect with the likes of the James Webb.

Their general conclusion is that it would be hard for the likes of the Webb telescope to detect atmospheric biosignatures in an ocean world like Earth, even though this would count as one of the most habitable types of planet. It may only become easy to detect life on a world like ours once it develops a terrestrial biosphere. Of course we only have our one example to judge this by, but if we can use our Earth as a "typical" example, it only developed one at a very late date. Another civilization looking at our Sun from a distant star using the likes of James Webb probably would not have managed a definite detection of any life here for most of Earth's history, at least, not using the methods we are likely to have available to us in the near future.

So what can we do? There are two main suggestions. One is to try to have a large enough sample to have a possibility of a terrestrial biosphere, which might have an easier to spot biosignature. The other is to look for statistical anomalies in studies of large numbers of planets, so that we can say "Probably one of these planets is life bearing, though we don't know which one". Either way, we need to study maybe a thousand Earth like planets, planets with large areas of oceans, to have a chance of finding evidence of life on one of them (possibly only statistically). That's assuming it is reasonably abundant and that most of those planets have it. And that probably needs something more powerful than JWST, a space telescope with a diameter of more than 10 meters.

"To be confident of finding a large enough pool of exoplanets to search for biosignature gases, we require the ability to directly image exoplanets orbiting 1,000 or more of the nearest Sun-like stars. The concept is that only with a large pool of Earth-like planets may we gain a probabilistic confidence of the existence of biosignature gases by mitigating the inevitability of false positives. Surveying a large number of stars will require a next-generation space telescope beyond JWST (an optical-wavelength telescope with a large diameter likely exceeding 10 m) "

In detail:

Quoting from the paper by Christopher Reinhard et al, published in April 2017, "False Negatives for Remote Life Detection on Ocean-Bearing Planets: Lessons from the Early Earth" (emphasis mine)

As a proof on concept, we briefly summarize the remote detectability of O2/O3, CH4, and the O2-CH4 disequilibrium throughout Earth's history. Our analysis suggests that the O₂ - CH₄ disequilibrium approach would have failed for most of Earth's history, particularly for observations at low to moderately high spectral resolving power (R≤10,000). In addition, it is possible that O₂ /O₃ may only have been applicable as a potential biosignature during the last ~10% of Earth's lifetime. As a result, most of our planet's history may have been characterized by either high abundances of a single biogenic gas that can also have significant abiotic sources (e.g., CH₄ ) or by a cryptic biosphere that was widespread and active at the surface but remained ultimately unrepresented in the detectable composition of Earth's atmosphere. Finally, we argue that cryptic biospheres may be a particularly acute problem on ocean-bearing planets, with the implication that many of the most favorable planetary hosts for surface biospheres will also have high potential for attenuation of atmospheric biosignatures.

and from the final Discussions and Conclusions section:

"Our analysis suggests that a planet with a biosphere largely (or entirely) confined to the marine realm will in many cases remain invisible to remote detection as a result of biosignature filtering by ocean biogeochemistry—a difficulty that may apply to both presence/absence and thermodynamic techniques. Our analysis suggests that the possible detection of oceans at a planet's surface (Robinson et al., 2010; but see Cowan et al., 2012) is a critical piece of contextual information for validating potential atmospheric biosignatures, and that planets with terrestrial biospheres (e.g., partially or entirely subaerial in scope) may be the most readily detected and characterized because of their more direct geochemical exchange with the overlying atmosphere. Ironically, in some cases planets that are very conducive to the development and maintenance of a pervasive biosphere, with large inventories of H2O and extensive oceans, may at times be the most difficult to characterize via conventional biosignature techniques."

In this earlier paper from 2014, "The future of spectroscopic life detection on exoplanets" by Sara Seager, she makes a similar point, that even with present day Earth then the methane signal is far weaker than the oxygen signal and that in the past, when one of these signals is strong, usually other is very weak.

She recommends that to have a decent chance of detecting life on a planet like Earth, we need a large pool of planets to observe. She also suggests that we also search for a single active gas well out of equilibrium, by much more than you'd expect from the dissociation effects of the sunlight, and that this might give results at an earlier stage, but would lead to false positives, so the result would just be a probability that one of the planets being studied has life.

"The Lederberg–Lovelock approach could be useful at the time when hundreds or thousands of rocky exoplanets have observed atmospheres—to increase the chance that two spectroscopically active gases that are redox opposites might simultaneously exist in the lifetime evolution of a planet. In the shorter term, a different approach is needed to optimize our chances to detect biosignature gases, if they exist, around a handful of accessible potentially habitable worlds. (Note that subsurface life is problematic for astronomical techniques because remote sensing may not be able to detect weak signs of life by biosignature gases coming from the interior.)"

"An idealized atmospheric biosignature gas approach is to detect a single spectroscopically active gas completely out of chemical equilibrium with the atmosphere that is many orders of magnitude higher than expected from atmospheric photochemical equilibrium. False positives will, in many cases, be a problem, and in the end, we will have to develop a framework for assigning a probability to a given planet to have signs of life."

In her conclusion she suggests that we have a sample of at least 1,000 Sun like stars, in order to have decent statistical evidence of the presence of life on some of them, though because of the problem of false positives, one might not be able to say of any of them definitively that it has life.

In short, Lovelock's method still remains our most valuable tool for remote detection of life on exoplanets. But its application has turned out to be far more tricky than he could possibly have imagined when he suggested it back in the 1960s. Given this background, perhaps we can be a bit more understanding of the astrobiologists. It's not as easy to spot life from a distance as one might think from the example of Earth with its easy to detect biosphere. Our planet may indeed be rather unusual in that respect.

So we shouldn't be too discouraged by the difficulty of finding life on Mars by studying its atmosphere. Based on what we now know, it was only to be expected for such a cold dry planet, and even for an ocean world, it might be hard to tell if there is life there from a distance. There may well still be life on Mars, and the atmospheric composition doesn't tell us much about that either way. If TGO finds a signal of methane or other gases from orbit it could possibly be a sign of life. If it doesn't find one though, or it finds a signature that turns out to be due to abiotic chemistry - it tells us nothing either way about whether there is life there.

So, then, what is the best way to find out about life on Mars, past or present. How do we search for it and study it? Especially if we can't find it from orbit,or can find it, but can't localize it to particular spot? Most space scientists will tell you we need to do return samples from the surface of Mars to analyse back on Earth. But is that the best approach? What happens if we "ask an astrobiologist" instead?

Idea of returning samples from Mars to Earth - how does it compare with an in situ search for astrobiology?

NASA consider a Mars Sample Return to be so important that they plan to use their "flagship slot" for funding for two decades to achieve this. The first decade of funding (through to 2022) is devoted to Mars 2020, a sample caching mission. It has better remote sensing capabilities than Curiosity, but they have removed its ability to do on board analysis of the samples, replacing this with the sample cache. The mission to return this cache is left to the next decade from the early 2020s to the early 2030s.

In detail, Mars 2020 will be able to do detailed chemical analysis of the surface of the rock using lasers to map the composition down to microscopic levels through Raman spectroscopy. It can also use X-rays to examine the composition of the rocks. Also, its binocular mastcam is much improved over Curiosity (more on this later).

However the downside is that they have got rid of the on board oven to analyse the samples which Curiosity has. So though it will be able to detect organics through the use of lasers and X-rays, it won't be able to do detailed analysis of them. So it is less capable than Curiosity in this respect. The whole thing is optimized around searching for interesting samples to return to Earth. Their idea is that instead of an oven or other method to analyse the samples in situ on Mars, all of that is done later back on Earth.

As Van Kane put it in a report for Planetary.org (where he describes its capabilities in detail): (emphasis mine)

"The 2020 rover will carry an instrument suite optimized for efficiently finding the best sample suite at its landing site for a possible return to Earth. If those samples do make it to our world, we likely will have a revolution in our understanding of the Red Planet. If they do not, the scientific community may come to wish they had asked for a more capable instrument complement to do more sophisticated science on Mars. But life is about choices, and NASA and the scientific community have bet that the samples collected will be so compelling that funds will be made available for their return to Earth."

For details of their sample caching mission, as it is at present, see also this paper in Nature.

They have also prioritized Moxie which just makes oxygen and carbon monoxide from the atmosphere. This has no direct value for Mars science and the other instruments. It just vents the oxygen and carbon monoxide to the atmosphere once made. It doesn't need to be moved either, doesn't require a rover. The idea is that if we can make oxygen from the Mars atmosphere, it will be useful for humans to breathe and for a propellant.

To make room for the cache, and for Moxie, they removed Curiosity's Sample Analysis at Mars. (SAM), with its ability to analyse samples by heating them in an oven and using mass spectrometry.

So, as you can see from this summary, technology has moved on since Curiosity. In other ways it is more capable than Curiosity as we'll see - but they have sacrificed a lot of payload for that sample return cache and for Moxie.

So, now let's look a bit closer at the plus side. Mars 2020 will have improved capabilities to search for organics remotely. Especially, its SHERLOC Raman spectrometry will be able to micro-map minerals and organics on the samples to a scale of 50 microns. Its SuperCam which replaces ChemCam has the same ability to "zap" boulders and analyse the resulting plasma but also has the ability to do Raman and time -resolved fluorescence spectroscopy which will let it map the distribution of organics remotely for the first time ever, and not confused by the presence of perchlorates. A Raman spectrometer gives information about arrangements of atoms. For instance it can detect a carbon atom double bonded to oxygen.

Its binocular mastcam has the capability to take stereo 3D movies and to zoom in on a target in stereo too, from a 23 degree wideframe through to a 6 degree narrow frame (nearly 4 times zoom capability). When zoomed in, it can resolve rocks at a fraction of a millimeter at 2 meters and to around 3-4 cm resolution at a distance of 100 meters. It also has a ground penetrating radar to get an idea of the structure of the rock layers beneath it, down to half a kilometer depth with a resolution of 5 to 20 centimeters and several useful filters it can use. It can take photographs and video with a resolution of 1600x1200 pixels and record video at 4 fps (faster for lower resolution subframes).

However, unlike Curiosity, Mars 2020 has no way to detect specific molecules in the sample. If it finds organics it just caches them for later return to Earth. We won't know what is in it until the sample is returned and analysed here on Earth, if it is.

It will also have improved cameras all round, not just the mastcam, and possibly a "helicopter scout" to search the local terrain up to a kilometer away, flying for three minutes a day.

Mars 2020 may have a Mars Helicopter Scout, to fly up to half a kilometer a day, solar powered to scout out the landscape around the rover.

However, though it has some interesting instruments to search for organics, the mission is focused on searching for organics, not life, and then with the idea that a future mission would return these samples to Earth probably in the 2030s.

There are likely to be many sources of organics on Mars, abiotic from meteorites and comets and produced locally, and the Mars meteorites we have already also have organics, but don't settle the question of whether Mars had or still has life. Will samples selected in this way be any more conclusive?

In short, NASA don't see in situ searches for life on Mars as a priority at present and have no plans to do it. In their road map they just have searches for past water, and organics. They don't see any need to characterize these organics in great detail before making the decision which samples to return to Earth.

Also, this is not meant as one of many sample returns (as astrobiologists would like), because of the huge cost. Their idea is to do this one sample return mission, returning a sample some time in the 2020s, followed fairly quickly by humans to Mars. in the 2030s. They are "betting the ranch" on this single sample return mission basically. I'd say that this is a high risk strategy. They have no "plan B" if the sample is uninteresting for astrobiology.

The ESA's ExoMars rover (in partnership with Russia) will use a rather different approach. It's a lighter spacecraft but its priority is to explore Mars in situ for biosignatures for past and present day life. It will also drill two meters below the surface (Mars 2020 doesn't have that capability). ESA also plan a sample return, but only after several more missions to the Mars surface.

So both want to do a sample return pretty soon, though NASA has it as a much higher priority than ESA.

This idea of returning a sample from Mars would seem to make a lot of sense. After all that's what we did with the Moon and we did learn a lot from the lunar samples. Indeed you could say our samples from the Moon were a great success. They lead to great steps forward in our understanding of the geology of the Moon. They also gave us enough information to disprove the possibility of life on the Moon (which was already thought to be rather unlikely before Apollo 11).

However, look at it a bit closer, and it's not so clear at all that this is the way ahead now, even though it was the way to do it in the 1960s and 1970s. After all they did return 382 kilograms of samples from the Moon , compared with less than half a kilogram for this Mars sample return. Also Apollo, together with the Russian sample return missions, returned our first ever samples from the Moon (the lunar meteorites in Antarctica weren't recognized until the early 1980s). In the case of Mars, we have quite a few Martian meteorites already.

Also, most significantly of all, our in situ analysis capabilities are hugely improved over the Apollo missions. We are now able to miniaturize our life detection instruments for Mars to tiny "labs on a chip" using minimal power. Just a decade or two ago. some of these instruments would have filled an entire room, and would have been totally impractical to send to Mars.

Also, in situ searches also have the huge advantage that they can study the materials in their natural environment and conditions. Also, it's possible to arrange for them to be uncontaminated by Earth life too.

It's not feasible at present to preserve a sample inside a Mars simulation facility for the long journey back to Earth. This means that the sample, especially if it has volatiles or is fragile in any way, will be altered by the environment inside the return capsule. It's also currently pretty hard to keep a sample return capsule uncontaminated by any Earth life, even a single amino acid as the designers of Mars 2020 have found out. Indeed, they have already decided that it's not going to be practical to keep it completely free of Earth organics. They will just try to keep the level of contamination reasonably low. More on that in the section Difficulty of keeping returned sample free of contamination from Earth (below) .

A sample return would be great for geology. But would it help with the search for life on Mars? As we'll see, many astrobiologists see it as more of a technology demo, mainly useful for proving that we can return samples of Mars life in the future. If you think about it like that, the price tag is very high for a technology demo.

Artist's impression of a sample return mission from Mars - image credit ESA. It would carry back samples totaling a little over half a kilogram of material, at a cost of many billions of dollars.

So, how did NASA end up focusing so much on a sample return mission? Their decision was based on the planetary science decadal survey in 2012, used to plan for NASA's missions for the decade 2013 to 2022. NASA asks for input from panels of space scientists to guide their mission planning for the next decade. One of the biggest decisions for 2013 to 2022, as for any decade, is what to focus on as their flagship mission for the next decade. With limited funding, they can't do everything.

Curiosity was their flagship mission for the previous decade. For 2013 to 2022, the decadal committee chose to recommend a Mars sample return mission, over the Jupiter Europa Ocean mission, which was the main contender. But with the funding available, they could only pay for the first half, the caching on Mars. They left return of those samples to Earth as a decision for the next decade.

So essentially, it's a double decade flagship mission. Although there was no way for them to commit funds for the next decade, they authorized it on the expectation that the next decadal review would make the decision to return the sample, and authorize a second flagship mission to do this. This makes it one of the most expensive decisions NASA has ever committed to in the field of planetary sciences in recent years. It would return less than a kilogram of material at a cost of millions of dollars per gram.

It's not just NASA that's prioritizing sample return from Mars in this way. They are just the first off the block. Other countries that may do a sample return in the near future include Russia who plan a Phobos sample return (again) (which is much easier to justify financially as it costs a lot less), but hope to do a sample return from Mars in the 2030s, Then there's China who aim for a sample return mission around 2030, and indeed the ESA themselves just mentioned, who have explored the idea for many years.

Where does this idea come from? Well, the idea to search for life on Mars in this way was much easier to motivate back in 2007, and much of it I think is motivated by a study back then, which would probably come to different conclusions if it was repeated today. So let's look at that report first.

2007 report favoured a sample return - what has changed since then?

This original idea for caching followed by a sample return goes back at least to this 2007 report. An Astrobiological Strategy for the Exploration of Mars. The scientists who wrote that report list the pros and cons of both a sample return and in situ study. This is all based on the technology of their time and their understanding of Mars which was much simpler back then. This report predates Phoenix and the many amazing discoveries about Mars of the last ten years. It also predates much of the recent rapid development in instrument miniaturization.

Even so it recommends a multi-stage approach.

1. Global reconnaissance that focused on the history of water and the identification of sites for detailed in situ analysis
2. In situ analysis of sites that hold promise for understanding the history of water
3. Deployment of experiments that address astrobiology science questions, including the nature of martian organic molecules and the presence of features indicative of present-day or prior life
4. Return of martian samples to Earth for detailed study
5. Human missions that would provide the detailed geological context for astrobiology measurements and the detection of modern-day "oases" of life

They also say

"Finding. The very successful intellectual approach of "follow the water" should be expanded to include "follow the carbon," along with other key biologically relevant elements."

It then says

"“the greatest advance in understanding Mars, from both an astrobiology and a more general scientific perspective, will come about from laboratory studies conducted on samples of Mars returned to Earth”"

So what has changed since then? Well first, step 3 has barely started. It's turned out to be far harder to detect organic carbon on the surface of Mars than anticipated, with even meteoritic organics clearly being removed by surface processes. There just isn't much by way of organics on the surface, and the little that has been found all seems to have come from meteorites, not life. We have also found oases of habitability for present day life on the surface that haven't been investigated. And we still haven't sent any astrobiologically focused experiments to Mars. So, I think it is fair to say we haven't done stage 3 yet. Unless you count Viking but that's clearly not what they had in mind, writing in 2007.

Also, our technology for in situ analysis has moved on so far in the last decade. Gene sequencers, electron microscopes and many other instruments that were once huge just a decade ago are now just a few kilograms in weight or smaller, and with minimal power requirements. We have many "labs on a chip" that require little more than a single chip, microfluidic manipulation, and a trickle of power, to do everything. We now have many in situ instruments that would not only detect if there is life on Mars, but study it in some detail too.

Yes, it's true that some instruments still can't be miniaturized. We can't miniaturize accelerator mass spectrometers. But we can do other forms of mass spectrometry to analyse isotope ratios, as Phoenix for instance did with its carbon and oxygen isotope measurements of the Mars atmosphere,

Many of the instruments astrobiologists want to send to Mars have been miniaturized already. Some have been miniaturized by the astrobiologists themselves, designing new in situ instruments. Others have been dramatically reduced in size for use on Earth, in commercially available hand held instruments. The last decade since this report in 2007 has seen many major breakthroughs in miniaturization throughout the field of life and biosignature detection. Indeed nowadays astrobiologists have been known to design Mars in situ instruments by combining together miniaturized off the shelf commercial devices that were developed only in the last few years. See the section on In situ instrument capabilities below for details.

Some of the astrobiologists' instruments are already in a mature state of technological readiness, ready to fly right away - these are the ones that were selected for previous missions to Mars but then descoped. Others have gone through some of the stages of the NASA development process, but would take a fair bit of work to get them to the right technological readiness to be able to fly. Then, of course,, it's more work still to adapt a completely new instrument to work on Mars and make it space hardy. However, think how much instrument development you could do for the multi-billion dollar cost of mission to return the cached sample from Mars?

Then there's the plus side that, you'll be able to study the samples in pristine conditions in situ, and make new decisions based on what you discover there. You can examine one sample, and then make decisions about what to study next, based on that previous measurement. There is no way to do any of that with a sample return. The sample was chosen for its geological interest plus detection of organics, and what you have is what you have to work with. If you decide from your analysis that you really need to look closely at the next rock two centimeters away from the one sampled - well tough.

Also if you can design an astrobiological instrument and send it to Mars, you have a new space tested instrument design for all future missions to Mars and other destinations in our solar system such as Europa and Enceladus.

With this background, perhaps time has moved on and this 2007 conclusion needs to be re-examined. The main differences since then are:

• Detection of "oases" of liquid water in the RSL's, and many other potential sites to investigate "in situ", none yet studied robotically on the ground
  
• Miniaturization of instruments makes in situ searches much more versatile. They are also so low mass, that for the same payload, we can send a suite of many more instruments than we could before, including "labs on a chip" consuming hardly any power. We are no longer restricted to a few large, heavy and complex life detection instruments as was the case for Viking.
  
  They are also exquisitely sensitive. The Viking labeled released experiment used to be the only way to detect minute traces of life on Mars Now we have many other complementary ways to look for both life itself, and for biosignatures, and these work for both past and for present day life. Some are so sensitive they can detect a single molecule of a biosignature in gram. It would be hard to keep a sample return so free of Earth based organics that it can compete with this. With so many instruments, they can complement and provide sanity checks for each other. It also means that when we meet unexpected situations, like the unusual chemistry encountered by Viking, we are likely to have other instruments in the toolbox of the in situ instruments on our rover that can help unlock the mystery.
  
• Surprising lack of organics on Mars - it's now abundantly clear (as it wasn't so much in 2007) that organics from comet and meteorite infall are actively removed. This makes it much harder to return past or present day organics, since when we do find a few traces, they may well be all that's left of this infall of organics from space. Similarly, traces of past and present day life may have been removed from many locations altogether, by processes of destructive chemistry that don't operate on Earth.
  
• We have a much more complex understanding of present day and past Mars than in 2007, and know of many geological processes acting both locally and globally that they did not know of ten years ago. It's proving to be a diverse, varied and surprisingly active planet. This has a lot of potential for the search for life, but it's going to make selection of samples to return more difficult, because there is so much variety and so many choices of types of sample to study.
  
• It was hardly a unanimous view even then. They reversed the decision of the previous 1995 study which argued for in situ study first, mainly as a way to extend the time before a sample return, so we'd know more and it would pass scrutiny for planetary protection more easily. For another example. Paige's paper from 2000 only seven years earlier, argued strongly for an in situ search first.

With that background and the complex geology of Mars, it's not at all clear that we can hope to find evidence of past life on Mars easily, or at all, by sample return missions. That is, not unless we already know it is there, and know where to look, and what to return. For all we know, Mars life may be easy to find once we know where to look, but so far, we don't know what we need to return.

So, anyway - let's look a bit closer at the advantages of in situ searches.

Impossibility of sampling everything - example of the clay layers of Mawrth Valles

This photograph of a small part of the Mawrth Vallis area of Mars which gives some idea of the complexity of the geology there. These are clays which are of especial interest as they are good for preserving past organics. They also show evidence that it had many wet environments in the past. See this assessment of the Mawrth Vallis as a potential landing site for ExoMars.

Close up image of a region of stratified clays in the Mawrth Vallis region of Mars

The final decision for ExoMars was to go to Oxia Planum, which is another layer with the same layered clay deposits, with up to 50 meters depth of clay. It also has a probable delta, and areas of the surface have been eroded as recently as 100 million years ago, giving a chance that it may have biosignatures preserved from billions of years ago, only exhumed previously. There they will be drilling to a depth of two meters and doing in situ searches. It is close to Mawrth Vallis, which is now their second choice.

How much material would you need to return to Earth, to just begin to get a reasonable picture of what happened when these clays formed? How do you find the biologically interesting layers? And now, imagine that once you find a layer of interest, you also have to drill deep into the surface to find a sample of organics less degraded by ionizing radiation? If you do this by sample return, then you need to return the samples of the various surface layers back to Earth first, before you can tell whether any of them are biologically interesting enough for it to be worth drilling deep to get a less degraded sample.

By comparison, an in situ search on the surface such as the one ExoMars will do is not restricted in any way. It can just continue studying new samples indefinitely, for as long as the rover survives on Mars. It can also home in on regions of interest. If one of our instruments finds that a particular band of rocks, or type of rock, for instance, is of especial interest - then the in situ rover can then focus the search on other rocks of that type in the area. Perhaps it finds a chiral signature, or it finds amino acids or other biologically interesting molecules in just one of those many clay layers. Then the question is, is it due to meteorites or organic life?. It can focus the search on that layer or those rocks, and follow the signal. The astrobiologists back on Earth can examine the data, and choose where to send it next to answer the questions that arise.

Similarly if it finds suggestive but degraded organics in an outcrop of one of the layers it can drill deeper into the layer to get less degraded samples and so on. The scientists on the team operating ExoMars can make decisions about where to go next based on the analyses already done.

How can we possibly do even a preliminary survey of any moderately complex region of Mars like this one by returning less than half a kilogram of material? Surely we are likely to miss anything elusive such as faint degraded biosignature traces of past life. It would be easier if it weren't that Mars has so many processes that would destroy life based organics, and a constant rain of organics from comets and meteorites to add to the confusion. And we don't know if life on Mars was ever abundant, or where to look for it if it was. For instance, what if it never developed photosynthesis? This is just one interesting spot on Mars for the search for past life, of many. Mars is geologically so complex, with a billions of years old stratigraphy, dust, water and dry ice processes, and a surface chemistry that is unlike anything we have on Earth.

If you have to return the rocks to Earth to search for biosignatures each time you find something that might be a clue to the location of well preserved biosignatures, this is impossible. There's no way you could do this in a single "betting the ranch" mission. It could take centuries and cost trillions of dollars.

Well so it seems to me. What do you think? Let's look into it some more and see some of the other issues with NASA's approach (inspired by decisions by the decadal review advisory committees) for searching for evidence of astrobiology on Mars.

Will the samples be out of date already when they are returned?

This is another potential issue with NASA's approach. Of course, you can understand the reasoning behind the decadal review's proposal to separate out the caching and the sample return into two separate missions. It means you can have a rover dedicated to finding the samples, and then later a second rover that doesn't have to do anything except trundle around picking up samples already cached and returning them to Earth. The first one intelligently selects samples. The second one can be relatively dumb and just pick them up. If we tried to combine both in a single mission the result would be far too expensive and complex for a single "Flagship" style mission.

However with this approach, there's a big delay between the choice of sample, and its final study back on Earth. The sample return mission is not equipped to select new samples to return (probably).

Suppose Mars 2020 finds a rock that looks a bit like the Tissint Mars meteorite. This is a meteorite with organics and a carbon 13 depletion signature (see Tissint meteorite - a great example of what we might get in a sample return from Mars (below) . If Mars 2020 found a rock like this on Mars it would stand out as one of the most important samples to return. It caches it in the 2020s. Scientists back on Earth get their first chance to study it some time in the early to middle 2030s, and that's the first point at which we'll know whether it has any other interesting biosignatures. Meanwhile, maybe other missions have already studied such rocks elsewhere on Mars in situ and shown that they don't contain life. By the time they can return it, then it may just be confirming something they already know from other missions.

So, the sample return mission, in this approach, will go to Mars to pick up samples that were selected based on a decade old understanding of what is interesting about Mars. Also, the samples may well raise as many questions as they answer. Well their idea is to send humans next of course.

Meanwhile an in situ mission in the 2020s such as ExoMars provides immediate results, which they can follow up immediately in situ by studying other rocks on Mars. They can lead to a new mission design ready to fly some time in the 2030s. These comparatively "instant" results seem a major advantage of in situ study to me.

The NASA approach seems to be based on the sample returned as an objective in its own right, which it is of course for geology. That's what it was like for the Moon. The astronauts brought back huge amounts of sample material. We'd love to go back to the Moon, and there are many things we could learn from new samples from the Moon, but we have got so much material returned that we can do a pretty good job with what we have already. It's also a much lower cost mission than a sample return from Mars. Scientists have excellent reasons for wanting more samples returned from the Moon.

It's much the same with comets and asteroids. They are relatively simple. Even with the Moon, it turns out that it's far more complex than we thought, ice at the poles, still producing volatiles, a few areas freshly resurfaced, probably vast lunar caves, many variations in the mineralogy, puzzles about the crater dating not answered by Apollo. There are many follow up missions we'd like to do, more sample returns. Still, it's a relatively low cost robotic mission (the Russians could do it already with Apollo era technology). Also, the questions they want to answer are all geological, and the cost is acceptable for the amount of science return.

However for astrobiology - you have the possibility, ever present, of a sample that has no evidence of life in it. You can't have a rock sample returned with no evidence of geology, or with only ambiguous evidence of whether there ever was any geology. If it is a rock sample, it had a geological past and there is much for geologists to study. So it's a very different situation.

Sample returns could easily not return any biology at all, or only ambiguous unprovable biology. The astrobiologists are saying that at this stage, in situ searches are likely to be vital for finding that exobiology, if it is present there. The example of the Tissint meteorite, not so well known as ALH84001, may help to illustrate this point more clearly.

Tissint meteorite - a great example of what we might get in a sample return from Mars

We've already covered ALH84001 (see What if Mars has really tiny cells - like the structures in the Mars meteorite ALH84001? (above) ). However there's another great example of what we might get in a sample return from Mars, the fascinating Tissint meteorite.

It's a witnessed fall, and so is one of the least contaminated of all the Mars meteorite we haves. This makes it most like a sample returned from Mars of any of them. The fragments were collected by nomads in the remote deserts in Morocco. Fell on 18th July 2011, and the first fragments were found in October 2011. More about the story of its discovery here. It has some contamination by Earth organics - but in minute amounts, and since the sample tubes would not be 100% sterilized, the same would apply to the samples cached by Mars 2020.

Again, for various reasons, some scientists see this meteorite as good evidence of early life on Mars.

(click to watch on YouTube)

(click to watch on YouTube)

They found organics, but not just any old organics. It also had a much lower ratio of carbon 13 to carbon 12 than the present day Mars atmosphere. (Original paper here). Here carbon 12 is the light stable isotope of carbon which gets taken up preferentially by biological processes through kinetic fractionation. The energy costs are lower if the carbon in the organism uses the lighter isotope. Carbon 13 is also stable but not so much favoured by biology. What's more, the difference between those two ratios was similar to the difference between the ratio for a piece of coal (biological in origin of course) and our atmosphere.

(Techy note, this is not to be confused with carbon 14 dating - carbon 14 is radioactive and unstable. Carbon 12 and 13 are both stable and don't decay at all.)

If Mars 2020 finds a rock like Tissint on Mars then the organics with this isotope signature would make it a top priority rock to return to Earth for analysis. So I think this discussion gives us an excellent "preview" of what could happen after a sample like this is returned from Mars. This meteorite, despite apparently clear isotope evidence, is proving as controversial as for the ALH84001 controversy

Gillet himself says about the Tissint meteorite

"Insisting on certainty is unwise, particularly on such a sensitive topic. I’m completely open to the possibility that other studies might contradict our findings. However, our conclusions are such that they will rekindle the debate as to the possible existence of biological activity on Mars – at least in the past."

And indeed, as he predicted, their isotope ratio evidence, and other lines of evidence, have been challenged. As interviewed by Live Science, Steele said

"I think the onus is on [the researchers] to provide that extraordinary evidence. I don’t think they have it at the moment."

The main problem is that, though life is the best known way of reducing the concentration of carbon 13, there are other ways that this can happen. For instance volcanic gases are often depleted in this isotope. On Earth that's usually due to biological organics getting into the carbon when it was originally buried. In this way, ancient biosignatures return to the surface millions of years later as volcanic gases. So if that was the cause on Mars it would still be an indirect effect of much earlier biology.

However, it can also happen in other ways. For instance, it can happen through slow processes of crystallization of liquid rock which can favour one isotope to precipitate out first. So, that's one possibility, but they think that their observations are difficult to explain in this way.

Mars also has a constant influx of meteorites and comets. The Tissint isotope ratio is not likely to be from comets because those generally have a huge excess of carbon 13 relative to Earth. That's the opposite of what they find. But meteorites could confuse their study, particularly the most common ones, the carbonaceous chondrites. These have widely varying carbon 13 abundancies, which dip down to the kinds of figures they found at times (example figures indented so they are easy to skip).

As an example, one study of 21 falls and five finds of carbonaceous chondrites found abundancies of between -31.5% and +12.73% with error bars of ± 3.37 and ± 0.41 respectively.

In the analysis of the Tissint meteorite, they found abundancies of -12.8% to -33.1% That would put it within the range of chondrite meteorites so though suggestive, it's not enough to prove their case.

For how the percentages are calculated, see δ13C.

For a comparison with the results from biology, plants have values of from -10% or so down to -30% or less, clustering at around -13% and -28%. Algae have a similar range of values, from higher than -10% down to -30% or less. Coal and marine petroleum typically has values around -25%, terrestrial petroleum around -30%, and land plants average around -25%, but with a fair bit of variation around those figures. See figure 1 in this article.

However, the organics from the carbonaceous chondrites would be insoluble. So how would they get into the cracks in the meteorite where they found the carbon? So they think that's unlikely, but that's not very conclusive reasoning of course.

Another way to get low carbon 13 values is through hydrothermal vents. Abiotic methane can have carbon 13 depleted to as low as -50%. See page 3 of this paper.

The end result of all this is that they have several abiotic explanations. Some are more plausible than others, but they can't rule any of them out totally. When it's something as radical as announcing detection of life on Mars, then they simply can't do that if there are reasonable alternative explanations of what they observed.

This is just one example to show how if you use simple ways to try to identify organics through biosignatures, you may easily confuse traces of life with non life organics. Charles Cockell and other authors discuss these and other issues in section 3.2. of this paper. They say that though sometimes one biosignature will be enough to confirm life detection, if it is really clear, such as chirality or the organic molecular composition, you often need to have multiple biosignatures simultaneously to be sure that it is life.

Even detection of RNA is not by itself clear and unambiguous evidence of life. Hauke Trinks found that strands of up to 400 bases can be made by freezing processes in artificial sea ice - without any intervention of life. Its polyadenylic acid - so RNA with just one nucleobase. See Did Life Evolve in Ice? (Discovery Magazine) and for the original paper, Ice and the Origin of Life. Other research shows that from the basis of hydrogen cyanide, and water, with various metal ions as catalysts, then it's possible to build all the nucleobases present in life, with Formanide as an intermediate step. For details see their paper. I go into this in a little more detail in the section about the Viking labeled release experiment: Exquisite sensitivity to life - no need for the microbes to reproduce - and not confused by non life organics (below)

Here you can see a fragment of this enigmatic meteorite from London's Natural History Museum, discussed by Caroline Smith, their meteorite expert.

(click to watch on YouTube)

It would be wonderful to have a few more samples like these two meteorites, the Tissint and ALH84001 meteorite. Most especially so, it would be great to have the context, the exact location they come from on Mars, which is the main thing we lack with the Mars meteorites. Also the Mars meteorites come from deep below the surface of Mars (surface rocks are not easily ejected). If we could do it at a low cost of a few thousand dollars per gram, say, for sure. I think most would say it is worth it at that price. But at millions of dollars a gram?

Everyone agrees that a sample return would be great for geology, also, just as it was for the Moon. But NASA is selling this sample return mission as a way to search for life. What happens to their plans, if they go ahead with this, and return samples at great expense which are similar to the Tissint meteorite?

What if they return a rock with a low carbon 13 isotope ratio for instance. And then the astrobiologists study it and are simply unable to tell whether the organics came from life originally or not. Instead they just say this "has rekindled the debate as to the possible existence of biological activity on Mars – at least in the past." What happens if the result is another decade of inconclusive discussion, just as we had for ALH84001 and the Tissint meteorite.

Surely, this will be a huge embarrassment for NASA, to spend millions of dollars per gram to return samples that turn out to be roughly equivalent for astrobiology to new Mars meteorites, like the ones we already have? If this happens, it's not the fault of the astrobiologists, as we'll see, in the next section. With that background perhaps you can understand why the astrobiologists aren't gushing over with enthusiasm for a sample return. Here is my "Future Possible News" story again from the section Should we return samples from Mars right now?in the introduction - this is something that could easily become a reality and unfold pretty much like this, if NASA go ahead with returning this sample in the 2020s.

The image here is a detail of one of the less well known close up electron microscope photographs of ALH84001, the controversial meteorite that was first announced as the potentially the first discovery of life on Mars, but later the announcement was withdrawn as premature. It remains controversial to this day, with astrobiologists arguing both sides of the case.

I made this “Future Possible News” story with this online spoof newspaper generator

Anyway those are a few preliminary thoughts that may get you thinking about it. So, now let's see what the astrobiologists themselves give as their main reasons for preferring in situ studies over a sample return?

Chris McKay's view - just grab a sample of dirt as a technology demo, and return it - one day on the surface, no rover - or SCIM - sample a dust storm in a flyby

The planetary scientist Chris McKay, at NASA Ames, straddles the worlds of astrogeophysics and astrobiology (he majored in physics, has a PhD in astrogeophysics and since then has done many research papers in astrobiology). He is involved in mission planning for Mars and you find his name on many of the papers in this subject area of extremophiles, life on Mars, Mars analogue habitats, planetary protection and the search for life in our solar system. He has also written papers on Mars terraforming.

He recommends we just grab a sample of the Mars soil to show what we can do and return it to Earth. Spend one day on the surface. Design the simplest lowest cost way to return a sample from Mars, no Mars 2020, no rover at all. Just grab it and return. In this interview he says

"The first thing is getting a mission that scoops up a bunch of loose dirt, puts it in a box and brings it back to Earth. If I was an astronaut, what I would be worried about is not the rocks. It’s the dirt. The discovery [by NASA’s Phoenix lander] of perchlorate in the dirt is cause to worry. It’s toxic, and the second cause to worry is the fact that it took us so much by surprise. There was no prediction or premonition that there would be perchlorate in the soil. The fact that it took us completely by surprise makes me wonder if there are other surprises in the soil. In fact, I would be surprised if there are no other surprises. Bringing back dirt is easy because it’s everywhere you land. You don’t need precision landing. You don’t need a rover. You land, grab some dirt and launch it back to Earth. The ground time on Mars could be one day."

"...I’ve said for many years that the sample return should be motivated by a combination of human exploration and science. The science community, I think, does itself a disservice by taking the attitude that there will be just one sample return ever in the history of the universe, so it has to be perfect. And a sample return mission that falls short of perfect shouldn’t be considered. I don’t understand where the logic is behind that. Let’s make a first sample return a quick and easy sample grab, demonstrate the key technologies. It builds enthusiasm for the idea of round-trips to Mars. It would also make getting a second sample return easier, both programmatically and technically. That argument falls on deaf ears when I try and bring it up in the community."

One of his main concerns is that there is no alignment at present between the NASA Mars strategy and astrobiology. He covers this twice in the interview - near the beginning, and towards the end (emphasis mine):

"If we’re going to search for life, let’s search for life. I’ve been saying this to the point of exhaustion in the Mars community. The geologists win hands down as they are entrenched in the Mars program. The favorite trick is to form a committee to decide what to do. The people that are put on the committee, of course, are people who are funded to study rocks. So the committee recommends that we study rocks. They’ll say these rocks will give us the context of how to search for life on Mars. Then you say, well, that’s not right. But NASA Headquarters will say they asked the science community and they told us that this is what we ought to do. It’s kind of circular. The reason the committee told you that — it’s because you put a committee together of people who study rocks. It’s almost a Catch-22. "

"...Right now, as far as I’m concerned, there is no alignment between the Mars strategy and astrobiology. What we have learned from studying Mars is that astrobiology has to go underground. You’ve got to start drilling. Curiosity has a drill and it had problems and we are now very cautious about using it. We’ve got to get back on that horse and send a bigger drill."

In that interview I don't think Chris McKay is suggesting that his "grab sample return of dirt from Mars" mission is likely to be of astrobiological interest. Rather he sees it as of interest for understanding the conditions in the current Mars dirt for future missions to the surface, and human missions particularly, as the dirt is thought to include chemicals harmful to humans. It's main interest for astrobiology would be as a technology demo to show that we can return a sample from Mars, at a later stage, once we know how to select the samples intelligently.

There's another similar proposal, that would be even lower cost, the "Sample collection to investigate Mars" mission which would just dip into the Mars atmosphere during one of its global dust storms, and pick up a sample of dusty air, to return to Earth. It would use a "free return" trajectory. As soon as it leaves Earth's vicinity, it's on a trajectory to skim the Mars atmosphere and return to Earth with only minor course corrections after that. Again this is mainly a geological mission. Laurie Leshing, one of the directors of the Boldly Go institute, interviewed by Space.com, says

"Think of it as a microscopic average rock collection from Mars"

Though these rocks would be tiny, micron scale, the Stardust sample analysis has shown how much science return you can get from tiny samples. Papers continue to be published leading to new results about comets including the discovery in 2011 that some comets actually get warm enough for liquid water to form. See summary in Wikipedia of some of the Stardust science results. Incidentally, Chris McKay's proposal would also include dust and larger particles that got there from distant parts of Mars during the dust storms, so her remark would apply to his idea as well.

BoldlyGo is a Colorado based privately funded non profit, much like the B612 foundation. They have ambitious plans to raise a billion dollars for this and other scientific space projects, partly through wealthy philanthropists, for private exploration missions.Jon Morse, chief executive and co founder remarks, as interviewed by spacenews.com in 2017 that there is an estimated $4 trillion of dollars available by way of funding from wealthy philanthropists looking for worthy causes to support, but it's not easy to convince them of the value of providing some of that money for space projects. It's a new approach. Crowdfunding of course does work well for low cost science projects, (a recent example is the crowdfunding for observing time to study Tabby's star) but it's not so easy to scale this up to projects that may cost tens or hundreds of millions of dollars per mission.

There's an interesting discussion of this mission on NASA Spaceflight.com. One of the contributors to the discussion says that it was originally proposed as a Mars Scout

(click to watch on YouTube)

This was originally proposed as a Mars Scout mission in 2002 and was one of the four semifinalists. See also the Wikipedia page about it, and discussion on NASAspaceflight.com which has some insightful comments on the idea.

So, there are some possibilities for interesting low cost geological sample returns. They would have planetary protection issues - but one simple way to deal with that would be to just sterilize the sample before it is returned to the Earth, with ionizing radiation - after all Mars gets lots of ionizing radiation already, add a few million years worth equivalent of gamma rays, and it's still geologically interesting, but now sterile of extant life. Anyway we haven't come to the planetary protection discussion yet. I go into this later under Why sterilizing a sample from Mars is not like sterilizing a dinosaur egg and If it is just an astrobiology "technology demo" - we could sterilize the entire sample returned to Earth (below).

Many astrobiologists argue strongly in the same way as Chris McKay and more so. A Mars sample return right now would be mainly a technology demo. It would be of geological interest of course, but 's not likely to answer any of the central questions in astrobiology about Mars. So there is no point in trying to make it as perfect as possible. We don't know enough at present to intelligently select samples for return for astrobiology, and it's also likely that we have to drill to find such samples, at least for the search for past life.

As for the search for life, then Chris McKay thinks that to find out about past life on Mars, we need to search in situ, and to drill. From the same interview I quoted earlier:

"Mars has been singularly disappointing on the surface. There are places on Mars that I recommend to drill to search for evidence of life. "

He gives three specific recommendations by way of examples. The first is to drill in the low northern plains (the floor of seas in ancient Mars) at a place with ice and salts close to the surface.

He was the PI (Principal Investigator) of the proposed Icebreaker Life, a proposal to send a second copy of the Phoenix lander with additional drilling capabilities. Phoenix found evidence of ice just centimeters below the surface and salts. The whole area is of interest as the floor of ancient shallow seas. So it's a likely place for early life on Mars. However, Phoenix couldn't drill. So the idea was to update Phoenix with the ability to drill to a depth of one meter and also to add sensitive life detection capabilities to it, and send this second copy of Phoenix to a similar area of Mars. It costs a lot less to build a new spacecraft that is a more or less exact copy of one already sent to Mars.

So, its mission was to drill one meter below the surface to search for evidence of habitability and life on Mars in the same area that Phoenix landed. This may be an ideal place to preserve ancient organics, shielded from the surface chemistry and from cosmic radiation. Its proposed payload included a mass spectrometer based zapping the sample with a laser, Phoenix's wet chemistry lab, and for life detection, SOLID with LDChip which has an antibody library that would let it detect up to 300 different organic compounds including proteins, nucleic acids, amino acids, etc. For more details of this instrument, see Detection of trace levels of organics and of chirality below.

Artist's impression of Chris McKay's proposed "Icebreaker Life" a near copy of Phoenix but equipped with ultra sensitive biosignature detection instrument SOLID with LDChip as well as a laser desorption mass spectrometer - pulses of laser light liberate ions similarly to the MOMA LD-MS mode for ExoMars. This gets around the problem of the reactive perchlorates on Mars reacting with organics when heated for mass spectrometry. It also has an updated wet chemistry lab as for Phoenix, an instrument that wets the soil - this is the instrument Phoenix used to discover perchlorates on Mars.

These conditions should also preserve organics from meteorites too. So the idea was to search for life as well as the meteorite organics, and study and compare both types of organics (if found).

As a second example site, he suggests that Yellowknife bay in Mount Sharp crater, a site investigated by Curiosity, deserves more in situ investigation. This is a spot where Curiosity drilled down just two centimeters, and it got down to "Gray Mars" already. He says (emphasis mine)

"The second site is a place that the Curiosity Mars rover has explored, but it didn’t get the attention that I think it deserved. Yellowknife Bay, at two drill sites, we drilled down 2 centimeters. We got through mud stone and we reached gray Mars — below the red covering on the surface. As far as we can tell, this is sediment that piled up in the bottom of a lake 3.5 billion years ago. We need to get well below the surface so that we’re seeing stuff that’s shielded from radiation. Drilling down, say 5 meters. "

His third example is to drill to a depth of 100 meters in places with strong magnetic fields in the southern highlands, as those are signs that they have been undisturbed for billions of years.

ExoMars of course has a drill, able to drill two meters. So, with ExoMars at last we will be able to get below the surface, and find out what is beneath it - ancient organics, or nitrates, or whatever there might be. Who knows what is down there. We can't tell much from orbit, except for density variations and searching for ice with subsurface radar.

This is just a start. We have various designs of robotic moles that should be able to drill tens of meters in the Mars conditions (dry, near vacuum and water can't be used for lubrication). There are others that could drill hundreds of meters and even kilometers there, even through ice and solid rock. See Robotic and telerobotic drilling versus human drilling (below) . In all the papers I've read, astrobiologists are agreed on this point, that any reasonably thorough search for past life on Mars has to have the capability of drilling to at least meters, ideally to ten meters (below the effects of cosmic radiation) and if they can drill deeper, hundreds of meters or kilometers, so much the better.

So let's look at this in more detail. I haven't found any papers by Chris McKay on this topic of whether we do a Mars sample return, or in situ investigation, and the merits of both. Just this interview, but there are several other papers by teams of astrobiologists. They have written detailed studies arguing strongly for an in situ search first for astrobiology.

Astrobiologists arguing strongly for an in situ search on Mars first

There are several papers on this topic. Let's take a look and see what they say. I'll start with the white paper from 2009 submitted for the decadal review

Decadal review white paper by Bada et al - sample return of "pristine" Mars meteorites is not worth the price of millions of dollars per gram - we need to search in situ

Would a Mars sample return help to address central topics in astrobiology? This was the question eight exobiologists from JPL, the SETI institute, the Scripps Institution of Oceanography and the University of California Berkeley addressed in their 2009 study. After considering it from all angles, they came down strongly in favour of in situ exploration at this stage of our exploration of Mars. Emphasis mine:

"Two strategies have been suggested for seeking signs of life on Mars: The aggressive robotic pursuit of biosignatures with increasingly sophisticated instrumentation vs. the return of samples to Earth (MSR). While the former strategy, typified by the Mars Science Laboratory (MSL), has proven to be painfully expensive, the latter is likely to cripple all other activities within the Mars program, adversely impact the entire Planetary Science program, and discourage young researchers from entering the field."

"In this White Paper we argue that it is not yet time to start down the MSR path. We have by no means exhausted our quiver of tools, and we do not yet know enough to intelligently select samples for possible return. In the best possible scenario, advanced instrumentation would identify biomarkers and define for us the nature of potential sample to be returned. In the worst scenario, we would mortgage the exploration program to return an arbitrary sample that proves to be as ambiguous with respect to the search for life as ALH84001."

(white paper by Jeffrey L. Bada, Andrew D. Aubrey, Frank J. Grunthaner, Michael Hecht,Richard Quinn, Richard Mathies, Aaron Zent, andr John H. Chalmers)

Some of the points they make are that the samples:

• Would probe only a tiny area of Mars
• May well not contain traces of life, as, at best, they would be selected on the basis of ambiguous biosignatures
• Only tell you about a few selected minute spots, and so don't let us explore spatial variation in geochemistry or indeed, of life.

The mission

• We would need to have exceptionally interesting samples to justify the high cost. We won't know enough yet to intelligently select samples like that for astrobiology
• Is seen by much of the scientific community as a technology demo at present. We need to know much more about Mars before we can motivate it astrobiologically.

Instead of a sample return at this stage, they recommend

• Thorough in situ searches, and increased mobility, to look at the many possible habitable environments on Mars.
• Follow the nitrogen, essential not just for Earth life but probably any similar form of life, and in very short supply on Mars. See Follow the nitrogen, dig deep and look for biosignatures (above)
• Drilling to depth
• Searching for biosignatures in situ
• Search for amino acids and amino sugars and test for their chirality. For instance more than 80% of the dry mass of E. coli consists of amine containing organics. The easiest to be preserved may be the cell wall components, such as petidoglycans. These might be preserved in mineralized biofilms.
• In situ searches can target clays and sulfates as a good place to search for life.

The in situ searches must

• Be highly sensitive to low parts per billion or parts per trillion . In the Atacama desert, then life is localized to microhabitats with huge variations in the amount of life present on both the macro and the micro scale. If the instruments are not sensitive enough, it would be easy to miss the signal of life altogether and return rocks with no life in them, even if there were sites only millimeters or meters away with plenty of life to find. By analogy with their test missions in the Atacama desert, for the best chance of success, the instruments have to be sensitive to organics at levels that would not be detected any of the instruments sent to Mars so far.
  
  The oxidizing surface conditions on Mars though make it more challenging. They say (page 7): "For this reason, it is paramount that technologies exist for highly specific quantification of the target biomolecular compounds at trace levels, equivalent to low parts-per-billion (ppb) or parts-per-trillion (pptr) sensitivity... Extremely high sensitivity is necessary due to the harsh environmental conditions on Mars and the fact that only the top few meters of regolith will be accessible"
  
  Also they identify a major problem with the thermal volatilization instruments including Viking's GCMS, Phoenix's TEGA, Curiosity's SAM or indeed ExoMars' MOMA due to the high levels of perchlorates on the surface of Mars. All those instruments could easily miss the levels of organics from microhabitats in the Atacama desert altogether. Some other approach is needed. It's the same for past life on Mars, as surface organics will be degraded by cosmic radiation and again the signal is likely to be very weak.
  
• Be able to extract the organics from the sample "as is" with very low amounts of degradation. They mention several ways to extract and concentrate organics without damaging them. For details of some of them see the Detection of trace levels of organics and of chirality section (below)
  
• Be capable of drilling to a depth of meters. "Drilling to the greatest depth possible will allow for the greatest chance of success of detecting organics and potential biosignatures on Mars "

The time to plan a sample return mission is later. Once we have done all that, we might well be able to identify and return samples of exceptional astrobiological interest. However, right now, we don't know enough to be able to say where to look, what to test for, and what to sample to return to Earth.

They say that this approach aligns most closely with NASA's 2008 Astrobiology Road Map for Mars, which recommends in situ searches for biosignatures as the first priority before a sample return.

So, it's the timing which is the main difference in the perspective of these astrobiologists, and the geologists, not the idea of a sample return. They see great value in a sample return, just, not now. They recommend a sample return at a later stage, once we have explored Mars more thoroughly and identified unambiguous or multiple simultaneous biomarkers on Mars. Alternatively, after we have exhausted all the in situ technologies available to explore for the biomarkers on Mars itself directly, and find nothing, we could do a sample return to attempt a more comprehensive search for them here on Earth.

I'll cover a couple of papers by other teams of astrobiologists, in a minute, but first, let's have a look at the issue of cost, which these astrobiologists highlighted when they said "In the worst scenario, we would mortgage the exploration program to return an arbitrary sample that proves to be as ambiguous with respect to the search for life as ALH84001."

I will also look at the issue of contamination of the sample return, given that the astrobiologists stressed the importance of sensitivity to low parts per billion down to parts per trillion.

Reducing the levels of contamination is an issue for in situ searches too. ExoMars has a target of a total of 40 ppb of reduced carbon, however it has a target of only 1 ppb of amino acids and of DNA, and 2 ppb of amines or amides. See table 2 in this report. An in situ mission can also scrub the sample handling chain- as in the case of detection of organics by Curiosity, where the researchers write: "Terrestrial contamination from the sample handling chain is unlikely because it was scrubbed multiple times with Rocknest scooped material prior to the first drilled sample at JK".

With such sensitive searches the ideal is a 100% sterile lander, or at least, a totally contamination free in situ sampling pathway. Could this be possible? It might be easier to do with a smaller rover, more easily sterilized in its entirety - and as time goes on scientists have found ways to achieve more and more, with more sensitive experiments, with lower payloads and less by way of power requirements. Could our future, a decade from now or more, be tiny 100% sterile very capable in situ rovers on Mars? I discuss this in Can we achieve 100% sterile electronics for an Europa, Enceladus, Ceres, or Mars lander? (below).

Mars samples would cost as much per gram as the world's most expensive diamonds

Early prototype of the Mars 2020 sample cache in 2013. At that stage it consisted of 31 tubes. The tubes are about 1 cm in diameter.

The latest version of the sample return capsule has 30 sample tubes,capable of holding 15 grams each for a total of 450 grams. It has 7 "witness caches" filled with materials that trap environmental contaminants, which will be opened one after another, with the first one opened during the journey there, then sealed on arrival, then the next one opened for a while, and so on. They will then compare them with the samples from Mars to allow for the effects of contamination.

Here is an animation by Honeybee Robotics showing one way that it might work.

(click to watch on YouTube)

With their idea, then each sample tube is actually a single use drill bit that acquires a core, and then the drill bit together with the core is placed inside the cache. For details see this paper.

Their plans have evolved since then. Since 2015 the idea now is to use "adaptive caching". Instead of a single sample container for all the samples, they will instead leave the samples in piles on the surface. The problem of a single cache is that they would need to decide to deposit the cache at some point - and they would have to do that before the rover stops functioning, which is of course, hard to predict. If they don't manage to deposit the cache in time, then it could be trapped inside the rover and impossible to recover. There might be a lot of pressure to deposit the cache during the prime mission, maybe even before all the sample tubes are filled.

With adaptive caching then they can just leave samples on the ground as the rover travels across Mars. They can also keep spare slots free for late in the mission if they find a particularly interesting sample after the prime mission is long over.

Their scientific rationale for this is explained in detail here. The downside is that the sample return mission will need a second rover to follow the path of the first one to pick up the cached samples. However it would need to travel many kilometers to get to the sample cache anyway, unless they reduce the size of the landing ellipse. Their vision for the follow up rover is that it can be much simpler without any systems to identify interesting samples or to acquire the cores, which could reduce its cost and payload. All it has to do is to pick up the samples (see page 5 of this 2017 report). See also Mars 2020 Depot Caching Strategy on the NASA website for a summary of how it works.

The cost is $2.4 billion for their first half of this plan, Mars 2020. That's $5.3 million per gram just for caching the samples. That does not include the cost of returning the samples to Earth, which as a second flagship mission is likely to cost as much again or more. The rover is simpler so costs less, but you then have to factor in the cost of the Mars Ascent Vehicle, and a separate spacecraft to pick up the capsule from Mars orbit and return it to Earth. Nor does it include the costs for a Mars Sample Return Facility to receive the returned sample (likely to cost at least half a billion dollars).

So, as a rough estimate, let's say that it would probably cost well over $10 million per gram for these 30 samples.

Of course Mars 2020 is not just a sample acquisition rover. It's also going to do a lot of useful science in situ on Mars. The price tag of the sample return flagship mission however will be almost totally spent on sample return. It's clear that once we pay for a sample return mission as well, that the cost will be at least many millions of dollars per gram to return those samples. This makes the samples roughly the same price per gram as the world's most expensive diamonds. Indeed, they probably would cost more per gram.

The Wittelsbach Diamond which was sold at auction for £16.4 million in 2008, or about $20.6 million, and had a mass of 7.112 grams before it was recut as the Wittelsbach-Graff Diamond. At the time it was the most expensive diamond every sold at auction. That makes it around $2.9 million per gram. It's hard to give an exact price per gram for the Mars sample return, but as a ballpark figure, perhaps in the region of $5 - $10 million per gram, possibly more, at any rate, more than this.

I think you can see why the astrobiologists argue that we need to have samples of exceptional astrobiological interest on Mars to be worth such a high price per gram to return them. They say we don't know how to select such samples intelligently for astrobiology yet. Mars 2020 is probably not equipped with instruments sufficiently sensitive to biosignatures to select samples of such exceptional astrobiological interest as to merit this price tag (unless it is very lucky).

Difficulty of keeping returned sample free of contamination from Earth - Mars 2020 will have a permitted 1 part per billion of Earth originated biosignatures

The gene sequencing pioneer Craig Ventner and Tessi Kanavarioti raise another issue with the sample return. They wish to send a DNA sequencer to Mars, and at least for their particular instrument, if we return a sample to Earth and analyse it to search for DNA, they are skeptical that we can sterilize a returned sample sufficiently for the task. That's why they want to send their instrument to Mars.

Their reason, in a nutshell, is that if you find DNA in a returned sample "Nobody would believe you"

"The reason to take a device all the way to Mars and not bring back the sample is because of contamination. No one would believe you,” says Tessi Kanavarioti, a chemist who carried out early theoretical work on Martian biology and was involved in studying rocks brought back from the moon in the 1970s. Sequencing machines are so sensitive that if a single Earth germ landed on the sample returned from Mars, it might ruin the experiment."

And indeed when a panel of scientists and engineers met to discuss sterilization of the sample return container for Mars 2020, they decided that they couldn't do 100% sterilization. Alex Sessions, of the California Institute of Technology, interviewed by Astrobiology Magazine put it like this:

“Zero contamination is effectively impossible to do. Whether in nanograms or picograms, at some level, there’s always going to be something there.”

The problem is that our air is full of organics from plants, fuels, paints, and ourselves. The sample tubes themselves can be sterilized in a hot oven at 500 °Celsius (930 °Fahrenheit), Any organics get burnt away into CO₂. But how do you keep the air around the tubes from recontaminating them? They considered one solution, to put the tubes inside a bag to keep them clean. But as Sessions says:

“The engineers were very worried about this. Imagine getting to Mars, and you can’t get the bags open.”:

So they won't do this. The final decision was:

“Ultimately, the panel recommended a level that is near the bottom of what can probably be achieved without containment. Thus complete isolation of sampling apparatus from air is not required, but might ultimately be an easier solution to achieving such stringent levels.”

You can read their detailed proposals here (see their executive summary, page 6)

They recommend a maximum level of contamination of the samples by Earth organics of one part per billion for organics associated with life (one nanogram per gram).

• 1 part per billion (one nanogram per gram) of organics that are associated with life (so that would include DNA and other biosignatures)
• 10 parts per billion (ten nanograms per gram) of other organics.

This may seem a small amount, but the astrobiologists expect traces of biosignatures of past life to be present at very low levels.

• Astrobiologists recommend sensitivity to organics at levels of parts per billion down to parts per trillion, in searches for past life degraded by cosmic radiation. That's why they consider it so important to send in situ instruments with exquisite sensitivity, for instance the astrobionibbler able to detect even a single amino acid in a sample. (see Decadal white paper by Bada et al, (above) ).
• Gene sequencing of ancient microbes in ice or permafrost requires scrupulous levels of cleanliness. Even dead microbes and fragments of DNA are a concern. Typically they handle this by removing of 1 - 3 cm of the surface with a pre-sterilized microtome knife, and making sure that the sample isn't cracked (see page 144 of this paper). However, they won't be returning ice cores, at least not with Mars 2020, and the most likely place to find extant life might be in the regolith or the sand or salt layers.The rock cores for the sample tubes also will be only 1 cm in diameter.

It seems clear from this is that Tessi Kanavarioti is right. If a gene sequencer finds DNA in a sample returned from Mars, then there would be no way to show that it was Martian life - unless it was abundant, above that 1 part per billion level. Even then it would be tricky, because of the way PCR amplifies small amounts of DNA.

Their reason for choosing this figure is that many amino acids in Mars meteorites are present at levels of tens and even hundreds of nanograms per gram, so would be detectable. They agree that many are present at levels of less than 1 nanogram per gram in the same meteorites, and so could not be detected with this limit. They felt this was an acceptable compromise as it would make it possible to detect some of the amino acids found in terrestrial Mars meteorites, and confirm they come from Mars. You can read their reasoning in section 4.2.2.1 of their report.

However, astrobiologists do expect to find amino acids on Mars anyway, brought there from meteorites and comets and produced in situ by inorganic processes. Detection of these amino acids in the returned samples would not count as detection of life. Even detection of a chirality imbalance would not count as detection of life, because we get chirality imbalances in meteorites on Earth that have never been in a planetary environment and are unlikely to contain life. See Organics from meteorites on Mars may boost a molecule over its mirror image, mimicking biosignatures (chirality) (above).

That's where the parts per trillion figures come in. If astrobiologists are to have a decent chance of finding past life on Mars, they need to not just detect organics, but amongst all the organics from meteorites, they need to search for the probably much fainter trace of organics from possible past Mars life. As it is, with parts per billion contamination by Earth organics, they will be able to do a study of a rock from Mars that is cleaner than almost any meteorite that we find on Earth. However it will not be quite free of Earth life, with an up to a 1% chance of it having a complete viable Earth microbe in it. Also, it's rather small, a 1 cm diameter rock core of fifteen grams of material. Also volatiles may be lost by heating during the drilling process (which since it is open to the Mars atmosphere, means that the volatiles are lost from the sample) also while the capsule is sitting on the surface of Mars waiting to be collected.

I think the main problem is that they take studies of martian meteorites as their paradigm of what a biological investigation of a Mars sample would involve. But for astrobiologists, these studies have been interesting, but controversial and haven't helped resolve the central questions in their field. For gene sequencers in particular, contamination is a very major issue for a sample return.

Another issue the Mars sample return study panel touch on is that a Mars sample returned by Curiosity might have conditions that permit growth of Earth microbes in it. They recommend a less than 1% chance of a viable microbe per sample.

"The conditions for cell division have recently been studied by Rummel et al. (2014) and have been found to be primarily dependent (at least for Mars applications) on temperature and water activity. Active refrigeration of the samples on Mars is unrealistic, and the water activity would reflect the nature of the samples (which is currently completely undefined)."

"... As a good experimental practice, therefore, OCP recommends that Mars 2020 be designed so that the sample tubes would be sterilized, and so that they could be sealed with a sample inside with a probability of less than 1% of a single live terrestrial organism per sample"

(quote from section 4.1.5 of their report, replaced 1e-2 by 1% which means the same thing)

That is a very high level of sterility for sure. The aim is that 99% of the time the samples won't have as much as a single viable microbe in them.

They don't give any examples in the report, for this concern about microbes reproducing during the journey back. However, Mars has many deposits of ancient clays, so I'm guessing that this is what they had in mind (do say if you know more). Curiosity studied a sample of Martian clay and found it formed in conditions habitable for life. These are likely samples to return in the search for past life. Clays can hold onto water even in the near vacuum conditions of the Mars atmosphere. These can't be kept refrigerated during the sample return, giving a possibility of Earth microbes reproducing in the sample.

Contamination is of course an issue for in situ searches too, but it is more addressable in that case. Anyway let's return to the other astrobiological papers now. The focus is on the difficulty of finding samples of astrobiological interest without in situ life detection instruments.

Paper by David Paige: "Mars Exploration Strategies: Forget About Sample Return!"

This is an earlier paper from 2000 by David Paige, professor of planetary sciences at UCLA. He refers to a 1996 study requested by Michael Meyer of NASA’s Exobiological Program Office This divided Mars exploration into five phases

• Phase 1. Global Reconnaissance, Search for water, past and present, and selection of sites for detailed study
• Phase 2. In-Situ Exploration of Promising Sites, Focus on the geology minerals, elemental and isotopic composition, and volatiles and organics.
• Phase 3. Deployment of exobiologically-focused experiments, search for past biosignatures and present day life.
• Phase 4. Robotic Return of Martian Samples to Earth, to examine the organics more carefully and verify any biosignatures and present day life found in phase 3.
• Phase 5. Human exploration of sites of unusual interest for biology, or inaccessible to robotic exploration.

Paige reasons that Mars exploration is still in phases 1 and 2, and that we need to complete phase 3, to search for clear signs of past and present day life on Mars in situ, before going on to phase 4. He refers to the ALH84001 discussion. At early stages of that discussion, the scientists thought that confirming the discoveries would just be a matter of returning a similar pristine sample from Mars for detailed analysis. But as they researched into it further and got involved in the intricacies of the debate, they no longer think this way. In detail he says:

"...Also, the fact that credible scientists found “evidence for ancient life” in the ALH84001 meteorite was interpreted by some to suggest that such exciting evidence may be much more ubiquitous on the surface of Mars than had previously been imagined, and that confirming the ALH84001 discoveries would only be a matter of returning a suitable sample to Earth for detailed analysis. However, in retrospect, we now know that much of the evidence for ancient life found in the “Mars rock” is ambiguous or debatable, and that similar issues are likely to arise when robotically acquired samples are eventually returned to Earth. We also now have a deeper appreciation for the fact that Mars is a really big place with a complex history to unravel, and that it will take quite a lot of evidence to prove that life ever existed on Mars, or quite a lot of searching to prove that it never did.

...There will always be scientists with laboratories who will advocate that NASA provide them with Mars samples for them to analyze. The fact is, however, that we don’t yet have the technology to do this within acceptable levels of cost and risk. Those who are anxious to move the program forward toward sample return have more than enough to do in the areas of basic technological development, risk reduction and testing. "

Paper: "New priorities in the Robotic Exploration of Mars: The Case for In Situ Search for Extant Life"

This is a more recent paper, this time from 2010, by four more astrobiologists, including Charles Cockell, and Dirk Schulze-Makuch, New Priorities in the Robotic Exploration of Mars: The Case for In Situ Search for Extant Life

They argue for in situ search for present day and past life first, before sample return. They prioritize the search for present day (extant) life over past life. They also prioritize search for past life in ice and salt over rock (because of the better chance of preservation in ice and salt for millions of years).

"Given the most updated knowledge we have about Mars’ environmental evolution, we call for a long-term architecture of the Mars Exploration Program that is organized around three main goals in the following order of priority: (1) the search for extant life; (2) the search for past life; and (3) sample return. We argue that this is the most efficient approach by which to address, with a high degree of certainty, the question as to whether life exists on Mars. "

(Alfonso F. Davila, Mark Skidmore, Alberto G. Fairén, Charles Cockell, and Dirk Schulze-Makuch)

In more detail they say (emphasis mine):

"Priority 1: In situ search for extant life"

"The search for extant life should be conducted in environments with the highest potential to support active organisms or preserve dormant organisms. These environments should be selected based on studies conducted in the best terrestrial analogues on Earth, particularly the Atacama Desert, the Antarctic Dry Valleys, and basal ices of polar ice masses. We assume the martian biosphere is carbon based and follows nutrient requirements similar to those of terrestrial microorganisms."

• "Rationale: Extant life on Mars, if present, is not currently detectable at a planetary scale but might be detectable at the local scale. This is observed in terrestrial analogues that are closest to martian conditions, where life is present and can be relatively abundant but is only detectable in specific niches with enhanced habitability potential (e.g., ice, interior of salts) and the use of microscopy or simple molecular techniques. "
  
• "Approach: The in situ search for extant life should be dedicated, focused, and relatively inexpensive (Discovery-type missions), with the use of instruments that can provide indisputable evidence for the presence or absence of organisms, for example, via microscopy in addition to measurements based on activity or metabolic state.""

"Priority 2: In situ search for extinct life"

"The search for extinct life should be conducted in environments on Mars that are known to support active microbial communities in terrestrial analogues on Earth. "

• "Rationale: Based on the above, niches with the highest potential to host extant life also have the highest potential to have fossil traces of a biosphere from the recent past. Two of the target sites proposed for in situ search for extant life, ice and salt, also have a high potential to preserve organic compounds for extended periods of time in the range of millions of years."
• "Approach: The in situ search for extinct life could be carried out by independent missions or in tandem with missions that are searching for extant life. If neither extant life nor evidence for extinct life are found in these icy and salty targets, then it will be reasonable to refocus the search for traces of past life in the rock record."

"Priority 3: Sample return"

"After lander missions have satisfied Priorities 1 and 2, we will have an informed perspective of the potential for life on Mars. Sample return would be most efficient and logical once we have information from a variety of environments, particularly if evidence of extant or extinct life is found at any of these sites"

Decadal summing up doesn't discuss this issue

The decadal survey was published in 2011. It does list Bada et al's white paper amongst the submitted references at the end of the report. But it is not cited in the body of this report or discussed. What's more, this view that we should prioritize in situ searches over a sample return at this stage is never mentioned in the final video presentation.

Here Steven Squyres is describing Max-C which later became Mars 2020. He says (35 minutes in):

"The view that was expressed very clearly by the Mars science community is that Mars science has now reached the point where the most fundamental advances are going to come from study of returned samples. Now Max C would do a little in situ science but fundamentally it is about collecting and caching a well characterized set of samples that would then come back to Earth as part of a three mission sample return campaign that continues on into the next decade. One mission that caches the samples, a second mission that lands alongside it, grabs that sample cache and puts it into orbit around Mars, and a third mission that captures that sample from Mars orbit and brings it back to Earth.

" ... Only Max-C is recommended for the decade that we talk about ... This campaign is multi-decadal in character. It has to be... We based its priority on its anticipated total science value, that is the science return from the samples, and the cost to NASA of the entire campaign... If that goal of $2.5 billion (for the first decade) cannot be achieved for whatever reason, our recommendation is that Max C should be deferred to a subsequent decade, or canceled. And there is no plan B. There is no alternate plan for Mars exploration recommended. This is what was told to us by our Mars panel. Mars sample return is the next right thing to do. And if you can't do Max-C for less than $2.5 billion, then there are other high priority missions on that list that take preference over something else that you might do on Mars".

(emphasis mine)

So - there's no doubt that the Mars panel for the decadal review put an exceptionally high priority on the Mars sample return. So high that it recommended this as the only mission to Mars they thought worth funding as a flagship mission, even with its cost so high it would span two decades or more.

The printed decadal survey conclusions are similar:

The Mars community, in their inputs to the decadal survey, was emphatic in their view that a sample return mission is the logical next step in Mars exploration. Mars science has reached a level of sophistication that fundamental advances in addressing the important questions above will only come from analysis of returned samples.

The site will be selected on the basis of compelling evidence in the orbital data for aqueous processes and a geologic context for the environment (e.g., fluvial, lacustrine, or hydrothermal). The sample collection rover must have the necessary mobility and in situ capability to collect a diverse suite of samples based on stratigraphy, mineralogy, composition, and texture. Some biosignature detection, such as a first-order identification of carbon compounds, should be included, but it does not need to be highly sophisticated, because the samples will be studied in detail on Earth.

Vision and Voyages for Planetary Science in the Decade 2013-2022

There is an enormous disconnect here between these statements of the decadal survey and the recommendations of the astrobiologists. It would be more accurate to say that the astrobiologists were emphatic in their view that a sample return is not the logical next step.

Let's take some of the things from the summing up, and compare them with the statements in Bada et al's paper.

• Summing up: "fundamental advances in addressing the important questions above will only come from analysis of returned samples"
  
• Bada paper: "We have by no means exhausted our quiver of tools, and we do not yet know enough to intelligently select samples for return

And

• Summing up: "Some biosignature detection, such as a first-order identification of carbon compounds, should be included, but it does not need to be highly sophisticated, because the samples will be studied in detail on Earth."
  
• Bada paper: "We argue here that when in situ methods have definitively identified biomarkers, or when all reasonable in situ technologies have been exhausted, it will be time for MSR. We are not yet at that crossroad."

How did this happen? Given that the main objective of the sample return is to look for life, you'd expect the views of astrobiologists to have top priority. After such a strong warning against a sample return, by these experts, why didn't it trigger at least an in depth study of some sort. Yes the astrobiologists could all be wrong, people make mistakes. But they have given it a great deal of thought, probably a lot more than the planetary geologists and space colonization enthusiasts. So why didn't it trigger a review?

And how could they say that the Mars community are emphatic in their conclusion when there was such a divergence of opinion in the white papers they studied so carefully? I don't know, as the decadal summing up simply doesn't explain. They must have read the paper but they offer no comments on it.

So this can't be more than speculation. But I do have one suggestion. It's based on another paragraph in the decadal review, Connections with human exploration:

"Mars is the only planet in the solar system that is realistically accessible to human exploration; it has been proposed as a target for orbital flybys and future landing by human explorers. To reduce the cost and risk for future human exploration, robotic precursor missions would be needed to acquire information concerning potential resources and hazards, to perform technology and flight system demonstrations, and to deploy infrastructure to support future human exploration activities. The elements of the Mars Sample Return campaign, beginning with the Mars Science Laboratory, will provide crucial data for landing significant mass, executing surface ascent and return to Earth, and identifying potential hazards and resources."

(emphasis mine)

They cite the 2002 report "Safe on Mars". This report recommends a sample return as the first step towards discovering whether it is safe to send humans to Mars. They seem to place a great deal of emphasis on this report, as it is the only Mars related cite of a short bibliography of seventeen citations in their earlier section "National and International Programs in Planetary Science" (see page 83 of this report).

If that's their reason for prioritizing a sample return, it's tragic because that's now a way out of date report. It doesn't reflect present day technology or our understanding of Mars. "Safe on Mars" only recommends a sample return because back in 2002, they felt that most of the instruments they would want to use were too large to send to Mars. We have now miniaturized all those instruments.

If you read "Safe on Mars" carefully, it says that in situ searches (if they can be done, as they can now) are a far better way to find out if Mars is safe for humans than a sample return. I go into this in detail in the section "Safe on Mars" - could a sample return tell us if Mars is safe for astronauts? (below)

They gave no reason for ignoring the Bada et al paper. So I don't know if that is the reason, and we just have to leave this as a mystery unless anyone reading this happens to know the answer. Anyway will they actually return a sample this time, and if they do, is there any chance that it could be hazardous for us or the environment of Earth?

Will the sample return happen this time?

It's at least a two decade project, and given the expense and technical challenges, it still may not happen. Many earlier plans for a sample return never came to anything. Mars 2020 is definitely going ahead and will cache samples on Mars. But will NASA send a mission in the next decade to pick up those samples? Well that's for the next decadal review to discuss.

Antaeus Orbiting Quarantine Facility (1978)

The idea of a Mars Sample Receiving laboratory was first studied in 1978. The idea then was for an orbiting quarantine facility called Anteus to receive the samples.

Other proposals were explored in the 1980s, including direct entry of a sample container to the Earth's atmosphere, recovery by the space shuttle, recovery to the space station, recovery to a dedicated Antaeus space station, and several intermediate proposals. None of this ever happened. See Mars Sample Recovery & Quarantine (1985)

Perhaps this time it will happen however? If so, what can we do to help make it a success, and a valuable part of our space program?

Well one of the things that could slow down the process, and maybe prevent the sample being returned at all is the issue of public safety. Can it be done safely, given the issues for back contamination of the Earth? The measures to protect Earth are also likely to be time consuming and very expensive, as we'll see. So can we do anything to reduce the huge cost?

Why we can't prove yet that Mars life is safe for Earth

It's natural to ask if Mars life could harm us directly, and the answer to that is, we just don't know, but it is possible. Yes, if Mars life is only distantly related, or not at all, it would not be adapted to attack us. You might think, naturally enough, that

"Okay, it's not adapted to attack us, so it will be harmless."

However that's reasoning by analogy with Earth microbes. Our bodies are well protected against them, with defenses adapted to a wide variety of Earth microbes. A harmful microbe has to overcome those defences to attack us.

We've never encountered extra terrestrial life. Yes, it won't be adapted to attack us. However the flip side of this is that Earth life won't be adapted to defend itself against it either. Our bodies and the bodies of other creatures, plants etc on Earth have never encountered an extra terrestrial microbe and may have no defenses against it.

Carl Sagan, who was a physicist with a long term research interest in biology, who wrote a thesis on the origins of life already as an undergraduate, put it like this on page 162 of his "Carl Sagan's Cosmic Connection: An Extraterrestrial Perspective" (emphasis mine)

"Precisely because Mars is an environment of great potential biological interest, it is possible that on Mars there are pathogens, organisms which, if transported to the terrestrial environment, might do enormous biological damage - a Martian plague, the twist in the plot of H. G. Wells' War of the Worlds, but in reverse. This is an extremely grave point. On the one hand, we can argue that Martian organisms cannot cause any serious problems to terrestrial organisms, because there has been no biological contact for 4.5 billion years between Martian and terrestrial organisms. On the other hand, we can argue equally well that terrestrial organisms have evolved no defenses against potential Martian pathogens, precisely because there has been no such contact for 4.5 billion years. The chance of such an infection may be very small, but the hazards, if it occurs, are certainly very high.

"…The likelihood that such pathogens exist is probably small, but we cannot take even a small risk with a billion lives."

Joshua Lederberg was a Nobel winning microbiologist, and microbe geneticist, and closely involved in early searches for life on Mars, summarizes it like this (emphasis mine):

"If Martian microorganisms ever make it here, will they be totally mystified and defeated by terrestrial metabolism, perhaps even before they challenge immune defenses? Or will they have a field day in light of our own total naivete in dealing with their “aggressins”?

That’s in his "Paradoxes of the Host-Parasite Relationship"

And in more detail in Parasites Face a Perpetual Dilemma:

"Whether a microorganism from Mars exists and could attack us is more conjectural. If so, it might be a zoonosis to beat all others.

"On the one hand, how could microbes from Mars be pathogenic for hosts on Earth when so many subtle adaptations are needed for any new organisms to come into a host and cause disease? On the other hand, microorganisms make little besides proteins and carbohydrates, and the human or other mammalian immune systems typically respond to peptides or carbohydrates produced by invading pathogens. Thus, although the hypothetical parasite from Mars is not adapted to live in a host from Earth, our immune systems are not equipped to cope with totally alien parasites: a conceptual impasse."

So, he is saying that our immune system and defenses are keyed to various chemicals produced by Earth life. such as peptides and carbohydrates. It's entirely possible that Mars life doesn’t use those chemicals at all.

To take an analogy, an artificial hip replacement doesn’t get rejected from our bodies, because the material it's made of is carefully selected, so that our bodies doesn't recognize it as something to mount a defense against. Well similarly, our defense systems might not recognize the Mars microbes as foreign material, any more than an artificial hip. So, there is no way to know until we study them, but it is possible that Martian microbes could colonize our bodies, and our bodies might be naive in face of it, never having had to face the like of this before. Our bodies defense systems might not "realize" that they have to put up any defenses at all. They might let the Mars microbes do whatever they want on and in our bodies, much as they would for an artificial hip joint replacement.

So what about other creatures on Earth and our biosphere?

Biosphere, crops, sea, and environment of Earth

Another microbiologist who was concerned about this early on is Carl Woese. He raises a much wider concern here, the effect on our entire biosphere. This is what he said in personal communication with Barry DiGregorio, a science journalist and scientist who often interviewed pioneers in the early years astrobiology (emphasis mine).

"I consider it a very adventuristic assumption that Earth is safe from Martian organisms. Obviously we know too little about life on Mars (even whether it exists) to make such assumptions. These are hand waving arguments whose only purpose is to rationalize the technological feats these people wish to accomplish. It appears that those making the case for sample return have not even considered organisms that would change the global organismal balance once they took hold - and what that would lead to is totally unpredictable. Indeed I get the feeling that those making the argument for sample return equate lack of knowledge (that it is unsafe) with lack of danger. This whole argument brings to mind the attitudes in NASA that Feynman wrote about in his book; Try it and if it doesn't work, then fix it. In this case you couldn't fix it if it didn't work ."

--Carl Woese, quote shared by Barry DiGregorio here. Also:

"When the entire biosphere hangs in the balance, it is adventuristic to the extreme to bring Martian life here. Sure, there is a chance it would do no harm; but that is not the point. Unless you can rule out the chance that it might do harm, you should not embark on such a course"

(quoted in Barry DiGregorio's paper here)

You can hear Barry DiGregorio talk about his interviews with these early pioneers in his SpaceShow guest appearance here.

Carl Woese was famous amongst fellow microbiologists for recognizing and defining the "third domain" of microbial life, the archaea (you can read about his discovery of the archaea here in a personal retrospective). He was also originator of what came to be called the "RNA World" hypothesis about the origins of life. This is the idea of early life that is based on RNA only, without DNA and not using proteins. As he says, it's not just us that could be affected. Microbes from Mars with novel capabilities and perhaps totally different biology inside could affect our crops, the sea, animals, the environment of Earth generally, and any lifeforms here.

Let's try a few examples, for instance, what if they out compete photosynthetic life in the sea? And what if they are inedible to sea creatures, or they produce chemicals that poison them - not because they are adapted to harm us, but just because their chemistry is so different from ours? They can harm

• By competing for resources
• By being poisonous when eaten by unsuspecting Earth life
• By altering habitats in ways that Earth life doesn’t find congenial

I go into this in more detail in Many microbes harmful to humans are not "keyed to their hosts" (below)

Every review of a sample return since Sagan, Lederberg and Woese has come to the same conclusion as they did - that we have to take precautions in case of a possible adverse impact on either us, or more generally ,the environment of Earth.

As Carl Sagan said in Cosmos,

"There may be no micromartians. If they exist, perhaps we can eat a kilogram of them with no ill effects. But we are not sure, and the stakes are high. If we wish to return unsterilized Martian samples to Earth, we must have a containment procedure that is stupefyingly reliable... there are nations that develop and stockpile bacteriological weapons. They seem to have an occasional accident, but they have not yet, so far as I know, produced global pandemics. Perhaps Martian samples can be safely returned to Earth. But I would want to be very sure before considering a returned-sample mission.” (emphasis mine)

The NRC study of a Mars sample return concluded:

"The committee found that the potential for large-scale negative effects on Earth's inhabitants or environments by a returned martian life form appears to be low, but is not demonstrably zero. Changes in regulations, oversight, and planetary protection controls over the past decade support the need to remain vigilant in applying the requirement to protect against potential biohazards,whether as pathogenic or ecological agents" (emphasis mine)

The NRC study and other studies of this nature have said the same thing as Carl Sagan, Carl Woese and Joshua Lederberg. The chances of harm to us, or our environmnet from micromartians seem to be low. However we do need to take precautions, as we can't show that the risk is zero. Also, we need to protect against biohazards that may be harmful to us and other inhabitants of Earth directly, as well as ones that may have indirect ecological effects on the environments of Earth.

Example of Apollo sample return - learning from our mistakes in the past

Nothing harmful happened during the Apollo missions. But were we just lucky?

You might well think that they worked it all out for Apollo, so we could just do the same as they did, put the astronauts in quarantine for a few weeks on return to Earth. But those quarantine precautions never had any peer review. They were published on the day of launch. They were of course based on our understanding of biology of the 1960s and 1960s technology. And they were not even applied properly at the time.

One of the most amusing breaches (in hindsight) is described by Buzz Aldrin. He noticed that ants found their way into the quarantine facilities while he was in quarantine. He writes, in his book "No Dream is Too High":

“The unit was comfortable, but there was little to do and nowhere to go, so we got bored in a hurry.

"One day, I was sitting at the table staring at the floor, and I noticed a small crack in the middle of the floor, with tiny ants coming up through it! Hmm, I guess this thing isn’t really tightly sealed, I thought. Imagine, if we had brought some sort of alien substance back with us, those ants could have contracted it and taken it back out to the world!”

Earlier, the command module hatch was opened while it was still floating in the open ocean, because the crane that was going to pick it out of the sea was considered unsafe, and the mission planners thought it was more important to get them out right away to prevent seasickness, than to wait while they tried to figure out a solution. Dust from the Moon surely went into the sea at that point.

It was opened twice, to pass protective suits in to the astronauts, and again to take the astronauts away in a dinghy and then helicopter

(click to watch on YouTube)

Diver opens the door to the command module as it bobs in the open sea and crew get out, already dressed in their decontamination suits (which were handed into them by the divers earlier on). Some of the dust from the Moon surely got into the sea at this point, as after all, it got everywhere inside the module.

This was not part of their plans, for them to evacuate from the command module in the open sea. It was an “on the spot” decision by mission planners. The crane that was going to take the module out of the sea had malfunctioned, and they felt that it was unacceptable to keep the astronauts waiting until they fixed it.

Also, before any of that, a vent of the Apollo 11 command module was opened after re-entry. This would have released any contaminants present in the module into the Earth's upper atmosphere (see page 8 of this paper).

There were other breaches of protocols as well, due to inexperience of the staff at the sample receiving facility, which was constructed too late to give them adequate training. Also, scientists arrived at the receiving facility after it was up and running, too late to become familiar with its operational procedures.

Also, even if it was done perfectly, it wouldn't have protected Earth from microbes from the Moon on the remote chance that it had any. We can see that now, with better understanding of microbiology and extremophiles than they had back then.

The duration of the quarantine of three weeks was also arbitrary. Leprosy, for instance, has a latency period of decades.

From left to right, Neil Armstrong, Michael Collins and Buzz Aldrin in their quarantine unit after return from the Moon. This was largely a symbolic gesture and didn't do much to protect Earth from microbes on the Moon, if there had been any. They had already been taken out of the command module into an open boat in the ocean, and there were many other lapses of protocol. Even if they had carried out the protocols perfectly, they wouldn't have protected the Earth according to present day understanding.

Their quarantine protocols also had no peer review as they were published on the day of launch.

As Carl Sagan put it in “Cosmic Connection”:

"The one clear lesson that emerged from our experience in attempting to isolate Apollo-returned lunar samples is that mission controllers are unwilling to risk the certain discomfort of an astronaut - never mind his death - against the remote possibility of a global pandemic. When Apollo 11, the first successful manned lunar- lander, returned to Earth - it was a spaceworthy, but not a very seaworthy, vessel - the agreed-upon quarantine protocol was immediately breached. It was adjudged better to open the Apollo hatch to the air of the Pacific Ocean and, for all we then knew, expose the Earth to lunar pathogens, than to risk three seasick astronauts. So little concern was paid to quarantine that the aircraft-carrier crane scheduled to lift the command module unopened out of the Pacific was discovered at the last moment to be unsafe. Exit from Apollo 11 was required in the open sea."

The Apollo 11 procedures are of most interest to present day planetary protection researchers as a valuable lesson that has highlighted issues like this.See 7 Lessons Learned from the Quarantine of Apollo Lunar Samples

Quarantine is actually very hard to do well. Indeed, I'm not sure if it is even possible, legally and ethically, once humans are involved. Also, even for a robotic return mission, there are issues of human quarantine, if someone gets exposed to the materials, through error, accident, or an executive decision to override the protocols. See The knotty problem of human quarantine - and what about exposure of humans during a robotic sample return? (below)

Why Carl Sagan and others thought that planetary protection was necessary for Apollo 11

Now, it's true that already, back then, before the Apollo landing, astronomers were well aware that the Moon has no atmosphere.You can't have liquid water survive for any length of time in a vacuum. So there was no chance of liquid water on the surface, and life there seemed unlikely.

However, before Apollo, they still had the idea that perhaps we could find liquid water in habitats just a few meters below the surface. They expected to find moisture below the surface, from comets. They thought that radioactive heating was possible as well. So, with their understanding of the Moon back then, there might be niche habitats below the surface, insulated from the surface extremes by the dust. Also back then, they had no idea how the Moon formed, and they felt that there was a chance that the early Moon started off as habitable as early Earth. Perhaps, in its lower gravity, it gradually lost its oceans and atmosphere. If so, life might have evolved just fast enough to keep up with the changing conditions, and so might still be there today. Carl Sagan calls this "the admitted very speculative possibility" in his 1960 paper Biological contamination of the Moon. This paper describes the putative Moon habitats for life though the focus is on forward contamination of the Moon.

If that "admittedly very speculative" theory was right, then if there is life in habitats a few meters below the surface - well - there could be hardy spores on the surface of the Moon. These ideas weren't that far out. Speculative yes, impossible, no. Even now, we have evidence that shows clearly that the Moon is still slightly active, with some small regions that show signs of disturbance from below rather than above. To find out more, see Volcanoes erupted on the Moon within the past 100 million years which also has a link to the full Nature article which you can read through their article sharing initiative. Also we now have some evidence that the early Moon did have some water which was involved in the formation of rocks during the process of the accretion of the Moon.

Indeed even today there's a minority view too that there could be ice just a few meters below the surface of the Moon over large areas of the Moon. This is a hypothesis developed in a series of papers by Arlin Crotts of Columbia University in studies of the enigmatic and controversial "Transient Lunar Phenomena" - bright patches that may appear briefly on the Moon, reported by many experienced lunar observers. He noticed a correlation of the TLP's with sites where argon and radon gas is detected. Radon gas can't come from the solar wind and it is generally agreed that it must be outgassing from below the surface. The usual explanation is that it's the result of slow leaks as a result of radioactive decay from below the surface. Arlin Crotts thinks that there may also be explosive outbursts of gas which may lead to the TLP's. He developed these ideas in a series of papers called "Lunar outgassing, transient phenomena, and the return to the Moon",

• I: Existing data
• II: Predictions and Tests for Outgassing/Regolith Interactions
• III: Observational and Experimental Techniques

In the third paper he proposes using ground penetrating radar in orbit around the Moon to search for subsurface ice, which he thinks might be present just a few meters below the surface, replenished from below. He also suggests various ways to monitor the Moon from orbit in searches for outflowing gas as well as ways to observe the TLP's. directly.

I don't know of anyone who suggests there could be present day life there any more - it seems to be an idea that was dropped after Apollo. However, Carl Sagan's ideas for subsurface conditions, though very speculative, weren't so wide of the mark as all that, and back in the 1960s, there was no way they could rule it out before Apollo.

So the Apollo quarantine precautions were meant to protect against a scenario that they thought was speculative, and unlikely, but on the basis of their best scientific knowledge at the time, not impossible. The lapses in the protocols are understandable on a human level in the circumstances, because everyone knew that the chance of any spores from subsurface lunar life was minute.

Were we just lucky? Do extraterrestrials sometimes become extinct after their first Apollo style mission to their nearest Moon or planet?

Everybody knew that it was almost certain that there would be no life on the Moon, and it's hard to take such a low probability seriously.It is totally understandable that the mission planers will be more concerned about whether the heroic astronauts will get seasick. When they balance that against a speculative risk that even the scientists think is extremely unlikely, it's obvious what they are going to decide. It is just how we tend to think in situations like that. But if you can open the command module hatch, and let dust into the sea just to avoid the inconvenience of seasick astronauts - well you might as well not take the precautions at all.

This would have applied even more so if the Apollo astronauts had got seriously sick in the quarantine facilities. Does anyone seriously think they would have kept them in there and left them to die? Of course not, surely they'd have been rushed to the nearest hospital or intensive care unit right away. But if you take them out of the facility if they ever get seriously ill, then again, you might as well not take precautions at all.

For that matter, could you legally and ethically keep a sick astronaut in a quarantine facility if their health or their life is at risk and they can be saved by hospital treatment, and the risk of harm is a speculative one that scientists can't assign a probability to? For more on this, see The knotty problem of human quarantine - and what about exposure of humans during a robotic sample return? (below).

I think it is fair to say that, though seriously intended, in reality the whole thing was pretty much a formality. They showed that they cared about it enough to try. But they didn't actually achieve anything. We were just lucky.

I think it's impossible to assess what the level of risk was, until we know more about other planets in the galaxy.

Who knows, out of many planets with large moons, maybe some of them do have hazardous life on them? Our Moon could easily have had traces of ancient life if the alternative theory was correct that it started off as a habitable world in the early solar system - a larger version of Ceres, say (see Search for early life on Ceres (below) ). Also, with present day technology, with biocontainment suits, with our nuclear submarines, with military bunkers to protect against biological weapons, I don't think we'd go extinct. Most of us might die, we might have to cover Earth with enclosed greenhouses to keep out the microbes, but we'd survive. See Worst case, almost certainly not an extinction event - rather - debilitating to habitability of Earth (below). But back at the time of Apollo they didn't have that technology to the extent we do today.

It might be that the risk we ran there was only one in a million, that could mean more than extra terrestrial intelligences in a million goes extinct soon after sending its first Apollo style mission to its nearest moon or planet, assuming that most attempt that already at a similar stage of technology to ourselves back at the time of Apollo. If there is at least one extra terrestrial intelligence with technology evolves per galaxy, with an estimated two trillion galaxies in the observable universe, that could be over a million ETIs that go extinct in this way in the universe as a whole.

At any rate, whatever the ins and outs for the Apollo quarantine, we've learnt from their experiences. The Mars Sample Return studies often use the Apollo return - not as an example of how to do it, but as a great example of mistakes we must avoid. That's one reason why all the studies to date require that the receiving facility is built well in advance. They require that it is not only built, but the staff thoroughly trained and the facility up and running and all the glitches worked out, even before you launch your mission to return a sample to Earth.

As a result, you can't launch a mission to get a sample from Mars, and then postpone the receiving facility to a later budgetary cycle and complete it at the last minute. To make sure that can't happen, the Mars sample return studies recommend that the facility has to be up and running well before the launch of the spacecraft to return samples from Mars. If it isn't up and running early on, then, at least based on the experiences with Apollo, we have increased risk of lapses of protocols (see Example of Apollo sample return - learning from our mistakes in the past (above) ). For details of their recommendations for the facility see Hardest part - what do you do next once you have a sample on the Earth?.

Mars according to our present day understanding is far more habitable than the Moon seemed to be in the 1960s according to their understanding back then. We actually have many potential habitats on Mars, not confirmed, but we do have actual very clear evidence of liquid water there, which we never had for the Moon. The main question is whether any of this liquid water is simultaneously warm enough for life and also not too salty.

Also, Mars life might be able to tolerate a wider range of such conditions than Earth life, for instance it needs to tolerate large swings of temperature between day and night, if close to the surface, and needs to be well adapted not just to desiccation but to ionizing radiation, and there would be strong evolutionary pressure to reproduce below -20 °C - but that doesn't remove the advantage of an ability to reproduce at higher temperatures too when they occur.

So it's well possible that we find indigenous life on Mars in habitats that Earth life can't tolerate, , and that they still could survive on Earth.

Also Mars of course, unlike the Moon, has a thin atmosphere which has carbon dioxide and water vapour in amounts that are useful for life. We also have examples of lichens and cyanobacteria that can quite possibly survive there with no liquid water at all, using the 100% night time humidity when the air gets cold at night, and in the early morning. See Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere

Just about everyone who has looked at the matter in depth says we need to take precautions in the event of a sample return from Mars. There is one exception, however. No prizes for guessing who it is. But let's see what he says.

Zubrin's arguments in: "Contamination from Mars: No Threat"

There is only one dissenting voice here as far as I know, in the entire published literature on sample returns from Mars. That's Zubrin again, in an article he wrote for the Planetary Report in July / August 2000, also available online here:

• "Contamination From Mars: No Threat".
• He also expresses similar views in this transcription of a tele-conference interview with ROBERT ZUBRIN.

The November / December 2000 edition of the Planetary Report has the replies to him, by specialists in planetary protection, under the title "No Threat? No Way":

• A Case for Caution by John Rummel, NASA'S planetary protection officer at the time, and previously, NASA senior scientist for Astrobiology
• Hazardous Until Proven Otherwise, by Margaret Race, a biologist working on planetary protection and Mars sample return for the SETI Institute and specialist in environment impact analysis
• Practical Safe Science by Kenneth Nealson, Director of the Center of Life Detection at NASA's JPL at the time.

Again, I'd better answer his arguments right away, because these arguments get enthusiastic support from the Mars colonization community. If you have heard them, and been convinced by his arguments, it is possible that, you may well think that there is no reason to take any precautions at all. Why do we even discuss this? Surely all those astrobiologists must just be wrong in their views?

That's certainly what Zubrin says, saying

"The kindest thing that can be said about the above argument is that it is just plain nuts.".

In more detail, he says:

"Of all the dragons infesting the maps of would-be Mars explorers, one stands out as not only illusory but hallucinatory. This is the "Threat of Back Contamination.""

"The story goes like this: no Earth organism has ever been exposed to Martian organisms, and therefore we would have no resistance to diseases caused by Martian pathogens. Until we can be assured that Mars is free of harmful diseases, we cannot risk exposing a crew to such a peril, which could easily kill them or, if it didn't, return to Earth with the crew to destroy not only the human race but the entire terrestrial biosphere.

The kindest thing that can be said about the above argument is that it is just plain nuts....".

He then outlines his main arguments against it, which are (this is my paraphrasing of his main points)

• That diseases are keyed to their hosts, and so Mars life would not be able to survive on, or in a human body. That if there were indigenous Mars host organisms, they'd be as distantly related to us as elm trees, and Dutch elm disease can't infect humans. Also, there's no evidence for macroscopic hosts on Mars anyway, so how can Mars microbes harm humans?
• Any Mars life that could survive on Earth is already here, transported on meteorites - we receive half a ton of meteorites from Mars every year, and so Earth life is already exposed to Mars life.
• That Mars life would not be able to survive on Earth because the environment is so different here.
• That to sterilize a Mars sample would be like sterilizing a dinosaur egg, a terrible loss to science.
• That every time you turn over the soil you are returning past Earth life to the present which could potentially harm us. So since we don't take precautions when we do that, we don't need to take precautions for samples returned from Mars.

As Margaret Race put it in her response:

"When I read the opinion piece by Robert Zubrin .... I didn't know how to react. As a biologist working on planetary protection and Mars sample return at the SETI institute, I wondered how an engineer and Mars enthusiast like Zubrin could make such irresponsible and inaccurate statements. Obviously, Zubrin is entitled to his opinion, even if it's based largely on misuse of facts. But what about the readers of The Planetary Report? Don't they deserve more than op-ed humour?" (emphasis mine)

So, let's answer his arguments as before. The arguments are rather easy to demolish, but the details are interesting I think, so it is worth going into it in some depth. You can read them below under:

• Many microbes harmful to humans are not "keyed to their hosts"
• Zubrin's meteorite argument for safety of a sample return
• Why Mars life would be expected to survive the warmth, and oxidative stress of Earth
• But we unearth ancient life all the time
• Why sterilizing a sample from Mars is not like sterilizing a dinosaur egg

However, first I'll share a few highlights from "No Threat? No Way". These responses don't get anything like the amount of publicity of Zubrin's arguments.

Some highlights from the rebuttals of Zubrin by astrobiologists in "No Threat? No Way"

To read the articles themselves you need to join the Planetary Society for a year, and then you can download them here. Sorry, I can't share them. But I can mention a few quotes:

John Rummel, planetary protection officer for NASA opens out with

"There are days when I ask myself, "Is it worth it?" After all, given the heightened awareness about Earth organisms newfound capabilities in extreme environments - to say nothing of the troubles that immune-compromised patients face with normally benign microbes - I figure the need for contamination controls for missions to places possibly harboring life should be obvious. So I sometimes wonder if I as Planetary Protection Officer, can really make a difference. "

"I want to thank Bob Zubrin for providing this week's job satisfaction. His opinion piece in the July/August 2000 issue The Planetary Report was so off the mark that I found renewed joy in simply contemplating an answer. "

He goes on to make the point that microbes that have not co-evolved with humans can be dangerous and uses Robert Zubrin's Dutch Elm disease example as a reminder that microbes which are not human pathogens can still cause damage.

He makes a rather telling point when he uses the example of Radiodurans - the microbe which is able to survive in reactor cooling ponds, as an example to show that microbes can survive in environments that they couldn't have evolved in. There is no way radiodurans could have evolved in reactor cooling ponds. Yet it is able to survive in them just fine.

'One canard to point out, however, is Bob's assertion that "microorganisms are adapted to specific environments," and thus Mars microbes would refrain from living on Earth. This is not a reliable speculation. A notable counterexample from Earth is Deinococcus radiodurans, an organism first isolated from nuclear power plants environments that did not exist prior to the 1940s. Where did this microbe come from? Deinococcus radiodurans has since been found in natural environments (dry lakebeds) quite unlike Three-Mile Island. ' (emphasis mine)

Margaret Race talks about the basis for planetary protection in the 1967 Outer Space Treaty, and the recommendations of the study by the National Research Council, "hardily an alarmist group". She also mentions a previous survey for the Planetary Report that found that out of 4,300 members of the Planetary Society, an overwhelming majority agreed to the statement:

"all materials brought to Earth from Mars should be considered hazardous until proven otherwise."

She likens our precautions for a Mars sample return to installing smoke detectors and fire extinguishers in a building, saying:

"He's confident in our impressive technological prowess; he's raring to go and doesn't want anything to slow down or stop our exploration of Mars - especially not burdensome regulations based on very small risks and scientific uncertainty. Yet when he suggests that there's no need for back contamination controls on Mars sample return missions, he's advocating an irresponsible way to cut corners. If he were an architect, would he suggest designing buildings without smoke detectors or fire extinguishers?" (emphasis mine)

Kenneth Nealson says that the technology for containing biohazards is not out of reach, and he also, already back in 2001, predicts that in the future we will be able to use in situ searches, writing

"Second, a number of measurements could be made onsite (on Mars) that would help in the search for life. The technology of in-situ life detection has lagged behind many other efforts; now may be the time to push for the development of instruments capable of detecting without ambiguity the presence of life at a given site, or more particularly, in a given sample. Sending data back should be a major part of the planetary program, especially as we venture farther from Earth to where sample return is more difficult and expensive. To become more expert in this procedure while on Mars would seem a reasonable and useful endeavor. Why not be safe, have pristine samples to study, and take on our duty as responsible scientists and citizens? I believe that is not too much to ask; in fact, it is prudent and wise to follow such a course."

"Doing solid science in a clean and safe way will help ensure the future of the space program. Alternatively, denigrating those who would argue for safe measures regarding the unknown is ultimately irresponsible." (emphasis mine)

So anyway now I'll do a detailed rebuttal of each of his arguments. I've got no page limit here (unlike the authors of those articles) so can go into in as much detail as is needed to rebut them.

Zubrin's meteorite argument for safety of a sample return

• Any Mars life that could survive on Earth is already here, transported on meteorites - we receive half a ton of meteorites from Mars every year, and so Earth life is already exposed to Mars life.

Yes we get half a ton of Mars meteorites hitting Earth every year, that's true.

Half a ton of meteorites reach Earth from Mars every year, though most of them are never discovered and quickly blend into the landscape, like all the other unrecognized meteorites that fall here. As with the other meteorites, we can spot them most easily in deserts and in ice sheets because there are no other rocks to confuse them

As he says, many of those meteorites have been traveling through space for millions of years, and the youngest, for hundreds of thousands of years. But is there any viable life from Mars on them? That's the main question.

As he says in his article, it's true that some life could survive the transit even for millions of years, deep within a many meters in diameter large chunk of rock.

However, life near the surface of a meteorite is going to get ablated away during the fiery entry into Earth's atmosphere, see the section : Case study - can photosynthetic life be transferred from Earth to Mars or vice versa? (above)

Also, if the rock is small, or the life is close to its surface, as most life is likely to be, even the most hardy life would be sterilized within 100,000 years. The last time a meteorite left Mars for Earth was probably about 600,000 years ago, as that's the youngest cosmic radiation exposure age of our Martian meteorites.

The EETA 79001 meteorite, with the youngest cosmic radiation exposure age of all our Martian meteorites. It left Mars only 600,000 years ago. It is also a very young rock. It formed on Mars only 180 million years ago.

This is long enough ago so that even radiodurans, tough enough to survive in reactor cooling ponds, wouldn't be able to survive the cosmic radiation dose near the surface of the rock

Techy details: In a study of the effects of cosmic radiation exposure on the Mars surface done for ExoMars, then after 134,000 years there would be no viable life left in dormant state to a depth of 1 meter (see their table 1). That's for the rather lower doses of radiation on the Mars surface. The rocks are shielded on one side by the planet itself, and they are also shielded to some extent by the Mars atmosphere. A meteorite in the middle of a two meters diameter rock in interplanetary space would experience more than double that amount of cosmic radiation, which makes it less than 70,000 years for a depth of 2 meters.

Those calculations are for radiodurans, one of the most radioresistant microbes known (although no longer the record holder). It's for a million-fold reduction in the population.

So any life left, throughout a rock of diameter of up to two meters would be thoroughly sterilized by the 600,000 year passage of EETA 79001. Of course that's also true for any other material ejected as a result of the same impact.

So how big could the rocks be that get here from Mars? The meteorites we have so far all had pre-atmospheric sizes of 23- 25 cm. According to impact modeling, typical meteorites will be on average 30 cm in diameter. The largest meteorites likely to get to Earth are 2 meters in diameter, at least for the sizes of impacts that sent material here for the last twenty million years. See page 1355 of this paper. So there is no chance at all of a meteorite large enough to protect life from cosmic radiation for as long as 600,000 years.

Perhaps some martian life can withstand higher levels of ionizing radiation than radiodurans. Indeed, we do have a more radioresistant microbe on Earth, Thermococcus gammatolerans - an obligate aerobe from hydrothermal vents able to withstand 30 kGy of gamma radiation, and still reproduce. Conceivably there could be viable life of some sort inside these meteorites, but it would be sterilized of almost all known forms of Earth life in transit long ago.

The upshot of this is that of the half ton of Mars meteorites we receive every year, probably all of it is thoroughly sterilized of Martian life now, if it ever contained any.

But did it have any life in it originally? Though there seems no chance of life arriving from Mars right now, could life have transferred to Earth soon after the impact on Mars? Let's look at this a bit closer.

Can life get into the ejecta from an asteroid impact on Mars?

Most suggested habitats on Mars are deep below the surface, or in the top few millimeters and centimeters. So, could life from these surface habitats get into the meteorites arriving on Earth from Mars right now?

Up to 2002, scientists thought that meteorites could only get to Earth from Mars from relatively large craters on Mars, 12 km or larger. However their measurements of the cosmic radiation exposure ages of the meteorites told a different story. The meteorites were exposed to cosmic radiation only during the transition between Mars and Earth. By adding together the cosmic radiation exposure age and their terrestrial age, then it's possible to work out their ejection age - how long ago they left Mars.

They were able to show that the meteorites in our collection must have left Mars on many separate occasions, roughly once every one or two million years for the last twenty million years. So they couldn't have come from a single impact, because they left Mars at many different times. However, there was no way that Mars could have had that many 12 kilometer diameter craters in the recent past. The chance of one such crater in twenty million years is low.

So, how could this be? There is no way Mars could have so many large meteorites (based on the present day lunar cratering rate, then it was unlikely to have even one impact that large), yet there was clear evidence that the meteorites in our collections came from multiple impacts, and no evidence of the meteorites from surface layers of Mars that we might expect to find occasionally from a larger impact.

These puzzles were resolved by a paper in 2002. The authors used a computer simulation and showed that though a large impact, with a crater 20 km in diameter is needed to eject materials from the crumbly Mars regolith or ocean beds, a smaller impact into the harder, younger rocks of the Martian uplands could send material with the Martian escape velocity of 5 km / second all the way to Earth. The crater could be as small as 3 km wide. Since then, later work showed that even smaller craters could send material to Earth after a glancing blow on the Mars uplands.

They also found independent evidence that the Mars meteorites in our collections came from some way below the surface of Mars. Meteorites from layers close to the surface of Mars would have evidence of long periods of exposure to neutrons generated by cosmic radiation hitting the surface rocks. None of the meteorites have this signature of surface rocks. This shows that our meteorites come from at least three meters below the surface of Mars.

Then in 2003, Zunil crater was found, a young and large rayed crater on Mars, followed by several others in 2006. The rays are only visible in thermal infrared images, and so remained hidden until they were discovered using THEMIS the thermal emission imaging system on Mars Odyssey (see figure 1 of this paper for a photograph of the rays). The rays extend as far as 1600 km (see summary and conclusions of this paper, page 377) with secondary non ray forming craters possibly as far as thousands of kilometers from their source. It's much like the Tycho rayed crater on the Moon, but the rays are much less easy to see on Mars. Many other smaller craters are now known. They usually the result of moderately oblique impacts (angles between 15 and 60 degrees) into young volcanic plains, and into volatile rich surfaces. This makes them an attractive source for the Martian meteorites - see the conclusions section of this paper for an overview.

Detail from figure 6 of this paper showing the rays around the 6.9 km Gratteri crater, one of several possible source craters for our Mars meteorites. The dark markings are rays only visible in infrared, similar to the rays of Tycho crater on the Moon. Rayed craters are usually the result of oblique impacts, which send more ejecta into space, so there's a better chance of the rocks reaching escape velocity.

The caption says "This THEMIS band 9 nighttime thermal infrared mosaic of Gratteri is an excellent example of the unique thermophysical signature rayed craters possess in the nighttime thermal infrared"

Some of our meteorites may come from a much larger crater, Mojave Crater, 55 km in diameter,, though this theory has some problems. This crater may have formed as recently as three million years ago according to their crater counting. A crater that large should form only every 35 to 50 million years. However this is a problematical theory because the Shergotittes seem to be geologically young, by their crystallization ages, and the crust there seems to be much older than that, around 4.3 billion years old. ALH84001 seems to have left Mars 15 million years ago (section 4.5 of this paper). The authors suggest that the Shergottites may be much older, not six hundred millions of years old, but 4.1 to 4.3 billion years old based on lead isotope ratios. They also suggest a break up in space to account for variation in cosmic ray exposure ages for the Shergottites. See articles in New Scientist. and in Sky and Telescope for more about this theory.

(click to watch on YouTube)

Flyover animation of Mojave crater and nearby region. The vertical scale here is accurate, not exaggerated. This 55 km diameter crater could be the source of some of the meteorites from Mars, if it is true that it is as young as only 3 million years old. If so, perhaps the Shergottites were formed much longer ago than they appear to be, over four billion years old, a controversial date that is suggested by lead isotope dating of these meteorites.

Anyway, the outcome of the research is that:

• The meteorite collections we have are biased towards the younger material of the Mars uplands, because the stronger the material, the more likely it is to reach escape velocity after the impact fracture.
  
• The meteorite material we have all comes from at least three meters below the surface - because if any of them were closer to the surface, we would see evidence of radiation by secondary neutrons from the surrounding rocks. We could spot this through the amount of Krypton 80 produced by neutron capture from Bromine 79. Krypton 80 is a stable isotope but the amount of it in our martian meteorites is too low for a rock that has spent much time close to the surface at any time since formation. See page 1355 of this paper.
  
• The meteorites we have had pre-atmospheric sizes of 23 - 25 cms. Modeling suggests that the average size is about 30 cm and that the meteorites ranged in size from 20 cm to a maximum of 2 meters diameter on ejection from Mars. See section "pre-atmospheric sizes", again on page 1355 of this paper.

So what does this tell us about the chance of present day life on Mars getting onto the meteorites? Well the young volcanic rocks are amongst the least likely places for present day life on Mars. They don't sample the ancient softer areas of Mars, the regolith and the ancient ocean beds. Also, they don't sample the surface layers, because our meteorites come from at least three meters below the surface. If Mars has hot spots, the ones that are close to the surface are likely to be rare compared to Earth. So, that seems unlikely too, a direct hit on a geothermal hot spot sending life from Mars to Earth.

The one possible large impact is Mojave crater, an impact into ancient 4.3 billion year old crust. The impact crater shows fans and channels (see end of this page), suggesting impact into subsurface ice that flowed across the surface as water or ice, but these formed as a result of the impact. It's not the most likely place for present day life. So, it would seem that any life transferred from Mars to Earth probably got here more than twenty million years ago.

There are several other factors that make transfer from present day Mars life to Earth tough even after the largest impacts on present day Mars.

• Many of the proposed habitats are in the surface layers of dirt, ice and salts. So would any of that get transferred to Earth after an asteroid impact on Mars?
  
• Other habitats are just millimeters below the surface of rocks. These layers would ablate away during entry into the Earth's atmosphere
  
• Mars life might be localized to particular regions. Even if it is widespread, still, if it is similar to the most inhospitable dry cold habitats on Earth, some rocks may have life beneath the surface and others none. Some areas may have the right conditions, mix of salts, humidity, for the liquid brines below the surface to have life and others not.
  
  It's also possible that life on Mars is extremely localized to only a few square kilometers over the entire planet, for instance, only to the RSL's, to take one example. If this is true, it makes it unlikely that a meteorite ejected from present day or geologically recent Mars will sample one of those habitats - the same problem as for the geological hot spots.

It may seem so difficult now that one wonders if it happened at all. The transfer would be easier

• When Mars had oceans and lakes
• If these oceans and lakes had abundant life
• During the very largest impacts at the time of the "late heavy bombardment" around 4 billion years ago.

Artist's impression of the Moon during the Late Heavy Bombardment (top) and as it is now (bottom) by Tim Wetherell of the Australian National University

This may well have been the most likely time for life to be transferred from Mars to Earth. This was between 3.8 and 4.1 billion years ago when there was a late surge of impacts by large debris that still remained within the solar system before it settled down to its present state.

In the early solar system, Mars had oceans, it may have had life, and it surely had many impacts by asteroids up to a hundred kilometers in diameter or more. Those large impacts into oceans would be ideal conditions to send large rocks impregnated with life to Earth

Do we know if Mars life has caused extinctions on Earth? - Example of Great Oxidation Event

Zubrin's reasoning is based on the premise that Mars life has never caused extinctions on Earth. But how can we know that? What if it has? We haven't had any extinctions in recent times, but then life transfer from Mars was unlikely in recent times also as we just saw.

To illustrate this idea, here is another of my speculative sections based on a bit of synthesis of various ideas. Could life from Mars have caused what was possibly one of the largest mass extinctions ever, the Great Oxygenation Event? This happened well before the time of complex multicellular life and easily recognizable fossils. Microbes are not easy to identify and characterize as fossils. This makes it hard to be sure - how can you tell from the fossil record how many species of microbes went extinct? However, all that oxygen in the atmosphere and sea may also have caused a mass extinction of anaerobic microbial life. This is often listed as the first of our great extinction events.

It's really hard for photosynthetic life to transfer from Earth to Mars, but it's a little easier to it to get from Mars to Earth (though still only barely possible). As Charles Cockell says in his paper,

"The exogenous arrival of oxygenic photosynthesis on a planet would profoundly change the direction of biological evolution on its surface. However, the likelihood of the transfer of photosynthesis between planets is less than for chemotrophy or heterotrophy."

For more on this, see Case study - can photosynthetic life be transferred from Earth to Mars or vice versa? (above)

Then, it's certainly a respectable theory that Earth life came from Mars as Steven Benner has suggested, also Paul Davies (Was Mars the cradle of life?), and others (see What we could learn - some examples - Early life or proto life - Life originated from Mars - Distantly related life - life not based on DNA style double helixes - life with capabilities Earth life hasn't developed yet (above) ). So, what about photosynthetic life as well, later on, once it evolved on Mars? After all, Mars had an oxygen rich atmosphere early on. It could be due to photosynthetic life (though there are non biological explanations as we saw in oxygen rich atmosphere for early Mars (above)).

Interestingly, one of our best candidates for a microbe able to survive on Mars, the oxygen generating photosynthesizing cyanobacteria chroococcidiopsis is an extremely radioresistant microbe. Usually this is explained, as for radiodurans, as an indirect effect of desiccation resistance. It evolved to resist damage to its DNA in a desert environment, the story goes, and the repair mechanism also works to protect it against damage from ionizing radiation. But what if it was the other way around? What if it actually evolved radioresistance in a place with high levels of ionizing radiation like Mars? Could the radioresistance have come first. Radiodurans is an aerobe - but it could have come from Mars too, back when it had oxygen in its atmosphere. This is very much a minority view, but it's something a few scientists have considered, see for instance this paper. Was earth ever infected by martian biota? Clues from radioresistant bacteria.

Earth probably had photosynthesis already, in the form of the haloarchaea, but this would have had no effect on the atmosphere as this form of photosynthesis, based on bacteriorhodopsin, doesn't generate oxygen. If cyanobacteria like chroococcidiopsis came from Mars, could this mean that Mars life was been responsible for the Great Oxidation Event? If so, it was great as far as we are concerned, as we wouldn't be here without it. However, it was a huge change in the environment of Earth, which also cooled down the Earth by converting methane in the atmosphere to carbon dioxide. Eventually the end result was that most or all of Earth was covered in ice sheets about 2.4 billion years ago. The oxygen was also poisonous to many microbes.

That's very speculative of course. However, I'm using it to illustrate a point. It would seem to be possible, at least in principle, for life from Mars to cause mass extinctions here. There have been many unexplained mass extinctions in our past, or ones with controversial explanations. Not just major ones but minor ones too. The minor extinction events were not so catastrophic, but they still made many species extinct.

It would be very hard to prove conclusively that none of those major or minor mass extinctions were caused by life that got to Earth from Mars, in a meteorite. After all, if Earth and Mars life are closely related, then invasive microbes from Mars can blend in with the other microbes already on Earth. How can you tell if they come from Mars, or evolved here?

The National Research Council looked into this question in their "Assessment of Planetary Protection Requirements for a Mars Sample Return". They don't go into their reasoning in any detail, but they say that they were unable to rule out the possibility that life from Mars could have caused mass extinctions on Earth in the past. They put it like this (page 48):

"Despite suggestions to the contrary, it is simply not possible, on the basis of current knowledge, to determine whether viable martian life forms have already been delivered to Earth. Certainly in the modern era, there is no evidence for large-scale or other negative effects that are attributable to the frequent deliveries to Earth of essentially unaltered martian rocks. However the possibility that such effects occurred in the distant past cannot be discounted. Thus, it is not appropriate to argue that the existence of martian meteorites on Earth negates the need to treat as potentially hazardous an samples returned from Mars via robotic spacecraft. A prudent planetary protection policy must assume that a potential biological hazard exists from Mars sample return and that every precaution should be taken to ensure the complete isolation of any deliberately returned samples, until it can be determined that no hazard exists." (emphasis mine).

But we unearth ancient life all the time

• That every time you turn over the soil you are returning past Earth life to the present which could potentially harm us. So since we don't take precautions when we do that, we don't need to take precautions for samples returned from Mars.

So what about Zubrin's argument that we unearth ancient life all the time. Yes, it's true that we do often unearth ancient life. For a more dramatic example, we often uncover ancient life in ice cores, and after deep drilling, kilometers below the surface. We often find microbes that haven't contacted the surface for hundreds of thousands, or millions of years. However this process also happens naturally, that life from deep within the ice or deep underground is returned to the surface. This doesn't cause mass extinctions, which shows that present day Earth life is well able to cope with such things.

This is the Natural contamination standard (see below) which I go into in more detail later. It works just great for asteroids and comets. Earth has a constant natural influx of these materials, just as happens with ancient ice and rocks returned to the surface from deep underground. That's the main reason we don't have to take any precautions to protect Earth for comet and asteroid sample return missions. We don't need to know in advance if there is life in the sample or what it is like. If there is life there, then whatever it is, we are clearly adapted to it.

However this argument only works for Mars if it is correct that we get frequent microbes from Mars on Earth, and if these samples also include the full complement of microbes we can get from a sample return. We could use it if the meteorite argument works, as then we'd know that any life on Mars can get here on meteorites. As we've just seen, the argument has flaws in it.

Also - is what he said there even true for Earth? In some special situations we might need to take precautions when we unearth ancient microbes, if something unusual happens. For instance, there's concern that global warming may lead to melting of mass graves in Siberia of victims of the Spanish Flu in 1918 and people with smallpox, which could return these viruses to the present population not adapted to them any more. Also, an ancient "giant virus" from Siberia was revived which had been trapped in Siberian permafrost for 30,000 years, and this rapidly became infectious in laboratory tests. It was only infectious in amoebae and was not a hazard to humans, but this got researchers thinking, about whether there could be risks from ancient microbes released due to global warming. See "There are diseases hidden in the Earth and they are waking up".

Again, a good way to look at this is to think in terms of the Natural contamination standard (see below)". If you can prove that what you are doing is equivalent to what happens naturally, then the mission is not a biological issue. The Siberian melting is something new, that hasn't happened before in human history, and so by the natural contamination standard, it also could lead to new issues, that we never encounter when we just dig over a patch of soil.

Why Mars life would be expected to survive the warmth, and oxidative stress of Earth

• That Mars life would not be able to survive on Earth because the environment is so different here.

Zubrin doesn't spell out why it is that he thinks that Mars life would not survive on Earth. But one idea is that Mars life is not used to oxygen, or to warm conditions, and could only survive in cold dry conditions where there is no exposure to oxygen - which would be very restricting if true. Now it is of course possible that some forms of Mars life can't survive here. The life in the section: Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells (below) would be an example. It would just self destruct if warmed up and exposed to damp. There wouldn't be many places on Earth where it could survive.

However, assuming what we find on Mars is not an unusual very fragile lifeform like that only able to survive in very cold conditions on Mars - well first, the Mars surface at midday can occasionally reach temperatures of up to 20 °C in equatorial regions. It's not like Titan with temperatures of -180 °C (I discuss whether life from Titan could survive on Earth in - Titan as potentially the easiest place for humans to live outside Earth below).. The conditions on Mars have a broad region of overlap with Earth's temperature range.

Daytime temperatures in Gale Crater as measured by Curiosity often go well above 0 °C and as measured from orbit then 20 °C temperatures in midday are not unusual on the surface of Mars.

The temperatures on Mars overlap with Earth. Mars life might well be able to tolerate typical temperatures we have on Earth just fine.

As for oxidative stress - though Zubrin didn't raise it specifically, it's often used in the context of Mars sample return studies. See for instance section 2.3 (page 14) of the ESF sample return study.

"It should be clear that the introduction of a possible organism from Mars, or a population of Mars organisms, would be very difficult to accomplish even if it were being done on purpose. The Mars environment (cold and dry) is very different from most environments on Earth (largely warm and wet). Free oxygen in the Earth’s atmosphere may be an even greater hazard for Mars organisms: it has the ability to strip electron from (organic) molecules and is therefore poisonous for any organism that has not developed the ability to produce antioxidants (the Great Oxygenation Event around 2.4  billion years ago wiped out most of the early Earth’s anaerobic organisms). Earth does have cold and dry environments, some of which are anoxic, but there is only a limited chance that Mars organisms would find their way to those places. Adding the presence of predatory and competitive Earth organisms, the chances for survival for an alien microbe (and its potential hazard) becomes even lower. The challenges of contaminating the Earth are daunting for an invading Mars microbe, and certainly the probability of success for such an invasion is much less than one."

This report is from July 2012, so it's well possible that the authors would phrase it differently now, as there have been so many developments in Mars habitat studies since then. Also even in the same report, later on they bring up the possibility of extremophiles from Mars that could survive on Earth:

"Due great differences in the environmental conditions of Mars and Earth, it is very likely that any life form existing on Mars would have great difficulty surviving under Earth conditions. However, the possibility must be considered that a Mars life form that is not adversely affected by Earth’s environment could be present in a returned sample. It is evident that biological organisms can survive in extreme physical and chemical conditions on Earth (extremophiles), and there is a considerable body of information on how they have adapted to survive under these conditions. Replication either in the external environment or following intake by Earth organism(s) Depending on its ability to cope with a new set of physico-chemical environmental conditions, a surviving Mars organism could undergo replication in the external environment or following uptake by an Earth species. Survival and ability to replicate would allow the Mars organism to colonise the Earth’s biosphere. These organisms would potentially disturb the functioning and equilibrium of the ecosystem they settle in, by, for example, competing for resources or representing new resources, and thereby have an indirect consequence on Earth species. "

Anyway let's look at it, why might this oxygen argument be less than totally convincing?.

First, we already know that some of the microbes from Earth that are best suited for living on Mars have no problem dealing with oxygen. For instance one of our best candidates for a microbe able to survive on Mars, the polyextremophile Chroococcidiopsis grows just about everywhere on Earth, from the tropics, through to Antarctic dry valleys and the dry desert core of the Atacama desert. So long as there is sunlight and some source of water or humidity, then it is right at home. The constantly changing Mars surface conditions also seems likely to favour polyextremophiles over specialist microbes that can only flourish in a very narrow range of conditions.

Also there is a small amount of oxygen in the Mars atmosphere already, around 0.145% as measured by Curiosity. In the past it may well have had an oxygen rich atmosphere. See Oxygen rich atmosphere for early Mars (above) . Microbes there may well retain adaptations to oxygen in its atmosphere from its early history, as they are often conservative and retain capabilities they needed in the past. As well as that, its surface is much more strongly oxidized than Earth's surface. Would microbes adapted to tolerate both hydrogen peroxide and perchlorates in their environment be killed by an oxygen rich atmosphere?

Also, it's well possible that Mars life generates oxygen on its own account, for instance if it has photosynthetic life based on fixing carbon from carbon dioxide, and releasing oxygen. The same is true also for any life that uses perchlorates as a food source. Most microbes on Earth that metabolize perchlorates produce oxygen as a byproduct in the process.

Also, Mars had warm and wet conditions in the past and may still have them locally, either in the salty brines, or even as fresh water trapped beneath clear ice. Since it may have oxygen producing microbes, surely we can't exactly know for sure that it has no oxygen rich environments either. Yes, perhaps most of its habitats, or all of them, are anoxic. But any suggestion of that nature is based on no data so far. For instance might photosynthetic life, or perchlorate metabolizing life create locally oxygen rich brines or water?

The fungal component of the lichens in the DLR experiment was able to survive in Mars simulation conditions, not just survive, it could grow as well, using only the humidity of the atmosphere. They speculated that it got enough oxygen from the algal component of the lichen for its limited needs. If there are lichens or similar organisms on Mars they could survive in a similar way, and would probably not be troubled by an oxygen rich atmosphere any more than lichens on Earth are (see Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere (below) ).

Based on all that, no, it doesn't seem that likely that Mars life would find the oxygen in Earth's atmosphere impossible to cope with. Also, why would it be troubled by moisture or warmth? That also is within the range of possible conditions of microhabitats on Mars. Of course warm wet habitats are much rarer there and smaller scale, and most of the proposals are microhabitats. However, a microbe adapted to a microhabitat that on Mars is only cms or even microns in thickness could manage fine in a similar habitat on Earth with a volume of cubic kilometers. Why should increased volume of the habitat be a problem for it?

What do you think?

Many microbes harmful to humans are not "keyed to their hosts"

• That diseases are keyed to their hosts, and so Mars life would not be able to survive on, or in a human body. That if there were indigenous Mars host organisms, they'd be as distantly related to us as elm trees, and Dutch elm disease can't infect humans. Also, there's no evidence for macroscopic hosts on Mars anyway, so how can Mars microbes harm humans?

Zubrin argued that microbes have to be keyed to their hosts to harm them. Actually, it's usually the other way around. Diseases are at their most virulent when they first transfer to a new host. That's why "Bird flu" for instance is so dangerous. The main problem with this disease is that it isn't "keyed to its host" as it hasn't had enough time to adjust to a human host. It's in the interest of most diseases to keep their host alive for as long as possible, and in some cases they may eventually become beneficial symbionts. So a microbe from Mars, if it can survive on and in humans at all, is if anything probably going to be more harmful rather than less harmful than a microbe that is "keyed to us"

So could there be such organisms on Mars that are harmful to us? That was the subject of this study, lead by David Warmflash of the NASA Johnson Space Center: Assessing the biohazard potential of putative martian organisms for exploration class human space missions.

First the authors look at the ways that Earth microbes harm us. They can be

• Infectious, causing damage only if they multiply inside the host - either invasive throughout the host or local in their effect
• Toxic - hazardous components of the cells, or the products of their metabolism may harm other organisms incidentally - not targeted at us but still can kill us.

Those two categories are not exclusive - an infectious organism might also produce toxic byproducts.

Now, most microbes on Earth are absolutely harmless to humans. They may live on our skin, and even if not at all adapted to us, they cause us no harm at all - unless of course you have a wound and they get inside the wound. Our bodies are well able to cope with them.

However, there are a few that are harmful to us, and those are not necessarily adapted to humans or even to macro organisms. One of the best known diseases of this type is Legionnaire's disease. Normally it lives inside amoebas, inside the cellular fluid. However, it can also live independently without any host in biofilms. Yet it can also live inside the lungs of human beings. It uses the same adaptations that let it live inside a biofilm, or an amoeba, to live inside our lungs.

This could easily be duplicated on Mars. A similar organism could survive in a biofilm on Mars just as Legionnaire's disease does on Earth. Or it could survive inside a single cell larger microbe on Mars. All that it needs to be a potential risk to humans is to live naturally in an environment similar to what it might encounter later in a human host. As they say in the paper:

"The causative bacterium, Legionella pneumophila, is a facultative, gram-negative rod that is one of several human pathogens now known to be carried in the intracellular environments of protozoan hosts. Additionally, L. pneumophila can also persist, even outside of any host, as part of biofilms. In essence, all that a potentially infectious human pathogen needs in order to emerge and persist in an environment is to grow and live naturally under conditions that are similar to those that it might later encounter in a human host. On Mars, these conditions might be met in a particular niche within the extracellular environment of a biofilm, or within the intracellular environment of another single-celled Martian organism.".

Next, they give an example of an organism that produces a toxin. Their example is the anaerobe Clostridia, which often lives as spores in soils. Some of its species are locally infectious in wounds. and can release life threatening toxins at times, including C. tetani which causes Tetanus, and C. perfringens.

As another example, they mention another species of Clostridia, botulinum, contaminates food stored in anaerobic conditions, which releases a toxin that interferes with the way our nerves work. This microbe can lead to the fatal paralysis of Botulism when ingested. Of course it is not adapted in any way to paralyse humans - there is no evolutionary advantage in that. This is just a byproduct of its metabolism that happens to be harmful to hum ans. So the same could be true for Mars microbes, they might produce byproducts that happen to interfere with our metabolism in ways that harm us.

Photomicrograph of Clostridium botulinum bacteria. Though it is not adapted to humans, it produces a toxin which coincidentally is a nerve toxin which causes the rare but serious Botulism disease. This toxin is paralysing if ingested, and can be fatal. This is one way in which Mars microbes could be harmful to humans directly

Another example is Clavicepts purpurea which produces ergot disease, in crops. When humans eat those diseased crops, it can lead to limb loss, convulsions and hallucinations, and again there is of course no evolutionary advantage in this for the microbe. There is no need for the Mars life to be related to us in any way for it to produce coincidentally toxic substances like this.

Here is another example of my own to add to the list. Aspergilliosis, a frequent cause of hay fever in humans. It's a fungus which is capable of surviving in extreme conditions. For most people, it's no trouble at all. For others it's a minor nuisance. It's not adapted to be virulent, and is normally easily kept out by our immune system, However, it can be harmful and even deadly to people with a damaged immune system.

It seems an interesting example, seems to me because of Joshua Lederberg's "On the other hand, microorganisms make little besides proteins and carbohydrates, and the human or other mammalian immune systems typically respond to peptides or carbohydrates produced by invading pathogens." which I mentioned above in Why we can't prove yet that Mars life is safe for Earth and his .

If some microbe from Mars for some reason is able to adapt to survive in our lungs, and if it is only remotely related to Earth life, our immune system might not recognize it as harmful. If so, we might all respond as if we had damaged immune systems, like the patients who die from Aspergilliosis. So perhaps this may give us an idea of what to expect.

Another example comes from Chris Chyba: cyanobacteria killing cattle

Lake Eyrie in October 2011 during its worst cyanobacteria bloom for a long time. The cyanobacteria produced microcystins which is a liver toxin and can cause sudden death in cattle within hours, also often kills dogs swimming in the water and is a skin irritant for people.

As Chrys Chyba summarizes the situation in his abstract:

"It is unlikely that these cyanobacteria evolved the toxins in response to dairy cows; rather the susceptibility of cattle to these toxins seems simply to be an unfortunate coincidence of a toxin working across a large evolutionary distance"

This is of no advantage at all to the cyanobacteria. Cows are neither predators on them, nor do they eat cows. It's just a coincidence that they happen to produce a chemical that is toxic over a very wide evolutionary distance.

In the same paper he gives the example of Serratia marcenscens, a bacterium which is found in water and soil, as a free living microbe - and is an "opportunist pathogen" of animal species as widely diverse as humans and elkhorn coral.

This sort of "poisoning by coincidence" is quite common, and you may be able to think of many more examples for yourself.

For instance, cocoa plants produce theobromine which kills dogs if they eat too much chocolate. The cocoa plant doesn't need to defend itself against dogs. Cyanobacteria also produce BMAA which is implicated in Alzheimer's. Again there is no advantage to the cyanobacteria to give humans Alzheimer's. It is just a chemical that happens to resemble one of the amino acids L-serine, and so gets misincorporated into proteins causing Alzheimer's. Even life that is based on a different biochemistry from Earth might easily, through near coincidence, produce chemicals that Earth life misincorporates in this way.

So, in the same way microbes from Mars could quite easily produce deadly toxins for Earth life and would not need to be adapted to us in any way at all to do that.

Why sterilizing a sample from Mars is not like sterilizing a dinosaur egg

First, I think most would surely agree that we shouldn't do science based on taking risks that imperil the entire Earth, even if the probability of that happening is thought to be rather small. For most of us, a chance of a major scientific discovery is not worth even a tiny risk of a devastating consequence for the environment of Earth.

This is not an "anti science" stance, as those who have this view include keen and enthusiastic scientists like Carl Sagan. You could hardly find someone less likely to be motivated by any "anti science" platform. He would have been more interested than just about anyone in what we might find in a Mars sample return if it was of biological interest. The same is true of Carl Woese and Joshua Lederberg amongst others of those urging that we take care about a sample return. They are keen scientists, and innovative ones who made major discoveries what's more.

Also I think it's important to realize, the idea isn't that we have to take precautions against all extra terrestrial life no matter what it is. It's not an unthinking blanket ban on returning life to Earth!

Once we know what is in it and know a lot about it, then depending what we find out about it, it could be perfectly safe to return to Earth. For instance if it is a fragile life with perchlorates and hydrogen peroxide inside its cells that would just fall apart and self destruct in Earth conditions, then we might be able to prove that. The same could apply if it is some very fragile early form of life that modern life would make extinct. We may be able to show that it can't survive on Earth and would be immediately eaten our out competed by Earth life. There might be many other cases where we quickly decide that it is harmless to Earth.

However, as we find out more, we might find that what we have is potentially quite dangerous (though interesting). Perhaps it is a very robust form of XNA based life, and we can see that almost nothing on Earth is a match for it or few Earth microbes or lifeforms could affect it in anyway. Or perhaps we can tell from studying its biochemistry that it would be invisible to our immune systems and also the defenses of just about all Earth lifeforms.

Or we might sequence its DNA, and find that it is related to Earth life but is several billion years ahead of us evolutionarily, with far more non redundant nucleotides than any Earth species (this is a number that has steadily increased and is an indicator of evolutionary complexity). Or perhaps we discover that it has capabilities we don't have, such as the ability to reproduce in extreme cold and remain active well below the temperatures of our freezers, or has a more efficient metabolism or novel form of photosynthesis, or whatever it is.

In all those cases, and more, we'd take extreme caution. There are many things we can do, return to a telerobotic facility above GEO for instance, or to the Moon. This may make the sample return and study a little more expensive, maybe even a lot more expensive. However, when it is a case of protecting Earth from life that we know is a hazard, or at least, don't know yet to be harmless, well we just have to match the price tag somehow.

What can we do about the possibility of life in the Mars 2020 cached sample - or Chris McKay's"Grab a sample of Dust" or the SCIM sample from a dust storm? - Sterilize!

Of course there are good geological reasons for returning a sample from Mars, if the cost is low enough. Especially, we can't put particle accelerators into orbit at present, or build them on Mars, so can't do accelerator mass spectrometry without returning a sample to the Earth's surface. That's a drawback, because it's our most sensitive method of spectrometry for very rare isotopes. This in turn is needed for accurate age dating of the samples. However those geological age dating studies can be done as easily with sterilized samples, so it's not such a major drawback.

It's also true that we can do a lot more science even with tiny samples, if we can share them with many teams of scientists. Just a single grain of Mars dust or a tiny fragment of a rock could be enough for a lab to test and look at in many different ways, as we found out with Stardust.

As we saw, astrobiologists are pretty much unanimous in saying that a sample return is unlikely to be ground breaking for their discipline if it only has a weak and disputed signal of past or present day life, as for our current Mars meteorites. To make progress, they need a strong unambiguous signal of that life that nobody could deny. They see that as unlikely at this stage, because we don't know how to select a sample intelligently, because Mars 2020 won't be able to drill for life, or detect life in it prior to sample return, is not going to visit one of the places most likely to have present day life, and the sample will be contaminated by Earth life and altered by the return journey, which further muddies the question.

Everyone else does see a sample as of great importance for their discipline. However they give as the main reason for sterilizing the sample, that they want to do this the astrobiologists. Nobody else is likely to care if it is sterilized, so long as it is done in such a way as to not harm their own investigations. Some parts of the sample could be heat sterilized, other parts sterilized using the equivalent of a few million years of surface Mars ionizing radiation, and others maybe in other way.

We can do biosignature testing on sterilized samples too. Even after an extra few million years equivalent of ionizing radiation, it would be easy to recognize present day Mars life - if there was any there. Well at least, so long as the sample is reasonably free of Earth originated organics, and there is a reasonably strong signal of Mars life there to start with. It's the same for past life. It's likely to have had hundreds of millions or billions of years of past ionizing radiation, and adding a few extra million years equivalent is not likely to make a big difference, not for the samples Mars 2020 is likely to return. The chance of it returning past life with equivalent of less than a million years of Mars surface ionizing radiation over its entire cycle from when it was first buried must be remote indeed.

Why are the space colonization advocates so keen on returning an unsterilized sample? Could it be that they have the idea, from "Safe on Mars", that if we return an unsterilized sample and nothing happens, that it helps prove that Mars is safe for humans? We will discuss this in "Safe on Mars" - could a sample return tell us if Mars is safe for astronauts?. We will find that it would do nothing of the sort. There could easily be life in the next rock or patch of dust along from the returned sample, a hardy spore that would wake up on Earth. We can't prove that it is safe to return a sample from Mars by returning samples that we find don't harm our biosphere, other creatures, or ourselves, not without a deep understanding of where to look for it on Mars.

So, there is no particular reason why we can't keep part of the sample in orbit and study it with in situ instruments operated remotely from Earth, and sterilize and return only part of it for study on Earth. That was always the plan anyway for the Mars Receiving facility on Earth - to keep the sample well contained except for a few sterilized samples which would be released to the wider scientific community at an early stage in the investigations. The unsterilized samples would probably be studied for the first time through telerobotics anyway even in a facility on Earth, at least that's one possible way to fulfill the design requirements for a Mars sample return facility.

So, I suggest it is better to keep this contained unsterilized sample in a high orbit, until we know if it is safe to return to Earth. If we find indications of life in it, we can still find out a lot about it from studying the biosignatures and chemistry of our sterilized sample, and meanwhile study the unsterilized sample in orbit as well remotely.

Or even simpler. If the first sample has a very low chance of life in it - why not just sterilize the whole thing? For astrobiologists, it's mainly a technology demo, with only a minimal chance of either present day or past life in it. They see it as a demo for future missions that might return astrobiologically more interesting rocks.

A Mars sample receiving facility is likely to cost over half a billion dollars, as we will see (Hardest part - what do you do next once you have a sample on the Earth? (below) ) . Sterilizing the whole thing could save hundreds of millions of dollars for the next flagship mission in the late 2020s, or early 2030s, which is not to be sneezed at. That amount of saving is enough to fund an entire new discovery class mission for the next decade, like Mars pathfinder, or the Dawn mission to Vesta and Ceres. Yet it would still prepare the way for future return of unsterilized samples once we know where to look for them.

It would also be hugely simpler in legal terms as well. It would save what is likely to be a decade or probably much spent passing all the necessary legislation. That's just going through the legal processes, assuming there are no issues with any of them. For details, see Current requirements for sample return and legal situation (below) . It would also address the reasonable public concerns for safety in a much more convincing way than a return of an unsterilized sample to the Earth's surface, however carefully that is done.

So, that's the suggestion:

• sterilize the whole thing (simplest)
• or return to somewhere safe such as above GEO, and then split the sample there, via some telerobotic method in an orbiting spaceship which could become the core for a telerobotic exobiology handling facility later on - and then sterilize half of the sample for immediate return to Earth for the geologists to look at carefully.
  
  That also could be low cost, especially since a rendezvous in orbit is a reasonable extra step to do for a sample return for Earth protecting reasons anyway. By the late 2020s it may be very easy to send two spaceships, one to rendezvous and collect the sample, then split it telerobotically, and another spaceship takes it back to Earth or it just gets "posted" back to Earth on a small rocket with return capsule.
  
  Keep half of each sample in orbit temporarily, in case the astrobiologists decide that there is something there of interest that requires study of the unsterilized sample at a later date.
  
  Return the other half, sterilized, for everyone to study right away (including the astrobiologists for preliminary studies, searching for biosignatures).

Even if there is no life detected in the sample, it's best to continue to sterilize it - after all the astrobiologists won't mind in that situation. That saves much by way of legal complexities, receiving facilities, and public concern.

If there is life detected in the sample, then scientists will be strongly motivated to find a way to study it safely. Then it is a known hazard which makes a huge difference.

So that's my proposal. I go into all this in detail in the sections

• Using ionizing radiation to sterilize the sample returned to Earth
• Subsequent sample returns
• Suggestion to return samples to above GEO

So, no, I don't think that sterilizing a sample that could potentially contain a viable Mars microbe is wrong. Indeed, I think it is the only prudent thing to do if you don't know what is in the sample.

The main alternative is to build a facility rated to contain all possible extraterrestrial forms of microbiology that could be returned from Mars. You then have to devise a way to get the sample into that facility without any risk of it getting released into Earth's environment even in the case of an accident or malicious interference, or understandable human lapses of protocols as for Apollo. Sterilizing the sample seems likely to be much easier, and to cost far less.

However, would that first sample contain indigenous Mars life at all? If they return it from the equatorial regions and the main objective is to study past habitability, does that not make it very unlikely that it contains any life? If we know it can't contain life at all, then it might mean we can tell in advance that it is going to be harmless.

Well, there are two ways it could have life in it. It could have hardy spores brought to the site via the dust storms. Or there could be a habitat close enough so that the spores come from a nearby habitat again in the dust. It's not unlike the situation for the Moon in the 1960s. They thought that the possibility of life on the surface was actually zero in that case, but according to some models of what the Moon might be like, there could be viable spores from habitats a few meters below the surface. In the case of Mars the chance of life in the sample is surely rather higher than for 1960s models of the Moon. There are actually some proposed habitats in equatorial regions, surface habitats, and close to surface (top couple of cms or so). Mars also has its thin near vacuum atmosphere, and the global dust storms. which the Moon doesn't have.

So, this seems a good time to look in detail at the Viking results, and the interesting question, could Viking have detected life already in the 1970s? Then I'll look more generally at some of the ways researchers have suggested that present day life could be present on Mars even in the equatorial regions.

I will return to this sample return discussion in How the future might unfold if Viking did find life in the 1970s and then Mars sample return would return rocks interesting for geology and would be useful as a technology demo - but would it help the search for life? and following (below). Then I'll return to safety issues for a sample return in Safe return of unsterilized sample (below)

Rhythms from Martian sands - what if Viking detected life already in 1976?

There is one scenario that could mean we return life from Mars right away, maybe even in the first samples from the planet. What if there is life there already in the sand dunes? It could be in dormant form, life which can be brought to life by the addition of warm liquid water, or it could live in one of the proposed habitats for surface life in the equatorial regions of Mars. See .Ice and water in equatorial regions on Mars (below) .

As we'll see, there are many suggestions for how it could survive there, using night time humidity, or the evaporating morning frosts. Indeed, there's also Curiosity's new indirect detection of a cold layer of liquid brine just below the surface of sand dunes on Mars. That last is likely to be either too salty, or too cold for Earth life - but Mars life perhaps can tolerate it, or it might be able to exploit it using biofilms.

Viking wouldn't need to sample the habitat directly. All that's necessary is for there to be viable spores in the dust and dirt which it sampled, for instance brought there in the dust storms, or churned up from deeper in the sand as a result of the moving sand dunes. None of that is at all impossible on current understanding of Mars. Also the chance of life at the Viking sites seems more likely now than it was with our understanding of Mars back at the time of Viking. It seems at least possible that there could be viable spores of martian life in the sand dunes.

Our best analogs of Mars habitats on Earth are the driest areas of the Atacama desert, or the McMurdo dry valleys in Antarctica. These are places with patches of microbes that are so hard to detect, that in some of the habitats, life was discovered for the first time in the last decade. Many of the microbes live beneath surfaces of rocks or below the surface soil, slowly growing; sometimes with individual microbe lifetimes of thousands of years. If there is life on present day Mars it's likely to be like this. It's likely to be hard to spot visually and difficult to detect with our instruments also. Curiosity, for instance, wouldn't have much chance of finding it.

Only one of the experiments on Viking had this capability, and that's the very one that seemed to detect life, the Viking labeled release. We have alternative hypotheses to explain their results involving complex chemistry with perchlorates instead of life. However, without follow up experiments, those also are just hypotheses. Astonishingly, more than forty years later, we still haven't sent anything to Mars able to answer this question for sure. None of our spacecraft since Viking would be able to spot these cryptic microbes which we now know to eek out a living in the driest deserts on Earth.

Given that we now think there's at least a possibility of life there, and given that one of the possible hypotheses for the Viking results is that it did detect life - could it be that it actually did, already, back in the 1970s? Gilbert Levin has been saying this for decades. Well, there is some new research that somewhat boosts the possibility that this happened. A few years back, to everyone's complete surprise, Joseph Miller, a specialist in rhythms of life, spotted smooth daily cycles in the data from 1976. To his expert eye, this strongly suggested life processes. So did Viking spot life or are these smooth cycles signs of something else, perhaps some very complex chemistry?

This section comes from my Rhythms From Martian Sands - What Did Our Viking Landers Find in 1976? Astonishingly, We Don't Know, also available as a separate kindle ebook.

Carl Sagan standing next to a model of the Viking Lander - so far the two Vikings are the only spacecraft ever sent to another planet to search for life. Curiosity's main focus is on habitability of ancient Mars. It can search for organics, and has indeed found them, but it is not equipped to detect present day or past life itself, at least, not in the low concentrations likely in the Mars surface conditions.

Two of the three Viking experiments drew a blank, but they were not sensitive enough to detect life in small concentrations.

• Viking's ovens couldn't detect the organics at up to several million bacterial cells per gram. In tests in the Atacama desert using the same methods Viking used, they were not able to detect any life at all in these ultra low concentrations. (see also original paper).

Our later rovers such as Spirit, Opportunity and Curiosity also are not sensitive enough. They would have had no chance at all of spotting the life that exists in the most inhospitable areas of the Atacama desert or McMurdo dry valleys.

Then, as another twist in the story, this time about Vikings original non detection of organics.

• A recent re-analysis of the Viking data suggests that these two Viking experiments might perhaps have detected organics after all. They detected chlorohydrocarbons which were dismissed as contaminants at the time. However, now we know about the possibility of surface perchlorates, these might possibly be the result of reactions of perchlorates with the organics (Phoenix discovered perchlorates in the Martian soil in 2008).

The upshot is, we've only sent a single experiment to Mars with a decent chance of finding life there - and it came up with results that are strongly suggestive of life, though most would say, not conclusive. NASA, perhaps understandably, discounted its results at the time, because the other experiments found nothing (and perhaps James Lovelock's argument was a factor also, see James Lovelock's argument for a lifeless Mars (above) ). However, we now know that the other instruments had almost no chance of spotting life there, in the harsh conditions Viking found. It's also possible that they did detect organics.

These reasons don't seem as convincing now as they did to many back in the 1970s.

Close up of the labeled release experiment on its testing rig in the laboratory. This is a photograph from the original paper published before Viking landed on Mars. According to modern understanding of the harsh conditions there, this is arguably the only instrument humans have ever sent to the planet which had any chance of spotting present day life there in the sparse populations we'd expect.

This experiment was sensitive enough, but the unexpected chemistry of the Mars surface made their results only suggestive, and inconclusive. There's an alternative explanation involving unusual chemistry, but it's quite hard to match this hypothesis to the data. There just isn't enough data yet to decide definitively, one way or another, whether Viking discovered life in the 1970s.

I'd like to look a bit closer at the Viking results here, and also look more generally, at the possibilities of life able to survive in the equatorial regions of Mars. Then, let's look forward at experiments we could send to Mars in the future, to settle this question about the Viking results. Also, let's look more generally at how an updated version of this labeled release and other experiments could help with the search for life on Mars. If Viking didn't find life, perhaps we can use the same method (updated to avoid confusion from the surface chemistry) to search for life in other parts of the planet, or in other habitats in the the same region.

How the experiment works

It's a simple idea. You prepare organics with ordinary carbon 12 replaced by the radioactive isotope carbon 14. Most microbes, as they digest this food, release some of the carbon in radioactive waste gases such as carbon dioxide, and methane (they may also produce some hydrogen and hydrogen sulfide). You then detect this radioactivity in the air.

In preliminary tests before the mission, published in 1976, the experiment detected life in all the samples from Earth, except for a few samples from Antarctica that might have been sterile. These preliminary experiments also involved a careful selection of the nutrients so that they would work with early forms of life and even pre-biotic precursors. The nutrients also had to create no false positives, and they made sure to include both mirror image forms of asymmetrical chemicals (racemic mixtures) to make it as sure as they could be that the microbes would find something there to their liking.

If there was any life that eats those nutrients in the sample, so long as it it started to metabolize the organics into gases containing carbon (and also was not killed by the medium), then Viking's labeled release would detect it.

Detects life "doing something" - with no need for the microbes to reproduce - and not confused by non life organics

The great thing about this experiment is that it doesn't require the organisms to reproduce. So, it works for microbes that either don't reproduce at al in the medium, or only do so over long timescales.

Microbes from cold dry places such as the Antarctic dry valleys may well reproduce only every few months, and some can even continue living for a thousand years in a slowly metabolizing almost dormant state before they reproduce. When you bring these microbes into the laboratory, they often keep up this slow pace of life, and are reluctant to reproduce. You might need to wait several months, or even years. before any new cells appear. So, the usual approach of putting some of them on a petri dish to see if they form visible colonies doesn't work for these slow growing microbes.

Also anything that depends on the microbes forming large visible colonies will only work for microbes that are "cultivable" - you have to provide just the right environment for them to reproduce. Even in ordinary conditions, we don't know how to do that for 99% of the microbes in a typical sample. However, if you test for evolved radioactive carbon, it doesn't need to reproduce, just metabolize, and only by a little bit. This makes the experiment exquisitely sensitive.

He was able to detect microbial respiration from a few cells, even if they couldn't reproduce in the culture. The team were able to show that it could detect life even in places such as the cold, dry Atacama deserts, and the Antarctic McMurdo dry valleys. Yet at the same time it was not confused by other signals, for instance, of organics in lunar soil. (Lunar soil has some organics in it from the solar wind and from meteorites, in parts per billion). This is one of the best ways to prove that there's life there, to actually catch present day life "in the act".

The search for past life is much more tricky, as it involves studying dead microbes. If we find organics on Mars, that doesn't need to mean that it ever had life. Even if we find chirality signatures, then again that's not a proof of life as sometimes meteorites have a chiral excess too, sometimes be surprisingly large. In this 2006 analysis two meteorites had chiral excesses ranging from 31.6 to 50.5%.

GRA95229 - a chondrite meteorite collected in Antarctica, had chiral excesses of +31.6‰ for a-AIB to +50.5‰ for isovaline, while the EET92042 meteorite ranged from +31.8‰ for glycine to +49.9‰ for L-alanine. It's thought that these excesses are extraterrestrial and not due to contamination by Earth life.

More about this in Organics from meteorites on Mars may boost a molecule over its mirror image, mimicking biosignatures (chirality) (above)

Life, if it's like Earth, should have 100% of one of the mirror image amino acids, and none of the other. You'd think that would make it easy to spot. However, amino acids can swap handedness in warm conditions (racemization). Ancient organics may be thoroughly racemized if they have been warm for some period of time. Also, the organics will get degraded by ionizing radiation, which can break up the amino acids into gases like methane, carbon dioxide, and water vapour. Meanwhile, organics are bringing new organics to the surface, and it may get new organics mixed in from more recent life, or present day life.

So if you are lucky, this signal from past life may be clear. If it's been kept at reasonably cool conditions, and not mixed with anything else, you can work backwards by looking at the rates different amino acids transform. In that way, ideally, you may be able to prove that it was originally a 100% only one of the mirror image forms, which would be a clear signal of life (or something else similarly complex). However, more generally the signal from past life is likely to be confused and need a fair bit of detective work to find out what was there.

Again, if we find RNA or some complex molecule not normally found in meteorites, it would be a reasonably good evidence of past life or present day life. But still, strictly speaking, it's not proof, as you'd still ask if there is any inorganic way of making it. It might be prebiotic chemistry that hasn't yet formed life.

One recent paper found that RNA strands of up to 400 bases long can be made by freezing processes in artificial sea ice (simulated in the laboratory) - without any intervention of life. Its polyadenylic acid, not that realistic, as it is RNA with just one nucleobase. However, it's a start. See Did Life Evolve in Ice? (Discovery Magazine) and for the original paper, Ice and the Origin of Life.

Hauke Trinks who studied sea ice in the Arctic regions, and came to the conclusion that RNA gets formed in the ice through inorganic processes, producing RNA chains of up to 400 bases. He thinks this may be a possible mechanism for the origin of life. Whether this is how life began or not, his experiment also shows that if you detect RNA, it does not by itself prove that you have life. You would need more evidence, because RNA can form naturally in rare conditions.

Other research by Raffaele Saladino et al shows it's possible to build all the nucleobases present in life on the basis of hydrogen cyanide, and water, with various metal ions as catalysts. See their Scheme 1.

Figure from a survey paper that shows how all the nucleobases that occur in RNA in modern Earth life can be made from Formanide, a product of water and hydrogen cyanide, in the presence of metal oxides, clays and other minerals. The nucleobases are shown multiple times in this diagram but only named on their first occurrence.

They go on to show how you can also link the nucleobases to a sugar, to form nucleosides which are necessary as an intermediate process for assembly to form RNA.

So, in principle complex RNA with multiple bases can be synthesized entirely from naturally occurring chemicals, without any life involved yet. For details see their paper. So, a chiral biosignature, and even RNA, could be pre-biotic chemistry.

However, if we find evidence of active metabolism, and transformation of organics into gases, that's a good sign of present day life actually doing something. If we can find evidence of circadian rhythms, and the rhythms go out of sync. with the Martian sun under constant temperature conditions - that would be hard to explain as anything except life. If it isn't life, it has to be some really really complex chemistry which has getting on for the complexity of life itself. If we could find all that - and at the same time, we find independent biosignatures, such as complex chemicals with chirality - then the two observations would complement each other.

The great thing about labeled release is that it is one of our experiments that can detect life that is extremely different from Earth life (e.g. not based on DNA, or even proteins). All the life needs to do is to eat organics and produce a gas, such as carbon dioxide or methane, as a byproduct.

The experiments can also tell you a fair bit about what kind of life it is. You can study how the evolved gases change depending on the conditions. You might learn whether it can photosynthesize, and you could find its preferred temperatures for metabolism. In more refined versions of the experiment (never sent to Mars as yet), you can learn what gases it produces during metabolism, what amino acids it eats, which of the two mirror image forms it finds edible, and so forth. That would all build up to a convincing case for life in the sample, as well as giving useful information about what kind of a lifeform it is.

It's not perfect. It could miss some types of alien microbes. We have to make some assumptions, some guesses about the environment life on Mars likes to live in. For example, Mars life might find the culture we provide so alien, to it that it can't metabolize at all, or it might even be killed by it. As a worst case example, life that has hydrogen peroxide and perchlorates inside the cell would probably "self destruct" if warmed up and introduced into a medium of liquid water with nutrients. See Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells (below).

Also, it depends on the Mars life exhaling a gas that contains the carbon such as carbon dioxide, or methane. Most forms of Earth microbe do this.

Why there was controversy back in the 1970s

The Viking lander had three main biological experiments, but only one of these experiments produced positive results.

• The Gas Chromatograph/Mass Spectrometer searched for organics, and found no trace of them.
• The Gas Exchange experiment searched for any gases that evolved from a sample of the Mars soil left in a nutrient solution in simulated martian atmosphere for twelve days. This experiment did detect gases, but so did the control, which repeated the experiment with a sample heated to sterilize it of any possible life. This suggests a chemical explanation.
• The labeled release experiment used nutrients tagged with ¹⁴C. It then monitored the air above the experiment for radioactive ¹⁴CO₂ gas as evidence that the nutrients had been taken up by micro-organisms. This experiment produced positive results. Also, in this case, the control experiments came out negative. Normally this would suggest a biological explanation. The microbes, as we said, just have to metabolize the organics and produce the ¹⁴CO₂ gas in the process.

The conclusion at the time, for most scientists, was that the Labeled release experiment had to have some non biological explanation involving the unusual chemistry on Mars. One early idea put forward by Albert Yen of JPL was that first carbon dioxide could react with the soil to produce superoxides in the cold dry conditions with UV radiation, which could then react with the small organics of the LR experiment to produce carbon dioxide.

At the time, the other two experiments seemed to rule out any possibility of a biological explanation.

Some of the LR data remained hard to explain as chemistry and the experimenter's Principle Investigator (PI) Gilbert Levin maintained from the beginning that his experiment probably detected life.

Here are some of the things that any theory has to explain, in addition to the non detection of organics by the other instruments:

• The LR response produced a lot of radioactive carbon dioxide rapidly, which some criticized as "too much too soon" for the levels of life expected there.
• A second injection of nutrient actually lead to a 20% decrease in the previously evolved ¹⁴CO₂ (or ¹⁴CH₄ )
• A sample maintained at 10 °C in darkness for two months at one site and three months at another had no response to the nutrient
• A sample heated to 46 °C produced 60% less gas
• A sample heated to 51 °C became erratic and produced 90% less gas

His comments on how this could be explained biologically are that (see this paper)

• The amount of ¹⁴CO₂ released is comparable to a sample from Antarctica and less than is usually released in tests on Earth.
• The second injection seems to have just wetted the sample and lead to absorption of ¹⁴CO₂ and his conclusion is that the life responsible for it died during the experiment, which is not too surprising given that most microbes even on Earth can't be cultivated in the laboratory.
• The difference in effect between 46 °C and 51 °C he considers to be strongly suggestive of life and hard to explain chemically for such a small change
• The results for the samples kept in darkness he also considers to be hard to explain without biology.

Most other scientists at the time continued to regard the experiments as inconclusive. However, work since then has suggested a possible re-evaluation of those results, and there has been much debate since then between a small number of scientists who think that the Viking missions did detect life, and the majority of scientists who think that it did not.

Perhaps the top non biological candidate at present is the reaction studied by Quinn et all in 2013. They suggested that the perchlorates were decomposed to hypochlorite (ClO^-), trapped oxygen, and chlorine dioxide. Then the release of oxygen from the trapped salts, plus the reaction of the hypochlorite with the amino acids can explain the results. There are many things to explain, particularly that two of the labeled release experiments got inactivated after storage in darkness for several months, and that the activity of the soil is significantly reduced at 50 °C. Levin and Straat in a paper published in 2016 review the proposals. For the study with formate in the Atacama desert mentioned there see this paper.

(this section duplicates some material from a section on the Viking results I wrote for the wikipedia article Modern Mars habitability).

Joseph Miller's analysis of rhythms in the Viking data

Circadian rhythms are the rhythms of life. We get hungry in the daytime and sleep at night, and suffer from jet lag, all because of these rhythms. Micro-organisms have them too.

They follow day and night cycles - but not exactly. Rather they run on their own internal clocks. They use heat, or sunlight or both as a reset, to keep them in sync with the days and nights. That's why you get jet lag, as your body gets back in phase with the shifted rhythms. Microbes are the same; they also have a form of "jet lag", a delayed response to changing external cycles.

Joseph Miller is a specialist in circadian rhythms who spotted something unusual in the Viking data which nobody else had noticed, It seemed, to his expert eye, to strongly resemble terrestrial circadian rhythms, and with a period of a Martian sol. This is what he saw:

He was able to get hold of the original raw data. This was not easy. It had all been stored in microfiche format, but format was confusing. To make it harder, the data from the labeled release was mixed up with data from the other experiments as well as engineering data. Also some of the microfiche images were not so easy to read, making it a long project to extract the data. It was also stored on CD - but again, mixed up with huge amounts of engineering data - and the documentation of the CD didn't give enough information to restore it.

Luckily, they discovered that Gilbert Levin's co-researcher Pat Straat had kept most of the LR data in the form of printouts. So finally the team were able to use these to restore most of the data. They then used the microfiche images to fill in gaps.

Allison Lopez (left), planetary curation scientist Dr. David Williams (center), and Lois Hughes (right) who helped restore the missing LR data - using printouts from Pat Straat and the microfiche images data

For the details of what he found, see his paper Periodic Analysis of the Viking Lander Labeled Release Experiment. Let's look at some of his main results here.

He analysed the data using the same mathematical tools that you use to detect Terrestrial circadian rhythms, and the results all came up positive. The experiment was isolated from the fluctuation of day night temperatures on the Mars surface, but imperfectly, so it still had temperature fluctuations.

His main alternative "null" hypothesis was that the rhythms were cased by temperature dependent solubility of evolved radioactive CO₂ in the soil. However this didn't seem to be able to explain all the effects seen.

• Temperature dependent solubility was not quite enough of an effect to explain the data.
• The LR data follows temperature fluctuations, but not exactly. In particular, you get some changes in temperature with no changes in the CO₂. If it was a purely chemical reaction, you'd expect it to follow the temperature changes more exactly
• More generally, the curve is smoother than you'd expect from the noisy temperature data. It has many regularities which the LR data doesn't respond to.
• The LR response is also delayed by 2 hours after the temperature fluctuations.

That last point, the long delay, is the one he found most telling here. He looked carefully at the design of the experiment, and he concluded that though a 20 minute delay was possible, due to variation in CO₂solubility, 2 hours seemed too much of a delay to explain by chemical reactions.

Shows temperature fluctuations in red, and the radioactivity measurements in black. The black lines are almost synchronized with the temperature fluctuations, but delayed by two hours. Also, the labeled release response (shown in black) is smoother, and doesn't follow every variation in the temperature (shown in red) exactly as you'd expect from a chemical reaction.

He also spotted another thing. There seems to be a sign of a change of rhythm when they introduced new food into the experiment with the second injection. This is something that often happens with circadian rhythms. In this diagram, look out for the change in slope in the lines drawn through the white dots

Techy details: this is an actogram. It looks complicated perhaps but the idea is simple. It's just a long timeline broken up into pieces stacked on top of each other. So, you read it from left to right and top to bottom. It's double plotted so that each line shows two Martian sols, each row starts one sol later, and the right half of each line is the same sol as the first half of the next line

The black lines show times when the labeled release values are above the two sol average. The white circles show the moments of maximum LR for each sol. The white diamond at day 7 show the moment of the second nutrient injection.

If you draw a line through the white dots, as shown, you find that it slants down towards the right to start with, delaying each day, with a period slightly over a Martian sol, 25.46 hours. After the second nutrient injection it advances two hours to a new stable position and starts sloping to the left with a period a few minutes short of the Martian sol (24.66 hours) by around 2.5 minutes per day.

The new pattern is not surprising as it follows the measured temperatures in the head assembly. This in turn tracks the advancing sunset times on Mars during the Martian winter. However the initial slope to the right is surprising for a chemical reaction.

This change of slope he says is a common feature of circadian rhythms after nutrient injection. However there are only six days of data before the nutrient injection. With so little data, it could just be a statistical anomaly. If the slope did change, he says that it's hard to think of a good explanation not involving biology.

To a trained eye, apparently this is all suggestive of a circadian rhythm, though not conclusive.

Then there's one other thing that strongly suggests biology rather than chemistry as an explanation. The "active ingredient" whatever it is in these experiments, is not only deactivated by high temperatures. It is also deactivated by keeping the soil in darkness, at a temperature of 10°C for two months.

This was an accidental experiment. They used samples collected at the beginning of the experiment and stored, out of concerns about whether the scoops could stand up to repeated scooping. It turned out that after this period of two months, in cold and darkness, the samples lost whatever it was that gave the LR activity. It's hard to think of any chemical explanation, why two months of storage in these conditions would inactivate an active chemical ingredient of the soil. How could it inactivate a chemical, if the only difference from the original surface conditions is that the temperature was kept constant and that the sample was excluded from light?

So the main things are

• Temperature response delayed 2 hours. Non biological explanation only explains a 20 minute delay.
• Temperature responses smoothed out more than you'd expect from a non biological explanation
• Change in slope of line through points of maximum LR response, though based on only six days data, would strongly suggest circadian data if it is valid
• Inactivation of the active compound by two months in darkness at 10°C - hard to think of a non biological explanation for that.

Joseph Miller, interviewed for USC news (he is a researcher at USC), said

“To paraphrase an old saying, if it looks like a microbe and acts like a microbe, then it probably is a microbe. The presence of circadian rhythmicity and a high degree of mathematical complexity or order in the LR data most likely means Viking discovered microbial life on Mars over 35 years ago.

... “This research is not a smoking gun .A smoking gun would be taking a picture under a microscope of Mars bacteria. But the case is getting stronger. We know there is subsurface water ice and perhaps liquid water in regions that seem to release methane gas into the atmosphere. Water is necessary for life, and methane is a potential signature of biology. There’s enough circumstantial evidence that strongly suggests NASA or the European Space Agency should consider explicit life detection experiments on Mars.”

In a later paper they used cluster analysis which lead to a similar conclusion (see also their paper). Once again it's no smoking gun but suggestive.

"We just plugged all the [Viking experimental and control] data in and said, Let the cluster analysis sort it. What happened was, we found two clusters: One cluster constituted the two active experiments on Viking and the other cluster was the five control experiments.

"It turned out that all the biological experiments from Earth sorted with the active experiments from Viking, and all the nonbiological data series sorted with the control experiments. It was an extremely clear-cut phenomenon."

"It just says there's a big difference between the active experiments and the controls, and that Viking's active experiments sorted with terrestrial biology and the controls sorted with nonbiological phenomena,"

Whatever is going on here, it's either something biological, or some rather complex chemistry. The complex chemistry would be interesting to disentangle too - why did it do that if it is the result of chemistry? We have hypotheses, but they don't explain it fully. Either way, it's a highly unsatisfactory situation that over forty years on, we still have no clear idea of what it is that Viking found.

Joseph Miller also was the lead author for a later paper in which he suggests that the 30% reduction of radioactive gas at the second injection could be due to methanogens in the soil. Perhaps the non reabsorbed gas was methane, which is insoluble in water at those temperatures?

Is the drop in the levels of radioactivity at the second injection due to the presence of methane gas from methanogens? Carbon dioxide would be absorbed by the nutrient, but the methane would not be.

So now let's briefly look at some of the ways that life might perhaps be able to survive in the equatorial regions of Mars which the two Viking landers explored. Is it at all possible that it detected life, based on our modern understanding of the conditions there?

Ice and water in equatorial regions on Mars

We've already looked at the warm seasonal flows or RSL's (Habitats for life on the surface of Mars - warm seasonal flows - now known as Recurring Slope Lineae or RSLs (above) ).These seasonal streaks are almost certainly caused by water. They occur in just in a few places here and there, on steep sun facing slopes. Other apparently identical places don't have them.

Anyway the interesting thing here is that though many of them occur at higher latitudes, some of these are in the equatorial regions (especially around Valles Marineres).

Warm seasonal flows on Mars. This is in Palikir crater, which is 41 degrees North. They have also been spotted in regions close to the equator.

See the section above: Habitats for life on the surface of Mars - warm seasonal flows.

From that section the three main theories are:

• Ancient ice just been deposited there millions of years ago and not yet exhausted (hard to see how there could be enough of it)
• It's replenished, indirectly, by deep down ice, possibly all the way from the hydrosphere
• Deliquescing salts just below the surface feed water into the warm seasonal flows (again a bit hard to see how there could be enough water this way)

This shows that potentially habitable liquid brines can occur in the equatorial regions. However the two Viking spacecraft didn't land near a known RSL. However, there's one way that Mars could support life almost anywhere, just using night time humidity of the air.

Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere

This is research originally sponsored by the German aerospace centre DLR. There are many lichens in tundra and cold high dry mountain conditions with the remarkable ability to survive and grow without any source of water at all. They manage this feat by taking in water from the atmosphere, which has high relative humidity when its very cold - the colder the air, the higher the relative humidity for the same water content. These lichens are also protected from UV and drought, and the ones in polar regions and at high altitudes also survive in conditions with high levels of UV. So, their natural habitat has many resemblances to the Mars surface. What happens if we put these lichens into a Mars simulation chamber?.

Researchers at DLR (German equivalent of NASA) testing lichens in Mars simulation experiments.

They showed that some Earth life (those lichens and strains of a green algae called chroococcidiopsis) can survive Mars surface conditions and photosynthesize and metabolize, slowly, in absence of any water at all. They could make use of the humidity of the Mars atmosphere.

Though the absolute humidity of the Martian atmosphere is very low, with hardly any water at all, the relative humidity at night reaches 100% because of the large day / night swings in atmospheric pressure and temperature.

Lichen P. chlorophanum on a Mars analog substrate for the DLR Mars simulation experiments.

These lichens and algae (cyanobacteria more precisely) were not adapted to live on Mars particularly, as far as we know. However their adaptations to polar conditions and high latitudes turn out to make them astonishingly resistant to Mars conditions. In Mars simulation chambers, which reproduce the thin atmosphere, the UV radiation, and the day to night variations in humidity. They are able to grow apparently completely normally even metabolizing and photosynthesizing, for weeks on end. They found that in these conditions, CO₂ concentrations at normal Earth atmospheric pressures reduce photosynthetic activity, but if you reduce the atmospheric pressure to Mars levels, then this has a compensatory effect, and the levels of photosynthesis return to the same levels they have at normal Earth atmospheric pressures and CO₂ concentrations.

Five of the species of the lichen used in recent astrobiological studies - these are able to metabolize and photosynthesize in simulated Martian environments, and can withstand the UV light through protective chemicals such as parietin. They can take up the moisture directly from the atmosphere at times of high relative humidity, so have no need for deliquescing salts.

The lichens studied not only survive, metabolize and synthesize in Martian conditions - but in protected conditions (such as partially shaded or protected by a thin layer of dust) both algal and fungal components not only continued to metabolize, but quickly adapt to their new conditions. Within 34 days, the algal component of the lichen increased its photosynthetic activity to compensate for the harsher conditions. Their experiments suggest, so far, that these lichens could survive almost anywhere on the surface of Mars provided you have sources of light, the CO₂ from the atmosphere, trace elements from rocks, moisture from the air - and some source of nitrogen would also be needed, maybe from nitrates which we now know occur on Mars.

These extremophile lichens can also survive cosmic radiation, UV, and the vacuum of space. Gilbert Levin has been saying for years that he thinks green patches on Mars might be lichens. Whether that's right or not, these results certainly vindicate the idea that lichens on Mars are a possibility, and it does make sense to search to see if we find any. Many cyanobacteria also have an amazing resistance to space and martian conditions.

These are ongoing experiments. A mission called BIOMEX, part of EXPOSE-R2 flew on the ISS on July 23rd 2014 to explore their tolerance of Mars conditions further. This flew a wide variety of organisms, including lichens. They were exposed to a simulated Mars environment, including the UV light, and a Mars simulated atmosphere, on the outside of the Russian module of the ISS. The aim was just to test to see if they could survive, not to try growing them with simulated Mars day / night temperature and humidity cycles as with the Earth experiments. Amongst the survivors were a species of the green algae Sphaerocystis collected from Svalbard, Norway, and a species of the cyanobacteria (blue green algae) Nostoc collected from Antarctica. This time the experiment was about survival rather than photosynthesis and growth. They were able to survive 15 months of simulated Mars surface conditions, including the UV radiation (so in simulated sunlight, not in shade), and despite the harsh conditions and the damage from UV and cosmic radiation, they grew new colonies on return to Earth. The algae were again chosen from species adapted to dry and cold polar regions and UV light. They were dried out slightly before flying them in space.

An effect that looks like desert varnish on Mars, photo by Spirit rover

Curiosity made some accidental measurements which might possibly give information about its properties. But generally it's not well understood either on Mars or on Earth. Of course any dedicated life mission to Mars should examine the desert varnish and see if there are any organics there, or any signs of life.

Equatorial frosts - and a source of water for the deliquescing salts

This is an observation that goes back to Viking. You get frosts even in equatorial latitudes. This shows that though there is hardly any water vapour in the Mars atmosphere (absolute humidity is low), yet at night it gets cold enough for 100% relative humidity.

Relative humidity is what makes cumulus clouds form, on Earth for instance. As the warm air rises, it cools down. There is no more water vapour in the rising air than before, but as it gets colder it can't carry so much water so it condenses out as the droplets of water in a cloud. This is also the reason you get morning dew, ground fog, and frosts. As the temperature falls at night, then again the absolute humidity doesn't change. There is no more water vapour than before, but because of the reduced capacity to carry water vapour, the relative humidity rises until, when it reaches 100%, it has to come out of the air as a cloud, dew, fog, or frost.

On Mars, when the same happens, the water vapour forms frost, or ice crystal fogs or clouds. On Earth, cirrus clouds are similarly made of ice. Conditions are too cold on Mars, and the air too thin for water vapour clouds (except just possibly at deep locations like Hellas basin). And actually on Mars the frosts only form when the air gets so cold that dry ice precipitates out of the air. The dry ice takes water ice with it.

Frosts on Mars - this photograph from Viking 2. Mildly enhanced to bring out the colour of the frost.

This shows that there is a hundred percent relative humidity of the air on the surface at night. In the day time, when it melts it probably evaporates instantly into the air. But deliquescing salts could trap this evaporating moisture, and retain it long enough to be useful.

When the frosts melt, then though they evaporate too quickly to be of use for life directly, this creates a temporary source of high humidity at soil level, which could be exploited by life somehow.

At least that's what most think. But Gilbert Levin and his son Ron Levin have come up with a way that liquid water might form briefly in the morning. Their idea is that in the very early morning, the atmosphere up to a meter above ground level remains cold, as it was at night, and stays too cold to hold the water vapour, as the surface ice melts. So, they think, there might be thin films of water that form in the early morning, just briefly before the air warms up and the water evaporates into the atmosphere. Chris McKay, commenting on this scenario, said that it is certainly possible. He agrees that it could form a very short lived layer of liquid, though it may not last for long. In these comments from 2001, he was skeptical about the idea that life could exploit it. For their idea, with Chris McKay's comments on it see Can Liquid Water Exist on Present-Day Mars? For Levin's many papers on the topic of whether Viking detected life, see this Mars page on his website. See also Rhythms from Martian sands - what if Viking detected life already in 1976?

Many scientists now think that life can exploit the humidity of the atmosphere directly, as the DLR experiments suggest (see Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere). So, might it not be able to exploit these thin layers of liquid water on the surface too, if they form, however briefly? What do you think?

If nothing else, perhaps this ground hugging layer of cold air, if that's what happens on Mars, could help to extend the period of high humidity in the early morning.

Life in the Atacama desert without rain

There are other ways that life on Earth can survive without any liquid water. In coastal areas, 80% of the quartz inclusions can have life, microbes huddling beneath the rock. These get their water from the fog, and all quartz inclusions over 20 grams were colonised.

This net is used to collect the fog in the Atacama desert for human use. The fog is called camanchaca by the locals. See trapping humidity out of the fog in Chile. Life in the Atacama desert can also use these fogs as a source of water.

Fogs on Mars (digitally enhanced) in the Valles Marineres - however these are fogs of ice rather than water. See Fog phenomena on Mars. (first page and five minute free preview available at DeepDyve).

This shows quartz inclusions in the coastal regions of the Atacama desert. The green in the photograph at bottom left shows algae underneath a rock, probably Chroococcidiopsis. Quartz inclusions on Mars could be a promising habitat for similar species. Chroococcidiopsis particularly is a prime candidate for a life form on Earth that could survive on Mars because of its multiple extremophile adaptations, and its ability to survive as primary producer in a single species ecosystem, and tolerance of ionizing radiation in quantities that would kill most microbes.

Though Mars has fogs of ice rather than water droplets, that's just because the air is so thin. It does have high humidity at night.

In the driest part of the Atacama desert, there's no fog and no rainfall for years on end. It isn't even very humid, doesn't normally reach 100% or anything near to it. Yet you still get life. It can get this through salt deposits which take in water from the atmosphere, a process known as deliquescence. This makes the salt slightly damp, and microbes can make use of this.

Hyper-arid core of the Atacama desert. Originally it was thought, conditions here are so inhospitable that it would be the "dry limit" of life. But in 2002 researchers found life here by DNA sequencing. Originally it was thought, you could only have dormant life that relied on the very rare rains here. But now it's known that life there can survive without any rain at all.

Life relying on deliquescing salt has also been found 2 or more meters below the surface of the Atacama desert. These microbes use mixtures of halite and perchlorates for deliquescence, and they use sulfates, perchlorates (ClO₄), and nitrates, together with organic acids such as acetate, or formate, as sources of energy. They are quite Mars like conditions really. See A Microbial Oasis in the Hypersaline Atacama Subsurface Discovered by a Life Detector Chip: Implications for the Search for Life on Mars

Similar habitats such as colonies inside halite crystals, or beneath quartz inclusions, could occur in equatorial regions of Mars.

We've also seen (above) that microbes could find tiny oases of water inside micropores in salt pillars even at levels of humidity far too low for deliquescing salts in normal conditions. Perhaps even at relative humidity levels down to 50% for ordinary salt (the Earth habitats involve salt rather than the Martian perchlorates which deliquesce more readily than normal salt does).

Salt formations in the hyper-arid core of the Atacama desert, where microbes were found, living inside the salt, and getting their water entirely by deliquescence. Perhaps the same process might work on Mars. See Paul Davies' Blog The key to life on Mars may well be found in Chile. Cassie Conley also talks about these as a possible habitat for life on Mars.

Desert varnish

I should mention this - but I won't spend much time on the topic here, as it is controversial, and not well understood on Earth or on Mars. It's a manganese rich shiny coating that forms on rocks in desert landscapes. What nobody knows for sure is whether it is caused by life processes or abiotically.

Petroglyphs carved in a desert rock, newspaper rock, Utah

Nobody is quite sure what causes desert varnish on the Earth. The curious thing is, it doesn't seem to be a weathering effect as the composition of the varnish is independent of the rock it forms on. So where does the manganese come from?

It might be the result of life, but might also be due to non life processes. The problem is that it's easy to find life in the varnish on Earth - but it might just be coexisting with it. Since the varnish forms slowly over thousands of years, you can't run experiments for long enough to duplicate the exact conditions that lead to its formation.

Mars also has desert varnish.

An effect that looks like desert varnish on Mars, photo by Spirit rover

Barry DiGregario, writing in 2012 suggested that these could be the result of manganese deposition from microbes. He examined desert varnish from New York state and found what seemed to be microbial structures. He thinks they were easier to observe because the conditions weren't so dry and the varnishes formed more quickly. He predicted that Curiosity could spot rock varnishes as it is landing in a perfect place to look for it:

"If Curiosity lands without incident I believe it is in the perfect landing zone to look for evidence of past or current life. If manganese-rich rock coatings are confirmed as I think they will be, then it would mean Mars likely has microbes still making this varnish today.”

As it turns out, Curiosity made some accidental measurements which might possibly be detection of rock varnish. They found that it took five shots of the ChemCam sampling laser before they got down to the bare rock. That's too many for dust, which should get blasted off with the first blast, very effectively. The ChemCam team generally discard these first five shots. In 2013, postdoc researcher Nina Lanza described in a poster session how these five shots have chemical similarities over many different rock types.

Later, she found clear evidence of a spike in manganese over the first three shots of ChemCam, suggesting a surface layer (beneath the dust) similar to the desert varnishes on Earth.

Quoting from their shorter conference abstract: (Mn is manganese)

"Because of the close association between Mn minerals and microbial activity on Earth, Mn-oxides have been suggested as a potential biosignatures for Mars missions, although Mn-rich coatings may also form abiotically"

... "Environments conducive to Mn oxidation and deposition such as hydrothermal systems or a freshwater lake may have existed in Gale crater shortly after its formation; however, the more recent day environment could also support Mn depositing processes, most notably the alteration of airfall dust on rock surfaces by thin films of water and UV photolysis. While the nature of the high, trending Mn detections from ChemCam is not yet clear, the detection of Mn in excess of a typical martian basalt provides intriguing evidence for water-rock interactions in a habitable environment. "

For more details see the discussion section of this longer paper.

So, she was not able to determine the source of the manganese n the varnishes. This is not proof of life on Mars. Even on Earth the nature of the rock varnishes is controversial. However, manganese is a sign of rock water reactions in a habitable environment. See also this report in Earth magazine.

Barry DiGregorio interviewed by Earth magazine said

"“The Martian and terrestrial comparisons in the Lanza et al. abstract look remarkably consistent, Mars has all the right ingredients for varnish formation today: clay minerals, suitable pH” and evidence of “brief wetting periods from melting snow and ice.” Still, he says, it is “difficult to know” exactly what they’ve found at this point."

Generally desert varnish is not well understood either on Mars or on Earth. Could we understand it better by studying it on Mars? Surely, any dedicated life mission to Mars should examine the desert varnish and see if there are any organics there, or any signs of life.

Transgressing sand dunes bioreactor

There's another idea for a way to have Mars life in equatorial regions. For the original paper here, see Habitability of transgressing Mars dunes. This is a paper written in 2013 just after the Curiosity landing. It looks at conditions in Gale Crater, and found that there's a possibility of present day life there, if it can exploit temperatures of -71°C to -77°C, well below the usually quoted minimum temperature of -20°C for reproduction for Earth life in liquid brines (though this is a bit fuzzy, see Temperature limits for Earth life).

The paper suggests that as the frosts melt, any salts in the soil will force the humidity downwards. If so, liquid water could be present at least briefly in Gale Crater (they use Gale Crater as an example because the paper focuses on possibilities for Curiosity).

They suggest magnesium perchlorate and calcium perchlorate would be good for capturing the water. They suggest a redox gradient for life as well, with ferrous iron (Fe²⁺) as the "electron donor" combined with either ferric iron (Fe³⁺), or perchlorates as the oxidant as the energy source for the life. They suggest the surface would be oxidized but the surface would be reducing, in a static dune. As the dunes move in the winds, the surface areas would be constantly put into an oxidized state by the UV light and the oxygen in the atmosphere, and these layers would make contact with lower reducing layers and microbes could use this chemical gradient to make organic matter with carbon dioxide as a carbon source. The other major nutrients and trace nutrients are all readily available in the rocks, apart from nitrogen. They suggest that the reduced nitrogen for life to use could be made from nitrogen from the atmosphere, catalyzed by iron oxides in the presence of UV light.

So, in short, the idea is that the moving sand dunes would churn up the soil and mix the reducing lower parts of the soil with the oxidizing surface layers and bring up nutrients from below the surface. Life then could exploit those chemical gradients and the nutrients.

(click to watch on YouTube)

This striking image from Mars Reconnaissance Orbiter shows quite how much sand movement you can get on Mars. In places, sand dunes as high as 200 feet (61 meters) are moving over the surface of Mars - a surprising result with its thin atmosphere (but strong winds and months long dust storms sometimes).

Sand dune movement could churn up the soil and bring deliquescing salts to the surface, and also mix up reduced and oxidised chemicals, making conditions easier for life. The salts below the surface would help capture liquid brines. These would be low temperature, well below the -20 °C usually given as a limit for life to grow, but it might be possible for Mars life to exploit it.

And as it turned out, Curiosity did find liquid brines just beneath the surface of the sand dunes.

Liquid brines beneath the surface of sand dunes at night - beneath the sand that Curiosity drives over

This was a serendipitous discovery announced in April 2015. Liquid brines that form through deliquescing salts (perchlorates) - the salts take in water from the atmosphere (same principle as the salts you use to keep equipment dry).

They found it indirectly. They noticed that when Curiosity drives over sand dunes, then the air above them is drier than it is normally. When it leaves the sandy areas the humidity increases. This shows that something in the sand dunes is taking up water vapour from the air, and rather a lot of it too. They calculated that the perchlorates in the sand must take up so much water at night that the liquid brines would be habitable, except that they are too cold for Earth life.

As the day progresses the brines warm up but any brines close to the surface would dry out, and become too salty for Earth life. That's for any water in the top five centimeters or so. They suggest that it could have permanently hydrated brines about 15 centimeters below the surface, and at that depth, the liquid would never get warm enough for metabolic activity for Earth microbes, never mind replication. Top temperatures in summer at that depth would be around -40 °C (I'm reading this off their figure 2a, grey shows the temperature range 15 cm below the surface).

The authors of the paper concluded that the conditions in the Curiosity region were probably beyond the habitability range for replication and metabolism of known terrestrial micro-organisms. For a summary see  "Evidence of liquid water found on Mars (BBC)" and for the article in Nature "Transient liquid water and water activity at Gale crater on Mars" (abstract, the paper is behind a paywall, but you can read it via the link in the BBC article through Springer Nature Sharedit).

However Nilton Renno, who is an expert on Mars surface conditions suggests that Earth microbes may still be able to exploit this liquid brine layer through biofilms::

"Life as we know it needs liquid water to survive. While the new study interprets Curiosity's results to show that microorganisms from Earth would not be able to survive and replicate in the subsurface of Mars, Rennó sees the findings as inconclusive. He points to biofilms—colonies of tiny organisms that can make their own microenvironment."

Mars liquid water: Curiosity confirms favorable conditions.

Nilton Renno was one of the first to suggest that liquid brines could occur on the Mars surface after he noticed what look like droplets forming on the legs of Phoenix.

Also, of course, these liquid brines could of course be well within the habitability range for Martian life if it can tolerate colder conditions than Earth life.

So we now have several ways that life could be possible in the equatorial regions

• Somehow using the frosts that form at night for 100 days of the year as they evaporate in the morning
• The warm seasonal flows (or RSL's) since some of them occur in equatorial regions
• Advancing sand dunes bioreactor
• The liquid brine layers found by Curiosity (related idea)
• Micropores in salt pillars
• Directly using the night time humidity

The dust could also have spores in it from nearby habitats even if there were no habitats at the exact location of the Viking landers.

How the future might unfold if Viking did find life in the 1970s

So, what if it turns out that Viking did find life way back in the late 1970s? Yes many would say that it seems a remote possibility. The skeptics think that they have shown that what Viking found didn't have to be life, by finding alternative explanations. But even if that is right, which is disputed, they didn't prove that it wasn't life and the equatorial regions may be more habitable than we realized back then.

What if Gilbert Levin, Joseph Miller and the others turn out to have the correct interpretation of the results after all?

If that is correct then our near future prospect for finding Life on Mars is rather good. The two Viking landers didn't land anywhere particularly special on Mars as we now understand it.

They did choose the sites for scientific reasons, based on their best science of their day. For instance, they chose the site for Viking 1 because they thought that there was a chance of finding fossil water in the Chryse Planitium washed down from what seemed to be a number of "stream" channels. For details see the end of chapter 9 in the NASA book "On Mars: Exploration of the Red Planet" by Noel Hinners. And yes, this area may well have had a lake or ocean in the early history of Mars, but Viking 1 landed nowhere near any of the outflow channels, which we now know also, formed billions of years ago. It landed 23 °N, 48 °W. Viking 2 landed in Utopia Planitia, rather further from the equator, at 48°N latitude, 226° W, an area that we now know has a lot of underground ice deep below the surface (discovered from orbit), and interesting scalloped and polygonal surface features associated with ice. Their top priority was safety though.

The mission planners were avoiding rocks, and preferred to land on sites at least partly covered in sand or dunes (see chapter 10 for their deliberations). They could see signs of the larger dunes from orbit, and the team thought that it was safer to land in a place with many of the more rocky features partly covered by dunes Viking 1 landed in a rocky region partly covered by dunes. Viking 2 landed in a rocky desert region, with rocks scattered across the sand.

So, if both Vikings did find life, well it probably means that life is present in low concentrations over much of the equatorial regions, and indeed to some distance away, ar at least, the areas covered in sand dunes or rocky desert terrain. It could be present as spores, or it could be that it somehow takes advantage of the evaporating dry ice / water ice frosts, or uses the 100% night time humidity, forms in salt pillars, or is sustained by the advancing sand dune bioreactors.

In this scenario, it's so easy to find life on Mars that ExoMars will probably detect biosignatures of life quickly, perhaps right away. Its instruments are sensitive enough to detect life even in the Atacama desert (where levels of organics are too low for Curiosity and Viking to detect anything). In this scenario, ExoMars finds biosignatures in trace quantities almost everywhere it looks, in the Martian sand dunes.

If ExoMars doesn't find it, perhaps not quite sensitive enough, then perhaps when the more sensitive SOLID3 or astrobionibbler gets sent to Mars, it might detect traces of present day life almost everywhere on the surface.

In this scenario, Mars 2020 will have life in the samples it collects too, and perhaps this gets returned for analysis in the late 2020s or early 2030s. If so, then we may confirm that there is life on Mars rather soon.

Of course this doesn't go the other way. If the sample return in the 2030s doesn't contain life, it doesn't even conclusively prove that Joseph Miller and Gilbert Levin's interpretation of the Viking data is incorrect. Perhaps Curiosity's successor is unlucky (after all its main focus is on rock rather than dirt) or both Viking landers were very lucky. The plan is to return just a few hundred grams of samples in total, with most of that rock rather than dust from the surface, so it would be easy to miss a very sparse signal in the dust.

A null result for life detection for Mars 2020 certainly doesn't mean that there is no life on Mars, or even, no life in the equatorial regions, or even, in the immediate vicinity of Mars 2020! Mars 2020 could trundle over sand dunes with microbial spores in the dust almost everywhere, yet it could easily collect no life in its less than a half kilogram of samples. That's a half of the weight of a typical bag of sugar, with most of that rock cores.

So, unless you have a lot more context to interpret the result, all you can deduce from a sample return with no life in it is that there are some rocks and patches of dust on Mars which don't have life in them. That would hardly be a huge surprise even if Viking did find life. It's easy to find samples with no life in them in the dry desert core of the Atacama desert and in some places in Antarctica.

A Mars sample return would return rocks interesting for geology and would be useful as a technology demo - but would it help the search for life?

The decadal survey summing up motivates the Mars sample return using examples of previous returns of comet and interplanetary dust, and moon samples.

Tracks of particles from comets collected in the stardust aerogel, first sample return of a comet to Earth

These undoubtedly were hugely valuable in advancing our understanding. They also showed the value of returning even tiny samples for geology. But those are all astrogeological missions, and their value was geological.

Geological specimens don't deteriorate in the same way as organics. There is no problem of racemization, or of lifeforms eating them. For most rock types, there is usually no problem of them being washed out by flooding. They are also easier to find. They are easier to preserve for transport back to Earth. They are also relatively easy to identify. The sample return from Mars may well be of great value for geology. Nobody controverts that as far as I know.

What though about its value for the search for life?After all that is given as the main motivation for doing this sample return.

If the motivation is astrobiological, listen to the astrobiologists

As we saw in Idea of returning samples from Mars to Earth and Astrobiologists arguing strongly for an in situ search on Mars first (above), many astrobiologists argue that a Mars sample return is not of great interest to their subject, likely to return a small half kilogram yield of samples that are likely to be of no more interest to astrobiology than the Martian meteorites we already have, at great expense, and not likely to address the central questions in their field.

Geologists would love to get hold of a sample from Mars. However, it's hard to motivate a multi-billion dollar mission to return such a small half kilogram or so of samples from Mars on the basis of geology only. That's mainly because of

• At the price tag of many billions, it costs so much more than a sample return from a comet or asteroid, (by comparison the Osiris Rex mission to return a sample from an asteroid will cost $800 million excluding the cost of the launch vehicle)
• We are making great strides in understanding the geology of Mars with our rovers, landers and orbiters on Mars.
• We have had many missions that do in situ geological studies
• We also already have a wide variety of meteorites which give us geological samples from Mars to study already.

We can't do accelerator mass spectrometry on the samples, so the resolution for isotope mass spectrometry on Mars is not as good as we'd like it to be. However we can do radiometric dating of some rocks looking at ratios of isotopes, as with this in situ dating of a mudstone to 4.21 billion years ago, done by Curiosity using potassium / argon ratios. We can also do relative chronology via meteorite crater counts, as well as make a pretty good stab at absolute chronology.

Yes, it's a huge plus, that we can send parts of the sample to labs throughout the world, for different teams to look at. We also not limited to any particular suite of robotic instruments but can bring in new instruments to answer questions we never had when the sample was acquired. Today we continue to use instruments only recently invented to answer questions we didn't know to ask before about the lunar sample returns.

So, it's understandable that the geologists are dead keen to return a sample from Mars. We could learn a lot from it, undoubtedly. However, is it worth that huge price tag?

If the motivation is geological, you listen to the planetary geologists, and ask them how to design the mission and what to do. But if the motivation is astrobiological - surely you need to listen to the astrobiologists? Methods that work for astrogeology may not work so well for astrobiology.

Could Mars 2020 pivot to support astrobiology?

The scientists in support of Mars 2020 are having a lot of difficulty convincing NASA and then the US government to provide finance for its vital follow up missions. Casey Dreier has highlighted some of these issues in a recent Op-ed in Planetary magazine: "The Fading Fortunes of Mars" (accessible to society members only). He summarizes some of the issues here, and in a detailed report here which he prepared with Jason Callahan.

Mars 2020 may have an extended mission, as for Curiosity, which may take several years, and it uses adaptive caching, leaving the samples in caches along its route. Though the follow up mission won't have to stop and do the analysis and can take short cuts, avoiding science detours, still, it's going to take a year or two probably just to drive the distance to pick up the samples. It also has to drive back to the MAV, unless you have two follow up missions, in which case it has to rendezvous with the MAV.

Also, if it has a landing ellipse similar to Curiosity, 20 kilometers by 7 kilometers then it may be up to many kilometers away from the first sample to collect, which at 100 meters a day means 10 days to travel a kilometer. With Mars 2020 they hope to improve this to a diameter of 10 km. by using "range trigger" - it decides when to release the parachute in order to reach closer to the target instead of releasing it at the earliest opportunity as in all missions to date. If they can target the cache at the center of the landing ellipse, they could get it down to 100 days there and back. Still, you are talking about a prime mission that would span more than 100 days on the surface plus probably several hundred days to follow the tracks of Mars 2020 to pick up the sample.

The samples can be cached for 20 years and possibly longer, no known failure mode,

They have a provisional launch date of MAV and rover to Mars in 2026. For a list of launch opportunities for Mars, see Table 1 page 7 in this paper).

ncidentally, some have suggested we could depend on the Red Dragon for sample return but Elon Musk recently announced he is canceling the Red Dragon.

It is harad to get the 5 kg sample container to orbit within the mass constraint of the skycrane. But a study in 2017 found that they could do it if they get oxygen from the atmosphere by electrolysis of the carbon dioxide in the atmosphere to carbon monoxide and oxygen. The carbon monoxide would just be discarded as there aren't any mature rocket designs using it as fuel, and they would use solid rocket fuel brought from Earth. The alternative was to take hydrogen and make a methane / oxygen fuel by the reverse Sabatier reaction. They discuss various possibilities for the fuel and settle on HTPB with decaborane as an additive to improve performance, and running with more than the optimal amount of oxygen because the oxgen can be sourced locally.

There's also the cost. The sample retrieval program has to include both a rover able to travel long distances over the surface of Mars, and the MAV. The MAV particularly is radical new technology which means it is likely to be high budget and subject to delays. Then there's the Mars 2022 orbiter NeMO (Next Mars Orbiter) - a very important Mars orbiter to boost communications to and from Mars with "broadband" laser communication with Earth, as well as high resolution imagery of the surface at 30 cm resolution similar to the now aging HiRISE. With the reduced budget for the Mars exploration program, these two missions together are likely to amount to the entire NASA Mars program for the next decade. It's been shelved but a new Mars orbiter will be included with the sample retrieval mission in the 2020s.

Of course other countries are likely to step in and so it's not such a dry patch for Mars as it would be otherwise.

We then need a separate mission to retrieve the sample from low Mars orbit and return it to Earth, which will need new technologies to capture the sample capsule from Low Mars Orbit, and to keep the capsule contained and monitor it for micrometeorite breaches during the journey back to Earth. The samples are good for 10 years in Mars orbit, so that means this mission has to retrieve it some time around 2040 at the latest. So that makes it another decade. Then you have the Mars sample receiving facility which is currently around half a billion cost, but that may increase. So that seems to account for most or all of the Mars budget for the 2030s.

So how can we sell all that to Congress? The Planetary Society and others are doing their best to sell it. But what if we pivot?

I think part of the problem here is that the astrobiologists are not behind this. Even Chris McKay who is as close to an astrogeologist as can be amongst the astrobiological community is not calling vocally for support of a sample return at this stage. The absence of their voice in support makes it much more of a hard sell. As far as I know, no prominent astrobiologist is saying about the Mars sample return that "This is what we have to do right now".

If they were united in their support, you'd have astrobiological enthusiasts telling Congress that this is the way to go. Usually the experts and enthusiasts for a discipline can sell it much better, more vigorously and with conviction, than those who are not involved in the research directly and who are mainly motivated by its geological value. And the main motivation is supposed to be astrobiological after all.

So can we pivot it to astrobiology? Well one way to do it, but rather radical, is to modify Mars 2020 to match what astrobiologists want right now (as we saw above in Astrobiologists advocating strongly for an in situ search on Mars first).

Let's make it another in situ mission. Postpone the launch if necessary to give the opportunity to upgrade it with new flight certified instruments, and replace the entire sample caching system with in situ life detection instruments such as the astrobionibbler, Solid2, automated gene sequencer SETG, chiral update to the labeled release. Have a call for proposals from the astrobiological community, and drop any requirement that the life detection instruments all have to be simultaneously capable of geological studies.

More radically, add a robotic mole with the ability to drill, in place maybe of the Mars helicopter (cool though that is) or Moxie. The astrobiologists are pretty much unanimous in saying that the ability to drill at least a couple of meters is vital for the search for past life on Mars at least. Moxie doesn't need a rover - is there any way it could be added to the Insight lander instead?

For the sample return, then do this separately, perhaps in the 2030s if it can't be done in the 2020s. The simplest approach is to return a sample from Phobos first. NASA could do their own Phobos sample return or combine forces with France and Japan or with ESA or Russia with their mission plans. Phobos samples can be returned unsterilized according to current planetary protection guidelines. Its regolith may have 0.025% Mars materials from throughout the past history of Mars. It would also be an opportunity to study Phobos up close, a likely target for human missions in the future. See Search for past life from Mars - on Phobos and on the Moon (above) for more on this.

Or, return it from Mars but in a much simpler Discovery class mission, such as Chris McKay's

"You land, grab some dirt and launch it back to Earth. The ground time on Mars could be one day.".

The dust and dirt includes miniature "rocks" from throughout Mars, much like a comet sample return. The main thing is the lack of provenance for those rocks. To make a start on that, they could have a Sojourner clone too, to drill some samples from nearby rocks - such a small rover wouldn't add that much to the payload, and would of course be far more capable than Sojourner.

The tiny Sojourner rover investigating a rock. Perhaps the "grab a sample of dirt" mission could be upgraded with a tiny rover like this to extract samples from nearby rocks as for the pathfinder "Rock garden"?

Image caption: "A panoramic view of Pathfinder's Ares Vallis landing site reveals traces of a warmer, wetter past, showing a floodplain covered with a variety of rock types, boulders, rounded and semi-rounded cobbles and pebbles. These rocks and pebbles are thought to have been swept down and deposited by floods which occurred early in Mars' evolution in the Ares and Tiu regions near the Pathfinder landing site."

"The image, which is a 75-frame, color-enhanced mosaic taken by the Imager for Mars Pathfinder, looks to the southwest toward the Rock Garden, a cluster of large, angular rocks tilted in a downstream direction from the floods. The Pathfinder rover, Sojourner, is shown snuggled against a rock nicknamed Moe. The south peak of two hills, known as Twin Peaks, can be seen on the horizon, about 1 kilometer (6/10ths of a mile) from the lander. The rocky surface is comprised of materials washed down from the highlands and deposited in this ancient outflow channel by a catastrophic flood."

Or simpler, just return dust from a dust storm as in the SCIM proposal in an atmosphere skimming flyby and return mission.

If we can do simpler sample return missions like that, we can decouple them from the idea that a sample return has to be a huge multibillion dollar mission. That means we then have the opportunity to do more and more increasingly capable missions.

So, we don't treat it as a

"this is our one golden opportunity for samples from Mars and let's, bet the ranch on it, and spend billions of dollars on it, make it perfect and get it right"

but -

"This is the first of what should be many follow up discovery class or lower Mars sample returns - let's show we can do it and also return something interesting, and let's show that a sample return can be done on a budget ".

For more on both these ideas, see Chris McKay's view - just grab a sample of dirt as a technology demo, and return it - one day on the surface, no rover - or SCIM - sample a dust storm in a flyby

There are many interesting targets that Mars 2020 can visit. The top candidate at present is this ancient delta in Jezero crater.

Jezero Crater, first choice for the Curiosity2020 mission, in a purely advisory ballot by scientists, is a delta system that seems ideal for finding traces of past life.

However Mars 2020 will only be able to drill cms, like Curiosity. Astrobiologists say that it may be hard to find clear traces on the surface as organics degrade away completely through cosmic radiation over billion year timescales. Their advice is that to get reasonably intact samples of billions of years old organics, sufficiently preserved for a decent astrobiological analysis, it's best if we can drill to 2 meters depth, and ideally more than that. Failing that, we are restricted to any materials that may have been excavated in recent meteorite impacts, or through scarps that are eroded by the winds on Mars.

Detail of Jezero Crater - from HiRISE image here - i
Image caption: "Jezero Crater is an ancient crater where clay minerals have been detected, and with a delta deposit indicating that water was once flowing into a lake. Since clays form the in presence of water, this crater would be a very good candidate for a lander to explore and build on what we've learned from the Mars Science Laboratory. Could some form of ancient life have existed here and for how long?"

The astrobiologists argue strongly that our search for past life needs to be done in situ, with the ability to drill at least 2 meters below the surface and that we need to do in situ sensitive biosignature detection. NASA plan to return a sample of 450 grams from this region in a hugely complex and expensive series of missions involving a second flagship mission just to pick up the samples. It's also going to be hard to get the second mission to Mars before the 10 year expiry date of the samples. Then you have the cost of returning it to Earth on top of that. All of this is of great interest to geologists, but not so interesting to astrobiologists, who think that at best, it may return samples as ambiguous for the search of life as the martian meteorites we already have.

Is it too late to just pivot Mars 2020 to an in situ mission, and do our first sample return as a much simpler lower cost mission such as SKIM to return a sample of a dust storm from the upper atmosphere, or Chris McKay's "Grab a sample of dust and return?"

With a mission like this, the astrobiologists would all be behind it, and sell it enthusiastically. Especially, if they get to send the first ever life detection instruments to Mars since Viking, this would be a great public outreach opportunity too and it would be easy to explain its vital significance to astrobiology. There would be genuine excitement amongst the astrobiological community and a renewal of interest in the entire field, also inspiring young astrobiologists to work on the instruments and giving astrobiologists the first ever chance to be Principal Investigators on astrobiological missions that actually fly and are not descoped, since the 1970s.

I don't know if there is any chance of this. But it does no harm to suggest it.

Apart from that idea, perhaps they can downplay it a bit, not so much "betting the ranch" on the most perfect sample return ever? We don't ask it to pick up all the caches, so saving maybe an extra year or so of travel time (bearing in mind that it then has to return to the MAV at the end). This also reduces the launch mass back to Earth.

The main problem there is that it needs the witness samples and blanks in order to help assess the amount of contamination. So our sample retrieval system has to pick those up. But perhaps the half dozen best of the remaining samples?

Then for the return from Mars orbit, sterilize the sample so you don't have the legal complexities or the costs of a Mars receiving facility and makes the whole thing simpler. That is, unless we already have very clear evidence of life within it. If there is clear evidence, then I think the extra funding for handling that should be easy to obtain, and return to above GEO and split it there and sterilize materials to return to Earth, as in my above GEO suggestion.

This wouldn't have the astrobiologists behind it so enthusiastically. But it would cost less because you return less mass, which reduces the cost and complexity of the MAV. Also by sterilizing it, then you remove the complexities and costs of a Mars Receiving facility, while still preserving any astrobiological value it may have for past life, leave some possibility for detecting sterilized extant life, and preserve its geological interest,

All this could easily trim a billion dollars or more (you save well over half a billion dollars for the receiving facility). Also it means you don't have to go through the complex legal process. Though that may not be topmost on the minds of the astrogeologists - once it becomes clear that Congress will have to pass probably decades worth of laws to permit us to return this, it is likely to be an additional disincentive some time down the line.

I.e. simplify it as far as is possible at this late stage. Then sell it as a geological mission and a technology demo. If they sell it as an astrobiology demo, it's going to be a big anti-climax if it returns samples that are as ambiguous for astrobiology as the Mars samples we already have. So it's a case of full disclosure right away, that astrobiologists see it as a technology demo rather than a way to solve the main questions in their field. Better to make that clear now, than in the mid 2030s. Of course, if before then, we find very clear signs of extant or past life on Mars, it can then pivot to an astrobiology sample return, as much as possible.

Why did the decadal survey choose a sample return?

To return to our earlier question, why did the decadal survey choose a sample return mission over in situ exploration? Why didn't they listen to the astrobiologists? I don't know the answer to that, since they don't discuss the Bada paper at all as far as I can see (do correct me if you know of anything they say on this subject, anyone, or you have any more information on the background to this). But I can offer a few thoughts that might be relevant.

First, there may be an element of what Chris McKay said in this interview - that the committees that make these decisions are made up of geologists, so their conclusions tend to reflect the views of geologists. When they choose scientists for future committees to make these decisions about how to plan future astrobiology missions for Mars, then again, generally they will choose more geologists to make the decisions:

"The geologists win hands down as they are entrenched in the Mars program. The favorite trick is to form a committee to decide what to do. The people that are put on the committee, of course, are people who are funded to study rocks. So the committee recommends that we study rocks. They’ll say these rocks will give us the context of how to search for life on Mars. Then you say, well, that’s not right. But NASA Headquarters will say they asked the science community and they told us that this is what we ought to do. It’s kind of circular. The reason the committee told you that — it’s because you put a committee together of people who study rocks. It’s almost a Catch-22.

If this is what is going on, then it might not be surprising that their ideas for a sample return reflect the priorities of geologists rather than astrobiologists. I covered this already in Chris McKay's view - just grab a sample of dirt as a technology demo, and return it - one day on the surface, no rover (above). Perhaps that is all there is to it.

Another thought, though, is that NASA, of all the space agencies, is the one most focused on the aim of an eventual human landing on Mars. So could it be connected with that?

"Safe on Mars" - could a sample return tell us if Mars is safe for astronauts?

This is just a guess, but I wondered if it is possible that they were motivated at least partly by the Safe on Mars report in 2002, especially since it is cited by the decadal survey and featured prominently in their citations list. As I mentioned before, it's the only Mars related cite of a short bibliography of seventeen citations in their earlier section "National and International Programs in Planetary Science" (see page 83 of this report).

In the section Connections with human exploration of the Decadal review summary they say, citing "Safe on Mars":

"Mars is the only planet in the solar system that is realistically accessible to human exploration; ... The elements of the Mars Sample Return campaign, beginning with the Mars Science Laboratory, will provide crucial data for landing significant mass, executing surface ascent and return to Earth, and identifying potential hazards and resources."

(emphasis mine) - see also Decadal summing up doesn't discuss this issue (above)

This 2002 report recommended a sample return from Mars to check to see if there are any biological hazards for humans on the surface. This is a 2010 animation showing how a sample return from Mars might have been done with 2010 era technology (they show it returned to the space shuttle).

(click to watch on YouTube)

If so, it's interesting to note that Safe on Mars recommended a sample return only because at the time they wrote the report, there were no instruments sensitive enough to do a good search in situ, in their view. They say:

"As stated above, there are currently no measurement techniques or capabilities available for such in situ testing. If such capabilities were to become available, one advantage is that the experiment would not be limited by the small amount of material that a Mars sample return mission would provide. What is more, with the use of rovers, an in situ experiment could be conducted over a wide range of locations."
(Page 41 of Safe on Mars) (emphasis mine)

So, actually, when you read it in detail, it's a similar recommendation to the one by the astrobiologists. With in situ searches you are not limited to such a small sample, and can explore a wide range of locations.

Well, now, these capabilities are available. "Safe on Mars" was written just two years after completion of the first draft of the human genome, the three billion dollar human genome project that finished in in 2000. At the time it would have seemed almost inconceivable that we could send a gene sequencer to Mars, as this is how they did the sequencing, with many work stations simultaneously and a lot of human interaction:

Some of the equipment used to sequence the human genome for the first time as part of a three billion dollar project, first incomplete draft released in 2000.

This is what a DNA sequencer looked like around the time of the publication of “Safe on Mars”. A whole room full of complex equipment.

Now Oxford Nanopore markets the MinION, a gene sequencer on a USB stick, which can do the same thing!

Image from Smithonian magazine announcement of the minION before it was in production. Now it is widely used.

This is what a DNA sequencer looks like today.

The advance in technology in the last 16 years in this field has been just incredible. This commercial miniaturized gene sequencer, combined with a commercial miniature nucleic acid extraction device is the basis for SETG - a gene sequencer to send to Mars. They have all the components in place, but with some steps still done manually. It combines together two commercial devices - the minION just mentioned, for DNA sequencing, along with SimplePrep X1 for DNA extraction.

The miniaturized commercial gene sequencer minION A, with the DNA extraction device SimplePrep X1 form the heart of SETG which astrobiologists hope can be sent to Mars to do DNA sequencing on the planet. See figure 3 of this paper. It's currently at technological readiness level 4 (some steps still have to be done manually).

The final instrument will have the whole process automated, end to end, in a miniature device you can hold in your hand, from sample acquisition all the way through to DNA sequencing. It's at technological readiness level 4 (some steps still have to be done manually), moving towards levels 5 and 6 (final stage before testing in space conditions). Techy details here. Such a device would be inconceivable with the technology of the year 2000.

It's the same with many other instruments that were huge laboratory filling machines back in 2002. They have already been miniaturized and a fair number also tested in space simulation conditions, and could easily be sent to Mars. The instruments we could send to Mars now include electron microscopes, and ultra sensitive biosignature detectors able to detect a single amino acid in a sample. Our instruments also include the exquisitely sensitive electrophoresis "lab on a chip" methods mentioned by Bada et al. Another new idea is the Solid3 approach of using polyclonal antibodies - which can detect, not just the organics you find in animal bodies, but a wide range of organics, again with exquisite sensitivity, in a "lab on a chip". See In situ instrument capabilities (below) .

A sample return can only tell us that there are some patches on some rocks on Mars which are safe for humans

The geology of Mars is much more varied than realized in 2002 when "Safe on Mars" was written, and conditions for habitability even more so. As we saw, there are many ideas now for potential habitats for life even in equatorial regions such as the advancing sand dunes bioreactor and the warm seasonal flows (see above). If the situation is similar to the Atacama desert and McMurdo dry valleys, their habitability could depend on small local variations in the concentrations of various salts in the soil. Also there are ideas for ways that life could survive (perhaps just below the surface) using the night time humidity with no water at all.

It doesn't seem likely that a few samples returned from the surface even of a large plain of sand dunes, for instance, would be able to confirm or deny the advancing sand dunes bioreactor hypothesis. There might be only a few sand dunes with the right mixtures of salts to give conditions for life in the entire plain. Or life might have colonized some rocks and not others, just through chance. This often happens in our driest coldest deserts. If you can't detect the organics at the very low concentrations typical of life in these habitats, and you can't detect biosignatures, how likely is it that you choose to sample a one centimeter diameter patch of rock that happens to have life in it? Even with thirty such tubes, with the main focus on the geology of the rocks, how likely is it that any of them have life in them? It could happen but it's not their priority obviously. Some may have organics, as they are searching for that, but most likely meteorite organics.

So, we can't hope to deduce that much from a small sample return about present day life on Mars. At least - not without a lot more context and understanding than we are likely to have by then. If it has no life in it, all you can say from a selection of samples like this is that there are some rocks and sand dunes on Mars that have organics, but don't have life in them. That's no great surprise.

Also, suppose we do detect life in the sample - again - it doesn't tell us that much about the range of possible lifeforms on Mars. Surely it's rather unlikely that if it has life that it has only the one species or even only one phyla throughout the planet? Once you accept that it could have multiple species, how likely is it that they will all be present in the first samples returned from Mars? If we find a cyanobacteria in one of the samples, for instance - does that mean that the only life on Mars is cyanobacteria? It doesn't seem too likely that we could conclude that much from just a few samples returned from Mars.

Mars astronaut at night, by Dmytro Ivashchenko. With the complex understanding of Mars we have now, then all that a sample returned from Mars could prove is that there are some patches on some rocks on Mars that don't contain life and some dust on Mars that doesn't have viable life in it.

If somewhere in your half kilogram of samples you find a lifeform from Mars, all it does is give you evidence that a particular species occurs on Mars. It doesn't tell you much about any other species that might also occur on the planet.

If you read Safe on Mars carefully, it recommends in situ studies as the way to find out if Mars is safe for humans. It only recommended a sample return because at the time the authors thought that it was too difficult to study in situ. If the report was redone today, surely it would recommend in situ study.

In conclusion, given what we now know about the variety of conditions on Mars and the varied possibilities for habitats there - it doesn't seem likely that a sample return from Mars at this stage would settle anything at all about the safety of the surface conditions for astronauts.

Sample return needs to take account of the possibility of life in the sample even if it is a low probability

The European Space Foundation (ESF) study, which carried out the most recent study of a Mars Sample Return, says that it is generally agreed that the probability that any martian micro-organism is biohazardous is low. However they also said that, as no life forms outside of Earth have yet been studied or characterized, it is impossible to do a standard probability assessment. This is covered in their section 4.2 Approaching the unknown and considering consequences in the ESF report, under unknown unknowns:

"This lack of knowledge, or uncertainty, prevents definitive conclusions from being reached on major factors that would allow for a real assessment of the risk of contamination posed by an MSR mission, including:

• Whether life exists on Mars or not

• If there are living organisms on Mars, it is not possible to define the probability of a sample (with a given size and mass) actually containing organisms

• If there are living organisms in the sample, it is not possible to definitively assess if (and how) a Mars organism can interact with the Earth’s biosphere."

So, we start from a position where we don't know if there is life on Mars. If there is, we have no way to know what the probability is of finding life in the sample. And if there is life in the sample we don't have any way to assess what the effect would be on Earth's biosphere. With so many unknowns, how can you make a start at carrying out a risk assessment? There's no way at present to assign probabilities to any of those.

They concluded that risk assessment has to be carried out by combining knowledge of Earth life with knowledge of Martian geology. They did come to the conclusion that it is possible to establish the risk as low, as a consensus of the beliefs of the experts in the field as represented by their experience.

"On the latter point, there is consensus among the scientific community (and among the ESF-ESSC Study Group, as presented above) that the release of a Mars organism into the Earth’s biosphere is unlikely to have a significant ecological impact or other significant effects. However, it is important to note that with such a level of uncertainty, it is not possible to estimate a probability that the sample could be harmful or harmless in the classical frequency definition of probability (i.e. as the limit of a frequency of a collection of experiments). However it is possible to establish the risk as low, as a consensus of the beliefs of the experts in the field as represented by their experience."

Risk assessment survey of microbiologists

The ESF study didn't actually quote a survey of astrobiologists, so it's a little hard to know what they meant by "it is possible to establish the risk as low, as a consensus of the beliefs of the experts in the field as represented by their experience." It seems likely to be an informal study as they didn't give any statistics or cite any research papers on the topic.

I thought I'd try to find out more about this. I don't know of a recent attempt to assess this, but I did find one study from back in 1998, the ecologist Margaret Race of the SETI Institute with Donald MacGregor of Decision Research^] carried out a survey of microbiologists attending a special five-session colloquium titled “Prospecting for Extraterrestrial Microorganisms and the Origin of Life: An Exercise in Astrobiology”.

• Asked if they thought there is life on Mars, 40.3% agreed and 32.3% said “don’t know".
• Asked whether life on Mars could pose a biological threat to Earth, 42.8% said “don’t know.”, 34.4% disagreed, and 22.9% agreed.
• Asked about our ability to predict with reasonable certainty how life elsewhere would impact our environment, 71.2% disagreed and 10.4% said "I don't know".
• Asked about various quarantine proposals, approximately half of respondents said that the (then) current proposed methods of quarantine of the samples are either moderately or highly adequate. About a third said “don’t know,”
• Asked if materials returned to Earth from Mars should be considered hazardous until proven otherwise, all agreed except for 1.5% saying “don’t know,”
• Asked about the potential for in situ experiments done on the Martian surface to sufficiently determine the safety of Mars samples; 58.2% disagreed, and 21.9% said "Don’t know.”

The authors caution however

"We have at present no evidence that life exists on other planets or bodies in our solar system, thus the cautious views expressed by respondents in the present study reflect the professional responsibility that most members of scientific groups would express when faced with a paucity of real data.

We now know that Mars is far more habitable than it seemed back then. Also of course technology for sample containment has moved on, but so also has our awareness of extremophiles and very small organisms and gene transfer agents. Our in situ technologies have also moved on hugely so the answers about the potential for in situ experiments would surely be far more optimistic nowadays too.

So we can't really use this to gauge the opinion of present day microbiologists. I think this survey is perhaps most interesting for the diversity of views amongst microbiologists, when asked for opinions on, for instance, whether there is life on Mars, and whether it could pose a threat to Earth. With the caveat that they were showing professional caution given the responsibility in absence of real data, 22.9% thought that it could present a biological threat to Earth, and all except 1.5% said materials returned should be considered hazardous until proven otherwise.

It's the only survey I can find.

If anyone reading this knows of a more recent survey, do say!

Is this true: "We cannot take even a small risk with a billion lives"? The bold, cautious and intermediate

This is from Carl Sagan's quote mentioned above (in Why we can't prove yet that Mars life is safe for Earth):

"…The likelihood that such pathogens exist is probably small, but we cannot take even a small risk with a billion lives.".

Carl Sagan posing with a model of the Viking lander in Death Valley Colorado - he said that "we cannot take even a small risk with a billion lives".

I've found when talking about this to others that to some people that is as plain as the nose on their face that we shouldn't take a risk of the type he describes.

Others when faced with this argument might well echo Zubrin's "just plain nuts". Now I should be clear here, Zubrin isn't arguing that the risk is so small, we can ignore it. He says the risk is zero. He doesn't think that his proposal to take no precautions is a risk for Earth's biosphere.

However, others do argue that we can ignore the risk because it is tiny. You even see this in the Sample Return studies. They have to come up with a level of risk that is "small enough" because otherwise they simply wouldn't be able to return a sample to Earth. They go so far as to explain in some detail why they came to their decisions.

Let's look in detail at the ESF study on how to deal with samples returned to Earth from Mars where they studied various versions of the Precautionary Principle in the context of Mars Sample Return

 This study found that the ones that were most relevant are:

• Best Available Technology Precautionary Principle: Activities that present an uncertain potential for significant harm should be subject to best technology available requirements to minimise the risk of harm unless the proponent of the activity shows that they present no appreciable risk of harm.
• Prohibitory Precautionary Principle: Activities that present an uncertain potential for significant harm should be prohibited unless the proponent of the activity shows that it presents no appreciable risk of harm...

They continue:

...It is not possible to demonstrate that the return of a Mars sample presents no appreciable risk of harm. Therefore, if applied, the Prohibitory Precautionary Principle approach would simply lead to the cancellation of the MSR mission

They therefore argue that the Best Available Technology Precautionary Principle should be used instead.

The definition of Precautionary Principle and the associated conditions presented above align perfectly with the potential risks posed by a Mars sample and the ESF-ESSC Study Group recommends that the Best Available Technology Precautionary Principle is applied when considering the potential release of unsterilized Mars particles.

So basically there they are dismissing Carl Sagan's argument that we can't take even a tiny risk with a billion lives on the basis that if they took that approach we wouldn't be able to do a sample return at all.

I don't think that's a good argument myself. The whole point in the Prohibitory Precaution Principle is that if you can't demonstrate that you can do it safely, you don't do the mission. However, what you think about that depends on your philosophical and ethical views. It's a moral and philosophical question rather than a scientific one.

They then need to assess an acceptable level of risk. They adopt a "one in a million" chance as the "gold standard" for containment of the Mars sample return. They give their reasoning for using this figure in a section "4.5.1 The use of ‘one in a million’. Their reasoning is that this figure has become accepted through custom as a "gold standard" for assessing risks, and is generally taken as meaning "essentially zero", though they acknowledge that it is almost impossible to find a justification for it:

"When investigating what is considered to be an acceptable – or tolerable – level of risk, one often comes across the figure ‘one in a million’.... This value originates from the concept of de minimus risk contained in a 1973 notice in the US Federal register, where de minimus risk is considered to be ‘essentially zero’ and therefore below which no further regulatory action is required From there, the concept of ‘one in a million’ spread beyond the United States as being a standard when defining thresholds above which adverse effects are not considered tolerable. It is primarily applied when considering the risk of an individual to adverse health effects due to exposure to chemicals, toxic waste or radiation....

...While it is almost impossible to find a justification for it, it appears that the [one in a million] value has been accepted and is now considered by regulators as being the ‘gold standard’ to be met to demonstrate excellence in risk management "

They reason that the chance that the particle is hazardous is already low, and then by making the chance of release as low as one in a million, the combined risk is low enough to be acceptable. So, they are planning for a "less than one in a million chance" but how much less, they can't say, as nobody knows what the original chance is, that the particle is hazardous. All they have there is expert opinion that the chance that it is hazardous "is low". Or "The likelihood that such pathogens exist is probably small" as Carl Sagan put it.

There this "one in a million" probability was developed for situations that involve individual risks or risks of death to a small number of people. E.g. the risk of exposure to chemicals, toxic waste etc. But the central question here, the one that Carl Sagan raised, is - should we use that same "one in a million" for an experiment that could have adverse impacts not just on a few people but on a billion lives? Experiments that in the worst case could significantly impact on the habitability of Earth or even potentially lead to a scenario close to extinction.

I don't think it would actually make humans extinct, at least not with present day technology, for my reasoning see Worst case, almost certainly not an extinction event - rather - debilitating to habitability of Earth (below). But in the worst case it could severely impact on Earth's habitability and future human life prospects on Earth.

I find in discussions that there just seem to be two different ways of thinking about this. Some people, who I call "bold" would do an experiment that risks everyone on Earth so long as the chance of this is really tiny. For them it is not much different from risking their own life. A risk that has a one in a million chance of impacting on the habitability of our Earth seems just fine to them. It's so unlikely, so why be concerned?

Now I'm not saying at all that they are selfish. They might well be unselfish people who would without any thought risk their life to help someone else. It's a different thing from that. They are just "bold". If there is some significant benefit to a group of people, or most especially, to all of humanity, or indeed advancing scientific understanding, then they think that it is well worth taking that small risk to achieve that benefit.

Others are far more cautious when it comes to experiments that may be a risk to all of humanity, and think like Carl Sagan that it is simply unacceptable to take even a tiny risk, even a one in a million risk, with a billion lives.

The ESF is perhaps between the two. They treat the one in a million risk as acceptable, but they think it is acceptable only because they think the chance of a hazardous particle in the sample is already very low.

In the hope of some mutual understanding, I'll outline the two attitudes. Many of you will lie on a spectrum of views between these two. I've encountered people on the entire spectrum of views from very bold to very cautious in my discussions.

Bold

First, on the level of individual risk, it makes no difference. If it is a one in a million risk then what difference does it make if you are just risking yourself or risking everyone? Either way, you die in the worst case, but it is exceedingly unlikely that this happens.

That's especially so if you are the adventurous type and plan to go to Mars in an early rocket ship, when there may be a great risk of dying, maybe even 50%. You are taking many much greater risks than the risk of a sample return. I think few would judge a sample return to be even remotely as risky for Earth's inhabitants as the risk of crashing on Mars, or dying on the journey out, in such a mission.

So, for someone who is prepared to do this, the individual risk from a sample returned from Mars to Earth is tiny compared with the personal risks they would be willing to take to go to Mars themselves. They see colonizing Mars as of such importance, that these risks pale into insignificance for them. That's understandable. We all understand how you can be passionate about something and maybe even occasionally to the extent that we'd risk our lives for something. We can probably all understand how you could take a significant risk of dying, if perhaps few of us would take such huge risks for the sake of going on a mission to Mars in particular.

So then, it's like they identify with the rest of humanity as being like themselves, willing to take risks that they think may have great benefits. For instance, you could save half a billion dollars for a sample receiving facility if you don't take any precautions, and with that half billion dollars you could do a lot of space exploration, or whatever you think is an important thing to do. So, they don't see why we bother to build the facility. Not if the risk is only one in a million or less.

You may also think that Earth is at great risk of becoming uninhabitable for humans, and that we have only a small window of opportunity during which we still have spaceflight. If you think that way then protecting Earth may seem less important to you than setting up a new civilization. You may think that a new civilization on Mars will overcome the many social problems in our own, that by doing a sociological "reset" you can start a new more "advanced" civilization on Mars. Well at least that is how many space colonization enthusiasts think about this.

If you think this way, then, is it really true that Earth is at great risk of becoming uninhabitable for humans? If you start to think about how robust and habitable our world is, especially compared to Mars and anywhere else in our solar system, and how resilient and adaptable humans are, then that could lead to a re-evaluation of this. Extremely large asteroid impacts could make Earth uninhabitable, but they don't happen any more (we are protected by Jupiter). Also, over the last couple of decades or so, we have got to know our stellar neighbourhood so well through ultra sensitive astronomical searches that we can now say with confidence that neither a supernova nor a gamma ray burst can in the next few thousand years at least (and very improbable long term). I go into this in detail in my section Not our "only precious window of opportunity" below.

Cautious

This is how I think myself so I can put this in the first person. Also, I'll take the example of a sample from Venus instead of Mars for reasons I'll explain in a moment.

So, maybe yes if I was the only one affected, I'd take the risk of harm from Venus cloud microbes, by flying into the Venus clouds if I had a good reason to do that. After all there's quite a level of risk just flying to Venus in a spaceship. The tiny risk of being harmed by native life there, given that I think it's pretty unlikely there is native life there, is surely much less than the risk of going there.

I wouldn't be surprised for instance if the risk for humanity of a Venus sample return is a lot less than the risk I, or anyone else, runs every year of being killed in a traffic accident. So I'm not afraid of a Venus sample return on the individual level. I wouldn't lie awake at night scared that it's going to make the Earth uninhabitable if I heard that NASA was going to do a Venus sample return.

But I'd still argue very strongly and vigorously that, for as long as there is a small chance of life in the Venus clouds, that they should study it in situ first. I would also argue strongly that they shouldn't return a sample of it to Earth, until they know for sure if there is some hazardous lifeform there or not, or unless they can guarantee pretty much 100% containment of the sample on Earth (perhaps return to above GEO). I'd continue to say that even if all the experts agreed that the risk is really tiny like 1 in a million.

I used Venus instead of Mars in this analogy because I happen to think the risk from Mars could potentially be rather higher than most suggest. I'll discuss this in the section Drake Equation style approach to Mars sample return risk probabilities - could risk of harmful degradation of our biosphere be as high as 1 in 1,000? (below)

For Venus at least, I think the risk is pretty low, because of the sulfuric acid in the clouds and because any life there has to have survived for at least hundreds of millions of years in the clouds. It's not impossible - the clouds are a very stable habitat, but I would rate the probability of life there as a low probability, though intriguing possibility. If there is life there, previously adapted to more clement conditions on the Venus surface before the global stagnant lid and runaway greenhouse, it may be able to survive on Earth, and then be hazardous. So the reason I think the risk is low is just because it seems an unlikely possibility to find life at all, despite the indications that may possibly be signs of life there. I go into this in the section Life in the clouds of Venus and following (below).

If you think like this, that it's a very low risk but not zero, like the risk of a fire starting in your house, then it's just like Margaret Race's smoke detectors analogy. Just the other day, the battery went on my smoke alarm and I had to get a new one. For a day or two I had only two smoke alarms in my house instead of the recommended three. I didn't panic that my house was about to burn down in an undetected fire :). I know that the chance of a fire on any particular day is low, and after all I still had two other operating smoke alarms. I just went and bought a new battery and replaced it.

It's like that. You may think it is very important to take precautions, e.g. to have the recommended number of smoke alarms in your house. Yet you may think the chance of a fire is tiny, and you won't have sleepless nights worrying that you have one less than the recommended number of functional smoke alarms :). Even if you have no smoke alarms for a day or two, it's not a big deal, as the chance is so tiny.

If you add to that, that the tiny chance is not just a fire affecting your house, but an ecological disaster affecting the whole world, then if you are one of those in the "cautious" group here, you'll do a lot more than just buy a battery for your smoke alarm. Even if the chance of the disaster is tiny, if you think this way, you go to a lot of trouble and effort to make sure that you have protected Earth against even the tiniest risk from your experiments. In a situation like that you will not only replace the battery when it runs out. You would make sure you have smoke alarms with batteries that can't run out, or ensure you have redundant smoke alarms, with plenty of spare batteries available at all times.

For those of us who think that we shouldn't take even a small risk with a billion lives, then taking precautions is like installing a smoke detector.

Though the risk of a fire is tiny, still many of us wouldn't think twice about whether our houses should have smoke detectors. It is similar with a sample return. Though the additional costs for precautions for a sample return are high, the potential risk is also much greater in its effect than your house burning down. So even if the chance of it is tiny, many of us including the planetary protection officers and just about all of those who write articles on the topic still think that it is very important to take precautions to prevent the risk to the environment of Earth.

Can't resolve this

I've found that this argument about tiny risks affecting billions of people has no force at all on some people, I've found, in discussions. Meanwhile, in the other direction, their very understandable attempts to urge people like myself to "be bold" and ignore the warnings of the likes of Carl Sagan, Joshua Lederberg etc are not likely to convince those of us who want to be cautious about such risks.

I don't think there is much one can do to resolve it. Except hopefully, to maybe understand each other better, that we have these different ways of thinking about things. This bold approach, taking risks for everyone, just as you would if it was only you involved, may well be a useful quality at times. It leads us to new and interesting situations. And maybe this more cautious approach is useful too at times, as it helps keep us out of trouble.

Yes maybe sometimes you can save some money by not taking those precautions, maybe millions of dollars sometimes. But for those who think they are important, then it's just not worth it. For us, it's like saving money on a house by not buying smoke alarms. Nearly always you end up with some extra money in the bank. But there's a tiny chance you end up with a burnt down house. We just don't think that risk is worth the saving in money from skimping on the smoke alarms.

There is another argument you can use for the cautious approach here. I'm not sure how much effect it would have on those who are bold by temperament in this topic area. I think this argument only really is effective if you are already disposed towards Carl Sagan's way of looking at things already. If you do think like that, however, this argument may motivate you to take even more care than you previously thought was necessary.

So let's look at that next.

Nick Bostrom's existential risk

The Oxford philosopher Nick Bostrom studies existential risk, which is to say, a risk of human extinction, or of significant degradation of the habitability of the Earth. He argues that we haven't evolved to be able to reason about such risks,either socially or evolutionarily, because, obviously, they have never happened to us in the past. After all, if they had, we wouldn't be here, or the Earth's habitability would already be severely degraded, which it isn't, not in this extreme sense. He suggests that this means that we have no trial and error experience to rely on, and there is no way to develop direct trial and error experience either.That makes decisions difficult as that is what many of our decisions are based on.

So, this makes it a tricky thing to even think about never mind come to decisions about. Before I get down to the details, let's just say one thing. We are used to messages of apocalypse, "the End is Nigh", "we are all doomed" etc. But that's not his message at all. He is not saying that we need to prepare for the end of the world.

His is a different message, it is a positive one. The reason for looking to see if there are any potential existential risks is not to give up, but rather, to identify these risks and so prevent them from happening. So it is a positive way of thinking. A way of ensuring that "the End is Nigh" won't happen.

He gave an entertaining talk on it (dry humour) which you may enjoy:

(click to watch on YouTube)

The end of humanity: Nick Bostrom at TEDxOxford

For more see his website Existential Risk, Threats to Humanity's Future. Related is the Cambridge Project for Existential Risk.

The reason these things may happen more easily now is because of the rapid forward pace of technology. It used to be that about the only existential risk humanity faced was impact from a giant asteroid. Now there are numerous potential risks that thinkers such as Nick Bostrom have identified. We need to look soberly at each one, and decide what to do about it.

I don't actually agree with him on many of the things he identifies as potential existential risks. I would say that even a giant asteroid impact is not actually an existential risk, see the section Not our "only precious window of opportunity" for space exploration below. But severe degradation of the Earth's environment at least for a while, certainly, I see that as a risk. And a Mars sample return is a perfect example of what he is talking about, I think.

Example calculation: a 1 in 10 billion probability of permanent degradation of Earth could harm an expected one million future people

So, Nick Bostrom has an interesting way of looking at this, a way of explaining mathematically why it makes sense to take precautions in the case of tiny probabilities that would have long term effects on the habitability of Earth. Let's try it out with the example of a Mars sample return.

Most likely, nothing at all would happen after a Mars sample return. Though we can't calculate it easily, I think most would agree that you are probably far more likely to be killed by lightning. Then if anything does happen, the chances are that it will be a minor incident, easily contained. There seems to be general agreement on this by just about all concerned (though I'm not entirely sure that this is correct myself, more on this in a minute).

However, as Nick Bostrom has argued, if you have a possibility of an existential risk, you need to treat it with especial care when it comes to decision making. Even a one in a trillion chance or less might be significant if you are talking about an existential risk. Nick Bostrom mentions figures such as  10¹⁶  as the total number of humans likely to live in the future billion years on Earth. If you take into account possibilities of space colonization and humans surviving into the far distant future, he quotes a figure of  10³⁴ human life years or more than 10³² current human lifetimes

To see how his approach works, let's take his lowest figure for the number of people potentially affected as 10¹⁶. Now, let's suppose you achieve a 1 in  10¹⁰ probability of existential risk (after containment). That's a tiny chance. One chance in ten billion of anything happening. Normally such a low figure would be considered  negligible, and it's certainly well below the "gold standard" of the ESF report of one in a million. However when you combine these two figures, to work out the effects of a one chance in ten billion of permanent degradation of the habitability of Earth, affecting 10¹⁶. people, you get an expected value of a million people who are severely affected by it. So, though the probability of anything happening at all is so tiny, you can argue following Nick Bostrom. that for decision making that example should be treated as equal in weight to any other risk expected to harm one million people.

We would go to a lot of effort to prevent harm to a million people. So that's his basic idea, to focus on the "expected number" of people harmed, by multiplying together the number of people potentially affected by the probability of something happening.

Worst case, almost certainly not an extinction event - rather - debilitating to habitability of Earth

Even for a sample returned from Mars, and in the worst case where it turns out to be highly dangerous to the environment of Earth, it's a bit hard to see it making humans extinct. It's the same also for artificial life created in a laboratory. That's because it can't instantly infect the entire world in one go. It has to spread somehow. So, let's look at two possibilities, a microbe that is almost immediately fatal, and one with a latency period.

Let's take an extreme worst case first. Let's suppose what we return is something very hazardous. It can colonize our lungs, and skin, eats our bodies and our bodies are totally naive as Joshua Lederberg suggested could happen in his:

"Or will they have a field day in light of our own total naivete in dealing with their “aggressins”?"

Our body's defense systems just don't see this microbe from Mars as harmful, and our cells "roll over" and let it do whatever it wants, accepts everything that happens. It also has hardy spores that can withstand just about anything on Earth as might well be the case for spores adapted to Mars.

Let's suppose it can just be distributed in the wind. Let's suppose that it replicates easily, eating anything organic that it finds. Let's suppose that people die quickly of its effects. Well after a sample containment breach, we'd soon have news stories that all the staff of the Mars Receiving facility have died and everyone in its immediate vicinity. Or the return capsule crashed or the helicopter or train carrying it to the facility and again everyone in the region died quickly.

Well in this case, it still takes some days or months for it to spread around the world. It might remain in the Northern hemisphere because there isn't that much air movement between the two hemispheres. It might find seas a barrier, or it might not, maybe it reproduces in the sea, but then ocean currents take months to transfer material. Whatever the situation, it's not going to get everywhere instantly. So, that's time enough to figure out how to protect yourself. You immediately know it's from Mars and a microbe probably, and spreads in the wind. from the speed with which it spreads. Probably rather quickly you get huge increases in sales of full body suits and respirators adapted to filter out even tiny microbes. People build airtight enclosing tents like big plastic bags they live in, to keep it out, with airlocks. Like "bubble boy" tents.

If nothing else, there are many military bunkers designed to protect their inhabitants for months against anything including biological warfare. The military also have suits such as the NBC suit

Two Canadian soldiers wearing NBC (nuclear, biological, chemical) suits

Then, the crew of nuclear subs would be able to survive for months or years on end below the sea, only surfacing from time to time to take in food and water.

Perhaps this was different at the time of Apollo? Perhaps if we were limited to the technology they had back, we could have gone extinct as a result of the return of a problematical microbe from the Moon. See Were we just lucky? Do extraterrestrials sometimes become extinct after their first Apollo style mission to their nearest Moon or planet? (above). In the present though, one way or another, there would surely be at least some survivors in the whole world.

Then what happens next? Well, these are only microbes after all, and well studied microbes too, by then. We could sterilize against them. Once a few survivors have biological containment suits sorted out, they would be able to go outside their tents, subs and bunkers, help others by supplying them with similar suits, and they would find plenty of food which they could sterilize of the Mars life. They gradually develop ways of coping with it. Even if the human body never evolves a way of coping with it, still they can grow crops inside greenhouses, much as they would do in a space habitat but far more easily.

So, we'd get some survivors. It would be a horrific disaster but not an extinction event for humans.

On the other hand, suppose its effects are not so immediately fatal. Then that means there's a latency period. Some of us will be more adapted to resist it than others. The longer the latency period, then the further it can spread without being noticed. However, if it is like that, you probably have variation in susceptibility and months or years in which to begin to develop countermeasures and ways to either remove it, keep it out, and to lessen its effects on the human body.

If it doesn't harm us directly, but has an effect on our biosphere then you can set up greenhouses and other facilities that are sterilized of it, and within those humans can develop again and spread like a space colony.

In all these cases you end up by basically "paraterraforming the Earth" - making it into a place where humans can live only inside huge enclosed areas. They can go out but only with protective suits and air breathers. Even if the effect on our biosphere is very rapid, we could still do that. Eventually, if we so wish, we could paraterraform the entire world, cover everything, the land anyway, with huge greenhouses.

So, though you could perhaps make up a science fiction story that leads to human extinction after a sample return, I'm not sure it can happen in reality. It's the same for just about all the existential risks.

We could of course go extinct voluntarily easily, if we could agree on it. If everyone agreed to stop having children as for the voluntary human extinction movement then there would be no more humans within a little over a century. There could be many other ways to achieve this with full co-operation of everyone. But apart from that, it needs some extraordinary science fiction situation.

Amongst the most plausible sounding is the idea of a strong AI that decides to get rid of humans. It's based on the idea that an artificial intelligence can be much more super intelligent than we are and so unstoppable. I know that this is a scenario quite a few take seriously, but to me it's just science fiction, at least, in the form of a computer program. Of course a super intelligent human or indeed, maybe biologically augmented creature - that does seem possible. But I'm not at all sure that being super intelligent, gives you a strong advantage either given that you have to work in a society with others. You may be interested in the reasons, see AI Apocalypse scenarios below.

This doesn't make much difference to the calculation though, whether we can go extinct, or whether the main thing we risk is diminished prospects. Even if the risk of extinction is so tiny we can ignore it even in this sort of calculation, Nick Bostrom's argument still applies as he uses his "existential risk" to cover not just human extinction but long term diminishment of our life prospects and of the habitability of Earth. That, I think, is something that is possible after a Mars sample return. So in that sense the worst scenario, though probably very unlikely, would lead to a less habitable Earth for humans not just now but for maybe even all future time. Maybe we never are able to get rid of the problematical microbe.

So then in the calculation itt's no longer about beings that won't exist but about beings in the future that would be restricted. The numbers affected are similar. So again, suppose the result of the risk calculation is an expected one million people who have to live in biological isolation tents for the rest of their lives - that's still something you'd put a lot of effort into preventing.

However as I said, this argument is only really is effective if you are already disposed towards this cautious way of thinking. If you have the "bold" approach where you boldly take a risk not just for yourself but for the whole of humanity, and see this as a natural thing for humans to do, this argument may still not have much force for you.

One of the reason that people have that "bold" attitude is that they think we have very little time in which to do space exploration. Sometimes they are in a huge rush to get into space this decade or next decade, or very soon, next few decades at least, thinking that if we leave it too late we've had it. If you think like that, it's no surprise really if a 1 in a million chance of harming Earth seems an acceptable risk to take, if you think the upside is that it is the only possible way to save humanity long term.

So let's look at that idea.

Not our "only precious window of opportunity" for space exploration

So, we can also answer the argument about this being our only precious window of opportunity to get into space and that it will soon be gone. This is something Elon Musk often says. Quoted in an interview:

"I ask Musk how often he actually thinks about colonizing Mars. Every day? Every week? "I do think about it a fair bit," he answers, explaining that part of his urgency is that we might not always have the technology to get there. Most of us instinctively assume that technology relentlessly marches forward, but there have been times before now in human history—after the Egyptians built the Pyramids, for instance, or after the multiple advances of the Roman Empire—when the civilizations that followed could no longer do what had been done before, and perhaps there's a complacency and arrogance in assuming that this won't happen again.~"


" "There's a window that could be opened for a long time or a short time where we have an opportunity to establish a self-sustaining base on Mars," he reasons, "before something happens to drive the technology level on Earth below where it's possible. So does the base become self-sustaining before spaceships from Earth stop going?...I mean, I don't think we can discount the possibility of a third World War. You know, in 1912 they were proclaiming a new age of peace and prosperity, saying that it was a golden age, war was over. And then you had World War I followed by World War II followed by the Cold War. So I think we need to acknowledge that there's certainly a possibility of a third World War, and if that does occur it could be far worse than anything that's happened before. Let's say nuclear weapons are used. I mean, there could be a very powerful social movement that's anti-technology. There's also growth in religious extremism. Like, I mean, does ISIS grow…?""

(emphasis mine)

I don't buy this argument at all. Let me explain why, and perhaps this may give you some points of interest to think about if you are persuaded by his arguments.

Note, I have not heard Elon Musk discuss whether we need to take any precautions for a Mars sample return. He is keen on Robert Zubrin's ideas, at any rate he often echoes the same arguments, for instance the ideas that we may be able to terraform Mars rapidly, and his idea that a Mars colony could sustain itself financially by exporting intellectual property from Mars to Earth. But that doesn't mean he agrees with Robert Zubrin on every point. Robert Zubrin as we've seen thinks that we don't need to take any precautions. What Elon Musk thinks about this, I've no idea.

Does he think that because we have such a precious opportunity, that we don't need to take precautions to protect Earth during a sample return? I've no idea. However it's relevant here, because, whatever he thinks on the matter, these ideas are often used in online discussions as reason why we should rush into space and not spend money and time on planetary protection precautions.

World war III

His analogies with the Romans don't work - after all the Chinese, Indian and Arabic civilizations continued to make great strides in science while the West languished in a dark age - which also wasn't quite as "dark" as it is sometimes made out to be. The Romans themselves took on the torch of civilization from the Greeks.

After that , the torch was passed from the Arabic civilizations (mainly) to Europe, and Western Europe went into the lead again. Many countries in Europe have been world political leaders for a while. As an example, Portugal was a leader of the world, in terms of ideas, exploration, and political power too, for a fair while. Columbus came from Spain of course but many other early voyagers were from Portugal. Vasco da Gama reached India in 1498, and Pedro Álvares Cabral discovered Brazil in 1500. It was the head of a world spanning empire for several centuries. They made many discoveries, especially on their long sea voyages, and were great inventors. The university of Columbia is one of the oldest universities in continuous operation.

Now it's just a small country which doesn't play a large role on the world stage. Spain also was a major world power with many inventors and discoverers. The UK was too at one point. There are many other examples. So I don't think it is right to make an analogy of the Romans with the whole of civilization. More like an analogy of the Romans, with, say, Portugal, or Spain, or the UK. Or indeed, China or India, or the Mayans in South America.

So, if some disaster leads to the US losing its dominance in the world, it is a national disaster for the US indeed. However, it's not a disaster for the world as a whole. It would just lead to some other country taking their place. Especially now with education so widely spread. Just about every country has universities and other places of higher learning. Nine tenths of all adult men and eight tenths of all adult women in the world are now literate. I can't see a political or financial disaster in the US taking down technology and civilization throughout the world.

It's true that there are some countries with low levels of literacy, especially for women. It could be a lot better. But there's no way a disaster could end up with a world in which nobody can read books, or understand basic ideas of technology. At most we lose the ability to send rockets into space for a short while. Maybe in the worst case, the US or the UK, or Europe generally or North America become third world countries with high levels of illiteracy, and almost total ignorance of modern science. Maybe in the worst case larger areas of the world, even most of it, get knocked back to the industrial age, nineteenth century science. I think even that is pretty unlikely myself. There would be some pockets of higher learning where people remembered and were able to study and understand books written about technology in the twentieth century, and some books would survive.

At any rate, even if it was like the nineteenth century, it's nineteenth century technology in a world with with lots of waste products and machines and so on from the twenty first century to give us clues to what can be done.

Meanwhile, all that could happen on Mars also. With them so dependent on technology, then a development that lead to loss of technology, and spaceships no longer arriving from Earth, would be far worse for them. If they lose the ability to make oxygen and to maintain a closed system atmosphere, they are finished. If their habitat deteriorates to the extent it needs to be replaced, or they need a new spacesuit, again they are finished, unless they can do all that themselves.

I don't find it credible that the whole world will get taken over by ISIS style politics / philosophy either. And if it was, why would it not spread to Mars or indeed, originate there? If nowhere on Earth is immune from that political upheaval, how could Mars possibly remain immune to it? Once you have millions on Mars, if that ever happened, and thousands of people going back and forth, probably getting there and back in weeks or even days by then - it could also be the source of strange ideologies, based on ideas maybe even inimical to Earth, who knows? Elon Musk couldn't direct its future political development, or anyone else, unless it develops as a dictatorship, which would have its own problems.

Also, I can't see a nuclear war setting the world back to the nineteenth century. After all pretty much the entire southern hemisphere is now a nuclear free zone and would not be hit by nuclear weapons in such a war, so why would it lose its technology, education, modern universities etc? Also most experts agree that a global nuclear war wouldn't cause a "nuclear winter" with controversy over whether it would cause a "nuclear autumn" or have no global effects (the early models predicting nuclear winter turned out to be flawed). For more on this see my Doomsday Debunked - Nibiru Is Nuts - What About Nuclear War, Asteroid Impacts, Runaway Warming,... it's about doomsday scenarios generally - and many people contact me scared about the BS nonsense planet Nibiru which is why I cover that. But I also cover scientific scenarios too.

Some of this section comes from that page and my kindle book Doomsday Debunked.

Doomsday Debunked also available to read online on my website (free)

AI Apocalypse scenarios

I'm skeptical about the idea that we could create artificial intelligent computers that take over the world. Yes many people do say that, including Stephen Hawking, and Elon Musk - but there are others who say it's impossible. I just don't buy it. So, in this section, I will open it out by present a personal perspective on the matter, which might perhaps be of interest.

What I will say here is based on Roger Penrose's arguments that understanding truth is not something that can be programmed. His basic argument is a proof that if you program a computer to recognize truth in mathematics, then there will always be truths that we can see to be true, which we can also see, from inspecting its program, that it can never recognize as truth. So, he argues,,a computer can only be programmed to simulate an understanding of truth, and not to truly understand it as we do.

On the basis of that reasoning I happen to think that whenever we understand something to be true, then it is based on non computable physics. So, what is "non computable physics" here? It's any physics that can't be simulated in a computer program, however fast and however much memory it has.

Non computable physics is certainly possible, as there is one simple example of it. Radioactive decay is based on non computable physics. You can't simulate it exactly in a computer program. That's not to say that adding true randomness, such as a hardware radioactive source, would give you the non computable physics needed for understanding truth.

Indeed, even conventional quantum computers, though they greatly speed up many calculations, don't introduce anything essentially new by way of computing power. Everything they do can be simulated by using slower non quantum computers. However, the example of radioactive decay does show that it is possible to have non computable physics. Where there is one example there may be others.

Professor Penrose's arguments are complex, and I know there have been many challenges to them and responses. I happen to think his reasoning here is significant and powerful, but don't want to go into defending it myself. The discussions rapidly get complex and technical if you do this, and he has answered those challenges just fine himself (in my view). I don't see that there is anything to add except to day that I agree with him.

However, there is a simpler way to get started thinking about this. A small change in the code can lead a programmed machine to say or do anything. E.g. it would be very easy for a knowledgeable programmer, familiar with the code, to program Alpha Go or Deep Blue to lose every game. It would be as easy to program a self driving car to always crash into the first car or lamppost or sign that it sees on the road, and to stop when traffic lights go green and move when they go red. There is nothing there in the program to care one way or another if you rewrite it like that. There's nothing that understands what is happening, and nothing to object to any of this at all. A computer will just do whatever you program it to do.

We can certainly program computers to do clever things. However, this is all based on human understanding, at some level. That is human understanding of truth. The computer programs may surprise us with new discoveries. But I don't think we can program a super intelligent or even a very dumb being that actually "understands" what is going on and can make plans of its own. There is nothing in the Alpha Go program that has any idea what the game of Go is, and there is nothing in Google's self driving cars software that has any idea what a car is or a road.

And yes, a program can certainly exceed its programmer's expectations, but without oversight, it is as likely to do something very dumb as something that seems intelligent, because there is nothing there to know anything. It's the programmer overseeing it that makes sure it does things we see as intelligent rather than dumb.

Computer programs can certainly do harmful things, that's for sure. Programmers can make mistakes, or even be malicious, and they can write accidentally harmful and dangerous programs that make use of machine learning in ways that surprise the programmer. But it's not through artificial intelligence making decisions for us. The difference is that there is nothing there that knows what it is doing.

Deep learning changes none of that as no amount of deep learning prevents the programmer from using what comes out of that part of the program, however they choose. They can program as much deep learning as they like into it, but then on the basis of that, program it to do whatever they want it to do, including ignoring the deep learning, or doing the opposite of what the deep learning suggests. The same is true if you introduce true randomness, or parallel computing, or quantum computing.

Whether or not you think Roger Penrose has succeeded in this proof - certainly nobody has proved in the other direction that all the laws of physics have to be computable. So - until someone comes up with a totally convincing proof either way, we are free to take either possibility as a hypothesis.

The strong AI enthusiasts are optimistic that human understanding can be simulated exactly with a computer program. However they are saying this without any proof. There is no particular reason to suppose that all physics has to be computable. Also the way our brains work don't much resemble computer programs, no more than they resemble clockwork. There are parallels but the resemblances seem rather superficial. For instance, you can't switch a human off and on again. We have nothing there that functions like a hard drive. There are no bit sequences to study. There is nothing remotely resembling a central processor. There are some resemblances to the ideas embodied in "neural nets" but they are a huge simplification of what is going on in our brain. Quite possibly a lot of the most interesting stuff has got lost, if we model the brain as a neural net.

So, what if our understanding of truth happens to be non computable as Roger Penrose suggests? Whether or not you accept his argument, what if he has got to the correct answer anyway?

If you think that way, the first consequence would be, that we will never simulate physics completely with a programmable computer machine. That includes also,

• Quantum computers if they act according to the currently known laws of physics with entangled qbits
• Hardware neural nets (if they can be simulated in an ordinary computer as software neural nets)
• Self modifying computer programs
• Machines that use probabilistic methods to solve things (e.g. probabilistic annealing),
• Massively parallel computers with billions of processors acting in parallel.
• Hardware cellular automata

All of those have been shown to be logically equivalent to a Turing machine and can't introduce anything essentially new. All they can do is to speed things up, make our computers faster.

If this is true, and if you go one step further as Roger Penrose does and say that understanding truth requires non computable physics, it doesn't rule out artificial intelligence. However, it means that a "strong AI" can't consist of just digital data plus a program. It also means it can't be digitally copied.

I think that the metaphor of a computer program does work up to a point. It's like the earlier seventeenth metaphor that humans are like clockwork. Indeed it's a similar metaphor since anything a digital computer can do can also be done in a sufficiently complex piece of clockwork, at least in principle. Basically it's the same clockwork metaphor updated using examples from modern technology. I just don't think you can make a clockwork automaton, even speeded up and implemented electronically, that understands anything.

So, I don't think that we can use the methods of computer programming to create creatures that understand truth and make intelligent decisions. In other ways, yes, our society could give birth to such beings perhaps, but not through programming.

If true artificial intelligences can't be programmed, and they can't be digitally copied, you start to look at them rather differently. You can't "prime them" with a digital copy of an earlier more capable Artificial Intelligence. They wouldn't start off understanding the world, programmed with knowledge of everything, suddenly able to take over from us in a boost of intelligence we can't imagine. They'd be more like babies.

Yes, they'd be babies with potential for things we can't do maybe. Perhaps potentially brilliant mathematicians, or physicists, or musicians or whatever once they mature. But they wouldn't start up like that. We'd have a huge responsibility if our society gives birth to super intelligent babies - and actually I think this is far more likely to happen through genetic manipulation than advances in computer programming. They might have potential for very fulfilling worthwhile lives, but they'd also perhaps have potential for huge suffering. If so, we might find it hard to help them with this.

They would also need to be social beings, to interact in our society and persuade us of things. They would probably differ in their views and opinions and ideas of what to do as much as we do. If brought up by us, they would inherit ideas from our own society too. Either adopt them, or rebel against them, or take intermediate positions, like human babies.

What if you think that a programmable AI is possible? It's a similar situation to the non programmable AI's in some ways, except that you have access to the code, and there's the possibility of copying an AI you like. There's also the possibility of AI's spreading like a computer virus. But if that's true, there's also the possibility of deleting them, or modifying them. You can also have anti virus defence, or indeed, the option to disconnect your computer from the internet if necessary. The AI's can't do anything without the help of the humans that they have to interact with.

Then there's the possibility of the "singularity". Or an "intelligence explosion". As Irving Good put it:

"Let an ultraintelligent machine be defined as a machine that can far surpass all the intellectual activities of any man however clever. Since the design of machines is one of these intellectual activities, an ultraintelligent machine could design even better machines; there would then unquestionably be an ‘intelligence explosion,’ and the intelligence of man would be left far behind. Thus the first ultraintelligent machine is the last invention that man need ever make, provided that the machine is docile enough to tell us how to keep it under control."

That's more plausible as a near future possibility if you think that understanding truth is a matter of computable physics and can be simulated with a computer program or not. If it is all non computable physics, then you could still get something similar. Genetically modified humans, modified by genes that give them greater intelligence in the areas needed for designing genetic modifications maybe could design genes for yet more intelligent creatures in a similar acceleration, though over a longer timescale.

At any rate, we are nowhere near strong AI at present. Our chatbots are pathetic, if you think of them as a representatives of AI. Though they are fun to interact with for a short while, they are nowhere near answering the Turing challenge, not if you talk to them for any length of time. They impress newbies perhaps, in the same way that the clockwork automata impressed people in the seventeenth century. The ones that are most successful succeed by simulating humans with handicaps, or special situations where the program is expected to ask many of the questions.

"Deep learning" has lead to impressive capabilities in speech recognition, recognizing faces, recognizing the objects and creatures in photographs. But there is nothing that even remotely resembles a computer program that is capable of recognizing truth. This was a news story: "Supercomputer models one second of human brain activity" about a super computer that modeled a neural net thought to be as complex as the human brain in terms of the number of neurons. It took it forty minutes to simulate one second of neural activity.

A single cell creature such as an amoeba moves around, has complex decisions to make, do I go here or there, eats food etc.

Amoebae

(click to watch on YouTube)

Amoeba Eating - and rejecting food

(click to watch on YouTube)

If you modeled the behaviour of an amoeba with a neural net you would need thousands of neurons. But it doesn't have any. So - this is just a plausibility argument - it's not proof. But - if our brains were as simple as these neural nets suggest - then a creature with a single neuron that takes advantage of its internal structure would be easily outsmarted by another creature with a brain with many thousands of neurons which it treats as just nodes in a neural net.

Surely our neurons can't be acting just as simple neurons in a neural net? Roger Penrose thinks that the microtubules inside every cell help, working like cellular automata. Whatever the reason, an amoeba does have a form of simple intelligence, and so far, nobody is sure how it achieves that. Surely our individual neurons can have access to similar levels of intelligence to an amoeba, if they need to, and if so, isn't it natural that they work together and pool that intelligence, rather than just each emulate something as simple as a neuron in a neural net?

Either way, whether you think of it as strong AI programmable computers, or as genetically engineered super humans, whales etc, if this does happen, then again it's as likely to happen in a highly technological society like a space colony as on Earth. Indeed, it's probably more likely there. So we can't "run away from it" to space. It's as likely to originate in space, or in technology developed for space missions, as anywhere.

For more on this see my:

Why Strong Artificial Intelligences Need Protection From Us - Not Us From Them

If A Program Can't Understand Truth - Ethics Of Artificial Intelligence Babies

The last one is also available on kindle

If Programs Can't Understand Truth - Ethics of Artificial Intelligence Babies 

Natural disasters - resilience of humans

When we look at past natural disasters, such as asteroid impacts and see figures such as over 90% of species extinct, it is easy to assume that means a high chance of humans going extinct. But humans are more robust than you might think, if you look at it carefully.

I suggest that even if only 10% of species survive, or even 1%, humans are such great survivors as a species, that we will be in that 1%. We are omnivores and can eat just about anything including shellfish, the staple of early humans in cold parts of the world like Northern Europe and North America.

So, as long as we retain at least stone age technology, there isn't much that could make us extinct. Even if we have to go back to beachcombing and surviving on shellfish, which was a staple of early human diet in cold places such as Canada, South America, or Northern Europe, for instance, Scotland (where I live), one way or another some humans would survive.

Conchero al sur de Puerto Desead - a shell midden in Argentina. For long periods of time ancient humans survived on shellfish. This was their staple, especially in cold places, for so long that they built up these huge shell middens in many parts of the world. See Shell Midden

We are omnivores able to survive on:

• Shellfish
• Insects
• Fish and other marine life
• Nuts
• Fruit
• Roots
• Seeds and cereals
• Some seaweeds and algae (like spirulina)
• Birds,
• Animals
• Reptiles.

Homo Sapiens is listed in the IUCN Red list of threatened species - as one of the species of least concern

"Listed as Least Concern as the species is very widely distributed, adaptable, currently increasing, and there are no major threats resulting in an overall population decline."

So long as any of our food species survive the extinction event, anywhere in the world and so long as humans retain at least stone age level of understanding of technology - then there would be many survivors and we would not go extinct, even if more than 90% of species went extinct. The dinosaurs weren't a patch on us as far as survival goes.

Many lifeforms survived Chicxulub, with no technology at all, including, turtles, crocodiles, alligators, small mammals, flying dinosaurs (the birds), dawn redwood trees, pine trees. After the Chicxulub impactor humans would have found plenty to eat. We can make clothing, use fire, make boats. We are able to walk and run very long distances easily, and swim to cross rivers. We know now how to cultivate crops, and raise animals and other creatures for food. We aren't going to forget to do all that . Even in the worst natural disaster, most likely billions survive, and at least many millions. Also there would be many relics of our technology, even tinned food to start with. We'd probably restore just about all our technology at least up to early twentieth century within a few decades - though it's far more likely we never lose it. Even a Chicxulub impact would leave many remote areas of the Earth, remote from the impactor, reasonably unscathed, even an impact that lead to global firestorms.

Those who claim we could go extinct often point to past evolutionary bottlenecks, when the entire population of humans in the world was down to a few thousand. But that's from when we were one of several distinct species of hominids, a sub species confined to tropical areas. Yes, it may have happened to humans in sub-saharan Africa, before they spread to Europe and India, as recently as 70,000 years ago, just locally. At that point the human population may have been reduced to as low as 2,000.

However , extinction event of course does not apply to the other hominids that had left Africa millions of years ago (in the case of Homo Erectus) and hundreds of thousands of years ago (in the case of H. Heidelbergensis, likely ancestor to modern humans, Neanderthals and Denisovans). There were plenty of intelligent hominids living outside Africa at the time, and they didn't go extinct until much later as a result of competition with modern humans. None of the other humanoids that co-existed with us in colder climates went through the same evolutionary bottlenecks. We nearly went extinct, yes, but in a world with many hominid species that were competing with us, that occupied most of the world apart from tropical areas.

Also, those 2,000 people, though they probably had fire and the ability to make log boats, may or may not have had clothing, and didn’t have the most basic ideas of modern science. Most especially, they didn’t have agriculture. That didn’t happen until 10,000 BC onwards: Neolithic Revolution.

Although they did belong to our species, anatomically and biologically modern, they didn't have our culture, It simply would never have occurred to them to try to cultivate plants or animals or birds, fish etc for food. How likely is it that some global catastrophe causes all humans to lose their knowledge even of agriculture?

As for ideas that modern humans wouldn't know how to survive because we don't have to do that any more - it doesn't take a lot of knowledge to eat shellfish or insects, those old staples of early humans. We would soon relearn to build a fire, make rudimentary shelter, and make clothing. There would be plenty of us with some knowledge of basic survival skills too. Plus in even the worst conceivable disasters there'd be much that remained from our civilization for the transition period.

See also my

• Why Resilient Humans Would Survive Giant Asteroid Impact - Even With Over 90% Of Species Extinct

Large impacts

Impacts large enough to sterilize a planet simply don't happen any more in our current solar system. Yes there were many impacts like that in the early solar system, which created the Hellas basin on Mars, Aitken crater on the Moon and the Caloris Basin on Mercury. However, those all date back to the time of the "late heavy bombardment". Perhaps this was caused by migration of giant planets, in the early solar system before they had cleared out all the dust and gas, deflecting comets and asteroids into the inner solar system. There are other hypotheses as well. Something did cause large numbers of huge objects to hit planets in the inner solar system. However, this is something that happened well over 3 billion years ago, getting on for 4 billion years ago, and we have had none of those for well over three billion years.

Jupiter protects us from the largest 100 kilometer scale comets from the outer solar system. It breaks up or deflects the largest comets, so that they hit the Sun, Jupiter, break up into smaller pieces through tidal interactions with Jupiter, evaporate in the heat of the Sun, or are deflected from the solar system. One way or another, they are destroyed or deflected away before they can get into an Earth crossing orbit. Jupiter doesn't do such a good job of protecting us from the 10 km sized asteroids, though it takes many of those "for the team" as well.

As for the asteroid belt, between Jupiter and Mars, its larger asteroids are in orbits stable over hundreds of millions of years timescales. Yes, true, our solar system is not completely stable even now. There is a small probability of Vesta eventually hitting Ceres, and a chance of a future resonance of Jupiter with Mercury disturbing it out of its orbit, but neither of those can happen any time soon.

This is confirmed by the cratering record. All the impacts that large on Mercury, Mars, the Moon are well over 3 billion years old. Earth has some impact craters significantly larger than for Chicxulub, the largest, the Warburton Basin in Australia is the result of two impactors both probably larger than 10 km in diameter probably from a larger asteroid that split just before impact. But that also would not make humans extinct. For other examples see this list of large unconfirmed craters (wikipedia)

Something like that would put so much soot into the atmosphere that it would turn day to night for years. Photosynthesis would be impossible. Only hardy spores and seeds of photosynthetic life would survive. At least in nature. But small mammals, and birds survived, also turtles. A few humans would survive also. Even large numbers with ingenuity, doing things like growing mushrooms on all the decaying wood, using insects as a food source directly or indirectly, fish, and any stored supplies. They could also use methane gas to grow methanogens as a food source directly or indirectly.

If given time to prepare we could also stockpile food, using food otherwise grown for biofuel or to feed to animals. So something like that does seem survivable, at least not an extinction event.

What about using LED lighting and growing crops as for BIOS-3?

Of course solar power won't work, but everything else will. The current world energy use per capita is 1,920 kilograms of oil equivalent (koe) in 2014 according to the World Bank. To get kilowatt hours, you multiply the koe by 11.63. To get average power in kilowatts, divide by the number of hours in the year, or 8766 (365.2422*24), to get 1920*11.63/8766 = 2.5 kilowatts. So bearing in mind that the BIOS-3 system needs only 1.5 kilowatts per person for lighting with efficient LED for crops (100 watts per square meter for 12 hours a day for 30 square meters per person) then it would be feasible to generate the power we need for crops - if we sacrifice other things to compensate.

However that's for all forms of power. We need it as electrical power. Some countries could find that much power from their electricity generation alone. Iceland has electricity consumption of 53,832 per capita per year (figure in 2014). That's a constant 6 kilowatts per person average. Only a quarter of that would be needed for LED lighting. Norway could also manage it with 2.6 kilowatts per person. Canada and a few other countries just squeak in with hardly anything to spare. The US generates 12,987 kilowatt hours per capita of electricity. That's just under our 1.5 kilowatts at 1.48 kilowatts average power usage per person. The world average is only 357 watts per person of electricity generated.

You could get enough power for LED lighting to feed a population of hundreds of billions that way for sure, and even perhaps a billion or more,if we were able to get all the infrastructure in place in advance and devote most of the electrical power to agriculture. But for the whole population, it would need a big increase in electricity power generation. You could do things such as to use the fuel for cars instead to power generators, and travel less.

Basically, I think it is fair to say that if you had space habitat type colonies on Earth they don't ned to use more than their "fair share" of Earth power generation to produce all their food using LED lighting. But they would have to be very efficient in how they use it and how they do everything else and it would be a huge effort to upgrade the whole Earth to a system like this.

However we don't need to worry that something like that could happen right now. Our asteroid surveys are already complete for NEO's of 10 kilometers upwards, and we have plotted their orbits. We now know that there are no asteroids as large as the Chicxulub one headed our way at least for the next several centuries.

What about comets? Well , less than 1 in 150 NEO's are NEC's (see the numbers of NEO's and NEC's in this table) so comets are very rare at present. That makes chance of one of those probably less than 1 in 150 million per century (given that there is about a one in a million chance of a 10 km diameter asteroid impact per century).

So, we can forget about ten kilometer diameter comets and asteroids, at least for a few centuries. If we find one headed our way, with a lead time of a couple of centuries, we have plenty of time to do something about it.

Now if we go back to a bit over three billion years ago, that's a bit different. We had impacts back then of objects of 30 miles diameter or more. There is evidence that they boiled our oceans. The ocean levels dropped probably by of the order of 100 meters, temperatures 500 °C for weeks, sea boiling for a year. After something like that, the land would be completely sterilized of life. We couldn't survive that, except perhaps in a colony on the ocean floor. A subsurface self sufficient sea colony could survive something like that, perhaps more easily than a space colony. But luckily we don't have to prepare for this. More on this in the next section.

We haven't found all the one kilometer asteroids yet, but that risk is pretty much retired too with only 5% left to find, less than 50 probably.

By far the most likely impact is of the order of tens of meters up to perhaps 100 meters or so. However we shouldn't exaggerate the likelihood even of those. A 100 meter object would be far larger than the largest meteorite strike in all of recorded human history. We certainly have plenty of reason to track asteroids and protect ourselves from them. It is the one natural disaster we can predict exactly to the minute (and so, evacuate the impact zone and take precautions for those who live further away) - and given enough time,we can prevent it too, by deflecting the asteroid. The largest ones are devastating, but they are low probability, far less likely than volcanoes, earthquakes and tsunamis. And they can't make us extinct.

So, small impacts are possible, yes. Larger impactors 10 km across are possible, though there is none headed our way for a few centuries. However those would not make us extinct. The very large ones 100 km across probably could make us extinct, melting continents and boiling oceans, but we don't get those any more, and haven't had their like for billions of years. We haven't had their like since the end of the late heavy bombardment, and Jupiter protects us from them in the present day solar system.

So, there is no way we are going to go extinct from an asteroid impact.

What about a comet as large as Hale Bopp (60 km in diameter)?

We do occasionally get large long period comets come our way. And this one is indeed a whopper. At 60 km in diameter, it would be likely kill all land life and also destroy surface life in the oceans too, boiling away perhaps a hundred meters depth of the sea. .

Luckily there is no chance of this in the near to medium future. In the long term, it has a chance of about 2.54 per billion of impact with each encounter, in an orbit that currently takes over 2,500 years to complete. Right now the orbit crosses the ecliptic some distance away from Earth’s orbit, further away from the sun.

(click to watch on YouTube)

It passed us by in the 1990s and we won’t see it again for another 2,500 years. So, our civilization doesn’t need to worry about it for a fair while. Its impact if it were ever to hit Earth would be particularly devastating because of its inclined orbit, with an impact velocity of 52.5 km / sec.

We do have historical impacts to compare it with, from over three billion years ago, as described in the Smithsonian magazine:

“Researchers led by Don Lowe of Stanford University describe the effects of two asteroids measuring 30 to 60 miles across that hit about 3.29 and 3.23 billion years ago. (For context, the asteroid that killed the dinosaurs was probably a measly six miles across.) The dual impacts sent temperatures in the atmosphere up to 932 degrees Fahrenheit for weeks and boiled the oceans for a year, long enough that seawater evaporated and they dropped perhaps 328 feet. The researchers reported their findings in the journal Geology.”

Asteroid Impacts Once Made the Earth's Oceans Boil for A Whole Year

In metric units, that’s air temperatures of 500 °C and evaporating 100 meters of ocean.

The paper is here: Geologic record of partial ocean evaporation triggered by giant asteroid impacts, 3.29-3.23 billion years ago

“recent studies in greenstone belts indicate that asteroids 20 km to 70+ km in diameter were still striking the Earth as late as 3.2 Ga at rates significantly greater than the values estimated from lunar studies. We here present geologic evidence that two of these terrestrial impacts, at 3.29 Ga and 3.23 Ga, caused heating of Earth’s atmosphere, ocean-surface boiling, and evaporation of tens of meters to perhaps 100 m of seawater”

Geologic record of partial ocean evaporation triggered by giant asteroid impacts, 3.29-3.23 billion years ago

Something like that is just not survivable. Not even a 30 to 60 miles (50 to 100 kilometers) impact. There would be no higher life left on land with the 500 °C atmosphere. The land would be sterilized to some depth but rock is a good insulator. A few hundred meters below the surface, you wouldn't notice it.

With the oceans boiling there'd be hardly any photosynthetic life left either. Just microbes - and deep sea fish and giant squids and the like would survive. Not whales though, they can’t hold their breath for weeks for the atmosphere to cool down.

If this happened naturally on some other planet like Earth, without intelligent species to do a "backup", then perhaps the land would be recolonized by the descendants of squids , and crabs, which have the ability to survive on land for a short while already.

I suppose theoretically humans could survive this without leaving Earth,t if they could set up a deep sea colony, or one buried deep below the surface of the land in caves, insulated from the surface. The ocean colony seems somewhat easier perhaps. But it might well be easier to survive such an impact in space, rather than to be trapped beneath a 500 °C atmosphere for weeks and a boiling sea for a year.

This would be the type of impact where our backup on the Moon would be really useful, and would let us recolonize the land quickly, not having to wait for the squids, crabs, and whatever might remain of photosynthetic life to do the job.

However the chance of Hale Bopp hitting Earth is tiny as we haven’t had an impact this large for over three billion years, and nor has the Moon, Mercury, or Mars. Also even close flybys by comets are rare. The closest is Lexell's Comet which passed by at only 6 lunar distances on July 1st 1770.

I have tried to find an article on the frequency of Hale Bopp or Swift Tuttle sized impacts but can’t find anything. Do say if you know of anything. Meanwhile, as a “back of the envelope” type first estimate, perhaps they happen at intervals of at least three billion years since that’s how long since the last one? That’s probably an over estimate of how often they happen, as back then Earth was still getting hit by more large objects than it is today, in the very tail end of the so called “Late Heavy Bombardment”.

That would make the risk per century of a land sterilizing impact like this less than one in thirty million, and probably far less.

So, how does that fit with the existence of Hale Bopp? Well it’s in an orbit that will let Jupiter deflect it from time to time. Over that time, it can hit Jupiter or the Sun, or, since it’s made of ice, then it can gradually evaporate away during close approaches as will probably happen to comet Halley. Also, it’s likely to be disrupted by Jupiter into multiple smaller comets during close flybys, as happened to comet Shoemaker Levy before it finally hit Jupiter. It could also just break up into two or more comets as it evaporates away some of the ice holding it together.

At any rate, Hale Bopp won't hit us for thousands of years.

The object that’s often mentioned as most dangerous for Earth in the very long term is Comet Swift Tuttle. We now know that it can’t get closer than 80,000 miles (130,000 km) for the next several centuries. But over thousands of years it can get closer. There’s a there is a one in a million chance that it will hit us in 4,479 AD. It’s not very likely and we have thousands of years to develop the technology to do something about it if needed. See my answer to What will happen if Comet Swift-Tuttle strikes the Earth in 2126?

Comet Swift Tuttle does close approaches to Earth - most recently in 1737, and 1862. The approaches below the shaded area are visible from Earth. The next such are in 2126, 2261 and 2392. However it can’t hit Earth on its current orbit as we now know, after working it out more exactly.

There’s a chance that it can hit Earth 4479 years from now, but it is only a one in a million chance. It would be devastating, not as bad as Hale Bopp but far worse than the dinosaur era asteroid impact. But we have more than 2000 years to develop the technology to do something about it in the remote case that we need to.

Even large asteroids and comets can be deflected given a timeline of decades or more to do something about it as then, only a tiny nudge to change its velocity will be plenty to change an impact into a miss decades later. A deflection of only two centimeters per second, the speed of a garden snail, can change a direct bullseye hit into a miss a decade earlier. If it does a fly past of Earth through a keyhole, before it hits, and you can deflect it a decade before the flyby, then you are talking about a nudge of microns per second. On those timescales, if we continue with our present day technology, never mind what we might develop by way of new methods by then, we could have many ways to deal with Hale Bopp. With over 2400 years before the encounter we have plenty of time to do something about it.

For more about impact hazards see my

• My answer to "How vulnerable are we to a catastrophic asteroid impact? Are there people whose job is to watch for them?"
• Supplementary material to that answer

See also:

• What can we do to retire the risk from asteroids quickly?
• My answer to How vulnerable are we to a catastrophic asteroid impact?
• Effects of asteroid impacts, from tens of meters to hundreds of kilometers
• Has anyone died from being hit by an asteroid?

I also look at all this in more detail in my

• Giant Asteroid Headed Your Way? - How We Can Detect And Deflect Them

Also available on kindle as:

Giant Asteroid Is Headed Your Way? : How We Can Detect and Deflect Them (Amazon)

(Though as of writing this, it needs an update with new material on long period comets like Hale Bopp).

Supernovae, gamma ray bursts and passing stars

We know our stellar neighbourhood so well now that we can say with a certainty that there is no near future risk from a supernova. The type 1a (or b or c) supernova have harder to spot precursors than a type II supernova, but we can now say for sure that there is no possibility of one of those either. Also the only gamma ray burst candidate facing our way is now known to be angled away from Earth by more than 40 degrees (and is far too far away to make us extinct).

As for a star or rogue planet, the chance is so tiny we can forget it.

In detail: Let's start with this account of an interview with Elon Musk, the author Ross Anderson presents the main risks as:

"A billion years will give us four more orbits of the Milky Way galaxy, any one of which could bring us into collision with another star, or a supernova shockwave, or the incinerating beam of a gamma ray burst. We could swing into the path of a rogue planet, one of the billions that roam our galaxy darkly, like cosmic wrecking balls. Planet Earth could be edging up to the end of an unusually fortunate run."

But they give no figures here. So let's supply them. Calculation indented, and coloured dark red, to make it easy to skip:

There's a formula, we can use here, from Perturbation of the Oort Cloud by Close Stellar Approaches. Our sun has approximately 4.2×D² encounters with other stars every million years where D is the diameter in parsecs of the region. Neptune's semi major axis is 4.49506 billion kilometers so the diameter of its orbit is around 0.00029135 parsecs. So, using that formula, every million years there is 1 chance in 1/(4.2×0.0002913²) of a star passing closer to the sun than Neptune.

That makes it about one chance in 2.8 million of a star passing closer to the sun than Neptune every million years. There may be twice as many rogue planets as stars, so that means one chance in 1.4 million of one of those passing closer to the sun than Neptune in the same time period. There may be as many brown dwarfs as stars in the galaxy as a whole, but the WISE survey showed that there is only one brown dwarf to every six stars in our vicinity.

The combined probability then is a little over 1 in a million that either a brown dwarf, star, or rogue planet passes that close in the next million years, so around one in a billion that any of these types of object get as close as Neptune in the next thousand years. Neutron stars or black holes are even more unlikely, about one star in twenty is a neutron star and only one in a thousand is a black hole. So we don't need to worry about any of these on the thousands of years timescale.

For more about this, see my Debunk: Our Sun or Earth could be hit by a rogue planet, neutron star, black hole, brown dwarf or star

Gamma ray bursts are possible also, but would not make humans extinct, even if very close. Our atmosphere shields us completely from gamma rays, which is why gamma ray telescopes have to be flown in space. The only reason we can see gamma ray bursts at all is because of our space observatories. The main effect of a gamma ray burst is on the upper atmosphere, and particularly, on the ozone layer. There was a theory at one point that this could through various interactions lead to increased nitrous oxide levels which could then lead to elevated ozone layers at ground level and so cause extinctions. The theory predicted elevated ozone layers at ground level, and reduced ozone layers and an ozone hole at the level of the ozone layer.

However this theory has been shown to be false by more detailed modeling. Yes, a gamma ray burst does raise ozone levels at ground level, but it doesn't do so by nearly enough to harm us. To find out more see the research announcement from NASA here: How Deadly Would a Nearby Gamma Ray Burst Be? The paper itself is here.

 

The gamma ray burst not only reduces the amount of ozone in the upper atmosphere. It also creates ozone depleting nitrogen oxides. They took the example of a gamma ray burst which hits the south pole most severely, as that has down drafts of air constantly. Those would bring the nitrogen oxides down to the lower atmosphere which is why you see the red regions descending with time. This causes a series of pulses of ozone depletion in the upper atmosphere which then leads to increases of ozone at sea level as the red regions let more UV through to the lower atmosphere. The model assumed a 100kJ/m2 burst from the direction of the South Pole, for a gamma ray burst within a few thousand light years of Earth (that’s very close compared to the diameter of the galaxy of 100,000 light years).

A very nearby gamma ray bursts could raise the ozone levels at ground level temporarily to 10 ppm. To be harmful to animal life it would need to reach 30 ppm. It is also not enough to be harmful to ocean life. Even if all the ozone created at ground level got absorbed in the sea, it would not be enough to be harmful to ocean life. So this disproves the hypothesis that a gamma ray burst could be the cause of the late Ordovician mass-extinction.

As a result of this research, the idea that gamma ray bursts could cause extinctions at all, on any scale, is not easy to establish. The main effect would be its damaging effects on the ozone layer which could lead to elevated levels of UV for a number of years. At any rate, if perhaps some other species were affected by the increased UV enough so that they go extinct, they would not make humans extinct. Also the gamma ray burst candidates are a rare kind of object, young Wolf Rayet stars. Of the hundred or so known, only one seems to be pointing our way. That's WR104, 8,000 light years away.

It does indeed look from that photograph as if it is facing us nearly face on. However, spectroscopic observations of the star suggest it’s axis is at an angle of 30° - 40° (possibly as much as 45°) which would mean it would miss. See WR 104 Won't Kill Us After All - Universe Today

For more details see my Debunked: A gamma ray burst could make humans extinct

It's the same story also for a nearby supernova. They are short, violent events, and again we are protected by our atmosphere from the worst effects, equivalent to ten meters depth of water in mass above us. See What’s a safe distance between us and an exploding star? And for more details, the paper here: Could a nearby supernova explosion have caused a mass extinction?

They find that a supernova within 32 light years (ten parsecs), which should happen every few hundred million years would not heat up Earth significantly, and would not be bright enough to harm the ecology through the light alone. In the year after the event you’d get as much ionizing radiation as you get normally in between a decade and a century. So the increase in ground level ionizing radiation is significant but it doesn’t seem to be enough to be devastating.

Also, are there any nearby supernova candidates? We can't predict when a star will go supernova exactly, but the only stars that can go supernova are ones that are at a particular stage in their life, and they have to be massive too, for Type II supernovae, and for type Ia the supernova candidate needs a white dwarf companion. Our sun can't go supernova at all, because it's too light.

The Type II supernova candidates are easiest to see, bright massive stars, larger than our sun, which collapse to a neutron star or black hole at the end of their lifetime. The red supergiant Betelgeuse will explode some day. We know this for sure. It could happen today, but is much more likely to be a long time into the future. It could happen a million years from now. However, it is far too far away to be any problem for Earth, nor is it close enough to be a second sun in our sky. It will just be a very bright star for us. It will be an interesting sight for astronomers, and give them a great chance to study a supernova close up. For everyone else, just a very bright star. Briefly, the brightest star in the sky. For details see: Betelgeuse will explode someday.

Eta Carinae is another star that can go supernova. It’s a “blue supergiant” - which shows it’s not just red giants that can go supernova. This is a very young, super hot star 8,000 light years away and it may explode in the next few hundred thousand years. It’s also far to far away to harm us. There are no type II supernova candidates close enough to harm us.

The other type of supernova is a Type Ia supernova (with some variations on it as Ib and Ic). What happens is that a red giant star dumps gas on a white dwarf companion, which then explodes. These used to be the "dark horses" which we couldn't detect easily, leaving the possibility that there might be a nearby one that would cause problems. However, as a result of the recent modern sky surveys, we now know that there are no nearby candidates for a type Ia supernova either.

The closest type 1a supernova candidate is IK Pegasi. At 150 light years away, this star system is far too far away to harm us. It’s moving away from us and the scientists think it won’t go supernova for several million years, by which time it will be perhaps 500 light years away. It would need to be within 30 light years to be harmful.

So, what about the type Ib and type Ic supernovae? These happen when a star loses its outer envelope, for instance to a companion star - and then the naked core of the original star collapses. Type Ib and Ic supernovae. But there are none of those nearby either. Here is a list of the nearby List of supernova candidates See also: The closest supernova candidate? - Bad Astronomy

So, though a nearby supernova within 30 light years could harm our ozone layer, right now there are no candidate stars that could go supernova, that are close enough to harm us. We get supernovas quite often and they leave rather beautiful remnants. This happens roughly once a century, though many are so obscured by dust and gas that they can't be seen with the naked eye from Earth. For instance Cassiopeia A which was recorded in the mid seventeenth century as very faint sixth magnitude star by John Flamstead on August 16 1680, he didn’t know what it was. For more details see my Debunked: Earth is threatened by a supernova

For those that worry about such things, I'd like to just add, that both of these are extremely unlikely events. They are rare events that happen occasionally in an entire galaxy, and can be seen from an immense distance, and are most often spotted in distant galaxies as well.

• We are at no risk from a supernova for hundreds of thousands of years into the future, as the nearest supernova candidate is 150 light years away.
• We are almost certainly at no risk from a gamma ray burst as the nearest gamma ray burst candidate is tilted at an angle of 30° - 40° (possibly as much as 45°) which would mean it would miss.

Also, supernovae and gamma ray bursts wouldn't send noticeable amounts of ionizing radiation down to ground level. Our atmosphere is equal in mass to ten meters thickness of water above us. That's plenty to keep out the radiation from supernovae and gamma ray bursts.

In both cases, it turns out that the main damage is to the upper atmosphere, to the ozone layer, leading to increasing UV for a number of years until the ozone layer heals. At the moment of the supernova or gamma ray burst then large amounts of UV light do penetrate our atmosphere - but UV is easily blocked out by a shadow. Also, a supernova or gamma ray burst only affects the side of Earth facing the event. None of this could be a human extinction event. We’d just have to wear broad brimmed hats, or protective clothing or use sun cream to avoid the risk of skin cancer from UV light as a result of damage to the ozone layer. I go into this in detail in my "Debunking Doomsday" gamma ray burst answer so take a look at that article to find out more.

Supervolcano

Here we are talking about a massive “supervolcano” eruption, which happens rarely. The main effect is on the climate. It would reduce the temperature globally by about ten degrees centigrade for a decade. Crops in the region near to the volcano would also be smothered by layers of ash in the year of the eruption - how much of a difference that makes depends on when it happens in the year. Jets would be grounded after the eruption for some time, over a wide area until the dust settles.

The numbers of supervolcanic eruptions for the whole world (not just Yellowstone) vary between 1.4 and 22 every million years, making the chance of a supervolcano in any century between one chance in 500 and one chance in 7000 approximately. That’s quite a low probability.

They are very rare and for Yellowstone, for instance, it’s far more likely that the next eruption is non explosive, with flowing lava. There have been 80 non explosive eruptions in the last 640,000 years since the last supervolcano eruption. An eruption like that would disrupt activities in the Yellowstone national park itself, but it’s likely to lead to few deaths and would not be catastrophic. The average of the two intervals between the last three major past eruptions is 740,000 years and that’s the basis of the often quoted 1 in 740,000 chance of an eruption per year - or 1 in 7,400 per century. But that’s not a very compelling argument. Some scientists think that it may not erupt as a supervolcano again ever. They think it may be winding down in its activity.

Also our understanding of these large volcanoes has moved forward and with modern understanding they think that if it did happen, the build up to a supervolcano in Yellowstone would be detected weeks in advance, perhaps months or years. Volcano Hazards Program YVO Yellowstone

We could prepare for a supervolcano by testing the crops we’d need to grow in the cooler world for that decade, With enough warning we could also store crops that are normally used to feed animals or to make ethanol to help tide through the first year after the eruption. A super volcano doesn’t threaten our survival as a species, and is also very unlikely, but it is something that is worth giving some thought to.

“The term “supervolcano” implies an eruption of magnitude 8 on the Volcano Explosivity Index, indicating an eruption of more than 1,000 cubic kilometers (250 cubic miles) of magma. Yellowstone has had at least three such eruptions: The three eruptions, 2.1 million years ago, 1.2 million years ago and 640,000 years ago, were about 6,000, 700 and 2,500 times larger than the May 18, 1980 eruption of Mt. St. Helens in Washington State.” Yellowstone Volcano & Supervolcano

Grand Prismatic hot spring, Yellowstone National Park. Estimate of 1 in 700,000 chance of an eruption per year (1 in 7,000 per century). which could kill 90,000 people. See What would happen if Yellowstone’s supervolcano erupted?

But how much effect would it have?it’s not at all certain the next eruption would be a supervolcano - indeed it’s far more likely to be an ordinary eruption. There have been 80 non explosive eruptions in the last 640,000 years since the last supervolcano eruption. So that’s the most likely eruption by far. The last 20 of those were mainly lava flows. An eruption like that would disrupt activities in the Yellowstone national park itself, but it’s likely to lead to few deaths and would not be catastrophic.

The average of the two intervals between the last three major past eruptions is 740,000 years and that’s the basis of the often quoted 1 in 740,000 chance of an eruption per year - or 1 in 7,400 per century. But that’s not a very compelling argument. Some scientists think that it may not erupt as a supervolcano again ever. They think it may be winding down in its activity.

Also our understanding of these large volcanoes has moved forward and with modern understanding they think that if it did happen, the build up to a supervolcano in Yellowstone would be detected weeks in advance, perhaps months or years. Volcano Hazards Program YVO Yellowstone

First, how likely are they? There’s an estimate here of the probability of a supervolcano happening anywhere in the world, and its effects. They say that there have been 42 supervolcanoes in the last 36 million years. However those came in two pulses and the rate varies between 22 events per million years and 1.4 events per million years.

The worst supervolcano in recent times was the one that created Lake Toba in Indonesia about 75,000 years ago. It’s 100 kilometres by 30 kilometres, maximum depth 505 metres.

Danau Toba

Its ash covered Malaysia to a depth of 9 meters, there’s an ash layer from it in central India that’s still 6 meters thick today, and ash from it is detected as far away as Lake Malawi in East Africa.

It injected 2500–3000 km³ of debris into the atmosphere, and probably killed 60% of the human population worldwide, mainly through climate change impacting on their food supply.

That’s the picture generally, that the main effect is through global climate change, which reduces the temperature globally by about ten degrees centigrade for a decade, together with the direct effects of the deposits of ash on their crops. A large supervolcano like Toba would deposit one or two meters thickness of ash over an area of several million square kilometers.(1000 cubic kilometers is equivalent to a one meter thickness of ash spread over a million square kilometers).

If that happened in some densely populated agricultural area, such as India, it could destroy one or two seasons of crops for two billion people. It would also mean that you can’t fly jets in the affected area for as long as the air is filled with ash, but that’s a minor effect compared with the rest of the devastation. As for the “noxious gases” such as sulfur dioxide - these mainly make a difference to the upper layers of our atmosphere by combining with water vapour to create clouds that block out the sun. It’s not like you’d have trouble breathing or anything like that globally - the amounts of gas are far too small for that, it’s manly the effect of dust and clouds on the climate.

We could prepare for this. With just a couple of years of warning, we could do large scale tests to work out which crops we’d need to grow in the cooler world for that decade, and store crops that are currently used to feed cattle or for production of ethanol, for human consumption instead, which would be enough to give us a buffer for the first year. In that way we would be able to avoid perhaps all deaths from starvation, so impact would be far less than it was for early man.

For details, see Extreme Geohazards: Reducing the Disaster Risk and Increasing Resilience from the European Space Foundation, and for the Yellowstone park eruption simulation,: Modeling the Ash Distribution of a Yellowstone Supereruption (2014) which summarizes Modeling ash fall distribution from a Yellowstone supereruption

This answer is based on those two sources, together with the USGS FAQ here Volcano Hazards Program YVO Yellowstone plus I used some details from the Wikipedia article on Lake Toba - you can check the citations there for more details.

There is also an idea of a way to prevent Yellowstone from a supervolcano eruption and at the same time generating lots of low cost clean energy for many thousands of years. The idea is to do a geothermal power generation using water circulating BELOW the magma chamber. By cooling it from below, there is no risk of triggering an eruption.

The rock in the magma chamber would solidify at a meter per year, so it's only making a difference long term, centuries or thousands of years from now. The originators of the project would never get to see their work make a significant difference but a supervolcano eruption is very unlikely anyway, geologists don't even know if it can still do them (all its past eruptions have been normal ones for thousands of years). So it's rather forward looking thinking, preventing a super eruption centuries or thousands of years from now. The same thing could be done with all our volcanoes eventually if we wanted to. Details here

Anyway - it's nowhere near a human extinction event, though it is potentially very devastating.

That covers all the natural disasters that get mentioned. So - it doesn't seem likely to me that there is any natural disaster that would even set us back a century, never mind stopping space exploration altogether.

Global warming and climate change

As for human caused global warming, the effects are often exaggerated. They are serious for sure, but It's not an apocalyptic scenario. It doesn't lead to an uninhabitable world.

It leads to a world that is more habitable in some ways, less so in others. Indeed one long term effect of global warming is to prevent the next ice age, so on the very long timescale like that, it actually makes Earth more habitable and the climate more stable. The problem is the pace of the change.

We can't tip Earth into a new Venus. That idea dates back to a speculation by James Hansen in his book "Storms of my grandchildren" which was soon disproved. If this was possible, then it should have happened already in the Palaeocene-Eocene Thermal Maximum when the temperatures at the poles reached 10 °C at the poles, compared to -2 °C today. They went on to do detailed studies and models, and showed that it just can't happen. We'd need to burn at least ten times the global reserves of oil, gas, shale, coal etc. There is a "moist greenhouse we could reach, but that too is not possible right now, even if we burnt all the available fossil fuels.

We could make the hotter areas of the world uninhabitable to humans without technology, but that the Paris agreement pledges so far should be enough to prevent that.

Now, it's true that if we had "business as usual" we could make Earth significantly less habitable. We can make some cities in the Persian gulf so hot that they become uninhabitable for a few hours every decade or two, that's well within what could happen in the near future. We can flood many coastal cities in worst case. We can make the oceans more acid - but they have been acid many times in the geological past, it's a disaster for corals but it's not an uninhabitable world - we'd end up with sponges thriving in the place of corals, eventually, as has happened many times in the past.

If we persisted with business as usual, increasing temperatures by ten degrees or more, then eventually in the more distant future we could end up with a world which is only habitable to humans in colder places in the higher latitudes. However, none of those come remotely close to making humans extinct.

In short, climate change won't make us extinct. For details see my Hawking Says Trump Could Tip Earth To Hot Venus Climate - Is It True? What Can Earth's Climate Tip To? and my Debunked: Climate change will make the world too hot for humans

Also, we do not face a food shortage crisis in the future either, or if we do, the reasons will be social and political rather than scientific. The population is headed for 10 billion, maybe 11 billion with middle of range projections. Our world has a food surplus. We prevented starvation for billions through the often forgotten Green Revolution between the 1930s and the 1960s. If it is needed, then with biointensive agriculture we can grow the same food in about a tenth of an acre that normally requires an acre. Then, we can take this far further, if needs be, with the advanced methods suggested for space settlement with aquaponics, aeroponics, and conveyer agriculture. We could get the same amount of food that normally requires an acre from just 30 square meters using space settlement style agriculture. See Human settlement and exploration - hugely positive or hugely negative - it all depends how it is done (above) with links for more on that.

Risks from self replicators

I do agree with Nick Bostrom and others on the risks from self replicators. First with nanobots - yes they may need great care. But I think we are a long way away from achieving that. I'll believe it is imminent when we have a self replicating city on the Moon, or self replicating solar panels built from the sand in the Sahara desert.

I also agree that we have a significant risk from artificial self replicating life made in a laboratory by modifying Earth life. We can already create life with six bases instead of the usual four. That's old research now but the researchers have just gone one better and found a way to add an extra pair of bases that is retained indefinitely, not just expunge it after a few generations. They added various features to their microbe including a "spell check" that let it hold onto the new bases for 60 generations which they think means it can hold onto them indefinitely.

They don't mention it but I think we are at risk from a sample return of extraterrestrial biology. Not an extinction risk, from either of those, from the reasons given in Worst case, almost certainly not an extinction event - rather - debilitating to habitability of Earth, but a significant risk of decreasing the over-all habitability of Earth for humans.

Summary - humans are hardy - no natural disaster can make us extinct - and technology unlikely to do so also - though there is much we can do to make our world less habitable or to help preserve its habitability

So, in short, no natural disaster can make us extinct That's not because we are immune from such things. It's because we are lucky enough to live in a quiet time of our solar system in a quiet part of the galaxy, far from the central black hole or anything hazardous to our solar system. We are also lucky enough to be an omnivore able to survive almost anywhere on our Earth with minimal technology. We are a very hardy species, so long as we have minimal technology and basic understanding of things such as fire, clothes, making a shelter, and agriculture.

There is no need to rush into space and ignore safety precautions for potentially risky actions, such as returning a sample of extra terrestrial biology. Indeed, that could be the very thing that makes ETs extinct, why wouldn't it? I don't think it will make a species as hardy and adaptable as us extinct, but it could in the worst case make our Earth significantly less habitable for us.

Further ahead, if the space colonists like Elon Musk achieve their dreams, then that's a future with space technology in the hands of millions of people who will have the ability to move objects at many thousands of miles an hour. How is that a safer future than one in which we focus on working to protect and save Earth? When Zubrin and others are advocating to push ahead into space, and stop being so safety conscious about what we do there - that may be the very thing that we need to watch out for.

The only way we can be harmed is by us doing careless things without the right safety precautions. One of those ways, and our main focus here, might be returning extraterrestrial life to Earth without checking first (by studying it carefully) to see if it could harm our biosphere. We can prevent this happening by being careful. We need to take similar precautions to the ones we'd use for working with new forms of life in a laboratory not based on DNA. But more so, because at least in that case we know a lot about the new forms of life, while if it is extraterrestrial biology returned before we have a chance to study it in situ, we have no idea what form of life is in the sample, what its biochemistry is and what its capabilities might be..

'm not sure we need to focus so much on the very improbable event of humans going extinct. I've looked hard at all the scenarios and I don't see it as plausible in the near future (apart of course from voluntary extinction).

However habitability of Earth is another matter altogether. Should that not be our priority instead, to work to ensure long term optimal habitability of the Earth? Which might well involve use of resources in space and knowledge from studying other planets. It might even be that things we learn from studying Martian biochemistry lead to revolutions in understanding biology which would then help us here on Earth.

Perhaps, rather than focusing on risk taking to attempt almost impossible goals of starting a new colony in an airless lifeless desert six months travel away by interplanetary spacecraft, our focus should be to protect the habitability of Earth? If so, then we need to take especial care about a sample return of extra terrestrial biology as well as anything else that can significantly impact on its habitability and the well being of billions of people.

What do you think?

Need for public debate - decision for all humanity

Whatever you make of those arguments, I think there is one thing most of us may all be able to agree on. That is, that if something that can potentially impact on the habitability of Earth itself, even if the risk is low, then it is a decision for all of humanity.

I wouldn't even call it ethics or philosophy quite, or at least not in the sense of the ethics or philosophy you do as an individual in your study, or the sort of thing you could delegate to experts on ethical or philosophical matters to sort out for the rest of us. Instead, it's a political decision, I think, or closer to politics than anything else. It is a major decision that could potentially be made by a few people that would have repercussions for large numbers of people, potentially, the whole population of Earth.

It is similar, for instance to the political decisions on climate change. At a certain point, you've done all the research you can do. You haven't achieved certainty, but if you wait for certainty, it's too late because you have to make the decision now, or in the near future, too soon to complete all the research you'd like to do. The ethics and philosophy consists in clearly presenting what the issues are. The science consists in presenting the results of the research, again, as best one understands it, including any divergences of opinion. But none of that can lead to a decision - not in a situation like this of imperfect knowledge. So then, the final decision I think enters the realm of politics.

In this political debate, which so far hasn't spread far beyond the discussions of enthusiasts, some people judge that it is okay to take small one in a million or less risks of degradation of the environment of Earth, for the benefit of all of humanity. Others, following the example of Sagan, don't think we should take such risks, tiny though they probably are. Does any small group of colonization advocates, or scientists, or philosophers, have the right to take such a bold decision for everyone else? Given that we are all potentially affected by it?

The theologian Richard Randolph (in 2009) examined this in detail from a Christian perspective and came up with some recommendations. First he looks at why it needs public debate:

"the problem of risk - even extremely low risk - is exacerbated because the consequences of back contamination could be quite severe ...the consequences might well include the extinction of species and the destruction of whole ecosystems."

"Humans could also be threatened with death or a significant decrease in life prospects"

The last case of "Humans could also be threatened with death or a significant decrease in life prospects" brings this into the region of existential risks. He argued that this makes it not a technical problem for scientists to study based on the principles they already know how to apply, but an ethical problem requiring extensive public debate at the international level.^/

He puts forward four criteria to ensure a full and open public debate.

1. Follow best practices of planetary protection (already being done).
2. Opportunities should be available for open comment from those concerned about back contamination. These should be taken seriously and NASA should publicly respond to them.
3. A committee should review the measures, including experts in ecology, biology, chemistry, risk analysis and ethics. Ethicists should represent a diversity of philosophical and religious perspectives.
4. The entire process must be transparent to the interested public.

Legal liability in case of damages

The ESF report considered the legal situation, and came to the conclusion that if there is a release of the contents of the MSR capsule during return to Earth, then the situation is clear, the state responsible has unlimited liability, without any ceiling, in respect to any detrimental consequences on Earth

"Under the Liability Convention (United Nations, 1971), the launching State is liable for “damages caused by the space object”. If a sample has detrimental consequences on Earth, it may be considered that the State having launched the spacecraft is liable under this convention (absolute liability without any ceiling either in amount or in time; Liability Convention Article 1 – loss of life, personal injury or impairment; or loss of or damage to property of States or of persons, natural or juridical, or property of international intergovernmental organizations)"

(emphasis mine)

They also examined the case where the release happens after the capsule has returned to an Earth laboratory, and damages follow. They concluded that in this case the situation is less clear. The unlimited damage clause may still apply, or they might instead be responsible for an illegal act under general international law in violation of Article IX of the Outer Space Treaty, which doesn't have the same provisions of unlimited liability.

How can we protect Earth during a sample return mission?

So, a sample return is not only an expensive mission, it also raises unprecedented issues of planetary protection for the Earth, and especially so if the sample is returned at such an early stage, when the planners of the mission can have no idea what is in the sample of biological interest.

If NASA does go ahead with the second half of their proposal, and they do a sample return, at that point they are entering into a course of action that Carl Sagan and others have argued is something we should only do with great caution. Many (though by no means all) would say that a 1 in a million chance of sample release is not good enough. So what can we do to protect Earth. One way or another, we will want to return samples eventually. The astrobiologists want to do this later, and prioritize in situ studies right now, the geologists sooner, but both are agreed that it is something to do at some point in the future.

Safe return of unsterilized sample

Just about all are agreed, that if we return an unsterilized sample from Mars, we have to take precautions to protect the Earth. That's the conclusion of all the studies into the back contamination risks of a Mars sample return to date.

Others take it further and say that we simply shouldn't return such a sample to Earth, until it is certified as "biosphere safe". That's the view stated in the ICAMSR charter.

"Having planetary/cometary samples certified as "biosphere safe" in space or in-situ before they are transferred to the Earth’s surface is our main goal and intention"

Though I might not agree with everything else they say there, I also think that by far the safest way to deal with a sample return is to return it to somewhere else, perhaps above GEO is my own suggestion. I also would say that we should only return sterilized material to Earth until the rest of the sample is certified "biosphere safe".

But first, let's look at the approach recommended in the Mars Sample Return studies by the ESF and the NRC. Their approach is based on the idea that we are going to return an unsterilized sample to Earth itself and have to find a safe way to do this.

Note, that in these studies the experts were not given the option to recommend a return anywhere else apart from Earth, for instance, to the Moon or to above GEO. That was not part of their remit. It was "above their paygrade". They were tasked with finding a way to return such a sample safely to Earth itself.

For instance, ESA asked ESF:

" to perform a study regarding planetary protection regulations for a Mars Sample Return (MSR) mission. Specifically, ESF was asked to perform a study on the level of assurance of preventing an unintended release of Martial particles into the Earth’s biosphere in the frame of an MSR mission."

(see mission statement)

This makes it impossible to say what their views would be on ideas to return the sample to other locations. We have the statements of the ICAMSR that we should not return the sample unless it is certified biosphere safe. Then we have the studies of the ESF and NRC, which just assume, without any discussion, that we are going to return an unsterilized sample to Earth. It is so polarized at present.

So far I haven't found a single comparison study that looks into the question of whether it might be safer, and simpler, and cost less, to return a Mars sample to some other location other than the Earth's surface, or to sterilize it before return to Earth. Do let me know if you know of any study like that.

So anyway - let's see what the experts think we have to do, if we return it to the Earth's surface.

Easier part, sample return to surface in a container

The (comparatively) easy part is to return a sample to the Earth's surface. That's pretty much worked out. The idea is that you have to break the "chain of contact" with Mars. You make sure nothing that has contacted the Mars surface or contacted anything else that contacted the Mars surface is exposed to Earth environment. The easiest way to do that is to use nesting capsules. The Mars sample is placed inside a larger capsule in Mars orbit. On return to the Earth system, you could indeed put it inside an even larger capsule enclosing both.

Then, you have to make sure that there is no way those capsules can be damaged and release the internal material even in event of a crash landing on Earth.

The main issues there are - that a micrometeorite could pierce the capsule, or orbital debris. Also is there any chance of a bad re-entry that burns up the capsule so that the interior is exposed? Those are valid concerns, but a carefully designed mission might be able to deal with all those. They seem to be addressable issues. For instance, though it would be hard to detect whether there has been any release of materials from the capsule, it's a much easier problem to detect if there has been a breach of it from outside through a micrometeorite or similar. If there has, you can simply say that you can't return it to Earth unless you put it inside a larger container.

As the ESF study put it,

"While sensory systems that detect leakage might be limited in risk protection, potential sensory systems that would detect any penetration of the Earth Return Vehicle to a high level of reliability should be feasible."

This may become a tough challenge if you have to achieve a 1 in a million risk of sample breach, and even tougher if you make the target risk much less than that. Still, it seems a matter of engineering - that if we make the outermost capsule shell thick enough and the capsule robust enough, we could make it so that it can survive anything that happens to it during the re-entry to Earth.

Harder part - human error

Another issue is human error, such as some mistake in design of the capsule that is never picked up all the way through the design process. The ESF study outlined four main ways that Earth could be accidentally exposed to materials from the sample. Most of these involve some element of human error, in the design of the capsule, or the protocols, or deliberate acts bypassing the protocols.

"In principle, there are four main ways for an environmental exposure to be initiated from the accidental/deliberate release a Mars sample into the Earth’s biosphere:

• A break-up of the container during atmospheric entry (due to a design fault or sabotage),
• An unsuccessful full sterilization of the Earth Entry Capsule, potentially having Mars particles attached to its outside surfaces,
• Damage to the vehicle due to heavy impact with the Earth,
• Escape of material during transport or from the laboratory. "

They say that the first two scenarios here could have potential for contamination over a wide area (especially if it breaks up at a high altitude). The last two are scenarios with a localized release. They say from past experience that "it is reasonable to assume that the most likely cause of a release would be due to human error or a deliberate human act following the introduction of the material into the laboratory".

They weren't tasked with considering these factors, as they say in the introduction:

"Upon return to Earth, the sample would still have to be transported from the landing site to the curation facility. While the Study Group was not tasked with considering human factors, it has to be highlighted that the use of human handling in this process and the transport itself entails the risk of human error and the potential for accidental release. For this reason, care must be taken to minimise human interaction with the sample and to provide adequate protection via transport containment to guard against an accident during transport to the curation facility. "

This is the part of the process that's perhaps hardest to quantify. How can you set a probability on human error, or on deliberate sabotage, or ignoring protocols, perhaps because a researcher gets impatient with the precautions?

Hardest part - what do you do next once you have a sample on the Earth?

The hardest part though, is, what do you do when it returns to Earth? If you just wanted to keep the sample in its container for ever, then it is simple, bury it deep below the ground. Maybe you enclose it in synthetic rock. Or simpler still just sterilize it completely with ionizing radiation, or through heat, or through chemical methods, and all is safe and dandy.

But of course that's not what we want to do. We need to study it, in a laboratory, cut bits out of the sample, and move those fragments around. We wish to look at them in many different machines. Eventually to send those samples to other laboratories around the world. You’d think this was easy, but it turns out to be surprisingly complex and difficult.

This is something that started off as something apparently easy to do. Back in the 1990s the general idea was that we can just return samples to a glove box facility in a biohazard 4 laboratory. The idea was - that since we know how to contain hazards such as the Ebola virus etc, surely there would be no problem containing a sample from Mars.Use the same techniques we already use in biohazard laboratories. In 1999 then they recommended use of Class III Bio Safety Cabinets (BSC's) in combination with HEPA filters.

This was still the general idea in 2002. They described the facility like this:

"The initial processing of returned martian samples should be restricted to a BSL-4 laboratory in the quarantine facility. A very modest gas-tight glove box (Class III cabinet) in a "clean room" (class 10; however see following g section) will be sufficient for this purpose. "

The main problem, as they saw it, was to deal with the contradictory requirements of keeping the sample clean, and protecting the worker and facilities from the sample. Biohazard level 4 facilities tend to be rather dirty, with the materials contained by a lower pressure reducing the tendency for biological materials to escape, but doing nothing to prevent contamination in the opposite direction. They recommended a double wall facility with a lower pressure inside the double wall around the glove box, to prevent contamination in both directions - but then if you have gloves that pass through the double wall, that then gives the possibility of making the sample dirty. Glove boxes are clumsy to use also. It might be easier to study it telerobotically, do only simple experiments of the unsterilized material, and sterilize any samples that need to be study in any more detail.

After a series of studies, however, it was realized that it's not as simple as it seemed at first. The precautions got more and more complex. The most recent studies require a facility costing perhaps half a billion dollars or more, with capabilities never tested before.

The main problem is that it is easy to contain a known pathogen, say smallpox, or anthrax, or the Ebola virus etc. Because you know that it needs an animal host (maybe a human host), and know what kills it. But what can you do when you don’t know what is in the sample, what its capabilities are, what size it is, or even what biochemistry it has? And if it possibly doesn’t use DNA? And perhaps it is a spore in resting state, that is highly resistant to ionizing radiation, to oxidising agents like hydrogen peroxide, and other chemicals, able to survive vacuum conditions, etc etc - all of which are very likely to be the case for Mars life?

Also, could the cells be tiny, far smaller than any Earth life? The smallest size for early cells if they don't contain all the machinery of modern life, is generally estimated as about 50 nm.

Successive studies by the National Research Council (NRC) in the USA (two studies) then the European Science Foundation (ESF) (one study) gradually lead to more and more stringent requirements.

First came the reduction to 200 nm by the NRC after discovery of the ultramicrobacteria. Then, that was reduced to 10 nm by the ESF. as a result of discovery of how readily archaea can share their DNA through the tiny Gene Transfer Agents (GTA's) Archaea can transfer genes between phyla that are as different from each other as fungi are different from aphids. It is an ancient mechanism and so may also be able to transfer genes from life that had last common ancestor with us in the early solar system. As we've seen already, In one experiment 47% of the microbes (in many phyla) in a sample of sea water left overnight with a GTA conferring antibiotic resistance had taken it up by the next day. The upshot was that if the life is at all related to Earth life, you have the possibility of this exchange of DNA bringing new capabilities to Earth microbes from space. Even if the microbes themselves don’t survive, their genetic capabilities might.

Another thing that makes the design more complex is the need not just to contain the sample (which is usually done by a positive air pressure from outside) but also to protect it from outside organics (which needs a positive air pressure from inside). You end up with some kind of a double walled facility and they cite this as one of the main reasons why you have to have a new design of building, never tested before. This is one of the designs they came up with in 2008, with telerobotics.

The LAS sample receiving facility uses a fully robotic workforce, including robotic arms that manipulate samples within interconnected biosafety cabinets. Carrier robots would transport the samples around the facility. Credit: NASA/LAS

This is for just a less than 1 kg of samples returned most likely, yet you have to build something like this. And even then, it might not be sufficient.

Every Mars sample return study to date says at the end that their conclusions have to be reviewed continually, based on new research. For the next study, whenever it is - well there is much active research at present into into a semi synthetic minimal living cell or an artificial minimal cell. Does the 50 nm size limit still apply based on the recent research? In the Programmable Artificial Cell Evolution project, the smallest artificial minimal cells were as small as 103 atoms, based on PNA instead of DNA, making it possible to simulate the whole cell as a quantum mechanical system in a computer. These “cells” were just a few nanometers across.

Also there's much more work been done on possible XNA based life, since the most recent ESF study in 2002. I would expect that to feature more in a new study than in previous ones.

Also, none of the studies to date address issues of human error, accidents, terrorism, a crash during transport of the sample to the facility, a plane crashing into the facility etc. The studies done to date do mention these issues, as we saw in the example of the ESF study, but only to say that these issues were not part of their remit. Then, the thing is, that after all this work - we might find that the sample receiving facility wasn't even needed. The samples returned might be completely harmless, as harmless as the Moon samples. Yet just about everyone is in agreement, that we can't assume this in advance, if we take our responsibility for protecting Earth seriously. At least, we can't do it based on our understanding of Mars to date.

It seems a back to front way of proceeding to me. Wouldn't it be better to characterize the sample first before we return it? Then design the facility around the samples once we know what they are?

For more about this see my Need For Caution For An Early Mars Sample Return - Opinion Piece

Legal complexities

Margaret Race (of the SETI institute) covered these in an excellent paper, Planetary Protection, Legal Ambiguity, and the Decision Making Process for Mars Sample Return. There’s far more to it than you’d think, if you are going by the way it worked with Apollo. Although there was a lot of preparation behind the scenes first, the quarantine rules for the Apollo 11 return were only published on the day that they launched to the Moon. This was intentional, to provide no opportunity at all for public discussion and challenges which might delay the mission.

That would simply not be permitted today. The Apollo regulations have been rescinded, so they can't just be "dusted off" and used again. In our current more legally complex world, then the whole process would take far longer than for Apollo.

The process involves several stages that have to go one after another, mainly, an Environmental Impact Statement under the National Policy Act, which would require involvement of multiple agencies, not just NASA, is likely to take several years, and could be held up by legal challenges on top of that. The courts wouldn't tolerate shortcuts. Then next comes the Presidential Directive NSC - 25, which has to be carried out after the EIS is completed, involves multiple agencies again and ends with Presidential approval to launch. It would also involve numerous international treaties and conventions that need to be negotiated and quite possibly also, domestic laws of other nations too, since they would be impacted in the worst case of harm to the environment of Earth.

There are also many questions to be considered when it comes to quarantine.This was never opened to public debate for Apollo. But it leads to major questions of ethics, and of human rights, if you consider the consequences for a worker accidentally exposed to the sample, for instance through breach of the containment, or indeed exposed through an executive decision to ignore the protocols, or some lapse and human error. There is much more to this than you'd realize, once you start to think it through. It was never opened to public debate in the case of Apollo, so was never given attention by lawyers and ethicists.

After reading what she said, I have thought it through in detail, and I think myself that this may be the hardest issue of all to address. Indeed, I think that this may be so difficult to address that it may be more than just a major issue that could take up years of legal process to reach a resolution. To see this ask yourself, what if a human is exposed to microbes that are shown to have a potentially harmful effect on the environment of Earth if they are removed from quarantine, and that there is no way to sterilize them of these microbes? Yet perhaps we have no proof that this is inevitable, it is just a significant possibility. What then happens next, legally and ethically?

She doesn't give an estimate of the timescale, but it's clearly many years, and perhaps getting on for decades, to complete all that legislation. NASA could do things to make it easier to pass the laws, but it's still bound to be a long process, and so far they haven't even begun on it. I go into this in more detail in Current requirements for sample return and legal situation (below) .

Also (my own comment on her paper) - especially bearing in mind the quarantine issues, is it possible that there is no ethically or legally acceptable way to navigate through to a solution? Surely we can have no a priority guarantee that we can find a legal solution, until the issue is studied in depth. I discuss these tricky questions in The knotty problem of human quarantine - and what about exposure of humans during a robotic sample return? (below) . So what do we do if the legislation is actually impossible to pass?

Natural contamination standard - great for asteroids and comets but doesn't work for Mars

This is one way you can simplify issues with a sample return hugely. If you can prove that what you are doing is equivalent to what happens naturally, then the sample return is no longer a biological issue. This is the "natural contamination standard" due to Richard Greenberg.

The basic idea is a good one, of the "natural contamination standard" due to Richard Greenberg.

"As long as the probability of people infecting other planets with terrestrial microbes is substantially smaller than the probability that such contamination happens naturally, exploration activities would, in our view, be doing no harm. We call this concept the natural contamination standard."

From Infecting Other Worlds. (You need a free JSTOR account to read it)

He states it in the forward direction, infecting other places in our solar system with Earth life. However we can also use it in the reverse direction. This approach is used for sample returns from comets and asteroids to Earth. Since fragments of comets and asteroids hit Earth all the time, then it can't add to this hazard, to return a sample from a comet or asteroid. If there is any life on comets or asteroids, which is not ruled out, well clearly we've evolved to be o

The problem with an attempt to apply this principle to Mars is that sending humans to Mars or returning samples from Mars to Earth is not mimicking a natural process exactly enough to count, as we saw in the discussion of Zubrin's meteorite argument (see Zubrin's meteorite argument for safety of a sample return (above) ).

If you wanted to apply the natural contamination standard to a sample return, then it would somehow have to simulate the equivalent of a century of interplanetary cosmic radiation, and vacuum and the cold of space. That's not enough though, as the last sample to get here from Mars so quickly would have got here hundreds of thousands of years ago, in rocks typically around 30 cm in diameter, up to a maximum of 2 meters. To simulate the meteorites arriving from Mars right now, you'd need the equivalent of hundreds of thousands of years of cosmic radiation.

It's worse than that though, as it also depends on whether any meteorite that hit Mars not only hit a habitat for life there, but also sent life into orbit - given that the material that goes into orbit comes from a distance of a few meters below the surface. The surface layers which we will sample in a sample return mission are also the ones most likely to have viable spores if there is present day life on Mars. They consist of dust, clays, and ice which would most likely just be scattered back into the atmosphere and not reach escape velocity after the largest impacts. As for the layers of ice and dust on Mars which could preserve ancient frozen spores, again, those aren't likely to get through the atmosphere at escape velocity either.

So, sadly, we can't apply the natural contamination standard for a Mars sample return.

This is based on zero data about extraterrestrial biology - so doesn't that mean we can ignore the risk?

All of this is based on zero data. However, that goes both ways. We have no proof that it could cause us harm. But we have no proof that it is harmless either.

Perhaps, out of millions of ETs or more that do a similar sample return mission, none of them has ever gone extinct or even experienced any major issues as a result of returning extra terrestrial life to their planet.

However, for all we know, it could be that all Mars like planets have such greatly accelerated evolution that they almost inevitably develop life that is hazardous to all Earth like planets. It may be the main way that ETs go extinct. It might also be the same for Venus like worlds. Or indeed any planet with independent evolution, that it is always extremely hazardous to introduce independently evolved life to a new biosphere like Earth. For more on this see Were we just lucky? Do extraterrestrials sometimes become extinct after their first Apollo style mission to their nearest Moon or planet? (above)

n the case of Venus I'm skeptical of life there, because - how could it survive for hundreds of millions of years or billions of years in the clouds? On the other hand the clouds environment is very stable - and I haven't seen any papers that show a way to disprove it, the data is intriguing, and there are astrobiologists saying it could be life. I'd rather like it to be true :).

In the case of Mars, then I think that though the probability that it is harmful seems likely to be small, it could also potentially be high too. It all depends on the way you think about it.

Here by a "high probability" I don't mean close to 100% (though that also is not impossible). For a sample return from Mars, a 1 in 1000 probability that it is hazardous to Earth's environment seems pretty high to me, given what we would risk.

It's not going to be easy to assign concrete figure to the probability of returning a lifeform from the Venus clouds or Mars hazardous to the environment of Earth. But perhaps we can make a start at thinking about it along the lines of the Drake equation.

Drake Equation style approach to Mars sample return risk probabilities - could the risk of harmful degradation of our biosphere be as high as 1 in 1,000?

The Mars Sample Return studies generally assume that the chance of returning hazardous life is non zero, but very very low. But we don't have any data. Could the risk be actually rather larger than most would think?

Let's try breaking up the problem along the lines of the Drake equation, which is used to estimate the number of extraterrestrial civilizations in our galaxy by breaking it up into component probabilities, e.g. average number of planets per star, fraction of those planets that have life on them etc. We are in a similar situation here, or indeed a worse one, that we have no data on which to base the calculation (we do have some data for the Drake equation on star formation and numbers of stars with planets etc). Still, even though we can't plug in any hard facts as numbers here, breaking it down in this way may be illuminating.

When we don't know what the probability is, I'll just assume that the cases are all equally likely. I'll also be optimistic throughout. The idea is to show you how,if one is optimistic about present day life on Mars, that this can lead to a rather high probability, better than 1 in 1000, that Mars 2020 returns a sample that's hazardous to the environment of Earth. You can try plugging in your own numbers, as for the Drake equation, and see what figures you come up with.

So, first, for the purposes of this calculation, I take it as a 100% certainty that Mars has water that could potentially be available for Mars life to use (not necessarily habitable for Earth life). I'm including the RSLs, (a near certainty of liquid brine in some form), the liquid brine layers Curiosity found indirectly but reasonably conclusively, the droplets on salt / ice interfaces of Nilton Renno's experiments in Mars simulation chambers (again. very likely as they formed so rapidly and perfectly explain the Phoenix let observations), and the idea from the DLR experiments that life could survive almost anywhere using the night time humidity.

Since this is in the direction from Mars to Earth, we don't need to assume that Mars life is limited in the way that Earth life is. It might be able to reproduce at lower temperatures, tolerate higher levels of salinity than Earth biochemistry, and so on. If there is life on Mars and it survived to the present, evolving to the changing conditions there, I'd actually be rather surprised if none of these proposed habitats are in fact inhabited, anywhere on the surface. So with that background, with my "optimistic hat on" as it were, the possibility of liquid water habitable to Mars life seems a near certainty.

So then our first question is, did life evolve on Mars? Or alternatively, did it evolve elsewhere and transfer to Mars? That also depends on how far it evolved before Mars became less habitable.

So that's hard to say. But in the absence of any way to calculate a figure, let's just use a single figure to cover all the possible ways life could get to Mars in the past. It's not likely we can get more exact than this anyway. So, following our approach of assuming two possibilities are equally likely if we have no way to estimate them, let's say:

• 1 in 2 chance of life on Mars in the past

We could also add a probability here that it is related to us or not. But it's hard to say whether being related to us makes it more of a risk, or less so. So I'll ignore that. Let's just treat both cases in the same calculation, for simplicity.

So, is it still there? There's no way to calculate this but let's guess

• 1 in 2 chance that it evolved far enough to be robust enough to persist after Mars got colder and dried up. Either it evolved photosynthetic life, or some form of chemotrophy that it can use in widespread locations on Mars (e.g. eating perchlorates).

Now what is the chance that this life is actually present on the surface right now? We don't know, as it could be in subsurface habitats only, including the deep hydrosphere. So, let's say 50/50 again.

• 1 in 2 chance that if there is life on Mars now, that it is present on the surface

These probabilities combine to a chance of 1 in 8 so far.

So now, is it dangerous? Well, a crude way to look at that is to start off by asking if it is more highly evolved than us. There is no particularly reason to expect it to have reached exactly the same stage of evolution as us.

• Does evolution on exoplanets always proceed at almost exactly the same pace as it does on Earth?
• Or could it be billions of years equivalent of evolution behind us?
• Or billions of years equivalent in evolution ahead of us?

Perhaps Earth life is so far ahead of Mars life, that what we find on Mars is like early RNA world life or similar, long extinct here. But it could be the other way around that the Mars life is well ahead of us because of its turbulent frequently varying past climate and conditions, and that we will only evolve similar life here on Earth a billion years into the future. You can argue both was as we saw in What are the effects of these frequent ups and downs in habitability? (above).

Or perhaps we are both roughly equally far evolved.

With no way to set a probability, let's make all three of those all equally probable, as is our practice in this section. So then there is

• 1 in 3 chance that Mars life is way ahead of us evolutionarily.

So far, the chance of present day life on Mars that is more advanced than Earth life comes out as 1 in 24. In the case where the life is unrelated to Earth life, then I interpret this as meaning that it is all round better than Earth life - better metabolism, more efficient, smaller cells ,etc.

Now, we have to also ask - what is the chance this life occurs in the region Curiosity lands in if there is life on Mars? So then - I'll just guess again, but the evidence that Viking may have discovered life is much stronger now that they found evidence of circadian rhythms. Also, if there is life on Mars right now then there are many potential habitats in the equatorial region,and it only needs spores to reach Mars 2020 to have life in the sample. I don't think it is a sure bet or even more likely than not. But let's call it 50/50 for now as we have no way to estimate this accurately.

• 1 in 2 chance that if there is life on Mars, that there is life in the region that Mars 2020 lands in

(If you think it's unlikely that Viking discovered life and unlikely that there is life in the equatorial regions, then you'd make this number much lower of course).

However, Curiosity might not sample that life. Viking might have been lucky. Also the samples are very small, only a gram, and the total is just under half a kilogram. Given that it has no biosignature detection, the sample might or might not contain life. So

• 1 in 2 chance of returning life in the sample return, if there is life on Mars, and there is life in the region that Mars 2020 samples

(Again there are plenty of good reasons for making this much lower, e.g. 1 in 10 or 1 in 100 chance that it is lucky enough to sample life. But if we suppose life is pervasive in equatorial regions then it might be rather high).

So putting that all together we have 1 in 100 risk so far that Mars 2020 returns life from present day Mars in one of its samples (well 1 in 96 but we might as well round it up).

But it might not be able to survive on Earth, for instance, if it has a biochemistry based on perchlorates and hydrogen peroxide inside the cell and it can only survive in conditions of extreme cold and aridity. So let's call that 1 in 2 as well.

• 1 in 2 chance that it can survive on Earth.

Highly evolved, doesn't mean it is harmful, so let's just make that 50 / 50 again that it is harmful:

• 1 in 2 chance that a more highly evolved lifeform is harmful in some way - to us or to the environment, or to our crops or whatever

Then, if harmful, it could be a minor nuisance or it could be hugely damaging.

• 1 in 2 chance that a more highly evolved lifeform is hugely damaging to the environment of Earth or to us, making our planet significantly less habitable for us.

So now we have around 1 chance in 800 that it is hugely damaging to us. Let's round that to 1 in 1,000. That's far higher than most would guess I'm pretty sure. Especially with this first sample return unlikely to have life in it.

In summary the procedure here is to work out probabilities for:

• Life on Mars in the past
• Life that evolved far enough to be robust when Mars got colder and dried up, and so, still present today on the surface or deep below it
• That it is present on the surface
• That it is way ahead of us evolutionarily.
• That it is in the region that Mars 2020 lands in
• That Mars 2020 caches it.
• That it can survive on Earth.
• That a more highly evolved lifeform is harmful in some way
• That it is more than a minor nuisance (like spoilt food) but is hugely damaging to the environment of Earth or to us, making our planet significantly less habitable for us.

Then you multiply those probabilities together, Drake style, to work out a probability of Mars 2020 returning a sample that is hugely damaging to the environment of Earth.

I have used high probabilities throughout by assuming that if we don't know how to assess the probability, that all the cases are equally likely. In particular I've used a very high probability of life in the equatorial regions, but one I think is not impossible, if you happen to think that it is possible that Viking discovered life already in the 1970s, or are optimistic about the possibility of habitats in the equatorial regions of Mars.

The idea is just to focus our ideas. There is no way we are going to get an agreed on answer, not without much more information, for instance, contact with an extra terrestrial ancient civilization. I make no claim for objectivity, and you will come up with dramatically different figures just by using more or fewer steps, even with the approach of setting the probabilities to be equal if unsure.

However, we may learn something from the variety of possible answers.Try going through putting in your own numbers and see what you think the final probability might be. Are there any steps I missed out that you think should be added? Any steps you think aren't needed? Or, can you come up with a better way of assessing whether or not a Mars sample return is going to be hazardous to the environment of Earth and to assign a probability to it?

Zero data - anything from 0% to 100% risk of sample return degrading the habitability of Earth

That may be an interesting exercise, but whatever you say in answer to those questions is still going to be, at best, a well informed guess. It doesn't matter whether you are an expert astrobiologist either, as their views have often changed. What you say may well change again once we know more about Mars. The real risk could be anything from 0% to 100%.

If we find life on Mars and it turns out to be far advanced over Earth life, and with many capabilities Earth life doesn't have, and it has spores spread through the dust almost everywhere on Mars, the risk from a sample return could be close to 100%, especially if we can prove, following Joshua Lederberg's ideas, that Earth life will be totally naive and have no defenses against it.

The real risk could be 0% also. For instance, if we find Nilton Renno's liquid water on salt / ice interfaces throughout Mars, and it's habitable to Earth life, then that would give us lots of data. If there is no native Mars life in those habitats, or only feeble early life - that might be a good indication that an unsterilized Mars sample return is safe, as otherwise, why isn't it colonized yet. That's not enough by itself. You'd need to know a bit more than that. Perhaps there is some reason those particular habitats aren't colonized which doesn't apply to some Earth habitats. However we might decide quite quickly that the risk from an unsterilized sample return is effectively 0%.

It's the same if we ask the question about Mars like planets generally,

"What percentage of Mars-like planets have lifeforms that are severely harmful to Earth like planets?"

The answer could be anything from 0% to 100%. Perhaps Mars-like planets have greatly accelerated evolution compared to Earth-like planets, and always or almost invariably develop invasive highly capable microbial life. Or perhaps they always have greatly slowed down evolution and have only the most feeble life or no life after 4.5 billion years. There's no way we can find out using near future space probes. If we can't get an answer from Mars, how can we find out by looking at tiny single point images of distant exoplanets?

About the only way that we could get an answer is if we encounter an "encyclopedia Galactica" or similar, some vast store of extraterrestrial knowledge beamed towards us and we can read the section on Mars like planets - and verify that it is a reliable source from other information in it.

That's one way we could have the answer to this, even before the next sample return from Mars, but I wouldn't want to count on it :).

Risks from Mars life if it is at a similar level of development to us

So far I've looked at the example of Mars life that is further evolved than Earth life, maybe with an evolutionary advantage equivalent to billions of years of future evolution.

However, Mars life could also be hazardous if it is at an equivalent stage of evolution to Earth life. It could be ahead of us in some areas and behind in others. In that case, it could still be problematic like the invasive species including invasive diatoms in the Great Lakes (see Invasive diatoms in Earth inland seas, lakes and rivers (above) ) .

It is enough for it to be more highly evolved in one area - e.g. photosynthesis, say, or a more efficient metabolism. That would be like the analogy of placental mammals out competing marsupials in Australia. It could be enough for it to be hazardous to Earth by out competing native life, even if at a similar overall level of development to us.

On that basis, I think there could still be a significant risk of Mars life being hazardous to Earth life if both are at a similar stage of evolution. It all works similarly in the other direction too, of Earth life being hazardous to Mars life.

Niggly minor issues like easily spoilt food in freezers

Then there are also the niggly minor issues. So for instance, assuming it has evolved to a similar level to Earth life or more so, Mars life might very well be far better adapted to extremely cold conditions. We may get blooms of Mars life perhaps in lakes in very cold places where it can survive more easily than Earth life.

Suppose that it can reproduce below -20 °C? The none of our current freezers would be cold enough to stop the food from going off. The main reason Earth life is not able to reproduce in our freezers might just be because it requires several significant evolutionary steps to do this, and it's never been an advantage for it before in the wild. Or, it might be that the Mars life has an unusual biochemistry involving use of perchlorates and hydrogen peroxide as "antifreeze" yet, is also able to survive in warm and wet conditions too.

If that happened, we'd have to build new freezers to freeze food down to -40 °C or whatever it is that Mars life can tolerate.

It's also likely to be far better adapted to ionizing radiation than any Earth life. If our life can already live in reactor cooling ponds - when it has never encountered high levels of ionizing radiation before - then what about life from Mars? It would also be better adapted to intense UV light and to low atmospheric pressures, probably. All of this would make it far harder to sterilize food and equipment of Mars life than of Earth life. This could make it harder to sterilize tinned food, perhaps.

Anyway - that's enough, to see the possibility of hazards from Mars life, either major, or niggly. . Let's go ahead now and think some about how we could protect Earth from Mars life - however probable or improbable you think the hazard might be.

It is easy to protect from known hazards such as rabies, legionnaire's disease, Dutch elm disease, oriental fruit fly etc. But how do you contain a sample when you have to protect Earth from any possible form of extra terrestrial life and don't know anything about it, whether it is nanoscale, what range of temperatures, levels of ionizing radiation etc it can tolerate etc etc? That's the challenge that the experts face who were consulted about safe methods to return samples from Mars, and that's why they developed such elaborate precautions to contain it.

One approach that you might think is an obvious way ahead is to use quarantine. After all that's how they did it for Apollo, both for the astronauts, and for the returned lunar samples.

The knotty problem of human quarantine - and what about exposure of humans during a robotic sample return?

This section may seem controversial, because, after all, the idea for Apollo surely was that quarantine would protect Earth. But as we saw, the Apollo regulations have never been debated publicly, as they were published on the day of launch to prevent the delays that would result from a public discussion of them. So, even if those were applied perfectly, with present updated knowledge, would they protect Earth?

You might well think this is only an problem for humans returning to Earth. How is human quarantine an issue for a robotic sample return?

Well, when you think it over some more, we have to think about what we do in the case of accidental exposure of humans to the materials returned from Mars. This could happen in many ways:

• Accidental breach of the container before it reaches the facility. Or in the facility after it gets there.
• Sabotage, or terrorism
• Someone decides to ignore the protocols.
• Human error
• Failure in the design of the facility leading to breach of confinement
• Natural events - hurricane, tornado, earthquake, meteorite impact

It doesn't matter much how they got in contact with the Mars samples, whether that happens on Mars or on Earth, so the situation is rather similar actually. If that happens to anyone, surely they would have to be quarantined too just as for astronauts that contact the materials on Mars?

It's the same also if the sample is returned to the ISS or to the Moon and examined by astronauts there. An astronaut exposed in the ISS or on the Moon is much the same as a sample handler exposed in a facility on Earth. If they contact the Mars sample, they become a potential carrier of any life that is in it. It's an issue in any scenario that has humans in close proximity to the samples and with some possibility of an accident or intentional act leading to them contacting the samples.

So you can't really separate out the procedures for return of astronauts from Mars and for robotic return of a sample, unless you can guarantee that there is no possibility at all of human contact with the materials in the sample.

An example of a return that could keep this clear separation between robots and humans would be a return to a satellite above GEO. So long as it is studied only robotically and any material returned from the satellite to Earth is sterilized, then there is no way humans can contact any life in the sample. I cover this in the section If likely to be of greater astrobiological interest - return samples to above GEO

But, so long as humans can contact the sample, accidentally or through intent, then there will have to be provisions in place to deal with that situation. So then, the ethical and legal issues are similar to the case of an astronaut who visits Mars.

First, what kind of a hazard are we talking about here. If you haven't come across the scientific papers and workshops and studies on this issue before, the chances are you're first thought will be of the "Andromeda strain" or some other science fiction scenario. In that case it's viruses from outer space. But as we've seen in this book, viruses aren't a likely problem for humans going to Mars, because they generally have to be adapted to their host, or something closely related. Any life on Mars has never encountered humans before so can't be adapted to us (though it may be capable of genetic transfer to our microbes)

The situation is rather similar to the forward direction of contamination of Mars by Earth life, with the additional complication that we have to look at effects on higher animals and humans, as well as human activities.

We have already looked at the many ways that Mars life could be hazardous to humans and also to the biosphere of the Earth, so here is a quick summary.

• Life not based on DNA which has chemical signatures that Earth based life is not designed to respond to. Joshua Lederberg's "If Martian microorganisms ever make it here, will they be totally mystified and defeated by terrestrial metabolism, perhaps even before they challenge immune defenses? Or will they have a field day in light of our own total naivete in dealing with their “aggressins”? in his "Paradoxes of the Host-Parasite Relationship"
  
  This is not just a problem for life returned from outer space. It's also an issue for XNA studies in the laboratory. The situation is rather similar. Escape of XNA from a Mars sample is not unlike escape of a sample of non DNA based XNA life constructed in a laboratory. The main difference there is a chance of exobiology that is more different from Earth life than anything we can engineer in a laboratory. Not just the XNA but every detail of its biochemistry may be different. Some biologists studying XNA based life think that we should take extreme precautions, and ensure that any chance of accidental release is basically impossible. If you agree with them, the same surely applies to XNA from Mars. I discuss this in more detail in Hazards of XNA from Venus (below)
  
• Life from Mars could harm us directly through toxic byproducts, or indirectly through effects on our agriculture, or our pets or other creatures we value. As an example, cyanobacteria produce toxins that kill cows. I go into this in more detail in Many microbes harmful to humans are not "keyed to their hosts" (above)
  
• Life from Mars could harm us indirectly through effects on our or environment - for instance if it harms or replaces the algae in the sea or the microbes in our soil, it could have major effect on micro-organisms that our ecosystems depend on. Perhaps also on small creatures such as earthworms, or anything of that nature which we seldom think about, but depend on all the same.
  
• Life that out competes Earth life (as for forward direction). For instance, what if Mars has some fourth form of photosynthesis different from the three main types on Earth. Or not based on DNA, it may have a more efficient metabolism. Maybe it has only one biopolymer. Maybe it can do more with less. The microbes might be smaller, the encoding more efficient, less need for error correction, enzymes much smaller to do the same thing. It could be all round more efficient, and so able to manage on less by way of resources, with a more efficient metabolism. It might out compete Earth life everywhere where it can survive on Earth, by making do with less.
  
• Life returned from Mars might have a hidden capability that it uses only in rare conditions, like harmless grasshoppers which in response to some trigger can turn into locusts. There could be some condition on Earth which triggers a different behaviour or capability that never turned up when the life was encountered on Mars or in transit.
  
• Life returned from Mars could be harmless until it adapts to Earth conditions, but then evolve to be a major problem later. For instance it might need to adapt to the warmth of Earth, or to water that is less salty than on Mars, or to the lack of perchlorates (if it tends to depend on perchlorates for food) or the denser atmosphere. It may be a polyextremophile with some capability inherited from an earlier Mars that needs to be unblocked, or it may get these capabilities by lateral gene transfer from Earth microbes. Or indeed it could happen the other way, Mars life transfers a capability, say a novel form of photosynthesis, to Earth life via gene transfer, but not straight away, and that novel hybrid Earth / Mars life is what causes the problems.
   
• Gene transfer agents. These are much smaller than viruses, and they can transfer small fragments of DNA from one species to another to give them new capabilities. It's an ancient mechanism, and works between distantly related species. GTA's can transfer capabilities between species as unrelated as fungi and aphids (example of a GTA that gave an aphid the capability to create carotene, from a fungus). Also it works very quickly between microbes in sea water, if the GTA's ever got into the sea. In one experiment a GTA conveyed antibiotic resistance on 47% of the microbes in sea water, all types, just the microbes you have in sea water naturally, after they left them exposed to the GTA's overnight. This is relevant if Mars life is distantly related to Earth life. Even if the life got transferred from Earth to Mars or Mars to Earth billions of years ago, it could still exchange capabilities with Earth microbes readily using GTA's
  
• It could be a slowly developing problem even without adaptation. E.g. if we go back to that example of photosynthetic life, if it has an advantage, it might be slight. Suppose it is just slightly better than Earth life. It might take decades before sufficient numbers build up in our oceans to replace the green algae and other photobionts. Nevertheless, with exponential growth, however slow, the result may be inevitable. Once we notice that it's a major problem, there might be nothing we can do to stop its inexorable advance.
  
• Or, of course, Mars life could also be totally harmless, as Carl Sagan said in Cosmos,

  "There may be no micromartians. If they exist, perhaps we can eat a kilogram of them with no ill effects. But we are not sure, and the stakes are high. If we wish to return unsterilized Martian samples to Earth, we must have a containment procedure that is stupefyingly reliable...here are nations that develop and stockpile bacteriological weapons. They seem to have an occasional accident, but they have not yet, so far as I know, produced global pandemics. Perhaps Martian samples can be safely returned to Earth. But I would want to be very sure before considering a returned-sample mission.”

You might think "surely we know how to do this", looking back at Apollo. Couldn't we just handle it as they did, put the astronauts in quarantine for a few weeks on return to Earth. But those quarantine precautions never had any peer review. They were published on the day of launch. And they were not even applied properly at the time, as we saw above in Example of Apollo sample return - learning from our mistakes in the past (above). Buzz Aldrin noticed ants found their way into the quarantine facilities while he was in quarantine. Earlier, the command module hatch was opened when they landed, and dust from the Moon surely went into the sea at that point, and before that, the vents were opened to the atmosphere after re-entry, and there were other breaches of protocols as well. It's thought nowadays to be more useful as an example of what can go wrong.

What if we did this today, better, applying the rules perfectly and using modern technology ? Would a three weeks quarantine, or even a ten years quarantine protect Earth? There was no opportunity for anyone to discuss such questions at the time of Apollo. But the situation is changed. These issues would have to be debated at length today. There is simply no possibility at all, in the world as it is now, of avoiding public debate, as they did for Apollo.

So what sort of issues are there?

If we were to attempt to use quarantine today, for humans who have been in contact with the returned samples, or for astronauts returning from Mars, then problems with this approach include:

• What do we do if we find, during the quarantine period, that they have been exposed to microbes that are hazardous to other humans or to the environment of Earth, and that they can't be sterilized of them? After all that's why they are in quarantine, because there is a chance of this happening. In that case, is the plan to confine them to the facility for the rest of their life? How could that work, legally and ethically?
  
• What happens if someone exposed to microbes from Mars needs immediate urgent medical help, for instance a heart attack or other life threatening condition, even if we know for sure that it is not due to exposure to the sample?
  
  Perhaps they need surgery, even an artificial heart or a stent. Can they be taken out of the quarantine to be examined and treated in a better equipped hospital? Or do we equip the facility with a telerobotic surgeon?
  
  Even with a telerobotic surgeon, there's the need for nurses and an anesthetist, and it doesn't seem too likely all that could be done telerobotically in the near future. Do we send in an anaestheticist and nurses too on condition that they remain in there after the operation until the sample is proven safe (if it is)?
  
• What do we do if they have a serious illness during quarantine and we don't know yet whether it is the result of their exposure to the sample? And what if we know it is due to exposure to the sample, but we are unsure what the long term prognosis is, or whether it can be spread to other humans? What do we do in these cases?
  
• What about their human rights to freedom of movement and not to be imprisoned? This is not such an issue if the quarantine period is short, like the Apollo three weeks, but what if we decide it has to be decades long or even indefinite?
  
  After all the whole point in this process is that there is a risk, however small, that they have been exposed to something that can't be released from the facility. So what happens f we prove that their bodies carry microbes that could severely impact on the environment of Earth if it is exposed to them?
  
  We can't simply rule out that possibility in advance, as if we do, the whole thing becomes a pointless symbolic gesture that would be ignored in the rare and unusual situation where it proved to be necessary. It's not a precaution at all, if it is going to be ignored in the only situations in which it is needed.

Well in practice, I think it is pretty clear that if anyone who has been exposed to the sample becomes seriously ill, they will be rushed to hospital and not permitted to die in the quarantine facilities. If you try quarantine in orbit, they will be returned to Earth as soon as they encounter any really serious health issue, especially if it requires surgery.

Could you ethically keep them inside anyway, if they get seriously ill in the quarantine facilities? It's different perhaps if you know for sure that it is a hazard for Earth to take them out. But if you don't know that for sure?

In practice, they might well be taken out of quarantine just for cases of minor discomfort, if the chance of the sample being hazardous to Earth was still thought to be very low. This was another of the lessons from Apollo. Carl Sagan wrote about this, in his "Cosmic Connection"::

"The one clear lesson that emerged from our experience in attempting to isolate Apollo-returned lunar samples is that mission controllers are unwilling to risk the certain discomfort of an astronaut – never mind his death – against the remote possibility of a global pandemic. When Apollo 11, the first successful manned lunar lander, returned to Earth – it was a spaceworthy, but not a very seaworthy, vessel – the agreed-upon quarantine protocol was immediately breached. It was adjudged better to open the Apollo 11 hatch to the air of the Pacific Ocean and, for all we then knew, expose the Earth to lunar pathogens, than to risk three seasick astronauts. So little concern was paid to quarantine that the aircraft-carrier crane scheduled to lift the command module unopened out of the Pacific was discovered at the last moment to be unsafe. Exit from Apollo 11 was required in the open sea."

At the very least it's a very tricky ethical and legal area. Even if they consent beforehand to be left to die there to protect Earth, can you hold them to that in the event that it happens, especially if you have no idea whether there is some extra terrestrial cause? It might well be some Earth based illness that needs to be diagnosed in the advanced facilities of a modern hospital to save their lives.

Then there's what Carl Sagan called "The vexing problem of the latency period", again from "Cosmic Connection"

"There is also the vexing question of the latency period. If we expose terrestrial organisms to Martian pathogens, how long must we wait before we can be convinced that the pathogen-host relationship is understood? For example, the latency period for leprosy is more than a decade."

• You have to guess at the latency period. Some diseases of humans such as leprosy can remain latent for decades before anything happens. There was no scientific reason for choosing any particular quarantine period for the Apollo astronauts. The three weeks period was just a guess. At least, I haven't seen any attempt at a line of reasoning to explain this choice. Quarantine does work fine when you know the maximum latency period, and if that is a reasonably short period. It doesn't work so easily, if that period is very long or you don't know what it is.

Also though, all that is based on the assumption that we need to protect humans from diseases of humans and that nothing else matters.

• It only protects from diseases that affect humans or other lifeforms in the facility. You can't take all the higher animals we depend on, the trees, grasses, sea water, fresh water, camels, horses, cows etc. etc. into the facility for testing
  
• It gives no protection from problems that manifest later. E.g. quarantine won't help at all if the microbes that humans carry with them need some time to evolve to adapt - either to terrestrial conditions or indeed even, to adapt to humans. This is a bit different from a latency period. Suppose for instance there was a microbe that could live in our lungs, like Legionnaire's disease, but needed some time to evolve to adapt to the higher levels of oxygen on Earth before it could? In that case it might have a very low latency period, but the problem doesn't raise until it has evolved. That evolution might include swapping genes with Earth microbes if it is related to us.
  
  It only gives protection if you keep the astronauts in quarantine indefinitely, in the case that life is confirmed in the sample that could survive on Earth - indefinitely, or until you prove it is safe. Potentially for ever if it is proved unsafe. But that's ethically and legally problematical even if they agree to it.

If quarantine is going to be effective as a way of protecting Earth, we have to think through such questions. It's obviously not going to be an effective method to protect Earth, if we have a policy that mission planners can drop any of the requirements whenever there is any problem that impacts on the humans involved.

Yet, surely human rights and health have top priority? Even if all the scientists and workers with a risk of contacting a sample sign a voluntary agreement to not be taken out of quarantine in any circumstances, even in event of serious illness and threat to life, can they legally, be held to such an agreement? It seems to conflict with basic human rights.

What if they change their mind once their life is threatened, or if it becomes clear there is no way to sterilize them of a serious threat to the environment of Earth and they have to stay in there for the rest of their life?

Incidentally it's also similar for people who volunteer on a one way mission to Mars. Apart from issues of forward contamination, what if they want to come back to Mars later on? That's like someone who agreed to stay in the quarantine facility changing their mind once they discover they have to stay there for weeks, months, years or even the rest of their life to protect Earth.

For all these reasons I think there is almost no point in attempting to use quarantine to protect Earth or to protect humans on Mars. It's largely symbolic and would give a false sense of security. It doesn't matter where you do the quarantine either, on Mars, or the journey back, or in orbit around Earth, or on the Moon, or back on Earth, none of that helps, so long as you accept that the humans involved have to have the right to return to Earth and to get medical treatment and help in the event of life threatening conditions, or to change their mind and make the decision to exit any quarantine facilities whatever their previous decision was, and whatever legal documents they might have signed.

For the same reason, I can't see how it can work for robotically returned samples either, unless you can somehow ensure that there is no chance at all of accidental exposure of humans to the materials.

I think that the answer here is that there is no substitute for knowing what is in the samples before they are returned to Earth. For as long as you don't know if there is life in them, or don't know what it's capabilities are, then you need to sterilize any samples returned here.

Returning samples from Mars - what if we know already that it is of astrobiological interest

NASA has made it a priority for the next twenty years to return a sample from Mars for analysis on Earth. ESA has also proposed it as a flagship mission.

Artist's impression of Mars sample return vehicle launching from Mars - credit ESA.

As we've seen, since we don't know where to look yet for early and present day life, and since Mars is such a complex planet, with such varied terrain to such, there is quite a risk that such a sample might fail to return material of biological interest. Some of the samples will surely contains organics, since Curiosity found organics there. But on the basis of our Mars meteorite studies, it's entirely possible, indeed probable, that the come from meteorites, comets and other abiotic processes, even if they have biosignatures such as life-like carbon isotope signatures. It's true that once returned, we can analyse them over and again with more and more instruments - but that's only useful to exobiologists if we return the right samples in the first place.

Meanwhile we can send increasingly sophisticated instruments to Mars to study it in situ and explore a far wider range of possibilities there. In those circumstances, then it's no surprise, I think, that exobiologists argue strongly for in situ searches in preference to a sample return.

However, what if ExoMars, or Mars 2020 for that matter, finds clear biosignatures of life on Mars at an early stage? It's not impossible It could happen, for instance, if life on Mars is very common in equatorial regions, with reasonable densities of spores throughout the sand dunes Perhaps there are spores there, beyond the possibility of detection by Curiosity but detectable, say, by ExoMars with reasonably high confidence?

It's a shame that ExoMars won't be taking SOLID and LDChip to Mars, but it is still highly capable for organics detection, and its capability to drill two meters could make a significant difference for in situ life detection too. For instance, what if Nilton Renno is right in his suggestion that life could exploit biofilms in the liquid brines detected by Curiosity indirectly, perhaps 5 to 15 cm below the surface? Then, perhaps ExoMars finds easily detectable life there right away, or after a few drillings. We don't know enough to rule this out. Also there's an outside chance that Mars 2020 finds unambiguous evidence for life too.

If that happens then exobiologists will be as keen as the geologists to return these samples to Earth for analysis. It will then become a top priority for them. Their caution is just because of what they see as the unlikelihood of finding life there if we haven't detected unambiguous biosignatures yet, based on their experiences studying martian meteorites.

However, in that case, the quarantine measures particularly seem an almost impossible tangle to navigate, legally and ethically, from the previous section, once it is open to full public debate. Is there some way to bypass these issues? Can this be done safely?

Legal requirements in detail

Margaret Race looked at it. You may be astonished, at how many domestic and international laws need to be passed - which were not needed for Apollo because the world nowadays is legally far more complex. After reading her paper, I think it could easily take well over a decade just passing all the laws even if everyone agrees and there are no objections, and surely longer if there are objections.

In more detail, summary of Margaret Race's findings:

She found that under the National Environmental Policy Act (NEPA) (which did not exist in the Apollo era) a formal environment impact statement is likely to be required, and public hearings during which all the issues would be aired openly. This process is likely to take up to several years to complete.

During this process, she found, the full range of worst accident scenarios, impact, and project alternatives would be played out in the public arena. Other agencies such as the Environment Protection Agency, Occupational Health and Safety Administration, etc, may also get involved in the decision making process.

The laws on quarantine will also need to be clarified as the regulations for the Apollo program were rescinded. In the Apollo era, NASA delayed announcement of its quarantine regulations until the day Apollo was launched, so bypassing the requirement for public debate - something that would be unlikely to be tolerated today.

It is also probable that the presidential directive NSC-25 will apply which requires a review of large scale alleged effects on the environment and is carried out subsequent to the other domestic reviews and through a long process, leads eventually to presidential approval of the launch.

Then apart from those domestic legal hurdles, there are numerous international regulations and treaties to be negotiated in the case of a Mars Sample Return, especially those relating to environmental protection and health. She concluded that the public of necessity has a significant role to play in the development of the policies governing Mars Sample Return.

When you bear in mind that even relatively simple legal cases at this level can take a decade, and the complexity of all the laws that would need to be passed, and the international nature of some of the deliberations, probably NASA should have started on the legal processes already if they want to be legally cleared for a sample return in the 2030s. That's assuming there are no serious objections to the plan. She remarks that there have often been legal challenges that last for long periods of time for the Environmental Impact Statements.

Also, she says that the courts would not tolerate shortcuts. NASA would not be the only agency involved in these legal decisions.

Quarantine regulations are likely to be particularly tricky, as they could lead to incarceration of government employees and other people (in the case where they are contaminated by the sample accidentally or for some other reason). Commenting on this, she says

"Quarantine regulations are thus intimately linked with constitutional questions about deprivation of liberty and property over which NASA has questionable authority. Clearly before a sample return mission can be mounted, significant clarification will be needed on many scientific, operational and legal questions related to quarantine and exposure concerns, in order to minimize the opportunity for challenges and at the same time ensure public security"

For more on this, see The knotty problem of human quarantine - and what about exposure of humans during a robotic sample return? (above) . What happens, for instance, if the sample contains microbes potentially hazardous to Earth, and a human has been exposed to these microbes and can't be sterilized of them? Do they stay there for the rest of their life? I think they would have to consider questions like that. If the courts don't consider such issues, the whole thing would become largely symbolic and the precautions would not protect us in the situations in which protection is most needed.

If we know what we are returning in advance, it is so much easier. If we can show that it is harmless, it becomes like samples returned from asteroids or comets. Those required no new laws to be passed. The rest of the process was relatively straightforward. All these legal complexities arise because we can't show that it is harmless.

Surely there is no substitute to finding out what is there first. So how can we do that?

If it is just an astrobiology "technology demo" - we could sterilize the entire sample returned to Earth

I have two suggestions here.

First, if you think the sample is very unlikely to have clear signs of past or present day life, for instance, because neither ExoMars nor Curiosity has found any clear biosignatures on Mars yet- then why not just sterilize the whole thing, maybe even right away in Mars orbit or during the voyage back to Earth?

On the other hand, if by then we think that there is quite a high chance that it could contain past or present day life (perhaps ExoMars or Mars 2020 detected it somehow), my second suggestion is to return it to above GEO.

This first suggestion is based on the suggestion of astrobiologists that it is mainly a technology demo for astrobiology. ESA at least will continue to search for life on Mars in situ, others also eventually, and perhaps by then NASA have decided that we need in situ studies as well. They might have decided that Mars 2020 is unlikely to have collected evidence of life, past or present, or at least, not unambiguous signs of life.

Well in that case we can save a lot of expense and difficulty by simply sterilizing the whole thing before returning it to Earth. Depending on how it is done, this sterilizing won't do much harm to their geological interest or even their astrobiological interest. After all the Mars meteorites are still of great geological interest, and of astrobiological interest also, although they have all received ionizing radiation doses equivalent to hundreds of thousands of years, and in many cases such as ALH84001, they have had millions of years of ionizing radiation, enough to sterilize them thoroughly of any viable life from Mars. The scientists who research into them don't seem to have much problem disentangling the effects of the ionizing radiation during their transition from Mars to Earth from other effects of interest, even effects of radiation doses they received on Mars.

So why not just sterilize the whole sample return capsule, artificially, like a meteorite that spends a few million years getting from Mars to Earth? Sterilize to equivalence to ALH84001 for instance. The main problem currently with sterilizing spacecraft to Mars is that dry heat or ionizing radiation destroys semiconductors (this may be solved in the future with more heat resistant electronics).

Well that's not a problem for a Mars sample return. You don't need to sterilize an entire spacecraft. The only part that gets returned to Earth is the sample itself, in its container. The rover remains on the Mars, just launches the container to orbit around Mars, and the container is picked up by a separate orbiting spacecraft. So it should be straightforward to sterilize the sample container along with anything that's inside it.

This is similar to ideas to sterilize samples taken out of the Mars Receiving Facility at an early stage before we have had a chance to analyse the sample carefully. But instead, sterilize the whole collection of samples returned to Earth.

So, then, how should we do the sterilization. Well, for the first sample returns from Mars, why not use ionizing radiation? Subject it to enough ionizing radiation to thoroughly sterilize even the most radioresistant microbes known such as radiodurans and chroococcidiopsis or halobacterium. Even though the organics would be damaged and degraded, modern analysis techniques would still permit us to learn a lot from a sample sterilized in that way.

We do have a bit of a problem here - how do we decide how much ionizing radiation is enough to sterilize it of Mars lifeforms? The problem is that the radioresistant organisms on Earth don't seem to have adapted to high levels of radiation particularly, as they can never encounter those environments (except in very rare situations such as natural nuclear reactors in deposits of enriched uranium in the early Earth). Instead they probably developed radioresistance as a side effect of adaptations to extreme dry conditions and UV, which damage DNA in a similar way. Radiodurans itself was a big surprise because it was adapted to much higher radiation levels than anyone thought was possible at the time. However, even Radiodurans was not pre-evolved to survive high levels of ionizing radiation. Before they encountered our reactor cooling ponds, nothing on Earth ever encountered radiation levels like those on Mars, as far as we know. Perhaps it or other microbes will become even more radiation hardy in the future?

Meanwhile, any life on Mars would have evolved radioresistance over billions of years, not as a byproduct of desiccation resistance, but specifically in response to ionizing radiation. So, life on Mars may be even more resistant to radiation than the most radioresistant microbes on Earth.

The most extreme example of radioresistance on Earth seems to be Thermococcus gammatolerans - an obligate anaerobe from hydrothermal vents which was able to continue to grow after irradiation by 30 kGy of gamma radiation (applied at a rate of 60 Gy per minute).

Thermococcus gammatolerans - an obligate aerobe from hydrothermal vents, the most radioresistant organism known, able to withstand 30 kGy of gamma radiation, and still reproduce. That's about 400,000 years worth of surface radiation on Mars at the radiation levels detected by Curiosity during the current solar maximum of 0.073 Grays a year - possibly it could survive surface radiation for longer than that when you include periods of solar minimum.

This micro-organism didn't evolve in an environment with high levels of radiation, but developed this resistance as a side effect of other effects that can damage DNA. Microbes on Mars would have evolved in an environment with high levels of radiation, and adapted specifically to that environment. They might be even more radioresistant than this.

The 2002 study studied the levels of sterilization that would be needed to extract part of a sample from the quarantine facility in order to study it, before it has been proven to be safe. So that's a rather similar problem to the question of what level of sterilization would make the whole sample safe. They suggest dry heat sterilization and gamma ray sterilization. They say: "Both gamma-ray sterilization and dry heat have the advantages that they penetrate to the centers of samples, and they can be implemented on a relatively modest scale."

How much radiation would we need? It's not an easy question to answer. They suggested that if we were to sterilize a sample with levels of radiation similar to the shortest transit time of a Mars meteorite then this should be sufficient as no Mars life has got to Earth that way in recent times, or if it has, then it has been harmless. Some meteorites get here within years. Radiodurans and many other radiation hardy microbes would survive that transit.

"The preceding section in this chapter argued that if there are Mars organisms sufficiently robust to survive a realistic sterilization treatment in the quarantine facility, then some of these resistant organisms also would have survived transit to Earth in meteorites, and our planet already has been infected by them. Thus sample certification as “effectively sterilized” is appropriately based on verifying that the treatment used kills the most resistant known terrestrial organisms, and that the treatment is at least as harsh as that experienced by recent meteorites in Mars to Earth transit. Being substantially harsher than this will not be necessary."

They are using the natural contamination standard there. But before we can apply that, we have to be sure that the sample return is an exact analogue of the natural process. But we've seen that there are many differences. For instance our meteorites all

• Had surface ablation on entry to Earth's atmosphere.
• Didn't come from surface layers on Mars, but rather, from at least three meters below the surface (as we can tell by the lack of asymmetrical ionizing radiation patterns).
• Most of our Mars meteorites seem to be the result of impacts into the Martian highlands.

Also, even the way the fastest meteorites to get here were subjected to the cold and vacuum of space for several years doesn't exactly match what happens to a Mars sample return. So I don't think subjecting the samples to the same levels of ionizing radiation as the Mars meteorites is necessarily sufficient "as is". Instead we need levels that could sterilize even very radiation hardy microbes.

They do discuss what is necessary to sterilize it even of the likes of Radiodurans. They also remark that Radiodurans was a huge surprise and was about ten times more radioresistant than previously known microbes. So, we should sterilize the samples to higher levels than for Radiodurans. What level that should be, they leave open, saying

"In practice the sterilization dose used should be the minimum dose that will kill everything known, with some extrapolation factor for the possibility of unknown, more resistant, life forms. The key question is the size of the extrapolation factor. Before D. radiodurans was discovered, the most radiation-resistant organisms known were about a factor of 10 less resistant than it is. Thus extrapolation of a sterilizing irradiation dose by a factor of 2, or even 5, from that adequate to kill the previous record holder might easily have led to a dose that would not effectively sterilize a sample containing D. radiodurans. (Radiation resistance of D. radiodurans appears to have been a consequence of adaptation to conditions of severe dessication.5) However, in spite of previous bad extrapolations, the strategy is sound. There must be a dose at which sufficient chemical damage is done to a cell for recovery to be impossible."

You need levels of ionizing radiation that seem sufficient to sterilize even a microbe that is far better adapted to ionizing radiation than our most radioresistant candidate, which I think is now Thermococcus gammatolerans which can withstand 30 kGy. So, it looks as if you'd need to use at least hundreds of kGy to be safe. There wouldn't be much left of complex molecules after that, sadly, such as the carotenoids.

However the chiral signal of amino acids would remain at much higher doses. We can get an idea from this paper The effect of ionizing radiation on the preservation of amino acids on Mars by Gerhard Kminek and Jeffrey Bada. They found that materials in the deep subsurface would experience 740 kGy over a period of 3 billion years from natural radioactivity of the rocks, see page 3 of their paper. This would decompose the amino acid glutamic acid only to 12%. With a detection limit of 0.01 ppb then it would be possible to detect a low population of 1000 cells per gram (compare 100 to 100 million cells per gram in permafrost).

The radiation doses are much higher in the topmost layers, but still acceptable if you dig deep enough. They conclude that we need to drill 1.5 to 2 meters depth to have a decent chance of recognizable amino acid signatures from ancient martian life.

At a depth of 2 meters, according to the most recent Curiosity measurements of surface radiation levels, then the dose per year is 8.7 mGy (table 3 in this paper), so as an accumulated dose over three billion years, it would be 26.1 MGy. At a depth of 1 meter, the accumulated dose over half a billion years would be 18.2 MGy. And at a depth of 10 cm, then it would accumulate 9.6 MGy in as short a time period as a hundred million years. This suggests that an extra sterilizing radiation dose even as much as a few MGy might not be such a huge issue, if you are looking for signatures of past life. Mars 2020 will be lucky to find samples that have been exposed to less than 20 MGy since they were deposited, at least for ancient life.

So - that's my suggestion. For the very first sample returns from Mars, when we have very little idea of what is in them, the idea is to irradiate the samples with several MGy of radiation before you open it. This would need review by astrobiologists of course, to determine what is a sufficient level of sterilization to make the samples safe.

The safest way to do this would be to irradiate it in Mars orbit, before it returns to Earth, perhaps with gamma radiation, using Cobalt 60, as is standard in food and medical gamma ray sterilization.

So, how could we do it without damaging the spacecraft? One idea is that the Cobalt 60 could be included in a shielded container sent along with the spacecraft that picks up the returned sample in Mars orbit. That way, even before the sample leaves Mars orbit, it is put straight into the container along with the Cobalt 60 source, with the whole thing surrounded by thick layers of lead to protect the spacecraft itself from the radiation.

This would add a fair bit to the expense, because of the mass of the cobalt 60 source. Typically these industrial cobalt 60 based irradiators are one meter diameter by one and a half meters tall. So you are talking about something quite substantial in size .I'm not sure how much it can be miniaturized for orbit, as most of the mass is for shielding. The spacecraft would need to be shielded from the ionizing radiation. However, I wonder if you can reduce the shielding mass if you only shield it on the side adjacent to the spacecraft that carries it? That might interfere with other electronics in the rocket used to launch it to orbit, but you could have it fully shielded on launch, then discard most of the shield for the transit to Mars and back, and discard both the source and the shielding before return to Earth as by then it would be thoroughly sterilized.

Self contained dry storage cobalt gamma ray irradiator (image from this paper). This is used for irradiating small specimens. It's still quite large and heavy because of all the radiation shielding. typically about 1.5 meters high and 1 meter in diameter. The core of it consists of cobalt 60 pencils. Perhaps for a spacecraft then the part that gets sent to Mars only needs to be shielded on the side next to the "mother ship" so reducing its mass?

My idea is to send something like this to Mars orbit to sterilize the sample before it is returned to Earth. The advantage of ionizing radiation over dry heat is that it is similar to the natural sterilization that a rock experiences if left on the Mars surface for millions of years. While dry heat sterilization is something new that may transform the sample in undesired ways.

As is usual for Mars sample return proposals, you would do this in such a way as to break the chain of contact with the Mars surface. A spacecraft sent from Earth orbits Mars. When it picks up the capsule, it carefully positions one capsule inside the other in the vacuum conditions of space without ever letting the exterior of the capsule touch any other part of the orbiter. The exterior capsule there is the one with the gamma radiation source. So then only the interior of your return capsule, the part strongly irradiated with Cobalt 60 during the return journey, has any contact with material which has touched the Mars surface.

I don't know if that is practical; it is just a suggestion. The advantage is that it would damage the sample in a way similar to the natural radiation on Mars, while dry heat involves adding an extra new form of sterilization that doesn't correspond to anything it was subjected to on Mars. Also sterilizing at Mars deals with the issue that the capsule could be damaged by a micro-meteorite during the return journey. It also replaces the immense complexity of the Mars handling facility on Earth with a relatively simple addition to the sample return mission.

You still want to take every possible care when handling it, and you might as well still return it to a biohazard handling facility (after all it might have harmful bioactive chemicals in it still). But the chance of release of extraterrestrial life with the ability to reproduce on Earth seem remote even at the 1 in 10²⁰ level when you have a sample already thoroughly sterilized with gamma rays - and you no longer need to attempt the perhaps almost impossible task of containing it at the 10 nanometer level.

Even if we do find life before the sample return, we don't have to return it right away. Perhaps it turns out to be simpler to sterilize the whole thing for the first sample return, and do preliminary studies of the life in situ on Mars, so that you can characterize it on Mars before working out how to return it to Earth safely. If you want to do DNA sequencing of present day life, or perfectly preserved early life - or even XNA sequencing, you can do that on Mars using the ideas for a miniaturized DNA sequencer to send to Mars (SETG, already built and pretty much ready to fly). If you want to attempt to revive revivable ancient life, again do that on Mars first, and other experiments that need unirradiated specimens would be done on Mars, to start with.

Subsequent sample returns

After the first samples are thoroughly studied, then the situation can be reviewed. But probably we should continue to apply extreme caution until we have a very thorough understanding of the situation on Mars, because there might be a variety of forms of life on Mars.

For instance if the first sample contains DNA based life, this doesn't rule out the possibility that Mars also has XNA based life. You can easily imagine a scenario where past XNA life co-exists on Mars with more recent DNA based life introduced on meteorites, for instance, either in different habitats or in the same microbial colonies - and the XNA life might be hazardous for Earth life, and never made the transition here via meteorite.

I'd suggest that we need to continue to take these precautions even if we think the chance of contamination of Earth is extremely low. After all even if there is as little as a one in a billion chance or less of returning XNA to Earth able to out compete Earth DNA and establish a separate ecosystem here or take over from some Earth life-forms - that would still be a completely unacceptable level of risk to take according to many ways of thinking. Even such tiny probabilities may be unacceptable when what we risk, in the worst case, however unlikely, is the habitability of the biosphere of Earth.

This is just a suggestion which I present for discussion. Would this ionizing radiation, perhaps 2 or 3 MGy or so, be sufficiently sterilizing to make a sample return from Mars completely safe for Earth life even at, say, the 1 in 10²⁰ level that seems necessary for novel existential risks? Would it also preserve the science value of the sample? What do you think?

However, if astrobiologists think there is a significant chance of life in the sample, another possibility might be to return unsterilized samples, not to Earth itself, but to cislunar space.

If it is likely to be of astrobiological interest - return samples to above GEO

This is for the situation where we think that the sample could potentially contain life but that if it does, we don't know enough about this life to say anything about it or its capabilities. Either Curiosity or ExoMars detects something that suggests that it is life (even with its limited biosignature testing capabilities), or perhaps the Trace Gas Orbiter comes up with evidence of this, or we somehow get convincing evidence that Viking did detect life in the 1970s, and this becomes widely accepted.

In this situation, I think that given that we have no experience at all in handling extraterrestrial biology, that it's better not to return the samples to Earth at all. Instead, what if we return it instead to a telerobotic facility above GEO? I've chosen this location because it is the orbit furthest in terms of delta v from Earth or the Moon of any point in cislunar space.

This also has the great advantage that it is well outside the range of any Earth orbital debris from LEO or MEO (medium Earth orbit). Any debris from the GEO satellites themselves will be slow moving, because they are almost stationary relative to each other. So any collisions are going to be of low relative velocity, and can't to send debris to much higher orbits. It is also far from Earth and from the Moon in terms of delta v, and it is a dynamically stable orbit long term. All this would seem to make it by far the safest place to return it to. It is also easy of access from Earth.

The obvious outer circle of white dots in this image shows the satellites in GEO together with the orbital debris in the graveyard orbit above GEO. Notice that it is stationary relative to the Earth's surface so any satellites there have very low relative motion. Satellites in GEO are retired to this "graveyard orbit" 300 kilometers higher at the end of their lives. For more images see the orbital debris gallery here.

I suggest a good place to return a sample from Mars would be a few thousand kilometers, perhaps ten thousand kilometers above GEO. Well away from GEO or the graveyard orbit. The furthest you can be from either the Moon or Earth in cislunar space in terms of delta v.

You need more than one kilometer per second delta v to get to either the Moon or to Earth from there. Yet it is easy of access from Earth, which should make it relatively simple to send telerobotic equipment up there

(click to watch on YouTube)

This video shows the orbital debris in motion, and from different angles.

As with the previous idea, we could return some samples to Earth right away so long as we sterilize them first. That should satisfy the geologists. I suggest using ionizing radiation to sterilize these samples again, for the same reason as before, because that happens anyway on Mars, and would still preserve some evidence such as chirality and complex chemistry for astrobiologists too, if there was any life there before it was sterilized. It's also easy to take account of for the geologists, who already disentangle the ionizing radiation effects of the journey from Mars to Earth when studying Martian meteorites.

If the unsterilized samples are shown to be harmless quite quickly, we just return them as is (perhaps sterilizing them to be sure to start with), much as we did with the Moon rocks. This saves years of legislation (probably a decade or more to pass all the laws), and hundreds of millions of dollars of expense for designing, building and operating a facility that is never needed.

Returning to above GEO simplifies the whole process hugely.

• No new legislation is needed to do that. It can be done within all the existing laws, as for sample returns from comets and asteroids. This eliminates the need for many years going through legal processes, and perhaps even into decades, just passing all the legislation to return it to Earth
  
• You don't have any concern about the staff not using the right protocols because it is all operated from Earth. Nothing the staff can do, by mistake, laziness or attempts to cut corners can lead to life from the spacecraft above GEO escaping into the environment of Earth.
  
• There is no risk of sample release to the environment of Earth as a result of natural disasters, human error, etc such as hurricanes, or earthquakes, lapses of protocol, design issues, or even terrorists, or the fiery heat of entering the Earth's atmosphere from orbit.
  
• Saves half a billion dollars of up front cost - the Earth based sample receiving facility has to be built and up and running before the sample return mission is launched. This way, all the missions to study the sample above GEO can be treated as extended missions. That's equivalent to an entire new Discovery class mission. The mission to retrieve the sample to above GEO corresponds to a stage we might well do anyway, to retrieve the capsule before return to Earth to deal with the possibility of sample container breach in transit from Mars.
  
• If the samples are rapidly shown to be of no great astrobiological interest - as most astrobiologists expect at present - you can just sterilize them and return them to Earth and don't need to do any other special handling, but treat them like the Mars meteorites. This is a huge cost saving. In this case you don't have to study it telerobotically in GEO, but just sterilize the whole thing and return it to Earth as in the previous suggestion.

It also has the great advantage that we design for what we discover as the mission progresses. There is no need to design an all purpose "swiss knife" of a faculty able to deal with all conceivable biochemistries. For instance if it is viable early life, based on RNA or even just primitive autopoetic cells, it might be easy to establish at an early stage that there is no possible hazard for Earth at all. In that case again, perhaps it doesn't need to be studied in a biohazard containment facility at all, but just protected to keep Earth life out of the sample. We don't need to establish that all life on Mars is safe for Earth, just that there is no hazardous life in the sample itself.

On the other hand, we might decide it needs extreme caution . For instance, we might do that if it is some exotic form of life which is not based on DNA at all, or if we decide that it is has a much more complex genome than any Earth microbe, billions of years advanced on us and we can't figure out what it does.

Also, this is a big plus when you consider the natural human inclination to ignore low probability risks of extreme events - if we do need precautions - the planners and staff will know it is potentially hazardous and will take great care. They would never do something the equivalent of opening a hatch because the astronauts might get seasick. They will design a protocol that really does work, and think through all eventualities, and take great care to make sure that it is going to work and be effective, and is not just a symbolic gesture.

In short, the three possibilities are:

• If we find possible life and precautions are needed, we apply them based on knowledge, not trying to second guess any possible extraterrestrial biology. Everyone is highly motivated to take suitable precautions
• If we find life that is so "feeble" that no precautions are needed, then there is no need to build what would then be a totally unnecessary facility and the legislation will surely be simpler and at any rate passed easily with no opposition.
• If it has no great astrobiological interest, we sterilize the sample and return it to Earth.

So then the main remaining question is - is this idea to return a sample to above GEO itself safe against accidents? Is there any risk of any of the material in the sample getting to Earth?

So, first, the only thing that could damage and release the sample above GEO is an impact but there wouldn't be any risk from spacecraft debris, as any debris in GEO or the graveyard orbit a few hundred kilometers above GEO wouldn't travel far because of their low relative velocity. So the only real chance of an impact leading to release of the material from the sample, would be from natural debris from asteroids and comets. Assuming it has thrusters to position itself as a satellite, then it could also maneuver to avoid such hazards, just like the ISS, but it might be an issue detecting small debris at that distance from Earth and its radar systems. It would of course have Whipple shields to protect from micrometeorites.

So, is there any chance that a natural meteorite hitting a spacecraft above GEO, with Whipple shields, and able to "dodge debris", could send viable life to Earth from the satellite? I leave this for experts to look into in detail, if the idea seems to have merit.

Another question to look at is, is there any chance of a failure to retrieve the sample to above GEO. Well there, the approach would be similar to the Asteroid Redirect mission. So - the intermediate stage would be, perhaps, to return it to a Distant Retrograde Orbit. This is an orbit that is stable over time periods of centuries, and is within easy reach of GEO in terms of delta v. It's an orbit that is in synchrony with the Moon, so a 28 day orbit around Earth, but it is also highly elliptical. The satellite orbits Earth more slowly when further away than the Moon and faster when it is closer to Earth, and so as seen from the Moon it seems to orbit it in a retrograde fashion. It's a more stable orbit than a prograde orbit around the Moon and has the advantage that it can be as large as you like in diameter, even continue all the way to LEO in a retrograde orbit around the Moon. But it's also a prograde orbit around Earth so it's easy to get from it to GEO

As a 28 day orbit around Earth, DRO is also far more accessible to a spacecraft from Mars than LEO, assuming it doesn't do aerobraking in Earth's atmosphere, or return directly in a fast re-entry to Earth. This calculation is for the opposite direction, from Earth to Mars, but it gives a good idea. The authors find that you need a delta v of 3.29 km / sec from DRO to LMO (Low Mars Orbit) compared to 5.758 from LEO to LMO. The delta v would be similar for a flight from LMO to Earth. The main drawback is that the delta v depends on the position of the Moon at the time of departure from Earth, so optimal trajectories repeat only once or twice a month.

This is actually similar to ideas in the literature to return the Mars sample to DRO around the Moon, and then for astronauts to retrieve it and return it to Earth. The only difference is that my suggestion here is to retrieve it robotically, rather than asking astronauts to retrieve it manually, and then return it to above GEO instead of to LEO. See this suggestion by Lockheed Martin to use astronauts to retrieve a Mars sample from DRO using the Orion spaceship (once ready). Other proposals though assume a fast re-entry to Earth's atmosphere at 12 km / sec (see for instance page 49 of this report). It would have a higher delta v requirement than that of course.

For more details see my:

Will NASA's Sample Return Answer Mars Life Questions? Need For Comparison With In Situ Search
No Simple Genetic Test To Separate Earth From Mars Life - Zubrin's Argument Examined
How To Keep Earth Safe - Samples From Mars Sterilized Or Returned To Above Geostationary Orbit - Op Ed

Need For Caution For An Early Mars Sample Return - Opinion Piece
Concerns for an Early Mars Sample Return - background material
Mars Sample Receiving Facility and sample containment
Mars Sample Return - Legal Issues and Need for International Public Debate

If there is Life in Venus Cloud Tops - Do we Need to Protect Earth - or Venus - Could Returned XNA mean Goodbye DNA for Instance?

Or return samples to the Moon

The Moon seems a promising place to return the samples to. If you return samples to a human occupied base on the Moon, then it's got the same issues as returning them to a human occupied facility anywhere. As we saw, quarantine simply can't work unless you know what is in it and what precautions are needed. Even if astronauts signed consent forms to let them be confined to the facility for the rest of their life, if needs be, it's not at all clear you can ethically or legally commit humans to such an agreement, should it turn out to be potentially hazardous for humans or any other creatures or the environment of Earth (e.g. carried to Earth on the skin or inside bodies of humans). For instance, what do you do if they become ill and Earth is the only place they can be treated effectively? For more on this, The knotty problem of human quarantine - and what about exposure of humans during a robotic sample return? (above)

However, if you return it to a robotic facility on the Moon - well that's another matter altogether. t's far better isolated than anything we could achieve on Earth, yet perhaps easier to build and work with than a large facility in orbit, especially if we develop infrastructure on the Moon, such as railways, and in situ industry and mining. As with the facility in orbit, then it could start simple. It would be absolutely fine to start with a single module, and then to send instruments to it, so long as it only goes that way, and any materials are sterilized in the reverse direction.

 

Hazardous Biology Facility on the Moon, telerobotically attended, surrounded by vacuum - Artist's impression, illustration by Madhu Thangavelu and Paul DiMare © from The Moon: Resources, Future Development and Settlement

It could be useful for any hazardous biology generally, like an extra biohazard level above biohazard 4. So for instance if we wanted to experiment with synthetic biology using XNA in place of DNA, then we could use a facility like this on the Moon, to minimize any risk of it affecting Earth.

Even if the life did escape from the facility, e.g. after a meteorite strike, where would it go? About the only way it could be transported is via the levitating lunar dust, but that would surely be thoroughly sterilized by UV radiation before long. You could also turn the region around the facility into glass and remove any dust that strays onto that glass regularly.

You would have to think about the effects of larger meteorite strikes. And it would need to be evaluated by exobiologists, but seems very promising to me for hazardous biology!

You could have humans there too, so long as they are in separate habitats, with no chance of contamination by the Mars life. It's a lot safer than anything we could do on Earth.

One other suggestion, what about putting the hazardous biology facility in a lunar cave? There are many cave entrances discovered on the Moon now, and some of them might be not needed for human habitats and just lead to a small cave the right size for the facility. It might have smooth walls like a lava tube. Ideal for the facility.

The advantage of putting it in a lunar cave is that it would now be protected from impacts by all except the very largest of the near Earth asteroids. Also, you could use a liquid airlock for the entrance. Some of the machines inside might need air, or we might need a simulated Mars atmosphere for the samples, but with a liquid airlock, there would be no risk of dust / air getting out onto the surface.

Returning samples to the Moon is a lot safer than returning them to the Earth's surface. However, there is an issue here. The COSPAR guidelines for category 5 (sample return) missions currently say that

"The Moon must be protected from back contamination to retain freedom from planetary protection requirements on Earth-Moon travel".

So before samples can be returned to the Moon, that would need to be discussed and the guidelines altered to protect not just Earth but the Moon also. One issue I can see that would need to be looked into in detail is - what if the sample return mission crashes on the Moon somewhere different from its intended landing site?

However it doesn't seem insuperable. They say that the Moon must be protected from back contamination "to retain freedom from planetary protection requirements on Earth-Moon travel". So long as the facility is geographically isolated, then perhaps we can achieve a sample return to the Moon without adding any significant risk of contamination of any habitats occupied by humans, or any spacecraft that are going to travel back to Earth.

Perhaps all that is needed is an update of the COSPAR guidelines for category 5. That of course is a matter for future discussion and for COSPAR to decide. But, since the original decision was by COSPAR you'd think it would only require agreement of the scientists. It doesn't seem to require new laws to be passed, so long as there is no significant risk, even with very small probabilities, to humans and spacecraft traveling from the Moon to Earth.

The Moon is relevant to astrobiology in another way too. Although Mars gets all the attention, it's not the only place where we might be able to find out about past biology through a sample return. in the near future. And I'm not talking about Europa or Enceladus (more on them later).

Rather surprisingly we could also learn a lot about the past history of biology and exobiology in our solar system from a sample from the Moon - if we can find just the right samples to return.

Search for past life from Mars, Earth or Venus - on the Moon in meteorites!

I was quite surprised when I first learnt about this. The Moon can also help bring us with the biological search for early life throughout the inner solar system, through remains of life that landed there in meteorites. During the Late Heavy Bombardment, large meteorites impacting on Mars, Earth, Venus, must have sent rocks throughout the solar system. After the Moon formed, it was a prime target for these rocks to land on. So, the experts say, we might well find meteorites from any of these places on the Moon. The best place to look may be the lunar poles, where the ice deposits would help to keep the meteorites from drying out. We can also search for meteorites deep below the surface, protected from cosmic radiation.

Our Moon has continued to get meteorite impacts from Mars and Earth through to the present day. Sadly, the current Venus atmosphere is so thick that any meteorites from Venus must surely come from at least several hundred million years ago. However, if it is true that Venus developed a thick atmosphere only half a billion years ago or so, then we might be able to find samples from its history all the way through from formation to the time when it first developed a stagnant lid and became the dry hot acidic pressure cooker of a planet it is today. We might even find life floating high in the clouds of present day Venus to complete the picture - if it did develop robust forms of life.

Search for past life from Earth on the Moon

Jordi Guntierrez was first to look at this (AFAIK) in a paper from 2002. He didn't know of any earlier work on the subject, as he says in his chapter in the conference proceedings here. He studied the possibility of finding Terrene meteorites - meteorites from early Earth in the lunar highlands on the Moon.

This was before recent ideas of ice at the poles, so he looks at the possibility of Terrene meteorites in the lunar regolith. He found that there could be meteorites buried several meters below the surface of the Moon. At that depth, they would be protected from cosmic radiation and also from the extreme temperature changes on the lunar surface. Though the shock of impact would convert 20% of the rocks kinetic energy into heat, the impact velocities for meteorites ejected from Mars would be low enough so that the "terrene meteorites" could include organics and even complete micro-organisms preserved for billions of years. He mentions that some of the lunar rocks have unshocked millimeter sized fragments of chondrites in them, which is promising for preservation of moderately sized micro-organisms since they would have hit the Moon at higher velocities than rocks from Earth.

Also micro-organisms would be able to survive the shock of re-entry and still be viable -of course they would not find anywhere to grow but they could be intact and freeze dried, and indeed with their DNA intact as well presumably. By analogy with the martian meteorites, any rocks that reach the Moon should also include bubbles of the early Earth atmosphere, including any gases produced by early life.

Wherever the microbes come from, some of them might then be unearthed recently as a result of meteorite impacts on the Moon. He also discusses the idea that early life on Earth may have developed multiple times, each time erased by giant impacts that sterilized the entire surface of Earth during the Late Heavy Bombardment. If so, the Moon would have ejecta from those impacts too, so it might potentially preserve organics which preserves evidence from multiple separate attempts at evolution on early Earth, with different forms of biochemistry

 

We could recognize the meteorites by their fusion crust, which of course is not the result of passage through the very thin lunar atmosphere - but from their passage through Earth's atmosphere when they left Earth. Some might have distinctive banded formations of sedimentary rock, so proving their origins from Earth. Nearly all the processes that degrade organics on Earth would not act at all on the Moon, with no water, no oxidizing chemical reactions. Another early paper from 2007 suggests that robots could search the lunar surface for Earth meteorites, which they could recognize by the presence of carbonates and hydrated silicates.

We now know that the Moon has extraordinarily cold conditions at the lunar poles, in craters that have never seen sunlight for billions of years. The conditions there are far colder even than Mars, so we could find organics there as well, preserved, hardly changed, since the early solar system. The lunar poles have ice too, which will help to preserve the organics. There may well be organics there dating back to soon after the formation of the Moon itself.

Also, we aren't talking about just a few meteorites here. There may be as much as 200 kilograms of material from early Earth per square kilometer of the lunar surface. That's 200 milligrams per square meter on average. It might be buried deep but some of it might have been unearthed by later impacts. We might find fossils also, fossil diatoms are still recognizable after a simulated impact on the Moon, indeed the smallest ones are intact, complete fossils. There must be a lot of material from the Chicxulub impact on the Moon - so perhaps there are fragments of ammonites and other sea creatures from the Cretaceous period, with organics still preserved, there as well. Perhaps the Moon will turn out to be one of the best places for diligent fossil hunters in our solar system, outside of Earth.

Artist's impression of Cretaceous period ammonites, courtesy of Encarta. The Chicxulub impact made these creatures extinct. It hit shallow tropical seas and the ejecta could have sent fragments of cretaceous sea creatures such as ammonites all the way to the Moon. Fragments in the cold polar regions may even have the organics preserved.

Indeed (though he doesn't say that) I wonder if the main issue for recovery of more or less intact and originally viable ancient microbes might be whether the microbes are able to withstand the shock of ejection from Earth, leaving the surface at more than Earth's escape velocity, rather than the shock of impact with the Moon. If that is possible, then the impact with the Moon is a much lower level of shock to withstand, it would seem. See General case of transfer of life from Earth to Mars (above), The most intact microbes might perhaps be ones that came from early Mars, if there was life there and we find it on the Moon.

We have dinosaur bones on Earth with evidence of protein preserved from 195 million years ago. So that at least seems something that we could find on the Moon in principle. Could we go further than that? What about DNA, can we sequence the genomes of dinosaurs and ammonites from DNA fragments left on the Moon all this time?

This is another of my speculative sections as I haven't found any research into whether we could find sequenceable ancient dinosaur or ammonite DNA on the Moon (though I have found some material on ancient DNA on Mars, more on that later).

It seems a natural question to ask though, so let's have a look at it. So, at first sight this may seem unlikely, because the most recent impact likely to send material to the Moon is 66 million years ago, and DNA is rather fragile, and easily degraded, especially in warm or damp conditions. Our oldest DNA on Earth dates back to between 780,000 and 560,000 years ago, from the remains of a horse found frozen in the Canadian Arctic permafrost.

Przewalski's horse, Photo by Claudia Feh. This is a wild horse that was preserved in zoos in Prague and Munich after it became extinct in the wild. By the end of the 1950s there were only twelve of them left in the world, but through careful selective breeding, they have been brought back from the brink of extinction as a viable genetically diverse population. They have now been reintroduced into the wild.

DNA recovered from the remains of a wild horse preserved for over half a million years in Canadian permafrost proves that this is indeed the last remaining species of wild horse in the world.

In the early days of gene sequencing, there were many claims of DNA recovered from dinosaurs, but so far, they have all turned out to be mistakes.

The degradation of DNA is slowed down in drier and colder conditions. But even in dry conditions at -5 °C, then the hope is for sequenceable DNA a million years old. Older DNA, tens of millions of years old still seems unlikely.

For more evidence on this, one paper worked out a half life of DNA decay using bones from the extinct New Zealand Moa. The average half life was about 521 years for a 242 base pair sequence. It was 158, 000 years for a 30 base pair sequence. This let them develop a model from which they predicted that there would be no sequenceable DNA left after 6.8 million years. They wouldn't expect to find even two base pairs attached together. They say:

"Still, the results indicate that under the right conditions of preservation, short fragments of DNA should be retrievable from very old bone (e.g. greater than 1 Myr). However even under the best preservation conditions at −5°C, our model predicts that no intact bonds (average length = 1 bp) will remain in the DNA ‘strand’ after 6.8 Myr."

They couldn't completely rule out DNA tens of millions of years old however. This was a decay rate for DNA in bones. There could be variations in decay rates depending on the tissue type. However, on the basis of their research, it seemed very improbable to them that a long sequence of dinosaur DNA could be preserved. No intact bonds after 6.8 million years and the dinosaur era is around ten times that age.

Also, for the best chance of survival, it would have to have been preserved in cold dry conditions. They don't discuss this, but where on Earth could we find dinosaur bones that have remained cold and dry for all that time? At the time of the Cretaceous period, while dinosaurs still roamed the Earth, Antarctica had already reached the south polar region, which it shared with Australia,, but it's climate was different from any today. The world as a whole was warmer than it is today and there were no polar ice sheets. Antarctica was warm and ice free in summer, and dark and bitterly cold at night. Polar seas wouldn't keep it cold either, even now in a cooler world the polar seas are only -2 °C at the sea floor.

Yes, there would have been permanent ice on high mountain tops in Antarctica back then - however, could a fossil in Antarctica, originally on a mountain top, have stayed continuously at temperatures of −5°C right through to the present? Through erosion and formation of mountains? It seems a long shot. At any rate, there is no chance at all of preserved DNA from earlier in Earth's history when the entire land mass was in the tropical zones

But the Moon is different. The craters at the pole are extremely cold, -249 °C. This is well below the freezing points of nitrogen and oxygen, also cold enough for liquid hydrogen (though as a liquid, it would only last momentarily if exposed to the surface) and not far above the freezing point of hydrogen. What's more, they have remained like that for billions of years. If an asteroid impact such as the Chicxulub impact or an earlier one sent dinosaur DNA and ammonite DNA, and other forms of DNA to the Moon, it would have the best chance of preservation of DNA anywhere.

So, if I can venture to speculate - might Cretaceous period DNA on the Moon still be sequenceable today? Even at a temperature of - 5 °C then it may be just within the bounds of what might be possible, depending on tissue type perhaps, if there is anywhere on Earth that's remained that cold for that long. So what about DNA stored at -249 °C for 66 million years or more?

Well, this paper studying preservation of ancient DNA in the Martian polar ice caps would suggest that it could well survive that long, not just for a few tens of millions of years, but maybe for billions of years too. See page 146:

"The Martian polar ice caps are believed to sustain temperatures of between -50 °C and -110 °C, and are thus likely to be a suitable place to look for simple chemical molecules, such as amino acids. The same is true of the Martian permafrost. Rough calculations using the Arrhenius equation suggest that 100 bp of DNA can theoretically survive 3.4 × 10^9–3×10^21 years at -50 °C and -110 °C, respectively (Figure 2, main text). Although the calculation is highly simplified, it does suggest that any nucleic acids on Mars would be preserved for periods of time significantly longer than can be expected on Earth"

That's 3.4 billion years at -50 °C and 3 sextillion years at -110 °C. So at -249 °C, if it is buried deep enough to be protected from degradation by cosmic radiation, what then... He does warn that his calculation is hugely simplified. However, it's a thought, could there even be sequenceable DNA from early Mars, Earth and Venus?

The ice at the poles must also contain organics from comets and asteroid impacts in the early solar system - and possibly, made locally too. Some prebiotic organics could have been synthesized in volcanoes in the early Moon too. See page 769 of this paper. So, study of the ice would give an insight into the pre-biotic chemicals delivered to the Earth as well - their quantity and diversity, and indeed chirality.

Although these researchers don’t mention it, there’s another result that’s relevant here too. It is just modeling at present - no direct evidence it happens. But if they are right, then microbial spores are constantly lifted from Earth’s atmosphere in its “dust tail” due to fast moving dust flows of interplanetary dust hitting the atmosphere. This dust travels at up to at up to 70 km a second and could hit spores and other particles at 150 km altitude and higher, and take them to the Moon and elsewhere. This could be billions of years history of microbial spores from Earth to study. Maybe even small creatures like tardigrades. Unlike material thrown up from meteorite impacts, a continuous record through the Earth’s biological history.

“Some bacteria, plants and small animals called tardigrades are known to be able to survive in space, so it is possible that such organisms – if present in Earth's upper atmosphere – might collide with fast-moving space dust and withstand a journey to another planet.”

Space dust may transport life between worlds - paper here

Another thought, another of my speculative ideas that you might enjoy thinking over. It's about the ice at the lunar poles (if it is ice). This is very speculative, but there's a suggestion that it could be "fluffy ice" or snow.

"We do not know the physical characteristics of this ice—solid, dense ice, or “fairy castle”—snow-like ice would have similar radar properties. In possible support of the latter, the low radar albedo and lower than typical CPR values for nonanomalous terrain near the polar craters are 0.2–0.3, somewhat lower than normal for the nonpolar highlands terrain of the Moon and are suggesting the presence of a low density, “fluffy” surface."
(page 13 of Evidence for water ice on the moon: Results for anomalous polar)

If there is fluffy ice like that - it could help decelarate those microbes, tardigrades etc in the dust tail. It could also help with the finer material in meteorite debris thrown up from Earth. That would be especially so for the occasional long grazing impact through the polar regions, that passes through hundreds of meters or more perhaps of "fairy castle" ice or snow in a lunar crater, pretty much horizontally. That would have much less shock of impact than a direct impact into ice or rock. The lunar polar regions might turn out to be the gentlest place in the solar system for catching meteorites from elsewhere.

Not just for Earth meteorites. Even meteorites from Mars, Venus or Ceres are likely to get there after many flybys of the other planets, Earth and the Moon so could have comparatively low relative velocities once they finally hit the Moon.

Search for past life from Venus on the Moon

The Moon may perhaps have meteorites from early Venus too, from before its atmosphere became as thick as it is now. Early Venus might have had oceans and might have been as habitable as early Earth and Mars. Also some scientists think it is possible that Venus remained habitable to life on its surface until at least 715 million years ago (see also NASA press release, and techy details in paper here). That would give many opportunities for life to be spread from Venus to Earth in giant impacts. Also if the Venus atmosphere only thickened up as recently as that, we should have meteorites from Venus on the Moon right through to that time, as a Chicxulub sized impact on Venus would then be large enough to send material to Earth and the Moon eventually (with escape velocity slightly less than for Earth).

Perhaps Venus might have been habitable right through to the global resurfacing event 400 - 600 million years ago (or more generally, it could be anywhere from 200 million to 1 billion years ago), which took about 100 million years to complete. . It could have had a thin atmosphere much like Earth's until that upheaval which put so much carbon dioxide into its atmosphere that it transformed into its present state.

Annabel Cartwright of Cardiff university - in her "Venus Hypothesis" - a non peer reviewed preprint on arxiv.org, goes so far as to suggest that large impacts on Venus combined with the global resurfacing event could have sent material all the way to Earth as recently as the Cambrian and Ordovician periods. She thinks that this might have been a time when it would be particularly easy for life to be transferred from Venus to Earth. Not directly through volcanic action but through large meteorites hitting Venus before its global resurfacing event, while it still had a relatively thin atmosphere and (perhaps) continental drift, before the stagnant lid formed that lead to the recent global resurfacing.

Though her paper has not been published as a peer reviewed article, I think it's interesting with many stimulating ideas, to think over. So I felt it was worth summarizing here.

She suggests that the increasing day length on Venus would have given an evolutionary advantage to life capable of extended deep hibernation states, high resistance to extreme temperatures and radiation. She cites examples of early life with these capabilities such as the tardigrades, nematodes and Triops cancriformis.

(click to watch on YouTube)

Tardigrade (water bear) drying out and rehydrating. While dried out it is one of the hardiest of all multicellular lifeforms,, able to survive even the vacuum of space, extreme cold and heat, and quite high levels of ionizing radiation, a thousand times the levels that are fatal for humans. It can survive in the dormant state for at least a decade and is the top candidate for multicellular life that could be transferred on a meteorite.

What about shock of ejection from a planet? Small things less than 100 microns across can survive, but in tests of impacts on a planet or the Moon, plant seeds break apart (for instance). So what about Tardigrades? They are complex multicellular creatures with around 40,000 cells. It turns out that resisting impact shock is another of their many talents! They survive impacts of 3.23 km / sec, the highest speeds tested in the experiment with shock pressures up to 7.548 GPa. After rehydration there were some tardigrades still swimming around :). Here is another experiment where they survive impacts of up to 5.49 km / sec. They can also survive accelerations of 16,000 g for one minute, in another experiment (this is different from an asteroid impact scenario though which involves accelerations up to tens of thousands of gs in 30 ms).

So they might be pre-adapted, due to the increasing Venusian day length, to survive passage on a meteorite from Venus to Earth. Also life might have evolved more rapidly on Venus because of the higher levels of radiation there. She suggests this as a possible explanation for the species that arose during the Cambrian explosion with apparently no predecessors and only surviving for a short period of time on geological timescales. She also makes an interesting point that early Earth had connected seas, but early Venus might have had disconnected seas so permitting different forms of life to evolve in each one, so it might have had a lot more genetic diversity in its seas than Earth had, similarly to the way isolated islands on Earth have divergent populations of land animals.

She suggests that periods of rapid increase in genetic diversity on Earth could be due to influxes of life on meteorites from Venus.

You'd have a similar problem with meteorites from Venus as with the idea of panspermia from Earth to Mars, that the shock of ejection would be hard for life to survive. But the Venus escape velocity is 10.36 km / sec . The Earth escape velocity is 11.2 km / sec. So the shock of ejection would be less for Venus, though depending also on the thickness of its atmosphere. It would be easier for it to happen with the larger meteorites with a larger spall zone, but material still would be heavily shocked and the spall zone for a large meteorite is below the surface. It would be much easier if the meteorites were large enough to punch a hole in the Venus atmosphere, or if it had a very thin atmosphere, thinner than Earth's, for some reason. For more on the problems of transfer from Earth to Mars, see General case of transfer of life from Earth to Mars (above).

If her hypothesis was right, then we'd expect to find evidence in life in meteorites from Venus on our Moon, from time periods on Venus as recent as the Cambrian explosion and the Ordovician period. We could then compare those with the life on meteorites from Earth at the same time and our fossil record.

If the more generally accepted hypothesis is right that Venus was habitable to life and had a relatively thin atmosphere as recently as 715 million years ago, then we'd still expect to find many meteorites from Venus on the Moon up to that time and would probably find evidence of any life there too, including organics, and microscopic diatoms or the like and fragments of larger shells and other lifeforms.

Search for past life from Mars - on Phobos and on the Moon

The Moon must have meteorites from Mars too, for us to pick up. See also section 3.1.1 of Back to the Moon: The Scientific Rationale for Resuming Lunar Surface Exploration. Even today, Earth gets around 500 meteorites from Mars that are as large as half a kilogram every year. though most fall in uninhabited areas, or the sea. The Moon also must have meteorites from Mars right through to the present. They may be easier to find there, without the erosion processes that quickly merge them into the landscape in most places on Earth.

Jordi Guntierrez mentions this briefly in his paper too, he suggests that for present day meteorites they wouldn't have a fusion crust hardly any atmosphere to get through on the way from Mars to Earth, but would have a fusion crust from early on if the Mars atmosphere was thicker.

Again the best place to look may be in the polar ice. The Moon must have numerous meteorites from Mars from the Late Heavy Bombardment particularly.

What about native lunar life?

But not native life, at least that’s unlikely unless it had a very fast early start there, or was seeded from Earth or elsewhere. This is another of my fun speculative sections, not meant as a scientific paper. Just something to ponder over.

According to some calculations from volatiles that should have been emitted during the formation of the Mares - the “lunar seas” - vast plains of solidified lava - it should have had an atmosphere of around 1% of Earth normal around 3.5 billion years for up to 70 million years. It was thicker than the current Mars atmosphere, only marginally so, 1% of Earth normal, so could have done some weathering, and had water vapour and other gases. Deeper spots on the Moon would have had thicker atmosphere of course, as for Mars.

There Ga means billion years (before present) and 987 Pascals are about 0.1% of Earth normal atmosphere.

The amount of water would be quite substantial, enough, they think, to account for all the ice that our spacecraft have suggested may exist at the lunar poles, so making this one hypothesis for their origin. If only 0.1% of the volatiles released from the magma was retained at the poles, that would be 100 billion metric tons of water ice.

• An Atmosphere Around the Moon? NASA Research Suggests Significant Atmosphere in Lunar Past and Possible Source of Lunar Water - Astrobiology Magazine
• Paper here

The modeled atmosphere consists mainly of carbon monoxide, some water vapour, hydrogen, and sulfur. The basalt itself has all the trace elements life needs. That leaves only nitrogen. They don’t say what the temperature would be as far as I could see, but it has to cool down enough for ice to form before the atmosphere dissipates.

There would be some organics for it to use, from meteorite and comet impacts, though photosynthetic life could also make it in situ fixing carbon dioxide, if that was present (as seems likely, indeed, microbial communities could also include carbon monoxide oxidisers). Nitrogen would be in short supply, at least, not in the atmosphere, but again, perhaps it could be delivered on comets. After all the ice at the lunar poles is thought to have a fair bit of nitrogen now. By the LCROSS impact experiment. Relative to H2O at 100% they found H2S at 16.75%, NH3 at 6.03% SO2 at 3.19%, C2H4 at 3.12%, CO2 at 2.17%.

Could there have been life on this very early Moon? After all, there are potential habitats for life on Mars, even with its very thin atmosphere (average 0.6% of Earth’s). By some of the same methods - life using 100% night time humidity, or in ice at the poles, in liquid water trapped beneath transparent ice warmed by the solid state greenhouse effect. Maybe seeded by life from elsewhere, e.g. early Earth?

We know that life got off to an early start within a few hundred million years. But - there's a vast amount of complexity to build up, to get to DNA based life. So how long did that take? It must have got to RNA cells pretty quickly. Is 70 million years long enough?

However whether that’s possible or not, 3.5 billion years ago would be well after the origins of life on Earth. There is clear fossil evidence from 3.5 billion years ago and some possible fossils from 3.77 billion years ago or earlier, as well as suggestive isotopic "Evidence for life on Earth before 3,800 million years ago - increased quantites of the isotopically light carbon 12 atom in apatite.

There would be constant bombardment of Earth by huge meteorites up to hundreds of kilometers in diameter back then. So, if it has no indigenous life, this early Moon might well be seeded by life from early Earth. It could also be seeded by life from Mars, which was habitable for life earlier than Earth. Or indeed, Ceres, or Venus of course.

Of course it doesn't mean life will happen there if it is suitable for life. Indeed quite possibly most places in our universe suitable for life are uninhabited.

I haven’t seen that suggested anywhere else so it is just a very speculative suggestion of my own. If there was, then could it be trapped in the cold traps as organics preserved for billions of years at temperatures of liquid nitrogen?

Anyway, if not native lunar life, there is a definite possibility of life from elsewhere in the solar ssytem, still preserved there for us to find.

Search for past life from Mars - on Phobos

The Moon and Mars aren't the only places we can go to, to try to find out about early Mars. What about the moons of Mars?

Phobos especially, the moon closest to Mars is expected to have large amounts of meteorite debris from early Mars which could possibly tell us things about the planet from the early Noachian period onwards. A recent study showed that perhaps 0.025% of the material in the Phobos regolith might be Martian in origin, most from present day Mars but some of it from the early solar system. Earlier layers in the regolith would have concentrations 10 to 60 times smaller than that because Phobos orbited further away from Mars in the past. (Mars impact ejecta in the regolith of Phobos: Bulk concentration and distribution : 128). Most fragments would be at least 0.3 mm in diameter (because smaller fragments are either de-orbited to the Martian atmosphere or to solar orbits within several years) and most larger fragments would reimpact with Phobos many times before they are captured, hitting opposite hemisphere of Phobos each time.

There is also a chance of larger meteorite fragments from Mars, on the surface of Phobos in similar numbers to lunar meteorites on Earth (Original paper: Mars impact ejecta in the regolith of Phobos: Bulk concentration and distribution

A sample-return mission to Phobos would return material both from Phobos and from Mars. Credit: NASA

The interesting question is, could meteorites have impacted into habitable regions of Mars and sent materials to Phobos?

We have Martian meteorites on Earth, but these all come from recent impacts on Mars, within the last few tens of millions of years. After an impact on Mars, the debris from the impact gets completely removed within a few tens of millions of years by impact onto Earth, or the other terrestrial planets, or Jupiter, or the Sun or ejected from our solar system. So though we have evidence of rocks from Mars that were formed long ago, especially ALH84001, they are all from recent impacts within the last 40 million years- the ancient rocks were excavated from Mars relatively recently.

Of course, meteorites that hit Mars billions of years ago sent ejecta to Earth as well, of course - but those ancient Martian meteorites on Earth must have eroded away long ago - and would be hard to distinguish from other rocks here. They are amongst the most interesting targets to look for on Earth's Moon - but also - what about Mars' closest moon, Phobos?

How Martian surface material can get to Phobos

Ancient Martian ejecta on Phobos should still be there, intact, preserved for billions of years. There is no process to destroy it. It can only get hidden by the accumulation of later material on the surface. It would also be uncontaminated by Earth life. If we find amino acids or DNA, or other biosignatures in an ancient Martian meteorite on Earth then this is likely to be inconclusive. The same discovery in a Phobos sample would be clear evidence of life on Mars.

As it turns out, rather a lot of the Martian surface material can end up on Phobos after a meteorite impact.

Shows trajectories of debris from an impact on Mars and the orbits of Mars's two moon's, Phobos (innermost moon) and Deimos

As reported by Purdue university in 2012, Evidence of life on Mars could come from Martian moon

"The team concluded that a 200-gram sample scooped from the surface of Phobos could contain, on average, about one-tenth of a milligram of Mars surface material launched in the past 10 million years and 50 billion individual particles from Mars. The same sample could contain as much as 50 milligrams of Mars surface material from the past 3.5 billion years.

"'The time frames are important because it is thought that after 10 million years of exposure to the high levels of radiation on Phobos, any biologically active material would be destroyed," Howell said. "Of course older Martian material would still be rich with information, but there would be much less concern about bringing a viable organism back to Earth and necessary quarantine measures.'"

(Gardner, 2012, Evidence of life on Mars could come from Martian moon)

The 2012 paper is updated as: (Chappaz et al.,, 2013. Transfer of impact ejecta material from the surface of Mars to Phobos and Deimos)

The interesting thing about this is, that if there was life in the sample, still alive when a large meteorite hit Mars, it has not been exposed to chemical processes on the surface of Mars or alteration by water.

If some of it then got buried on Phobos, perhaps as a result of the impact of a nearby large meteorite on the surface there could be some potential for preserving material from early Mars on Phobos if it was reasonably abundant.

It seems a possibility at least. Failure to detect life on Phobos doesn't mean that there was no life on Mars. But if life was abundant on Mars, in the past, then it might be easy to find it on Phobos, perhaps more easily than on the surface of Mars itself, at least in the early stages of exploration.

The surface area of Phobos is large, a fair amount of territory to explore, 1548.3 km²(Same area as a square island 40 km by 40 km). Its surface is larger than Hong Kong or Singapore, and three times the size of the Isle of Man. If looking for larger meteorite fragments from Mars, we may need to explore Phobos also.

However the regolith is thought to contain many smaller particles from Mars also, and these are distributed throughout its surface. This suggests at least a possibility that any sample returned from Phobos could contain ancient Martian meteorite debris, and possibly also, evidence of ancient Mars life.

Ideally we could send rovers to Phobos to search the surface for traces of organics from Mars, using the ultra-sensitive detectors developed by exobiologists, and dig a little way beneath the surface to find older deposits, and perhaps, intact ancient meteorites of biological interest. Then we could return the most interesting samples for analysis on Earth.

We still have the process of cosmic radiation, to contend with which will destroy most of the organics over periods of many millions of years. Still there are signals, such as chirality,that could be preserved. Also there is no atmosphere to interact with the organics. Perhaps also we might find larger fragments buried deep enough below the surface to have some protection from the cosmic radiation.

It might be a bit of a long shot, because it would depend on the organics and life on Mars being sufficiently abundant. Certainly worth a try though.

Also, there is much else we can learn about Phobos from the mission.

Other things we could find out from the mission to Phobos

Phobos's origins for instance are a mystery, it's hard to explain how it could enter its current orbit if it originated in the asteroid belt, but is not easy to explain as debris from a collision with early Mars either.

It would be great to resolve this mystery. Also either way, whether Phobos is made of materials from early Mars, or is an old asteroid from the asteroid belt, it is interesting to study it in detail for its own sake as well.

A mission to Mars could also access lower layers of Phobos by sampling the Stickney crater which dug deep below its surface.

This mission would also be useful as a way to assess the potential of Phobos for in site resource utilization for Mars orbital missions. Phobos might contain ice, which could be a major asset for future manned missions to Mars orbit,to explore Mars telerobotically, for rocket fuel or life support. See Phobos as first pit stop in manned Mars exploration.

Exploration of Phobos

There have been many plans to explore Phobos and return samples from it. One of them actually got as far as a launch, but its rocket failed soon after the launch, the failed Fobos-Grunt mission by Russia. This would have returned a sample from Phobos. The Russians still have plans to follow it up

The ESA is planning a mission for a sample return from Phobos in 2024 or 2026 called Phootprint.

Phootprint (Artist impression, credit ESA)
For an overview of this mission as slides, see ExoMars - Mars Exploration Programs - and European Robotic Exploration Programme (EREP)

Another proposal is a mission under study by Stanford researchers, in collaboration with NASA, to send "hedgehog rovers" which would bounce over the surface of Phobos. This would study Phobos over a period of several years. It would be a technology demo for a system that could study other small bodies in the solar system in similar detail, and designed for high levels of autonomous operation.

Finally, Russia planned to follow up their failed Fobos Grunt with a new improved Fobos-Grunt 2 possibly as soon as 2024, probably in collaboration with ESA. They have also proposed a "Mars Grunt" which would be a Mars sample return, but have proposed that it does a Phobos sample return first.

Artist's impression of Fobos Grunt approaching Phobos

France in collaboration with Japan plan a sample return from Phobos

There have been several other proposals for missions to Phobos.

Planetary protection for samples returned from Phobos

For the failed Fobos-Grunt sample return, Phobos was categorized as an "unrestricted category V". This means that no special precautions need to be taken. However there was some uncertainty about this categorization. So the Russians, planned to take precautions anyway, similar to those for a restricted category V return. See Categorization of Phobos-Grunt, Also, see Realization of the COSPAR Planetary Protection Policy in the Phobos Grunt Mission - citing from that article:

"Nevertheless, given the fact that the purpose of the mission is the delivery of soil from Phobos, a celestial body that is free of atmosphere and water, the developer of the spacecraft (Lavochkin Scientific and Production Association) with the Institute of Biomedical Sciences RAS, the status of the final phase of the mission was previously defined as a safe return to Earth of category 5 (“unrestricted Earth return”).

"However, with allowance for some uncertainty in assigning the subcategory for this stage, and being aware of the responsibility for the safety of Earth, we have undertaken a number of measures (adopted by the representatives of COSPAR) to meet the requirements of planetary protection of Earth in delivering the samples

"(1) Ensuring the integrity of the container with the Phobos samples at all stages of the mission until landing on Earth.

"(2) Performing specific actions in Earth quarantine after delivering the Phobos samples to a special laboratory for studying the physical and chemical properties of the soil.

"(3) Breaking the contact chain of Earth with the equipment delivered from Phobos by means of the following:a lander with the soil intake device (manipulators) is left on Phobos;the rocket delivering the landing module with the soil to Earth has no direct contact with the surface of Phobos, it being burnt in the Earth’s atmosphere, and its fragments exposed to natural sterilization; the outer surfaces of the module carrying a container with the soil are heated up to ~1500°C during free fall in the Earth’s atmosphere thereby providing for their natural sterilization.

"For ensuring personal and public safety and protection of the environment, following the Federal law “On Sanitary and Epidemiological Welfare of 30.03.1999 no. 52FZ”, the medical and technical requirements have been developed for the organization, sanitation and epidemic prevention measures when working with the samples of Phobos soil and biological objects included in the experiment “BioPhobos” in a specialized laboratory (Institute for Medical and Biological Problems of Russian Academy of Sciences). Compliance with the requirements of Russian legislation is a priority for all types of work and mandatory for all organizations working with samples of Phobos soil and biological objects in the experiment “Bio Phobos”, regardless of the type and scale of the organization."

It's an interesting question for planetary protection. For sure, the risk of planetary protection for Phobos is far less than for the Mars surface. I think we can all agree on that, because any present day sample from Phobos has almost certainly been exposed to cosmic radiation for a sterilizing hundreds of thousands of years minimum. The most recently ejected Martian meteorite on Earth is EETA 79001 which was launched from Mars 600,000 years ago.

The EETA 79001 Meteorite, youngest meteorite from Mars, radiation exposure age 600,000 years. This is old enough so that it has been thoroughly sterilized by cosmic radiation during its journey to Earth. Many meteorite fragments from this same impact on Mars would reach Phobos after a much shorter journey (typically). However they would have rested on the surface of Phobos for a similar period of time, exposed to similar levels of cosmic radiation.,

 (Incidentally, this meteorite is of historical interest as the one that was used to prove conclusively that the SNC meteorites were of Martian origin by measuring isotope ratios in trapped bubbles of gas in the Martian atmosphere).

Theoretical studies suggest meteorites able to send materials to Earth would hit Mars once every one or two million years. However not every impact able to send material from Mars to Phobos will also be able to send it all the way to Earth. So, it is possible that Phobos may have more recent samples of Mars than that. The chance of contamination from Phobos must be extremely low, but still, is there s the remote chance of more recent micro-organisms from Mars? In the very remote chance that there is viable life in the sample, then there may be a tiny chance of severe consequences, just as for any Mars sample return.

This is all very unlikely, as it's going to be hard for any of the fragile surface life to get to Phobos. If it has past organics and evidence of life on Mars, probably that's from times when the surface of Mars was at least a bit more clement with rivers and seas, or lakes. That's long enough ago for it to be thoroughly sterilized.

So, is a Phobos sample return is properly classified as an unrestricted category V? Should we be cautious and classify it as restricted category V at least until we have some idea of how recent are the most recent samples from Mars on Phobos?

Well, on the other hand, we may be able to to apply the Natural contamination standard here. Survival of a sample for what must be at least many centuries on or very close to the surface of Phobos is not unlike survival of that sample in transit from Mars to Earth. Also, ejection of a sample from Mars to Phobos again is very similar to ejection from Mars to Earth. Somewhat less delta v is needed to get to Phobos, but it is a very similar situation.

Could we argue that any samples returned from Phobos are similar to the Mars meteorites we already have (including any that may land undetected, from more recent Mars impacts), or at least, to samples like that which got to Earth 600,000 years ago within a few years of the impact on Mars? There doesn't seem to be any evidence of severe consequences on Earth from Mars microbes as recently as 600,000 years ago.

We need to be conservative about this if unsure. On the other hand, classifying it as an unrestricted Category V of course greatly simplifies the legal situation, and removes the need for a dedicated Mars receiving facility with all the complexity that involves. Classifying it as restricted category V might make it unfeasible. So, we need to be careful to classify something as restricted Category V only if that is really needed.

One suggestion. What about sterilizing the sample with the equivalent of 600,000 years of ionizing radiation before return to Earth? That then would bring it to parity with the natural contamination standard for the most recent meteorites to leave Mars. So then, on the remote chance that there are more recent samples that have got to Phobos but not reached Earth, then this would bring us into parity with the samples received from Mars directly to Earth. We could use gamma radiation for the sterilization. I'm not sure this is necessary but it would be a way of sticking to the letter of the natural contamination standard. Then we could use the same reasoning as is used for a comet or asteroid sample return to classify it as unrestricted category V, and perhaps then we can do that with no need for that slight uncertainty there is at present?.

What do you think about a sample return from Phobos? Is it okay to classify it as an unrestricted category V? In the circumstances, do you think Russia's proposals for how to handle it are adequate? Or is anything else needed?

Phobos could of course also have meteorites from early Earth, Ceres, and Venus, like our Moon, but is of especial interest for Mars.

These sections include material from my article: Why Phobos Might be the Best Place to go for a Sample Return from Mars Right Now.

Search for early life on Ceres

Mars, Earth, and Venus are all likely origins of life. However, there's another possibility too, there's a chance that Ceres was the origin of life for both Mars and Earth. Some things in its favour are:

• It seems to have got off lightly in the bombardment by giant meteorites in the early solar system,
• It see ms likely to have had hydrothermal vents in the early solar system, and large amounts of water.
• The ESA space telescope Herschel tentatively detected water escaping from Ceres at a rate of about 6 kg per second. in four observations from November 2011 to March 2014, and then two additional observations.

Then the Dawn spacecraft found bright spots on Ceres, for instance in Occator crater. These turn out to be carbonate salts probably brought to the surface in cryovolcanoes - liquid water erupting onto its surface, much as lava does on Earth. This is commonplace further out in the solar system, but Ceres is rather close to the sun for cryovolcanism. Then, more strikingly, it found evidence that Occator crater is all that remains of a former central mountain from an impact as recently as 34 million years ago, what's more, some of the white material is only four million years old, and there may be some activity even today.

The white mound here is in the center of Occator crater on Ceres. It's thought to be the remains of a cryovolcano, erupting liquid water instead of lava. And there's evidence of a light haze that forms in this region, varying in brightness through the day, suggesting that perhaps there is ice there still exposed to the surface. That shouldn't be possible so close to the Sun and on a surface warmed up by the Sun, unless the ice is replenished from below. So the cryovolcanism that created this white dome four million years ago may still be somewhat active. Press release here.

There are two ideas about how such recent could happen. In both cases an asteroid hits Ceres. With one of them, the asteroid creates an impact hydrothermal system of molten rock and water, and the other idea is that there is watery material below the surface (a kind of mud), already liquid, which was released by the impact itself.

Ceres probably started off as an ice covered ocean similar to Europa and Enceladus (in between them in size). The surface pure ice would all have evaporated by now because it is so close to the sun compared to comets, within the "ice line". But its subsurface muddy salty oceans would have remained, leaving the surface covered in clays, which we see and ammonia mixed in as well.

Its clays particularly are a strong smoking gun of an early ocean. The whole surface is covered with these magnesium clays mixed with ammonia, carbonates and some darker material. Here is Carol Raymond talking about Ceres and whether it is an ancient ocean world (about half way through).

(click to watch on YouTube)

The water might not all be gone. There could still be liquid water beneath the surface of those bright salty features in Occator crater. Ralf Jaumann, planetary scientist and co-investigator from DLR says

“I’m pretty sure that there’s also water in Occator in the subsurface,” Jaumann said. “The problem we have is all of our instruments are not able to penetrate the surface. We just get the first millimeters of the surface in our data.”

Ceres also has bright spots in cold traps in some of its permanently shadowed craters, near its poles. They are in conditions where ice could be stable for billions of years (temperatures of 110 K, so-163 °C or -260 °F). Dawn was able to analyse one outcrop which extended into sunlight allowing Dawn's spectrometer to confirm that it was ice. It could be that all the bright spots in permanently shadowed craters contain ice.

Interestingly, one small crater, Oxo crater, has ice in a sunlit region. It can't have been there for long, as it would disappear over timescales of decades to centuries.

Water ice in Oxo crater. This ice will vanish quickly compared to most geological timescales, on a timescale of decades or centuries. It must have got there recently somehow. Carol Raymond talks about this in her presentation video in the second half.

With these discoveries, Ceres seems a prime target for the search for origins of life. It must have had liquid water in the early solar system, probably a global ocean similar to Europa and Enceladus. It may have subsurface water, or mud, still there to this day beneath some of the white deposits.

Earth, Moon and Ceres to scale, for comparison. One theory suggests that Ceres could be the origin of life for Earth and for Mars. The Moon could be interesting for the search for life also, as it would preserve meteorites from impacts on early Earth, also on Mars and probably Venus too from the earliest solar system.

Also, to share a speculative thought again. With ice stable for billions of years in its permanently shadowed craters - I wonder if it could be another interesting place to search for meteorites which could preserve organics and fossils from life in the early solar system, as for the Moon, once we have the capability to do this?

Myth of automatic terraforming, or easy terraforming

This is an idea that may be familiar to you from many science fiction stories. The idea that if you add microbes to a planet, no matter what they are, even if you introduce them by mistake, that this will automatically turn the planet into a second Earth or the closest to Earth that's possible for the planet. I call this the "myth of automatic terraforming".

To make a start at challenging this idea, think of a future Earth too hot for life, a billion years into the future. It might well have life for a while, living on the edge in a planet that is just barely still habitable. Most, or all of the life there would consist of microbes that can tolerate extremely hot conditions (extremophiles), such as the ones that live at very high temperatures in our hot springs.

As Jack O’Malley-James put it in his thesis "Life at the End of Worlds":

"As the limits of life on Earth are explored, the boundary conditions for habitability are pushed further away from the constraints of the traditional HZ. This allows for planets to be considered habitable, but not necessarily Earth-like. Ocean planets, arid planets, ice planets and even rogue planets that have been ejected from their original systems and are traveling through interstellar space can all potentially be considered habitable. For these cases, it is the extremes of life on Earth that hint at the possible biospheres."

(page 36 of his thesis)

So a far future Earth, might only be able to support what we on Earth call extremophiles. He suggests that we may be able to study analogues of that far future Earth right now, spectroscopically, by studying planets around sun like stars that have reached a later stage in their star's main sequence progress. He gives several examples, including HD 197027. which is a star that's very similar to our sun, but rather older, about 8.2 billion years old, so around 3.6 billion years older than our sun. Though this star is not known to have planets yet, it may well (as most stars are now known to have planets). So, suppose it had a planet similar to Earth in its habitable zone in the past. What would it's state be now? It could be a way to get a glimpse of the future of our own Earth, not just in theory but as actual observations of a similar planet. He finds that if it had a planet similar to Earth in the past, then by now it would be barely habitable, with the possibility of a "very small, fragmented biosphere today". See page 120.

"If an Earth-like temperature is assumed for an Earth analogue planet at a point in the host star’s main sequence evolution that is equivalent to the Sun’s current main sequence stage (approximately 3.23 Gyr ago for HD 197027), the temperature evolution of that planet would render it largely uninhabitable within 3 Gyr (cf. Figure 6.5) of that point (0.23 Gyr before present). Isolated habitable regions could persist for up to a further 0.5 Gyr for high obliquity values (> 60◦ ), which would allow for the possibility of a very small, fragmented biosphere today" (emphasis mine)

So, with a planet so hot as to be barely habitable, then why would adding a lot of microbes from present day Earth cool such a planet down automatically? If it could sort itself out, it would have done it already.

Now, just possibly there might be some biological way to use microbes to do something to help cool down that future Earth or present day planet orbiting HD 197027.or other similar planets - but there doesn't seem to be any reason to expect that to work automatically. You might well have to design the solution around the situation and use artificial methods to establish the necessary feedback loops.

Mars may be in a similarly extreme state, Earth-like and very habitable in the early solar system, except, that instead of getting too hot, it got too cold for most forms of life. He points this out in section 6.3.

"Mars could provide a local example of a planet in the very end stages of its habitable lifetime, albeit a planet that has become largely uninhabitable as a result of low temperatures and losing an atmosphere, rather than from runaway heating."

Perhaps Mars had life early on and what we see there now may already be a planet "at the end of its life" with only fragmentary habitats, much like the possible future Earth around HD 197027.

If so, its native life has not terraformed it, and if so, why would life from Earth terraform it? Why would life from Earth push any other planet with completely different conditions into an Earth like state?

For that matter why would the addition of life from Earth push even an exact copy of Earth without life into a copy of Earth's present day biosphere? Even if the life came from Earth originally? We don't know if our current Earth is in a unique state that all Earth like planets tend towards. Perhaps Earth had many different states it could have ended up in? Some might have been less habitable and others habitable to many creatures, but not to the likes of us.

For instance, Earth is not that far off a "moist greenhouse". If our Earth was an entirely ocean world, with no land, and the sun was three percent warmer, or we were closer to the Sun, we'd be in that state now. average sea surface temperature of 330 °K or about 57 °C. It would be moist heat too. Those temperatures are far too high for habitability by humans - we can survive dry heat but not moist heat, with a limit of a 30 °C wet bulb temperature. Yet for the creatures who live in such a world, it would be home, ideal for them. Adding Earth life to that planet again would not make it into an Earth-like planet habitable to humans. It's just fine as it is for whatever lifeforms live there. They would want to find ways to turn our Earth into a moist greenhouse to make it comfortable for them.

We can't get into this state even if we burn all the available fossil fuels of all types. But our Earth could enter such a state in the future over timescales of hundreds of millions of years, long before it ends up in a Venus style greenhouse. I discuss this moist greenhouse, and the runaway Venus possibility more in Hawking Says Trump Could Tip Earth To Hot Venus Climate - Is It True? What Can Earth's Climate Tip To?

Indeed, the situation may be worse than this. Adding life from Earth might actually make a planet less habitable to humans. Indeed, might that have happened already? Could Earth originated, or Earth-like life have made Mars less habitable than it would otherwise be?

Could oxygen generating photosynthetic life set up an "anti Gaia" feedback on Mars?

This another of these synthesis / speculation sections. As far as I know, it is my own idea, suggesting it as an example of the sort of thing that could happen, as an interesting possible counter example to this myth of automatic terraforming.

Could photosynthetic life be self limiting on Mars? Here we assume of course that it somehow developed oxygen generating photosynthesis or got it somehow from Earth. In that case, perhaps photosynthetic life on Mars would actually set up feedback cycles that limit its own growth.

This is how it would work:

• Start with Mars warm enough for a fair bit of photosynthetic life
• This takes carbon dioxide from it's atmosphere
• This thins its atmosphere and cools it down.
• This makes it less habitable

This feedback loop continues until it stabilizes at a level where it is just barely habitable. At that point, it would be so cold and dry that its photosynthetic life that it has negligible effect on its atmosphere. It then could warm up from changing eccentricity, but as it does so, the life then takes more carbon dioxide from its atmosphere and cools it down again.

This would be exactly the same feedback loop as operates here on Earth. Photosynthetic life works just great here, as it's exactly what we need, to cool the planet down. Before photosynthetic life evolved, Earth was getting too hot because of the warming sun. It didn't work perfectly - it threw Earth into a snowball phase for many millions of years, so it "overshot the mark". But eventually Earth ended up in a nice stable state, with photosynthetic life as part of the system that keeps it all going nicely by removing excess carbon dioxide from the atmosphere (on long timescales anyway)..

But Mars never was at any risk of getting too hot as the sun got hotter, and as it lost its atmosphere its problem was that it was too cold for life, not too hot. So on Mars, this same feedback loop would have made it less habitable.

Indeed, wild speculation again, what if the present situation on Mars is the result of this process, with photosynthetic life there taking carbon out of the atmosphere, not only cooling the planet, but also leaving free oxygen in the atmosphere, which reacts with the crust? If that was the case, you'd expect there to be some photosynthetic life on Mars, but in such tiny amounts that it's effect on the atmosphere is only barely detectable, if at all. It would look pretty much the same as Mars does to us so far, except we haven't found the life yet. We haven't really looked thoroughly though, not for such small trace amounts of life. You'd expect to see more life at times when Mars warms up and its atmosphere gets thick, temporarily, but even then, it would be acting all the time to cool it down yet further.

If this was the situation, Mars could be locked in a kind of "anti Gaia" feedback loop, which continually keeps it only just marginally habitable. Whenever it warms up and gets a bit more habitable this "anti Gaia" feedback loop would kick in making it less habitable again.

As far as I know, this idea of an "anti Gaia" of photosynthetic life on Mars is my own. Do please let me know if you have come across any papers on this topic!

I don't know how likely or far fetched that is in the case of Mars. However, it is just an idea to illustrate a point here. Life might not necessarily automatically make Mars more habitable.

Indeed also, if it hasn't developed photosynthesis yet, it might be that adding photosynthesis is the worst thing you could do right now, by way of attempting to make it more habitable.

Closing off future options - hard enough to roll back introduction of feral camels to Australia, never mind of microbes to Mars

The main thing is that adding life to a planet can push it in many different ways, as it has often done on Earth, and there is no way of knowing if it would make the planet more or less habitable to life. At least, not unless you hold to the strong Gaia hypothesis. Even then, why would it be more habitable for humans specifically? More on this in Myth of automatic terraforming and the strong Gaia hypothesis (below)

The one thing it definitely does do however is to close off future options. After you've introduced new life to another planet, and it actually takes hold there, especially if that new life is in the form of hardy microbes, what can you do about it? It is hard enough to roll back rabbits, cane toads, rats, Kudzu or Japanese knotweed, and other higher lifeforms from a continent or even a small island, sometimes. When you have a large continent like Australia, it can be next to impossible even to roll back the introduction of camels! (Australia still have more than 300,000 feral camels, after an expensive program that reduced the initial population of a million camels).

How could you roll back a problem microbe from a planet with a surface area as large as the land area of Earth? We can't even roll back the effects of humans on the Lascaux cave paintings, as I mentioned near the start of this book, in the section Touching Mars . Like the microecologists studying the Lascaux cave paintings, you may have to conclude that we are stuck with the new situation on Mars whatever it is and can only look for a new equilibrium. Quoting from the summary of the Lascaux Cave Paintings Symposium:

"Isabelle Pallot-Frossard and Geneviëve Orial presented the micro-biological context and the strategies chosen to control it (biocides and repeated actions, more than climatic management and cleanings), whereas Claude Alabouvette detailed the microbiotic ecology (stressing the augmentation of micro-biological diversity in the zones affected by anthropic activity). The work carried out at Lascaux was compared to the efforts of conservation of mural paintings in Japan (Takeshi Ishizaki). With this debate, it became clear that, for the hydrogeologists and climatologists, the main paradigm seems to be the recovery of the equilibrium prior to 2000, whereas for the microbiologists this is not possible, the aim being the search for a new equilibrium."

So, it seems pretty clear that once a new microbe has established itself on Mars, you can never roll back. If you later find that one of the lifeforms you introduced is a major problem on the planet, there is nothing you can do about it, not if it is a microbe, and probably not that much you can do about it if it is some hardy form of multicellular microscopic life.

This new equilibrium could rather easily work against your longer term aims for Mars. Maybe you want to increase oxygen levels but you introduced aerobes that eat the oxygen? Maybe you introduced secondary consumers that eat the algae that you want to use to introduce oxygen?

Maybe you want to increase carbon dioxide levels, but some microbe is busy fixing the carbon dioxide and turning it into oxygen and reduced carbon. Maybe you want to increase methane levels as a greenhouse gas to warm the planet, but you accidentally introduced methanotrophs that eat it all the methane you produce?

Maybe you introduced a microbe that poisoned water supplies throughout Mars so that they are harmful to humans, and need extra treatment? Or, the main focus of this book, you introduced a microbe that is making native Mars life extinct, and you only discover it too late.

There are many things could go wrong as a result of microbes you introduced by mistake.

Imagined colours of future Mars. This is just to suggest the idea that there could be many possible futures and accidental or intentional attempts to transform the planet could push it in many different ways, and we might not have much control on what happens after that especially if something takes it in an unexpected direction.

The one in the middle is the aim of terraforming. But it could as easily be any of the others or something else altogether. And once we start to introduce life to Mars, there is no way to take any of it back again if it causes problems, or evolves rapidly into something problematical. See Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?

As one simple example of how microbes introduced by mistake could mess things up quickly, some bacteria convert water to calcite, and if you introduce them by mistake, you might find that these microbes have converted the underwater aquifers to cement. That's an example from Cassie Conley, current planetary protection officer for the USA - she is a microbiologist and astrobiologist.

"Conley also warns that water contaminated with Earth microbes could pose serious problems if astronauts ever establish a base on Mars. Most current plans call for expeditions that rely on indigenous resources to sustain astronauts and reduce the supplies they would need to haul from Earth."

"What if, for example, an advance mission carried certain types of bacteria known to create calcite when exposed to water? If such bacteria could survive on Mars, Conley says, future explorers prospecting for liquid water instead might find that underground aquifers have been turned into cement."

Going to Mars Could Mess Up the Hunt for Alien Life

In more detail, Mars has almost no oxygen, which changes how microbes behave. What she is talking about there is anaerobic oxidation of methane, which leads to the formation of calcium carbonate in anoxic conditions . It's done by a consortium of methane oxidising and sulfate reducing bacteria. See summary here in wikipedia: Calcite - formation process - which links to this technical paper which goes into more detail.

Geoffrey Landis, NASA scientist and "hard" science fiction writer, gives another fun example of how it could go wrong. The idea of "ecopoiesis" is to set up an ecology, not necessarily Earth-like, and to use that to initiate an ecosystem. So with Mars you want to release a lot of CO₂, if it is there, and then, you don't want any oxygen producing life, because that would cool it down

So the plan would be to have an anaerobic ecology in a sulfur rich environment. As Geoffrey Landis tells the story:

"So ecopoiesis and warming up and giving an ecosystem to Mars would in fact mean adding more carbon dioxide and making an anaerobic ecology. And Sagan was sort of a little bit sarcastic about this possibility and said,

“Well, let's see, let me see if I get this straight -- basically, you're proposing converting Mars into a sewer. Anaerobic bacteria! Yay, sewer bacteria!”

So, the sort-of working title of Brown Mars was going through my head. So this was a story about Mars that had been not terraformed, so it wasn't Earth-like, but yet it had an ecosystem, an anaerobic ecosystem that, as our astronauts going to it many years after that ecopoiesis event, take off their space suits and say, "Oh my God, Mars smells like shit." So that was my first Mars story. "

The story's final name was Ecopoesis and you can find it in his collection of short stories "Impact Parameter: And Other Quantum Realities". Here is the part where his characters discover that Mars stinks:

Suddenly she broke away from me. "Oh, god!"

"What?"

"Take off your rebreather."

Puzzled, I reached up, snapped the strap free, and pulled it forward over my head. The silicone made a soft "poik" as the seal popped loose. I took a breath, and gagged on the sudden odor.

The smell was as if I'd been wading through a cesspool in the middle of a very rotten garbage dump. I looked down. My shoes were covered in brown. One leg, where I'd knelt on the ground, had a brown spot on the knee. Leah was even dirtier

Shit.

Tally popped through the lock, accompanied by a fresh burst of fecal odor. I held my nose and suppressed my instinct to gag.

"Of course," said Leah. "Anaerobic bacteria." She thought for a second. "We're going to have to find some boots, and maybe overalls. Leave them outside when we come in."

I started to giggle.

"What's so goddam funny?" Tally said.

"I've decided you're right," I told her. "Mars stinks. Take off your rebreather. You'll see."

Why bringing Earth life to Mars could be like making wonderful yoghurt - or bad smelling gone off milk

If we do eventually introduce Earth life to Mars, well it could be like milk and yoghurt. It's rather a close analogy actually.

Depending on the microbes you use, milk can turn into butter, cheese, yoghurt, or be spoilt by pathogens, Mars might be the same. If you want yoghurt then only two species will do the trick, of the countless different species you could introduce to the milk.

A bowl of yoghurt, photograph by Swabian (wikipedia)t. Let's use this as an analogy for "good Mars" - whatever it is you want to achieve there.

Then of the many species you introduce to the milk, only two of them will create yoghurt. The rest are undesirable and will frustrate you in your objective to make the milk into yoghurt, and will probably make this impossible. If you are lucky you may end up with some variety of cheese, but most likely you just end up with sour milk that's "gone off" or perhaps something that's even harmful to you.

The other species you might introduce to the milk are most likely to spoil it, make it inedible, or even make it poisonous for humans. If we are very lucky, introducing random microbes to a bowl of milk without vetting them, perhaps we get cheese when we were hoping for yoghurt. But the most likely thing to happen is that it just goes off.

That's a good analogy for Mars, if it has habitats there already (as many think it has). It could well be that the entire future biology of Mars could depend on the chance event of some microbe that got there accidentally in the first crash of a human occupied spacecraft on the planet. Just as the future of a bowl of milk depends on which microbe gets to it first, and the current condition of the Lascaux caves depends on the sequence of microbes that colonized it after its discovery, so the future of Mars could depend on some accidentally introduced microbe that gets into the habitats there first before the others.

This can't be the right way to introduce Earth life to Mars, even if we do decide to introduce life to it eventually.

Once the milk is gone off, there is no longer any possibility of making it into yoghurt.

If we are interested to find out what happens to Earth life in a Mars like environment, we don't have to do that on Mars. We can simulate huge Mars type environments in lunar caves or habitats made from materials from asteroids, and see what it would do there. We can build kilometer scale Mars simulation Stanford Torus type habitats too eventually.

In that way, we can try out our ideas for Mars, not just on a small scale, as we do today, in Mars simulation chambers a few meters in scale. We can try this out in entire Mars simulation habitats. These would be expensive, yes, but they would cost far far less than the countless trillions of dollars needed for transforming Mars in any kind of a directed meaningful way.

Also, if we use habitats instead of Mars itself to try out our Mars colonization and terraforming ideas, we'd get results in decades without having to wait for thousands of years, or even hundreds of thousands of years, to see what happens. Even then, even with tests in multi-square kilometer artificial habitats, the chances are we'd not be able to make an accurate predictions of what Earth life would do to Mars in advance. We'd still get many surprises. But we might at least manage to winnow out some of the worst surprises that could happen to us if we attempt to terraform Mars. More of this in Instead of terraforming Mars in a multi-millenium project, why not terraform a lunar cave in a multi-decade project? (below)

Myth of automatic terraforming and the strong Gaia hypothesis

I think this myth of automatic terraforming is based originally on Lovelock’s Gaia hypothesis in its strong form, the idea that life makes planets more habitable for itself.

The weak Gaia hypothesis is widely accepted - that the Earth has many systems that work together to help keep it in a habitable state, mediated by life, is widely accepted. However, the idea that such a system arises automatically on all terrestrial planets with life is not at all universally accepted, although it is a staple of science fiction. That’s the “strong Gaia hypothesis”.

Some things about our own planet are puzzling, if you try to interpret it in terms of "Strong Gaia". For instance, why did photosynthetic life evolve, or proliferate when it did? Why did it turn a warming carbon dioxide rich atmosphere into a cooler oxygen rich one, at just the right time to cool our planet down to keep it habitable? Why didn't it arise too soon, making our planet too cold, or too late, leaving it too hot? The strong Gaia hypothesis would say that this is not accidental, but happened at just that time because the Earth needed an oxygen atmosphere to stay cool. But the details of why it would happen just then aren't very clear. And after all, it wasn't perfect, with an overshoot to a planet that was too cold, with Snowball Earth.

In science fiction the strong Gaia hypothesis has been exaggerated to mythology. You often get the idea that introducing life to a planet not only helps keep it habitable for that life, but that it also automatically makes it habitable for humans too. The story goes, that you head off to a lifeless but potentially habitable planet, add more water and atmosphere if necessary, scatter Earth microbes, come back a few thousand years later, (or millions of years later) and you have a second Earth.

Is that true? Suppose that they are right, that somehow, for reasons not yet fully understood, life is always able to make its planet as habitable as possible for itself. If that was true, would it turn Mars into a second Earth? Well something that could lead you to start to have a few questions about that is that an Earth type biosphere is seriously sub optimal for Mars. It would leave temperatures there well below freezing. Our atmosphere is not a warm enough "blanket" for Mars, which is so much further from the sun. Would a strong Gaia on Mars really make it into a second Earth, even approximately?

Why "Strong Gaia" should give Mars a methane / hydrogen / carbon dioxide / hydrogen sulfide atmosphere - not an oxygen rich one

The way to make Mars the most habitable it could be for life would be for methanogens to evolve to create methane, which is a strong greenhouse gas. A mixture of 3% methane, and 3% hydrogen would be ideal, with the rest carbon dioxide as we saw in Greenhouse gases on early Mars. Or alternatively perhaps it could somehow generate lots of hydrogen sulfide (a gas produced by sulfate reducing bacteria). That too could keep it warm, though it's not as strong a greenhouse gas as sulfur dioxide (which comes mainly from volcanic eruptions though you can also get it by burning hydrogen sulfide in oxygen). Hydrogen sulfide is highly toxic to humans except when in low concentrations. Carbon dioxide is toxic to humans in doses above 1%.

The last thing Strong Gaia would do on Mars is to generate an oxygen rich atmosphere like Earth's.

I don't know of anyone who has worked out what the implications of the strong Gaia hypothesis would be for a Mars like planet (again do let me know if you know of such a paper) - but if the strong Gaia hypothesis was true, then surely also the life would evolve over time to generate stronger and stronger greenhouse gases on Mars to keep it warm? That would make it more habitable for its native lifeforms, but not an environment humans could live in.

We may have spotted methane on Mars. If so this figure from NASA / JPL shows possible sources. One possibility is methane clathrate storage. It's possible that early Mars had large amounts of methane in its atmosphere which helped keep it warm. The only natural way for a Martian version of Gaia to keep it warm today is through generating greenhouse gases.

If so, a methane atmosphere is one way it could do it, or a mixture of methane and carbon dioxide, or other stronger naturally produced greenhouse gas. The result would be habitable possibly for ancient Mars life, but not for humans. This could be a way to "Mars-form" Mars to return it to conditions that it enjoyed in the early solar system. But if so, and if this is indeed what early Mars was like, then whatever lead to the methane disappearing would probably happen again.

That would be a very strong version of the Gaia hypothesis - the idea that

• Life on planets like Earth evolve oxygen generating photosynthesis to make it colder as it gets too warm as the sun gets a little brighter
• Life on cold planets like Mars evolves methanogens and other microbes that create increasingly more powerful greenhouse gases to warm it up as it cools down through loss of its atmosphere

If that was right, it would not be too promising for making Mars Earth-like. It would mean that whatever you try to do to Mars, the biosphere would tend to converge back to an oxygen poor and carbon dioxide and methane rich atmosphere or something similar. You'd be fighting against strong Gaia every step of the way.

Mark Waltham (in his "Lucky Planet") has argued that it is probably much more a matter of luck, at least partly. He argues that just by chance, life on Earth converted carbon dioxide in the atmosphere into oxygen at just the right time to cool it down.

It doesn't really matter whether you accept weak Gaia or strong Gaia. With this background, then introducing Earth life to Mars would probably do nothing to make it more habitable for humans, not without some long term plan, megaengineering, and careful selection of which lifeforms to introduce when.

You can't just leave it "up to Gaia" to do it for you.

• On the weak Gaia hypothesis, you can't rely on Gaia to do it for you, because it is a matter of chance. We just don't know enough about planets yet to "rig the dice" in our favour.
• On the strongest possible Gaia hypothesis, then it would surely evolve to an oxygen poor, carbon dioxide atmosphere enriched with methane,, hydrogen and hydrogen sulfide instead, maybe also sulfur dioxide - or something similar.

So, if you want an Earth type atmosphere, then you need need artificial greenhouse gases or large planet scale mirrors or both, indefinitely, to keep it warm enough long term. It then becomes a thousands of years project that goes on and on, trillions of dollars a year keeping it habitable, through into the indefinite future or until eventually you give up. On the weak Gaia hypothesis, you have to engineer the whole thing to work, or depend on a large element of luck. On the strong Gaia hypothesis, then if you want an oxygen rich breathable atmosphere on Mars, you have to fight strong Gaia all the way.

And what do you do if it begins to go in some unexpected direction? That could happen because you are fighting against "strong Gaia" or because you haven't managed to "rig the dice" quite right for "weak Gaia"?

It is a major issue on Earth just to keep the levels of carbon dioxide at the correct values from rising at levels of only 400 parts per million. Also, those tiny changes, significant though they are on Earth, are the result of a planet spanning civilization burning huge amounts of fossil fuel.

On Earth, a planet spanning civilization burning huge amounts of fossil fuel was able to increase the carbon dioxide levels in the atmosphere by about 10 ppm per decade, or 0.01%. And now we are running into issues trying to prevent it from going significantly above 400 ppm or 0.4%, we can't yet reverse that trend though we hopefully will be able to reduce the rate of increase. On Mars we need a much more radical transformation than just keeping levels of carbon dioxide from getting too high, or raising it by tens of parts per million per decade. We want an atmosphere with breathable levels of oxygen, no toxic levels of hydrogen sulfide, less than 1% carbon dioxide -and starting with an unbreathable atmosphere of pure carbon dioxide so thin that the moisture lining our lungs would boil.

The keen terraformers have worked out methods that they think might work, with significant levels of megatechnology. But what do you do, if, despite all your efforts, your Mars biosphere starts to produce methane, or hydrogen sulfide or some such in large quantities? To change it's atmosphere back again, you need a Mars wide industry. Do we have anything like the technology to attempt something like that in the near future? Not just to give the Mars biosphere a big kick in a desired direction, but to keep it on course for whatever outcome we wish to achieve after that, for centuries and millennia?

Who can say, maybe some day we will be able to do this. Maybe, decades or centuries into the future, we will have technology, and more important, the deep understanding of planetary ecosystems, that even makes it almost "easy". But meanwhile, we only have one example so far, of a planetary ecosystem, and that's our Earth. These ideas for terraforming are based on a lot of extrapolation on rather limited information.

Why not start a bit smaller than an entire planet?

Instead of terraforming Mars in a multi-millenium project, why not terraform a lunar cave in a multi-decade project?

I think it is great to think about terraforming ideas, yes. It helps us learn a lot about our planet and exoplanets and Mars itself to do those thought experiments. But as for practical experiments, let’s start a lot smaller. We haven’t yet managed a closed system ecosystem the size of Biosphere II on Earth. So that's probably the first step.

WE have already been able to create small scale habitats where humans grow crops and the crops generate oxygen, in a small space, such as the Russians with their BIOS-3. But here we are talking about something far more ambitious than that, a complete biosphere. We haven't done that yet.

Biosphere II was our best try at that to date, and it failed, though we learnt a lot that should help with the next attempts. Once we have successful small closed system ecosystems like that on Earth, then we can try the same ideas in space also, for instance in the possibly vast lunar caves, as vast as an O'Neil cylinder.

Artist's impression by Don Davis of the interior of an O'Neil style cylindrical space colony - from Space Colony art of the 1970s.

The Grail radar data suggests the possible presence of lunar lava tube caves over 100 kilometers long. They could also be as much as kilometers in diameter, in the low lunar gravity. So, lunar caves could potentially be as vast as an O'Neil cylinder . If so, maybe some day we could have colonies like this on the Moon, easier to construct than an O'Neil cylinder. Probably they would be multiple tiered and of course nobody can live upside down on the roof as the gravity pulls in one direction only.

During the lunar day, the light for the caves could come from solar collectors on the surface channeled through optical fiber. During the lunar night the illumination could come from efficient LED lights powered from fuel cells and other forms of stored power. Or they could be powered by strips or patches of solar panels that circle the Moon round to the day side.

Solar panels are easy to make in the hard vacuum, especially since the nanophase iron makes it easy to turn the lunar soil into glass using microwaves (as easy as boiling water in a microwave). Solar paving rovers could drive over the Moon making the surface into solar panels. See Solar cells from lunar materials - solar panel paving robot in my "Case for Moon First"

For more about all this see my An astronaut gardener on the Moon in Why Humans on Mars First are Bad for Science.

This would be a vast and ambitious project, lunar caves with a biosphere of a hundreds of cubic kilometers. However, it is far far smaller and easier to achieve than the planetary scale biosphere needed for terraforming. It is also a project that could, realistically, be completed in decades.

If you think this is an ambitious project, it's nothing compared to terraforming. A terraformed planet would take thousands, or hundreds of thousands of years to completion. On Earth the process of "terraforming" took hundreds of millions of years, or billions of years, depending how you measure it. Since when have we ever achieved a project that requires thousands of years to completion?

We could start smaller than this of course, much smaller, biospheres of cubic meters, then hundreds of cubic meters. Surely it would be a while before we feel we can tackle a biosphere consisting of hundreds of cubic kilometers or more. If we can't make those, we have probably got nowhere near the technological capability needed to terraform a planet.

Then once we achieve this, we can work up to larger maybe city dome or Stanford Torus type ecosystems and try filling entire lunar caves with air and converting them to habitats. Eventually we can try Terraforming or paraterraforming the Moon. Let’s leave off ideas to terraform planets like Mars until we know a bit more.

For more on this, see also

• Best places to introduce Earth life right now - while continuing with biologically reversible exploration of Mars
• Plenty of places to experiment with sending life to other places in our solar system - Asteroid belt resources, NEO's, caves on the Moon

Searching for a non confrontational way ahead

At the moment, there's a tremendous impetus amongst Mars advocates to get to Mars as soon as possible. Elon Musk even hopes to send humans to the Mars surface as soon as the 2020s, recently suggesting a first human mission in 2024, with NASA talking about the 2030s. I think it would be wrong though to suggest that Elon Musk doesn't care about the science impact of introducing Earth microbes to Mars. Here he answers a question on this topic, in the 2015 AGU conference in San Francisco, 30 minutes into this video:

Q. "I am Jim Cole from Arizona State University. I was listening to Chris McKay, another advocate of humans to Mars, and he was talking about how if we do go to Mars and we find life either there or extinct, we should consider removing human presence so that we can allow the other life to thrive. I was wondering what your thoughts on that were. "

A. "Well it really doesn't seem that there is any life on Mars, on the surface at least, no sign of that. If we do find sign of it, for sure we need to understand what it is and try to make sure that we don't extinguish it, that's important. But I think the reality is that there isn't any life on the surface of Mars. There may be microbial life deep underground, where it is shielded from radiation and the cold. So that's a possibility but in that case I think anything we do on the surface is not going to have a big impact on the subterranean life.".

So, it's clear (as I'd expect actually), he does think it is important we don't extinguish any native Mars life. But he thinks there isn't any present day life on the surface. But is that right?

As we've seen already, there's an almost bewildering variety of suggestions for habitats on Mars for life, including (these are all links to sections of this book, above):

• Nilton Renno's droplets that form where salt touches ice - why did he call a droplet of salty water on Mars "a swimming pool for a bacteria"?
• Recurring Slope Lineae
• Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere
• Liquid brines beneath the surface of sand dunes at night - beneath the sand that Curiosity drives over - that one was reported as uninhabitable but Nilton Renno was not so sure, biofilms might make it habitable
• Transgressing sand dunes bioreactor
• Desert varnish
• Sun warmed dust grains embedded in ice
• Southern hemisphere flow-like features - these may involve fresh water!

There's a wide variety of views also on the topic of whether any of these are habitable, and whether they actually have life in them, from almost impossible to very likely, see Views on the possibility of present day life on or near the surface, and for the idea that they may be inhabitable but uninhabited, see Uninhabited habitats, in my Are There Habitats For Life On Mars? - Salty Seeps, Clear Ice Greenhouses, Ice Fumaroles, Dune Bioreactors,... (long detailed survey article with many cites)

So when will we resolve this? Well it might not be easy, not if Mars is uninhabited. We probably can't detect most of the habitats from orbit, never mind prove that they are uninhabited (if they are). Most of these potential habitats would be hidden from view, a few millimeters or centimeters below the surface.

It's true that some of the habitats might be quite productive, for instance methanogens in warm humid locations deep below the surface heated by geothermal processes. There might be enough life there to cause obvious effects on the atmosphere, such as the methane plumes. So we could resolve it in a positive direction from orbit, by finding traces of life there. But as Mars changed from a warmish wet planet to a cold dry planet, any surface life would probably become more and more sparse, and have less and less effect on the atmosphere.

As Mars slowly changed from the warmish humid planet on the left to the dry cold planet on the right, then any surface life may have become more and more sparse, and had less and less effect. Image from NASA (Goddard space center).

So, if the life from early Mars still lingers but is sparse, it might easily have almost no effect on the atmosphere by now. I covered this in How much oxygen would surface photosynthetic life produce on Mars? (above) and the following sections. The most habitable areas of Mars such as the warm seasonal flows, if we are lucky, might be about as habitable as the Antarctic dry valleys or the high Atacama dry desert. If that's the way of it, life in those few square kilometers of the Martian surface would have almost no effect on the atmosphere.

Mars already has small amounts of oxygen (0.145% as measured by Curiosity). Even though TGO can detect a signal as weak as 0.000000001% of the atmosphere, that's no good as the atmosphere already has 0.145% of oxygen, unless somehow it spots local or seasonal variations.

At the moment we don't have any plans to send rovers to the surface to search for present day life there, or even to approach closely to the proposed habitats to see if they might in fact be habitable for Earth life. It doesn't seem likely that we will resolve this before the 2030s. ExoMars could spot life on Mars if there is life in the equatorial regions, but only as a secondary goal. It's main aim is to search for past life. TGO could detect a signal suggesting present day life from analysis of trace gases. NASA's sample return mission, could spot present day life too but that's a very long shot unless life is abundant on Mars as it would have no way to detect it in situ and it's not at all clear that it's going to happen.

There are a few ways we could resolve the question of life on Mars in a positive way if we are lucky, if it is reasonably easy to find. As for searching for subtle traces of present day life on Mars, well we don't have any plans even to send rovers to the most likely places to find it.

Value of a non confrontational approach

You might think, why not go out and out, confront this issue head on, let's have a grand debate or a contest of some sort, and whoever wins the confrontation gets the prize? Well, yes, sometimes confrontation can be good. Sometimes you have to do it. Or you tackle an issue head on and you get more clarity from clearly exposing your differences from another person. If you are lucky, you may find that something new comes up that transcends either of your views or the things you knew, something which you could only have found out by clearly exposing the differences. You might not convince each other, but you might be able to go different ways after the confrontation, each with a bit more clarity.

Sometimes, though, you are "in it" together and can't just continue your separate ways. Sometimes after battling away at a confrontation, you find it is going nowhere. In this case, Zubrin has been battling with those who want to protect Mars for many years. It doesn't seem likely that this is going to end by one or the other view "winning the argument" or conceding defeat.

At other times, you can compromise, find an approach that lets you accommodate both views at once to some extent. But sometimes a compromise satisfies nobody. As an example of this, travelers from Hawaii have to be careful not to bring the oriental fruit fly Bactrocera Dorsalis into California because it would devastate the crops.

Bactrocera dorsalis - a female oriental fruit fly. Travelers from Hawaii to California have to be careful not to introduce it, as it makes fruit unfit to eat. Similarly microbes from Earth introduced to Mars may have harmful effects on whatever is on Mars.

It would satisfy nobody to try a compromise solution for Hawaii imports to California, e.g. to import fruit only on the first day of each month. That would mean the fruit importers are severely restricted in what they can do, while the fruit growers are not protected from the oriental fruitfly, so it satisfies nobody.

So, sometimes, confrontation isn't wise, as it only entrenches views, and it makes it harder to look at the good points of what the other person is saying. And sometimes compromise is impossible because it is a situation that just doesn't have a natural compromise that would satisfy both parties in the confrontation. When that happens, it's time to look for a non confrontational approach, a way ahead that while accepting the differences of views, leads maybe in an unexpected direction, perhaps somehow just takes a detour around the confrontation that was looming up. That can then be satisfactory to both.

That is what I'm attempting here. I think it's a situation where direct confrontation will only polarize positions and entrench ideas, I don't myself think that the compromise approach of sending humans to Mars with some extra precautions is adequate (highlighted by the problem of a human crash on Mars which would effectively end all possibility of planetary protection). That's like the compromise of taking no precautions against the oriental fruit fly on the first day of each month. If we continue to need to take precautions to protect Mars from the microbes on our rovers, I don't think it's going to work as a compromise to give the microbes on human occupied ships a "special pass".

But I do think it's a situation where a non confrontational approach is possible, and can be satisfactory for both. Not a head on conflict, not a compromise, but a search for a new direction for humans in space exploration. That can give us some breathing space, which can lead to new ideas, discoveries and solutions for the way forward in the future, whatever it is.

And, we do have a good starting point here. As Cassie Conley said,

“To date, there has been a consensus that everyone will follow the same rules with the objective of conserving these things for future generations. From what I can tell, this is the first time in human history that humans as a global society made these kinds of decisions. And so far for the last 50 years we’ve managed to stick with them. We’ve never succeeded in doing something like this before."

Moving your house to avoid a pond for great crested newts

So, to try to see this in perspective, first lets try to look at something much smaller. Suppose you want to build a house and need to fill in a pond. You get an assessment done, and you are told that this pond is the breeding ground of a rare form of amphibian. In the UK it could be the great crested newt.

You might not give it a second glance, but this is a European protected species. Your would not be permitted to build on its pond, nor can you try to move it (normally). Instead, you would have to preserve the pond and build somewhere else.

Of course some people couldn't care less about protected species. But others do. It's not a big deal if you don't care for great crested newts. You accept that others do, and as a law abiding citizen, you just build somewhere else. I gave the example also of the oriental fruit fly which makes fruit unfit to eat and so you can't import some fruit and flowers into California from Hawaii. It's an annoyance I'm sure for fruit importers, but it is something they understand the need for, and so most will just keep to the regulations.

For another example, the Kakapo, a flightless parrot, is very trusting and vulnerable to cats, dogs, etc.

I think most people would understand and accept that you can't have cats and dogs on islands inhabited by the Kakapo. There are many examples of invasive life, of course, which can cause problems on Earth. The best known ones are the higher animals and plants, but as we'll see, there are invasive microbes as well.

Examples of invasive Earth life

There's a long list here of examples of invasive species on Earth, and more here, and more in Wikipedia article on invasive species. Here is a selection (leaving out any that are only pests or diseases of particular plants or animals):

Large animals and reptiles: cane toad, common myna, deer, goats, rabbit, cats, dogs, pigs, crab-eating macaque, Polynesian rat, bush tailed possum, hedgehog, north American beaver, Burmese python, American mink, Eurasian boar

Insects: bot fly, yellow crazy ants, Argentine ant, red fire ant, gall wasp and other wasp species, parasitoid insects, gypsy moth, harlequin ladybird, thistlehead feeding weevil, Asian longhorn beetle, Oriental fruit fly.

Aquatic creatures and amphibians: tilapia fish, trout (rainbow and brown), round goby, Chinese grass carp, sea lamprey, Chinese giant salamander, Signal crayfish, Chinese mitten crab, Physa acuta snail, zebra mussel, Mediterranean mussel, Japanese oyster drill, European periwinkle, Australian banded jellyfish, comb jelly,

Aquatic plants: Canadian pondweed, water hyacinth, water lettuce, water chestnut,

Plants: Chinese yam, kudzu, Japanese stilt grass, Mile-a-minute weed, Himalayan balsam, Asiatic sand sedge, prickly pear, giant hogweed, wild parsnip

Fungi: Fly Agaric,

Algae that form large structures: over 60 have invaded the Mediterranean, 28 along the Atlantic coast and many others world wide. Examples Caulerpa toxifolia and Kappaphycus alverezii

Although these are multicellular lifeforms, still, perhaps they may give us useful lessons in the diversity of the ways microbes from one planet may impact on another as well.

Many of them make native species extinct by changing the habitat in various ways, or by smothering the area with their form of life. This doesn't require the invasive species to be adapted in any way to attack the species it overwhelms.

The same may happen with microbes from Earth on Mars. There is no need for Mars life to have encountered Earth life before, for it to cause problems. It's the same in the reverse direction from Mars to Earth. Also, microbial adaptations can evolve quite quickly, so they may evolve to be a nuisance on the other planet when originally they were not.

So what about microbes, can they be invasive too?

Invasive diatoms in Earth inland seas, lakes and rivers

You could perhaps try to argue that only higher animals, plants and insects can be invasive. Until recently most people thought invasive microbes were just impossible on Earth, because with its connected oceans and atmosphere, they could be spread so easily. But as it turns out, microbes can be invasive too, most easily in fresh water and inland seas that are not physically connected.

These invasive microbes rarely hit the news, except for one particularly well known single cell invasive species, "rock snot" (didymo diatom) - it forms large easily visible structures like a multicellular plant, but it only needs a single cell to spread it. Diatoms are microbes that form cell walls of silica, rather unusually.

As it turns out, there are many other examples of invasive diatoms. This problem is very much under reported. Since most researchers didn't think this form of invasive species was possible until a couple of decades ago, probably there have been many invasions before then that nobody noticed. They just assumed that the problematical diatoms were native ones.

Here I'm summarizing Diatoms as non-native species, by Sarah Spaulding, Cathy Kilroy and Mark Edlund. Here are some of the more notable examples:

• Great Lakes: over 180 non indigenous species have been introduced to the Great Lakes in the US, and of those, a study in 1999 found 24 invasive algae including 16 diatoms. Several more have been introduced since then. Most had little effect, but S. binderanus blooms made the drinking water taste bad and clogged water treatment plant filters in the 1950s-1970s. Another species A. normanii f. subsalsa formed blooms in the summer that took silica out of the water for their diatom cells which had the result that cyanobacteria out-competed other species. T. balitica from the Baltic sea had a similar effect. Some of these invaders may also have made other diatom species extinct locally.
• French rivers: The French rivers have been studied for a long time, making it easier to track diatom introductions. Two examples of invasive species which are thought to have come from outside France recently are Eolimna comperii, Gamphoneis eriense and Encyomena triangulum. The non native species produce their largest blooms in summer. This also includes Hydrosera triquerta Wall. which blooms in estuaries and Diadesmis confervacea Kutz in the warm waters of power plant outflows.
• New Zealand: Asterionella formosa was introduced with European settlement around 1880 and is now found in 45% of New Zealand lakes with plankton records. It probably got there on salmon eggs.

But there is one recent and very clear example, Dydimosphenia geminata

This diatom has been widespread in the northern hemisphere for at least a century. Until recently, microbiologists assumed that it was always there and had spread to those places naturally. Then it began to form large blooms in Iceland in 2000, central Europe in 2003, and the US in 2009. But though unusual, these still weren't treated as invasions because everyone assumed it was a native species to these areas already, so something to do with local conditions.

But then, it suddenly appeared in New Zealand in 2004. That was a wake up call because there was plenty of evidence to show that it was not a native New Zealand species.

It now forms large blooms in New Zealand and the government are trying to stop its spread. It may well be spread in some way by humans, and perhaps that explains the other blooms too, maybe it was a variant with new capabilities. New studies showed that its cells stay viable for weeks if kept in cool wet conditions, so it could be spread for instance on damp sports equipment from place to place even from country to country. It's remarkable for its ability to form blooms in low nutrient waters. It can have economic impacts by blocking water intakes and interfering with angling and other recreational activities.

Didymosphenia geminata in South Island, New Zealand. The two asterisks at lower left show where it was found in 2004. The triangles show where it had spread to by 2005-2006. The open circles show its locations for 2006-2007, and the +s show its locations in 2007-2008. The government had eradication programs to get rid of it. But it is still spreading.

These studies probably are just the tip of the iceberg because it's only recently that researchers realized that invasive microbes could be a problem. It's a topic that has just not been studied previously, and the symptoms of invasive microbes have probably often been ascribed to other factors. For instance, were those earlier blooms of Dydimosphenia geminata in the US, Iceland and Europe due to an invasive species or not? We just don't know.

Risks of invasive microbes in lake Vostok and Antarctica

This next example gives us quite a close analogy to the planetary protection measures for Mars. Antarctica is at particular risk from invasive microbes, because it is so isolated. Also, there's intense interest in the lakes trapped beneath the ice sheets, such as lake Vostok, the only remaining major pristine body of water on Earth. It's not only pristine, but it may well be unique too, with lifeforms found nowhere else.

With lake Vostok, rather similarly to Mars, the aim is to keep microbes from the surface of Earth out of the lake. Here I'm summarizing some of the material from this paper: Non-indigenous microorganisms in the Antarctic: Assessing the risks. Scientists take great care not to introduce any non native species into its waters. They would dearly love to explore it. It's got highly oxygenated waters, surprisingly. The oxygen gets there in a kind of conveyor belt. First snow falls on the surface, with air trapped in it. Over thousands of years the ice gets packed, goes deeper and deeper, and eventually reaches the lake where it melts, bringing oxygen at high pressure into the lake. As a result they expect it to be the most highly oxygenated lake on Earth.

The ice above the lake helps keep it insulated which is why it's able to be warm enough for liquid water. At that depth, once liquid, it can stay liquid just heated from below by the warmth of the Earth itself. It also probably has hydrothermal vents and may well have unique complex multicellular life that's evolved there for millions of years independently from the surface. So as isolated, for instance, as the complex life in Australia. How we'd love to go down there and explore it and see what is there, but we just don't know how to sterilize our equipment to do that yet.

When the Russians drilled, they didn't know there was a lake there originally, and when they realized it was there, they they stopped drilling 120 meters short of the lake itself. Their drill penetrated into ice that had melted and refrozen at some earlier time, but stopped short of the liquid water itself. After analysing the drilling fluids the scientists found a large number of microbial contaminants which would get there in the fluids. That made the results of analysis of microbes in the ice controversial, and it underscored the need to keep the lakes free of surface microbial life. So the Scientific Committee of Antarctic Research has drawn up a code of conduct for Subglacial Aquatic Environments in Antarctica designed to prevent introduction of non indigenous microorganisms to these subglacial lakes.

Subglacial lakes and rivers on the Antarctic Continent.

Since then, the Russians have drilled all the way through to the lake but in a way that triggered a geyser that rushed up the drill hole for hundreds of meters, then froze from below. They believe that this let them sample it without introducing any invasive microbes or other life from above. Their first attempt in 2012 lead to sample contamination by the drilling fluids, so they tried again in 2015. It's a rather similar situation to Mars. They hope to find out if Vostok contains life and what forms of life it contains.

You wouldn't be able to get permission to melt through the ice above the Vostok lake, put a human occupied submersible into it and cruise around. Even if you were a multibillionaire.

It's not just the subglacial lakes in Antarctica though; there are issues with microbial life on the surface too. On average each person sheds a billion microbial cells each day, through hair loss, skin sloughing, sneezing, coughing etc (estimate from page 543 of this paper). On the plus side, most of these microbes would not be pre-adapted to Antarctic conditions. Also though microbes exchange genes readily through gene transfer in warmer conditions they do it more slowly in such cold conditions.

This shows a typical Antarctic dry valley field camp. It's perhaps the closest analogy we have to a Mars base:

The researchers stay within a fixed area around the camp in order to limit their impact, in a "corral" system. They give as a a typical example, a corral that's 50 meters square. After a ten day camp with 6 people restricted to those 50 square meters, they will leave an estimated sixty billion microbes in the soil.

Assuming those sixty billion microbes are evenly mixed into the top one cm of the site, then that makes it around a hundred thousand microbes in each cubic centimeter, which is between 0.1% and 10% of the natural population of microbes in those sites. Calculations from (Cowan et al., 2011. Non-indigenous microorganisms in the Antarctic: Assessing the risks : box 2).

This is perhaps our closest analogy to the Mars planetary protection guidelines on Earth and helps underscore how invasive microbes can sometimes be an important issue even on Earth when we encounter situations here similar to Mars exploration. But we don't have any previous experience of something as new as exploring a new planet in this way.

We even have an analogy for the microbes carried in the dust storms on Mars. One of the big questions in Antarctica is how easily the microbes can spread in the wind. As an example, Aspergillus fumigatus was found in an Adele penguin colony and then later in a remote dry valley. This is a fungus found in soil and in compost heaps, which also causes allergic reactions in humans and sometimes serious diseases such as Aspergilliosis, and a frequent cause of hay fever in humans. Incidentally it's a useful example to show how a microbe not adapted to harm humans can still be a nuisance for us. Anyway they don't know if it was already present in the remote dry valley or it spread there in the wind from the penguin colony.

They conclude that though this may not be a serious threat at present that our instruments are getting much more sensitive and that in the future studies may let us notice microbes that we wouldn't have spotted before. They conclude:

"...To counter such threats might require a new tier of Antarctic Specially Protected Areas, essentially ‘no-go, no-fly zones’ where access would be permitted only under the strictest of conditions of biological protection, designed to provide rigorous protection of the environment from human dissemination of nonindigenous organisms. Such zones would, at the very least, provide control sites for future comparative analyses of the impacts and consequences of the anthropogenic introduction of microorganisms...."

It may be an interesting paper to read for anyone who is thinking through effects of non indigenous microbes on Mars or in the other direction from Earth to Mars. Remember that Antarctica is globally connected with the rest of the world through the atmosphere and the sea, and yet microbes are still an issue there. How much more of an issue would this be if the only way life could get from the rest of Earth to Antarctica was through transfer on meteorites, which may have never happened or may have happened only billions of years ago, and most recently 66 million years ago, and probably not photosynthetic life? That's the situation for Mars.

And also remember that Astrobiologists wish to send exquisitely sensitive instruments to search for life on Mars capable of being confused easily by just a tiny fragment of a single cell from Earth.

Also, how much more of an issue would this be if the life on Mars is only early life, made extinct billions of years ago by modern DNA based life? Or in the other direction, if Mars microbes have evolved more rapidly and are billions of years advanced over Earth life in some or all of their capabilities.

What if there seems to be no alternative? - we must have Mars!

At the moment, there's a tremendous impetus amongst Mars advocates to get to Mars as soon as possible. NASA and the Planetary Society are talking about a human mission to Mars in the 2030s. Elon Musk even hopes to send humans to the Mars surface as soon as the 2020s, at one time suggesting a first human mission in 2024. I think it would be wrong though to suggest that Elon Musk doesn't care about the science impact of introducing Earth microbes to Mars. Here he answers a question on this topic, in the 2015 AGU conference in San Francisco, 30 minutes into this video:

Q. "I am Jim Cole from Arizona State University. I was listening to Chris McKay, another advocate of humans to Mars, and he was talking about how if we do go to Mars and we find life either there or extinct, we should consider removing human presence so that we can allow the other life to thrive. I was wondering what your thoughts on that were. "

A. "Well it really doesn't seem that there is any life on Mars, on the surface at least, no sign of that. If we do find sign of it, for sure we need to understand what it is and try to make sure that we don't extinguish it, that's important. But I think the reality is that there isn't any life on the surface of Mars. There may be microbial life deep underground, where it is shielded from radiation and the cold. So that's a possibility but in that case I think anything we do on the surface is not going to have a big impact on the subterranean life.". (emphasis mine)

So, it's clear (as I'd expect actually), he does think it is important we don't extinguish any native Mars life. But he thinks there isn't any present day life on the surface. But is that right?

As we've seen already, there's an almost bewildering variety of suggestions for surface and near surface habitats on Mars for life, which could well be impacted by surface operations by humans, including (these are all links to sections of this book, above):

• Nilton Renno's droplets that form where salt touches ice - why did he call a droplet of salty water on Mars "a swimming pool for a bacteria"?
• Recurring Slope Lineae - this is regarded as pretty much confirmed that there are liquid brines there, though we don’t know how salty or cold the brines are.
• Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere
• Liquid brines beneath the surface of sand dunes at night - beneath the sand that Curiosity drives over - that one was reported as uninhabitable but Nilton Renno was not so sure, biofilms might make it habitable
• Equatorial frosts - and a source of water for the deliquescing salts
• Microbes could find tiny oases of water inside micropores in salt pillars
• Transgressing sand dunes bioreactor
• Desert varnish
• Sun warmed dust grains embedded in ice
• Southern hemisphere flow-like features - these may involve fresh water!

There's a wide variety of views also on the topic of whether any of these are habitable, and whether they actually have life in them, from almost impossible to very likely,

• See Views on the possibility of present day life on or near the surface, in my Are There Habitats For Life On Mars? - Salty Seeps, Clear Ice Greenhouses, Ice Fumaroles, Dune Bioreactors,... (long detailed survey article with many cites)
• For the idea that they may be inhabitable but uninhabited, see Uninhabited habitats (above)

So when will we resolve this? Well it might not be easy, not if Mars is uninhabited. Most of these potential habitats would be hidden from view from orbit, or even from a rover passing close by, a few millimeters or centimeters below the surface. It's hard enough to even detect near to surface habitats from orbit, or with our rovers, never mind assess whether or not they are in fact inhabited, or what the detailed conditions are in them.

Potentially, some of the habitats might be quite productive, for instance methanogens in warm humid locations deep below the surface heated by geothermal processes. There might be enough life there to cause obvious effects on the atmosphere, such as the methane plumes, and if so we could, perhaps, resolve it in a positive direction from orbit, by finding traces of life through clear biosignatures we can detect even from orbit. But as Mars changed from a warmish wet planet to a cold dry planet, any surface life would probably become more and more sparse, and have less and less effect on the atmosphere. (See discussion in James Lovelock's argument for a lifeless Mars and following above)

As Mars slowly changed from the warmish humid planet on the left to the dry cold planet on the right, then any surface life may have become more and more sparse, and had less and less effect. Image from NASA (Goddard space center).

So, if the life from early Mars still lingers but is sparse, it might easily have almost no effect on the atmosphere by now.The most habitable areas of Mars such as the warm seasonal flows, if we are lucky, might be about as habitable as the Antarctic dry valleys or the high Atacama dry desert. If that's the way of it, life in those few square kilometers of the Martian surface would have almost no effect on the atmosphere.

Mars already has small amounts of oxygen (0.145% as measured by Curiosity). Even though TGO can detect a signal as weak as 0.000000001% of the atmosphere, that's no good as the atmosphere already has 0.145% of oxygen, unless somehow it spots local or seasonal variations. I covered this in more detail in How much oxygen would surface photosynthetic life produce on Mars? (above) and the following sections.

So, is there any chance that we find out more by the mid 2020s when Elon Musk hopes to send humans to Mars? Or even the early 2030s when NASA and the Planetary society want to send humans there?

Well, maybe. We don't even have any plans for anything to approach closely to the proposed habitats such as the RSLs to see if they might in fact be habitable for Earth life, or have life there already, not in the near future. Mars 2020's aim is to search for past life. ExoMars does have the search for present day life in the equatorial regions as a secondary goal, but its main aim is to search for past life. Its selection of landing site was based entirely on its value for the search for past life. It will be able to drill, so presumably it could drill down to the layer perhaps 15 cm below the sand dunes where Curiosity detected liquid brines. So, if there is life there, such as Nilton Renno's biofilms, or life able to tolerate temperatures much lower than Earth microbes, maybe it finds it. And if Viking did find life in the 1970s, maybe it will be sensitive enough to detect the organics.

TGO could detect a signal suggesting present day life from analysis of trace gases

NASA's sample return mission, could spot present day life too but that's a very long shot unless life is abundant on Mars as it would have no way to detect it in situ and it's not at all clear that it's going to happen. Also we wouldn't know this probably until the early 2030s.

So, in short there are a few ways we could resolve the question of present or past life on Mars in a positive way if we are lucky, if it is reasonably easy to find. As for searching for subtle traces of present day life on Mars, well we don't have any plans even to send rovers to the most likely places to find it quite yet.

It's easy to accept that you should keep non-native species out of an island inhabited by flightless parrots, as after all we have no shortage of islands on Earth. It's also easy to accept that you should choose a different place to build, if your new house endangers some species of amphibian. Again usually you can find an alternative house site. It's the same with the Hawaiian fruit. There are many other places to get fruit. Where it gets much harder to cope with is if there is something you are very keen to do and there seems to be no alternative, no way to keep your non-native species out of the place you want to build on or colonize, and nowhere else to go.

The Mars colonization enthusiasts want to colonize Mars. If the planetary protection rules were enforced as strictly for humans, as they are for robots, it would certainly keep humans away from Mars altogether. I think everyone would agree with that much, the enthusiasts included. There is just no way you can sterilize a human occupied lander to robotic standards, because of the trillions of microbes that live in and on the human body, also in our food, and in the air. Also, if you assessed human landings on Mars in the same way you do for a robotic mission, you'd have to do planetary protection assessments of the effects of a "hard landing", i.e. a crash on Mars, as I looked at in one of my earlier articles and kindle booklets and also in the section on Why do spacecraft crash so easily on Mars? (above):

This is my short kindle book and article on it:

Can We Risk Microbes From Human Crashes - On Mars? If Not, What Happens To Dreams To Colonize The Planet? - also available to read online.

So COSPAR would have to just choose not to take into consideration the effects of a crash of a human spacecraft on Mars (because that would count as an immediate fail of planetary protection). Also, the NASA mission planners or whatever space agency does the final planning would have to not take it into consideration either, as any mission to Mars right now with humans on board is bound to have a significant risk of a crash. If a human occupied spacecraft did crash on Mars, well I believe that just about everyone would agree that that would be an end for planetary protection for Mars. Sooner or later, so long as there are habitats for Earth life on the surface of Mars, or connected to it, life from the crash would get there. And whenever you detect traces of present day life on Mars, your default assumption would be that it's life that got there somehow from the crash site.

There is just no way we can achieve anything like parity with the robotic missions. So, the only way humans could be permitted to go to the Mars surface under COSPAR recommendations in the near future would be if COSPAR reduce the planetary protection requirements for humans to much less than their requirements for robotic spacecraft. Also, it seems that if humans go to the Mars surface, we have to relax the requirement of biological reversibility. Even if the microbes did not encounter any micro-habitats on Mars to their liking, the spores would be spread over the surface in the dust, in such large numbers, that it would probably be impossible to "put the genie back in the bottle".

Spores last for a long time, especially if they can get into a shadow, protected from UV light, and even more so if they get into a cave. They can sometimes last for millions of years on Earth. Eventually, in the global dust storms, some of those spores would encounter habitats, if there are any at all on Mars. They'd still be there thousands of years in the future also, to potentially cause problems with plans to transform Mars. For instance, if we try to roll back to early Mars, or to do step by step terraforming, or other transformations based on introducing some species before others (ecopoiesis), these pesky spores could scupper all our plans.

For the details see: Biologically reversible exploration of Mars (above)

It would still not be a confrontation if you could land humans somewhere on Mars isolated from everywhere else. But the Martian dust storms turn the whole planet into one connected system, apart from a few places perhaps, like the crater at the summit of Olympus Mons (on short timescales of thousands of years anyway, so long as it doesn't erupt).

Even if you aim for the crater at the top of Olympus Mons, there's a possibility that the spacecraft crashes somewhere else on Mars during the landing attempt. And it might not be totally isolated even at that height surrounded by the rim of the crater. We simply don't have any current spacecraft or plans for the future that could permit humans to land on Mars in a biologically reversible way, whether or not it is theoretically possible. We'll come back to this in more detail in the section on Could we send humans to the Mars surface in a biologically reversible way? (below)

Ponds and flightless parrots again - back to the Moon

So - it's like the example of the island of the flightless parrots, the Kakapos, or the pond for the great crested newts, except that it is now a planet sized "pond", and a planet that some humans want very much to attempt to colonize. Also, there seems to be nowhere on Mars that would be truly isolated where they could go and try out their space colonization experiments, without having an impact on the potential interest and science value of Mars especially for astrobiology.

Anyway - this becomes a confrontation when you think there is no way ahead. If you can just move your house to avoid the pond with the great crested newts, no problem. So that's when I realized, that what is needed is an alternative vision, somewhere else in the solar system that is as good as Mars.

So, first, you can use the asteroids, and Phobos and Deimos for materials to build habitats. Some space advocates are very enthusiastic about such ideas, and it was the "new big idea" in the 1970s with many enthusiastic supporters back then. There are still many who find it an inspiring vision, especially because of the vast amounts of resources in the asteroid belt, sufficient to build self sufficient habitats with living area equal to a thousand times the surface area of Earth. Enthusiasts say we could have embarked on such a project already using the technology of the 1970s.

However, for others, similarly minded to Elon Musk, the asteroids don't quite cut it. Living Mars may seem a lot easier in some ways than building habitats from asteroids. At any rate, whether it is easier or not, I think we can agree that it is a rather different kind of vision for near future directions in space settlement.

But what about the Moon? Could that help defuse the situation? It's got half the gravity of Mars, and some call it a "Moon Planet". It would count as one of our largest continents if it was part of Earth. I was already a "Moon firster" and was aware of some of the material on the subject. But until I wrote my "Case for Moon First" book, I had no idea quite how many points there were in favour of the Moon as a place for human habitats and ISRU (In Situ Resource Italicization). Depending on what we find when we explore, study and prospect further, the Moon might actually be better than Mars for this.

So - like the house builder moving the position of their house to deal with the issue of the great crested newt pond - is it possible that the Mars enthusiasts can move their base to the Moon, and use much of the same ideas for ISRU there, instead? Could they do that for the first few missions at least?

To take up the great crested newts analogy again, it actually seems to be a better building site for our house, more conveniently located, with also much better prospects for commerce with Earth (if it is possible anywhere in space, commerce based on the Moon seems the best bet). I go into this in a lot of detail in my Case For Moon First (on kindle) or also available online. This is also briefly summarized in the introduction under Human settlement and exploration - hugely positive or hugely negative - it all depends how it is done (above)

Meanwhile also of course, we would explore Mars robotically, and eventually send humans there, to explore it from orbit. They can also land on its moons Phobos and Deimos, study their interesting history, look for traces of ancient life on Phobos and look into the possibility of in situ resource utilization on Deimos especially (as it may be a useful source for ice / water).

We can also use many of the ideas for Mars Direct, and other Mars architectures on the Mars surface, but not yet for humans. We can use these ideas to support robots to the surface- highly capable fast moving rovers fueled by the methane fuel that are used for humans in their designs. Similarly, we could use the planned Mars stationary satellites over a human base for the rovers, to relay signals back to Earth via broadband and hugely increase the stream of information back from Mars. It's not often mentioned, but there's an enormous bandwidth issue for Mars. Our high resolution cameras there and other instruments, on both the orbiters and landers, could return vastly more data, if we could only have a higher bandwidth to return them.

In this way we can get some breathing space, of a few decades, hopefully, to find out about Mars on a scientific level. We can find out if there are potential habitats there for Earth life and search for exobiology. Meanwhile, all this work builds up an infrastructure on Mars and in Mars orbit that would be useful if we did decide to send humans to Mars.

Or indeed, it could be useful for other things too. Perhaps we decide to try ecopoiesis (duplicate the biological transformations of early Earth on much faster timescales), or we try to turn the clock back to early Mars to make it more habitable for any indigenous life, or we transform it in some other way. Or we might grow plants there telerobotically from orbit, for export of crops and other plant products to orbit and throughout our solar system (plants could be grown on Mars using sterile hydroponics without impacting on any native Mars life, since seeds can be sterilized).

There would be many possible futures, and if we follow this approach of giving ourselves a bit of a breathing space, they will all still be open to us at that point. Keeping Mars protected from Earth life is not limiting our futures at all.The opposite. It's expanding on what we will be able to do in the future.

Meanwhile we can work on space habitats, gain experience with closed systems, and eventually build city domes on the Moon and large closed systems in the lunar caves, continue to explore ideas for creating larger and larger self sustaining habitats. This question of whether we eventually get to the point of terraforming entire planets, I think can be left to later, until we have much more understanding than we have now, with these early experiments. It's hardly of imminent importance to start right away, with a project that would take thousands of years, and with the risk that early mistakes could add many thousand years to the timescale as we try to adjust the balance of atmospheric gases and microbes in the ecosystem to get it back on target for our desired end point. Or indeed, early mistakes there could simply make long term terraforming impossible (if it is possible at all). Or indeed, perhaps our attempts at terraforming frustrate something else which is humanities main aim there a thousand years from now.

So, then it becomes an open path, where instead of closing off futures, we open out to more and more possible futures, and wider vistas at every step. These vistas don't just include Mars as the "next step" after the Moon, but many destinations for humans throughout the solar system, such as Callisto, Venus, Mercury, etc.

If this approach is valid, I'm sure it will still be a slow process to win the Mars enthusiasts over to this idea of starting on the Moon for these planetary protection reasons and as a way to retain the science value of Mars. People don't change their ideas overnight, especially if they have been working for decades to try to get humans to Mars. Also what I present here is just one vision, which has to be part of a debate, to explore possibilities. Surely others will have many other visions. But at least it is another alternative vision, which may add to the possible future options to consider, and I think as non confrontational as one could be in the circumstances.

I hope perhaps that at least with these books I'm helping provide a greater diversity of visions for these debates about our long term future. :).

How many years are needed to do a biological survey of Mars?

This is a bit like asking how long a bit of string is. The surface of Mars is similar in area to the land area of Earth. How could you design an adequate sequence of missions to search for life in all eight continents, especially once you accept that if it exists, it is going to be cryptic life, impossible to see from orbit , and that it may very well be invisible even to a rover passing just meters or centimeters away from it. The rover could even drive over such a habitat and never notice it.

However, to make a start on it, Carl Sagan had to come up with a number of biological exploration missions, for one of his calculations. (See his "Decontamination Standards for Martian exploration programs") Writing in the mid 1960s, of course with very different ideas of what to expect on Mars, he assumed that about

• 60 missions to the Mars surface would occur before a human landing, giving enough time to get a first idea of the exobiology of Mars. He assumed
• 54 of those successful,
• 30 flybys or orbiters,
• In a twenty year exploration phase before a human landing.

That's a rather fast pace of exploration, with 90 missions in 20 years, so about nine missions for every opportunity to send spacecraft to Mars. Typically we send two or three missions, per opportunity (roughly every two years), or occasionally, none at all, and have averaged a reasonably steady pace of slightly under one mission per year since 1960 (including orbiters as well as landers, and including failures).

He also had no idea how hard it would be to search for life on Mars (this was back in 1961). So for that he was over optimistic, but to compensate, he also supposed far less capable spacecraft than we have now, with only two successful experiments per mission. So - perhaps those roughly compensate, we now have more capable spacecraft but also a much more challenging situation on Mars. Could his 54 missions be about right?

It's about eight landers per Earth continent, over a land area the size of Earth's land masses. It isn't that many, on the other hand, perhaps eight capable landers could give you a first idea of what a continent is like from a biological point of view? Even with hard to spot cryptic biology?

I don't have any other figure to use anyway, so let's just go with it for now and see what the implications are. At least it can help illustrate how we could do the calculations - you can try the same reasoning with your own figures and see how it goes.

Another big difference from Carl Sagan's vision in the 1960s is the large number of failures. So far we've had only seven successful landers in the six decades since he wrote that: the two Viking spacecraft, the Mars Pathfinder lander (with its tiny Sojourner rover), Spirit, Opportunity, Phoenix and Curiosity. In addition, we've had 14 successful orbiters if I counted it right, including the the ExoMars Trace Gas Orbiter. It averages at a little over one successful lander and two orbiters per decade. He envisioned 15 successful orbiters and 27 successful landers per decade.

Of those seven landers, we had / have three landers able to travel kilometers, the Spirit, Opportunity and Curiosity rovers, and one tiny rover able to travel of the order of meters, the Sojourner rover. However, we can't really count any of those seven rovers as part of his list. Only two of those missions could really count as biological exploration - the two Viking landers. I think you could call the Trace Gas Orbiter, and Curiosity early stage pre-biological exploration missions. They could in principle discover life, but both would need a strong signal to have a chance to distinguish it from non life easily.

So that's only 4 missions so far with a biological focus: two stationary landers, one rover, and one orbiter. And there we are being rather generous, as really only the Viking landers were strongly focused on in situ life detection. I'm not sure if you can really include any of the missions since Viking as part of his biological exploration phase. They seem borderline, as he may have envisioned something far more like Viking.

So, probably its just two landers, 52 to go. Even ExoMars is rather borderline, at least in comparison with Viking, as its main focus is on past life, including the choice of which site to visit and which instruments it has. That's mainly because for about thirty years, most scientists thought that present day surface life on Mars was impossible. This is now changing, and on the plus side, our tools for investigating it have moved forwards by leaps and bounds since Carl Sagan's time, but the habitats on Mars have also turned out to be much more elusive than Carl Sagan could have imagined.

So, let's try to venture a very rough guess, at how long it would take, just based on Carl Sagan's numbers. At our current rate of one lander a decade, it would take more than five centuries to complete the survey. So far there is no sign of the pace increasing, but it might do so in the near future with lower cost and smaller affordable orbiters and rovers for Mars. If we step the pace up and achieve a couple of new landers for each launch opportunity, it would be 52 years to complete his preliminary survey If we can get the pace of missions to Mars up to the levels envisioned by Carl Sagan originally of 45 successful missions per decade, about nine missions for every launch opportunity to Mars, maybe we can do it in twenty years or less. All of these are timescales for a biological survey of the whole of Mars.

I think that any estimate like this has to be a first order approximation at best. We shouldn't set a fixed date or fixed number of missions in advance. As you continue the survey you'd get data that gives you a better idea of how long it will take to complete than you realized at the start. It might well be faster, and it could also be slower.

Of course, if there is present day life on Mars, and we are lucky. we might find it with the first life detection mission to go to a promising habitat there. You never know. Even ExoMars, though I'm not sure if we could count it as part of a biological survey for present day life on Mars, because of its focus on past life.does have the capability to find any very strong indications of present day life (more so than Curiosity). So far we have only two missions that have searched for life, using 1970s technology and 1970s ideas about Mars.

We don't yet have any missions on the drawing board to search for present day life as their main focus, not since Viking. We do have many instruments designed by astrobiologists to do the task, for whenever they get the green light to send them on a mission to Mars (see In situ instrument capabilities).

So - it's quite a daunting task to do a biological survey of Mars unless we find a way to send more missions to Mars. Though the instruments to do it are all there, or can be developed quickly, as soon as the astrobiologists get a "green light" to search for life directly on Mars in situ.

Is there any way to speed this up?

Speeding the search up with miniature robots - what could we do if we had funding for a preliminary exobiological survey of the whole of Mars from Earth?

Miniaturization may speed it up, especially if we can scatter lots of highly capable smaller robots over the surface of Mars. See my Soaring, Buzzing, Floating, Hopping, Crawling And Inflatable Mars Rovers - Suggestions For UAE Mars Lander.

However, a swarm of 54 identical probes all sent to explore a single cave or a single region on Mars, and a fleet of 30 identical orbiters similarly wouldn't hack it. That's just numbers for the sake of numbers.

We can certainly do a lot more with a lot less mass than Viking, today, and maybe we could get dozens of highly capable miniature landers into a single launch of something like the Falcon Heavy, but they need to be carefully planned missions. Imagine trying to get a clear view of the biological diversity of Earth with 54 landers, so about eight per continent, or in terms of countries, one lander for every three countries? When you think about it that way, it's not a lot to try to find out about a planet with a total land area similar to Earth, with a complex and diverse geology, and a cryptic biology hiding beneath the surface of rocks, or in the sand, or beneath clear ice, or in partial shadow.

So, they need to be 54 carefully planned missions to the surface (if we go with that number for now). There are many places we need to explore and we need to devise dedicated missions for each, with different challenges. But if the rovers are small, many of them could go on the same launch vehicle ,and then sent to many different locations on Mars from a mother spacecraft in orbit around the planet. Perhaps the same mother ship could even wait, and send later missions to new locations on Mars a few months or a year or so later, in response to discoveries from the earlier landings there. A program like this might perhaps speed the whole thing up greatly, maybe by an order of magnitude or more.

So, what could we do if the funding was available to do an exobiological survey of Mars from Earth? I'd think you need a few missions to each of these targets myself, based on the habitats discussed in this book:

• Ocean bed regions where Phoenix landed, looking especially for Nilton Renno's droplets that form where salt touches ice. How did the phoenix lander leg drops really form?
• The dry ice geysers in Richardson's crater for the Southern hemisphere flow-like features
• Clear ice and snow in the polar regions for Sun warmed dust grains embedded in ice
• Steep slopes of Valles Marineres for the Recurring Slope Lineae
• Areas of high humidity - such as the Hellas Basin - as optimal for Lichens and cyanobacteria able to take in water vapour directly from the 100% night time humidity of the Mars atmosphere
• Sand dunes to study the Liquid brines beneath the surface of sand dunes at night - beneath the sand that Curiosity drives over and the Transgressing sand dunes bioreactor area. These rovers can also investigate whether Viking found life in the 1970s and hopefully settle that vexing question one way or the other.
• Rocks throughout Mars to study the Desert varnish

Seven habitats and a total of fifty four landers / rovers would give us eight missions to each one. There are bound to be other habitats to look into. For instance to try to find the source for the methane signals if those are confirmed, maybe the lava tube caves, maybe the summit of Olympus Mons if there is ice there.

Here is a longer list, with more subdivisions:

• The Recursive Slope Lineae (RSL's). You would need to send missions to more than one of those as they are especially interesting, geographically isolated and it's possible some have life and some don't.
• The flow-like features in Richardson's crater near the south pole. This may consist of fresh water trapped under ice, so are especially interesting for viability
• Clear ice in the polar regions generally - is there liquid water trapped beneath it?
• Dust grains in the snow or liquid water around meteorites caught in the polar ice?
• Check the polar ice caps for deep subsurface water, using radar, and if any is found, to drill down to search for life (if liquid water forms at depths of over 900 meters after a meteorite impact or through geothermal heating, it will remain liquid indefinitely through the flow of heat from Mars itself insulated by the ice above
• Equatorial sand dunes. Levin thinks Viking discovered life already, and recently with discovery of circadian rhythms in the re analysed experimental data, others think there is a possibility of that also. Then, whether Viking found life or not, there are ways they could be habitable. So we need to check up on that to be sure. Also Curiosity found a liquid water layer a few cm below the surface of the sand dunes indirectly. Nilton Renno has suggested that microbes could find a way to create a niche in it, by transforming the environment as it can do on Earth even though the data suggests it is always either too salty or too cold for life. [ExoMars may give the first ideas about this - though it's not quite as sensitive as Viking's labeled release, it could find life in the Atacama desert core which Curiosity couldn't]
• Salt / ice interfaces. Nilton Renno's "swimming pools for microbes" in droplets of water that form where salt touches ice. We'd need to search in places with different salt compositions as that would affect the habitability.
• Salt pillars and salt deposits, for deliquescing salts, and for water that can form in fine pores in salt pillars.
• Caves need a visit. Not so much lava tube caves as other types of caves, for instance ones formed by slippage, or ones that formed through erosion by water or dry ice. The difficulty is, they are hard to spot form orbit. We do need survey not just orbital images, which are also limited to particular times of day and such like. E.g. miniature planes to fly along the Valles Marineres to photograph it up close.
• We should explore the surface itself for life, for lichens and cyanobacteria that might be able to grow in partial shade, using just the night time humidity, according to the DLR experiments.
• The Hellas basin because of the icy mists that form there and because it is the densest atmosphere on Mars which could make a difference to habitability.
• Study the Martian dust as it might have spores in it from anywhere on Mars.

Penelope Boston lists these as some of the types of cave possible on Mars:

1. Solutional caves (e.g. on Earth, caves in limestone and other materials that can be dissolved, either through acid, or water)
2. Melt caves (e.g. lava tubes and glacier caves)
3. Fracture caves (e.g. due to faulting)
4. Erosional caves (e.g. wind scoured caves, and coastal caves eroded by the sea)
5. Suffosional caves - a rare type of cave on the Earth, where fine particles are moved by water, leaving the larger particles behind - so the rock does not dissolve, just the fine particles are removed.

She points out a few processes that may be unique to Mars. Amongst many other ideas she suggests:

1. For the solutional caves, the abundance of sulfur on Mars may make sulfuric acid caves more common than they are on Mars. There's also the possibility of liquid CO2 (which forms under pressure, at depth, e.g. in a cliff wall) forming caves.
2. For the melt caves, then the lava tubes on Mars are far larger than the ones on the Earth.
3. Mars could have sublimational caves caused by dry ice and ordinary ice subliming directly into the atmosphere.

There may be other places to target, but those are the main ones I can think of right away.With this longer list we now have about four missions to each type of habitat. However we can combine some of them together into a single mission, especially a rover.

I think a great place to go for instance would be a visit back to Phoenix's landing site - a study from ground level there able to detect life could also detect whether any of the Earth microbes from Phoenix have been able to replicate as Phoenix was crushed. Hopefully not, but if they have best to know at an early stage. Then this gives us, for the first time ever, some ground truth for robotic exploration sterilization to show our measures are adequate (hopefully) or to locate flaws in them if they are not, so we can fix them for future missions. On the (hopefully remote) chance that microbial life has started to spread from the location of the crushed Phoenix lander, it would then become an emergency containment procedure, to stop it spreading any further and sterilize the region, if we can.

The orbiters would do remote sensing. Some would be like the Trace Gas Orbiter, searching for traces of gases produced by life on Mars, photographing the surface, radar imaging for subsurface lakes in the polar regions, etc. For instance our photographs of the RSL's from orbit are all taken in early afternoon, the very worst time to spot effects of liquid water on Mars, because the spacecraft that takes those photographs is in an orbit that takes it closest to Mars when locally it's early afternoon there. We need orbiters to photograph Mars close up at other times of day such as early morning - orbiters that are put in ideal orbits for such observations.

We also need orbiters dedicated to broadband communications with Earth (probably doing their own observations of Mars as well). These would certainly be in place before any human missions for Mars. Best done right away at the biological survey stage. Assuming NASA's new relay satellite goes ahead, with 800 gigabytes of information a day, in the 2020s that would make a huge difference for both orbiters and landers. But as the numbers of orbiters and landers increase there, we need more relay satellites like that, probably a new relay satellite for every few new orbiters and landers.

With broadband communications, then instead of communicating with Mars once a day as is the current situation, you get delays of between 8 minutes and 48 minutes there and back. This would let scientists communicate with our spacecraft on Mars or orbiting Mars not just once a day, but between 15 and 90 times a day. They could do photography and investigations at night too, if you have heaters to keep the spacecraft warm and lights to illuminate the landscape. If you can do that, you can have two-way communication with Mars between 30 and 180 times a day.

When Mars is closest to Earth with the eight minutes light speed round trip time, if you have power to run your rover at night as well as in the day time, this broadband communication would let you do as much communication and control in two days as we currently do in a year.

You could do even more if you use artificial real time, which gets rid of this bottleneck of having to wait for data back from Mars before you decide what to do next. It's based on you and the Mars rover both talking to each other simultaneously, continuously. The idea is that this lets you build up a virtual reality copy of the Mars landscape on Earth and drive your rover in that VR world while the real rover drives simultaneously on Mars. This video describes the idea:

(click to watch on YouTube)

The Mars landscape changes only slowly, and most of the science is done in a small study regions, so you'd soon build up a complete 3D map of the region and could drive around much as you do in real time on Earth, and not notice the time delay until you have to do something interactive like lift up a rock, drill a hole, etc.

Even when driving over new territory, then most of the time you will have a 3D VR version of the landscape to drive on, except when you go over a rise on a hill, round a corner etc. With satellites photographing the surface at high resolution from orbit, and in 3D, then the landscape around the corner is already known at a scale of centimeters too (HiRISE achieves 30 cm resolution, and perhaps future orbiter cameras will do even better).

We would certainly need to do some building on previous expeditions in this way. However, however much you speed it up, it seems unlikely you could do all those 54 landers in one mission. Many of the habitats also are seasonal. Also, we could easily discover new potential habitats to explore as time goes on - the pace of discovery of new potential microhabitats on Mars or potential habitats through theory and simulation experiments has been quite fast for some years now and it shows no sign of letting up quite yet.

Also,you'd have survey missions and preliminary missions first, but if we had the funding, say a dozen missions every two years for twelve years :). Each wave of missions building on the previous ones, refining the search. Or some other combination including maybe an initial pace of a dozen missions every two years, building up to more and more missions as we get an idea of which places to target next.

You'd also be looking for uninhabited habitats - habitats with no life, because sending Earth microbes there is irreversible, whether there is life in them or not. Potentially, these are of great interest for exobiology, especially if they have complex organics, but no life, or "almost life". For purposes of astrobiology, of course we want to explore all habitats including ones that are outside of the temperature range and salinity for Earth life. This is also relevant for planetary protection in the backwards direction - can any of the life in such habitats (if there is any) also survive on Earth? However, for planetary protection purposes in the forward direction, this is specifically a search for habitats that Earth life can access and survive in - more specifically, also ones that are reasonably easily accessible from the surface.

If it's possible to get humans to Mars orbit, then they could oversee all these rovers on the surface, rather similarly to the game of Civilization. At any given time, much as in Civilization, many of those landers would be

• Doing routine tasks, things they can do autonomously. Deep drilling and retrieving cores, observations of the conditions on Mars
• Controlled remotely from Earth using broadband.

But from time to time they'd encounter something more challenging and interesting, or the scientists have a new objective they want to investigate - and that's where astronauts in orbit would step in. So in that way, towards the end when all the rovers are in place on the surface, a half dozen astronauts in orbit could work with all of those fifty four landers, and more, using telepresence, and supported by teams of people on Earth who take care of the more routine operations of the landers. The astronauts in orbit around Mars would be a precious resource and you wouldn't want to task them with doing things we can do as easily from Earth.

Civilization VI beginner's city. Astronauts in orbit around Mars could control rovers on the surface much as gamers play civilization. The rovers do most of their work autonomously, or controlled from Earth. The astronauts don't have to do anything to oversee them.

But from time to time they need to step in to do things that need supervision on the spot. They would "teleport" from one rover to another, trouble shooting, driving or drilling when the conditions are tricky, examining rocks close up via telepresence to decide what to do, and so on, dealing with all the most interesting issues that arise on the surface. They would use their human skills and expertise to the maximum, as that is the reason they are there.

If it turns out that we wish to keep microbes from Earth away from Mars for some decades, or indeed, indefinitely, we could still build mines, farms using sterile hydroponics, and many other facilities on Mars eventually, operated from orbit in this way. We could explore these on the surface, also using humanoid avatars controlled much as humans as avatars explore computer games at present. Like "Second Life" for instance.

I wouldn't like to estimate how long such a vigorous program of exploration would take. But on the face of it, it seems like you could complete something like this in years rather than decades, given the funding, the resources and the interest and enthusiasm to solve the many challenges. With at a dozen rovers and half a dozen orbiters sent every two years, it would take about decade, based on that number of fifty four landers which we rather "plucked from the air" for illustrative purposes. If we could speed this up, increase the pace as time goes on, it could take less time than that. We could discover life on Mars at any time during this survey.

Though there are no plans yet for something as dramatic as nine missions per window, this seems a good time to list some of the future missions that are already scheduled and some interesting proposals. It's no longer just up to NASA and the next decade may see a fair number of missions to Mars.

Planned future missions to Mars

We have depended on NASA for all the landers and rovers on Mars for many years. It's next mission is of course Mars 2020. ESA is going to launch ExoMars. Both launch in 2020 probably. As for orbiters, we have missions from the US and ESA operating, and now, the MOM mission from India. Russia, though successful elsewhere, including Venus (in some ways more challenging), has had many failures when it comes to Mars including its recent failed attempts to get a sample from Phobos. But perhaps that is about to change.

The entire Mars budget for NASA is pretty much tied up for two decades with Mars 2020 and the sample return, and it has a reduced budget for Mars exploration. But we do have many other exciting prospects from other nations.

The US does have one exciting mission in the near future, if it is approved - its Mars 2022 orbiter NeMO (Next Mars Orbiter) - a very important Mars orbiter to boost communications to and from Mars with "broadband" laser communication with Earth, as well as high resolution imagery of the surface at 30 cm resolution similar to the now aging HiRISE. This "broadband to Mars" could make a huge difference to future Mars exploration.

They also have the Insight Lander, to study the interior of Mars and its geological evolution. It will also be the first test of a self hammering probe on Mars, a heat flow probe that will drill to a depth of 5 meters, trailing a cable with sensors on it behind it. The UK's Beagle 2 mission in December 2003 was equipped with a self hammering mole PLUTO capable of drilling to depths of 1.5 meters, but the mission of course failed due to one of the solar panels not opening on the surface.

Curiosity actually has one experiment on SAM, that could test for chirality, as one of the six gas chromatograph columns detects chirality Sample Analysis at Mars (SAM). It uses GC4 chirasilDex (Chiral compound separation) And apparently in the mass range to detect amino acids. The Mars Curiosity Rover Can Detect Chiral Compounds. Whether or not Curiosity uses this capability, detecting chirality is well within the range of possibility for future missions to Mars. The MOMA Gas Chromatograph for ExoMars will have one chiral column of its four, coupled to its mass spectrometer and it will be able to detect a wide range of organic molecules.

What else do we have on the timeline?

• India is planning more missions to Mars after its success with MOM and may include a rover, See also Mangalyaan 2.
• United Arabic Emirates plans to launch its first orbiter Mars Hope to Mars in 2020. This is part of its plan for human settlement of Mars by 2117.
• China has its sights set on Mars in the mid 2020s with an orbiter and rover
• France in collaboration with Japan plan a sample return from Phobos
• ESA sample return from Phobos - Phootprint
• Russia planned to follow up their failed Fobos Grunt with a new improved Fobos-Grunt 2 possibly as soon as 2024, probably in collaboration with ESA.
• Russia "Mars Grunt" - a Mars sample return, proposed that it does a Phobos sample return first.

If those all go ahead, then it may be one Mars mission on average per year.

Proposed missions

• Icebreaker Life, - Chris McKay's updated version of Phoenix with the ability to drill one meter below the surface to search for evidence of habitability and life on Mars in the same area that Phoenix landed, with biosignature detection capabilities.
• BOLD - this would send six miniature penetrator impact probes to the Mars surface. They are miniature versions of Viking, each far more capable than the Viking lander. They have small parachutes to permit a softer landing. The impact would drive the probe a few tens of centimeters into the martian regolith and a spring action sampler then collects a sample for analysis. Amongst many astrobiological instruments, it will have a chiral version of the Viking labeled release and attempt to settle whether what Viking found was unusual chemistry or biology. It has ability to change course during the landing sequence and achieve pinpoint landing to within 10 meters. Each lander would be about 180 pounds or 59 kg, with a science payload of about 10 kg.
• MELOS is a Japanese proposal under study. It would be a rover sterilized sufficiently to access the RSLs, but has been downgraded to an engineering demo.
• MarsNet, collaboration by Finland with Russia and Spain- series of weather stations across Mars to study its climate
• PADME to study the dust in orbit around the Mars and also the composition of Phobos and Deimos including the water content of the top 30 cm of the regolith (by detecting hydrogen with a neutron spectrometer). To photograph parts of the moons to resolution of less than 0.5 m from a distance of 1 km and investigate theories about the origins of its moons. It could launch on a Falcon-9 rocket in May 2018 and reach Mars seven months later, in December 2018.

A rapid survey of Mars does seem possible - however none yet planned to guide our decisions as soon as the 2020s or 2030s

So, it's possible we could survey Mars more rapidly. Sadly though, there aren't any planned missions yet to suggest we are going to achieve such a huge increase in pace of the robotic study of Mars as nine missions per in the very near future, as in, the next decade (say). Nor are there any signs yet of using lots of small robots to speed it up as in Speeding the search up with miniature robots (above). In the circumstances, if we send humans to Mars in the 2020s or in the 2030s, then, unless there is some big change, and someone does a large number of robotic missions first, we can't have anything like enough information by then to know what effect the many microbes associated with a human mission would have on Mars.

If you share Elon Musk's certainty that there is no life on the surface - which to be fair was the scientific consensus right up to 2008, then you may agree with his conclusions that our activities on the surface can't impact on it (see his answer to Jim Cole 30 minutes into this video also discussed in Elon Musk's fun but dangerous trip to Mars (above) ).

But ideas about Mars are changing fast, and we can't be so sure now about those apparent certainties of the early 2000s of a sterile and uninhabitable Mars surface.

Clash and confrontation?

For this reason, I foresee a possibility of some kind of a confrontation, where experts who meet to make planetary protection guidelines for COSPAR just don't have enough information to say for sure if there is present day life or habitats for Earth life on Mars or not. So far this hasn't happened, there have been divisions of opinion but they have been able to come to an agreed recommendation. But if this happens, then it would come down to personal judgement.

Experts who are skeptical about life on Mars might say to the Mars colonist advocates like Elon Musk, "Sure, go ahead, you probably won't do any harm". While those who are optimistic about the proposed habitats are quite likely to say "Slow down, we need more data". I wouldn't be surprised if the workshop was inconclusive with some saying it is okay and others saying it is not. It would be their official professional opinion too, with much hanging on the outcome. Who would like to be the one responsible for irreversible contamination of Mars? Either they just say "sorry, can't approve it yet" or they are unable to come to a conclusion. I find it hard to believe that a COSPAR workshop could decide that it is okay to approve a human mission to Mars.

This potential confrontation was highlighted recently in my guest appearance on David Livingston's "The Spaceshow" on 3rd May 2016. I said, during the show, that it's possible that we might not have enough information for COSPAR to approve humans to Mars soon enough for Musk's plans, and also said that it is still a possibility that we could find out that there is vulnerable life on Mars. I was saying much the same things I've been saying in this book.

Anyway I expected this to be a controversial thing to say, knowing that many keen Mars advocates would be listening to the program - but I was surprised at quite how controversial it seemed. From the questions we got by email, it seems that many people would be very upset if their plans to attempt to colonize Mars were even delayed a few years because of issues such as this. David said that the possibility that planetary protection issues could delay their plans is never raised in the Mars colonization conferences at all, which are held every year in the States.

So, if I'm right about this projection, it will be a great surprise and shock for many Mars colonization advocates, if the planetary protection experts were to say that we have to keep humans off Mars for a while longer. What can we do to help defuse and resolve this possible future confrontation?

My Case for Moon First book actually came out of my deliberations after thinking over that show. It does seem to help for some people, to pivot to this positive focus on the Moon first. It also makes all kinds of practical and logical sense in its own right, as many Moon Firsters have been saying for years.

So that's my proposed solution for a way ahead here. Let's pivot to the Moon as our near term destination for humans in space. Meanwhile, let's press ahead with robotic exploration of Mars, which is something we can do without need to relax planetary protection requirements. Indeed, we should probably increase them, especially for robotic missions to potential liquid habitats on Mars, even if they are microhabitats only cms or millimeters or only microns in scale. Also let's work to increase the bandwidth from Mars, with immersive artificial real time, which would actually bring many of us much closer to Mars in a way we could only do vicariously in the case of an astronaut mission there. Eventually, as our mid term goal, let's aim to send humans to orbit around Mars, controlling what by then may be dozens, or even hundreds of rovers on the surface. And then let's leave what happens after that to future decisions as by then our understanding of Mars may be as much transformed once more as it has been transformed in the last decade. We may just not know enough yet to make wise future decisions for a decade from now.

For those who think this is the best approach, well perhaps there are signs that we are headed towards a future like that, which may at least give us a breathing space for now. Trump's administration is less focused on Mars than the Obama administration, with signs that it may pivot to the Moon, which has been everyone else's priority all along.

Even Elon Musk seems to be pivoting just slightly to the Moon. He recognizes its value in the larger scheme of things. He has also canceled the Red Dragon mission. Supersonic retropropulsion is still something he wants to achieve, but he now thinks of it as something to do later. He plans to return the Dragon 2 to Earth using parachutes instead of supersonic retropropulsion, for now

He is also considering pivoting to the Moon for the near future as the first step, saying (video link):

“If you want to get the public real fired up, we’ve got to have a base on the moon. That would be pretty cool, and then going beyond that, getting people to Mars

.... Having some permanent presence on another heavenly body, which would be a moon base, and then getting people to Mars and beyond. That’s the continuance of the dream of Apollo that people are really looking for,”

Asked about the Falcon Heavy, he says (video link)

"How are you managing the risks associated with the Falcon Heavy, and particularly the recently announced private launch round the Moon?"

"First, I should say, Falcon Heavy, that requires the simultaneous ignition of 27 orbit class engines. A lot that could go wrong there. I encourage people to come down to the Cape to see the first Falcon Heavy mission. It's guaranteed to be exciting. ..y. I hope it makes it far enough away from the pad that it does not cause pad damage. I would consider even that a win, to be honest. ".

"That dwindles the number of people who want to rid on that that first time"

"Yes, full disclosure here, I think Falcon Heavy is going to be a great vehicle, It's just like so much that's really impossible to test on the ground. ... It actually ended up being way harder to do Falcon Heavy than we thought. ... But no question, whoever is on the first flight, they're brave.

He explains in detail some of the issues, such as need to build an entirely new airframe to withstand the extra forces on the central core, its changed aerodynamics over the Falcon 9, with all the loads also changed, and the separation systems. It's so many things to test and most can't be tested on the ground. He is optimistic that it will end up being a great rocket, able to take 100,000 pounds (45.4 metric tons) to LEO and to throw the Dragon 2 around the Moon.

Here is his complete talk, he talks about the Moon 25:25 minutes in and about the Falcon Heavy in response to a question at 26:40 IN.

(click to watch on YouTube) .

Of course this may be a bit disappointing for those who are dead keen to get humans to the Mars surface as soon as possible. But myself, as you'll guess from this book, I welcome this pivot. Also his interest not just in Mars but seeing this as a first step to both Mars and beyond. So, maybe it doesn't have to be just Mars, Mars as the next objective after the Moon? Maybe Callisto for instance could be an objective, or materials from asteroids, or the moons of Mars? With a longer timeline, there's time for many things to change, and for decisions about Mars to be based more on knowledge rather than hopes and expectations.

This approach doesn't mean that humans can never land on Mars ever

The idea isn't at all to prohibit humans on Mars. The humans are not the problem. It's only our microbes that are. The aim is to keep our microbes out of Mars for now. But sadly we can't do that without keeping humans out of Mars as well, because of the microbes that go with us wherever we go. The suggestion here is to do it step by step and to make sure we understand Mars and understand the implications of our actions before making a decision about whether it is okay to have human boots on Mars.

I'm a spaceflight and science fiction enthusiast myself and I'd love to be able to say we can't do any harm by visiting Mars and to cheer on humans on an expedition to Mars. That's just for the childlike wonder of seeing humans doing things like that in space. So it would be fun to see humans go to Mars. Similarly it would be great to say that humans can't do any harm by visiting the Lascaux caves.

At least with this approach, we can send them to Mars orbit, no matter what we might discover about the surface. It's a huge challenge, and it would need to be done with care to make sure that they can't crash on Mars. But it seems to be a mission within our capability in the not too distant future, and similarly also, to visit Phobos and Deimos (again taking care).

What about humans on Mars later on?

I think it is impossible to project and try to see that far ahead. Our understanding of Mars has changed radically in just the last ten years since the Phoenix lander in 2008. It may change radically again, and especially so if we find life on Mars. We need to decide what to do later on, based on what we learn in the next decade or two. We could easily make discoveries that upturn everything we know about Mars, especially in the field of biology.

If we find that there is some vulnerable early RNA based life on Mars for instance, I think that public opinion might well swing in the direction of saying we need to go slow here, and study it first before doing anything that could make it extinct on Mars. The scientists would be on the TV talking about how exciting this discovery is, and I think nearly everyone would soon understand the importance of what we had found. If they then went on to say that humans on Mars would make it extinct by bringing our microbes there, then surely there would be a lot of popular support for keeping Mars free of Earth life for now.

In the other direction, however, maybe we have other findings that show that microbes would have minimal impact on Mars.

For one example, suppose that none of the proposed habitats turn out to be habitable for Earth life? We can't examine every single one, quickly, but suppose that after a while we have excellent reasons, complete confidence, that they are all either too cold or too salty for Earth life? I think that's an unlikely scenario myself, and it would be a disappointment for exobiologists, but it's a possible future as of writing this. We'd have to think about impacts on attempts to terraform Mars in the future as well, but maybe we could come to see that our microbes can't harm Mars - specifically, can't damage its value for us.

Or we find life there, and prove that it can "play nicely " with Earth life, in both directions. Also the likes of our aerobes, methanogens, photosynthetic life etc are all there already and the situation is such that adding new species from Earth can't impact on the planet in a global way, or impact on future terraforming or "mars forming" attempts. Maybe we can set up "planetary parks" on Mars and that is all that is necessary to preserve its science value.

Or maybe new technology gives us the capability to send humans to Mars in a biologically reversible way. Again, it's hard to see that with present day technology, at least not for an interesting mission for the humans involved. But the humans in a metal sphere idea (below) shows that it is at least possible, though perhaps not in a very useful way at present. Could there be some other more flexible and more interesting ways to achieve the same thing? If you have any other ideas for biologically reversible human exploration of Mars, do let me know.

I can't imagine how we could have useful biologically reversible human exploration of Mars with anything like present day or near future technology. But there are many technologies today that I couldn't have imagined in the 1960s when I watched the Apollo landing on the moon on TV as a child of 14.

Indeed right up to not long before the landing, leading science fiction writers published stories in which explorers to the Moon, or further afield, returned to Earth to report their findings. As they imagined it, we'd know nothing about what happened during the voyage until they returned. They knew that the Heavyside layer is used to bounce radio signals so that we can hear radio from the far side of the world. It may have seemed reasonable to them that we would not be able to transmit through it.

That is, except for one story that is. It was published in Amazing Stories, April 1947, All Aboard for the Moon, by Harold Sherman. It is the only example I've heard of a science fiction story predating Apollo in which the events are watched on global TV as they unfold. Perhaps Harold Sherman new of proposals to use the Moon as radio relay, basically as a natural communications satellite, dating back to 1928, or Project Diana which bounced radar off the Moon in 1945?

So sometimes your ideas about the future can be upturned like that, suddenly, in just a couple of years. In the same way our understanding of Mars, and of ourselves, our technology, and our understanding of the rest of the solar system, exoplanets, our galaxy, all of that might change hugely in the next couple of decades.

When will we know enough about Mars?

I don't think we can answer this at this stage in any definitive way. At any rate, I haven't come across any suggestion for an estimate. It's asking us to predict future science. You can't know what direction it will go and what we will learn about Mars. The sketch of 60 rovers, 54 successful and 30 orbiters in How many years are needed to do a biological survey of Mars? was just for illustrative purposes, using numbers originally suggested by Carl Sagan in the very different situation of 1970s technology and understanding of Mars. For him also, it was just a way to get numbers to work with for his probability estimates.

Let's take it step by step, and send humans to the Moon and asteroids and Mars orbit first. The main thing right now should be not to close off future possibilities. Let's do whatever we can do, to make sure that if we make that decision in the future, it will be an informed decision.

Of course whenever we do make a decision, we wouldn't know Mars completely, as that would take for ever. We don't understand Earth completely yet. But we'd know a lot more about Mars than we do today. After we've had a chance to do our first biological exploration of Mars from orbit and from Earth since the 1970s, then we may have many answers. We may already know that there is life there and know a lot about it. Or we might not have found anything yet, but we have looked hard, and have a clearer idea of what we don't know and what is needed to fill the knowledge gaps.

I think that with the rapid pace of science, we can only foresee the future technology perhaps ten or twenty years ahead in detail, and even on that timescale, many surprises are likely. As for the exobiological discoveries we can make on Mars, how can we anticipate those until we make them? Any guesses are likely to be wild of the mark, at least in detail.

That's why I think it is very wrong to set a time limit myself, not if you science. That's why I find it rather hard to be a fan of the Planetary Society.

So much of what they do is good. But then they say they think we should send humans to Mars orbit in 2023 and then to the surface in 2025. I don't know how long it will take to have a clear understanding of Martian biology and habitats and what the impact would be of humans on Mars surface. But surely two years from first attempts to study via telepresence, to landing humans and irreversible introduction of Earth microbes can't possibly be enough time to know what the effects of your actions will be. Also they present it as if the 2025 landing would be inevitable, in the sense that the timetable would not be changed as a result of whatever is discovered in 2023. Whatever we find out, no matter how vulnerable Mars is to Earth life, we still set down human boots on Mars in 2025.

If that is indeed how they understand it, how is that compatible with a scientific approach to exploration?

(click to watch on YouTube)

Bill Nye, CEO of the planetary society, saying we should send humans to Mars by 2023, and to land by 2025

I can't see a human expedition to Mars orbit happening by 2023 anyway, when we don't yet have experience of a two year mission to the Moon without resupply from Earth. The ISS has had lots of problems with its life support which were often fixed only because we could get equipment to the ISS within weeks or months. See a list of some of them here. Some of those weren't serious ,but several of these would have killed the crew if they had been orbiting Mars instead of Earth. Surely we need shake out cruises closer to home first, of at least a couple of years each. We also need to design those missions, get it flight certified, we need to do preliminary tests. We need the equivalent of Apollo 8, and earlier missions too, as we don't have that hardware any more. So some years just getting back to the stage we were at for Apollo 11. Then at least two years, probably more, to have a full length trial of the hardware and humans too closer to hand. With only six years to 2023, and given that we have to support the ISS through to the mid 2020s which takes up a lot of the NASA human spaceflight budget, I can't see humans to Mars orbit then - probably 2030s at the earliest?

And though I agree that Mars is very interesting, the Moon is too, and of course I don't agree that we should ignore the Moon and send humans straight to Mars. If we do take things a bit more slowly and do a biological survey of Mars of some sort before landing, our future there may go in different directions depending on what we find.

Those who are especially keen on humans in space may say

" I hope we find that we can land humans there as soon as possible".

But astrobiologists and those who are keen on the subject of the origins of life and exobiology may say

"I hope we find interesting astrobiology on Mars".

Let's look at those two futures a bit more closely.

Two main futures - humans landing on sterile Mars or a habitable Mars surface with humans in orbit only

The idea is that we just don't know enough yet to see that far ahead. We need to decide what to do later on, based on what we find out. This might or might not end up with humans on the surface of Mars, depending on what we find out. In some scenarios we land quickly, in others, maybe not for some time, and there could be situations where we just don't land there at all. We still have the rest of the solar system to explore. So, to get us thinking about how the future might unfold, if we decide to continue with planetary protection of Mars until we know enough to make informed decisions about what to do there, here are a couple of scenarios:

First scenario - uninhabitable to Earth life and humans (perhaps) land quickly

In this scenario, the Mars surface is uninhabitable by any Earth life, not even by extremophile microbes. This is the one that's optimal for those who say

" I hope we find that we can land humans there as soon as possible".

Also there are no subsurface habitats Earth microbes can get to either, anywhere on Mars. We become confident about this early on.

Humans land there just as they did on the Moon. That would be fun and exciting just as it was for the Moon. The young space geek, science fiction loving kid in me would just love that! In this future, Mars is pretty much like a larger Moon with more ice and some very cold or very salty uninhabitable thin layers of brine near the surface (we know they exist already). It may have water in the deep hydrosphere, but none of the accessible water is habitable.

In this future, humans could perhaps land early on .It does need thought even then. Could accidentally introduced earth microbial spores interfere with future plans to transform Mars? It is after all, an entire planet, connected through the dust storms, and with dust able to protect spores from UV light. Even if there are no habitats for Earth life, the spores would get into shadows and eventually caves and could have effects thousands of years later.

The first landing of humans there would be exciting, as for the Moon. However, if there is no interesting biology there - it may be a geological paradise, but after the initial excitement, it might well turn into "just another place" with humans cramped in habitats, complaining about the food and the difficulties of being cut off from Earth, posting homesick video clips talking about how they never get to see a blue sky or sunlight or trees or grass any more, how they can't get out of their habitat for weeks on end in the dust storms and complaining about how long it takes to put on a spacesuit etc etc.

I can't see them transforming Mars into a "second Earth" myself. It's just too cold, desolate and the major problem of no air to breathe. Earth might soon start to seem a paradise to them, that distant unreachable blue planet where there is abundant water and breathable air everywhere, temperatures just right for life and you can walk anywhere without a spacesuit and feel the sunlight and rain and wind on your skin. Living on Mars would probably seem a lot more mundane than it does at present in science fiction stories and movies. The general public got bored of humans on the Moon in the 1970s pretty quickly and I think all except a few geeks would get bored of Mars, except of course for planetary geologists.

Of course the Mars colonization enthusiasts expect Mars to be an exciting place with everyone on Earth agog for the latest news from the planet, more interesting to us than the Olympics if you go by Mars One's ideas about Mars. But I can't see that myself. And the optimists say we will quickly make Mars more habitable to humans, but I can't see that myself either.

Second scenario - habitats on or near the surface - exciting for astrobiology and humans remain in orbit for now

This other main future is one where the Mars surface turns out to be habitable to Earth life, and so to native Mars life too, if it exists. This is the one which is optimal for those who say

"I hope we find interesting astrobiology on Mars".

In the most exciting future here, perhaps nearly all those suggested habitats turn out to be habitable. Nilton Renno's salt / ice interfaces, the seeps of briny water, the pure water at 0 °C in Richardson crater due to solid state greenhouse effect through pure ice, it has lichens and other lifeforms on the surface using the 100% night time humidity, lifeforms in the Hellas basin, all of those inhabitable. Perhaps even, it's a planet with multiple genesis, not just one "Shadow biosphere", but several, all co-existing, based on different biochemistries. We'd learn so much from that.

Almost anything that could happen in this future is exciting. Even Zubrin's picture of a Mars with the same lifeforms as Earth would be exciting. How could that possibly happen? How did they all get there, and when, and why didn't they evolve in some different direction from their cousins on Earth? A future like that with uninhabited habitats would be interesting too. We'd learn a lot from both these futures.

The most thrilling future of all here, though, would be the discovery of indigenous life or early life precursors. Mars has such a different past from Earth, at its most habitable for just a few hundred million years, with oceans, early life could have evolved there and still be there. Those early life forms might never have gone extinct on Mars. Wouldn't that be just the most amazing thing to find there? Life that was made extinct on Earth, and they could be extremely vulnerable to Earth life. That would fill in a huge gap in our understanding of life.

Or, if we were to find some form of life that's followed a totally different direction of evolution from Earth life. Even perhaps more than one form of biology, different directions explored and perhaps none of them have yet taken over as the only form of biology on Mars.

That would be like discovering an exoplanet, complete with its own extraterrestrial life, in our own solar system. Ok, in this future, humans can't land on the planet, not early on anyway. But to compensate, they can explore it by telepresence, and we can all participate, looking at the streaming feeds from Mars and walking via virtual reality through the landscapes they uncover in their explorations from orbit. It would be exciting to follow the expeditions of the telenauts exploring Mars from orbit. And there's something also fascinating about a place you can't go to in person, for whatever reason. It would add to the interest and mystique of Mars.

For me, that's by far the most exciting future here. So, though the sci fi geek in me likes the future with humans on the surface like the Moon, as someone who is dead keen on science and astrobiology, I'm rooting for that other future where the habitats turn out to be inhabited - and most exciting of all, a future with some form of indigenous life, early or life precursors or alternative biology. This is a scientific possibility at present, something that could turn out to be true. I hope this is what we'll discover on Mars in our near future.

In that future, you'd still have humans exploring in many other places in the solar system including Mars from orbit and its two moons. Maybe eventually they land on Mars too, but only after we know a lot more about Martian and Terrestrial biology.

The two biospheres might be compatible, or they might not. There is just no way to predict this in advance. We just have to see what we find out. If they are incompatible, it might be that there is no safe way to have humans on the surface. There is no guarantee that Martian biology will permit humans to live there safely. Also no guarantee that we won't make the life there extinct by landing there.

But perhaps after studying it for some time, we decide that it is compatible, and we've done enough science study, or we can establish "parks" on Mars that we keep sterile of Earth life by whatever technology we have by then. So this future could end up with humans landing on Mars, later on, just not right away. It would depend on what we find out about Mars.

More scenarios

You can come up with many other variations. Perhaps Mars is uninhabitable to Earth microbes, but has its own native life, which can't harm Earth life and Earth life can't harm it. Or, as Zubrin suggests, a future where Mars life is pretty much identical to Earth life with exchange of microbes able to keep microbial populations close to identical to Earth populations. I think both those futures are rather unlikely, but we won't know for sure until we find out more about Mars. You can come up with many other possible futures, and there might be other possibilities that are so outside of the realm of our expectations that none of us can see them yet. Predicting the future is always a risky game!

In some of these scenarios, maybe humans can land early with no problem, in others they don't land for some time, or not at all. I hope though, whatever happens, that we don't end up in a future where we accidentally introduce Earth life to Mars like the Arthur C. Clarke Venus explorers in the introduction (Why don't explorers in science fiction have these problems when exploring other worlds?).

That would be so sad, to do that by mistake. As with the Venusian human explorers in his story, surely future humans would think back on with sadness for those vanished microbial species from Mars (and maybe lichens and other more complex lifeforms) which they will never be able to "meet", even through a microscope, for the rest of future time.

Only one Mars - no warp drive yet

These planetary protection issues arise so acutely for us because there is only one Mars within reach. If we could travel at warp speed to distant star systems within days or week, maybe we might know of thousands of Mars like planets with pre-DNA life, or with habitats but only life precursors and no life, or a different biochemistry from Earth life, or whatever it is that makes Mars unique in our solar system. Even then I think we would have a fair bit of responsibility for those planets.

Even in some far future with warp technology, although we would have many planets at our disposal, it would still be an irreversible change, for any particular one of them. We would have taken on some measure of responsibility for the future beings that might evolve on those planets which we transform, maybe millions of years into the future. Perhaps future intelligent lifeforms may arise there who may have a form, biology, and motivations that we may not even be able to predict at present. For instance, to start a new habitable planet that would have future intelligent creatures and that would un-terraform a few million years into the future might well be seen as irresponsible even if we have thousands of planets at our disposal to experiment with. In such a future, we might still have some kind of a "prime directive" that applies even to planets with only primitive life.

As it is though, we have only the one Mars in our solar system, just as we have only the one Earth. There is no immediate prospect of developing a realistic warp drive as far as we know. Yes, there are ideas, but nothing concrete.

There are issues with warp drives also, as if it is possible, it may permit travel backwards in time, which is an idea that challenges causality. Faster than light travel is common in science fiction, We do have theoretical possibilities for it in our own universe already, such as the Alcubierre drive, however this requires exotic matter with negative energy which we don't know how to make. It's not yet at all clear that we will ever be able to do it in practice.

Certainly we have not got any spacecraft able to do this yet, and I don't think many would say we should plan on the assumption that we will be able to do this in the near future. So we need to proceed carefully with actions that may change Mars irreversibly. We may never be able to access another planet like this, within a travel distance less than decades even traveling at a tenth of the speed of light. Mars could be the only place in our entire solar system, or at any rate the only terrestrial planet with extraterrestrial biology, or early life, or complex precursors without life, or whatever it is that is there.

This makes it the only terrestrial planet of its type (which is clearly different from Earth in many respects, and from Venus also) in the habitable zone of its star, not just here, but throughout the entire observable universe. This makes it potentially extraordinarily precious to us. For us, it is quite literally unique in the entire universe. We may find many other Mars-like planets, but there is no near future prospect of finding one that's accessible to us to study close up.

Star Ship USS Enterprise orbiting Earth from "Tomorrow is Yesterday" from the Star Trek the original series remastered.

If we had warp drive, Mars might just be one of many Mars like planets that we can visit in our galaxy. As it is now, Mars is absolutely unique to us. There is no way we can find "another Mars" if something we do messes things up there for us.

Even if we ever do develop a warp drive, Mars may still be unique. It might be the planet that Earth life evolved on. It might be the only planet other than Earth in our galaxy with DNA based life. There may be many other such connections, including just that it is a planet of exactly the same age (to within a few million years) around the same sun and with a shared history. Other planets around other stars may have other forms of early life in other solar systems but that close connection to Earth could make Mars of especial interest to us.

Also, it's not impossible that Mars, and Earth are unique in various ways in the entire galaxy. For instance, on the knowledge we have so far, if the origin of life is an event extremely rare and unusual, perhaps this is the only place it happened? On the basis of what we know so far it's not impossible that Earth, along with perhaps related life on Mars, Europa and Enceladus are the only places with life in our entire galaxy. Think how sad it would be if we were to destroy the only other forms of life other than ourselves in our galaxy?

At any rate if there are other forms of life, on planets outside our solar system, they are light years away, and it would be so sad to destroy the only other forms of life in our own solar system. So far we simply don't know if that is a possible outcome of landing humans on Mars. There is no way we can know that in the near future, not with reasonable certainty, without some attempt at a biological survey of Mars.

So one way or another, Mars could be unique and very precious to us. One thing is for sure, it is the only planet of its type accessible to us, right now.

Precautionary principle and super positive outcomes

The main principle here is that if we are to make wise, informed decisions, we need to do them out of knowledge and understanding, to the limits of our capability. For as long as we have fundamental gaps in our understanding,with significant unanswered questions about whether this course of action could cause problems for us and our children and future civilizations on Earth, we just shouldn't introduce Earth life to Mars irreversibly, or indeed, to anywhere else in our solar system.

Colonization advocates will often argue that since we don't know that we will cause problems on Mars, we don't have any basis to hold back. Let's go ahead and see what happens, and "learn on the job" what impact we will have on Mars.

But for me that's not nearly good enough reasoning to back up an action that risks potentially irreversibly introducing Earth life to Mars. This section comes out of attempts to make this clearer, and to explain why I think it is so important that we don't proceed here out of ignorance, but make sure we have a reasonably clear understanding of our situation first.

This section is based on the Precautionary principle guideline in International Law

"When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.

"In this context the proponent of an activity, rather than the public, should bear the burden of proof.

"The process of applying the Precautionary Principle must be open, informed and democratic and must include potentially affected parties. It must also involve an examination of the full range of alternatives, including no action."

Now, we can't apply the precautionary principle directly to our situation on Mars, at least, not in the forward direction. If we start an irreversible process that makes some unique or early form of life extinct on Mars, then there is no risk of direct harm to human health and the environment, at least not on Earth.

However what we do have here is a risk of destroying a potential future benefit of immense value. So we need a positive version of the precautionary principle. A principle based on the idea that with the technology we now have, we may meet situations where we can see a prospect of a vast future benefit, but we can't prove that this benefit will occur. We could call such an outcome a "super positive outcome".

So, a "super positive outcome" is one which has transformative effects on us, our children and all future generations and civilizations. In this case discovering some alternative form of life or early life on Mars could revolutionize biology, could potentially benefit medicine, agriculture, and indeed anything that we do that uses products of life, also nanotechnology. It could potentially, in the best case scenario be a hugely positive transformative discovery.

The principle then would cover situations where we have a potential, but not proven, future positive outcome, and actions that can undermine that outcome.

As with the precautionary principle, there may be no way we can establish the cause and effect relationships thoroughly before it happens. Until we can do a biological survey of Mars, there is no way to prove that Earth microbes could make Mars life extinct, or to prove the other way that they will cause no harm. To do that we need to find out whether Mars has habitats for Earth life, whether those habitats contain indigenous Mars life, and whether Earth microbes can make the Mars life extinct.

So, I suggest we should have similar guidelines to the precautionary principle for super positive outcomes like this - things of potentially overwhelming positive value that we could discover. So, let's try just replacing "threats of harm to human health or the environment" by "super positive outcome" and rephrase the rest of it accordingly in a positive way. We get something like this:

"When an activity impacts on a potential super positive outcome, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.

"In this context the proponent of an activity, rather than the public, should bear the burden of proof.

"The process of applying the Precautionary Principle must be open, informed and democratic and must include potentially affected parties. It must also involve an examination of the full range of alternatives, including no action."

So I'm not saying we should never land on Mars. But that if we do, then as with the precautionary principle, it has to be on the basis of knowing clearly the consequences of our actions. Also it can't be a decision of a few enthusiasts who are bound to be biased in one way or another. When there is a super positive outcome involved, we need public debate and open and informed democratic discussion of whether we should do it. And "no action" always has to be on the table as an option.

If we make irreversible decisions such as whether to introduce Earth life to a planet, we need to be sure that we do this together. And when we don't know the effect of our actions, the burden of proof has to be on the proponent for the activity, such as to land humans on Mars. They would be responsible for providing the evidence we need to assess whether it is going to impact on a super positive outcome. If they can't do that, then it's reasonable to ask for "no action" until we understand the situation better.

For more about this see my "Super Positive" Outcomes For Search For Life In Hidden Extra Terrestrial Oceans Of Europa And Enceladus

For discussion of how we could explore Mars first, in detail, see my section How many years are needed to do a biological survey of Mars? (above)

Let's look a bit closer at some of these potential super positive benefits.

Benefits to humanity from astrobiology

Those writing about the value of astrobiology often talk about how we might (if we find independently evolved life on Mars) discover we are not alone in the universe, that there is life perhaps everywhere in the galaxy and so on. And yes that would be a benefit.

However there are many much more pragmatic potential benefits. All of these also could be lost if we destroy some form of extraterrestrial life, even if it is only microbial, before we know it is there or before we are able to cultivate it outside of its native habitat.

New dimension to biology

First there's the impossible to predict. If we find life that uses new bases, that uses a new codon table for amino acids, that is RNA only with no proteins and enzymes that are based on RNA fragments (ribozymes) etc etc, then we would have new ideas about how biological systems work. Not even just XNA substituted into our DNA based life. It could be a form of life that is completely novel from the ground up.

I've used this analogy already, but it bears re-use. It could be like stepping up to a new dimension. Like all our biology has been in two dimensions and then we discover the third dimension (not to be taken too literally, it's a metaphor).

How could our understanding of how Earth based biology advance as a result of discovering a form of life that is based on a different biology? What new tools might we develop to study this life?

It's opening up a new field of discovery that would inspire young biologists and surely invigorate the entire field of biology in ways we can't predict. That by itself is an immediate benefit. There's a tendency for people to say that science has to have practical value to be of benefit. But they never say this of other fields. I studies pure maths, and nobody says that pure maths has to have practical benefits. You don't have to prove that the four colour theorem or Fermat's last theorem etc etc have practical benefits to study them.

It's even more so in the arts, music composition, painting, poetry. Most of us value these things and think that they add to our culture and civilization. So discoveries from astrobiology and the vibrant field of research is itself a benefit, I'd argue.

However because it is a science too, it has many possible practical benefits too. It's just that they are impossible to predict, based on knowledge we don't have yet.

This might lead to innovations in biology, medicine, nanotechnology, etc at a fundamental level. Though so hard to predict, it may be the most transformative too.

New biological materials

We have many products of microbes such as

• Food stuffs: cheese, butter, yoghurt, alcoholic drinks such as wine and beer, soy sauce, miso, protein (e.g. from spirulina),
• Drugs such as antibiotics, steroids,
• Materials such as polysaccharids used to thicken and stabilize food, as a base for cosmetics, to extend blood plasma
• biofuels
• vitamins, enzymes, amino acids,

Microbes from another planet might give us new products, or be able to generate the products more efficiently,

Genetic firewall

If the life is unrelated to Earth life, then it could be that Earth life has no resistance to it. But it could be the other way around that it has no resistance to Earth life, or that it can't survive in any of the habitats on Earth. If so, it could also be the ultimate in a genetic firewall.

For a clear example of how this might work, suppose what we have there is hydrogen peroxide and perchlorate based life with those reactive chemicals in place of salt in the water inside its cells, and it can only flourish at extremely low temperatures, below -120 °C say, and just self destructs when warmed up. See section Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells - habitable to Mars life and not to Earth life (above)

Perhaps astrobiologists can prove it can't survive on Earth. Another example of that type is if it is some early form of life that Earth life destroys easily. In both cases then it would be safe to use on Earth in industrial applications. Then - if it is not related to Earth life either, there may be no chance at all of lateral gene transfer.

If we had life like that, it could give us what researchers into XNA have been looking for, see this paper Xenobiology: A new form of life as the ultimate biosafety tool

"These changes make this novel information-storing biopolymer “invisible” to natural biological systems. The lack of cognition to the natural world, however, is seen as an opportunity to implement a genetic firewall that impedes exchange of genetic information with the natural world, which means it could be the ultimate biosafety tool. "

They are talking there about artificially modified life, that might result once we are able to insert XNA into DNA based life, which then replicates, but the same could also apply to naturally occurring organisms of he same type. Once we understand them well that is.We could use it to generate exotic chemicals with no risk at all that our microbes will escape into the wild and cause harm there, or hybridize with Earth life.

The authors say that though there have been no major incidents from millions of experiments in recombinant DNA in laboratories that we may have billions and even trillions of such experiments in the future.

"Whatever new or improved physical containment mechanisms are developed, there is one key problem that cannot be solved: all biotech (and nanobiotech) use the same “software program,” namely DNA. DNA occurs in all naturally evolved and domesticated microbes, plants, and animals. Instead of bug fixing, and poorly adjusting biosafety regulations, red taping R&D, or painfully trying to fight off public resistance, why not switch to a different genetic software program altogether? Why not prepare a safe foundation for all the billion and trillion future biotech experiments and applications? Why not switch to another hardware that is incompatible with everything nature has ever created. Why not construct a genetic firewall that solves this problem once and for all?"

They give a list of specifications for XNA based life as the ultimate biosafety mechanism. As you can see, it won't be easy to do:

• Xeno-organisms must not loose their auxotrophic character. [Auxotrophic here means unable to grow in the wild, dependent on laboratory conditions]
• Natural organisms must also not be able to produce these essential biochemicals, to avoid a symbiotic relationship with XNA.
• Natural DNA polymerase should not be able to transcribe XNA to DNA.
• Natural RNA polymerase should not be able to transcribe XNA to RNA.
• Artificial polymerase must not be able to transcribe DNA to XNA, or otherwise the XNA would have direct access to 4 billion years of evolutionary experience.
• XNA genes be taken up by DNA organisms should not be recognized by natural transcription factors.
• Preferably, single-strand XNA should not interfere with the transcription process in natural cells (like iRNA).
• Symbiogenesis (the merging of two separate organisms to form a single new organism) between XNA and DNA should not take place.
• XNA must not be a recalcitrant chemical, but should act as food for natural organisms after its death/destruction.
• Preferably, additional layers of orthogonality such as non-canonical base pairs, rearranged codon assignment, etc. should be used to increase the safety mechanism even further.

This is a big challenge if we make the XNA by modifying Earth life. But if we are lucky, we might find a form of exobiology that is naturally orthogonal, not able to do any of these things, such as the hydrogen peroxide and perchlorate based life.

Then there's another application here. If Earth life doesn't recognize the XNA based life as a threat - well it of course makes it dangerous to Earth life. But the same property would also make it great for manufacturing implants which would not be rejected - so long as we can make sure that the life is totally sterilized first.

Industrial applications of astrobiology already - enzymes from extremophiles

I found this in an article by Charles Cockell:

"In uncovering the secrets of life's survival on the Earth, astrobiology has some found remarkably prosaic applications. The powder that works in your washing machine at high temperature functions because it contains proteins extracted from microbes that grow in volcanic hot springs."

" They were first found by scientists (who would today call themselves astrobiologists) seeking to know how life adapts to such primitive, searing surroundings."

From "How the search for aliens can help sustain life on Earth", Charles Cockell op ed special to CNN, October 2012.

Intriguing isn't it?

So let's look into it some more. I thought I'd research into the background to this quote, and I was just astonished at how many industrial applications there are of enzymes from these extremophiles that the astrobiologists are so interested in and study avidly.

These enzymes are now used

• in the food industry, including bread making, fruit juices, for lactose free foods, for making syrups
• in the $1 billion industry of enzymes for detergents
• for wood pulp and paper processing
• in the textile, cement, cosmetic industries.
• in various research techniques for experts studying DNA and RNA

They are used to reduce costs, make the processes more ecofriendly, reduce CO2 emissions, more efficient faster processing, etc etc.

So, though we haven't yet found any extraterrestrial microbes, the astrobiological research into extremophiles has already had many commercial benefits in industry on Earth. What then might be the benefits of astrobiological research when we actually discover another form of life not related to Earth based life? Or forms of life based on DNA but adapted to novel conditions on Mars? Maybe some of these discoveries we've already made through astrobiological research into extremophiles on Earth can give an idea of how extraterrestrial life could benefit us in the future.

If what we find on Mars is related to Earth life, but evolved on a planet with night time temperatures cold enough for dry ice, extreme desiccation, high levels of UV, and of ionizing radiation, pervasive perchlorates and hydrogen peroxide etc - we may well find extremophiles there that push the limits far beyond what Earth extremophiles can do.They would have had billions of years of evolution on Mars in conditions that life on Earth never encounters, anywhere on our planet.

I'm going to summarize some of the examples given in a survey paper Cold and Hot Extremozymes: Industrial Relevance and Current Trends (paper from 2015) and this section is taken entirely from that paper, published in "Frontiers in Bioengineering and Biotechnology". I'll mention some of the highlights, They give many more examples and describe them in detail.

Example enzymes from cold adapted extremophiles used in industry today - detergents, for fruit juice, bread, textiles, cosmetics, cement, research

For examples, this Table of commercially available cold-active enzymes

These are more active, and active at lower temperatures. This means you need less of them to get a result, and you don't have to heat them up and they can work at low temperatures. As an example, this lets you reduce washing temperatures and to help people with limited access to warm water. The global market for detergent enzymes is valued at over $1 billion.

They may also be inactivated as they get hotter, which is useful if you want to use them only for a particular length of time, e.g. meat tenderization.

"The current trend in the detergent market is cold-water detergents that can work as efficiently as common detergents but at lower temperatures. The application of cold-water detergents would allow for reduced energy consumption and carbon dioxide emissions as well as improved fabric protection. Despite the appeal of this idea, it has taken time to become a widely spread option. Hot water remains the preferred standard method for cleaning clothes. However, recent efforts to discover and develop novel enzymes that can work efficiently at cold temperatures are helping to change the cleaning industries awareness and creating an excellent opportunity for the application of cold-wash detergents. "

Production of lactose free foodstuffs. The cold active enzymes let you do this at lower temperatures.

"A cold-active β-galactosidase isolated from a marine psychrophilic bacterium was recently characterized. This β-galactosidases hydrolyzed around 80% of the lactose in raw milk at 20°C and pH 6.5. In 2012, a patent was granted to Stougaard and Schmidtfor a cold-active β-galactosidase with stable enzymatic activity at temperatures <8°C. "

Fruit juice processing for clarification and to reduce viscosity, and extract natural oils.

Enzymes with "activity between 5 and 20°C and are used for the clarification of juices, musts, and wines"

Bread making. Let's dough prove at lower temperatures.

"A recent report demonstrated that three psychrophilic xylanases from P. haloplanktis TAH3A, Flavobacterium sp. MSY-2 and one from an unknown bacterial origin effectively improved dough properties and final bread volume (up to 28%) when compared to mesophilic xylanases from Bacillus subtilis and Aspergillus aculeatus "

Textile, research, cement and cosmetic industries -as a low temperature antioxidant enzyme.

"Recently, the gene that codes for the psychrotolerant enzyme catalase (CAT) from a psychrotolerant microorganism belonging to the Serratia genus was successfully cloned and expressed in E. coli. CAT is a very active and extremely efficient antioxidant enzyme even at low temperatures .... This enzyme kept 50% of its activity after 7 h of 50°C exposure and is active in a wide range of temperatures from 20 to 70°C. Due to these unique characteristics, it can be applied in the textile, research, cement, and cosmetic industries. This enzyme is now commercially available through Swissaustral Company".

Enzymes from heat adapted extremophiles used in industry today - manufacture of syrups from starch, baking, brewing, paper making industry

Table of examples of commercially available thermostable enzymes

The enzymes are active and efficient at high temperatures, extreme pH values, high concentrations of the substrate, and high pressures. They are also highly resistant to organic solvents, and other things that stop enzymes working (denaturing agents). They are easier to separate during purification steps (because they don't break up) and they catalyze faster reactions.

Enzymes from extremophiles are useful for production of glucose syrup from starches, for instance corn (maize). As part of a complex process the starch plus enzyme is heated to 105 °C briefly using steam, then it is held at temperatures above 100 °C to complete the gelatinisation process, then it is hydrolized in holding tanks at 90 - 100 °C for 1 to 2 hours. You can read the details of this process here. The resulting liquefied starch is mostly turned into sugar, but some is used for bulking agents and in baby food.

They are also useful for "baking, brewing, the preparation of digestive aids, and in the production of cakes and fruit juices"

"Currently, Novamyl  (Novozymes) a thermostable maltogenic amylase of Bacillus stearothermophilus is used commercially in the bakery industry for improved freshness and other bread qualities. AlphaStar PLUS is a food grade, bacterial α-amylase from B. subtilis used in brewing application"

Also they are useful for the paper making industry. The first stage is to extract pulp from wood, fibre crops and waste paper. This is normally done using extremely high temperatures of over 60 °C, alkaline pH and strong chemicals like sodium sulfide, sodium hydroxide and chlorine. So enzymes that work at high temperatures are very attractive for making the result more eco-friendly, but more profitable too.

"In efforts to complement the current pulping procedures, enzymatic bio-pulping is becoming an attention-grabbing approach as it offers an eco-friendly, safer, and profitable solution for the pulp and paper industry. The utilization of stable hyperthermophilic/alkaline enzymes represents a valuable addition to the current pulping processes by increasing the efficiency and reducing the use of dangerous chemicals. Despite the fact that currently the market for enzymes in the pulping and paper industry is small, it is expected to reach a size of $261.7 million in 2018, with a CAGR of 7% (2013–2018). Europe is positioned as the main geographical market "

Various enzymes from heat tolerant extremophiles are used for biobleaching, which makes the resulting paper whiter. Other enzymes are used for dealing with sticky deposits of pitch that degrade the paper. Yet other enzymes help make the paper sheets brighter and stronger, and increase the overall efficiency.

As a couple of many examples they give:

"Other hyperthermophilic enzymes that find application in the pulp and paper industry are cellulases.  These enzymes increase the brightness and strength properties of paper sheets and the overall efficiency of the refining process. In 2012, Dyadic International launched FibreZyme G500, a cellulase with a wide temperature and pH range for activity, from 35 to 75°C and pH 4.5 to 9. This is an adaptable product for most pulping procedures. In addition, a recent report described a novel recombinant archaeal cellulase with an optimal activity at 109°C, a half-life of 5 h at 100°C, and it is highly resistant to strong detergents, high-salt concentrations, and ionic liquid"

The enzymes are usually discovered either by searching for DNA / RNA sequences coding enzymes that are already known, which of course creates a bias towards enzymes similar to ones already known. They often then need directed evolution and protein engineering to turn them into something useful in industry.

The other way to do it is to look for enzymatic activity in vivo in large collections of microbes. The difficulty here is screening the vast numbers of enzymes that occur in nature. You need to be able to test hundreds of samples simultaneously, in "High Throughput Screening" technologies.

The next question then is, once discovered, how do you produce the enzymes? The best solution would be to use the extremophiles themselves to produce the enzymes. But it's more convenient to use microbes that work well at more ordinary temperatures, so they often transfer the gene sequences into host bacteria that can work at less extreme conditions.

To find out more please read the paper here.

Please note - it's quite a technical paper and I summarized it as a science blogger and mathematician, not as a biologist. So - as usual do feel free to correct this, if there is anything here that I haven't understood properly, thanks!

Searching for life on Europa and Enceladus

The main focus of this book is on the search for life on Mars. However, many of the same ideas for Mars also apply to searches for life on Europa and Enceladus. Also this goes both ways. We have a chance for a fresh start on Europa and Enceladus, to design new ways to search for life, which may then help us with future searches on Mars.

For instance, as we saw in Idea that we have contaminated Mars too much already, so there is no point in protecting it, we can't give a 100% guarantee that we haven't contaminated Mars, though the experts think that it is unlikely that we have. That's because our exploration of Mars was based on Carl Sagan's probabilistic approach, based on a 1 in 1,000 chance of contaminating Mars during a fixed term exploration phase.

However, we have instruments almost unimaginable to Carl Sagan writing in the 1960s. So far we have successfully studied Enceladus and Europa from orbit, and already learnt a fair bit about them. We have flown Cassini through the geysers of Enceladus, sampling them directly, with no chance at all of contamination. We crash our spacecraft into the gas giants Jupiter and Saturn, to prevent them contaminating their icy moons. This all means that we still have the possibility of an exploration strategy for Europa and Enceladus that achieves 100% protection from Earth life (for all practical purposes). So, is that what we should do?

Europa and Enceladus will challenge our planetary protection methods like nothing before. Microbes can survive so easily in ice when it is melted, as it would after an impact in an "off nominal" mission. Even the Jupiter radiation at Europa would not sterilize the remains of a lander that end up meters below the surface after a crash into a crevasse or geyser, or even into liquid water thinly covered by ice in one of the active continually changing chaos regions on Europa, with the ice constantly turning over like a calving glacier.

We may meet challenges like this on Mars too. The flow-like features in Richardson crater on Mars could according to some models be cms thick fresh liquid water under ice, not unlike Europa on a much smaller scale. The salty habitats of the RSL's might be challenging too, and even Nilton Renno's droplets on salt ice interfaces. Though these microhabitats may be minute on a human scale, for a microbe they would be like tiny subsurface oceans, mini oceans only centimeters or perhaps even millimeters thick.

Maybe we need to use similar care in our search for life on Mars once we start to explore these regions? Could we devise 100% clean mini rovers for Mars, to study habitats colonizable by Earth microbes close up without planetary protection issues?

Europa and Enceladus have one great advantage over Mars. They are actively throwing water up into space in geysers which we can sample without even landing.

Hubble's confirmation of probable geyser activity on Europa

Hubble has found new evidence of possible plume activity on Europa. In a series of ten observations, they saw them on three occasions. Here are the images they created.

Look at the seven o'clock position for the possible plumes, not far from the South pole. The central image has another possible plume close to the equator. They spotted the plumes in these three cases, out of ten observations by Hubble. These are composite images with a photograph of Europa superimposed on the Hubble data. The geysers seem to extend at least 200 km above the surface of Europa.

Image credit NASA / ESA / W. Sparks / USGS Astrogeology Science Center

Composite image showing one of their observations with three plumes at lower left. Credits NASA/ESA/W. Sparks (STScI)/USGS Astrogeology Science Center

For more details see this report in Nature, and the preprint of their article also this review in Sky and Telescope.

Detail from artist's impression of the geysers for one of the Hubble observations

Interestingly, they spotted them in roughly same position as the previous plume detection in 2012, though not close enough to it to be the "same plume".

Hubble Sees Evidence of Water Vapor at Jupiter Moon - and more details in this article in AmericaSpace and in Sky and Telescope.

Combines smoothed Hubble UV data with visible image of leading hemisphere of Europa.

Note Hubble didn't photograph take the photographs of Europa in any of these images - it is far too far away for that. It just made the observation of the water plume - those large blue pixels in this last image. The scientists were using Hubble at the very limits of its sensitivity. Also in all the images, then Europa would have been dark. They observed the plumes as absorption bands as Europa transited the smooth face of Jupiter. The pixels shown as bright in these images were in fact dark in the original photographs, as was Europa itself.

Europa is tidally locked to Jupiter, so every time Europa does a transit of Jupiter, we see the same face of it in the same orientation. This shows how it works:

That's why they think it could well be a recurring plume in the same place as for the 2012 observations. This is potentially exciting news because when Hubble first spotted a plume of water, it seemed to be a once off observation and didn’t get repeated. After the initial excitement many astronomers thought it might well have been just a rare asteroid impact on Europa sending water up into space. Europa's Elusive Water Plume Paints Grim Picture For Life - Astrobiology Magazine

However with three more observations from Hubble, this makes asteroid impact a very unlikely explanation, and especially so since it's been observed in the same position approximately, multiple times. It's now much more likely to be a huge geyser. Here is a NASA TV presentation about their discovery by Katrina Jackson:

(click to watch on YouTube)

This is very exciting news for astrobiologists, because such an enormous geyser would suggest that the water is escaping Europa at high pressure. This suggests it comes from deep below the surface. Could it even sample the subsurface ocean? This image shows some of the possibilities.

Artist's concept for possible geyser scenarios, credit Caltech / NASA

With Cassini's observations of Enceladus, then we have evidence that the material in the geysers was in direct contact with hot rocks only a few months earlier.

However the ice cover for Europa is much thicker than for Enceladus, at 100 kilometers which makes it rather difficult for tidal effects to keep a channel open all the way down to the subsurface ocean against the immense pressures of the ice closing the channel.

This seems a more likely scenario

Artist's impression of a rising lens of water - this is Britney Schmidt et al's proposal. The water originates in the subsurface ocean but rises slowly, rather like a rising plume of lava on Earth, until it breaks through the surface, but unlike lava on Earth, you have hundreds of square kilometers of molten ice suddenly liquefying and the ice forms ice bergs which turn over and move around chaotically. The geysers might well originate from a region like this.

There a hot plume of water from the ocean rises very slowly through the ice. We have evidence on the surface of chaotic terrain, which may be the result of one of these plumes reaching the surface. The water, denser than ice, would cause the surface of Europa to dip as it approached it, and then once it reached the surface, the water would freeze and break up into giant icebergs that would turn over and form the chaotic terrain. At least that's one explanation of how it might work. If so, these geysers could come directly from one of these giant rising hot plumes of liquid water.

This in turn is very interesting because this could make it one of our best chances in the solar system for finding extraterrestrial life and even possibly, complex multicellular life such as we have on Earth. There is almost no chance of life getting transferred from Earth to Europa or vice versa over the entire history of the solar system as models suggest that only a few meteorites have ever made that transition in the entire history of the solar system. And if the ocean material is indeed sent into space in a geyser, one of our spacecraft could sample it just by flying through it with no need for a lander.

A region like this would be a perilous place to land. Where will it land? Crumbling and tumbling ice blocks - or smooth ice moving as slowly as continental drift?

Hubble's confirmation of a repeating plume

One of those Hubble plumes from 2014 has turned out to be a repeater in a later observation from 2016. This is especially interesting as that suggests it is a geyser or something similar. What's more, with these two photographs, they know its position accurately enough to try to find out where it comes from on the surface. This shows the observations - as before with a photograph of Europa superimposed on the data.

Both photographs are by Hubble, taken in far UV light. The one on the left was taken on March 17, 2014, and the one on the right, on February 22, 2016. Image is from the Hubble site here.

When the scientists looked closely at that spot on the Moon, they didn't see anything obvious by way of optical features. But when they turned to infrared, this is what they found:

This shows the Galileo heat map of Europa's night side (at the time) overlayed on the ellipse showing the estimated position of the geyser observed by Hubble. I have just superimposed the two, images from the press release available from the Hubble site.

In detail: the green ellipse is centered on the February 22, 2016 event and shows the region of uncertainty for it. The earlier 2014 event has more uncertainty but its uncertainty ellipse is centred on the same spot. The area covered by the image is around 1320 by 900 km. For more details see figure 2 of this paper. It's superimposed on an image of the night time brightness temperature contours from Galileo from a paper published in 1999. The red region has a temperature of over 96 °K

This shows it in a broader context. The hot spot associated with they geyser is the hottest point on the Europan night side, temperature 96 °K, towards middle right roughly at longitude 270 and latitude around -15. Shows figure 1 from this paper.

Here is the press conference where the scientists talk about these results.

(click to watch on YouTube)

How can we find out more?

NASA has plans for a Europa multiple flyby mission, now called Europa Clipper, to be launched in 2022 on the SLS to get there in just 3 years. If you do a type I Hohmann transfer, spanning less than 180 degrees around the sun, then you can get from Earth to Jupiter in under two years as Voyager 2 did, taking one and a half years to reach Jupiter from Earth. So it is not a particularly long journey time from Earth.

Because of the strong radiation around Jupiter close to Europa, it actually makes most sense to do flybys of Europa rather than to orbit it. After each flyby, they will have plenty of time to send back the vast amounts of data they collect with each close approach. The end result is much more data sent back and a mission that can last for years instead of just a month or two.

(click to watch on YouTube)

Keeping safe - sample geysers

I think the best solution here is to focus on sampling any geysers as our main priority. We can definitely do that with a mission to Enceladus, and now it seems we may be able to do it for Europa as well. Enceladus is less known amongst the general public, but it also may have life.

Geysers on Enceladus (moon of Saturn). A spacecraft could fly through these geysers (Cassini has done so many times now). It could do a detailed analysis and even a life search as according to some theories, the water in these geysers was in Enceladus’ ocean as recently as a few months before they are ejected into space. Europa may have geysers also but with its larger gravity they may not go so high into space, so may be harder to spot.

With these new observations this now becomes a top priority. As with Cassini for Enceladus, a Europa flyby mission should be able to do multiple flybys of Europa. Cassini found out a lot about Enceladus's subsurface ocean from analysing the plumes, and that is with scientific instruments built 20 years ago (it was launched in 1997) and a mission that was planned at a time that we didn't know that geysers were even a possibility.

A flyby mission can go through the plumes at different heights, and at different times in the orbit and gradually build up a picture of what is in the material. It can actually sample the material directly and analyse it on board the spacecraft. The Europa flyby will do around 45 flybys of Europa so will have plenty of opportunities to fly through the plumes.

Why a Europa lander is likely to draw a blank - it looks so smooth and those brown lines seem biologically interesting, but what is it really like?

It's natural to go for the low hanging fruit first. The Europa Multiple Flyby Mission (now called Europa Clipper) will do exactly that. It will help us to understand Europa's surface as well as Enceladus after Cassini's multiple flybys, and indeed better. You can understand how a Europa lander seems like the next low hanging fruit after that. After all, Europa from a distance is the smoothest moon in the solar system. It just looks like a smooth ball, crisscrossed with brown coloured lines that seem likely to include organics from its subsurface ocean. It has no volcanoes, no big impact craters, and what we do have of past craters is smoothed out by the leveling effects of the soft ice. As a result it's subject to many science fiction stories including the Space Odyssey series and the Europa report and has seeped into public consciousness as the next "go to" spot to search for life.

So, which do you think is smoother, Mars or Europa? If you haven't looked into this in detail, most people I'm sure would say Europa. And indeed, later photos confirm a very smooth surface from a distance.

Part of a high resolution stitched together mosaic of the Europan surface

It looks like such an easy place to land.

However, these first appearances are deceptive here. Its apparently smooth surface, on closer inspection, turns out to be rough at every scale we can image it. It seems likely to be far rougher than Mars, right down to sub-meter scales. It's the opposite of what you'd expect indeed. Not only is Europa no smoother than Mars, actually, Mars is way smoother than Europa.

As Britney Schmidt put it, reported by Scientific American,

'Icy surfaces on Earth are incredibly complex, and Europa is rough on every scale we’ve ever observed it, so finding a flat spot might be impossible. It’s hard not to be worried about that. Mars has been difficult for us—and it’s way flatter than Europa.'

It's hard enough to avoid landing on a boulder on Mars. Viking 1 came close to disaster landing a few meters away from "Big Joe", a boulder two meters in diameter. And that was after careful selection of the site after much deliberation, by a combination of radar and photographic observations from orbit. Europa seems to be covered in features like this right down to the smallest scale we can see. What's more, it is an actively changing surface which may even look different in detail next time we image it close up. There may be no large smooth areas at all. Also the most interesting areas there for biology are amongst the roughest too.

This artist's impression gives an idea of what scientists now think the surface of Europa looks like, though it could easily be rougher than this as we have no idea what it is like at the sub meter scale:

Artist's impression of the surface of Europa courtesy NASA / JPL - likely to be very rough right down to sub meter scale.

The surface can't have mountains, because they would relax into the ice. So it doesn't even have any noticeable huge impact craters, very unusually indeed in our solar system. But on the smaller scale, many think that it has giant plumes, rather like lava plumes (not the geysers), of liquid water that rise slowly from its subsurface oceans.

As these huge hundreds of cubic kilometer scale plumes approach the surface they form dips in the surface ice to start with, when the surface is still liquid below, as may be the situation right now, with Thera macula. But as they break through to the surface, the water freezes. The whole interior of the plume expands, because ice is less dense than water, and the top part of it becomes a huge sea of icebergs which turn over as the interior freezes and pushes outwards. Those are of course like to be very rough on all scales down to sub meter resolution.

Shows part of the "Chaos region" of Europa with resolution of 54 meters. The small craters are probably secondary craters from ejecta from Pwyll, a region formed by a giant impact 1000 km to the South. This area is particularly chaotic but the close up images of Europa generally show a very rough surface. It is likely to be rough down to sub meter scale.

The macula regions, which are probably the most biologically interesting, may be the result of overturned icebergs separated by deep crevasses. According to Brittney Schmidt, then chaos terrain on Europa may be the result of upturned icebergs and may indicate liquid water not far below the surface in some places. These are amongst the most biologically interesting places, but the toughest areas to land on.

Alternatively they may be the result of impacts. The harder surface Europan ice may be quite thin (for an icy moon), for instance according to a study of its gravitational effects and movement of the crust over the subsurface it may be around 10 km thick. If so it may have a lower layer of soft ice with the ocean up to 100 km below the surface. Or it could have a thinner ice layer directly over a liquid ocean. With the thinner ice model then impacts of comets could break through the surface ice into its ocean every few hundred million years. In Jupiter's deep gravity well, even small comets come in with quite a wallop, on average 26.5 km / sec. A 0.5 km comet could breach 5 km ice and a 5 km comet could breach 40 km ice according to calculations published in 2015. With the thinner ice models, the surface could be breached every few million years. Even with a 40 km ice layer, it could be breached every 250 million years. Techy paper.

Estimates from study of the impact craters on Europa show that craters like Tyre wider than 30 km all have collapsed rims. But smaller craters like Pwyll at 27 km in diameter have not collapsed much, so the ice can't be very thin. It's more than three or four km thick and probably a fair bit thicker than that.

Whether they are formed originally by rising plumes of water from below or impacts that break straight through the ice to the subsurface ocean from above, any region that has been a macula in the past is likely to be extremely rough, and covered in treacherous crevasses. The most biologically interesting regions may be the hardest to land on.

For more about this, see Where will it land? Crumbling and tumbling ice blocks - or smooth ice moving as slowly as continental drift? (below)

Also, according to one theory most of Europa is also likely to be covered in "ice knives" up to meters high, that form gradually over millions of years through ice subliming into the vacuum preferentially when hit by the sun. These would form in equatorial regions but because of the movement of the ice over its subsurface ocean and locally, may occur anywhere on the surface of Europa. More on this in the next section. Rugged unknown surface of Europa - ice knives, crevasses, and upturned icebergs (below)

If Europa does have smooth surfaces anywhere on the meter scale, these may well be the biologically least interesting places to land. Though the macula may be the results of upheavals of water that actually originated in the subsurface oceans, the brown lines are, many of them, the results of much slower tectonic type processes.

The wide band crossing this image (from Galileo) is an example of a "dilational band" where the subsurface ice is gradually being pushed up through cracks in Europa's crust, a process similar to sea floor spreading on Earth. It seems also to have subduction zones where ice is buried, much as for Earth's continents again, though this required careful detective work.

This shows the subduction zones deduced in a paper from 2014, which suggests that Europa may have a system of continental plates and continental drift like Earth. Figure 4 of paper by Simon Katterhorn and Louise Prockter - and NASA release about their work.

These geological timescale processes of tectonic motion would bring ice to the surface very slowly. Anything in the top few meters would be damaged by radiation. Craters could bring material from deeper layers to the surface, but the largest known crater, Tyre, only samples material down to 3 km below the surface. Though that is pretty deep, it's nowhere near deep enough to sample a subsurface ocean. For more on this see this 2002 paper (from before the discovery of the Europa plumes). So though geologically interesting, these slow processes are likely to require a lot of detective work to find out if there was life in the deposits originally. More of that later.

Also those brown lines - though they may contain organics from its subsurface oceans, are likely to consist largely of tholins, or other organics, millions of years old, broken up by the ionizing radiation and converted into complex chemicals by UV light, and in the case of Europa, by impact processes. It's also likely to contain organics brought there by meteorite impacts. Those have probably got thoroughly mixed up with any organics from the oceans, sliced and diced and recombined into those complex organic molecules called tholins, many times for millennia.

Europa as seen by Voyager 2 during its closest approach on July 9, 1976. It has by far the smoothest surface of any moon known, and covered in these intriguing brown lines which science fiction has presented to us as cracks into a subsurface ocean. The ocean is now confirmed, but would be around a hundred kilometers below the surface.

The surface is now known to be very rough down to the smallest scales we can observe it. And the brown lines are probably not organics from the subsurface lakes, but rather, tholins, created by breaking up organics and recombining them, as a result of ionizing radiation, and to incorporate organics from meteorites and comets.

The one big ray of sunlight here is that Europa does seem to have geysers according to the Hubble measurements.It has repeating "water vapour plumes" expanding into the vacuum around Europa, which all seem to come from the same quadrant of Europa though probably not from the same exact geographical point. These could be the result of surface ice processes. They don't seem to be the direct result of impacts, as those wouldn't repeat.

But more optimistically, they may come from geysers, especially the repeating geyser of 2016. If so, this quadrant identified by Hubble may be a local weak patch in the crust, perhaps with liquid water not far below the surface. It might even have liquid water that comes to the surface all the way from its subsurface ocean. Enceladus does have geysers that send water from its subsurface ocean all the way to the surface including silica particles that give evidence that the water may well have been in contact with rock, not just that, but even hot hydrothermal rock, only months before it reaches the surface of Enceladus.

Here is another of my speculative thoughts again here. I can't remember seeing as a suggestion (do say if you know of one), but what about a subsurface lake created by meteorite impacts, an impact lake? Like the ones suggested for Mars in Ice covered lakes habitable for thousands of years after large impacts (above). Could that be an alternative hypothesis for the geysers? That would let Europa have extra geysers not necessarily associated with the Chaos regions. After all Europa must get high velocity impacts frequently, easily able to melt the ice below the surface and penetrate it to some depth. Even a small lake of a few cubic kilometers or less, under pressure, perhaps could generate a geyser that persists for some time.

At any rate, the new picture emerging is that the best way to find life from the Europa oceans, or at least, in any subsurface lakes, is to sample a geyser, not the surface. This also has minimal planetary protection issues.

It is a major technological challenge, because of the ionizing radiation. A multiple flyby mission can do geyser sampling just as Cassini did for Enceladus. The geysers are likely to have their larger particles rather lower in the higher Europa gravity, but especially with such a smooth surface, and no atmosphere, it can fly pretty close. Cassini flew within 175 kilometers of Enceladus during its closest flyby, and the Europa plumes . .

However, an orbiter would run into the same problems as a lander, that the electronics would fail after weeks or months in the high levels of radiation there.

However, just a suggestion again - if instead of a lander, we have an orbiter, then all the extra payload needed to land it on the surface of Europa could instead be used as additional radiation shielding. So perhaps it can last rather longer than the lander's 20+ days.

It's a pretty good "low hanging fruit" now, since we have probable confirmation of the geysers, though we don't know for sure if they are connected to subsurface ancient water directly or indirectly, or are some more recent temporary phenomenon.

We have one really outstanding "low hanging fruit" however, the Enceladus geysers.

Enceladus geysers as the low hanging fruit right now - with other interesting targets in the Saturn system - could Dione, Rhea and even Mimas also have subsurface oceans?

So, the Enceladus geysers provide the best low hanging fruit to try next, if you are interested in icy moons and their oceans. We know they exist. We know they can be sampled because Cassini has already done this, diving deep into the plumes. The particles are in the micron range and larger, easily large enough to contain microbes. They come from the deep subsurface of Enceladus.

Enceladus' crust may be very thin. We now know that Enceladus librates, and the amount of the libration, 0.120 ± 0.014° is only possible with a global ocean to let the surface ice move over the more massive interior. A study in October 2016 which did modeling of Enceladus based on those libration observations predicted that Enceladus' ocean may be 38 km thick below a 23 km thick crust, 7 km thick at the South pole (with an uncertainty in their model of 4 km for all those figures). And a study of thermal imaging data from Enceladus released in March 2017 suggests there could be liquid water as close as 2 km below the surface beneath the south pole.

So Enceladus is a very promising "low hanging fruit" for a flyby. With the SLS we can send a mission to Enceladus on a timescale of three years, not much different from the two years for Europa. With the demise of Cassini later this year, we won't have a single spacecraft in the outer solar system beyond Jupiter. An Enceladus orbiter would also do flybys of Titan, and many of the smaller moons of Saturn.

It could also study the intriguing Dione, next satellite out from Enceladus, which may have had geysers in the past, now frozen over. It may still have an ocean right now like Enceladus, that same study that predicted a thin crust for the Enceladus ocean predicts that Dione has a global ocean perhaps 65 kilometers thick (uncertainty of ±30 km) below a crust of 100 km (99 ± 23 km) of ice. Paper here.

They predict that Dione will be observed to librate like Enceladus (so proving the existence of its internal ocean) when we have the opportunity to look at it more closely. So that's one thing our flyby mission could do. Also it could study the evidence for past geyser activity there, close up. Dione is in a 2:1 mean motion resonance with Enceladus. (Enceladus orbits Saturn every 32.9 hours and Dione every ). Its gravity helps keep Enceladus in an elliptical orbit, which seems to be what helps keep Enceladus' ocean liquid. Meanwhile, Enceladus keeps Dione in an elliptical orbit too, which may in turn help keep Dione's ocean liquid. (Enceladus has eccentricity 0.0047 and Dione, 0.0022).

Also, the Cassini team has not given up on the possibility of Dione geysers. Their last Dione photograph will be a plume search on August 3, 2017.

The Enceladus mission could also study Rhea, as that also may just possibly have a subsurface ocean according to some calculations (and is also intriguing because of the question of whether it ever had a ring system, with marks suggesting a previous ring that de-orbited, which would make it the only known moon with rings, and it may just have very faint rings of fine dust to this day). And of course, Titan.

Another rather surprising candidate for a subsurface ocean in the Saturn system is another tiny moon Mimas.Its libration of six kilometers instead of three is evidence either for an elongated (non hydrostatic) core, or a global ocean. The ocean could be between 24 and 31 km below the surface depending on the viscosity of the ice

There is no way it could sustain an ocean into the indefinite future but its eccentricity is high, and perhaps it had much higher eccentricity in the past and so had an ocean through tidal heating that's still there today. The other option though, is that perhaps it has an oddly shaped core - which is not unreasonable. Perhaps it was formed by collisions of two other bodies for instance and the cores haven't completely coalesced. For more about all this see the end of this paper. Anyway a mission like this could go on and study all these moons of Saturn close up, like the Cassini mission but with more modern instruments.

We need to think about planetary protection,so as an end of mission, it could leave Enceladus, and go on to explore the rings and end up entering Saturn's atmosphere, as for Cassini. Or if it stays orbiting Enceladus throughout its mission, it could crash into the equatorial region of Enceladus. This is old ice, not been disturbed for billions of years, so would seem to have no planetary protection issues.

So then, the logical next mission after an Enceladus orbiter is a Europa orbiter, heavily shielded, to sample its geysers, if they exist. Or potentially, a second multiple flyby mission designed to look a the geysers. We could also create an artificial geysers by firing a sterile "dumb penetrator" at thin ice or the most biologically intriguing part of the surface. By now, we have the results in of the Europa Europa Clipper multiple flyby) and so have a far thorough understanding of Europa from close up observation.

Then all of that would logically prepare for Europa and Enceladus landers, which should have ice moles, to drill at least meters deep into the geysers or into any subsurface water covered in thin ice. The aim would be to make them 100% sterile. Then finally we have the option of sending subs into the Europan and Enceladus oceans, a major challenge but not technologically possible. Again these have to be 100% sterile in this vision.

This gives a much more measured approach, less "hit and miss". A Europa lander will only succeed in finding evidence of present day or past life on the most extraordinary of lucky accidents, a bit like trying to find fossils on Mars. This approach will give more and more information about the subsurface oceans with each visit, and with the possibility of finding direct evidence of life in the plumes of either Europa or Enceladus at an early stage - material that has recently left the subsurface oceans or lakes and until then was protected from radiation damage by the ice that covers them..

It's also far better for planetary protection, with the ability to achieve close to 100% confidence that we don't contaminate either of the oceans.

This mission is not currently on NASA's radar. Geoffrey Marcy, retired professor of astronomy, and exoplanet researcher, co-investigator of the Kepler planet finding mission, and principal investigator of the billionaire funded "Breakthrough Listen" has suggested that a dedicated mission to fly through the Enceladus geysers to search for microbial life might be an ideal candidate to interest billionaire sponsors interested in astrobiology. See Could a dedicated mission to Enceladus detect microbial life there?

Although NASA do plan to do multiple flyby missions of Europa first, they don't currently intend to attempt life detection in the plumes, and they have their sights set on a lander to follow up quickly after the orbiter - indeed the idea is to launch it before the orbiter reaches Europa. I'm not sure it is the way to go right now, for multiple reasons. Planetary protection is one of them, but the science value is another part of the reasoning too.

So, let's look into this in more detail.

So then what about landing on Europa?

It’s actually quite a challenge to land on Europa. Rather controversially, NASA have a mandate from congress that they have to include a lander on this mission. This means they have to do it, without any scientific evaluation of the relative merits of other missions, e.g. geyser sampling.

This is an earlier version of the lander from the mid 2000s,, see A Lander for NASA’s Europa Mission

It's controversial because this is an idea put forward by Congress. This is not usually how you plan science missions, that a politician tells you that you have to do it in a particular way. That's more like the way that human missions are done, where the objectives are often to a large part political.

Normally it's a case of asking the scientific community for suggestions, then detailed proposals, fully worked out with costings, and then they are compared with each other based on their scientific merits rather than their political merits. It seems odd to do it the other way around, for a pure science mission.

The original plan was to have both lander and orbiter on the same mission. Now Congress have mandated NASA to do them as separate missions, and have also said that both missions have to use the Space Launch System (SLS) - a heavy booster being developed in the States, which itself is rather controversial, since it is going to be high cost, it's going to fly infrequently, and each launch is going to be extremely expensive.

It will be a remarkable vehicle able to send large masses into space and send human crew on deep space missions. However some think it will be overtaken by the private sector who have their own independent ideas for ways to achieve heavy lift such as the Falcon Heavy. The Europa mission could cost upwards of 2-3 billion dollars not including the launch, possibly as much as 3-4 billion dollars. The SLS launches themselves will add $500 million to $1 billion apiece just for the launch. The whole thing is quite controversial. See Two SLS to Jupiter in the Space Review.

Anyway this is now a rather unique mission, a planetary science mission mandated by Congress, which in addition has its launch vehicle selected by congressional mandate too. This does have its advantages though for a Europa mission. First, the spacecraft can be far more massive. It can have more instruments, use a much shorter transit time to Jupiter of only three years, and it can have more radiation shielding to protect it from Jupiter's ionizing radiation. It can get there so quickly because with such a capable launcher, there is no need for a gravity assist.

For the orbiter, the main controversy would be about the cost. The mission itself will be much more capable than it could be without the SLS. Apart from that, then the main questions might be about which instruments to send. Now that we are pretty sure that Europa has geysers - could we add geyser sampling capabilities and in situ life detection? But nobody could question the value of a multiple flyby of Europa for science.

However for the lander, then in the case of Europa, I'd like to draw attention to several reasons that make a lander a tricky proposition.

• Rough terrain. The first issue is that, as we've seen in Why a Europa lander is likely to draw a blank - it looks so smooth and those brown lines seem biologically interesting, but what is it really like? (above), the surface of Europa has not yet been imaged in the detail needed to choose a landing site, but all the indications so far are that it is very rugged in detail. It's thought to be far more hazardous for a lander than Mars. It's also an unknown surface. We've never tried something like this before. It's not unlike landing Philae on comet 67p, though for different reasons. There is no risk of it bouncing off the surface, but it could crash on rugged terrain.
  
• Planetary protection . You have to sterilize the lander sufficiently for planetary protection, because the very last thing we want for Europa is to go there just to discover life that we brought ourselves. Is that something we can actually do at this stage? It's a huge challenge when the target is icy.
  
  A hard landing (crash) on the Europan surface could heat the ice to melting point, or even potentially contact liquid water beneath thin ice and the ice could then shield microbes from the ionizing radiation of Jupiter. After all, Galileo at the end of its mission was directed to crash into Jupiter to prevent the remote chance of it eventually hitting Europa in the distant future. Also, the surface of Europa is constantly changing, especially in the more interesting regions for astrobiology. If it lands meters below the surface of Europa in an iceberg that then flips over, it could end up in subsurface liquid water easily, if it doesn't break through right away after an "off nominal" mission.
  
• How much science value will it have? The surface ices are likely to be very degraded and mixed in with inorganic tholins that get there from comet and meteorite impacts. The lander could only last three weeks or so on the surface because of the ionizing radiation, also its batteries will run out as it doesn't have solar panels. It's a static lander - they aren't attempting a rover. It can't drill to any depth. How much is it likely to find out about past or present day life on Mars?.

The strange thing here is that Congress actually have mandated a launch date for the lander too. NASA has to launch it in 2024. That means they have to send it to Europa a year before the orbiter gets there, and so before they have any new data on the Europan surface conditions. The reason for this is because they want to keep up a cadence of at least one SLS launch a year. So if they launch the Europa missions in 2022 and 2024 then that means two early SLS slots are taken care of. So it's a political decision.

Actually the Trump administration hasn't included any funding for the lander, which may mean that it will be postponed, though this is just the first step in the process of working out the details of the 2018 budget for NASA.

So let's look in detail at some of the issues with sending a lander to Europa based only on the knowledge we have about it so far. Is it something that we can actually do, realistically, in this time frame - with a launch in 2024, and do we know enough to design it before detailed observations of Europa from the orbiter?

Also, would it be a good use of our resources for the search for life, past or present, on Europa? And can we design a lander to prevent forward contamination to land on Europa ?

Rugged unknown surface of Europa - ice knives, crevasses, and upturned icebergs

As we saw in Why a Europa lander is likely to draw a blank - it looks so smooth and those brown lines seem biologically interesting, but what is it really like? (above), the surface is very rough down to meter and probably sub meter scale, enough to give mission planners nightmares.

From an article in Scientific American, Feb. 17, 2017:

“Although mission planners have yet to map Europa at very high resolution, the lower-resolution images they have already seen show a topography rugged enough to give them nightmares, says Britney Schmidt, a planetary scientist at Georgia Tech and study co-author. 'Icy surfaces on Earth are incredibly complex, and Europa is rough on every scale we’ve ever observed it, so finding a flat spot might be impossible,' she says. 'It’s hard not to be worried about that. Mars has been difficult for us—and it’s way flatter than Europa.' ”

(emphasis mine)

We haven't yet imaged it at sub meter resolution, but it could easily be very rough at down to centimeters scale also.

As an example of how little we know, one theory that has not yet been disproved is that parts of the surface might be covered in closely spaced vertical “ice blades” or “ice knives” which would make a landing there hard to achieve. On Earth these blades form quickly, in special conditions On Europa they would take millions of years to form, but it’s the same basic process. As Daniel Hobley said: "Light coming in at a high angle will illuminate the sides of the blades, causing them to retreat away,"

These are called Penitentes. See Penitentes: Peculiar Spikey Snow Formation in the Andes

This video shows how they form on Earth and decline, time lapse:

(click to watch on YouTube)

Here is a photo from the European Southern Observatory site high in the Atacama desert:

Planetary Analogue, see also their Icy Penitents by Moonlight on Chajnantor, and Iconic, Conical Licancabur Watches Over Chajnantor

Ice knives on Europa

On Europa, if they exist, these structures can potentially be meter scale or higher. With no atmosphere, the conditions on Europa might well be ideal for their formation. Our missions to Europa so far haven’t taken high enough resolution photos to see them. Ice blades threaten Europa landing - BBC News

They wouldn’t be the result of ice or snow subliming into an atmosphere, obviously. It’s a slightly different process. Instead they’d be the result of the sunlight causing the ice to sublime to water vapour in a vacuum at very low temperatures well below 0 °C. Also they would form slowly over much longer timescales, of millions of years.

The surface of Europa is about 50 million years old, so when we ask if penitentes can form on Europa, one of the main questions is, how much can the ice there erode under the influence of sunlight in 50 million years? The answer to this question is extremely sensitive to the peak temperatures on Europa, to the extent that twenty degrees can make a difference between formations that are meter scale and ones that are on the scale of millimeters.

In the paper: HOW ROUGH IS THE SURFACE OF EUROPA AT LANDER SCALE? Hobley et al produce this table

So, for a surface temperature of 132 °K (about -150 °C) it loses about 5.66 meters over the average age of the surface of 50 million years. For a temperature of 128 °K (-154 °C) it loses 1.28 meters in 50 million years, tailing off to 1 cm at 116 °K (-166 °C), and only millimeters at 114 °K

So this is very sensitive to the peak surface temperatures of Europa. Also, the surface is eroded by sputtering from the Jupiter radiation and from bolide (meteorite) impacts. That would counteract the effects of the ice blade formation at temperatures of 126 °C downwards. They conclude in the paper that the knives could be from one meter to 10 centimeters in height, probably restricted to within 15 or 20 degrees of the equator.

However Europa also has “true polar wander” by which the entire crust moves over the subsurface ocean. This could reduce the size of the blades but also move the ice blades away from the equatorial regions.

In a study from 2018, then Daniel Hobbley and Jeff Moore used observatopma; data to calculate the sublimation rates and came up with a prediction of penitentes of up to 15 meters in height and a spacing of 7.5 meters between them. Press release here and paper on Nature here (abstract only, paper is behind a paywall).

Incidentally, though the Europa ones are only inferred indirectly, in the case of Pluto they have actually observed really huge probable penitentes.The ones there have a spacing of 3 to 5 km, and a depth of 500 meters in a series of aligned ridges.

Figure 1 from this paper. Possible penitentes on Pluto. The inserted compass rose shows how they are aligned with each other suggesting penitentes and they fit models for penitente formation in methane rich ice, so maybe these are layers of methane rich ice.

Upturned icebergs - for regions like Thera Macula - amongst the most interesting regions on Europa

Other issues could include a frozen landscape consisting mainly of upturned icebergs. According to some ideas, then hot plumes of melted water rise from the deep subsurface sea and eventually reach the surface and produce these irregular landscapes, as icebergs form on the freezing surface, and then turn over.

One of the most interesting regions, thought to be most likely to have thin ice over liquid water by the “thin icers” is the Thera Macula

NASA / JPL / UA / Paul Schenk This might be a region of overturned icebergs with, perhaps, liquid water still present only a short distance below the surface. Most of these chaos regions are raised, which suggests the ice below them that lead to their formation has frozen. But Thera Macula is actually a dip in the surface of Europa which may be a sign that it has the denser melted water still beneath it. See Is Europa's ice thin or thick? At chaos terrain, it's both!

The original of this photograph was taken by the Galileo spacecraft on 27th August 1999. It was made using two images at resolution of 220 meters per pixel and colourized with another image at a lower resolution of 1.4 kilometers per pixel.

Possibility of liquid water close to the surface or breaking through

Thera Macula could be a sign of 20,000-60,000 cubic kilometer of liquid water slowly approaching the surface and near to it right now. That's as much water as there are in all the Great Lakes put together. The surface features there are rough as a result and may well be actively overturning right now. There Macula may have changed noticeably already since Galileo photographed it at a resolution of 220 meters per pixel close up on the 27th August 1999.

The geysers also suggest water that reaches to the surface, at least inside the geysers. All this suggests at least a small chance of our lander crashing through thin ice or a soft surface, especially if we land it on one of the most interesting regions such as Thera Macula. Or it could fall into a crevasse and be unable to communicate.

I know the plan is to orbit Europa for a while before the lander gets there, but what if the orbiter doesn’t find any suitable spot for the design of lander, and decides a different design of lander is needed, or no lander at all? Maybe the lander has to land somewhere uninteresting, or they have to hold back from landing at all for planetary protection reasons? Also how certain can they be that the landing spot doesn't have subsurface liquid water in small quantities?

We don't yet know how to sterilize a spacecraft to Europa 100%

Then the other problem is that we don’t know how to sterilize a spacecraft 100%. Or more accurately, we can certainly sterilize a spacecraft completely, but the methods that do this, such as prolonged heat, or ionizing radiation, also destroy the electronics so it won’t work any more.

That includes of course the ionizing effect of Jupiter’s radiation. Although the surface of Europa is riddled with ionizing radiation that would quickly kill any human, any spacecraft there has to survive this, at least up to the landing. That also means that it is protected sufficiently so that microbes could survive also, up to the landing and for the duration of its mission on the surface.

If there are some microbes on the lander, and they survive to the landing, then it might impact into liquid, or create a liquid area due to a crash on Europa which might be deep enough to shield microbes so they can reproduce there. Or microbial spores brought to Europa with the lander could eventually in the future over thousands or years find their way into the ocean.

If we can achieve 100% sterile landers in the near future then this will no longer be an issue. See Can we achieve 100% sterile electronics for an Europa, Enceladus, Ceres, or Mars lander?

Analogy of a Ming vase - what is an acceptable level of risk for making all the life in Europa's ocean extinct?

This is a question mission planners and scientists have to ask themselves when designing a Europa lander. We don't have a way to make a lander like this 100% sterile at present. Also microbes from the lander could get into its ocean as the Europa surface may have connections with its deep subsurface. You might think that there is no way we could make the native life there extinct, perfectly adapted to a Europan ocean, for billions of years. However, just as for the discussions for Mars, there are many ways it could happen. For instance, Europan life could be very vulnerable if all the life there is based on a much simpler biochemistry, some earlier form of life that on Earth was made extinct when DNA based life evolved, or something similar to early life on Earth.

So far, we haven't anything that risks introducing Earth life to Europa. Indeed, we have taken great care to prevent this. The Galileo orbiter was crashed into Jupiter to avoid any possibility of contaminating Europa. So, do we continue in the same way? We certainly can do this if we decide to. We would run almost no risk at all of making the Europan life extinct, if we continue to study it from orbit. We can also safely do geyser flythroughs instead of landings, and meanwhile we can work towards the goal of future 100% sterile landers. If we did that, we'd not have any risk of making Europan life extinct.

However, that is not the current plan. The NASA lander would be based on the same principle used for Mars to date. The plan is to make the risk low, but not eliminate it completely.

Landers also run into the problem that the most interesting areas of Europa, the chaos regions, are probably in a state of flux. They are not only very rough, with fragile and soft ice, but also subject to dynamic changes with ice blocks calving or turning over, much like a calving ice sheet in Antarctica, and they are still active, with changes likely even since the Galileo flybys. The surface of Europa also may not be the best place to look for life either. Organics on the surface are likely to be mixed with meteoritic and comet organics, and broken up by the ionizing radiation from Jupiter, then recombined into tholins. The most interesting places to study close up may also be the roughest, with possibly liquid water close to the surface, thin ice, crevasses etc.

Meanwhile a fly through of the geysers has none of those hazards. It also bypass many of the issues of confusion with organics from meteorites or comets. Any organics in the plumes may have come from deep below the surface recently. They may even include still viable life.

As usual I write this to stimulate debate. This is a decision that can impact on us for all future time, if the mission makes Europan life extinct, as there would be no way to reverse those effects. So, we are all involved in the potential effects of it. I think it's important that we also engage in discussion of the acceptable level of risk. This is not a decision to be made for us by scientists on their own. It goes beyond science into areas of ethics.

This section is based on the planetary protection section of the recent report on NASA's planned Europa lander. This is how NASA currently plan to land on Europa:

There is no doubt that they are following the guidelines set out by COSPAR. See section 10.5.2 in this report. The published planetary protection guidelines that they intend to follow may seem good also. However look at it a bit more closely. The design aim is not 100% sterilization but a 1 in 10,000 risk of contaminating the Europan ocean per mission.

This 1 in 10,000, has become the "gold standard" figure for these planetary protection calculations, as an acceptable "per mission" probability of contaminating a planet. In 2000, the international committee of scientists who prepared the Preventing the Forward Contamination of Europa reviewed the situation, and they saw no reason to change that figure for Europa. So they just adopted the same figure as is used for Mars.

Techy aside on how this works for Mars. This 1 in 10,000 per mission figure is now treated only as a notional figure. Nobody does the calculations any more. Instead they use the Viking pre-heat sterilization requirements of 300,000 colony forming microbes per spacecraft. The 1 in 10,000 figure was the design basis for Viking sterilization, so you could say they are still using it, but indirectly.They reason that the harsh conditions on the Mars surface roughly correspond to the heat sterilization stage from the Viking calculations so that the missions are still, roughly speaking, sterilized to within that original 1 in 10,000 per mission guideline.

In a later review, a new COSPAR committee tried to come up with a rationale for this figure, but they were unable to do so.

"Unfortunately, the historical literature does not record the rationale for COSPAR’s adoption of the 10^-4 [1 in 10,000] standard. Nor, in fact, has the committee been able to come up with its own quantitative rationale for this number. "

Their main reason for retaining this figure was:

" The introduction of a new contamination standard into the deliberations will, in the committee’s considered opinion, complicate the resolution of more serious issues arising from the methodology contained in the 2000 Europa report."

So, in summary, it is done this way for reasons of historical precedence. COSPAR are not sure how the figure was arrived at originally, but by tradition this figure has come to be used as the "gold standard" for per mission probabilities for planetary protection calculations ever since. So, the reasoning goes, we might as well use it for Europa as well, since we have no reason to choose any other figure.

The Europa committee also cited a COSPAR resolution from 1964 as a precedence. (it is also cited in this earlier Europa planetary protection report). However, if you follow up their citation, the original text gives a much stronger requirement, one that was probably unachievable with the technology of the time. It's the probability of a single viable microbe on the spacecraft, not the probability of contamination of the spacecraft's destination.

"that the probability that a single viable organism be aboard any vehicle intended for planetary landing must be less than 1 in 10,000" (emphasis mine)

Though the COSPAR reports don't mention it, the 1 in 10,000 figure (though not the single viable microbe) also occurs in a chapter "Decontamination Standards for Mars Exploration Programs" written by Cal Sagan and Sidney Coleman, in "Biology and the Exploration of Mars", published in 1966 and reporting on a study carried out from 1964 - 1965. If so, this study started around the same time as the 1964 COSPAR resolution, and at this distance in time, it may be hard to discover which came first.

Sagan and Coleman's aim was to make sure that Mars remains uncontaminated during an "exploration phase" of 60 landers (54 successful) and 30 flybys and orbiters. But 100% certainty was impossible with the technology of his time. His calculation is rather complex and the details are based on 1960s ideas about Mars and about Mars exploration. It's also tied to restrictions of 1960s technology, that no longer apply, for instance, the idea that only one in ten of the experiments on a lander will be successful. This is a very early paper on the topic, from well before humans landed on the Moon. He just says

"If we desire 99.9% probability that 108 experiments can be carried out before contamination, ..." (he suppose two successful biological experiments per lander out of a total of 20 experiments per lander sent to Mars)

... From this he calculates his 1 in 10,000 figure

The calculation was intended as a demonstration of how to start from a probability of contamination for the entire exploration phase, and use that to derive a per mission probability. He never intended this as a "gold standard" for per mission probability for all future time. Nor did he attempt to justify or derive his 99.9% figure for the exploration phase probability.

So, the authors never suggested that there was any logical or scientific or philosophical way to deduce that figure. Nor has anyone suggested that you can since then. Also, the background to the paper no longer applies. We now no longer think in terms of a fixed number of missions in an "exploration phase", so the calculation he used to deduce his 1 in 10,000 figure is no longer relevant. Few probably have even read this now ancient chapter.

Greenberg and Tufts raised a concern about the use of this figure for Europa in EOS, the transactions of the American Geophysical union writing amongst other things:

"Even as a sterilization requirement, the COSPAR-adopted value is not relevant to the case of Europa. It was adopted in the context of Mars exploration, and even in that context its relevance would be questionable, in retrospect, one-third of a century later."

[They then discuss Carl Sagan's calculation, based on an exploration phase, mentioned above, then continue]

"Whether the COSPAR policy was appropriate for Mars is therefore highly questionable and has been questioned in the past [e.g.,DeVincenzi and Stabekis, 1984]. It is certainly not appropriate for a current discussion of Europa exploration, where there is a completely different environment than on Mars."

They propose the Natural contamination standard instead. That of course would be very strict for Europa - as there's almost no chance of exchange of life to Europa from Earth. It would seem to require 100% sterile landers.

The former task group chair Larry Esposito responded to their reply explaining how they came up with their 1 in 10,000 figure:

"The best justification is that it is the result of thoughtful deliberations of the Task Group members. A different committee might have reached a different conclusion"

In short, the whole thing seems rather arbitrary, with nobody able to give a rationale for the number. It's adopted through tradition, by citing precedents, none of which had any rationale for them either.

Before I criticize these plans - I'd like to say that I am not criticising the scientists who came up with those proposals for the Europa lander. They are just scientists doing what they were asked to do and have done a good job of it too, within the requirements set by Congress. I'm not criticizing COSPAR either, as they indeed have no rationale they can use for a different number, so what else could they do? Nor am I criticizing Carl Sagan for choosing those numbers, in a very different situation, time, and with different technology. He had to choose something to illustrate how his calculation would work. He never intended it as a "gold standard" and it was just a "for instance" which stuck.

Also, Carl Sagan and the others working on planetary protection for Mars had to choose: either don't send anything to land on Mars or accept a per mission chance of contaminating the planet. Having chosen the latter, they then had to choose a figure to use as a basis for their calculations. They had no reason to select one figure over any other. Carl Sagan had used those 99.9% and 1 in 10,000 figures in a published paper, and they simply had no rationale to choose anything different. So they ended up stuck with it as a "gold standard" which it was never originally intended to be, and so it has continued as such ever since. That seems to be the long and the short of it.

If Carl Sagan had chosen a figure of 99% or 99.99% to illustrate his calculation we would now be using a per mission probability of 1 in 1,000 or 1 in 100,000. The calculation is pretty much the same for each one. E.g. if he assumed 99.99% as his target for the probability of keeping Mars free of contamination after his 90 missions (30 orbiters and 60 landers) then for a per mission probability of 1 in 100,000 it works out as (1-(1/1000000))^90 = 99.991% certain that it remains uncontaminated after 90 missions. Presumably if he had used a 99.99% figure in his paper we would now be treating a 1 in 100,000 chance as our "gold standard" for planetary protection for Mars and now for Europa.

However, with Europa we have many more options than they had with Mars in the 1970s. We can study Europa in detail through flyby missions and orbiters, with instruments far more capable than any they had back then. We can also send geyser sampling missions to Europa until we find out more. For as long as we explore Europa in this way, we can keep a 100% reliability of planetary protection (for all practical purposes). This wasn't an option for Mars, with no geysers to sample from orbit, and less capable orbital reconnaissance technology. It was a case of landing there with 1970s technology, doing our best to minimize the risk of forward contamination, or not landing at all.

Europa also is more vulnerable than Mars was thought to be even at the time of Viking, with its probably globally connected sea, and water that may reach close to the surface in some of the chaos regions. Some of that water may erupt as geysers.

Also, we don't have to think in terms of an "exploration phase" before an inevitable human landing there. With Europa especially, our aim surely is to keep it free from Earth life indefinitely, or at least until we understand it much better than we do now (the "Europa Report" science fiction film notwithstanding). It's not even much good as a "pit stop" for ice in the Jupiter system. It makes far more sense to use Callisto for that, as it is outside Jupiter's worst radiation which would kill humans on Europa pretty quickly.

Callisto has plenty of ice, no planetary protection issues, and with less delta v to get to it from other parts of the solar system. There isn't any reason for humans on Europa except to study the life there. So, unlike the situation for Mars, it won't interfere with anyone else's aspirations for colonization, if we plan for indefinite future protection from forward contamination, if that is what the science demands. For more about the idea of humans on Callisto, see Sending humans to Callisto or Ganymede

Also as I'll look into later, it seems possible that with modern technology we can achieve 100% sterile landers if we work hard at it and develop the necessary technology. That wasn't possible in 1970s before the development of high temperature electronics . Indeed now, if we can use RTGs for power, we have the possibility of constructing entire spacecraft systems capable of operating at high 100% sterilizing temperatures.

So, in short, I don't think we have to adopt a 1 in 10,000 figure. We can achieve certainty, or near certainty, in the near future by using geyser sampling. Longer term, with new research and changes of "best practices" for sterile rovers, we can achieve 100% sterile spaceships whenever it is necessary. That much has even been proposed for a mission to Europa, with Brian Wilcox's work on a 100% sterile Europa probe designed to melt its way into the Europa ocean, using plutonium 238 as a heat source (as for RTGs). To find out more see Can we achieve 100% sterile electronics for an Europa, Enceladus, Ceres, or Mars lander? (below) and High temperature sterilization.

That 1 in 10,000 may seem a very low probability, and you might wonder, with such a tiny chance, why would 100% guarantees be needed. However, this depends so much on how much you value whatever it is that you are risking at that 1 in 10,000 level.

Would you hand a valuable Ming vase, say, to an art dealer to inspect if they said

"You may have read the news stories about people who lost their precious vases because of my clumsiness. That's true. I handle about a 100 a day and about three times a year I drop the vase and it shatters - but there is only one chance in 10,000 that I'll drop it"

"So it is perfectly okay to give me your precious vase! "

Or would you look for an art dealer who isn't so clumsy?

This Ming vase was sold for over ten million dollars on May 30 2006, making it the most expensive vase at the time. Steve Wynn bought it and donated it to a museum. If you had this vase, would you hand it over to an art dealer who told you he or she had a track record of breaking one vase in every 10,000 they handle?

A discovery of, say, RNA world life on Europa might well be of great financial value, far more than a Ming vase, leading to discoveries that lead to whole new multi-billion dollar industries in the future. However whether it has great financial value or not, the scientific understanding that comes from the opportunity to study such a thing in our solar system might well also be priceless, literally.

It could be a discovery that you couldn't replace in any other way, at any price at all. Totally beyond price, of incalculable value for science and understanding of biology, medicine etc. Why do we have to take such a risk? Especially if we can sample the geysers and take no risk at all, to all practical purposes? I don't see how it is a justifiable risk. How can we take such a risk of losing something as precious as an ocean inhabited by RNA world life or something equally wonderful?

Also, we know so little about Europa. Their planetary protection assessment is based on the idea that we have to prevent a viable microbe entering the Europan ocean, and the aim is to sterilize the lander, not remove all organic matter from it. But what about genetic material? If Europan and Earth life are genetically related, from a shared ancestor billions of years ago, it could take up capabilities of our life from just some fragments of material such as Gene Transfer Agents (GTA's).

If Europan life is not related to us, that might even be worse in some ways. Perhaps just a bit of RNA, or an enzyme, or a piece of cell wall or other biological product may get incorporated into a lifeform there, and change how it functions in some way. If it is able to replicate its new modified biochemistry somehow, to pass on whatever new adaptation resulted to other lifeforms there, it could give other life in the Europan ocean "new ideas". This is a hard thing to evaluate as we have no examples of extraterrestrial biology to test it on. We just have ideas for how other biologies might perhaps work. Perhaps early life or even pre-biotic cells might be most at risk of "learning new ideas" from Earth microbes? How can we know for sure until we have examples of life like that to study?

So far we have no idea what is in the Europan ocean. We might get a better idea from geyser flyby missions. I think that until we know what is in it, have at least some first idea, that we should assume that whatever is there is very vulnerable to Earth life. It's the only responsible way to do it.

So now to particulars of how they plan to sterilize the Europa lander to achieve that less than 1 in 10,000 per mission risk of contaminating Europa. Their aim is first to do dry heat sterilization. They say "typically > 125 °C" so that's higher than for Viking (112°C for roughly 30 hours). They would do it in controlled humidity <25% and typically partial vacuum or dry nitrogen. The effectiveness would depend how long they did it for, which they say might be for hours to days.

In their plans, the spacecraft is placed inside a a biobarrier, and is heat sterilized as a unit. However, some of the materials would be irradiated instead of dry heat sterilized, especially batteries, and "aseptically assembled". Can this be done without recontaminating their heat sterilized spacecraft with Earth life? The sterilization is only as good as the precautions taken to prevent recontamination when the spacecraft is assembled.

Then - they say that hardware that's subject to 10 million rads (100,000 Grays) is considered to be sufficiently sterilized by the journey so it doesn't matter if it is recontaminated. Thermococcus gammatolerans, the most radioresistant organism known, is able to withstand 30 kGy of gamma radiation, and still reproduce. Is 100 kGy enough to be sure that there is nothing viable left? That's more than three times that limit, but is that enough? Are there any other uncharacterized microbes more resistant than the ones we know of? Also, if the life is killed, what about fragments of genes and GTA's?

As an extra precaution, at the end of the mission, the spacecraft itself on the surface of Europa may be heated with an incendiary design to further sterilize it. This of course will only work with a "nominal mission". If it crashes through thin ice into water, for instance, or something goes wrong during the landing and it buries itself deep in the ice, this is the worst case scenario, and how can incendiaries help then? There would be no end of mission burning in that case.

Perhaps the main issue is the "aseptic assembly" - if only they could use an RTG instead of batteries, then they wouldn't need to do that as RTGs (Radioisotope Thermal Generators) can withstand heating easily.

If they could power it with an RTG (I know they are in short supply now, but the mission delay may help there), could they sterilize their lander by heat during the mission out to Europa? That's the idea of Brian Wilcox's 100% sterile Europa probe, that it would be heated up to over 900 degrees Fahrenheit (500 degrees Celsius) during its entire cruise to Europa . That's what I'd call a well sterilized spacecraft for something as potentially vulnerable as Europa.

If they could do that or similar, and heat it so much during the flight out that even fragments of DNA and organics are not an issue either, then planetary protection becomes a non issue. Well, so long of course as precautions are taken to leave no possibility of the sky crane (which they plan to use) contaminating Europa either, or any mission hardware that hits Europa.

Where will it land? Crumbling and tumbling ice blocks - or smooth ice moving as slowly as continental drift?

There is another way that the mission could be fine as regards planetary protection. If the surface is smooth, if the landing all goes fine, if the sky crane and the lander itself all land on the surface exposed to the ionizing radiation of Jupiter, and if it lands on a surface that's stable over long timescales, thousands of years - then I see no problem :).

The more stable surface ice on Europa changes only slowly. It's only going to be subducted over very long timescales, on a surface slowly moving at a pace of centimeters per year, if similar to Earth subduction - for details, see this paper in Nature from 2014, "Evidence for subduction in the ice shell of Europa".

If everything is as stable as this, and all the spacecraft debris remains on the surface, then Jupiter's ionizing radiation will soon sterilize the lander thoroughly. Not only that, before it can get into the Europan subsurface ocean or any other subsurface liquid, all the organics of Earth life will be long broken up into gases like carbon dioxide, ammonia. methane, and water vapour. A future like that is not a planetary protection issue. For all practical purposes it would not be running a risk at all I think.

Artist's impression of the Europa lander after a "nominal" landing on Europa on a smooth surface. Planetary protection issues would be minimal if they achieve a landing like this, because the Jovian ionizing radiation would soon sterilize it.

However, we have to prepare for all possibilities, not just everything going right. A crash that buries it deep below the surface will be enough to protect it from the harsh Jovian ionizing radiation, and any viable life on the lander would be preserved.

• Suppose that we find that the entire surface of Europa is so rough that its landing ellipse is bound to contain at least some rough terrain? That seems rather likely on the information so far. In that case, it would surely still try to land - after all the expense of sending a lander there. But then it could land on a cliff or in a crevasse,
  
• What if the surface is soft or fragile, as is the case in many icy locations on Earth? It could sink deep into soft fluffy ice.
  
• Suppose the skycrane fails or for some other reason it hits the surface hard? It could end up meters below the surface? In the worst case, it hits thin ice, or an ice bridge, or soft material or falls into a crevasse, and ends up deep below the surface in a pit of water melted by the impact?
  
• What if it lands on thin ice just centimeters thick, over an unsuspected subsurface plume or geyser, or a layer of freezing brine?

If it crashes like this, they can't use the incendiary device for the end of mission sterilization. The spacecraft might well be broken into fragments too.

• Or what if the most interesting spot to land on Europa is rough and actively moving?

There may be a lot of pressure from scientists to land it in Thera Macula for instance. This area seems rather likely to have subsurface water, with evidence that ice is actually calving, with huge multiple kilometer sized blocks breaking off right now. It even has the possibility of liquid brine refreezing right at the surface. All this makes it an exciting place to explore, but also dangerous for a lander there.

Close up view of the chaos region in Thera Macula showing probable calving ice blocks and a ridge probably expanding as a result of refreezing of brine.

According to the authors of this 2011 paper, Blocks A, B and C are calving at pre-existing breaks (reactivated). The edge of NS seems to be ready to break off at the point marked with the red arrow.

The blue arrow indicates a ridge which may be swelling due to brine filling it and refreezing. This would be an especially exciting and interesting area to land on Europa.

However, the ground is very rough. One of the reasons they think it may cover an area of liquid water not far below the surface is because it is lower than the surrounding surface of Europa. Gradually the entire lake beneath it will freeze if this is right, lifting the entire area above the surface. Abstract of this 2011 paper here. There are many other chaos regions on Europa which are raised above the surface unlike Therea Macula. These may illustrate the later stage in the process when the subsurface lake is frozen through completely.

There are some doubts about whether it is active right now. If it was, the surface should be much warmer than it is. But that could be due to an insulating layer of some sort, just a few cms thick, amongst other possibilities.

This entire area would be exciting to study on the ground, and it is one possible place to search for geysers too. Perhaps it has as good a chance of life detection as anywhere on Europa. It also may have liquid water not far below the surface, and possibly even breaking through the surface right now in spots.

However, on the face of it, a lander here seems to run a high risk of eventually being crushed under an iceberg and pushed down into liquid water. Or perhaps it could fall into a crevasse, or crash through thin ice into the water right away in a high velocity failed landing. In any case, it would also be a challenging spot to land because of the rough terrain. It might be impossible to choose a landing ellipse with a decent probability of landing on a reasonably flat surface.

Then, if this is indeed a layer of ice over a plume of liquid water, similar to the Great Lakes in volume, slowly rising from the deep subsurface of Europa towards the surface, then the surface is likely to be unstable over long timescales as the water freezes and huge icebergs turn over. The closest to what is happening on Europa right now might be something like this, which entered the Guinness book of records as the largest glacier calving event ever filmed.

has

(click to watch on YouTube)

Video description: "On May 28, 2008, Adam LeWinter and Director Jeff Orlowski filmed a historic breakup at the Ilulissat Glacier in Western Greenland. The calving event lasted for 75 minutes and the glacier retreated a full mile across a calving face three miles wide. The height of the ice is about 3,000 feet, 300-400 feet above water and the rest below water. "

It's easy not to realize how dynamic the chaos regions of Europa might actually be, once we can look at it in detail. Even if it happens more slowly than this - it still means a surface probably riddled with crevasses, and caves, and thin ice and with large blocks kilometers in size that break off and slowly turn over, crumble, etc. Also, it could be a rapid process, to the extent that we might well see changes in the surface of Europa even in the few years since Galileo's close flybys of Europa from December 1997 through to 2001. Quoting from Britney Schmidt et al's original paper (which is behind a paywall):

"At Thera Macula, we are probably witnessing active chaos formation... Today, a melt lens of 20,000-60,000 cubic kilometers of liquid water probably lies below Thera Macula; this equates to at least the estimated combined volume of the Great Lakes....

"Although it's unclear how rapidly the break up of Thera Macula took place, such a volume would take around 100,000 to a million years to freeze. Surface modification should be extensive just after the collapse and persist as long as the lens is mostly liquid, such that Thera Macula may have noticeable changes between the Galileo encounter and the present day."

From that same paper, the center of Thera Macula is sunken below the surface by around 800 meters, and the highest region is raised by 800 meters.

We can get an idea of what may happen to Therea Macula eventually by looking at another region of Europa, Conamara Chaos, which may represent the final stage of this process. It is raised above the surrounding surface by 200 meters at the highest points, its lowest points are 100 meters below the surrounding surface, and on average it is 100 meters above the surface.

"Conamara Chaos appears to have completely disrupted the ice fringed by its boundary scarp, and to have thickened relative to the background terrain. Matrix 'domes' reach heights typically of 200 m. These domes can entrain or tilt some smaller blocks. Large blocks represent the lowest points within the region, with heights equivalent to or up to 100 m below the background elevation. On average the entire region is raised by ~100 m."

An analysis of the Conamara Chaos region in 1998 shows that most of the blocks moved one to five kilometers. Many also rotated (on average by 11%) and more than half of the pre-existing terrain was destroyed during its formation, turned over or crumbled to pieces. There must have been many huge Manhatten sized areas that were totally disintegrated during the formation of Conamara Chaos.

Conamara chaos before (left) and after the ice movement. Figures 2 and 3 of this paper combined.

The original positions of the blocks were reconstructed using the features that traverse them. The white areas are the "matrix" of low lying areas, tilted blocks and peaks that seem to have been newly formed. So probably the entire white area represents areas of the original terrain that were destroyed during the process of formation of this terrain.

What happens if the lander ends up on a giant block of ice, perhaps as large as Manhatten or larger, which slowly calves or crumbles or flips over after the mission? Or what if it crashes through thin ice into liquid water, or breaks through an ice bridge or falls into a deep crevasse on landing? Should we not wait until we know more about conditions there, and design a lander capable of landing on the most interesting places on Europa?

As Catherine Walker, a planetary scientist at JPL says interviewed by Nature:

"Arriving there could be like trying to touch down on a glacier in Greenland spawning ice chunks. I don’t know if you really want to land on that”

If we must land on a potentially calving or fragile block of ice - should we not design our lander so that it really is 100% sterile? And if we avoid landing in such precarious places on Europa - is a lander a wise choice of mission at this point? Would it not be restricted to exploring some of the less biologically interesting areas of the Europan surface?

It might well be that, unless our rover is 100% sterilized, we have to avoid landing on the most interesting spots on Europa such as these. This need not even delay the pace of science at all. If we aren't ready for a lander on the most interesting spots on Europa, or don't know enough to select a landing spot, we do have an alternative, the geyser sampling, which is excellent science value. We can do that right away instead of a lander.

However, if it comes to it, it's worth bearing in mind that we do have situations on Earth where we just stop and don't do anything until we find a way to do it that's acceptable. Even if a landing on one of the most interesting spots on Europa, say Thera Macula, turns out to be the only way to find out about some aspects of the Europa surface. Even if it turned out to be the only way to check something, some interesting finding from orbit. If we can't do it yet within an acceptable level of risk to fulfill planetary protection requirements, then we need to focus our attention on doing things we can do already. Then we can search for a way we can do the lander in the future.

That's the situation for lake Vostok right now. Scientists would dearly love to send a sub to explore the subglacial lake and look for hydrothermal vents and for what may well be unique creatures, even multicellular life, living in those unusual highly oxygenated conditions and complete darkness. But they can't do it yet, not without risking contaminating the lake with surface life, confusing the science and possibly greatly reducing the science interest of lake Vostok. So we await the technology to be able to do it. Sometimes as scientists we are faced with situations like that here on Earth. It might be that we encounter other situations like that as we explore Europa and other places in our solar system.

This is a situation that might well arise if we send a lander to Europa that is not 100% sterile. We might have to choose a spot on the surface that is stable, flat, and with nothing interesting happening right now, to avoid planetary protection issues if we land it somewhere more interesting. We need to find a spot where even if the lander impacts on the ice, it is likely to be hard and slowly moving, at centimeters per year, eventually to be subducted perhaps a million years into the future. Or, in the worst case, if it is everywhere covered in rough terrain and soft ice, with a good chance that after a hard landing, the lander ends up many meters below the surface, shielded from cosmic radiation - how could we justify landing there, except with a 100% sterile lander? If we take planetary protection seriously, then we have to be prepared sometimes to say something like:

"This mission is super interesting, but we just can't do it yet consistent with planetary protection - go back to the drawing board and try again"

Just as the Russians did with Lake Vostok. We have to do that, even if it means leaving a multi million dollar mission in orbit around Europa because there is nowhere to land it consistent with planetary protection. I don't know if that is a possible future, but if it is, this is what it means to be really serious about planetary protection.

Design change suggestions that could make a lander close to 100% sterile

Though the guideline is a 1 in 10,000 risk of contamination, I expect that the scientists would just make it as sterile as they possibly can within the budget and technology they have available. If you are heat sterilizing it anyway for instance - why not leave it in the chamber for weeks or longer? After all, how could you design it to an exactly a 1 in 10,000 probability anyway? But the method suggested there of heat sterilization followed by aseptic assembly of the batteries sterilized separately seems difficult to make 100% sterile because of the problem of recontamination during the "aseptic assembly". It also still has the problem that it only sterilizes and doesn't remove organics, leaving the possibility of transfer of gene material and other unusual organics which could have effects of significantly modifying a Europan lifeform or life precursor.

Could they make it 100% sterile or close to it by some practical change in the design? I have a few suggestions here that may help, similar to the idea of Brian Wilcox's 100% sterile Europa probe,. They require a little more expense, but that's surely worth it if you agree that what we have in the Europan ocean may potentially be of tremendous value for us and all future generations. Indeed, of incalculable value, irreplaceable.

RTGs instead of batteries

My first suggestion is to use Radioisotope Thermoelectric Generators (RTGs) instead of batteries. The advantage is that RTGs can be sterilized by heat sterilization. Batteries can too, but as they say in the report, this reduces the shelf life of the battery which is no use if the battery isn't going to be used until years after it is sterilized. That's why the batteries had to be sterilized separately and then assembled together with the rest of the spacecraft after sterilization. RTGs are not affected in any way by heat sterilization. Indeed the main power source is hot anyway.

If it has RTGs then the whole spacecraft can be assembled first, then heat sterilized as a unit, enclosed in a biobarrier. There are then no concerns about recontamination.

I should just briefly look at the safety of RTGs. Since they contain radioactive materials, they are designed with special attention to safety, in the event of a launch explosion or a crash back down to Earth later on. For instance, the RTG designed for Galileo, New Horizons, Cassini etc consist of many small pellets each individually encapsulated in a welded iridium alloy, which in turn is enclosed in graphite impact shells which in turn is enclosed in a web (see section 2.2. of this report). Even if it breaks apart during a launch accident, then the iridium alloy around the pellets protect anyone or any creature that might ingest the particles. Normally the capsule would be recovered after an accident. Even in a launch explosion, yes, these pellets would contaminate the area around the launch site, but they would be of minimal danger and the area could be decontaminated.

They also considered the possibility that the probe crashes at a late stage, long after the launch, during a later flyby of Earth, lands in a remote place and someone discovers it before the scientists, with no idea of what it is. In that situation, they are not endangered unless they break it open and smear its contents on their body - a rather unlikely scenario.

There were anti-nuclear protest in the lead up to the Cassini launch in 1997. This lead to NASA looking very closely at the precautions, which they found had minimal chance of any harmful effect. They concluded that the chance of any breach of the container was exceptionally low, and in the event of an accident, not more than one extra person would die due to cancer over a 50 year period than if there was no accident. They also worked out that the chance of an accident during the Earth flyby was less than one in a million.

So, if carefully designed and assessed, RTGs are pretty safe. The main downside here is that RTGs are currently in very short supply.

It may seem a waste to use RTGs for a short lived lander, when batteries would suffice. But if they are the only way to achieve a 100% sterile lander, it's surely worth it. It has benefits too as it would also mean that it could continue to survive on the surface and do occasional science there even when the orbiter is long dead, so long as there are other flyby missions. The lander can be protected more thoroughly from ionizing radiation and also landed in a spot that has reduced levels of it so could last much longer than a dedicated Europa orbiter communications satellite.

We have a shortage of radioactive materials for RTGs. The US use Plutonium 238 (a short lived alpha particle emitting isotope, of no use for nuclear bombs, very different from the Plutonium 239). They stopped production in 1988, and since then have bought it from Russia, but Russia has also stopped production. US resumed small scale production in 2013 and aim to produce an average of 0.3 to 0.4 kilograms, increasing to 1.5 kilograms a year. Meanwhile the US has only 35 kilograms, half of which is not suitable for use in RTGs.

ESA propose to use Americium 241 since it doesn't have the necessary neptunium-237 feedstock to make Plutonium 238. The UK has approximately 2-3 tons of Americium 241 within its civilian plutonium stockpile of 120 tons (slide 9 here) - which is a waste product (as a decay product of Plutonium 241), though it is also used in smoke detectors. This 2016 paper describes a plan for a facility that can produce 25 - 50 grams of Americium 241 per day. or 9 - 18 kilograms a year. So potentially in the near future Americium RTGs may be readily available.

The articles on this topic can be confusing to read because they use three distinct concepts to rate the RTGs

• Thermal power - example, for Curiosity's RTG it's 417 watts per kilogram of the isotope
• Specific power - the amount of electricity generated - example Curiosity produces 23 watts per kilogram of the isotope
• The power density for the RTG as a whole including its shielding, structure, power generation - for Curiosity it's about 2.44 watts per kg

Also it's not always clear if the figures for the thermal or specific power are measured relative to the mass of the isotope itself or its oxide. This can make quite a difference, as the RTGs are fueled with the oxides rather than the isotope itself. Either Plutonium 238 oxide or Americium oxide. Plutonium 238 oxide is 71% Pu238 by weight. Then there is the question of whether the isotope is isotopically pure. For instance Plutonium 238 is typically 83.5% pure.

Comparison of the two approaches (using figures from this article)

• Plutonium-238, has a half-life of 87.7 years and heat output 560 W/kg, but it produces less than that because typically it's only 83.5% pure (see section 2.2 of this paper) for the first RTGs though it has varied for later generators. As an example, Curiosity's RTG has a heat output of around 417 watts per kilogram (74.46% of the heat output of pure Pu-238).
• Americium-241 has a half-life of 432.6 years and a heat output of 115 W/kg and is isotopically pure.

So Plutonium 238 needs only about a quarter of the mass of Americium 241 for the same thermal power output. But that's just the thermal power output per mass of fuel.

What really matters is the electrical power. Also the isotope is no use without the electricity generating equipment. So, what matters in practice is the power output per mass of the entire RTG. Curiosity uses a Multi-Mission RTG (MMRTG) which has 4.8 kg of Pu238 dioxide. It is designed to supply 110 watts of power at the start of the mission from two kilowatts of thermal power, or about 23 watts per kilogram of fuel. However when you take account of the entire RTG including the shielding and power generation equipment, that takes the mass up to 45 kg, making it about 2.44 watts per kg.

This shows Curiosity's RTG, the cylindrical object with the heat radiator fins (inside the red square), source of all its electrical power (self portrait). It uses Plutonium 238 which is a radioactive element which produces alpha particles, of no use for nuclear fusion, but great as a source of heat. It's so radioactive that it keeps itself hot.

ESA are aiming to develop RTGs with power output from 10 to 50 watts, with a power output of between 1 and 2 watts per kilogram. That makes power output per kilogram slightly less than for Plutonium 238 but not hugely so (1-2 watts instead of 2.44 watts). On the plus side, the americium-241 RTGs would have a much longer life, of centuries rather than decades. So you'd no longer have this issue of spacecraft running out of power as their RTG's age, not on the scale of decades, only on timescales of centuries.

In summary we have a shortage of RTGs right now but the situation seems likely to change in the near future. Any delays in the Europa lander may help here. If we need to use RTGs on a Europa lander to achieve 100% sterilization, this might mean postponing the lander until the US has increased its supply of Plutonium 238 and Europe has got its Americium 241 RTGs ready for use. But I'll suggest that it's better to do flyby missions anyway first.

If a short delay like this is acceptable, of a few years, until we build up the supply of RTGs, then the RTGs have major advantages for planetary protection. For more on the potential for RTGs see Would we use solar power or radioisotopes for missions to Saturn's moon Enceladus?

Other radioisotopes - what about Curium 242

There are several other alternatives we could use for radioisotopes, see the conclusion of this undergraduate thesis. One of those caught my eye as of special interest for Europa and Enceladus, if practical difficulties can be overcome. That's Curium-242 (different from the Curium 244 used for X-ray spectrometry on some spacecraft such as the Philae lander).

So, this is another of my speculative sections. Curium-242 has a half life of only 162.9 days, which might not seem that promising at first. However it's great for melting the ice. Pure Curium-242 would produce 119 kW of heat power per kilogram of the isotope at the start of the mission. The interesting thing about this is that it decays to Plutonium 238.

So, though it starts off with an output nearly 200 times that of Plutonium 238, and continues to produce a huge amount of thermal power for the first few years, in the long term it converts itself into a Plutonium 238 reactor and so continues to produce reasonable amounts of power for up to decades later.

If we could make an RTG based on Curium 242, and if we can get it to Europa within a couple of years of extracting and purifying the Curium 242, it would still have heat power output of more than 5 kW per kg when it arrives there. That is an order of magnitude more than for Plutonium 238. Perhaps this could be useful for ice melting? It's not enough to get into the hundreds of kilowatts for fast melting into the Europan ice, unless we can get it there much faster, or send large masses of Curium to Europa. But it's enough for a melt rate of months per kilometer for a small 20 cm diameter probe (of any length). Here I'm talking about melting the ice directly using the thermal heat from the RTG. See Ice moles or submarines for exploring Enceladus or Europa (above)

Curium 242 is an abundant byproduct of spent fuel but is soon gone after it's removed from the reactor because of is short half life. However it is also produced as a decay product of Americium 242, which has a much longer half life - and so can persist in spent fuel for hundreds of years. Details here.. In the future with faster methods of propulsion, if we can get it to Europa within days or weeks, Curium-242 would be able to produce vast amounts of heat for ice melting.

The main issue here is that we don't currently have any program to generate large amounts of Curium 242, and I don't know how practical or easy it would be. It's just one of several isotopes explored in a preliminary way in an undergraduate thesis. But I thought it was worth drawing attention to it.

Post launch heat sterilization

Another idea is to do an extra sterilization stage after the launch. This avoids any issues with recontamination. Assuming we use RTGs instead of batteries, then we can heat sterilize the whole thing, without risk of damage, after launch. In this case, the lander is already designed for heat sterilization as a unit, and what's more, it has a heat source on board, in the form of the RTG itself.

The idea here is that vacuum is a great insulator and the problem for most spacecraft is to reject heat, as much as to keep the spacecraft warm. Also if it uses solar panels similar to those for the Europa Clipper Mission for the orbiter and the mission itself before lander separation, then it has an excess of power at the start of the mission. It has 48 kW of power available to it on leaving Earth. That's a lot of power which you could use to power electric heaters. If it has RTGs, those can be used as a power source too, since they aren't needed until the landing, or indeed they can be used for direct heat also.

So why not have the lander section thermally isolated from the main spacecraft, and keep it heated to the highest temperature it can stand long term during the voyage out to Europa? We could just keep it at a sterilizing level of heat for months on end on its way to Europa.

This has another advantage also. We don't need to enclose it in a biobarrier before launch. We could use clean room assembly, and swabs etc, as for Viking to reduce the organic contamination to a minimum, and then the three years of continuous high temperature "in flight" sterilization does the rest. This should please the engineers, as they get nervous if the equipment is enclosed in barriers that need to be removed after the spacecraft is launched. What if they get stuck, and can't be removed? No problem, because now we have no barriers to deal with.

If the whole ship is sterilized then it might not make so much difference. The way they handle it post sterilization is to enclose it in its payload fairing in the clean room, and maintain a positive pressure of nitrogen throughout. The inside surfaces of the shroud have to comply with planetary protection standards. That continues until the launch, when the nitrogen source is disconnected, but then as the spacecraft rises through the atmosphere, it continues to have a positive internal pressure, which prevents it from being recontaminated with external microbes.

However, if only part of the spacecraft is sterilized, for instance because it's a rover that's going to depart from a non sterilized orbiter, then that idea of using the payload fairing to keep it sterile might not work. So I think the idea of post launch heat sterilization might be appealing to the engineers.

Deep cleaning with supercritical CO₂and CO₂ snow

Dry heat sterilization (actually with a small amount of humidity despite the name) is the most established method for planetary protection, used since Viking. The other methods that are approved by NASA and ESA are gamma radiation, and low pressure hydrogen peroxide. There are many other ideas under investigation. For a review in 2017, see Brushing Your Spacecraft’s Teeth: A Review of Biological Reduction Processes for Planetary Protection Missions, which covers both the methods already approved for planetary protection, and others that are under investigation.

Most of these techniques leave dead microbe bodies behind. That's normally seen as more of a problem for life detection instruments than for forward contamination. But it might be necessary to deal even with GTAs that can transfer capabilities in the forward direction, and also, would organics from Earth life give the extraterrestrial life "new ideas". Ideally we'd like to sterilize the spacecraft of all organics.

That makes the carbon dioxide snow rather attractive as one of the few techniques that removes everything, if it is used to sterilize a part that is already reasonably sterile. This is a new technique for spacecraft sterilization currently being evaluated by ESA. See Deep cleaning with carbon dioxide. and Science Daily article about it. JPL are also exploring it. The main difficulty is scaling it up to sterilize a complete spacecraft, also, although it is great at removing micron scale contamination, it's not so good at dealing with complete microbes.

Perhaps it might be ideal if you want to completely sterilize a small nanosat, or micro rover. So far, neither NASA nor ESA have approved it for planetary protection.

The great advantage of this is that it sterilizes at low temperatures, has no adverse effect on electronics, and also removes the organics completely if the robot starts off reasonably clean. It can also penetrate into tiny cracks and holes. It also has the additional advantage that it removes impurities that could interfere with the electronics. It seems to be a "win win win" scenario.

If this method can be made 100% effective, you not only get no life on the spacecraft, but no DNA fragments or GTAs, or indeed, anything organic at all.

There are two ways to do it. One is to use supercritical liquid carbon dioxide, and enclose the instrument in a pressure vessel. This is great for small and delicate parts. The other way is to generate tiny particles of carbon dioxide snow which impact on the surfaces. See also wikipedia article on carbon dioxide cleaning.

With the supercritical liquid method, the liquid carbon dioxide penetrates into tiny holes, dissolves the organics, and then, as it escapes as snow, it takes the organics with it, and changes back to gas, so leaving the spacecraft completely dry with no residue.

The pressurized supercritical method is:

• Inject CO₂ in a supercritical state. It behaves much like a liquid at 74 atmospheres upwards and 31 C.
• In this state, it dissolves organics readily
• It then forms snow which captures the organics and gets blown / sucked away.
• The snow evaporates rapidly into the gas state, leaving no residue
• Can be mixed with Hydrogen peroxide and other chemicals to increase effectiveness.
• Can be used even with sensitive electronics.
• It has been used to clean USB drives in testing and they functioned afterwards.
• Surface is left with no trace of organics, not even dead organisms. Major plus!

The impacting snow method is what ESA are investigating and use already in an operating spacecraft clean room. It relies on tiny explosions of carbon dioxide snow to clean the spacecraft.

They mix the liquid CO₂ in advance with clean dry air, where it forms tiny snowflakes. So they then hit the spacecraft with minute snowflakes which penetrate into every nook and cranny. The spacecraft surfaces are relatively hot for the snowflakes. So when they hit those "hot" surfaces, they suddenly expand 800 fold, in mini explosions, taking the organics and other matter away with them. Instead of injecting with supercritical liquid which forms snowflakes later, they hit the surface with CO₂ that has already formed into snowflakes, soon after it left the nozzle. They plan to use this method to help with sterilizing ExoMars due for launch in 2018. See this press release from August 2015.

"The method originates from the USA, and is used to remove paint from aircraft fuselage. A powerful jet of frozen carbon dioxide (CO₂) crystals, about the size of a rice kernel, blasts the paint right off the metal. The researchers made this crude instrument substantially more refined. Instead of CO₂ pellets, they use carbon dioxide snow to work on each individual component – from the highly sophisticated aluminum workbench to the ring washers. Here’s the rub: the beam that the jet emits is additionally accelerated with a blast of CDA (clean dry air) that encases it. This is how it penetrates into every nook and cranny, removing even the minuscule pollutant. As soon as the tiny snowflakes hit the relatively hot surface, they become gaseous, causing their volume to explosively expand 800-fold. The detonation pressure completely sweeps away every single bit of dust, even fingerprints which the cold gas had just turned brittle. “This approach involves a dry process that does not warp surfaces. When cleaning, these can be gently treated with CO₂. That makes it unnecessary to apply heat or chemicals,” Gommel says when explaining the advantages of this method. "

More about planetary protection for ExoMars here.

This shows scientists using a similar technique to clean all the organics from the surface of a test mirror:

Scientists using CO₂ snow cleaning to clean a test mirror - if the James Webb mirror gets contaminated by organics they can clean it in this way before the launch. The advantage of this approach is that it doesn't just sterilize the equipment. It also removes all trace of organics.

A similar method can be used to clean electronics and spacecraft and remove all organics from them.

We could use this, first on Earth before the final heat sterilization stage. For that matter, we could use it afterwards as well if we can figure out a way to do it without recontamination. Or we could to it "in flight" too - we could also take a container of CO₂ and use it for a final CO₂ snow sterilization stage for the lander during the mission to Europa. This is rather like the idea of using the carbon dioxide in the Mars atmosphere for an extra CO₂ snow sterilization stage after landing there.

Of course this adds somewhat to the complexity of the mission. But if you value the Europa ocean highly, then it's worth it, to make sure we have no practical chance of making life there extinct.

For more ideas see the sections below: Can we achieve 100% sterile electronics for an Europa, Enceladus, Ceres, or Mars lander?

However, whether we can make the lander 100% sterile and free of GTAs and other possibly problematical organics at this stage or not, another question is - is this the best thing to do at this stage anyway? And what are the best instruments to send to Europa to search for life?

Would a Europa lander be likely to spot life?

This section is a critique of the Europa lander report, which gives the astrobiological motivation for the lander, and the instruments they plan to include on it. See also this paper: Science Potential from a Europa Lander.

First, is a lander a good idea anyway at this point? In the section of the report where they discuss the motivation for the mission, they refer back to the 2011 NRC Decadal survey "Vision & Voyages for Planetary Science in the decade 2013 - 2022" that the study. They say that it: "explicitly mentions that a lander would enable scientific opportunities simply not possible from a flyby or orbital mission"

Now that is true. The 2011 decadal survey does explicitly mention the value of a lander. However,, it also qualifies this as a "far future" mission to do "ultimately" after first studying Europa with orbital and flyby missions and after flying through any Enceladus-style plumes if present. Those qualifications have got lost at some point along the way.

Here are the details, indented so you can skip it easily - or - if you are interested, take a closer look and see what you think:

In the Relevance to NASA Goals section they quote from the decadal report as:

“A key future investigation of the possibility of life on the outer planet satellites is to analyze organics from the interior of Europa. Such analysis requires […] a lander ….” and “...a lander will probably be required to fully characterize organics on the surface of Europa.”

It seems a strong endorsement of a lander. But let's look at it a bit closer.

The first part of their quote comes from page 240 of the report:

"A key future investigation of the possibility of life on the outer planet satellites is to analyze organics from the interior of Europa. Such analysis requires either a lander in the far term or the discovery of active Enceladus-style venting, which would allow analysis from orbit with a mission started in the next decade." 
(quoted passage emphasized)

And yes, it does mention the possibility of a lander, but the words they left out I think are actually rather significant".

"either a lander in the far term or the discovery of active Enceladus-style venting, which would allow analysis from orbit with a mission started in the next decade"

(the words left out of their quote in bold)

So, it's either a lander, or geyser flythroughs, and the decadal report strongly favoured geyser flythroughs over a lander as the mission to do first. The lander is described as "far term" and the geyser flythrough as something to do in "the next decade.

The second half of this quote is from page 239

"Observations of the surface of Europa should include the capability to determine the presence of organics, for instance by reflectance spectroscopy or low-altitude mass spectroscopy of possible out-gassing and sputter products. Observations should also provide correlation of any surface organics with surface features related to the ocean and provide site selection for a future landed mission. Ultimately, however, a lander will probably be required to fully characterize organics on the surface of Europa. "

(quoted passage emphasized).

Incidentally the sputtering which would help them analyse the surface chemistry from orbit using low altitude mass spectrometry is explained earlier in the report as "(i.e., ejection of particles from the surface by plasma bombardment)"

The key missing words there are "Ultimately, however" which again suggests a far future lander.

Before a lander, they envision studying the surface from orbit first to select a landing site, including studying any out gassing and sputter products (results of ionizing radiation hitting the surface). As it flies through the stutter products, and analyses the products in situ with its on board mass spectrometer, they can get a pretty good idea of the surface chemistry, and even work backwards to get a rough idea of where on the surface those sputter products came from.

I think the qualifications "far future" and then "Ultimately, however" for the lander, contrasted with "in the next decade" for a geyser flythrough, makes it pretty clear that the authors of the decadal survey thought of a lander as a late stage mission once we understand the surface of Europa well from orbital and flyby missions. Surely what they have in mind is not just a case of a few orbits to pick out an intriguing site to land? Rather, it's a case of careful study from orbit followed by a lander designed as a follow up mission to those orbital observations. If we are lucky we may find out a fair bit about it from orbit first, from remote study, and also from in situ mass spectrometry of the sputter products, which will let it "taste" the surface from a distance.

They saw a geyser flythrough mission is as especially useful if it has Enceladus-type venting. Well, we now have reasonably good evidence that it probably does have them. So it would seem that a geyser flythrough is the most logical next mission at this stage, rather than a lander, at least, going by that report. It may turn up many puzzles that will require a lander to sort them out, but we need to know what those puzzles are first before we send the lander.

Incidentally for Enceladus, which we know a lot more about than Europa because of the Cassini flyby missions, with its confirmed geyser plumes, which Cassini has already flown through and sampled many times, the 2011 decadal survey says:

"The committee commissioned a broad study of possible mission architectures including flybys, simple and flagship-class orbiters, landers, and plume sample return missions, and concluded that a simple orbiter would provide compelling science"

So even in that case they weren't thinking of a lander quite yet.

Earlier orbiter and geyser flythrough missions may turn up puzzles that influence the design of the lander. There may be surface features and phenomena that we don't yet know about which could influence the design of the lander. For instance does it land on a hard or soft surface? Is it going to be covered in spikes and crevasses? Is there a danger of landing on thin ice? Do we have to allow for the possibility that it gets upturned and deposited into a subsurface lake in the near future? Is it important for it to be mobile? Does it need to drill, and if so, how deep? Does it need to be able to navigate rough terrain after landing, and if so, how rough? And of course most importantly, how much of a planetary protection risk is there? Also the study from earlier missions may turn up puzzles to investigate that could guide the choice of instruments to send on the lander, when we send one, as we surely will eventually.

That's what happens when you have politics directing the goals for a science mission. With such a remit, there is no way they could recommend a geyser flythrough. It was "above their pay grade" to do that. Nor could they do a comparison study. The only thing they could do is to set out the advantages of a lander, as they do, in their quotes from earlier reports they highlight passages that cover benefits of a landed mission..That was a natural response, as a way to fulfill their remit. It's hard to see what else they could do, whatever their personal views might be about geyser flythroughs or landers.

However if you read the sources themselves, they rather clearly favour orbital study and geyser sampling over a lander, at this point in time. The lander is indeed seen as a useful mission, so what they said was correct, but it is seen as likely to be far more useful to send it once we understand the Europan surface through orbital missions, and so, they thought of it as a "far future" mission.

So in short, I found their motivation section unconvincing. They made as good a case for a lander as one could in the circumstances, but it wasn't persuasive if you look at it in the context of earlier work on the topic that they quoted.

Their proposed lander has a lifetime of only three weeks before its batteries run out, it can't move, and it can only drill centimeters. It would certainly be a useful a technology demo, and test the possibility of landing on Europa. If it crashes, then it will tell us something about risks involved in landing on Europa.

It could do good science too, for sure. If nothing else, it can do a nice study of the surface organic tholins on Europa - we have never had any mission to look at tholins close up before, although they are common in the outer solar system. For more on this see: Mixtures with other organics from comets, meteorites, and produced locally on Europa - and degraded organics - it could do a nice study of tholins but could it find life? (below)

However, the surface is not too likely to have abundant, or easy to spot life as we will see, and any organics would be severely damaged by the extremely intense ionizing radiation. It would be a first step, an early pioneer in the search for astrobiology on Europa. If the aim is to search for life, I think we need to have an idea of the larger picture here.

Icy world oceans exploration sequence - orbiter - geyser flythrough - lander - ice mole - submarine

Trying to study the oceans of Europa with a surface lander is a bit like trying to study Lake Vostok, four kilometers below the surface of Antarctica by studying the surface ice above it. If we are unlucky and it is like lake Vostok, then you'd learn nothing about life in its waters from a surface mission to study the ice on top. The subsurface ocean for Europa is much deeper below the surface than lake Vostok, and on the basis of what we know so far, even with the Hubble plumes, it is still entirely possible that similarly, many or all of the surface features on Europa including the brown lines just can't tell us anything about the biology of its subsurface ocean. We just know too little so far to evaluate their astrobiological potential or otherwise.

If we are lucky and there is some communication with the surface through geysers or in other ways, we might find outflows or geysers. Or there might be materials that come from the subsurface ocean, via the slow process of drift like continental drift, or perhaps there is life in melt plumes close to the surface and that gets to the surface and we can study that. If that's the case then the lander might have a chance, if it lands in the right spot. It could study the geysers too if it lands close enough to one to detect the icy dust that falls onto the ice from the plume as it erupts. With only three weeks on the surface, it would be very lucky to spot a geyser actually erupting, but there might be enough material on the surface for it to have a chance to detect traces of life in the plume.

In the case of Lake Vostok there are no geysers or outflows to the surface, but the Russians did trigger an artificial "mini geyser" artificially when they bored through into the lake, momentarily causing a surge of material a short way back up the drill hole from the lake which they could then analyse. It then froze through and got resealed from below. So the parallels are rather close.

One issue with sending a lander is - what if it produces a null result, as it most likely would? Wouldn't that possibly be as discouraging for Astrobiological searches on Europa as the Viking null result was for Mars?

I think, especially after the lesson of Viking, we must be careful not to treat a null result as meaning much at all for the lander. It just means no life was detected in the small region of the Europa surface that it landed on in the time period of three weeks with the instruments they had on board. It might be that life is easily detectable in other ways, maybe even where they landed, or if they could drill more than a few centimeters, or elsewhere on the surface, or in the geysers. After Viking, there were no follow up astrobiological experiments, and there haven't been any to this day. Could the same happen for Europa after a Europa lander? We are talking about politics here, not just science. It's driven by political decisions by people who think of the lander as our way to find out if there is life in the Europan ocean - so if it draws a blank - might they just close the purse strings for astrobiological searches on Europa? Especially as it is being presented as the way forward for astrobiology on Europa?.

Eventually what the scientists would love to do is to send a submarine down there. It might be the only way we can find out anything about the ocean, and a melt probe and submarine is certainly needed for a detailed understanding. So far we haven't been able to do this for lake Vostok, so it might be a while before we do it with Europa, though as we develop technology to do this for Europa the same technology may also let us explore lake Vostok. It's a major challenge, but perhaps not totally out of our reach, and many people are working on the ideas and the technology to do it. Before then, we can do ice moles, to melt or drill many meters below the surface. See the section on Ice moles or submarines for exploring Enceladus or Europa (below) .

So, there are many ways we could explore Europa:

1. Orbiters and multiple flyby missions (right now)
2. Preliminary geyser sampling (if it produces geysers) - Cassini has already made a start on that with Enceladus. The Europa Europa Clipper mission will do an even better job than Cassini but it is not yet focused on life detection. This stage could include artificially created geysers through impacts with "dumb penetrators".
3. Low relative velocity high particle mass geyser sampling for life detection - including missions that fly close to the surface to pick up particles several microns in size, and that use low relative speeds, as low as 100 meters per second, to pick up intact viable cells if there are any in the geyser. We would then use miniaturized low mass biology instruments to test to see if there is life or its remains in the particles
4. Lander - static
5. Lander - roving
6. Ice moles to drill well below the surface and perhaps into the vents of geysers and into subsurface liquid if available near the surface.
7. Submarines to explore the subsurface oceans with communication with the surface.

The main problem with searching for life on the surface of Europa is that if there is any life there at all, it is likely to be mixed up with organics from comets and asteroid impacts. It's also likely to be highly degraded by the ionizing radiation from Jupiter which can both break up organics, and also recombine them into complex molecules known as "tholins". The surface ice is also probably oxygen rich with perchlorates and a reactive chemistry. So a lot of the issues I will raise with the lander are to do with two problems

1. Would there be evidence of life on the surface at all?
2. If there is - would it be so much mixed up with non life organics or so damaged that it's next to impossible to tell it apart?

The methods they propose and the instruments they plan to send would work best if we can find undegraded and easily separated biosignatures. These could come either from the subsurface ocean, or from habitats closer to the surface. They have to get to the surface quickly or they will get degraded and transformed by the ionizing radiation.

The search for life and biosignatures on Europa has many of the same issues as for Mars, but for different reasons. Instead of a search for macro fossils, there's a search for macroscopic biofilms and multicellular life. Instead of a search for biosignatures of past life that has to be buried in a more habitable past Mars rapidly, and then exposed rapidly billions of years later to avoid the degradation from cosmic radiation, we have a search for biosignatures of life from the more recent past which has to get to the surface through an ice sheet many kilometers thick and be exposed rapidly to avoid degradation from the much more intense ionizing radiation from Jupiter. In both cases, then there's a possibility of finding trace signatures of life on the surface, but if so it may be localized and hard to spot and also in both cases meteorite and comet organics confuse the picture so we are looking not just for organics, but for the signature of life amongst what could be an overwhelming signal from abiotic organics on the surface. In both cases also, the surface organics are gong to be highly degraded and transformed by ionizing radiation and reactive surface chemistry.

Sampling a geyser may bypass many of these issues by sampling water directly from the subsurface. But what about a lander? Let's look at some of their detailed recommendations and potential issues with them.

Many of the ideas they present in the report would be useful for other ways to detect life including geyser flythrough missions, and later on, ice moles and submarines.

In the next few sections I look at some of the ways they suggest to search for life on the Europan surface. Some of their ideas are really interesting, especially the ones involving microscopy. I also look at their decision to require that all astrobiological instruments should also be able to provide results of interest for geochemistry. Then I go into the issues of whether life is likely to be present at all on the surface of Europa, whether it might be highly degraded, whether it could be too small to see, and what if it is mixed up with other types of organics?

The next few sections are a little more technical than the rest of this book, though not that much more so. If that's not your thing, just skip the more techy parts. Or if you want to skip this next section altogether, you can jump ahead to How easy is it for Europan life to reach the surface?

Many of the ideas they present in the report would be useful for other ways to detect life including geyser flythrough missions, and later on, ice moles and submarines.

Using microscopes to search for Europan life

• Microscopic search for cells with limit of 0.2 microns

I found this one of the most interesting sections of the Europa lander report. I'm sure most with an interest in the robotic search for life have had the thought - what if there is life there perhaps even easy to recognize as such with a microscope, and they can't see it because there is no high resolution microscope in the payload? Well, perhaps we get our first high resolution microscopes in space, hurray!

It is especially innovative, because, we have never successfully used high resolution optical microscopes in robotic missions to destinations outside Earth. Beagle 2 lander and the Rosetta Philae mission both had microscopes on board which were never operated. Apart from those two missions, the best microscopes so far have had no more resolution than a hand lens. That is apart from the atomic force microscope, but that's not an optical microscope. There was one of those on the Phoenix lander (which it used to image dust particles in 3D) and another on Rosetta.

The main issue with this approach is that we don't know how small Europan life might be. Could we see it even with our highest resolution an optical microscopes? The limit is 0.2 microns for diffraction limited optical microscopes, and by the results from the 1999 Limitations of Size workshop, yes that should be enough to spot even the smallest lifeforms.

However, that's based on Earth type biochemistry, and on living cells. As we saw in How small can a living cell be? there are several ways that relics of Earth life or life based on different principles could be smaller than this:

• Cells that shrink after death
• Fragments of larger organisms
• Pathogens or symbionts which depend on a host - e.g. viruses
• Live in consortia of smaller cells unable to survive independently on their own. They don't go into detail but I think they mean, like a biofilm, or even a primitive multicellular lifeform - but made up of tiny nanobes only a few tens of nanometers across, which provide biochemical products the other nanobes need, with none of them able to live independently for any length of time except in dormant states.
• Based on biological systems different from the ones we understand (such as the RNA World cells)

The last is especially relevant to Europa. For instance, an early life type cell, it can't have all the complexity of modern life as it's impossible for that to arise in one go. We could have a whole ocean there of RNA world type life, no protein, DNA or ribosomes, which lets the cells be far smaller. They calculated that the smallest RNA world type cells could be around 50 nm in diameter. See How small can a living cell be? (above) where I discuss this result in detail.

Now, scanning electron microscopes (SEM) and atomic force microscopes could spot smaller cells of a few tens of nanometers in diameter. However, the downside is that they may well turn up lots of ambiguous things that look like cells, but nobody knows if they are or not. We've seen that already with the ambiguity of the "nanobe structures" in ALH84001 and the many nanobe sized structures that resemble life in the searches for a shadow biosphere on Earth. That's especially so, if it is a mix of life based organics with non life. It could have a few genuine cells mixed up with many nanobe through to micron sized "cell like" structures that are not life.

So we have to approach all this with the caveat that it can only work if the cells are reasonably intact and also large enough to be seen in an optical microscope, about 200 nm in diameter or larger. If we do find structures this large, however, then they look in some detail into ways to distinguish living cells, dead cells and non life structures.

• Atomic force microscopy to measure the forces between molecules and properties of cell walls such as elasticity, hardness etc
   
• Fluorescent microscopy. Especially deep UV at wavelengths less than 250 nm can make some of the amino acids fluoresce. These are the aromatic amino acids (i.e. the ones that incorporate a ring of six carbons). Similarly chlorophyll fluoresces as well as other biological organics. We could also use fluorescent dyes that bond to specific macromolecules such as lipids, proteins and nucleic acids - though that depends on life on Europa having similarity to Earth life. See also Using an optical microscope to watch microbes swimming and Using an optical microscope to watch fluorescing microbes below
   
• Raman microspectroscopy coupled with microscopic images, both focused to the same depth in the field of view. This lets you do spectroscopy of individual cells (or cell like structures)

Raman microspectroscopy - the spectra on the left correspond to the in focus particles in the microscopic images on the right, letting the experimenter distinguish the spectrum of a protein rich particle, a lipid rich particle and a living cell. This could help work out what it is that the spacecraft sees in its microscopes. Figure 4.1.7 on page 4-35 of the report.

For more about ways to search for life with microscopes see the section Optical microscopy (below) which covers these instruments and techniques and some others in more detail.

As well as looking for biosignatures of life directly, they could also look for biominerals like magnetite, iron sulfides, silicon dioxide (used for diatom cells) and carbonate filaments and structures (see for instance, example of calcite evidence of early life).

Even with all that, this approach could still end up with ambiguous results where nobody is able to assess the astrobiological significance of whatever they find, especially if the signals are not very strong or the life is present but degraded and mixed with many non life particles. The astrobiological papers I've read e.g. by Charles Cockell and others in section 3.2. of this paper stress the importance of a wide range of different methods that complement each other because of this possibility for individual methods to be ambiguous unless you are very lucky in what you find. I think if we are serious about searching for life, we need to throw our entire tool box at the problem rather than bias it towards some favoured way of searching for life. See also the discussion of the Tissint meteorite again.

So what other methods should we use? The report has a very strong emphasis on biochemical approaches which they say must also give useful results for geochemistry. Many of the in situ instruments that astrobiologists have proposed and designed and even sometimes had selected for flight are of this nature. But not all.

What about instruments designed solely to search for life, which are of no interest or almost no interest for geochemistry? Should we have such an exclusive emphasis on dual purpose biochemistry / geochemistry?

Should we prioritize dual purpose geochemical / biochemical instruments over dedicated biology instruments?

I think the conclusions in this part of the report especially are rather controversial. There are many astrobiologists who have designed instruments to fly in space for in situ life detection that would be of little value for geochemistry. For instance, they may search directly for metabolism rather than biochemistry.

So why this difference of approaches, and why isn't everyone convinced by the arguments they present there? Well the report uses the Viking experiments as a reason for including only biochemical life detection experiments on the lander: Quoting from the report:

• "Were it not for the results from the gas chromatograph mass spectrometers (GC-MS) on the landers, the results from the biology payload may have been ambiguous. However, the GC-MS measurement of an upper limit of organics present at no more than parts-per-billion in the soil made the putative metabolic chemistry nearly impossible to interpret as the result of carbon-based life. Significantly, a biochemical definition – i.e., one in which detecting the building blocks for life are a critical cross-check for potential biosignatures – outweighed a metabolic definition "
  (emphasis mine)

However, whether you agree with that depends on how you understand the Viking experiments and indeed on whether you agree that they disproved the presence of life in the samples.

It is easy to look at the failures of the metabolic labeled release experiment and praise the gas chromatography / mass spectrometer experiment because it gave the "right answer". But that's judging them with hindsight, and also, based on an assumption that the gas chromatography / mass spectrometer experiment was right.

But we don't know for sure, even today, if it really did give the "right answer"s we saw in Rhythms from Martian sands - what if Viking detected life already in 1976?.

And even if it was right, it might have been right for the wrong reasons. What about the failures of the biochemical experiments on Viking to cope with the unexpected conditions on Mars?

First, the Viking biochemical instruments weren't very sensitive. They could have missed as many as several million cells per gram of bacteria - and would have no chance of finding life at the 100 cells per cc they later discuss in the report as the level of sensitivity needed for Europa (based on 120 cells per cc for subglacial accretion ice, see page 3-8), so they are a poor example to use. Also, the two Viking organic detection experiments actually detected chlorohydrocarbons which were dismissed as contaminants at the time, but might possibly be the result of reactions of perchlorates with the organics if the labeled release results did find life.

So the Viking biochemical experiment was unable to rule out life in the sample on two counts, because it wasn't sensitive enough, and because it too was confused by the unusual surface biochemistry, which may have turned organics into chlorohydrocarbons.

You can equally well deduce from the Viking experience that biochemical tests for life are prone to confusing results.At the time, I think it was natural to look at the labeled release experiments as the flawed ones, after the surprise of the harsh conditions on Mars, and also influenced by James Lovelock's arguments about an atmosphere in chemical equilibrium. But the truth is that both were flawed. That is not a criticism of any of the experiments - they were well designed for searching for life on Mars according to the ideas of their time. They just weren't designed for the harsh conditions as they found there and the totally unexpected perchlorates based chemistry of the Mars surface.

Also the report doesn't mention that just as there are more modern versions of the Viking biochemical experiments that addressed the issues and make them far more sensitive - there's an upgrade of the labeled release too, the chiral labeled release, which can provide unambiguous results even in challenging conditions like the Mars surface. If you mix the surface material with warm water, and it releases labeled carbon dioxide or methane when fed one chirality of an amino acid and not the opposite - what could it be except life?

If a chiral labeled release test like this were to turn up a positive result, it would be such a strong positive indication of life that it would outweigh a null biochemical result. Whatever was going on, it would have to be life or something else of a complexity approaching life even if it's biochemistry was so different from Earth life as to be hard to impossible to detect by other methods. That's one great advantage of metabolic experiments - they assume nothing about the life's biochemistry.

The other great advantage of the chiral labeled release is that it works for cells in semi-dormancy too. They just need to metabolize, eat some of the chiral amino acid, and you'd detect it. Even if it is slowly metabolizing, hard to "wake up" properly and not going to reproduce for months or years, or even if you can't find the right conditions to cultivate it, still you'd detect it, so long as it can metabolize at all.

Also, a major advantage for a situation like the Europan surface, it can detect a few cells of life even when mixed in with abiotic organics or dead life. Even if the abiotic chemistry has carbon isotope ratios resembling life, or a chiral signature resembling life, it won't give a false positive. It can't give a false positive, no matter how unusual the abiotic chemistry, which is a flaw of most biochemical ways of searching for life. It could miss some forms of life, but in the chiral update of this experiment, it's hard for it to produce a false positive.

There is a possibility of a false negative with the chiral labeled release however. If the chirality doesn't matter, it could mean that this is a form of life which is chirality indifferent (some ideas for early life or pre-biotic almost life especially would ignore chirality, see Joyce's ribozyme (below) and the ideas for "autopoetic" cells). If there is no response, it could mean that the experiment killed the life (maybe it can't stand the warm water conditions for instance). So you couldn't use it by itself to rule out life in the sample, but it's a great experiment to find forms of life that other experiments might miss.

All this depends on whether there is a chance of a viable spore where they land. If there is,, then the various metabolic experiments are worth considering.

Of course if you are pretty sure that any life there must be dead, for instance if you are far from any geyser and there is good evidence that the surface has been undisturbed for long enough to sterilize any life in the top few centimeters - then they can't do anything. But there are other astrobiological instruments that work just fine for dead and degraded life too, for instance astrobionibbler, DNA sequencers (if there is a chance it is related to Earth life), or SOLID3. These instruments are particularly designed to detect organics such as amino acids, and won't be of much use for geochemistry in general, sometimes of no use (a DNA sequencer is of no use for geochemistry). They are not as wide ranging in their capabilities as all purpose mass spectrometers, and so couldn't be included on the Europa lander according to their criteria that they have to be able to do geochemistry. But if there is biochemistry there, they are superbly sensitive. If it is an astrobiological mission, why not send those too? For more about these instruments, see In situ instrument capabilities (below),

Requirement that life detection experiments should also provide valuable non biological information

Also they lay down as one of their guiding principles:

• "Life-detection experiments should provide valuable information regardless of the biology results"

That does of course make sense if you think the chance of life is very low. Perhaps that is the case for a lander, in which case we can think of it as a geochemical experiment with a remote chance of finding life, and in that case, with the science payload limited, then we would choose geochemical experiments with a decent chance of also providing some preliminary information on the remote chance there is life there and it is reasonably easy to spot.

That could be worth doing but if so, I think it's not really a mission with the search for life as its priority. It's more like the Mars habitability assessment spacecraft. Perhaps all this reflects a pessimism about its chances of finding life?

But if you think there is a decent chance of life there, this principle seems to ignore the value of a null result. For instance if the labeled release comes up with a blank, this shows that there are no viable metabolizing cells capable of metabolizing the organics presented as food and producing carbon containing gases. To take another example, if there is a possibility of biology and the other experiments show that what you have is life, then it might be highly significant to show that it can't be sequenced and so is not DNA based - this would prove that it is different from Earth life, either a separate origin or it split off before the evolution of DNA based life. There is a remote chance of shared origin between Europan and Earth life, either from life that predated our solar system or from panspermia in the early solar system. So a null result from a DNA sequencer could be of great significance for Europan biochemistry.

It's obviously important to have some, or indeed, mostly experiments that provide valuable information regardless of biology results especially in a preliminary mission. However I'd venture to suggest that to require that all experiments have to be of this nature seems to be going a bit too far. It would be a case of balancing the mass / power requirements and the benefits of the results the experiments can provide. That's especially so, since many of the in situ biological instruments are low mass and have low power requirements as "labs on a chip". Why not let the astrobiologist pitch their instruments too, and see if there is a possibility of adding some of them to the payload?

So, yes, if you think the chance of finding life is very remote, it might make sense not to send pure biology experiments quite yet. But as soon as there is a decent chance of finding life, surely we do need to send biology experiments to look for it?

Also, another important point, those experiments shouldn't be judged a failure if they turn up a null result, or even ambiguous results. If despite all our care the results are ambiguous, as they may well be if we encounter some very unusual astrobiology, prebiotic chemistry, or abiotic geochemistry - then that's a basis for improving our life detection instruments in the future. The biochemistry instruments may very well turn up ambiguous results too. Many of the results of the Mars rovers are at least somewhat ambiguous in detail.

We've been sending mass spectrometers to space for decades. But we've only sent one design of dedicated life detection experiment to space, ever. The astrobiologists need to have an opportunity to design new instruments, send them into space, and learn from the experiences. Hopefully this will not be the last biological mission to Europa, and what we learn from it can feed into future instruments to send to Europa, Enceladus, Mars and throughout the solar system.

We have to start our astrobiological exploration of our solar system somewhere. And if serious about searching for life, surely we need to send some dedicated life detection experiments, selected on their merits for the search for life, whether or not they are of non biological interest. Well that's what I think anyway. What do you think?

Strengths and weaknesses of the "lego principle" approach

The report puts a lot of emphasis on James Lovelock and Chris McKay's "lego principle" for searching for life. This is the principle that life tends to favour a few types of molecule of rather particular atomic mass (e.g. only even numbers of carbon atoms, or only multiples of three), while abiotic processes have examples of just about all possible atomic masses. Also abiotic processes tend to favour low mass organics which are somewhat easier to form through random processes.

The idea goes back to James Lovelock, his principle A2 in this paper and is also favoured by Chris McKay, because of it's ability to spot life biochemistry even if it is based on very different principles from Earth life. This is one of the examples from the report to illustrate how you could recognize life through the "lego" principle:

These show the results of a gas chromatography (a process often used in space missions) to analyse abiotic and biotic organics.

The graph on the left shows the products of an abiotic process, the Fischer Tropsch process - which reacts hydrogen with carbon monoxide at moderately high temperatures to produce synthetic fuel and lubricants. The graph on the right shows the fatty acid fraction from a bacterial extract. The numbers show the number of carbon atoms in the molecule associated with the peak. If we spotted something like the graph on the right, it would be a potential biosignature indicating that what we have may be life.

Their examples show that this form of analysis could be informative, if you compare a sample consisting entirely of life based organics with one consisting entirely of abiotically produced organics. In ideal situations like this, it could suggest life even if it is very different from Earth life and also degraded too, with no viable life left.

This is especially so for some kinds of organics. For instance the most common fatty acids in their right hand graph are polymers of acetate, C₂H₃O₂ and so form with even numbers of carbons.

Fatty acids are used to make cell walls, are normally the result of polymerization of acetate which is why they have even numbers of carbon atoms.

Ball and stick model of the acetate ion. Carbon in black, hydrogen in white, oxygen ions in red.

It has two carbon atoms and when it is joined to other copies of itself to make fatty acids (polymerized) then the resulting molecules have an even number of carbon atoms.

Fatty acids can react to form hydrocarbons with an odd number of carbons.

Other organics used in life are made as a result of polymerization of isoprene which has five carbon atoms, so tend to come in multiples of five carbon atoms.

Isoprene. Present in many plants. It's polymers have multiples of five atoms and are common in nature, for instance, making up the main component of natural rubber. Combines to make polyisoprenoids

This lego principle is quite a robust signature as you continue to get some traces of it even in highly processed organics such as petroleum.

"In a typical sample of fossil lipids, for example, one would find a predominance of even-carbon numbered fatty acids, odd carbon numbered hydrocarbons, C15, C20 and C25 acyclic isoprenoids, C20 and C30 cyclic terpenoids including steroids, and C40 carotenoids (Eigenbrode 2007). Subsets of these traits are even identifiable in highly derived products such as petroleum where n-alkanes may exhibit weak odd over even or even over odd carbon number preferences."

So, it seems rather robust. It should work well even with rather degraded organics such as we might expect on the Europan surface.

However, we can only evaluate this approach properly if we look at its weak points as well as its strong points. One of the problems is, how do you use this principle to detect life in a mixture of both types of organics? This is a likely situation on the surface of Europa, in conditions rather hostile to life and very favourable to formation of organics through abiotic processes (as we'll see in the section Mixtures with other organics from comets, meteorites, and produced locally on Europa - and degraded organics (below)).

Their two examples of how the principle works are:

• Carboxyl acids, which are built up in chains of 2, or in the case of archaea, 5 carbon units at a time. We could spot this in regularities of the numbers of carbon atoms in the building blocks if life on Europa is based on a similar approach.
• Amino acids, life uses a small selection of 22 amino acids out of 70 detected in meteorites, with a difference in abundancies between biotic and abiotic use, for instance that high molecular weight amino acids are over represented in Earth life based organics and the 100% chirality of Earth life based amino acids.

Also, can it produce false positives? A "lego principle" type spectra looks like the spectrum of something produced by life - but the big question here is whether there are any abiotic distributions that can also produce it? This requirement is similar to the Knoll criterion for fossils which we came across in How would we recognize fossils on Mars? . It's not enough that it would be produced by life. Can we also say that it can't be produced by non life?

Well, there are other biosignatures that would be especially hard to mimic without life.

Cholesterol - an example of a molecule with a unique stereoisomer signature, one out of 256 possibilities

• Many molecules have multiple chiral centers that can be "flipped" independently, and again life processes tends to produce a 100% of only one of all those variations.

They link to this paper which goes into it a lot more, along with those issues as well: Molecular biosignatures. They mention there (section 2.2) the diastereoisomers - versions of a molecule that are related as in a mirror but only part of the molecule is flipped rather than the whole thing. One striking example on Earth is cholesterol which occurs in all animal cells as an essential element of animal cell membranes.

Theoretically it could exist in as many as 256 different stereoisomers, but life produces only one of them.

 

This shows cholesterol as it occurs in living cells above, nat-cholesterol. The lower image shows ent-cholesterol which is flipped in a mirror with every bond that was out of the paper in the top image shown into the paper in the bottom one and vice versa.

A bold line shows a bond that rises out of the page (sometimes shown as a wedge) and a dashed line shows one that goes into the page (the wedged / dashed notation).

The four carbons marked in red in the top images are stereocenters - the bonds there can be flipped independently of the others changing the structure of the molecule. For instance, a bond that is shown going into the page could be flipped so that it comes out of the page or vice versa. (If you need help identifying these sites, see this video). The result is that you could have 256 different forms of cholesterol, of which only one occurs in nature.

Techy aside: If you haven't done organic chemistry for some time you may need a reminder about how these diagrams work. Each carbon atom has four bonds - however by convention, to simplify the diagram, it omits hydrogen atoms that are attached to carbon. That hugely simplifies the diagrams, because they are so numerous in organic chemistry and it is easy to deduce them from the rest of the diagram.

If a vertex has less than four lines from it, the remaining bonds are all attached to hydrogen atoms. So for instance an end point with nothing shown attached to it ends in a carbon atom and the three remaining carbon bonds are attached to hydrogen atoms. A double line is a "double bond" where two carbon bonds are combined together.

Every vertex where two or more lines meet is a carbon atom. Every end point is also a carbon atom, unless it is labeled with a different molecule (as in the case of the OH).

If we found something like cholesterol in extraterrestrial organics in only one of its many stereoisomers, then it would be a good biosignature, or indicate some very complex pre-biotic chemistry. However, we still have the problem of racemization - spontaneous transformation of an isomer into another isomer which happens slowly in the presence of heat. Also a complex molecule like this is likely to be degraded and broken up by the ionizing radiation and indeed the resulting fragments can be recombined with other organics too as a result. So though discovery of cholesterol in only one isomer would be a very striking discovery, unless there is a lot of viable life in the sample, we would be more likely to find fragments of cholesterol and various isomers of it. That would be less striking and require a fair bit of detective work to discover if it could have started off as 100% one isomer of cholesterol.

More generally you can also get "structural isomers". You may have to break one of the bonds and reconnect it to a different carbon but in the process you get many versions of the organic that are similar in other respects and only differ in the connectivity of their carbon bonds. For instance the paper lists nerol, geraniol, ipsenol and cineol as structural isomers used in nature with the formula C₁₀H₁₈O

These all have the formula the formula C₁₀H₁₈O and are "structural isomers" as you can get from one to the other by varying the connectivity of the carbon atoms. Life normally uses only a few of the structural isomers available, while inorganic processes can produce many more. From figure 4 of Molecular biosignatures by Gene D McDonald

Again life based organics tend to include only a few examples of the many possible structural isomers, and often life "chooses" the thermodynamically least favoured structure. See section 2.3 and figure 4 of Molecular biosignatures

You also get a similar situation with polymers joining together asymmetrical units such as the five carbon atom isoprene. A similar example is phytol, a component of chlorophyll. It has three stereocenters leading to 8 possible isomers already. Then it also is made up of four isoprene units linked together "head to tail" which could be linked in two ways each. So there are numerous structural isomers only one of which is used in biology. See figure 5 on page 140 of the paper.

Mixtures with other organics from comets, meteorites, and produced locally on Europa - it could do a nice study of tholins but could it find life?

Europa is likely to have organics from comets and meteorites. Indeed, many icy surfaces in our solar system including Triton, Pluto etc are covered in an organic deposit called "tholins" a kind of processed tarry substance. It's the result of irradiation of simpler organics and other readily available gases and ices such as methane, ethane, ammonia, carbon monoxide, hydrogen sulfide, formaldehyde, nitrogen, carbon dioxide and carbon monoxide, which are common in the outer solar system. Hit them with UV light, electrons, ions and so on, and a bit paradoxically perhaps, they create more complex organics.

What happens is that they break up these simpler molecules into fragments which then recombine to make more and more complex molecules. As Sarah Hörst puts it:

"So what do scientists mean whey they say tholin? In general, we mean an abiotic complex organic solid that formed by chemistry from energy input into simple, cosmically relevant gases or solids. Shorter still, “abiotic complex organic gunk” works for me. You can see, perhaps, why Sagan and Khare felt the need to make up a word to capture this idea."
What in the world(s) are tholins? (emphasis mine)

So tholins don't have a single formation mechanism and it doesn't refer to any particular type of organic molecule. Instead the term refers to any "abiotic complex organic gunk" formed in or on icy moons and comets etc.

In the case of Europa then there's another way to form it, through meteorite impacts. That's because its ice is under a lot of tidal and tectonic stresses and the impacts into its surface could release these stresses in the icy crust, leading to electrical discharges which would form complex organics during the impact itself. These impacts could also form subsurface lakes that persist for up to thousands of years beneath the impact crater, which gives yet another way for complex organics to form then be released to the surface later on. See A new energy source for organic synthesis in Europa's surface ice

The report does touch on this issue but it is not discussed in any depth. There is only one mention of the word "tholin" in the report, in this paragraph.

"Given that material collected from Europa could range from predominantly water ice, to salt dominated, to an as-yet-to-be-discovered surface composition (e.g., organic “tholins”), some degree of phase change of the collected sample may be necessary to achieve the requirements detailed above. Including a phase change from solid-to-liquid (through solubilizing salts and/or adding a combination of heat and pressure to change the phase from ice to liquid) is one way to potentially filter or concentrate small, potentially biogenic materials onto a target for microscopic observations."
(emphasis mine)

So it would seem that their proposed solution to this problem of tholins and meteorite originated organics, and other abiotic organics mixed up with life based organics is to dissolve solid deposits in water, or to use heat and pressure to convert ice to liquid.

We haven't yet been able to study a sample of tholins from icy moons and comets. However, it seems rather optimistic to suggest that by such simple processes, the organics from life would get separated out from the abiotic tholins. Those same procedures would also extract any soluble abiotic organics amongst the tholins.

What about the amino acid glycine, for instance (NH₂-CH₂-COOH). This is soluble in water, so would be extracted by their proposed procedure. It's commonly found in meteorites and also detected by the Philae lander on comet 67p. Similarly what about all the other small organics produced by abiotic processes? One of the signatures of abiotic processes is that they tend to create lots of small chain organics. I'd have thought those would dominate in any attempt to extract the organics that are the product of life from the mixture? Surely the tholins on Europa are likely to be mixed up with organics from meteorites which certainly include them. And surely they will have many other organics of all sizes produced inorganically too?

So this seems to me like quite a major issue. Unless the sample consists mainly of life based organics, how could the "lego principle" be used to distinguish these from the organics from life? What do you think?

Abiotic organics that mimic biosignatures

Another problem with this approach is that though suggestive of life, "lego principle" distributions are not unique to life. Abiotic processes can produce "lego principle" type mass distributions as well in special situations which may occur on Europa. The surface organics will also include materials from comet and meteorite impacts for instance.

Here is an organic mass spectrum of the Murchison meteorite. It was obtained using a laser mass spectrometry system that could be miniaturized to send to a comet or asteroid

Figure 2 of this paper, showing the mass spectra for positive and negative ions from an organic extract from the Murchison meteorite. If material like this was mixed with organics bearing life - could we use the lego principle to distinguish the life based organics from the meteorite (or perhaps comet) originated organics?

For another example, this shows the spectra of PAH's (Poly Aromatic Hydrocarbons) from a chondrule (one of the small inclusions that give chondrite meteorites their name) from the Allende meteorite.

Organics spectra from exterior (A) and interior (B) of a 2 mm diameter chondrule from the Allende meteorite.

This is a similar issue to the Knoll criterion which we covered in How would we recognize fossils on Mars?

"The Knoll criterion is that anything being put forward as a fossil must not only look like something that was once alive -- it must also not look like anything that can be made by non-biological means.”

Oliver Morton, author of Mapping Mars: Science, Imagination, and the Birth of a World

The authors of Molecular biosignatures list some distinctive characteristics of meteoritic organics

• Molecules with more carbon atoms are less common, and usually a linear decrease suggesting that amino acids are created by adding one carbon atom at a time
• Branched forms of molecules are more commonly found in meteorites than in life
• Small organic molecules with few carbon atoms tend to occur in all chemically possible forms. For instance 57 amino acids occur out of the 159 possible with up to seven carbon atoms

As they conclude in the section 3.3. " Organic Chemistry of Carbonaceous Meteorites" in that paper:

"Such pathways tend to discriminate against high molecular weight compounds and favour formation of all possible constitutional isomers at lower carbon numbers by more or less random synthesis. Qualitatively similar patterns occur in both carbonaceous meteorites and laboratory prebiotic syntheses (see earlier discussion). Most importantly, none of the three patterns is exhibited by the classes of compounds used in living systems. These considerations lend confidence to the proposal that such patterns can distinguish between biosynthetic and abiotic organic matter."

The problem is, that biosignatures like these also depend on being able to isolate the biotic from any abiotic organics. If there are organics from meteorites mixed in for instance, it will already have the complete variety of smaller molecules,and branched molecules etc. Also, it depends on whether this molecular fingerprint biosignature is preserved when organics are degraded.

Though this "lego principle" surely is a good tool to help with the search for life, it seems likely to need a lot of other corroborating evidence, especially in the Europan conditions.

The report does mention one other approach, which may seem almost foolproof if they are lucky enough to find such biosignatures

• Detection of complex structures that may be impossible to build up through normal abiotic processes.

Yes, life produces many complex biological pigments which would be hard to duplicate with abiotic processes. But then you have the problem that in the unusual chemical conditions of Europa then abiotic processes may produce unusual organics.

One striking example is the way that certain abiotic processes can produce nucleobases and even RNA, as Hauke Trinke found out. See Did life evolve in ice? The authors they cite for this section, Cronin and Walker, make a similar remark

"For example, pores in rocks may have influenced chemical selection, leading to increasingly lifelike chemistries over time."

If we do find unusual chemicals like RNA in the Europan ice, even long strands of RNA, it would be interesting, would be suggestive at least of unusual pre-biotic chemistry - but not yet fully diagnostic of life.

So, though interesting, even the most complex chemistry may still be the result of unusual abiotic processes, and it might be quite hard to establish for a certainty that some complex organic was not produced abiotically. They may have developed from scratch in the surface conditions in unusual chemistry, as the result of interactions of frozen ammonia mixed with cyanide, for instance, and may not be a sample from the Europan ocean at all or tell us anything about conditions below the surface, even if the surface materials also do contain materials from the subsurface oceans.

Also to confuse it further, we have the possibility of pre-biotic chemistry - complex chemistry that could amplify some organics over others, on the way towards evolving life, but without necessarily producing life quite yet.

Degraded organics from life

It may be hard to spot life, if there is any, viable or damaged, amongst the other organics on the surface. The surface deposits may be highly degraded. This is their reason for drilling 5 - 10 cms below the surface ice, but is that enough?

Well, if they are very lucky, the ice they drill into originated in the subsurface ocean, and then reached the surface with no long period of exposure to the ionizing radiation in the recent geological past. It might even be an ice berg that rolled over recently,exposing ice that was buried meters below the surface a few years earlier, or ice exposed by a meteorite impact recently, or materials that were deposited as a result of a geyser.

However they might as easily find they are drilling into ice that has remained close to the surface for millions of years, or newly formed ice, or ice that was recently buried, or the debris from comet or asteroid impacts on Europa, or brought there through micrometeorites. Then, if they find organics from the subsurface ocean, it could be that originally it was biotic, pre-biotic or abiotic, or a mix of all three, broken up into tiny pieces by the ionizing radiation from Jupiter, and then built up into tarry deposits again by the tholin producing processes.

You might wonder why they think that drilling 5 - 10 cm is far enough. After all the astrobiologists say that it's 1 - 2 meters minimum for Mars. Why is it only 5 - 10 cms for Europa? the reason is because they are concerned about a different type of radiation. On Mars the past life would be up to billions of years old and damaged by the very penetrating cosmic radiation. On Europa, it's at most tens of millions of years old, and the most serious effects are from the Jupiter's ionizing radiation, which is much more easily blocked than cosmic radiation.

s explained in "Hiding from Jupiter's radiation"(NASA astrobiology magazine), the ions from Jupiter's radiation belts can penetrate only a millimeter below the surface. Electrons penetrate a centimeter, and so even they are easily blocked. However these collisions also emit high energy photons that penetrate to a depth of about a meter and so most of the degradation at depth will be from those.

So, the ionizing radiation from Jupiter's radiation belts consists of many more particles than the cosmic radiation particles, which is why the radiation dose is so high, deadly for humans. Yet, they are far less energetic individually and can't penetrate very far.

The radiation belts orbit Jupiter faster than Europa does, so the highest levels of radiation are on the trailing hemisphere, meanwhile the micrometeorites that hit the leading hemisphere can build up a regolith some meters thick which may help provide some shielding. So some areas on the leading hemisphere may get less ionizing radiation, and also have a regolith from the micrometeorites to protect materials there. So some areas may be more shielded from radiation than others though anywhere on the surface of Europa is going to get a lot of radiation.

Any surface organics that have been there for millions of years will still be degraded by cosmic radiation just as for Mars, and if it has been exposed to the surface even briefly in the past it will be rapidly degraded by the ionizing radiation from the Jupiter system.

It's not just ionizing radiation that can degrade the organics. The surface also may have magnesium chlorides and perchlorates as for Mars, according to an observation from the VLT reported in May 2016. They can now study Europa from Earth with spatial resolution as good as tens of kilometers, with adaptive optics.

From the original paper, these give a better fit than the sulfates for the surface materials in the darker regions and also the chaos regions - except on the leading hemisphere where the "bulls eye" is still thought to be due to sulfates deposited from Io and forming sulfuric acid hydrates. The perchlorates reach 17% concentrations in places, with concentrations of around 15% at the apex of the trailing hemisphere, so that includes the Conamara chaos region. The chlorides reach even higher concentrations of 25%. It has chlorates as well but they are barely detectable. Distribution of all of these is highly variable on the local scale.

I wonder if, as for Mars, such high levels of perchlorates may complicate the searches for organics by interfering chemically and helping to remove biosignatures? Especially since they are so variable in distribution, what if the lander lands in a patch with high concentrations of perchlorates?

Then as well as that, the surface is also thought to be oxygen rich as a result of the ionizing radiation which splits the hydrogen from the water leaving oxygen behind. That's the same reason that many expect Europa's ocean to be rich in oxygen. If so, then as for Mars, I wonder if it might have hydrogen peroxide in the ice? Perhaps it might also have nitrogen oxides, singlet oxygen etc. Oxygen tends to break up longer chain organics, into shorter chains and gases, as happens with deposits of petroleum. See section 5.5 of Molecular Biosignatures.

So there are many ways that the surface conditions could make it hard to spot life there. This is something that we may learn more about as a result of future orbiter and flyby missions to Europa. We might be able to find spots on the surface that are optimal for finding relatively undamaged traces of the ocean chemistry and even life.

How diagnostic is the carbon 12 to carbon 13 ratio?

• They suggest using carbon 12 / 13 ratios to find life, but again barely mention the drawbacks of this potential biosignature.

I covered this in the discussion of the Tissint meteorite from Mars (above) - a meteorite from Mars with a low carbon 12 / 13 ratio typical of life. The Tissint meteorite would indeed count as proof of past life on Mars, if it weren't that you can create such ratios through abiotic methods. As we saw in that section, carbonaceous chondrites can have low carbon 13 ratios. Europa is often hit by comets and asteroids. We actually have evidence of clay like materials on the surface of Europa which may include organics from meteorites. Comets by contrast tend to have a huge excess of carbon 13 relative to Earth - So organics from comets might mask the signatures from life, and organics from at least some carbonaceous chondrite meteorites might mimic those signatures. So with a mix of comet, meteorite and life organics, it's likely to be hard to disentangle them.

Then there's another way to get these signatures. Abiotic methane produced from hydrothermal vents can have carbon 13 depleted to as low as -50% on Earth (see page 3 of this paper). That's well below the range of -10% or so down to -30% or less for life on Earth. Europa may well have hydrothermal vents in its ocean floor that produce methane that then gets its way into organics produced by abiotic processes. They would be organics from its ocean, but the low carbon 12 / 13 ratio would not be a proof that it was from life.

Other atoms that can be studied in this way include hydrogen / deuterium ratios and nitrogen 15 / 14 ratios.

The authors of Molecular biosignatures does suggest one way that we can distinguish biotic organics - they tend to have a much more stable level of these isotopes, hardly varying. By comparison, the materials from meteorites have highly variable ratios. This shows some example ratios in organics from the Murchison meteorite

As you see, the ratios, even for a single meteorite, range from -20 to +40. By comparison, life has carbon ratios from below -30% to a bit above -10% typically, with much of it in a narrower range than that. The reason for this is probably because organics from meteorites sample different source materials - with varied isotope ratios and also formed at a wide range of temperatures. All Earth life shares a single isotope reservoir and Earth's biochemistry operates in a narrow temperature range too, from around 0◦C to 120◦C

So, if we did find a depleted C13 / C12 ratio, then it would be interesting, but not at all conclusive as a biosignature. It could just be a signature of meteorite organics or abiotic processes in its hydrothermal vents. The distribution may well be highly variable, including also augmented C13 from comet impacts with excesses of 1000% or more,, with all of this mixed in with any biotic signature.

If we find a depleted ratio that also shows little by way of variation, then it might be an indication of life, especially if combined with other biosignatures. If we find widely varying ratios then it might well indicate that there isn't much by way of biotic organics present.

However none of this seems likely to be conclusive as a way of ruling out life in the sample altogether, or confirming it. Not without a lot of other evidence to back it up, at least.

Would we be able to detect life using sub-millimeter precision photography - or could it be confused with abiotic organics?

• Searching for strands of biofabrics - for instance biofilms, and long strands of Europan microscopic life. Also pigments in the ice from life.

This is another idea from the report. The basic idea of photographing the region around the lander to millimeter resolution to look for strands and other macro structures that may be created by life processes is great. But could you not get strands of material from the tholins if looked at close up?

Actually it turns out that yes you do get strands, in sublimation experiments. These experiments were done to simulate conditions on a comet, icy satellites of Jupiter, Triton or Pluto and Charon, all of which may well have tholins on the surface. So, they didn't specifically simulate the Europan conditions, but it seems likely to be relevant.

The experimenters took a mix of 0.1% tholins with water ice particles of various diameters - in the examples I've selected below, they were around 4.5 microns in diameter, and then they let the ice sublime in vacuum conditions for many hours.

This is what they saw at the 1 mm scale. The tholins are the dull brown "fluffy" structures:

With a closer look, they are like this:

So, if Europa does have tholins, it seems at least possible that they would form structures at many scales of magnification that strongly resemble life, just like these structures.

Also, we need to bear in mind that this might well be our first ever close up examination of tholins on an icy moon or comet. We wouldn't have any other standard to compare them to at this stage. We might spot structures that we later find are very common amongst tholins, but because this is our first ever look at them close up, we mistake them for structures formed by life.

Also, how likely is it that macro structures such as seaweeds, biofilms etc from deep below the surface end up on the surface of Europa intact? Or indeed, at all?

I'd have thought that any that get to the surface lifted kilometers in the geysers would be likely to be micron sized - decayed or broken up into fragments. We may get some that rise intact through the slow process of continental drift, or they may grow in the plumes of liquid water that slowly rise and break through the surface.

So it doesn't seem impossible. It could happen and it makes sense to make sure we can't miss such signs. But it doesn't seem likely that we find obvious traces of macroscopic life if we land on a random site on Europa. We'd have to be very lucky, or else, these traces have to be very abundant and easy to spot. Again this is rather similar to the situation for Mars. It seems as optimistic as searching for fossils on Mars.

So, yes it's possible that these traces are so abundant that we find them at an early stage, or that we are just extraordinarily lucky with our first mission. We can't rule this out, but perhaps it's not very likely.

Cell counts in the most inhospitable environments on Earth - and application to astrobiology

The Europa Lander report looked at analogues of the Europa ocean. Though I've put this in the Europa section of this book, salty brines on Mars are a bit like tiny oceans, so perhaps it can give a rough idea for Mars microbes too, and I've added some examples more relevant to Mars such as the McMurdo dry valleys and sand dunes.

This is from section 3 of the report. The numbers here all come from that report, except for some additional entries that I've added, which link to other sources. Some of them are done by direct cell count. Others use various ways to estimate the numbers of cells.

It's sorted with the most numerous first:

• Cold dry desert locations such as the Atacama dry valleys - typically a few million cells per cc
• Blood Falls outflow, Antarctica: 600,000 cells per cc.
• Sand microbiota in migrating dunes in Qatar, 530,000 cells per cc.
• Lake Vida ice, 444,000 cells per cc.
• Cold icy brines: 200,000 to 200 million cells per cc.
• 200,000 cells upper bound - site 45 for UREY testing (para 25 here). This is based on E. coli cells as the norm.
• Circumpolar deep water around Antarctica: 74,000 cells per cc
• Salty brines in the Mediterranean Red and Black Seas - 19,000 to 150,000 cells per cc
• Driest parts of the Atacama desert, between 3,100 and 511,000 cells per gram of soil - with total organic content less than 0.01% (see Table 2 of this paper)
• Driest most inhospitable part of University Valley in Antarctica - between 1,400 and 5,700 per gram of soil. Many of their samples had microbes in dormant or damaged states, needing liquid enrichment steps followed by three to five months of incubation at 5 °C before colonies appeared on the agar plates
• Subglacial accretion ice - 120 cells per cc average (from the deepest ice core samples above lake Vostok in Antarctica - one layer of 260 cells per cc above another layer close to the lake of 80 cells per cc). From their table 3.1, the accretion ice with the higher cell density of 260 had only 60 micromoles of carbon per liter. I make that 0.000078% carbon by weight if I got the calculation right.
• Estimate for the Europa ocean - between 0.1 and 100 cells per cc based on energy considerations. See page 3-10 of the report

Another thought here. These cell counts are for Earth life, which has a minimum size, of about 200 nm in diameter, which is actually reached for the ultramicrobacteria.

Astrobiological cells easily be smaller than Earth based cells, nanobe size, especially if it is some form of early life. To get an idea of how small they could be, we can go to the workshop held in 1999 on Size Limits of Very Small Microorganisms, in response to the discovery of what then were thought to be possible microbe fossils in ALH84001. The panel discussing cells based on alternative biochemistry came to the conclusion that an early form of life based on a single biopolymer, such as, perhaps, an RNA world cell, may be able to fit into a sphere of diameter 50 nm, or 0.05 microns.

So, if the cells were as small as only 50 nm across, that would make the volume of the cell around 0.0000005 cubic microns. By comparison, two strains of E. coli were measured with volumes of 0.6 and 0.7 cubic microns (see page 95 of this paper). You'd fit more than a million of those cells into the volume of a single E. coli cell. Even a cell count of a million RNA world cells per cc would have less by way of organics than a single E. coli cell per cc.

How easy is it for Europan life to reach the surface?

This is the big unknown. The authors of the Europa lander report base their 100 cells per gram count on samples of subglacial accretion ice on Earth with abnormally low levels of life in it. But for all we know, what on Earth is a low level may be a high level for the Europan surface ice. They also suggest this.

Their more optimistic estimate is based on life dispersed throughout the Europan ocean, making use of much of the available energy in the geochemistry of Europa. However, even if life is abundant in the Europan ocean, there are many obstacles in the way of subsurface life getting to the surface. Even the geysers may not sample life in the oceans.

The main problem here, which they touch on in the report, is that most life on Europa is likely to use geochemical sources of energy rather than sunlight. The oceans may be filled with life, in low concentrations of a few hundred to a few ten thousand cells per litre throughout the ocean, which then would increase the chance of sampling it at the surface. But it could also be concentrated around the hydrothermal vents.

If so, we would need a lucky chance of life from hydrothermal vents getting to the surface.That's possible.

For Enceladus especially, we have a rather good chance of striking it lucky and sampling life from the hydrothermal vents in the plumes, as there's evidence that water in the geysers of Enceladus may have had direct contact with hot rock only months before it reached the surface and escaped into space.

But, what if the situation on Europa is more like the vents on Earth? For details see this survey of the literature on Earth's hydrothermal vents from 2013.

Earth's entire ocean is thought to cycle through hydrothermal vents over a period of 240,000 years, so any sea water you encounter has probably been in a hydrothermal vent at some point in the last quarter of a million years. But that's only on very long timescales. The individual plumes are neutrally buoyant. They rise by a few hundred meters and then disperse horizontally for hundreds of kilometers. If the Europan hydrothermal vents are similar, and if life is concentrated around the vents, then we may get very little by way of still viable life outside of the vents themselves. Maybe only after major subsurface catastrophic events that send life all the way to the surface. Though that would also depend on how long the life remains viable and whether there are other processes that decay them. We can't really argue by analogy with Enceladus here as the situation is different. Amongst its many differences, Europa has around half the gravity of our Moon, so significantly more than tiny Enceladus with almost no gravity.

We do have another way to detect Europan life if it is localized to the vents. Just as for Earth, the entire Europan ocean would pass through the vents over geological timescales. It would happen more slowly than for Earth's oceans because the volume of water is much greater, and the timescale would depend on the number of vents, but it does seem plausible that we could also still observe the metabolic products of life in the ocean. The Europa lander report suggests this as a possibility. If the rate of recycling was the same as for Earth, then the entire Europa ocean would pass through the vents every half million years or so, assuming it has around twice the volume of water of our oceans as some estimates suggest. See also All the Water on Europa.

For this to work, this assumes that Europan life is reasonably productive. It could be if it makes use of oxygen and the oxygen created on the surface does get into the ocean.

It's a neat idea, and this idea of an oxygen rich Europan ocean has become pretty much accepted by many scientists, but it is not at all proven yet. It is still based on theory, and there is room for some skepticism about it. The oxygen could all be released in the surface conditions before it reaches the subsurface ocean, as suggested in this study from 2004.

A recent survey looks into this (preprint here). They looked at fluxes of hydrogen and of oxygen into the ocean - life then could exploit them as redox pairs. The hydrogen is no problem as Europa should have some hydrogen fluxes, at least a tenth of the Earth's even if there is no hydrothermal vent activity. That is plenty of hydrogen for chemosynthesis. But oxygen fluxes are much more variable depending on the assumptions you make. However the data we have so far does seem to suggest that it has substantial fluxes of oxygen into the ocean.

"Estimates of present-day oxygen flux to Europa’s ocean vary based on assumptions of how the surface ice overturns, with values ranging from 3 × 10⁸ to as high as 3 × 10¹¹ moles O₂ per year . The higher values stem from assumptions that ridge and chaos formation bring fresh ice to Europa’s surface, and melting within the ice aids in carrying materials to the ocean ...

Recent research supports the idea that Europa’s ice is currently cycling surface materials into the ice and ocean. Subsumption and mobile subsurface liquids ] are tentatively identified in the few well imaged regions of Europa that may represent its global geology. I If Europa’s active ice shell delivers oxygen to the ocean at rates close to the upper bound of 3 × 10¹¹ moles O₂ per year , this is within two orders of magnitude of Earth’s net photosynthetic oxygen flux of 2 × 10¹³ moles"

This suggests that the amount of oxygen reaching the deep ocean could be anywhere between a hundredth of the amount that reaches Earth's oceans to a hundred thousandth of the amount that reaches our oceans, depending on your assumptions about the geological processes operating inside the ice shell.

The higher estimates of the oxygen levels seem most likely at present, and if so, the ocean could be quite productive and easy to sample from the surface. However, if the lower estimates are correct, Europan life may not use oxygen for metabolism much if at all, and it would be harder to detect from the surface. So, we just can't say, though at present it does seem reasonably promising that the life is present in some quantities.

So if we are lucky, Europa has reasonably abundant life and it may even have vents that send life all the way to the surface from the deep subsurface hydrothermal vents like Enceladus.

Or it may have life in the subsurface lakes below Thera Macula if they exist, and if so, it has life very close to the surface.

But the situation may be more complex. It may have very localized life, e.g. no life except close to the hydrothermal vents, and these may reach the surface only rarely or not at all as viable life.

Another thought, my own idea just now, writing this. What if it tends to congregate around the interface between the water and the ice, like some forms of life that live on the underneath of ice sheets on Earth? If so, then that would make the chance of the life getting into a geyser rather high.

Anyway, with this background, what is the chance of finding this life on Europa, in the five drill samples the lander does, each 5-10 cm deep, in the spot around its landing site, somewhere on the vast surface of Europa?

It's for this reason also that I think that there is no substitute to starting with in situ searches for life in the geysers, as our best bet, if they exist. These can either find life, or prebiotic intriguing chemistry, or else, through a null result, show that it is very unlikely that we find life in the plumes. If we turn up a null result in the plumes, we need to search harder or elsewhere.

Why top down mission planning may not be the best way to organize a scientific search for life in the Europan ocean

In general, I think this whole prescriptive approach is the wrong way of setting about it actually. It's not so much what they include in the report, as what they leave out. All their suggestions are great, but this is all driven from the "top".

• First we have the politicians telling the mission planners that we must have a lander, so that they can't add in a geyser / lander comparison study.
• Then the mission scientists see it as their role to map out the priorities for instrument selection, for instance by forbidding from the outset any pure biology instruments.
• Finally, the astrobiologists get to contribute instrument suggestions, so long as they fit the priorities set by the politicians and the mission scientists.

The message is:

"We will send a lander to Europa and we will send instruments to search for C12/ C13 ratio, mass spectrometers based on the "lego principle" and high resolution microscopes and we will search for millimeter scale biofilms optically."

"If there is space we may add other instruments as well so long as they are of value for geochemistry and are not designed only to search for biology. "

If we are indeed doing at least partly an astrobiological mission rather than a purely geochemical mission, let's not specify in advance how to explore Europa and what our priorities are and what types of instruments to send. We need to cast our nets as wide as possible to have the best chance of identifying life unambiguously.

So, instead, why not make it more of a two way, back and forth process of dialog between the mission planners and the instrument designers? Let's say to the astrobiological community:

"What are your suggestions for astrobiological life detection instruments to send to Europa? And what do you think is the best way to explore it, what are the priorities? A lander, a rover, ice drilling, geyser fly through, sample return? "

I think that with this approach, a science led rather than a politics lead mission - that the indications are that probably most astrobiologists would recommend a geyser flythrough over a lander.

Whatever we do, whether we send a geyser flythrough, a lander, a rover of some sort, an ice mole or eventually a submarine, we should have a completely open request for instruments for our first astrobiological mission since Viking. Then the mission planners need to evaluate them on their own merits, and on whether they contribute something new to the mix of experiments to send to Europa. After all we have almost no prior experience to go on. That's very different from geochemical searches, where we have lots of prior missions to build on.

One astrobiological experiment may be able to detect something that the other experiments miss. Many are "labs on a chip" and wouldn't add much to the total mass, so unlike Viking, we could have many instruments in the payload, especially of the smaller ones. I think we should aim to have as diverse a selection of robust ways of looking for life as possible. For some idea of the diversity of what is possible, again, see In situ instrument capabilities (below) .

Also, if we consider viable life is possibility, life that can actively metabolize - then I think specific tests for viable life should be included as well. For instance, we should definitely consider chiral labeled release and microbial fuel cells along with everything else in the list of experiments. And Viking did not show that biochemical instruments are superior. Its biochemical results were just as ambiguous and confused and flawed as its metabolic results.

For instance if we had a mix of tholins with a few viable cells in it, then the only way to detect the cells might be to use a chiral labeled release - and what's more it might be just about the only way to unambiguously prove that there is life in the sample. If all the other experiments drew a blank but the labeled release demonstrates an uptake of C14, and only for amino acids of one chirality and not the other, we'd already know for sure that there was life there, in a way that wasn't possible with the original achiral Viking experiment.

Whatever we do, I think we shouldn't do anything to limit the vision and inventiveness of the astrobiologists at the instrument proposal stage. We need to bear in mind that we have no experience at all of non Earth based biochemistry.

So, however experienced the mission planners, there is a limit to what experience and received wisdom can do to guide us here. The more minds we get to look at the problem and the more diverse the ways of searching for it, surely, the better. The better the chance of an innovative idea, the initially daft seeming experiment that turns out to be the key to the puzzle.

We know how to search for Earth life, and have become very good at that, but a too "Earth-centric" approach may easily blind us to some forms of astrobiology that we might find on Europa. Whether designing the mission or selecting instruments, we need to consider as diverse a range of robust ways of looking for life as possible.

Sample returns from Europa

We've seen that, just as for Mars, though for different reasons, we may need to search in situ for some time before we find evidence of life in the Europan oceans or closer to its surface. For those reasons also, I think we should not plan for a sample return, at least not for astrobiological reasons, until we know a bit more. The situation is very similar to a Mars sample return, though for different reasons. As with Mars, we need to be able to intelligently select astrobiologically significant samples to return. So far we don't know how to select such a sample intelligently. We risk returning a sample that tells us nothing about whether there is life in its oceans.

There is one difference from Mars, as we have no samples from Europa at all. So a sample return could be motivated more easily just for its geochemical interest.

However it might be going to far to pin your hopes for advances in astrobiology on a sample return, as for Mars. On the basis of what we know at present, it would be hard to guarantee any astrobiological interest in a sample returned from Europa unless we can run tests that would be able to distinguish, e.g. the Tissint or Murchison meteorites, or tholins, or the results of abiotic synthesis in hydrothermal vents, from samples of biological interest.

Even with geysers, we couldn't use a sample return to decide the question of whether there is life in the geysers, unless we are lucky and the oceans are so rich in life that it makes its way in abundance into the geysers. We may need to select from particular geysers and at particular times and there may also be particular heights in the geysers that concentrate the material of most interest to us. I think we will probably only be able to decide all that by doing in situ searches first.

Once we find samples of astrobiological interest, I think the planetary protection issues for a sample return could be dealt with most easily by returning it to above GEO as I've suggested for Mars samples (in If likely to be of greater astrobiological interest - return samples to above GEO above) . However the downside is that a sample return could use up a large part of the mass allowance for science payload. I'm not sure that is worth it until we are sure we can select the sample intelligently, at least from an astrobiological point of view.

Could there be life which uses the surface ice of Europa or Enceladus itself as a habitat?

So far we've been assuming that the life is in the subsurface ocean or in lakes trapped beneath the ice. But there is one other possibility, and this idea actually favours a lander as the best way of finding the life. That's if there's life actually in the surface ice. Perhaps photosynthetic life.

Andrew Martin and Andrew McMinn, sea ice microbiologists at Tanzania university, think that there could be life in stress related fissures in the ice. That's because of the way they seem to actively exchange water with the exteriors of the icy moons.

If that is right, then the life would be there right at the base of the geyser, and in cracks in the chaos regions, perhaps using photosynthesis in brief periods when the sunlight penetrates into the cracks. This is how he puts it:

"Stress-related fissures in the ice shells of Europa and Enceladus are complex, and our understanding of their topography is based on theoretical modeling. But fissures appear to actively exchange liquid from the subsurface oceans to the ice exteriors."

"The physiological demands on any microbial organisms would be exceptional, but these features could harbour small-scale, biologically permissive domains. Even brief periods of photosynthesis might be possible."

It mightn't be the easiest find though.

The lander would have to land in areas of the surface with cracks in the ice, possibly with an unstable surface in the case of Europa. Also, it may need to drill to get into the cracks, and as planned, it has no way to travel over the surface, even to sample materials from a nearby crack a few meters away. See the section on Ice moles or submarines for exploring Enceladus or Europa (below)

Could a Europa lander help find geysers for a geyser sampling mission?

In the report, they say that

• "Furthermore, observations of plumes feed forward to future missions that could sample plume material directly (e.g., fly-through sample collection). "

But would a lander be a good way to search for new plumes / geysers?

Europa has a much closer horizon than Earth, similar to the Moon because it's quite small. At 3,100 km in diameter it is similar in diameter to our Moon (3,474 km). It's also very flat. So, let's work out how far away you could see.

Using the formula for the distance to the horizon and let's put in a height of 1 meter above the surface.

Then if the surface was perfectly smooth it could see for sqrt(3,1000,001) meters, or around 5.57 km.

Europa is very flat with local variations in topography of at most 250 meters (though larger areas may be elevated as much as a kilometer). So if you landed on a higher point in the terrain you could see for at most sqrt(250*3,1000,250) meters, or around 88 km (probably much less as that assumes you are surrounded by lower terrain out to that distance).

If it is raised as high as 1 km around the surrounding terrain, then you can see for sqrt(1000*31001000) or 176 km.

So, you'd be able to see less than six kilometers in any direction, unless you were on a local high point, when you'd see perhaps several tens of kilometers at best.

How far away it can see a plume depends on how high it is. We can use the same formula, suppose the geyser was, say, 200 km high.

The horizon then (as seen from the top of the geyser) is sqrt(200*(3100+200)) , or 812 km.

So, our lander could see a large geyser like that at a distance of about 812 km. That's out of the total circumference of Europa of 19,478 km. It could just see the very top of a 200 km high geyser at a distance of about 8.34% of its circumference in all directions.

The possible plumes are in the seven o'clock position, not far from the South pole - though the central image here has another possible plume that's close to the equator. They spotted the plumes in three out of ten observations by Hubble. These are composite images with a photograph of Europa superimposed on the Hubble data. The geysers seem to extend at least 200 km above the surface of Europa.

A lander on Enceladus can be guaranteed a great view of its geysers because they all erupt in a small patch around the south pole, and they erupt continuously, what's more. But we spotted them just fine from orbit, so we don't need a lander there either, not to discover new geysers. However, it would be useful to have a lander there to observe them visually as a way to monitor them continuously.

On Europa though, the evidence of geysers, what there is, suggests they are spread out over a large area. We do now know about a probable repeating plume, see Hubble's confirmation of a repeating plume (above) , so that seems a good bet, except that the two outbursts were two years apart, and our lander has only around three weeks to spot any activity.

This suggests, it doesn't have much chance of spotting the large plumes. But that's not its objective. The idea rather is to look for very small plumes, too small to be seen from orbit.

Does Europa have thousands of smaller geysers only ten kilometers high?

This depends on Europa having numerous smaller plumes, which they think is quite possible. They might be caused by tidal flexing, and if so, then in 20 days on the surface, the lander would see 5.65 tidal cycles. Once you start to think about it this way, the idea of using a lander to search for plumes becomes more promising.

This is what they say about the plumes:

"The plume searches conducted thus far suggest that Europa's plumes are not tidally modulated, however some fractures on Europa correlate with the patterns of tidal stress throughout Europa’s 85-hour orbit, and thus. smaller, time variable plumes erupting from these fractures could be controlled by the tidal cycle. Small-scale plumes have been considered as a source of dark deposits) observed along some of Europa’s lineaments, the margins of large chaos features, and surrounding small chaos and other topographic features. Numerical modeling of the eruption process predicts plume heights between 2.5 and 26 km. A plume that can erupt material out to 10 km laterally from the source vent would have a height of 5 km if dispersed in a wide plume, and 21 km high in a narrow plume.

The Europa Multiple Flyby Mission [Now called Europa Clipper] will be capable of searching for plumes with several different instruments. Transient plumes of order 10 km or smaller, however, may be difficult to identify via remote sensing. The Europa Lander offers a highly-complementary approach in which one specific region could be monitored for activity over several tidal cycles with surface observations"
(emphasis mine, cites removed for easy reading)

So, how could those small geysers form? They give two cites to follow up. a paper from 2003 by Sarah Fagents, and a paper from 2013 by Quick et al.

Let's start with the paper by Sarah Fagents.

There are several features on the surface that could perhaps be explained by flow of liquid water over it, so suggesting cryovolcanism. Of course, just as on Earth with our volcanoes the cryovolcanism doesn't have to be eruptive. Often lava just flows onto the surface. One of their cites is to this paper, which has a summary of the main ways liquid water could form on the surface in figure 11

#

Figure 11 from paper from paper from 2003 by Sarah Fagents

The first of these is most promising for the geysers. If there's a rising melt plume from below, it could have dissolved volatiles from the ocean itself. From section 3.1 of their paper, those could in turn come from the hydrothermal vents, with CO2, SO2 and CO as some of the possible dissolved volatiles. This would lead to geysers in a similar way to the way you can make a diet coke / Mentos fountain which went viral in 2005 with Steven Spangler Science's video.

(click to watch on YouTube)

This is caused by the sudden explosive release of the carbon dioxide dissolved in the diet coke. Something like this could cause small geysers on the surface of Europa.

Explosive eruptions like this could explain the dark bands that outline some of the features on Europa. Sarah Fagents et al found in an earlier paper in 2000 that they could model them with eruptions wi:h a velocity of 30 to 250 meters per second. You can read the abstract here (paper is behind a paywall). Those would create plumes between 1 and 25 km high. Sarah A. Fagents writing in 2003 puts it like this (section 3.1 of this paper)

" This explosive venting mechanism was proposed to explain an enigmatic elliptical feature identified in Voyager images as a potential cryoclastic deposit , but this feature has since been discounted as an artifact produced by vidicon distortion. Cryoclastic eruptions have also been proposed as one possibility for the origin of low-albedo, diffuse halos associated with some lenticulae and triple bands, with the low albedo resulting from impurities in the erupting mixture or annealing of ice grains. However, the vapor-rich nature of such eruptions, and the likelihood that small water droplets would rapidly freeze in Europa's frigid environment, imply that it is unlikely that such a mechanism would produce significant surface flows."

Since she is one of the authors of the paper that suggested the dark bands were due to explosive eruptions in 2000, she is showing skepticism here about the results of her own earlier paper.

Lynnae Quick et al's paper. is more optimistic about finding them, arguing that since there are many other features on Europa that look like the results of cryovolcanism, then surface venting wouldn't be much of a surprise.

"Ultimately, since observations at Europa′s surface have revealed features indicative of effusive cryovolcanism, cryomagmatic intrusions, and cryoclastic deposits, it is not unreasonable to assume that surface venting in the form of plumes may also occur as part of a continuum of volcanic and magmatic processes"

Here is a photograph from Sarah Fagin's paper, reproduced by Lynnae Quick et al, showing the dark smudges possibly caused by small geysers.

The white arrows in this figure point to ark smudges around the canal like feature of Rhadamanthys Linea. They extend from 2 to 6.8 km away from the feature, suggesting plumes of height 2.5 to 25 km. Figure 1 from Lynnae Quick et al

They come up with a similar range of heights, between 2.5 and 25 km. They also discuss ways of detecting them, and they think they could be detected from an orbiter at a height of 100 km above the surface, looking for back illuminated plumes, or using stereo imaging of the surface. It would need a reasonably high resolution camera, but not impossibly so, it would be sufficient if it was able to take photos with a similar pixels per meter resolution to the one that was sent on the Galileo spacecraft, or slightly better. Galileo did search for plumes and would have been able to spot these plumes. It didn't find them but it only did a few observations at such high resolution and could easily have missed them..

"For typical imaging systems, features that take up at least 5 pixels in an image are usually reliably resolved. A factor of four margin on this resolution will ensure that plumes taking up at least 20 pixels can be reliably identified. These constraints imply that ... an imager should have resolutions between 25 and 125 m/pixel at 570 km ... Specific searches for cryoplumes were carried out at resolutions as high as 72 m/pixel during the Galileo mission, yet no plumes were detected, perhaps as a result of unfavorable viewing geometries at the time of observation or due to the fact that only a small portion of the surface was viewed at such high resolutions."

"We therefore suggest that imagers with 450 m/pixel resolution (angular resolution per pixel of about 88 µrad) should be employed in future searches for europan plumes. At this resolution, a plume source with H=2.5 km, at a distance of 570 km from an orbiting spacecraft, would take up 50 pixels in an imager and should be fairly easy to identify. Imagers should also be sensitive to our nominal visible wavelength of 0.6 µm."

"In summary, the most favorable viewing geometries that would allow for the detection of plumes on Europa at visible wavelengths include images taken at very high phase angles, when the terminator is just below the horizon and plumes are backlit by the sun and/or high-resolution stereo images of the illuminated disk taken at 40–801 emission angles."

Perhaps this possibility of detecting them from orbit makes the use of a lander for detecting them less important? But a static lander could observe the plumes through their cycle as tidal stresses change and if it can be landed close to them, observe them in more detail than the orbiter. It could also see the process in detail including observing the plume particles falling and so show conclusively that the surface dark features are created by falling particles from the geysers.

Also, (not in the report as far as I know, so my suggestion, it could also spot smaller scale local features). From Sarah Fagin's paper again, this shows what happens if you get a flow of liquid water onto the surface in a vacuum as on Europa. Just as molten lava forms a crust on Earth and continually breaks through the crust as it advances, the liquid water flowing on the surface of Europa would form a crust, and at the advancing front, it would be broken up with thin ice and liquid water exposed to a vacuum. The water exposed to a vacuum evaporates rapidly forming vapor sprays. I wonder if it would have a chance of spotting these, if it landed in the right place? They would be hard to spot from orbit, like the Martian carbon dioxide geysers.

Figure 14 from Sarah Fagin's 2003 paper. There are several features on Europa that may be due to flowing liquid on the surface, in a process similar to this.

How this could change the prospects for a Europa lander

If geysers are as common as that, it not only makes it possible for the lander to spot geysers. It also adds to the possibility that the lander is able to sample some recently deposit plume material on the surface.

Artist's impression of Europa lander observing a geyser. It could see the top of a 10 km high geyser at a maximum distance of about 176 km out of the total circumference of Europa of 19,478 km. If it landed in rough terrain, in a crevasse or at the base of a cliff, then like the Philae lander on Rosetta it might not be able to see far at all.

So, this all depends on whether Europa does have numerous smaller plumes. If it does, and if they erupt continuously at least with every tidal stress cycle, and they are present over much of the surface of Europa, then the lander may have a decent chance of spotting them. If they also bring fresh material from the subsurface to the surface, then it would increase its chance of spotting life in the ice.

So, how far away could our lander spot plumes only 10 km high?

If we go back to the same formula, as from the previous section, then

Using:

The horizon (as seen from the top of the geyser) is sqrt(10*(3100+10)) , or 176 km.

It's a lot of "if"s there. But if this was true, it would rather turn around the prospects of learning something from the Europa lander of astrobiological significance.

What though about the issue of rough terrain? It could easily fall into a crevasse and see nothing at all beyond a few meters, like the Philae lander.

Philae lander, our first spacecraft to land on a comet. This is a photograph of it taken by Rosetta. It landed on its side in rough terrain after "bouncing" off its original intended landing site. It couldn't see far. A lander on the rough terrain of Europa might well end up in a crevasse or alongside a cliff face and be unable to see far, and so, not see far. It could even miss a nearby geyser just hundreds of meters away hidden behind the cliff or crevasse.

In the case of Philae that was because it bounced away from its original smooth landing site. In the case of a Europa lander, we don't have the problem of bouncing, but it might well be impossible to select a large enough smooth landing ellipse to guarantee a smooth landing site.

The result could be similar. If it survives a landing into rough terrain, it could end up close to a cliff like Philae and unable to see anything in the direction of the cliff. The terrain of Europa may be very rough on the scale of meters. A one meter cliff or boulder ten meters away would hide a 10 km plume 1 km away. So that horizon of 176 km could be reduced to 1 km or less if it is surrounded by numerous 1 meter high boulders at a distance of ten meters or so. A ten meter high outcrop or hill a hundred meters away would have a similar effect.

So, if we want to search for plumes on the surface of Europa in this way, perhaps it's best to map the surface at high resolution at sub meter scale first, and see if we can land it, not just on top of a local elevation, but in a region reasonably clear of boulders. Or alternatively, equip it with a mast or a means of climbing to the top of boulders.

Here is an idea of a way to help with this situation. What if we add a "mini rover" with a camera and perhaps even means of gathering a sample to return to it as payload? Nowadays, this could be tiny, just centimeters in scale, robust and maneuverable, to scout out the surrounding area and to scale boulders and cliffs to find a good vantage point to survey the surrounding terrain. Perhaps the Origami rover which folds flat and is the size of a smartphone could be ideal for this?

Perhaps though a lander would be best used to study geysers that are already known, since, as we saw in the previous section, they can also be detected from orbit with a reasonably high resolution camera on an orbiter at a height of 100 km. (See end of Does Europa have thousands of smaller plumes only ten kilometers high? (above) )

With the lander now on hold, then maybe we will get to see the results from the Europa Europa Clipper (multiple flyby) mission before they consider sending a lander? If there is evidence of numerous surface geysers,erupting continuously, or almost continuously, then it becomes a different picture, much more like Enceladus, and the science case for a lander might then be very strong.

A geyser flythrough would still have advantages that you can sample the material immediately after it is ejected. It would be fresher, maybe give you a clearer view of the ocean chemistry, and a better chance of viable microbes. You'd be able to fly through many different geysers, the large ones and the small ones, and at different heights and times of the tidal cycle and of Jupiter's year. Some may be connected to deep reservoirs and some not, so you could sample all the different kinds of geyser too.

Also independently of the science return from a lander, we could still choose not to land until we can achieve 100% sterile landers or reasonable certainty (not just 99.99% sure) that the lander can't cause forward contamination of the Europa oceans.

What we could lose - not just us - but all countries on Earth, all our descendants, and all future civilizations and intelligent species that arise in our solar system

Just as flightless parrots, the kakapo, were perfectly adapted to New Zealand, but vulnerable to cats and dogs, which never evolved in New Zealand. In the same way an RNA world on Europa, or some even earlier form of life could be perfectly adapted to Europa. It could have diversified to countless species, perhaps even have primitive forms of biofilm or indeed multicellularity - yet it could all be gone soon after the first introduction of modern DNA life to the ocean, as after all, all the pre DNA lifeforms that once existed on Earth are gone now. There must have been a time before DNA when all the life on Earth was non DNA based. Though an early life RNA world is plausible, an early DNA world is not, it's just too complex a system to arise in one go from prebiotic chemistry. So that means there must have been something on Earth that predated DNA and since it is no longer here, surely it must have lost out in competition with DNA based life.

Such an ocean as that would count as one of the most interesting things we could find there. And if we are lucky enough to live in such a solar system, then accidental introduction of just a few Earth microbes that somehow get into its ocean might well mean that after a few years of exponential growth later there is not a single microbe left of all the RNA world (or whatever) Europan life that was in its ocean before the first visit.

Or it could have cells with metabolism only, and no exact replication. Or an RNA world ocean with no cell membranes yet. Or something else, which we haven't even thought of yet, not quite alive, that has been made extinct by Earth life long ago on Earth.

That would be just tragic. This is what we must avoid - another of my "possible future news" from an alternative future that hopefully won't happen:

Possible future newspaper story in a (hopefully) "alternative future" in which humans accidentally introduce Earth life to Europa's ocean, then regret what they did. Made with this online spoof newspaper generator. For the image I used a detail from Artist's impression of the Europa lander after a "nominal" landing on Europa

Imagine how awful that would be, especially if in follow up missions, we found only Earth life there. Then, perhaps after much work, we find some evidence that suggested there was a complete biosystem of many species of some other form of life for billions of years that became extinct soon after our first Europa lander crashed on Europa! Perhaps we even find evidence of large fish like multicellular creatures, like our fish, octopuses and cuttlefish, all now extinct.

If it doesn’t happen that quickly, but it's still irreversible, we could find life there, but know that it is going to go extinct in the near future, from the introduced Earth life, with nothing we can do to stop it. Again, imagine how tragic that would be!

For these reasons, I think we have to be sure to explore in a biologically reversible way.

If we do it this way, then perhaps some time in the early 2030s, we send a lander to Europa. By then we know the surface conditions very well so design the lander to be able to land on whatever it is we know is there. If we make it a priority, then by the early 2030s we may be able to sterilize the lander 100% – and perhaps make it a rover too, or a hopper.

Some time later we send a 100% sterile submarine too. This delay also means we can send a more capable spacecraft, with 2030s technology instead of 2020s technology. Given how far technology has moved in the last decade, e.g. the incredible shrinking DNA sequencers - what will it be like by the 2030s?

It's not just the scientists who can design experiments for Europa and write research papers about Europan exobiology that are affected. Nor is it just the nations that come together to take part in the mission itself All the countries on Earth lose this opportunity if we contaminate Europa.

Also it's not just us now. Our descendants lose this for all future generations. Also all future civilizations that arise in our solar system for the remaining billions of years of our solar system lose this opportunity of learning from whatever is in the Europan ocean just now. Even all future intelligent species that arise in our solar system lose it.

It could even be that there is nothing like it even around other stars, if there is something unusual about our solar system that lead to life developing here, and the Europan life has a very distant last common ancestor, maybe before DNA, for instance, then we might not find life anywhere else for thousands of light years. If life was seeded into our solar system's birth nebula, then there may be other stars, our stellar birth siblings, with life, but the nearest again could be thousands of light years away.

There might be more to it even than this. Could Europa's icy crust hide an even greater secret than an alternative biochemistry and second genesis?

If life in a Europan ocean can have the intelligence level of squids, basking sharks, octopuses and cuttlefish - what about a non technological civilization?

It's been suggested that there could be life in the Europan ocean, as intelligent as our crabs, squids, cuttlefish, octopuses or sharks. It couldn't have whales obviously, without air. But it could have equivalent large creatures like our plankton eating whale mouth shark, basking shark or the mysterious krill eating megamouth eating smaller microscopic or small creatures living in their oceans.

It could have enough oxygen for complex life, from the paper by Kevin Hand et al in 2007,. The idea is that ionizing radiation hitting the surface ice turns the water into hydrogen peroxide and then the entire ice crust could eventually become hydrogen peroxide rich. If the time it takes to deliver oxygen to the ocean is similar to the age of the Europan surface, then they found that it would have plenty of oxygen delivered to its ocean in this way:

"Europa’s ocean could well have enough dissolved oxygen to support any known marine macrofauna"

Oxygen may have been what triggered multicellular life on Earth, so could Europa have multicellular life?

In a later study they used Galileo results to map hydrogen peroxide abundance on Europa. They did find it, as expected, but in lower concentrations than the most optimistic figures in their model, and only on the trailing hemisphere which has more ice exposed to the radiation. However, interestingly, Europa seems to have more oxygen in its ice than expected, with the hydrogen peroxide not being enough to explain it.

Their conclusion was (end of this paper):

"Compared to models for seafloor production of reductants, such as methane and hydrogen sulfide, which yield ~3 × 109 moles per year delivered to the ocean, it appears that our new results for peroxide on Europa could lead to an ocean limited by oxidant availability. This conclusion also depends strongly on the global geographic distribution of O2, which may have concentrations significantly larger than peroxide"

So we do still have the possibility of an oxygen rich ocean, because of the mystery of the oxygen, that there seems to be more of it in the ice than they could explain. If it only gets there as a result of the hydrogen peroxide, then the ocean would still have oxygen, but in rather limited concentrations.

So, we don't know how oxygen rich the ocean is. But supposing it does have an oxygen rich ocean, then the main question here is whether it has enough biomass for a pyramid type complex ecology to build up like that, enough to sustain larger animals than microbes and microscopic or small creatures, and perhaps even larger creatures. That depends on the amount of methane and hydrogen produced by the hydrothermal vents, because the pyramid can't be based on photosynthesis but rather on microbes and microscopic creatures living on the products of the hydrothermal vents.

There are various estimates. Louis Neal Irwin and Dirk Schulze-Makuch in their book "Cosmic Biology" (page 192) talks about these attempts to estimate the Europan biomass.

"We and others have actually attempted to model what the energy availability on Europa might be, and what the resulting biomass could look like. The calculations are technical and full of assumptions that are little more than best guesses at our current state of knowledge. But the results of such models are instructive in guiding the nature and limits of our thinking about what life on Europa could be like."

He explains their results using the example of a one gram, tadpole sized predator (tertiary consumer). Their calculations yield various estimates, depending on who does the estimate and what assumptions they make, of

• one such creature every 6.76 cubic meters (a cube with edges 2.6 meters long)
• one every 32.5 cubic meters (edges 5.7 meters)
• and one every 729 cubic kilometers (edges 27 kilometers).

He discusses three main communities - the ice ceiling which he sees as perhaps colonized by biofilms grazed on by small crab like creatures, the ocean floor (benthic) community which could have seaweed like strands attached to them which take advantage of chemical gradients, and also creatures that move up and down in the water column to exploit chemical differences at various heights. Then there could be hydrothermal vent communities similar to the ones we have on Earth, localized to the vents. In between there could be some life, browsed on by larger creatures.

He thinks that life would be in the slow lane there, "By comparison with Earth, life on Europa would likely appear to grow and move (if at all) in slow motion".

He argues this on the basis of its small core which would not produce a lot of radioactivity supported heat, unlike Earth's hot core. He thinks that they would also not be very mobile and have limited sensory perception. If he is correct in this, he predicts that alien intelligence is not very likely there.

"The cognitive ability of animals on Earth is directly related to the complexity of their nervous systems, which is driven by the extent of their sensory capabilities, and their need for motor co-ordination ... If our speculation that Europa's biota would, for the most part, be neither very mobile, nor capable of sophisticated sensory perception is correct, we can surmise that the cognitive ability of even the smartest organisms in Europa's ocean is not likely to be great. Europa may house alien life, but alien intelligence is not likely to dwell there".

However, as he says, that's based on a lot of assumptions, and particularly, it's all based on an assumption that it has similar characteristics and limitations to Earth life. What if it has a more efficient metabolism? Could its biochemistry be adjusted to low temperatures, using reactions that are faster than the ones used in our DNA based life? Might the Europa core actually have more radiogenics than expected, and be hotter than expected? Or might it be more productive than expected?

Well, a study in spring 2016 turned this on its head yet again. They found that as the moon cooled down, since it formed, the rock forms cracks in it much deeper than the cracks in Earth's ocean floor, as 25 kilometers deep, into its rocky interior, so about five times deeper than on Earth. This gives much more room for water to percolate into the rocks and form hydrogen through the process of serpentization, even without any volcanism. The cracks deepen at about 1 mm per year so constantly exposing fresh olivine for the serpentization reaction. They found that even with a much less active interior than Earth, it could still produce comparable amounts of hydrogen to Earth's ocean floor because of those deep cracks. Indeed on a per unit area basis, it would produce nearly an order of magnitude more hydrogen than Earth (ten times as much). They note in their conclusion that some researchers have speculated that perhaps Earth life required continent type materials like granites, rich in potassium and other essential elements (KREEP), and if so then it's possible that Europa had mantle type convection as for Earth, to create granites and similar rocks, but that's speculation at present. The paper is here.

The NASA press release puts it like this:

According to Vance, researchers previously speculated that volcanism is paramount for creating a habitable environment in Europa's ocean. If such activity is not occurring in its rocky interior, the thinking goes, the large flux of oxidants from the surface would make the ocean too acidic, and toxic, for life. "But actually, if the rock is cold, it's easier to fracture. This allows for a huge amount of hydrogen to be produced by serpentinization that would balance the oxidants in a ratio comparable to that in Earth's oceans," he said.

So, could Europa's ocean have an active complex ecosystem similar to Earth's oceans? Nobody knows. Steve Vance, one of the authors, interviewed by Business Insider, in 2015 (before that research was published) was optimistic.

"My modest thought about what kind of life might be at Europa involves the kinds of things that we see at heads of thermal vents [on Earth], mainly microorganisms,"But in my bolder moments I wonder if Europa could have the kind of vigorous biosphere that Earth has that supports larger forms of life,"

Larger forms being anything from small fish to modestly-sized octopi. At that point, you approach a size of creature that on Earth can be rather intelligent. Also, perhaps with the new ideas for sources of hydrogen, the ecosystem there has enough energy now to drive life with a more active lifestyle than astrobiologists thought likely before.

So, with the proviso that of course this is all speculative, and based on the research which leads to the most optimistic projections of what's possible in the Europan ocean, let's look a bit further at the question: how far could that intelligence evolve?

The "coconut octopus" is a tool using mammal that carries a coconut shell around with it to protect itself.

(click to watch on YouTube)

Meanwhile the common cuttlefish, and other varieties of cuttlefish signal to each other using rapidly changing visual patterns on their skin, forming a "language" with dozens of "words". This figure shows some of the visual signals.

. Images typical of the 12 clusters of body patterns from page 1622 of this paper. See also the list on page 1618

If there can be life at that level of intelligence in creatures similar to ones that just possibly might be able to survive in a Europan ocean - what stops them from being even more intelligent like us?

It's impossible to compare exactly, but these octopi and cuttlefish may be on a par with the great apes in their own way, in intelligence. Our small increase in intelligence over great apes has lead to our ability to construct languages to describe so many things, to develop mathematics, science, writing and so on. How far away is a cuttlefish (say) from developing a civilization if it was similarly a bit more intelligent than its cousins? Perhaps indeed, it is more a matter of focus, that it needs to have its intelligence a bit more developed in some particular direction to have language and civilization?

If there is a civilization in the Europan ocean, or any of these ocean worlds, then it is likely to be millions of years old. That's because it would be an almost unbelievable coincidence for it to develop at the same time as ourselves, in an ocean that has a gestation period, including evolution, of billions of years.

When you take account of that possibility, then our clumsy mistake with a spacecraft could in worst case lead to extinction of another (non technological) ET civilization far in advanced of our own (except in technology) and our only chance of contacting ETs in our own solar system.

They would have no need for technology to develop mathematics, philosophy, music, millions of years ahead of us.They could pass on their understanding through to later generations through memorization (as some human cultures did before writing) especially if they have an excellent memory. Also writing is possible without any sophisticated technology. They could record their great works of maths and literature and poetry etc by scratching marks in the mud of the sea bottom or scratching symbols on a rock face with a sharp pointed rock. To preserve their great works for the future, they could continually remake the marks as they get eroded.

There is no way they could have fire in an ocean with a thick surface blanket of ice. Probably they have no chance of smelting, except perhaps by dropping ore bearing rocks into hot streams of lava or some such. So, they would probably have only the most primitive technology by our standards. They might be able to conceive of complex machines and devices, perhaps have their equivalent of our science fiction stories about them, but have no way to make them. They would have no way of knowing anything about the universe outside of their ocean.

They might perhaps have tried to venture up into the geyser channels from below, but if any of the got near the vacuum of space, they'd die. It would just be a place they can't explore, most likely ( apart from the science fiction possibility of developing a skin that protects them from the vacuum and radiation of the surface temporarily). If they encountered our robotic spacecraft, they would just be mysterious things they couldn't identify that would descend into their world, a world without technology.

They would know nothing about us until one of our robots enters their ocean. Perhaps for them the whole episode would be a bit like the clangers in this episode, that they would treat it as some kind of a strange creature. It would probably be even more unfamiliar to them than the space probe that the Clangers encountered in the episode "The Intruder"

(click to watch on YouTube)

The chance of a non technological civilization there may seem to be very small but I don't think you can say, on our understanding so far, that it has to be so small as to be negligible. It's hard to assess it, because if there was a civilization there, there seems to be no way that we'd know about it yet. For all we know, non technological civilizations in subsurface oceans of icy moons may even be common throughout the galaxy.

In the worst case, we might have no contact with them at all until we make them extinct, by accident, perhaps by crashing a spacecraft on the surface, in such a way that it crashes through thin ice and introduces Earth life to the melt plumes beneath Thera Macula and then through geysers, into their ocean.

When you start to look at it like this - then you may start to see a pristine Europan ocean as a "super positive" outcome of potentially incalculable value. We can't know what's there yet. It could be that it is of little interest to us as it is now, or can't be harmed by Earth microbes. But it could be of tremendous value and interest, intrinsically and for its value to us, and perhaps also, its value to intelligent creatures that live in its ocean too, if they exist.

For more on this see the section on Precautionary principle and super positive outcomes (below) .

Suggestion: geyser sampling in late 2020s, then 100% sterile lander in early 2030s

NASA plan to send the Europa Clipper (multiple flyby) mission to Europa possibly as soon as 2022 to get there by 2025, using the SLS. So far, great! But let's postpone the lander a little until we know more about the surface conditions, unless it is 100% sterilized.

I understand that it's perhaps too risky to send a geyser sampling mission to Europa until we know for sure that what Hubble saw were geysers. But there's a good chance that they are since they come from the same place on Europa each time and asteroid impacts are probably ruled out. If so the main question would be, are the water plumes from geysers from Europa's ocean directly, from a rising plume that originated in those oceans, or are they generated in some way from the surface ice. Again a deep subsurface origin seem pretty likely but not proven.

So why not just postpone the next mission by a couple of years until we know what the situation is on Europa. We can give us more flexibility also by targeting Enceladus as well as Europa. If we want to keep up a cadence of SLS missions, what about a geyser sampling mission to Enceladus as the next SLS outer solar system after the Europa clipper? That's already a sure thing.

Mission to Enceladus' geysers - and almost identical mission to Europa's geysers

NASA plan to send the multiple flyby mission to Europa possibly as soon as 2022 to get there by 2025, using the SLS. So far, great! But let's postpone the lander a little until we know more about the surface conditions, unless it is 100% sterilized.

I understand that it's perhaps too risky to send a geyser sampling mission to Europa until we know for sure that what Hubble saw were geysers. But there's a good chance that they are since they came from a similar area on Europa each time, one of them seems to have repeated, and asteroid impacts are probably ruled out. If so the main question would be, are the water plumes from geysers from Europa's ocean directly, from a rising plume that originated in those oceans, or are they generated in some way from the surface ice. Again a deep subsurface origin seem pretty likely but not proven.

So why not just postpone the next mission by a couple of years until we know what the situation is on Europa. We can give us more flexibility also by targeting Enceladus as well as Europa. If we want to keep up a cadence of SLS missions, what about a geyser sampling mission to Enceladus as the next SLS outer solar system after the Europa multiple flyby? That's already a sure thing.

Mission to Enceladus' geysers - and almost identical mission to Europa's geysers

We already know that we can do geyser sampling of Enceladus, as Cassini has done it multiple times, flying through its plumes and sampling them directly. Also the evidence so far suggests that the water it sampled comes from its subsurface ocean. Some time, if not right away, we are bound to want to sample these plumes and try in situ life detection and biosignature analysis.

So why not start work on the design for a mission right now, and make it dual purpose so it can visit either Europa or Enceladus, with the decision of which to send it to left until later. Then, if the NASA Europa Clipper (multiple flyby) mission shows that Europa has repeating geysers and they originated in its ocean or rising plumes from its ocean, I think just about everyone would agree that the best follow up is to sample the geysers. They would give us the freshest possible samples we could have of its ocean, and it would have almost zero planetary protection issues because the orbiter makes no physical contact with Europa at all, only with the material in its geyser.

It's tricky to do geyser flythroughs for Europa because of the ionizing radiation. Either you shield the spacecraft heavily, or you minimize the time spent in the heavy radiation zone or both. The earlier Europa Orbiter concept involved putting a heavily shielded orbiter into a science orbit around Europa. Details here. The Europa clipper approach is to stay in orbit around Jupiter and do repeated flybys, a bit like Cassini with Enceladus. It could do very low passes to sample the geysers if there are any to sample. So a follow up mission could use either of those approaches.

On the other hand, if the Europa multiple flyby mission doesn't find geysers on Europa, or they are a surface phenomenon in some way that is not of interest to the search of life, we send the next geyser sampling mission to Enceladus, without the extra ionizing radiation shielding.

Also, there is another way that we could sample a Europan geyser. If it doesn't have geysers already - let's make an artificial one!

Planetary protection friendly version - artificial geyser from a "dumb impactor" - Bernd Dachwald’s idea

This is an idea to create an artificial geyser, so the mission has two components, the "dumb impactor" consisting of just a metal slug, easily sterilized, and an orbiter to fly through the plume and analyse it. It's the same idea as the geyser flythrough mission, except that we create an artificial geyser for it to analyze.

This idea is especially useful if Europa has thin ice anywhere, which we may be able to find from orbit by use of radar. Then, we can attempt to make a hole right through into the water below. If the water is either under pressure (compressed by freezing) or is volatile rich, it could lead to an artificial "geyser" of ice or water that rises from the hole in the ice into the vacuum of space. It would splash ice and water into space anyway, but if we are lucky or know from orbital studies (including radar) where to target it, perhaps it can even cause a mini geyser that would last for some time.

If there is no thin ice, it's still useful, as the penetrator would still create a small crater and send a plume of ice into space, including surface materials.

The idea comes from Bernd Dachwald, director of the German IceMole project who suggested the idea to me, in a conversation in the pub when we discussed planetary protection issues for ice moles for Europa. He was one of the invited speakers for a one day conference on "Search for Extra Terrestrial Life - Europa & Enceladus" held in Oxford on 24th July 2015, talking about his IceMole, with his full talk here, and I was invited to talk about the science value of Europa and planetary protection, giving a presentation on “Super Positive” Outcomes For Search for Life In Enceladus and Europa Oceans. Full list of videos for the conference here.

Later I found out that the idea is also mentioned as a proposal for Enceladus in section 2.4.1.2 of the Enceladus Report (page 2-34) as an idea for Enceladus, to create a "control plume" away from the main geysers. It isn't given much attention as it is of less value for Enceladus, because we already have geysers erupting continually into space there.

"Kinetic or “dumb” impactors offer the opportunity to create a “control plume” away from the south pole, for investigating compositional differences at various locations on the surface, for example. They can also be useful for seismic sounding in conjunction with surface seismometers. However, large compositional differences are not expected elsewhere on the surface (which is coated with plume fallout and processed E-ring particles), and tidal flexing should generate sufficient seismic signals for internal sounding. Thus, this option was considered to be of low to modest scientific value. As dumb impactors require only basic technology, they do not require further investigation at this time and could be included if mass were available and if future scientific evaluation warranted their use."

However, though not so useful for Enceladus, it may be a very useful technique indeed, for Europa. That's because

• It might not have geysers or they may be intermittent and hard to predict.
• It may have other surface regions of special interest for the search for life (perhaps the chaos regions) that don't have geysers, but have surface material that would be great to analyze from orbit to search for it.
• It may have ice so thin, only centimeters thick if it has briny seeps, that a penetrator could punch through it creating a geyser
• If it has thin ice overlying liquid water under some pressure, or rich in volatiles, the geyser might even continue for some time after the impact.

I found a mission for Europa too, called Europa Clipper, but we'll come to that at the end of this section.

For the last case, if Europa has intermittent geysers, perhaps we could trigger a new outburst with the impact? If we want to try to break through thin ice to create a new geyser, then we want the fastest impact velocities we can manage.

The relative velocities could potentially be huge if we arrange things carefully. Europa is deep in Jupiter's gravity well, with an orbital velocity of 13.74 km / sec. So, perhaps we could arrange to have the penetrator orbit Jupiter in the opposite direction from the direction Europa orbit? (Astronomers call this a retrograde orbit, and they call the direction that Europa and the other inner satellites follow, a prograde orbit, orbiting in the same sense as Jupiter's orbit around the sun).

There is no need to circularize it. You get a few extra km / sec if it's in an eccentric retrograde orbit with the closest point to Jupiter (perijove) at the same distance from Jupiter as Europa.

The escape velocity from the Jupiter system at Europa's orbital radius of 670900 km is 19.44 km / sec (you can use this online calculator, select Jupiter and then change its radius r to 670900 to get the escape velocity from a planet the same mass as Jupiter but radius equal to Europa's orbit, which is the same as the number we need). So we could generate an impact velocity of up to 33.18 km / sec, or even more if the penetrator enters the Jupiter system with a hyperbolic orbit.

So, the impact into Europa could be at a speed of over 30 km / sec if the retrograde orbit is very eccentric.

For a larger hole, you could use two “dumb penetrators” with the second one closely following the first for more effect.

How could we do it? I expect the best time to do orbit changes to get the dumb penetrator into a retrograde orbit would be early on, as the spacecraft gets captured by the Jupiter system, during its very eccentric elliptical capture orbits. We could use flybys of one or other of Jupiter's moons, say, Ganymede, to flip it into a retrograde orbit around Jupiter. There's a simpler way to do it to separate the penetrator from the rest of the spacecraft early on, as it is approaching the Jupiter system, and hit it with a hyperbolic orbit relative to Jupiter. The main problem with that though is to slow it down, because it would reach Europa much sooner than the orbiter designed to fly through the plumes, which has to do many gravity assists of Jupiter's moons to get into position. I think it would have to be captured first, into a prograde orbit, then flybys of the moons could be used to flip it into a very eccentric retrograde orbit, which it would stay in until the time came to impact on Europa.

Techy note, in case of confusion, this is not the same as the idea of a "distant retrograde orbit" around Europa, which is the most stable orbit around the moon, and a likely choice for a Europa orbiter. That's still prograde around Jupiter. I'm suggesting a retrograde orbit around Jupiter itself to hit Europa from the opposite direction from the direction it is traveling around the planet.

Essentially, this is an artificial asteroid impact on Europa, simulating the highest velocity meteorite impacts Europa can get.

We would need to arrange it to synchronize the impact by the 100% sterile "dump penetrator" so that it happens just before a synchronized low flying orbiter flies past, to capture a sample from the geyser. It can also sample the dust flux from micrometeorite impacts into Europa and from ion sputtering, and the incoming particles from the Io volcanoes etc.

If it turns out to be of science value, we can do the same with Enceladus, which also has a high orbital velocity of 12.64 km / sec - the same idea of a dumb penetrator in a retrograde orbit around Saturn could achieve an impact velocity of more than 25 km / sec, and again we could add a few km / sec using an eccentric retrograde orbit around Saturn. The velocity to escape from Saturn at Europa's orbit is 17.86 km / sec so the most eccentric possible temporary retrograde orbit around Saturn with its closest point at Enceladus' orbital radius could reach over 30 km / sec.

All this would have minimal planetary protection issues in the forward direction, if the dumb penetrators can be 100% sterile - e.g. just lumps of metal heated before impact to temperatures where no Earth microbes could survive, and even DNA and RNA fragments, GTAs and other complex molecules get destroyed - or otherwise 100% sterilized before impact.

If Europa is indeed producing geysers naturally, we don't need to do this, we can just observe one of the natural plumes "as is". But we could have it as a backup plan if it doesn't have geysers, or if they are hard to predict, or as a way to study regions without geysers. But it would have to be 100% sterile.

I think the main planetary protection issue here in the forward direction would be with the delivery system. The dumb impactor has to be sterile. But to get into a retrograde orbit, it also needs guidance rockets and to be able to communicate with Earth, probably through the mother ship. We have to be sure that this delivery system is 100% sterile too, or else, that it has no chance of hitting Europa either during the maneuvers or after it is ejected on an impact trajectory. It's the same also if it is ejected from an orbiter around Europa, we need to be sure that no components of the delivery system hit Europa at any time, or else, to make sure that they also are 100% sterilize.

For instance, if the delivery system is not sterile, and at some point is on an impact trajectory to hit Europa - what happens if the computer fails at that point, reboots say, as a result of a cosmic ray hitting a sensitive part of its memory? Or what if the thruster to shift it away from the impact glitches? Then it's on a trajectory to hit Europa and we may not be able to regain contact until it hits.

One idea here - what about propelling the impactor from the spacecraft, much as one does with a bullet, using an explosive charge of some sort, or even high pressure gas? That might be a way to put the dumb impactor onto a trajectory to hit Europa, with no unsterilized spacecraft ever in an orbit that intersects with Europa.

Europa Ice Clipper (not to be confused with Europa Clipper)

This was a proposal for NASA worked up in some detail, to impact a small 10 kg mass into the Europa ice at 10 km / sec and predicted a crater of around 10 meters in radius (details depend on the ice, whether it is hard, or soft, and they studied three different models - like hard ice, wet sand and dry sand.

This seems to be a rather forgotten project now. I can't find a link to the file on the NASA website, as is usually the case for things like this, and can't find it in google scholar either - except as a citation - not even an abstract. I got my own copy of it as an attachment from a NASA Spaceflight.com forum post, so here is a copy uploaded to my own website.

The aim was to use a low impact velocity so that the debris would be mainly unmelted ice. So let's look at this mission. It's from 2002, so ideas about Europa have changed a lot since then. But it was interesting research still relevant today, and was a discovery class mission, and a project that could be revived. Their idea was to do a sample return.

"The Europa Ice Clipper is a flyby mission. To obtain samples of the surface of Europa we will use an impact sampling method. As we approach Europa a l0 kg hollow copper sphere is released on a impact trajectory. The spacecraft then diverts to fly through the plume of surface material that is created by the impact. The ability to create a plume, predict its properties, and sample the particles in the plume while protecting the spacecraft are the basis for this mission design."

It would be a Europa version of the Stardust mission. It would do on board analysis of the dust but it would also have an aerogel collector as for Stardust and return the materials to Earth, also with a particle collector (collects them after passing through a thin aluminium membrane), and a volatiles collector. Those samples would complement each other

"A primary objective of the Europa Ice Clipper will be to return a sample of the refractory material in the surface of Europa to the Earth for detailed analysis. As the spacecraft moves through the ejecta plume the Aerogel Collectors (AC) --- similar in design to those used on Stardust --- will be exposed and collect particles. It is not expected that water and other volatiles will be retained in these collectors. However, the silicate materials and refractory organics will be preserved for transport back to Earth."

"A second collector is also deployed during flyby, the Active Volatiles Collector (AVC). This collector is composed of substrates made of sapphire wafers onto which a low-Z metal film is deposited during encounter of the volatile plume. Encounter velocity is too low to capture volatiles by implantation, but they can be effectively trapped by co-deposition. Using this approach we will capture water vapor in sufficient quantities to allow for the measurement of the D/H and oxygen isotopes after return to Earth. A third collector, the Particle Capture Collector (PCC) will directly collect particles that penetrate a thin aluminum membrane. By sealing the collector volume immediately following the flyby, this material, including released volatiles, is returned to Earth also for isotopic and elemental analyses"

They worked out how much material it would collect, and the results were rather promising. Different results for ice, wet and dry sand models, but they were in general agreement.

"Based on these calculations we determine that the 0.1 m² area of the particle analyser (JEPA) will collect over 5,000 particles of size about I um (radius) over the course of the fly through. Collected ice ejecta may be in particles larger than I micron. "

Chris McKay assessed it as having no planetary protection risk for backward contamination to Earth.

"The Ice Clipper mission would impact sample the upper 1.2 to 3.4 m of the ice depending on the surface hardness. At these depths the radiation dose is expected to be 500 and 40 rads/year, respectively. These dose rates would kill dormant cells in less than 36,000 and 450,000 years even for the most radiation resistant strains. It is therefore likely that a Europa sample return mission such as Ice Clipper can be treated using the Stardust mission as a model for planetary protection, that is, the returned material can be assumed to pose no biological risk"

This was in 2002, before the discovery of possible geysers and the idea also of water flowing onto the surface of Europa and magma like plumes rising to its surface from the subsurface ocean. That might change things if it was reassessed because it gives a way for microbes to get to the surface more recently than a few tens of thousands of years ago.

If so, what about the solution of sterilizing it before return to Earth using gamma radiation? That's not likely to make it much harder to analyze the radiation damaged organics in it, and could be allowed for, while making sure that there are no viable cells from Europa in the sample.

There could be a planetary protection risk in the forward direction - the spacecraft is part of the delivery system and we need to make sure none of the delivery system impacts on Europa. If I understand right, the spacecraft is set on a course that would hit Europa, and it releases the "dumb impactor" which then stays on that course and hits the surface, and then it does a very small course correction to avoid Europa and instead to pass through the plume and return to Earth.

Normally missions such as Mars orbiters practice trajectory biasing to make sure that they are never in a trajectory that could impact Mars (so that in case of failure, loss of communication etc, then Mars is not affected). With this plan, the spacecraft would be on an impact trajectory until after it releases the impactor. Perhaps the impactor could be released using an explosive charge or pressurized gas instead? Or require the spacecraft to be 100% sterile. Or am I missing something here?

One neat thing about the original proposal is that it would be a free return mission - that as soon as it leaves Earth it's on a trajectory that will lead to a flyby of Europa and then all the way back to Earth again. It's a hyperbolic flyby - it uses the gravity of Jupiter to return it to Earth, and it flies past Jupiter at just the right time to synchronize this with a skim past of Europa on its way inwards towards the deeper part of Jupiter's gravity well.

This 2016 paper has a new re-analysis of the possible trajectories for this mission, and finds numerous ways to do it. Their reference mission leaves Earth in 2026, returns in 2040 and has a Europa flyby speed of 12.69 km / sec, but leaves Earth at only 3.94 km / sec and re-enters at only 10.86 km / sec. They can do a slower flyby of Europa of 7.7 km / sec, but only by increasing the launch velocity from Earth to 5.6 km / sec and return velocity to 14.2 km / sec.

Redesign of spacecraft before launch - leave the launch to one or two years after the multiple flyby gets to Europa

First, as I said before I'm glad that the lander is postponed, for planetary protection reasons, and also, because I think it was too soon, to not just design the lander, but to launch it as well, before we get any results back from Europa Clipper, and indeed, while the Europa Clipper was still on route to Jupiter.

So what could we do instead? Well these are just a few thoughts to share about how one could proceed after Europa Clipper if we don't do the lander right away.

First, w e could send multiple missions like the Europa Clipper right now, the more the merrier. So the ESA JUICE mission will help a lot too with its ice penetrating radar (able to see into the ice to a depth of 9 km), laser altimeter, imaging spectrometers in infrared visible, UV and sub millimeter wavelengths. It does only two Europa flybys sadly (its main objective is Ganymede which it will orbit and even has a lander for it) but it will fly past Europa at an altitude of 400 km each time at different latitudes, so quite close see page 33 of Hans Huybrighs' thesis).

JUICE also has a particle environment package that can do mass spectrometer analysis of the plumes. In a recent study Hans Huybrighs showed that it should be able to detect water vapour in plumes that release only one kilogram per second. The ones Hubble spotted are estimated to have released 7,000 kilograms per second. The paper itself is here. This question was also the subject of his earlier PhD thesis here.

If we want to send a lander, irrespective of planetary protection issues, even if it is 100% sterile, or we achieve certainty that it can't introduce forward contamination of Europa, I think it's good to wait at least until we have at least started to learn a bit more about Europa. There are such huge uncertainties about Europa just now, as we've seen. What Clipper and JUICE tell us about the Europan surface, and what they tell us about the geysers and any near subsurface water might change our plans about what's the best thing to do next.

Of course it is expensive to change the design of a spacecraft when it is all ready to launch. But if that is what needs to be done, it's a lot less expensive than finding soon after launch that the one we sent doesn't have the instruments we need to investigate puzzles raised by the orbiter. It's even worse if we decide on a lander, and soon after launch, find that its design can't cope with the landing conditions when it gets to Europa, so it just has to stay in orbit, or if we find that it can't land in the most interesting places on Europa.

If we are reasonably confident that everything is fine, we could have the lander all ready to launch, but just not launch it until we have some confirmation from the Europa Clipper that it's found a suitable landing site for our lander, and that our instrument suite is indeed what we want to send to Europa right now. Or, if it seems likely that we might want to make some changes as a result of the science return from the Europa Clipper mission, perhaps we could hold off on finalizing the design. Get all the components space hardened but leave the integration into a complete spacecraft to be sorted out. Design but not build. Or finalize the design and do a build - but even then keep the possibility open that you do a last minute redesign.

The lander is by far the most uncertain here as it requires a fair bit of new technology. We could also work on both a lander and a geyser sampling mission, with the expectation that the lander needs more development time, as well as new information from Europa - and also possibly developments in planetary protection methods too. Meanwhile do the geyser sampling mission first, inserted as a new mission which can fly before the lander. Design it to do in situ life detection, including metabolic life detection (if we think viable life in the plumes is possible), sensitive detection of organic biosignatures, optical and UV microscopy, and biosignature analysis.

We have opportunities to get from Earth to Jupiter every year because of Jupiter's slow twelve year orbit. It's not like Mars where we can only send a spacecraft there every two years. I suggest that we work on the flybys and take them to the point where the spacecraft are a year or two years away from final launch. Then if the Europa Clipper launches in 2022, the follow up mission, probably to sample the plumes, could set off in 2026 (Oct 23rd). By then we already have a fair bit of data from the Europa Clipper which could influence the design and selection of instruments in the final stages. We can then get there some time in 2028. It doesn't take that long to get to Jupiter if you can handle a reasonable delta v (about 7 km / sec from 200 km height LEO). Or set off in 2027 if you need more lead time to finalize the spacecraft.

If Europa doesn't have geysers, and we decide against the artificial geyser idea for now, then we can send our geyser sampling mission to Enceladus instead. It doesn't take so very much longer to get to Enceladus if you have a little more rocket thrust available.

By then, mid to late 2020s, then perhaps we can do a mission to Enceladus that departs LEO with a total mission delta v of 8.75 km / sec for the Saturn rendezvous. If so we can get to Saturn in less than four years (compared to two years for a fast mission to Jupiter). Depart LEO Jun. 17th 2026 and get to Saturn on Jun. 12th 2030. Because Saturn's orbit is so slow, there are opportunities for similar trajectories to Saturn every year (though some of course are a little better than others). Once in the Saturn system it can do repeated flybys of Titan and other satellites of Saturn to get into its final orbit with not much more delta v.

You can try other dates like that with the NASA Ames Trajectory Browser. It's a first approximation that only takes account of the gravity of the sun - a more detailed calculation would need to take account of the gravitational effect of other objects.

And if Europa does have geysers, we do an especially radiation hardened version and send it to Europa too.

There may be other differences too for Europa. We can work on those at the design stage too but don't need to actually build the final spacecraft until we know for sure that we are going to send a geyser sampling mission to Europa.

Also we can keep the idea of an artificial geyser on the drawing board for Europa, do enough work on it to be able to build this into the mission if we need to. Once we get the data back from Europa, we might decide it is worth the extra work and delay to add in ability to make an artificial geyser - especially if the previous mission has found evidence for thin ice covering an extensive lake / ocean from a rising plume from the subsurface ocean about to break through the surface (as may be the case for There Macula).

Sampling geysers - particle sizes

First, you might wonder if the Europan geysers are able to lift microbes high enough to sample them. Well, yes, they can, even with Europa with gravity 81% of that of the moon. From a paper published in 2015, full paper here, they assume

• A plume height of 100 to 200 km, as for the Hubble image
• That this geyser originates in water exposed to a vacuum
• 5 km / sec fly through
• Collector area of 10 square centimeters

They calculate that bacterial cells up to 10 microns in diameter could be lofted right to the tops of the plumes.

So, would a flyby detect living cells? That's harder to say. If the number of cells in the Europan ocean are low, 100 per cm³, similar to predictions for lake Vostok, they would be lucky to find a single cell. Expected number 0.15 cells for a single flythrough at two kilometers (and none at 200 km). For a million cells per cm³, similar to populations of hot springs, then it rises to 1,529 cells. If similar to deep ocean basins on Earth, then it's between 1.5 and 153 cells collected.

They emphasize the need for a large collection area to have a better chance of detecting cells. That shouldn't be hard to do as they considered rather a small collector area, 0.002 square meters. Let's try a larger one, of 0.1 square meters similar to the JEPA collector for the shelved Europa Ice Clipper. Then you need to multiply those numbers by 50 to get an average of 76 cells per flythrough if levels are similar to those predicted for lake Vostok. If it does multiple flythroughs those numbers go up.

One advantage of Europa is that it is very smooth on the scale of kilometers. It should be possible to do a flyby at two kilometers, so long as the spacecraft is shielded against the sizes of particles expected at that height. They found that no articles as large as 2 mm reach that high, a size of particle that you should be able to shield against.

For Enceladus with its lower gravity, even very large particles are lofted up to 100 km. From the 2007 Enceladus flagship concept study then a 10 cm by 10 cm collection area flown at a height of 100 km through a single geyser plume would collect between one and several thousand particles of 10 microns in diameter. That's large enough for a large terrestrial microbe. Particles as large as 1 mm in diameter could hit the spacecraft at that altitude so it might need shielding.

Shows diameter of dust particles expected for various power laws for a spacecraft flying through a single Enceladus plume at a height of 100 km. This is figure 2.5-1, page 2-37 from the 2007 Enceladus flagship concept study. The article says: "For a 100 cm² collection area, in this example, between one and several thousand plume particles larger than 10-micron diameter would be collected in one plume passage. The largest particle hitting a 10 m² spacecraft could be larger than 1-mm diameter, depending on the dust power law index, so shielding would be necessary if such particles were dangerous to the spacecraft."

So, yes the geysers do lift microbes and even larger particles high enough to sample them, even for Europa with its higher gravity. But how do we capture and analyse them from orbit?

Studying dust and ice by analysing the plasma created during impact of particles - method used by Cassini

Cassini was able to sample the Enceladus geysers using its Cosmic Dust Analyser. It's a simple design based on the Galileo dust detector subsystem (photograph of it here).

Galileo dust collector - simple instrument which detected the mass, charge and velocity of small dust particles. Launched to Jupiter in 1986. This in turn builds on earlier dust collectors going back to the early Pioneer spacecraft

Cassini's dust collector is vastly more powerful. It can still read the mass, charge and velocity of the dust particles, but it also has a mass spectrometer to inspect the plasma generated during the impact itself. From this, it can read out many details of the chemical composition of the dust. So this is a mature technology for space missions.

Cassini's cosmic dust analyser - designed to analyse the dust in interplanetary space and the ring particles in the Saturn system, but it also turned out to be just the thing to use for sampling the unexpected Enceladus geysers. It doesn't slow the particles to analyse them, but rather, makes a virtue of the dust particles' high impact velocity of many kilometers per second.

It has two targets, an outer gold plated one called the Impact Ionization Target (41 cm in diameter) and an inner rhodium one (16 cm in diameter) called the Chemical Analyser target. The dust particles hit it so hard that they turn into a mixture of fragments, neutral atoms, ions and electrons in an impact plasma. (See section 3.3.3 of this paper).

Then three collectors are used to analyse the plasma. There's one for electrons, one that measures the charge of negatively charged ions, and one for positively charged ions. It's the positively charged ions that are used for the time of flight mass spectrometer. The overall charge of the plasma gives an indication of the particle's mass and velocity, and the rise time for the plasma depends on its velocity only, so you can also read off the particle's mass and velocity - how these depend on the mass and velocity is checked before flight with calibration experiments. The collectors can also be used to decide whether the particle hit the gold target or the rhodium one.

The Rhodium target was chosen for the chemical analyzer because when it is impacted, it produces large numbers of ions and was less contaminated by other materials than gold or silver targets.

When particles smash into the rhodium target then the positive ions from its plasma are then accelerated by an accelerator grid, towards a detector mounted in the center of the tube. This grid is only 3 mm above the target, with a potential difference of 1 kV. Ions of different mass will be accelerated away from the target at different speeds and the results are analysed using a "time of flight" mass spectrometer.

Lighter ions travel faster, so you can read out the mass of the ion from how fast it is moving. This then gives a mass spectrum which gives the chemical composition of the ions. Of course this is mixed up with ions made of the target material Rhodium, so you ignore the Rhodium signal in the output.

It's a rather simple design as such things go, and as a result, the peaks are broad rather than sharp. However, it still gives a good read out of the masses of the different sized ions produced by the impact and so of the chemical composition. The researchers were able to make ingenious use of it to find out a great deal about the Enceladus geysers.

The instrument also has a couple of grids of thin wires across the entrance to help with evaluating the speed, direction and charge of the dust particle before impact. Details and technical drawings in section 3 of this paper. Diagram and more details from this paper.

So, most of the ideas for geyser follow up missions are based on this simple proven design. The Enceladus Life Finder proposal adds a reflectron, a kind of a magnetic field based "mirror", which helps to even out the time of flight for debris in the plasma traveling at different speeds. This gives it more sensitivity, and it is able to distinguish isotopes of single atoms. It can also find high resolution masses of larger fragments, and has a wider range of masses it can detect.

Enceladus Icy Jet Analyser - proposed for the Enceladus Life Finder mission. The brown line shows the path of a dust particle which comes in through a window in the side of the tube, hits the impact target made of Iridium this time, the plasma is accelerated into a reflectron which through electric fields "reflects it" through an angle into the tube with the ion detector.

This evens out the speeds of the ions leading to much higher mass resolution.

It can be operated in two modes, to detect either positive or negative ions. It's able to discriminate the masses of different isotopes of single atoms.

It can simultaneously measure individual atoms such as H+, C-, O- through to organics with masses of up to 2000 unified atomic units (one atomic unit is approximately the mass of a proton or neutron, or about the mass of a hydrogen atom) - so up to several hundred atoms in an organic molecule made up of hydrogen, carbon, oxygen etc, and over 100 carbon atoms.The instrument masses 3.5 Kg and has a peak power requirement of 14.2 watts. For details see this paper.

The Europa Clipper mission will have a similar reflectron based instrument, SUDA, though with a lower design mass range of up to 150 atomic mass units instead of the 2000 of the Enceladus proposal techy details. It's mass is about 24 lbs, or about 10 Kg, much of which is for the to protect it from Jupiter's ionizing radiation.

The mission also has a mapping image spectrometer for remote sensing of the Europa surface, and a mass spectrometer MASPEX for analysing the tenuous Europan atmosphere,

It also has a UV spectrograph to look for plumes in a way similar to the way Hubble spotted them from Earth, and a thermal imager to look out for the warmth of vent sources on the surface of Europa (a method that worked well for spotting the sources of the Enceladus plumes). So the flyby mission may well spot plumes if they are there. If it does find them, it can fly through them and study them with SUDA and MASPEX which are more capable instruments than the ones on Cassini. See NASA Goes First Class for Europa and scientific payload.

These instruments can also analyse the composition of the surface of Europa as it flies over it, from particles thrown up from the surface by micrometeorite impacts on Europa. By analysing the direction the particles come from, it should achieve a surface resolution of Europa of about 100 km for the origins of the particles. So there is much it can do even if Europa doesn't have geysers.

Also, in some ways the gravity of Europa is an advantage, even though it confines the geysers to closer to the moon than for Enceladus. We have to fly lower, but then it's very smooth so that's not a big deal. And we can learn something of the chemical makeup of the geyser even from sampling its very thin "atmosphere".

It turns out that the atmosphere of Europa remains transformed long after a massive geyser eruption. That's because the particles from the geyser land on the surface, then re-evaporate, over and over.

So, we'd see spatial changes in the atmosphere long after a geyser eruption, shifting chemical patterns days or even weeks later. This also could help discover freshly formed surface features.

There's also a proposal, not adopted, to send a second mini spacecraft with a miniature version of SUDA called Sylph (abstract), on board Europa Clipper, The idea is it would deploy it to fly as close as 2 kilometers from the surface of Europa. It would be about the size of a domestic propane tank, and would do a one off single three second fly through of a plume to get information about its composition from closer to the surface.

There is a significant chance of Sylph later hitting Europa. To be safe, they assume certainty that it will. So that means it would be sterilized as for a lander, with similar issues that after the bake out (at 128 °C for ten weeks), it's then assembled then baked out for an extra 2.5 weeks, then they then have to assemble it with batteries sterilized separately using ionizing radiation because though some batteries can be heat sterilized, they get severely reduced capacity after several years of storage after that.

They also have to load the propellant aseptically - can all this be done in a way that keeps it 100% sterile? As usual their aim is a 1 in 10,000 chance of contaminating Europa. See the spacecraft integration section of the paper. Can it be increased to near certainty?

See Design change suggestions that could make a lander close to 100% sterile (above).

It's just a proposal and I don't think it has been adopted yet.

For an overview of the Europa Clipper's science payload:

Detail from slide 3 of Bob Pappalardo's presentation. Shows everything except the Radiation Environment - responsibility of the Radiation Science working group.

Capturing the ice and dust particles more gently, in an aerogel - for sample return, or in situ analysis

All of these instruments work by smashing the particle into a plasma of charged ions and electrons, or studying the atmosphere of ions already present. But for biology experiments we want something that slows down the particle more slowly, and not smash possible microbes into smithereens as an ion plasma, or at least, if we do smash them into smithereens, we would like a few large recognizable organic molecules to remain.

The usual way to do a slow capture for a sample return is with an aerogel.

Peter Tsou, JPL Scientist, with the Aerogel used by StarDust to capture interstellar dust and dust from a comet and return it to Earth

The Enceladus Life Finder proposal includes an aerogel to capture dust particles for a sample return. It isn't gentle though, indeed they make the fast impact into a virtue. They deal with backward planetary protection of Earth by tuning the collection so that large molecules would survive but viruses and bacteria would not.

A sample return sounds good, but it has its downsides.

• Time delay: You can only analyse the sample when you return it, probably many years later
• Planetary protection issues - you need to be very sure that you aren't going to return a potentially invasive ET microbe to Earth.
• May have only a small sample size, compared with the amount of material you can investigate in situ.
• How do you preserve life and organics in the sample during the long journey back to Earth?
• Mass requirements, it's not just the sample return container - the payload for the fuel and rocket to return the sample adds to mass requirements which could have been used for more instruments in situ
• Or time limitations - alternatively, you collect it on a free return trajectory as for the Europa Ice Clipper, in which case you don't need the extra payload for the return fuel, but have just that one short flyby as your only chance of collecting a sample, and of studying your target.

The simplest form of sample return is a "free return" trajectory. In the case of Enceladus, it takes 13 years to reach Saturn, captures it in an aerogel with a single flyby of Enceladus, and then returns to Earth thirteen years later. It is simple but high risk as you have only the one chance to do a sample capture and if you don't successfully fly through an active geyser during those few seconds when it is closest Enceladus, that's the end of the mission - it returns to Earth with nothing.

A more complex mission would have more chances of capture and could return the sample more quickly than that. See the Enceladus Public Report page 2-33 (that is page "2-33" not pages 2 to 33) for a discussion of sample return trajectories.

However, do we have to do it this way? What about analysing the sample itself in situ? That would let us get an answer instantly. What's more, you can test geysers at various heights, times of orbit, even times in Saturn's year if that matters. Also you can follow up new lines of investigation based on the samples analysed so far. For instance if a particular altitude or time in the cycle of tides is particularly promising, you can investigate them further right away in situ. It might speed up the pace of discovery a lot. Most important of all maybe, the sample has no time to deteriorate during the long journey back. That's important for volatiles as well as any possibility of viable microbes in the sample, for instance.

If we want to do the analysis in situ, how do we get hold of the sample to analyse it? We know how to turn the particle into a plasma, by impacting it into a target, as for SUDA. But how about capturing it more gently?

We have a lot of experience in collecting a sample from a planetary or lunar surface, with a digging tool or a drill; that technology is well developed now. But those methods just don't work for a geyser flythrough. We can't just reach out with a digging tool and dig up a bit of the geyser, or drill into it. So how do we transport a sample from the geyser to our onboard suite of biology and biochemical detection instruments to analyse it?

In situ analysis of particles in an aerogel - the Enceladus Amino Acid Sampler

One way to analyse the particles in situ is to use an aerogel again, as for a sample return - but to analyse the dust and ice particles right away as soon as they are captured. That's the idea of the Enceladus Amino Acid Sampler.

The way they capture the geyser particles is with an aerogel. Aerogels are light fluffy materials that are able to slow down particles without damaging them. Again they are mature technology. The NASA Stardust mission for instance used an aerogel to capture material from a comet as well as interstellar dust.

The main challenge is how to transfer the samples from the aerogel to the mass spectrometer in situ.

First though, here is a brief recap of aerogels and their use for capturing particles in space missions.

The basic idea behind an aerogel is to make an ordinary gel - similar to jellies, the sweet desert - but instead of gelatin, they are made of silica, or sometimes of graphite and other materials, which makes them hard to the touch. The first aerogel made by Samuel Kistler in 1931 was made from sodium silicate (waterglass) as a precursor. Then you delicately remove the water and replace it by air (the tricky part), with various ways to do that. The resulting material is light, but also strong. It also turns out to be ideal for capturing dust particles at high velocities without damaging them. The basic idea is to start with a less dense aerogel near the entrance of the collector then the aerogel gets denser and denser the further you get into the collector, as the particle slows down.

The idea of using aerogels to capture particles started in earnest in 1992 with experiments flown on the Space Shuttle and then the "StarDust" mission in 1999 to bring back a sample of dust from a comet

Stardust aerogel collector under construction. All those compartments are filled with varying density aerogel. First low density aerogel to slow the particles down then higher density to bring them to a gentle stop.

Particle captured in aerogel - shows what one of the particles looks like after it has entered the aerogel. This method can stop small dust particles gently even if they approach at very high speeds of many kilometers per second, faster even than a high velocity rifle bullet.

With StarDust the researchers didn't plan in advance on how to remove the particles. They had to work this out after it was already in flight. They found a solution but it is very fiddly and it will take them decades to extract all the particles. There is no way we can do that in situ.

In the case of StarDust then the researchers hadn't planned in advance how to remove the particles from the aerogel. They had to work this out after the spacecraft was in flight.

They did find a solution, but it was exceedingly complex. They use a micromanipulator and a glass needle to extract small wedges of aerogel. Their paper about how they do it (written before the sample return, but it's the same as the method they are using now) is here.

It works, but it takes a long time - they may take decades extracting all the particles. Though they have found many interesting results already. That works for a sample return, if time consuming. But it's no good "as is" for our in situ analysis.

The main new technology needed is a way to get materials out of the aerogel for in situ study, and then to get it to the mass spectrometer.

The first step of their method is to enclose the container after the geyser encounter. Then, you heat the aerogel so that volatile molecules get evaporated (or more generally "desorbed") from it for analysis. Alternatively, you can introduce a liquid or gas as a carrier to carry the materials from the aerogel to the mass spectrometer. They haven't gone into this in any more detail than that in this paper.

Their main focus was on the next stage: how to work with the materials from the Aerogel. The extracted volatiles or the liquid with the organics dissolved in it need to be pre-concentrated and treated and then finally delivered to the Mass Spectrometer. Their idea is to use microfluidic channels, tiny channels to move minute amounts of liquid, and then trap the amino acids in resin beads for observation. They tested this and it seems promising, see their 2013 report. After that they would use the "lab on a chip" to find their chirality and the the mass spectrometer to verify the identity of the amino acids.

We have some advantages here though, the main thing, that we don't need to drill or dig. So long as we can get the materials out of the aerogel in situ, then we can go on to analyse it in as many ways as for surface missions.

As for how they analyse the particles, their approach is based on Cassini's Ion and Neutral Mass Spectrometer (INMS), a bit like SUDA, a time of flight mass spectroscope - but this instrument didn't capture dust particles. It just analysed atoms and ions in the tenuous "atmosphere" that the spacecraft passed through. It was very effective - for instance this and other similar instruments on Cassini helped the scientists to gather evidence that the geyser plumes originated in a region of high temperatures in the Enceladus interior of 500 - 800 K (227 - 527 C).

So anyway they took this instrument as their starting point, but updated it with modern technology so that it's more sensitive. During the geyser encounter, it would be able to analyse the plumes in the same way as Cassini's INMS by analysing any ions and neutral atoms it encounters in the atmosphere of Enceladus. However they suggest a dual use. They would use the same instrument to analyze the dust and ice particles captured in the aerogel.

As far as I can discover, the Enceladus Amino Acid Sampler is the first proposed "in situ" analysis tool for studying dust and ice particles in the vacuum of space, rather than for studying materials on a planetary surface. It's early days - when you do things for the first time, it is bound to take a while before you find the best way to do it. So now, time for some more of my speculative sections.

Suggestion - in situ analysis of the particles within a transparent silica aerogel using, e.g. Raman microspectroscopy

This is just a suggestion of my own. There are many kinds of aerogel, some opaque. But silica aerogels are transparent, with a slight blue cast due to the Raleigh scattering. So what about analysing the particles within the aerogel before we extract them?

This is mature technology used here on Earth, you can find out about its composition without even removing the particle from the aerogel by using Raman microspectroscopy. If we did this in space, it would be especially useful as it would give detailed information about the composition of the particle, with a spatial chemical map of its surface at a microscopic scale. We could also try other techniques of Optical microscopy (see below)

Also, do we have to capture into an aerogel? They are great for high velocity impacts. But I wonder if there might be better methods for low velocity impacts for astrobiology in situ searches?

Suggestion - for low relative velocities, capture directly into water, brine, ice, or foamed up water or ice

This is another of my speculative sections - do we need an aerogel for the slower relative velocities such as the 176 meters per second of the Enceladus Orbiter? What about water or ice or a "water / ice aerogel" (to coin a word)?

At high velocities, then liquid is hardly different from impact into a solid. But at these low velocities, capture into liquid or ice could be gentler than impact onto a solid target, in between an aerogel and a solid target.

Indeed if the microbe spores are as robust as some Earth microbes, they could remain viable even after a 5 kilometers per second impact into a culture medium. We don't need to speculate as scientists have actually tried the experiments already. See section 5.1 of this paper, also page 56 of this one which describes experiments with five kilometer per second impacts into agar.

Summary: I cover several ideas in this section but perhaps the most promising idea is to have an aerogel first to slow down the particles but not stop them completely. Behind that, the particle itself is captured in briny water, which would be foamed up to give a gentler slow down to remove what is left of their velocity. The advantage is that there is no need to extract them from the aerogel, which may well be tricky, to judge by the Stardust experience. The particles end up in briny water which could be adjusted to have the same chemistry as the Europan or Enceladus ocean. Then do the rest of analysis just as for Mars etc.

In detail: this is just my own suggestion as I haven't seen it in any of the papers I read yet (do say if you know of someone who has suggested it). So the first idea, and starting point was to ask - at such slow speeds, could we capture the particles directly into ice or water covered in a thin film of ice?

The water, turned to ice, would of course immediately start to sublimate in the vacuum of space, but the actual fly through of the plume only lasts for seconds. The sublimation rate for ice is about 0.7 mm per sec at 0 °C. So it would take 14 seconds to sublimate one cm of the material. For cites and the calculation see Liquid airlock in my Case for Moon First.

Advantages of this approach:

• There is nothing in the capture material to confuse the mass spectrometer results.
• The capture material could be adjusted to have the same composition as the Enceladus or Europan ocean (as best we know it from orbital measurements).
• Easy to extract the captured particles - as they are already in water, not caught up in an aerogel
• Easy to clean the capsule container for capsule re-use with a fresh sample

That could lead to the exciting prospect of actually capturing viable dormant microbes into a medium much like the sea they just left (if they do have robust dormant states). Perhaps they could even be revived for in situ observation with an optical microscope as suggested for the Europa lander, and for in situ metabolism tests like the chiral labeled release or microbial fuel cells.

If we are going to do lots of these sample captures, it might well help to reduce the rate of evaporation. The evaporation rate will be slightly lower if the water is salty like the oceans, which you might think would be another advantage, but this effect is not very significant, at most 10-20% for very salty water, see figure 1 in this paper.

Can we do better than that? Especially if we are going to do many sample flythroughs, it would be good to reduce the evaporation rate further than for salty brine, to conserve the water. Well, it can be done. You could use room temperature or lower ionic fluids. These have such a low evaporation rate that they have been suggested for liquid mirror telescopes on the Moon. You'd need to be careful of course to choose a material for the ionic fluid that would be easy to distinguish in the mass spectrometer and other analyses. Also, if you want to capture microbes alive (in dormant state), you need a liquid that is not likely to harm life adapted to the icy moon's ocean.

However, there's another idea which would need much less of the ionic fluid, just a thin layer on top of the collection brine. The ionic fluid, if immiscible with water, could be introduced in a separate thin layer on top of the sample collection medium (in between two covers, outermost one to protect the ionic fluid from the vacuum of space while the gap is filled, and inner thin cover to prevent it from getting into the main part of the sample collection chamber).

This is all done before the flythrough. If we choose our ionic fluid right, we can have one that gets frozen through exposure to space. We wait until it is frozen, then remove the cover behind it to expose the liquid water. The idea really is to create a temporary very thin and easily penetrated cover of frozen room temperature ionic fluid. So then the impacting particles of ice and dust would first penetrate this thin film of frozen ionic fluid, then impact into the foamed up water or ice behind it.

Another idea is to introduce a thin layer of vacuum stable light oils in between the sample capture medium and the vacuum of space. First let a surface layer of ice form on the surface of the capture medium of brine, before opening the aperture to the vacuum of space. Then once that happens, add a thin coating of a light oil at the last minute to prevent sublimation. It's the same idea as an ionic fluid, and like ionic fluids, these have very low sublimation rates, only 6 grams per square meter per year when exposed to a vacuum, for one example I mention in Liquid airlock in my Case for Moon First. The particles smash through the thin surface film of oil and ice, and the oil would also help to reseal the hole that they form. Probably the particles wouldn't be contaminated much by the oil.

There are many materials here to choose from, from the ionic fluids and the vacuum stable light oils, to see if any are suitable for a very thin layer that wouldn't confuse the in situ sample analysis unduly.

Perhaps we could also investigate ways to "foam up" the water with minute bubbles of an inert gas, with varying density similar to an aerogel to help slow down the particles without any damage? The basic idea is to create fizzy water, either with carbon dioxide in it - or perhaps some inert gas, such as helium etc (for minimal effect on the mass spectroscopy etc), depending on what is most appropriate. The foaming could be done with the lid closed, so in an enclosed chamber, then the cover removed seconds before the flythrough of the plumes. In that way the thin layer of the medium immediately exposed to a vacuum turns into ice during the encounter and hardly has any effect on slowing down the particles.

Another suggestion is to have a hybrid water / aerogel system.

In this idea, the capture capsule has an outer cover of a conventional silica aerogel to slow down the particles initially, and also to help protect the water from evaporation / sublimation into space. The thickness of the cover is designed to be just enough to slow the particles down to a halt when they reach the water which is just behind the aerogel.

The aerogel then could be kept in place, throughout the mission, perhaps with an additional protective cover which is removed during flythroughs. Meanwhile, the water behind it is moved to the in situ instruments for analysis using microfluidics or whatever, and replaced by fresh water.

Some smaller particles would get caught in the aerogel, but larger ones would travel all the way through into the water. Different thicknesses of aerogel cover could be used to optimize it for different sizes of particle or speeds of geyser flythrough. Or, we could have variable thickness aerogel. At least some of the particles would hit the part of the aerogel that's the optimal thickness to slow them down to a halt gently just before they hit the water.

Two of the comet particles captured by the Stardust mission. Their explanation is that the particles were like loose dust clods made of a mix of "large and strong rocks" and fine powdery materials. They broke apart as they entered the aerogel. The "Turnip head" like top part of the track consists of numerous small particles that were stopped quickly. While the largest particles made it all the way down to the bottom a distance of 1.1 cm, 0.85 cm, and 1.17 cm respectively in these three examples.

If the capture brine had an aerogel cover, then its thickness could be adjusted, maybe in separate collection chambers, or with a variable thickness, to capture either the largest or the smaller particles in the geyser optimally or anything in between.

For instance in this case an 0.8 cm thick aerogel cover over brine could have permitted a Star Dust like collector to collect the larger particles into brine while a 0.1 cm thick cover would favour the smaller particles. With the thinner cover, the larger particles would hit the water at great speed, and it would be like a solid surface for them - they would decelerate rapidly in the brine and be damaged. With the thicker cover, then the smaller particles never reach the water.

Of course the actual thicknesses would be much less than in this example for a slow velocity capture, and also they would depend on the materials of the aerogel.

I mentioned in the list of advantages that this idea of a "water / ice / possibly ionic fluid" aerogel could also help with capsule re-use. So how would that work?

First, the motivation behind this is that it might be hard to completely remove all trace of the previous sample from a silica aerogel before you analyse the next one. You might just have to have multiple aerogels for each plume flythrough.

However, if you use water, and you remake this "water / ice aerogel" each time by feeding minute bubbles of an inert gas into it each time, it would mean that we could potentially empty the capture container after use. We could then clean it completely of organics (perhaps with carbon dioxide snow since it is quite small so may be easy to cleanse in that way) and then re-use it by simply introducing more foamed up water. The water itself can be purified by distillation, easy to do in a vacuum, and then recondensed, or (if the volume involved is small) we can take enough water with us for a large number of re-uses.

I've no idea how practical this all is as I'm not a space engineer. I'm just suggesting ideas here, in case they are of interest to instrument designers.

At any rate - the idea of capturing dust and ice particles, and then analysing them "in situ" seems a very promising field, one which has just opened up recently.

It's very much in its infancy with hardly anything written about it in the scientific literature, not that I could find anyway. Surely eventually as it matures, we'll have more and more sophisticated ways to capture and analyse the samples, just as happened with all the methods that have been developed for the more mature field of sample collection and analysis on a planetary surface. One way or another, surely they will find a way to capture the material in a gentle way for analysis.

Once we have the particles captured in water, we can use any of the methods explored in In situ instrument capabilities to study them further. Some of the instruments might need adapting for zero gravity conditions.

If anyone reading this knows of more papers on gentle capture of particles from geysers for in situ study, do say!

Why Enceladus is so interesting

Enceladus is especially interesting because of its geysers. Nearly everything we know about these comes from measurements by Cassini, and by looking into this, we can also get some idea of what the Europa Clipper (multiple flyby) Mission could find out about Europa if it has similar geysers. It would even use similar instruments to Cassini, though of course, purpose designed for the task (when Cassini was designed, nobody expected it to encounter geysers). It's also based on later technology than Cassini of course, of much greater sophistication

Here is a photograph (not an artist's impression) of two of its geysers.

Photograph by Cassini of two of the warmest geysers on Enceladus' South Pole.

As you can see, Cassini has good cameras. It discovered the geysers in 2005, and since then has flown through them several times, from 2008 onwards. It had one more fly through scheduled for October 28 2015, within 30 kilometers of the surface. This one was also timed to pass through when the plumes are at maximum output (a first for the mission).

Detail from photograph taken by Cassini during its 2015 fly through of the plumes. The complete image is available here

With its suite of instruments including imaging spectrometers it can do spectrographic observations from a distance and can also capture and examine the material in the plumes close up.

As a result of its observations, we now know that the plumes erupt continuously, and are easily accessible from flybys.

 

Mystery of the Enceladus heat sources

Enceladus is a tiny moon much smaller than Europa. It's in a resonance with Dione which helps to keep it and Dione in eccentric orbits, but the resulting tides would be rather weak. So there was a mystery about how Enceladus got enough heat to keep its ocean liquid. Back in 2012, it was still quite a puzzle to get the heat budget to add up.

However now, research seems to be gradually converging towards a possible solutions. First, Enceladus is tidally locked. But it's also kept in an eccentric orbit, and as a result has eccentricity tides. So how do eccentricity tides work?

The resonance with Dione keeps it in an orbit which isn't perfectly circular. By Kepler's second law, it orbits Saturn more quickly when it is closest.

Kepler's second law. A line from a planet to the sun sweeps out equal areas in equal times. The same law also works for any two objects orbiting each other. So when Enceladus is closest to Saturn it orbits it more quickly and when furthest away it does so more slowly, as seen from Saturn. If you know basic physics actually this is just another way of saying that angular momentum is conserved. e.

Techy note: Strictly speaking the line here should join the planet or moon to the focal point of the ellipse. For instance in a binary star system, each star follows the second law on its own, tracing out equal areas relative to the focal point of its elliptical orbit. Also, it's a first approximation, the actual motion is more complex because of many body interactions.

There is no way for Enceladus to keep the same face pointed towards Saturn all the time as it would have to spin sometimes faster and sometimes slower to compensate. By conservation of angular momentum it has to spin at a constant rate.

So it has to librate. As seen from Saturn, Enceladus will wobble back and forth slightly. As seen from the side of Enceladus facing Saturn, then Saturn will move back and forth in the sky.

This creates the eccentricity tides, but as you can imagine they are rather weak. But just strong enough to keep its ocean liquid it seems.

The hot spots only occupy a small area of its surface. They are very hot compared to the rest of its surface, but also very concentrated along the narrow "tiger stripes" fractures which are all in a small region around its south pole.

This is a heat map taken by Cassini in 2008 and later in 2009.

Zooming in on Heat at Baghdad Sulcus Temperatures measured at up to 180 °Kelvin (-93 °C) at the hottest parts, confined to an area of a few square meters. The rest of its surface averages -201° C. So the vents are over 100 °C warmer than the rest of its surface and the interior of the vents may be even hotter, perhaps hot enough for liquid water.

There is no internal heat in violet coloured areas. Most of the heat from the warm flanks of the fractures, and the interior of the fractures may be warmer than that, possibly warm enough for liquid water just below the surface.

One now outmoded idea was that the geysers were the result of friction, the sides of the ice cracks rubbing against each other to produce heat. But the images show localized hot spots only a few tens of meters across, which suggests that the heating comes from the geysers themselves.

Another idea was that the ice itself deforms, with the tides.

But the simplest model involved the entire crust of Enceladus shifting back and forth over a global ocean, with a steadily spinning core, helping to keep it liquid.

Evidence for a global ocean for Enceladus

This actual physical shift of the surface back and forth is called the physical libration - as distinct from the apparent libration - as seen from Earth, our Moon seems to librate back and forth.

Cassini's instruments were sensitive enough to spot this libration, so then the search was on to detect it. And yes it does librate, as we found out in 2015. Here is the NASA press release, and here is a paper announcing detection of a large physical libration of 0.12 degrees. This strongly suggests that the ocean is global and that the core is not directly linked to the surface, because with a heavy core linked to the surface the libration would be much smaller. They were not able to conclude whether the ocean is temporary or has been there for billions of years.

The plumes wax and wane with the tides. However, puzzlingly, they do this with a 5.7 hour delay which is a challenge for the models.

There are two main solutions suggested as a way to keep the oceans warm enough. One way is through the lag in the eccentricity tides. Another way, favoured by the 2014 paper, is due to longitudinal libration of the entire crust over a global ocean.

(click to watch on YouTube)

Shows libration of our Moon, both latitudinally and longitudinally, as seen from Earth. That's the apparent libration. The Moon spins at a constant rate but because of its non circular orbit, it changes position as seen from Earth.

Enceladus would also librate like this as seen from Saturn - but also, Saturn tugs at it, and is able to move its shell over a global ocean so that it shifts back and forth somewhat less than the underlying moon. This causes a physical libration - Enceladus' shell sometimes spins just a bit faster than its core and sometimes more slowly, with the shell physically moving back and forth relative to its core.

To find out more, here is a short summary from 2017. See also this 2014 technical paper which predates proof of the global ocean, and Hugh Platt's blog post on those earlier results.

For more about the 2015 study, see the results of a libration study based on seven years of measurements of Enceladus by Cassini (paper here).

Although we understand the situation much better now, the source of the heat that keeps the ocean liquid still remains somewhat of a mystery. They suggested that perhaps Saturn's tidal effects on Enceladus may be greater than expected (this depends on how tides on Saturn respond to Enceladus' gravity as well as the response of Enceladus to Saturn). A 2012 paper suggested that Saturn tides may be ten times stronger than was previously thought.

Is Enceladus' ocean temporary or long term?

Originally it was thought to be a temporary ocean, because the heat budget didn't add up to keep it liquid. But these newer models favour the idea of a long term ocean.

If Enceladus was completely frozen through, there wouldn't be anything like enough heat from the tides to create an ocean. But once an ocean forms, by whatever method (e.g. impact) then the tides have much more heating effect and the oceans stay liquid. So it's thought that long term Enceladus can be in either of two long term states - either completely frozen, or with a liquid ocean. And it so happens to be in the state with a liquid ocean.

With these new models it has probably had a liquid ocean for billions of years.

On the other hand, other recent research looking at ridge patterns in the surface in equatorial regions suggests that the equatorial regions of Enceladus may have had thin layers of ice in the past, now frozen to greater depths - if so - the ocean may be gradually freezing.

If that's right, this doesn't mean that the plumes will stop next year or next century, but over a few more hundred million years it may freeze solid. If so then it may be a temporary ocean, perhaps an event that happens from time to time in its geological history - created as a result of changes in eccentricity of the Enceladus orbit from time to time, age of order hundreds of millions of years, maybe a billion years.

What we know about the geysers so far - and prospects for life

This is what we know about the Enceladus geysers.

(click to watch on YouTube)

3D model of the geyser basin of Enceladus showing the location and tilt of 98 of the 101 geysers identified in 2014. Five of these jets have images taken too close together to determine the tilt precisely - these are shown with dotted lines.

• 101 geysers counted in 2014
• Typical vent is a hot spot of about 10 square meters.
• Geysers occur along fracture lines
• They contain ice crystals, methane, ammonia, and silica
• The silica tells us an astonishing amount about the conditions there

The geysers come from an under surface ocean which has been detected indirectly by gravity anomalies. 

Water is denser than ice and the anomalies are explainable by a liquid ocean below the surface of the south pole at a depth of about 30 to 40 km and with a thickness of 8 to 10 kms. Extends up to a attitude of 50 degrees south, though it could also be global. It also has an internal rocky core beneath this made up of silicates, density about 2.4 grams per cubic cm, so there's the potential for water / rock interactions there. Sapienza Università scientist Luciano Less explains in this video.

From the silica we can deduce (Gabriel Tobie, March 2015 Nature):

• Enceladus subsurface sea has at most 4% salt - so not too salty for life
• It's pH is between 8.5 and 10 so moderately alkaline.
• Originates at an interface between rock and water at temperatures of 90 °C and pH greater than 8.5
• The silica particles probably get to the geysers within a few months of formation at the vents, or at most, within year

However as often in science, this is research in progress. Before we say "right that's confirmed" I should mention that another article questions whether silica particles would form in this way in the Enceladus ocean. They reason that Enceladus' core should be similar in composition to a carbonaceous chrondite's, which when mixed with water forms solutions that are undersaturated in silica.

Another line of research, combining geophysical modeling, and geochemical analysis of the plumes, suggests a different picture of the Enceladus oceans, that it may be very alkaline, about pH 11, with sodium carbonate ("washing soda") present in addition to the sodium chloride.

Interior of Saturn's Moon Enceladus. New research suggests pH of 11 so it may potentially resemble a soda lake.

Shows an ocean around the southern hemisphere only. Recent research suggests it is probably global.

This would make it similar in pH to a soda lake. These are also habitable on the Earth, so it could remain habitable to many species. 

Artemia Salina, a species of brine shrimp - popularly known as "Sea Monkeys" can survive in soda lakes such as Mono Lake (pH 10) (Wikipedia article on Mono Lake).

Kenya's alkaline lake Magadi also has a fish, the Lake Magadi Tilapia, Akolapia Grahami, adapted to live in water at pH 10, and up to pH 11. It does this mainly by excreting all its nitrogenous waste as urea instead of ammonia.

Lake Natron in Tanganyika. It's pH can sometimes reach as high as 12. The rocks that weather to make it salty have carbonates, but little by way of magnesium and calcium. Because the water is so caustic, it's an ideal breeding site for flamingos that nest there, protected from predators by the alkaline waters. (More about the geology and chemistry of Lake Natron)

See Ocean on Saturn Moon Enceladus May Have Potential Energy Source to Support Life. For the paper see The pH of Enceladus' ocean.

This paper has an interesting section at the end about the possibility of life in the Enceladus oceans. Some of their conclusions or suggestions include:

• It's possible that Enceladus never developed a differentiated core (if it did, basalt / water interactions would make the oceans less alkaline) 
• In these conditions, all the salt from the undifferentiated interior of Enceladus probably leached into the ocean
• Without the driving forces of sunlight and continental drift, the ocean could come into a near equilibrium with the core.
• Hydrothermal vents are attractive for origins of life because of the wide variety of physical and chemical gradients
• Hydrothermal vents don't by themselves guarantee that life evolves, because carbonaceous chrondite's show evidence of hydrothermal vents in the parent bodies - but no evidence of life. Perhaps these chondrites were on the way to life but hadn't yet evolved it, because they didn't have liquid water for long enough.

They suggest that one of the main questions to be addressed is whether or not the Enceladus ocean still has fresh anhydrous rocks deep in its core, allowing hydrogen to be produced today, via serpentization? Or might life have evolved there, but later on died out due to lack of food? See the paper for details.

More recently scientists for the Cassini mission reported that they have found hydrogen in the plumes, and they think it provides good evidence of hydrothermal vents producing hydrogen. See also NASA press release.

It took a fair bit of planning to do this, as Cassini is not designed to sample the hydrogen directly. They had suspected that hydrogen was present previously but didn't have conclusive data. So, they had to develop a new mode of operation for its Ions and Neutral Mass Spectrometer (INMS) which let it measure the hydrogen before impact, so that they could distinguish the hydrogen from hydrogen that might be created as a result of impact into the target in the instrument.

They found considerable amounts of hydrogen. 0.4 to 1.4% of the plume by volume mixing ratio. They also found 0.3 to 0.8% carbon dioxide, 0.1 to 0.3% methane, and 0.4 to 1.3 percent ammonia by volume. See table 1, page 3 of their paper. This is far too much hydrogen to be produced by chemical processes or radiation dissociating the water in the ice (which would also produce a signal of oxygen which they didn't find). It couldn't be stored in the ice or the ocean, as there was too much for that, and another way to produce it - shearing of ice mixed with silica at the tiger stripes also doesn't produce anything like enough hydrogen.

So they were left with hydrothermal vents in the ocean as the only remaining hypothesis standing, for the source for the hydrogen, by process of elimination. Discussion on page 3 of their paper.

What's more they also found that Enceladus ocean is likely to provide conditions that are energetically favourable for conversion of this hydrogen to methane by abiotic processes, as well as by methanogens that could use it as a source of energy. They found out that this is energetically possible so long as the pH of the ocean is less than 14. The upper limit on the ocean's pH in the published literature is 13.5, but it seems more likely to be between 9 and 11, so it is comfortably within the desired range.

Over this range of pH values then the levels of hydrogen they found in the plume "translates to a strong thermodynamic drive for methanogenesis in the ocean of Enceladus" (see page 4 of their paper).

(click to watch on YouTube)

These conditions are almost identical to those in the "Lost City" hydrothermal vent in the mid Atlantic.

These habitats occur when mantle rock (the layer below the Earth's crust) is exposed to the surface. The chemistry supporting this life is serpentization: olivine rock plus water and CO₂ reacts to create serpentine + magnetite + brucite + ammonia gas + hydrogen gas + hydrocarbons. Then the sulfate in seawater is reduced by hydrogen to produce hydrogen sulfide

Microbes may be using anaerobic methane oxidation as methane consumers (methanotrophs) There may also be methane producers (methanogens)

Temperatures of the inside of these pillars: 20-90°C and pH 9-11 (water emitted at 300°C but rapidly cools down). The inside of the pillar is dominated by biofilms of - Lost City Methanosarcinales (LCMS) Microbial community to a large extent independent of surface conditions

These hydrothermal vents show how you can have an ecosystem not dependent on sunlight in any way. This community could probably survive on Enceladus. It's not only a great place for highly evolved life. It is also one of the habitats suggested for the evolution of life in early Earth. Could it be a habitat where life could have started on Enceladus?

So this is the new picture of Enceladus that is emerging.

Image from April 2017 press release

A study in 2013 showed that, though there is a chance that life on Enceladus or Europa could be related to Earth or Mars life, opportunities for transfer would be rare. In their simulation, between one and ten meteorites get from Earth to Enceladus over the entire history of the solar system. The figures are similar for Europa, and similar figures also apply for transfer from Mars to Enceladus or Europa.

Also Enceladus' ocean may be comparatively young, and if so, there's no way that life could have survived there for billions of years to the present in a dormant state and revived, so presumably it would have to have a "second genesis" as the chance of a meteorite getting there from Earth right now must be tiny.

What could we find in the Europa or Enceladus oceans?

Let's go back to the graph from Half of the pages of the book of evolution have been torn out (above) , Alexei Sharov, and Richard Gordon's idea to plot the increase of complexity of DNA against the time of origin of the lifeform.

This diagram shows the complexity of the DNA as measured using the number of functional non redundant nucleotides. The graph is adapted from figure 1 of this paper which also explains in detail how it was derived. Notice that the prokaryotes; the simplest primitive cell structures we know; are well over half way between the amino acids and ourselves. Mammals have around 3.2 billion base pairs or 3.2× 10^9. The smallest prokaryote base pair has 500,000 base pairs (for Nanoarchaeum equitans and Mycoplasma genitalium) or 5 × 10⁵. There must have been much simpler ancestral species.

As we saw in that discussion, the graph doesn't necessarily mean that early lifeforms originated billions of years before the origin of life on Earth, as it could also be that early life ramped up in complexity much more rapidly than modern life.

If we find an ocean just a few tens of millions of years old, as originally suggested for Enceladus, then its ocean could be on the far left, at the start of the graph. Perhaps it has protocells or the simplest forms of life, just a few tens of nanometers in diameter. It could also still be like that even with billions of years of "evolution" if it is really hard for life to evolve.

Or it could be an RNA or PNA world in there, with no metabolism but replicating chemicals. Or a primitive early form of life. In that case it could help fill in the huge gap in our understanding of abiogenesis.

It could be like that, even if most of the life in our solar system is related. Perhaps we even have shared origins with the life in Europa's ocean, shared through asteroid and comet impacts in the early solar system. Or, perhaps most life in our solar system comes from microbes that got to us from other stars in our birth nebula - or even previous stars in our galaxy - as we explored in Distant cousins with last common ancestor from a planet around another star (above) . However even in that case, if Enceladus does indeed have a young ocean, it may be too young to be seeded in that way.

If so, it might give us a unique insight into the origins of life. This could be as exciting for astrobiology as the discovery of complex life. We have no way to replicate what can happen in an entire organics rich ocean with hypothermic vents evolving for a million years in a laboratory and so have no practical way to explore the early stages of evolution. We get interesting things happening in experiments in the lab, such as formation of RNA, of amino acids, of cell like structures, etc. But it is nowhere near the complexity of life.

But then - Enceladus could also be way to the right on the graph. We could find a billions of years old ecosystem that has evolved through as many stages as our own. It could even beyond us, even further to the right on the graph. Evolved to some future form of life that hasn't yet arisen on the Earth.

Also, though it may seem unlikely, there is nothing so far, actually, to rule out creatures in the ocean of Europa or Enceladus as intelligent as octopuses, or dolphins. It's the same as for Europa, that if we assume it is like us, then it would need oxygen or something similar before it can develop multi-cellular life of any complexity and especially also an active lifestyle. I don't think there is any suggestion at present that Europa's ocean could be oxygen rich, because it doesn't have either Europa's tectonic drift or Jupiter's ionizing radiation to split the ice to enrich it with oxygen.

However, that conclusion, natural though it may seem, relies on a lot of extrapolation based on an example of only one way of doing biology, as the astrobiologists who suggest it agree. Yes, it took billions of years for Earth life to develop multicellular life, at least in its most complex forms, and that development seemed to require oxygen, and lifeforms without oxygen tend to be sluggish. But can we generalize from that to all habitable oceans?

If they can be as intelligent as dolphins, the rest is as for Europa's ocean, why not as intelligent as ourselves? See If life in a Europan ocean can have the intelligence level of squids, basking sharks, octopuses and cuttlefish - what about a non technological civilization? (above)

As for Europa, a civilization in these oceans would be unable to use fire. Under conditions of high pressure, sealed from the surface by ice, they would probably be unaware of us and us of them, so far, anyway. They might have had a civilization that lasted for billions of years, be advanced in arts, music, philosophy, mathematics, anything that requires intellectual thought. May be advanced in understanding of life and biology - as far as you can go without fire and advanced tools. Yet they might have no idea there is anything to the universe outside of their ocean.

However it doesn't have to be intelligent life to be more highly evolved in this sense, way to the right in the graph. It doesn't even need to be multicellular. After all, microbes have evolved in genome complexity too. What if, for some reason it never developed conditions for multicellularity, but with no competition from multicellular life, it just developed more and more complex and capable single cell microbes, with genomes just too complex for Earth life to have got that far yet?

Also, since DNA is so very particular, and "Rube Goldberg" or "Heath Robinson" in the way it works, what if it the life that evolved in the Enceladus ocean works differently? Enceladus particularly is so far from Earth, and shielded from us by Jupiter, that it may well have had no exchange of life at all with Earth since the very early solar system.

Of all the places we know of so far to look for life, Enceladus and Europa are possibly the best places to look for life that may be totally unrelated to Earth life. Cells based on XNA or other principles. Either of these could easily have cells whose interior "ecosystem" of the thousands of components that make up a cell is totally unlike the way Earth cells work. See Life on Mars dancing to a different tune (above)

Enceladus orbiter proposal

This is an interesting proposal for an Enceladus multiple flyby. The original proposal for the decadal review was for a launch on Jan 28 2023. (21 day launch window). It flies past Venus twice and Earth twice, reaches Saturn July 29th 2031. It enters Saturn with a six month orbit.

The highest delta v is for the insertion into Saturn orbit. After that, it uses many flybys of the inner moons of Saturn to work its way in to Enceladus. First it flies several times past Titan, Then Rhea, then Dione, then Tethys and finally Enceladus. It has several close encounters with each one. So the journey to Enceladus is a very interesting science mission on its own. Finally it ends with an orbital insertion and gets into an equatorial orbit around Enceladus.

• "The Titan tour would include three flybys of Titan and would end with an encounter with Rhea.
• The Rhea tour would include 15 flybys of Rhea and would end with an encounter with Dione
• The Dione tour would include 10 flybys of Dione and would end with an encounter with Tethys.
• The Tethys tour would include 12 flybys of Tethys and would end with an encounter with Enceladus
• The Enceladus tour would include 12 engineering flybys and 10 science flybys of Enceladus and would end with Enceladus orbit insertion

See full study here

It would be great if it could go into a polar orbit around Enceladus, but this is probably not stable so this orbiter can't pass through the plumes on every orbit. Instead it would explore the equatorial regions mainly, and do an occasional flythroughs of the plumes. It would also have a mapping phase for the whole of Enceladus, with orbits at 62 degrees inclination and 184 km altitude, precessing by 360 degrees every 1.37 days as Enceladus orbits Saturn, so it would be easy to arrange to map the entire moon which would take six months (page 9 of the report). It could also do detailed mapping of particular regions to 1 meter resolution, at 50 km altitude, such as the regions around the south pole geysers (ten slow orbits - see table 1-1)

Close up image of Enceladus' north pole. which was hidden in darkness until 2015 due to the Saturn system's very slowly changing seasons in its long orbit of 29 years. This is the opposite pole to the one with the geysers. The Enceladus orbiter would map the whole of Enceladus to a resolution of a few meters and the south pole to resolution of a meter.

So, it spends much of the time mapping Enceladus. But it would do many polar flythroughs of the plumes, about 20 times in total. Finally it ends up in a lower equatorial orbit.

The equatorial region of Enceladus is thought to be ancient, with no connection with its ocean and so their idea is that they could end the mission simply by crashing into Enceladus anywhere in the equatorial region. If that's not acceptable then the mass of the mission needs to be increased to allow for some maneuvering to select an appropriate area of Enceladus to crash into at the end of mission.

I don't see any problems with that so long as there is no chance of the debris from the crash reaching the geysers at the South pole and so long as it's confirmed that the rest of Enceladus is stable on long timescales.

It can capture the ice particles in the geysers with low impact velocities of only 176 meters per second. That's because of Enceladus's low gravity. Europa by contrast has a 1.432 km / sec orbital velocity for an orbiter flying through its plumes, which is about twelve times the orbital velocity around Enceladus, but still very low as such things go (for Europa's orbital velocity, put its radius of 1560.8 km into this online calculator and for Enceladus choose its mass and put in its radius of 252 km)

I see that as the main advantage of an orbiter, that it lets you have low impact velocities, so it is easier to catch microbes and cell structures undamaged. It's also less hazardous for the spacecraft too, if you want to dip lower into the geysers to sample larger particles. Let's compare these speeds with how fast Usain Bolt runs:

(click to watch on YouTube)

He reached a top speed of 12.2 metres per second (see The physics of Usain Bolt's world record 100 meter dash). The impact velocity of 176 meters per second for an Enceladus orbiter is about fourteen times faster than Usain Bolt. Many microbes, would survive it intact, especially in dormant state and cushioned by an impact into aerogel.

This plan has zero chance of back contamination of Earth of course, as nothing gets returned.

In the forward direction, it could have planetary protection issues to consider but they seem rather minimal. Could impacts dislodge microbes that are still attached to the outside of the spacecraft - given that the plume itself falls back to Enceladus? What is the chance they could be viable and hit the planetary surface, and then eventually find their way down to the subsurface ocean?

A radioresistant spore can be viable after hundreds of thousands of years of cosmic radiation even on the surface of Enceladus not protected at all. Though the chance of that may be low, with these ideas of a "super positive outcome", this might mean we have to sterilize the exterior of the orbiter well, if there is a chance of that happening. There doesn't seem to be any significant risk of microbes enclosed inside the orbiter from reaching the surface. Maybe we can even use an unsterilized orbiter.

Another option is to use multiple flybys of Enceladus as well as Titan, Dione etc, with slow encounters at speeds of 1 km / sec with 50 or more flybys of Enceladus and 20 - 50 flybys of the other moons too. Though the flybys won't be quite as slow as for an Enceladus orbiter, it means we don't have to do the awkward polar orbit excursions to do a flythrough of the plumes. We may be able to get more flybys of Enceladus that way too, 50 instead of 20 by their example figures.

Finally, a lower cost mission of a similar type would have only 10-20 flybys of each moon at 4 km / sec similar to the Cassini mission. It would cost less because the total mission time is less and it has less staff to manage the encounters.

For a summary of these options for the decadal review, see here.

Though that's a now dormant proposal, the idea could be revived. Spilker said back in April 2017 that she is going to submit a new proposal for a New Frontiers mission to Enceladus (due in on 28th April 2017).

As reported by Popular Mechanics, she said:

"Here we are getting these free samples out of the geysers of Enceladus, Cassini has flown through those, but we just don't have the instrumentation to look for things like fatty acids, amino acids—the heavier molecules that might be indicators of life. So I'm working on a proposal that would carry the kinds of instruments that you would need to better characterize the ocean and to start to answer the question: Is there life in Enceladus's ocean?"

"If all goes well, Spilker will then have the opportunity to write a step two proposal, and if that's approved, we could see a mission launch specifically to search Enceladus for signs of life, similar to the Europa Clipper mission that will visit the icy moon with a large subsurface ocean that orbits Jupiter.

"Assuming everything is approved by NASA, a new Enceladus spacecraft would take five to six years to build, and eight to ten years to fly all the way to Saturn. Best-case scenario would be a launch in the mid-2020s, and perhaps later, which is all the more reason to get started now."

No details available for it yet AFAIK.

Would we use solar power or radioisotopes for missions to Saturn's moon Enceladus?

You might think that solar power was impossible as far from the sun as Saturn, but actually, as we'll see, no it isn't if you use large panels.

First, why use solar power. The reference study for the Enceladus orbiter for the Enceladus orbiter proposal (above) used three RTGs - Radioisotope Thermoelectric Generators. These are likely to remain the best option if the aim is to maximize the science payload as they produce a lot of power for a small payload.

However, one thing that's changed since that study is that we have a shortage of radioisotopes to use for the RTG's right now and perhaps for the next decade or so. So can we find a way to get there without using RTG's? Well, another option is to use solar panels.

Actually huge solar panels were an option for the Cassini already, perhaps surprisingly, even with the technology they had back then. They found that it would need a total area of 598 square meters (see page 2:56 of this paper). Those are rather huge solar panels, and far larger than anything previously sent into space on such missions. They found that it was possible but would seriously reduce the science payload, and increase complexity etc.

But newer studies with the technology we have today are more optimistic about this approach. We now have solar panels that are reasonably efficient at converting the sunlight into power even in when far from the sun, when the sunlight is low intensity, and they also work at low temperatures too. Rosetta used those panels and so did Juno, which is now operating on solar power in an orbit around Jupiter. Those two missions proved by doing it that the technology works.

So, that's the proposal for the more recent Enceladus Life Finder mission, to use the same technology right out to Saturn.

Juno proved that it works out to Jupiter, at 5.5 au, with solar panels with a total area of 45 square meters, and power output of 414 watts at Jupiter. The aim would be to extend this to 9.6 au for Saturn by increasing the size of the solar panels. The resulting system would mass more than for RTGs, and so would reduce the science payload. But how much would it reduce the payload by?

A 2008 study, on request of Alan Stern, gave some answers to this. It found that it would be possible to use solar panels out to Saturn (10 au), and beyond, even to Uranus if you use 500 square meters of panels, using these new solar panels. Using a reference mission with three RTG's with a maximum power of 335 watts slowly diminishing through the mission.The use of solar power reduced the science payload from 1,000 kg to 550 kg, with a solar power output of 48 kW in Earth's orbit diminishing to 337 watts at Saturn's orbit, degrading to 335 watts by end of mission.

The system would have battery storage during eclipses. It would have four large areas of solar panels covering a total of around 160 square meters once unfolded. Our technology has developed since then, and so may have improved on these figures, but it shows already that solar power is at least possible out to Saturn and beyond. The efficiency of the solar arrays can be improved using concentration approaches - Fresnel lenses or trough type solar power concentrators

Once we have a reasonable amount of radioisotopes for RTGs, we can use those of course, either as the primary source of power, or combined with solar power. NASA is resuming Plutonium 238 production at Oak Ridge, expect to be able to reach the 2 kg a year needed for planetary probes by 2019

To summarize, then the idea of a geyser flythrough mission to Enceladus is well within our technological capabilities, with many avenues we could explore to make it feasible. We don't have to use RTG's at all, though they may increase the science payload once they are more readily available perhaps a decade or so from now.

Suggestion - hibernation with solar power to permit Juno sized solar panels out to the Saturn system and perhaps even further, right out to Neptune?

This is another of my speculative sections, a suggestion of my own.This is for geyser flythrough missions and other missions that require power only for short periods of time, right out to Saturn and beyond. We can use larger and larger solar panels, as the 2008 study showed. But those assume we need continuous power. Often we need it intermittently, only for minutes or hours at a time, so might there be another approach?

This suggestion is based on the proposed 2016 Hera Saturn Entry Probe which would hibernate for most of the time, and gather solar power and store it in batteries for use when needed, including the brief period of entry into the Saturn atmosphere. It just doesn't need to have power all the time, and the science part of its mission is so short, they could do it this way.

Well we are in a similar situation with multiple flyby missions and with geyser flythroughs too. Especially for Enceladus where an orbiter would spend a lot of its time in the most stable equatorial orbit and only occasionally do excursions into the most interesting, but dynamically unstable polar orbits. So, why not do the same for extended missions to Saturn and beyond? The spacecraft could spend most of its time in hibernation, which reduces the solar power required considerably. We could use a combination of battery power with reduced size solar panels.

To illustrate this idea, if we sent Juno to Saturn, with its 45 square meters of solar panels, slightly higher power of 414 watts, and added sufficient battery storage, then at roughly a quarter of the size of the panels normally needed for a Saturn mission, it could use its panels to charge batteries in hibernation mode for around four hours for each hour of power for geyser flythroughs and other times when it wakes up for science observations and analysis.

This approach would work especially well with the Enceladus orbiter because it would have to spend most of its time orbiting Enceladus in an equatorial orbit anyway, and only do occasional excursions to the poles - for those reasons that an equatorial orbit about Enceladus is reasonably stable, requiring only occasional course corrections, while a polar orbit is not.

Also, even if it didn't wake up from hibernation when in an equatorial orbit, that wouldn't be a serious planetary protection issue, because with an orbit skimming not far above the surface of Enceladus, it would just crash into Enceladus in the equatorial regions. Those regions of Enceladus are old and an impact there would be into terrain that has no risk of contaminating its ocean,

So, it could spend three quarters of its time in an equatorial orbit, gathering sunlight and using a bit of power to send its data back to Earth, and hibernating most of the time as it charges its batteries. Then when it has plenty of power, do polar missions to study the geysers. For mapping orbits - then it could just have the power on for one orbit in four, and spend the rest of the time hibernating, so long as the orbit was one that doesn't cause issues for planetary protection. It would take four times longer to do a complete map of Enceladus.

If we also equip it with an ion thruster, like the Dawn mission, then that gives the ability for an extended mission which could take it from Enceladus back out to explore Dione, Titan or the Saturn rings - a bit like a Cassini mission but in reverse. It's primary mission is to study the Enceladus Geysers and its extended mission is to explore the rest of the Saturn system.

Again, for this extended mission, or a multiple flyby mission of any type, with only occasional flybys of Saturn's moons, the same idea of hibernation three quarters of the time might work rather well, waking up just to do the observations during close flybys of its targets, and for processing of the data and transmission back to Earth.

If the lifetime of the power supply and ion thrusters is long enough, it could also have very extended mission redirects to study more distant targets even out to Uranus, Neptune and beyond using the ideas proposed to extend the Cassini mission using the "Interplanetary Transport Network" which is practical for trajectories between gas giants, using very little fuel.

In the case of Cassini this wasn't possible because they didn't know if Cassini would be able to wake up, twenty years later in the case of a cruise to Uranus, 40 years later for Neptune and 12 years later for a redirect back to the Jupiter system. Cassini wasn't designed for this. But if our spacecraft is designed in advance to have a long enough working life to do this, it might be an interesting end of mission option as an alternative to a plunge into the Saturn atmosphere. It could become a long lived spacecraft in the outer solar system for future use for decades.

After all, the Voyager spacecraft are both still functioning and sending back data, limited only by the reduced power of their RTGs. Voyager 2 was launched on August 20 1977, so its extended mission has now lasted over 40 years. Even with Cassini there was some possibility that it could survive those longer missions, it was just that the risk it wouldn't wake up when it arrived was too high, on consideration, and they thought they can get good science return as well as be optimal for planetary protection by sending it to burn up in the Saturn atmosphere.

The same idea of using solar power and spending most of its time hibernating could work right out to Neptune. If it has solar panels similar to Juno's designed for Jupiter then at 30 au, then it would spend more than 99% of its time asleep, but would still be able to do science for 1% of the time.

So, even out to Neptune, with only 1% of full power, then every 100 days it could do 24 hours of science mission which might well be considered a good deal for a very extended "end of mission" scenario. After all, with Voyager 2, they did all the observations of Neptune and its moons, the only close up information we have so far, in a few hours. You'd have the equivalent of the Voyager 2 flyby, but for a full 24 hours, every 100 days.

I haven't taken account of degradation of the panels there, which the 2008 study put at 15% after a decade. But even if they degraded to 50% then it would still give us a 24 hour Neptune mission every 200 days. Or alternatively, a three hour mission every 25 days. Sounds like a good deal for distant Neptune if we don't have anything else there to study it by then.

Or if we use the larger panels from the 2008 study out to Saturn, at 160 square meters for , we could use them out to Neptune to generate 11% of the power it had at Saturn, so our spacecraft could wake up and do its science, or return images to Earth, for one day in nine. If we use its 500 square meter panels as suggested for Uranus, with 400 watts of power at Uranus, then in an extended mission that would work all the way to Pluto with 25% of the power available at Uranus, so waking up for one day in four.

If designed for geyser sampling of Enceladus, perhaps it could also try geyser sampling for the Triton geysers?

Detain from computer animation of Triton geysers by Mark Garlick published on YouTube in 2015. Perhaps this could be a very extended end of mission target for an Enceladus orbiter equipped with batteries and an ion thruster. There is a possibility of life in the subsurface liquid nitrogen of Triton, though of course it would be based on very different chemistry from Earth life. See Life in liquid nitrogen - Pluto or Triton (below) .

We could also achieve long lived extended missions using Americium 241 which retains close to full power on timescales of centuries, see RTGs instead of batteries for the details.

These geyser sampling missions to Enceladus could be precursors for technology we use further afield in dedicated missions sent to Neptune, or even Pluto which may have geysers to investigate in future missions, or even go to those places themselves if they use battery assisted solar power, or long lived RTGs based on Americium 247.

Ice moles or submarines for exploring Enceladus or Europa

Eventually we want to explore the Europan ocean. In 2004, NASA started work on an expensive and very ambitious project to do this with its "Jupiter Icy Moons Orbiter", or JIMO for short, powered by a small nuclear fission reactor (not to be confused with ESA's Jupiter Icy Moon Explorer). It would have used a fission reactor for power (not an RTG) as it needed vast amounts of power by spacecraft standards. It had a design weight 16 metric tons and a six ton nuclear reactor capable of 200 kilowatts of power.

So you are talking about a hefty spacecraft here. By comparison, Cassini has a mass of 2.15 tons. Estimated cost $16 billion excluding the costs of the three launches for its components into orbit (it would be assembled in orbit - for the costs, see page 178 of their final report).

They didn't get very far with it as the mission was cancelled in 2005. However they worked out the basics of it in considerable detail, continuing work that started in 2002, see their final report.

Here is a video showing how they could have sent a submarine to explore Europa's ocean.

(click to watch on YouTube)

Could we do it at a lower cost nowadays? And what is it that makes such a mission so costly and difficult? Well, there's been plenty of work on the idea since then, including work on the less ambitious but still challenging project of ice moles able to melt into the ice to depths of meters, or more, to investigate the geysers and other possible habitats closer to the surface.

In 2012, Steve Squyres, lead scientist for NASA's Opportunity rover on Mars, talking about the idea of sending a submarine to the Europan ocean, put it like this :

"This is fantastic stuff, This is the holy grail of planetary exploration right here...."

"This is one of the hardest missions you can imagine. You need a power system that will enable you to get onto the surface. You then have to some way find your way down through what might be 10 kilometers of ice. And then you have to release some kind of free-swimming vehicle that is able to go down to the bottom of that ocean and find out what's down there."

One issue with a submarine operating in an ocean beneath many kilometers of ice on a distant ice moon is:

"How does it communicate with the surface?"

Scott Bryant has suggested we could use transceivers left in the ice at perhaps 100 meter intervals. Though water is pretty much opaque to microwaves, ice is more transparent - especially if there isn't too much salt in the ice. His paper on the topic from 2002 is here.

Another issue s:

"How can we make it both strong and light enough to be buoyant at such high pressures?"

Carl Ross, an engineer who designs and improves submarines, says that to have reserve buoyancy, it needs a hull made of metal matrix or ceramic composite (2005) (paper is behind a paywall). See also article about his ideas on Universe Today.

More recently Rodrigo Fernandez and Antonio Sanchez Torres explore the idea further in a paper from 2015. They find that a 3:1 ratio of length to beam width would make it easiest to navigate, optimizing the combination of drag and skin friction. At a depth of 96 km it would experience a pressure of around 131 MPa, so about 1,310 times atmospheric pressure, They looked at Al / SiC fiber and found that assuming 60 cm internal diameter, the skin would be 2.7 cm thick.

Then we have the problem of power supply.

"How can we get enough power to it to melt through the ice?"

Here one new idea is Bill Stone's laser powered cryobot. Earlier designs take their power with them. But with his design the power is transmitted to the cryobot with a laser through an optical fiber just microns wide which is also used to communicate with the surface. It also uses the laser energy to heat water to melt the ice in front of it as it penetrates it.

Artist's impression of a probe released by a laser powered cryobot into the Europa ocean. Image: NASA/JPL. See also Robotic tunneler may explore icy moons

This assumes of course that the layers of ice above the cryobot are reasonably stable for long enough for it to function with the potentially fragile optical fiber intact.

Bill Stone estimates that the power needed for his submarine on Europa would be between 250 kilowatts and a megawatt.

Actually, you don't have to have quite as much power as that. It's possible to melt through the ice at much lower power levels, but it takes far longer, or you need to use a smaller submarine, or both. For instance according to one estimate, with a more easily achievable one kilowatt of power, it would take five years for a 20 cm diameter probe to melt through each kilometer of the ice. But that would just take too long, 50 years for every 10 kilometers.

With the same diameter probe, with ten kilowatts of power it would take a few months per kilometer. With a hundred kilowatts then it could melt through five kilometers in a couple of months. See section 2 of this paper.

That's a rather narrow probe. For a 60 cm diameter probe, the estimate is that 1 kilowatt would take three centuries to drill five kilometers. With ten kilowatts that same depth takes less than twenty years and with a hundred kilowatts, then it takes a little over a year to drill five kilometers (reading from their figure 1).They suggest maybe a combination of melting and drilling might be optimal.

These numbers are far above the typical power outputs of RtGs of up to a few hundred watts, but are within the range of small nuclear fission reactors, as for the JIMO project .

There is another approach which is theoretical at present, but perhaps worth a mention (from the conclusion of this undergraduate thesis). That is to use Curium 242 which has a huge thermal heat output to start with of 119 kW of heat power per kilogram but with a half life of only 162.9 days. But it decays into Plutonium 238 so is still useful for power after decay.

This is very speculative, but if some day we could make an RTG based on Curium 242, and if we can get it to Europa within a couple of years of extracting and purifying the Curium 242 then we have 5 kW per kilogram of power available. So, a hundred kilowatts of thermal power would require only 20 kilograms of Curium 242 and could do that melt through of five kilometers in a couple of months. It's just a theoretical idea at present. Curium 242 is produced as the main decay product of Americium 242, which has a much longer half life - and so can persist in spent fuel for hundreds of years. So, perhaps it could be extracted, but it's not being extracted and there are no plans to make RTGs in this way at present AFAIK. I cover this possibility in Other radioisotopes - what about Curium 242 (below)

Other ideas aim to penetrate below the surface, but not as deep as the subsurface ocean. One of these ideas is to use an ice penetrator which smashes into the ice and so can analyse the subsurface. A penetrator could get up to three meters below the surface without having to drill. Also a device like this would be light, and so you could deploy several of them from a single spacecraft orbiter.

Dr. Sanjay Vijendran's Europa Penetrator

Another idea is being explored by Honeybee robotics, Dr . Kris Zacny's Honeybee Robotics Auto-Gopher wireline drill. This is an ultrasonic rotary drill, using vibrations to hammer through rock foundations. It captures a core sample every 10 cm. The system then pulls the drill out of the hole and captures a core sample for analysis. They were able to drill to a depth of 3 meters and were able to penetrate at a rate of 1 meter an hour. The rest of the time, ten hours, was spent lifting and lowering the drill in a test in 2012.

Field testing the auto-gopher wireline drill for Hyoneybee Robotics

Then, there's DLR's Enceladus Explorer's Ice Mole recently tested at Blood Falls in Antarctica, which uses another approach - it's steerable and uses a drill to pull it forward, and steers through differential melting of the ice. this could let it drill into the side of a geyser.

See also their paper on how to deploy their ice mole on Enceladus.

Another fun idea, is that rather than sending a submarine, we could send an "upside down rover" to explore the ice that covers the Europan ocean - from below. After all the ice / water interface is a likely place to be of astrobiological interest. This has been tested in Antarctica, to explore methane seeps from the sea bed. These go up to the ice, and then the convection associated with the bubbles melts the ice right through to the surface (the bubbles don't change the melting point of the ice, they just help transfer heat from the ocean to the ice).

(click to watch on YouTube)

They call it the Buoyant Rover for Under Ice Expeditions, or BRUIE. If Europa has thin ice close to the surface, perhaps an upside down rover like this could be especially useful for exploring its undersurface?

These are exciting missions, and hopefully this is something that we can do in a responsible way, consistent with planetary protection, some time in the not too distant future.

These missions have the same planetary protection issues as the Europa lander, and more so because of the potential of a buried mole or submarine or submerged rover ending its mission far below the surface and contacting liquid water, protected from cosmic radiation and Jupiter's ionizing radiation by a thick cover of ice.

So, if you agree with the idea that we have to do rather better than the standard 1 in 10,000 chance per mission of contaminating these oceans, these ideas probably need to wait until we can achieve 100% sterile spacecraft. At least the submarine and mole would need to be sterile, if there is a chance of them encountering a habitat for life. We do have one idea to do this already, with Brian Wilcox's work on a 100% sterile Europa probe designed to melt its way into the Europa ocean, using plutonium 238 as a heat source (as for RTGs).

Is there any chance of doing it with no risk of contamination, before we can achieve a 100% sterile lander? Unlike the idea of the Europa lander, a static rover, only able to drill 10 cms, these missions are at least somewhat mobile.

If we could ensure a safe landing, they can land a little bit away from the most biologically interesting areas, or separated from them by a thick overlying area of ice. The mole or submarine itself of course has to be sterile as it may contact liquid water. But the rest doesn't have to be.

Still, there's the problem of landing safely. The Europa lander would have a "landing ellipse" like the one for Mars, though they don't say in the report how large it would be. Probably we can't know until the mission is worked out in more detail. The landing ellipse depends on the technology as well as the situation. Mars has rather large landing ellipses for Mars, 20 by 7 kilometers even for Curiosity and for ExoMars, 100 by 15 km (see section Why do spacecraft crash so easily on Mars (above). The main reason for this is because of the unpredictable effects of the Mars atmosphere which varies considerably in density from day to night and through the seasons.

However both Europa and Enceladus have no appreciable atmosphere. Once the landscape is surveyed, then we could in principle land with as much accuracy as for a lunar landing. Perhaps eventually we can achieve confidence in a precision landing, to land in a place we know is safe for purposes of planetary protection - and do it with the original flight path offset in such a way that it can't possibly impact on a sensitive area,

This would need accurate targeting for the landing, and a good understanding of the local geology. We would also need to know that new cracks can't open or new geysers form in the area where the less than 100% sterile equipment is landed. We don't have that yet even for Enceladus. However, I think Enceladus is likely to be the best place for this, because the geysers region is so localized, and because it seems to have none of the other processes that create the Europa chaos regimes or even slow continental drift. If we can know enough about local conditions then perhaps we can land in a long term stable area far enough from the geysers to be okay for planetary protection, but close enough for a sterile rover to go there.

We also need to be confident in the landing technology, that there's no risk of it penetrating the ice to deep below the surface.

If we can be confident in that, we could sterilize any part that contacts liquid water, and make sure that the rest of the equipment is either 100% sterile or can be sterilized at the end of mission or remains in a location that is stable, and will get sterilized by natural processes such as Jupiter's ionizing radiation.. For instance we could send a 100% sterile rover combined with an ice mole to approach a nearby geyser. Or we could drill with a 100% sterile ice mole or submarine from a lander that is less than 100% sterile.

We would have to have a lot of confidence in this though. What if it hits thin ice, or the rocket misfires and it ends up meters below the surface in a crevasse or soft ice?

It may be possible, but I'm inclined to think that the best approach is simply to make sure that one way or another, anything that lands on Enceladus or Europa is 100% sterile as discussed in the section: Can we achieve 100% sterile electronics for an Europa, Enceladus, Ceres, or Mars lander? (below)

I would suggest also that a submarine particularly that can actually explore a subsurface lake or ocean has to be 100% sterile both inside and out. The same would also apply for an ice mole that is expected to explore an area that potentially could have some liquid water such as perhaps the vent shaft of a geyser or designed to explore an unstable region. In both cases, our equipment would be below the surface, so protected from cosmic radiation.

Both a submarine and a mole could potentially be broken up through natural processes at the end of its mission, and contact liquid water before it is thoroughly sterilized of life (and organics like RNA, GTAs etc if relevant) by ionizing radiation or other natural processes.

Safe and easy landings for Europa and Enceladus - “ice breaking” instead of “aerobraking”

We can’t do aerobraking on Europa. However one idea for Europa is a penetrator, using what we could call "ice breaking" to slow it down as with Dr. Sanjay Vijendran's Europa Penetrator. If we can do 100% sterile electronics and instruments, then this could be one of the easiest ways to find out about the Europan subsurface. Especially if it has liquid water below thin ice.

This assumes that by the 2030s we may have the technology to sterilize a spacecraft 100% without destroying the electronics. I hope so! The easiest way to achieve that might be to use tiny spacecraft.

Chipsats for Europa - could they be 100% sterilized?

This is another interesting idea, an mission proposal from 2014 to send “chipsats” to Europa’s surface, each one rather “dumb” but lots of them, each one consists of just a few sensors on a flat chip. Some would fail but enough would get through to be useful. They would be able to survive impacts that a larger more complex lander couldn’t. It would be able to react quickly to events on the surface, by sending chipsats down to investigate. It could send hundreds of them to regions where the liquid water is emerging, spacecrafts on a chip.

My own suggestion again, it also sounds like a great way to deal with the rough Europa surface, especially if it is rough on a meter and sub-meter scale. Drop lots of small chip sats and even if most of it is too rough to land a normal spacecraft, some will find a smooth spot. Even if there is nowhere for a large spacecraft to land, they may find somewhere.

That sounds like a kind of a lander that is so minimal, perhaps it could be 100% sterilized by supercritical CO₂ snow or something similar?

Though CO₂ snow proves a bit hard to scale up to a large spacecraft, it's shown its ability to remove all the organics from the surface of an electronics chip without damaging the chip. It’s been shown to work with USB drives. I wonder if it is good enough to 100% sterilize chipsats? It would have to be 100% reliable. I discuss it in Deep cleaning with supercritical CO₂and CO₂ snow (above).

"ATTEMPT NO LANDING THERE" --New NASA Mission to Europa will Ignore Arthur C Clarke's Warning (2014 Most Popular)

Two dimensional planetary surface landers

Another idea is to use flat and flexible two dimensional landers. They would use the same technology used for flexible thin film electronics in ultra thin laptops etc. They have the advantage that you can stack many of them into a small space. But like the chipsats and more so, surely they'd also be much easier to sterilize.

Artist's concept of flat landers deployed on Mars

Because they are "bendy", they can actually move around on the surface by including actuators in the sheet. They could be made of Kevlar woven fabric, as it has strength and resilience down to cryogenic temperatures. They could be covered with Mylar to protect the Kevlar from UV. They could be powered by printed circuit board fuel cells or ultra thin flexible solar cells, or through power beaming from another orbiter or rover.

They can communicate using either a flexible RF transmitter, or else, though with a lower data rate, via modulating laser retroreflector array which would be interrogated by the mother ship - the idea is that it changes its reflectance a bit like a bar code for a bar code reader, except one that's continually changing. In this way it could communicate with n orbiter or rover that beams a laser light at it. It can keep warm through a radioisotope heater unit if necessary.

Many instruments can be included too. These include 3mm diameter imaging cameras, or an imaging array for 3D video. Spectrometers and gas sensing films.

It's even possible to add a really tiny mass spectrometer, ground penetrating radar with a range of 50 meters with 15 meter resolution, and a miniature laser based dust and particle analyser. They are talking about "off the shelf" commercial chips here.

Two alternative miniature mass spectrometers that could be sent to the Europan surface on a chipsat,

It's from their Table 8-3. Examples of highly capable science instruments that can fit within the 2D lander, see page 32 of the paper.

They also had plans to develop new miniature instruments such as a miniature gas chromatograph, and drills and penetrators that can be deployed from flat landers. For details see this article from 2014.

Origami rover

This is an idea that is in between a 2D and a 3D rover, and again perhaps may be easier to sterilize. It is an idea for a scout “origami rover” the size of a smartphone. Spaceships and rovers could deploy it to scout out places where it’s too dangerous for the main rover to go, or get to places they can’t reach (because it's so much smaller, and also good at scaling steep slopes and dropping into pits).

A larger rover could also deploy several of these origami scouts, to scout its neighbourhood, to see which is the most interesting place to go to next.

It can climb 45 degree slopes, crawl into overhangs, and drop into pits and craters. Because it can be folded flat, you can stack several copies one on top of each other a bit like a pack of cards. They can do parallel science increasing the amount you can do in a day. It can survive a fall of three meters on Mars so that would be about 6.86 meters on the Moon and 8.47 meters on Europa.

This is what it is like in the tests:

(click to watch on YouTube)

It has “whegs” instead of wheels - a kind of cross between a leg and a wheel that lets a rover traverse much rougher terrain than you can with a wheel.

It can travel quite long distances too as in this speeded up video of a 250 meter “hike”

(click to watch on YouTube)

It can travel about 2,050 feet (625 meters) on one battery charge on a flat dirt path like that.

This shows it climbing a steep slope - it can handle up to 45 degrees:

And slip into cracks

So far they have tested it in Rainbow Basin California, at a ski resort in Grand Junction Colorado, Big Bear California and on Mt Erebus in Antarctica.

They plan to field test them in the Mojave desert in the near future. PUFFER’s body inspired by origami is due to Jaakko Karras, at JPL who is also the project manager.

Its body is wrapped in Nomex, the same textile used for the air bags that cushioned the NASA Spirit and Opportunity rovers when they landed on Mars. The Nomex is integrated into the folding circuit boards and it helps protect it from high temperatures as it repels heat. It’s able to function in the Mars temperatures of –135 to 30 ˚C without a heater

The next stage is to find a way to integrate instruments into it. For instance to sample water for organics or add a spectrometer. It is a “Game Changing Development” program managed by GCD, and is part of NASA’s Space Technology Mission Directive. For more about it: Origami-inspired Robot Can Hitch a Ride with a Rover, PUFFER Prepares for Field Testing, JPL Robotics: Research Tasks, NASA's Adorable Pop-Up Rovers Are Designed to Explore Harsh Alien Terrains. If you want to see more videos of it, I did a page on quora here with the YouTube videos embedded into the page.

Can we achieve 100% sterile electronics for an Europa, Enceladus, Ceres, or Mars lander?

There’s nothing in principle to prevent 100% sterile electronics. You just have to find some process that electronics can withstand and life can’t.

If you heat metal to hundreds of degrees centigrade, no life will survive and the result will be 100% sterile. The problem is that this will destroy most spacecraft electronics too. So can we find a way to sterilize it of Earth microbes without destroying the delicate equipment? That’s the big question here.Also all this might be far easier to do with a chipsat than with a large conventional spacecraft.

The idea of deep cleaning with supercritical carbon dioxide might be just the thing for smaller spacecraft like the origami rover and chipsats. For those we could use the pressure vessel approach where you infuse supercritical snow throughout the whole thing at moderate temperatures and high pressures. Organics dissolve into the carbon dioxide, then you release the pressure and exhaust it as snow, which takes the organics away and leaves the electronics dry and free of impurities. For details see the first of the two methods I discuss in Deep cleaning with supercritical CO₂and CO₂ snow (above).

What about larger landers? Could you remove all traces of organics from the exterior of a lander in this way? And - can you also keep exterior and interior separate so there is no chance of leaking contamination from inside the mole? Or use deep cleaning for the interior as well?

The carbon dioxide snow has one additional advantage if you use it on Mars - you can also source the carbon dioxide from the atmosphere, and so, you could do in situ cleaning after you land there.

High temperature sterilization

This next idea is amongst the simplest. If you can make the whole thing able to withstand high temperatures, you can just heat it up to a high enough temperature to sterilize all life. The main issue with sterilizing modern spacecraft is that many instruments are quite delicate. They can be damaged or go out of alignment and even the sterilization temperatures used for Viking of 111 °C for 40 hours is too much for them. But there are electronic circuits now designed to operate at up to 200 °C . High-Temperature Electronics

Also, there are other developments that should permit temperatures of 200 °C upwards..High-Temperature Electronics Operate at 300 °C | EE Times and Designing for extreme temperatures. NASA is especially interested in this technology for exploring Venus and in 2007 it developed a silicon chip capable of 17,000 hours of continuous operation at 500 °C.

There’s an economic incentive on Earth too for developing these electronics, as they are useful in oil wells, aviation and motor cars.

• Oil wells now drill deeper than before. The temperature goes up by 25 °C for each kilometer of depth. Now that the shallower oil fields are exhausted, they now drill in "hostile wells" with temperatures in excess of 200 °C and pressures in excess of 25,000 psi (over 1,700 atmospheres). Cooling is not practical in those conditions so they need electronics that can work at those temperatures.
• Planes can save weight if they place electronics close to the engines, reducing complexity, increasing reliability and reducing weight, and if the electronics don't have to be cooled, it helps with cost, weight and most important, reliability (don't need to worry about what happens if the cooling system fails). Again this requires electronics able to function at 200 °C
• Electric powered cars can be simplified, more reliable, less weight, less cost, in a similar way if the electronics can be placed right next to the engine components at high temperatures.

It's a case of back to the drawing board probably for a lot of the designs for instruments. They need chips, solders and other components that work up to high enough temperatures for 100% sterilization. But it seems like it may be possible!

It is not a new idea for spacecraft. So far, it's been explored mainly for Venus surface missions. This is for ideas of a rover to operate at Venus surface temperatures for long periods of time. One option is to cool it to 200 - 300 °C. Another is to design it to run at 500 °C with no cooling.

For 200 - 300 °C they say (already back in 2010) (page 16)

"1) Medium-Temperature Semiconductor-Based Electronics: Medium-temperature (200–300°C) electronics are not only technically less difficult than electronics that operate at Venus surface temperatures but also have terrestrial commercial applications. A broad set of component options, including microprocessor and memory devices exist. "

They go on to discuss high temperature electronics capable of operating at 500 °C saying that the level of technology was similar to that of early 1960s development of silicon electronics. But we wouldn't need that if the aim is only to make sure it can be 100% sterilized of organic life using heat treatment. Since then there's been work on high temperature electronics including silicon carbide integrated circuits able to function for weeks (521 hours) at 460 °C - it was also exposed to the high pressures and sulfuric acid of the Venus atmosphere. They used the Glenn Extreme Environments Rig. Here is a summary of the research at Phys.org.

The great thing about high temperature electronics also is that the whole thing could be sterilized. Instead of the present system where the computer section has to be isolated from the rest of the spacecraft with filters to keep the microbes inside it, the whole spacecraft can be 100% sterile.

Then as well as that - if the lander is as robust as a Venus lander, we can heat it in space too. We could have a small oven and heat the lander to 300 °C in the vacuum of space on the way out to Europa - or in LEO or wherever is convenient. Even have a separate "autoclave" type unit in LEO which we use to sterilize our spacecraft before they leave Earth.

Brian Wilcox is working on a 100% sterile probe to descend into the Europan ocean. It would have vacuum insulation like a thermos flask, a blade that cuts ice chips that the body then melts and analysed. It would be heated to over 900 °F (500 °C) during its cruise to Europa which would not only kills microbes but also decomposes organics that would confuse the results.

Vacuum insulated probe for Europa (screenshot from this YouTube video) - it doesn't heat the ice directly. Instead a blade at the tip cuts the ice into chips which the probe then melts and analyses. The probe would be heated to over 900 °F (500 °C) throughout the cruise out to Europa. It uses plutonium 238 for the melting - and so, presumably for its power source too, so there is no problem with batteries vulnerable to heating.

He describes it in a paper here (abstract, the paper itself is behind a paywall).

In his abstract he says

"A central thesis of this work is that we must start by addressing the Planetary Protection constraints, and not to try to add them on at the end. Specifically, all hardware in the probe would be designed to survive heat sterilization at 500 °C for extended periods, as required to meet the COSPAR 1-in-10,000 probability per mission of biological contamination of the ocean"

The NASA summary says

"To ensure no Earth microbes hitched a ride, the probe would heat itself to over 900 degrees Fahrenheit (482 degrees Celsius) during its cruise on a spacecraft. That would kill any residual organisms and decompose complex organic molecules that could affect science results."

So, he doesn't actually say 100% sterile in the abstract, though the summary implies that it is. Anyway, surely after a three year cruise at a constant temperature of 500 °C, there would be no viable life on it, and organics decomposed too.

It's just the probe that's sterile here, not the lander. But combine that with ideas for a Venus lander and we may be getting close to a 100% sterile complete system.

The first such lander and rover would be expensive until we figure out the designs. But from then on, it could just be a standard part of every mission to a vulnerable place in our solar system - like areas of Mars with surface liquid brines or even fresh water under clear ice - and Europa, Enceladus, Ceres etc. There will probably be many such missions eventually. So why not "get it right" now so we can do them all without any risk of contaminating them with Earth life? At least - not until we have had a chance to study them properly first to see what the effect would be of introducing Earth life.

I can see some problems. What about the organics and other sensitive components of the experiments themselves? The chiral labeled release needs to have chiral amino acids as food. SOLID3 needs its polyclonal antibodies.

Perhaps we have to restrict the on site instruments to more robust instruments and leave those to later? Or we could use a sterile rover or lander to collect a sample and return it to less sterile instruments at some distance from the site of interest or in orbit. With Europa's low gravity and fairly low orbital velocity especially with Enceladus, with hardly any gravity and very low orbital velocity, it might be relatively easy to return samples to orbit for the less well sterilized instruments to study.

Thanks to Adeel Khan for suggesting this in a Quora comment. At that time I didn't know that Brian Wilcox was working on it already. But the idea doesn't seem to have had a lot of attention so far, that's the only example I've found to date. Anyone know of anyone else who is working on using heat for 100% sterile spacecraft?.

Multiple methods at once - e.g. 200 °C or higher to sterilize, CO₂ snow to remove organics

Perhaps for the best results, to achieve 100% sterile spacecraft, both can be used one after the other. High temperatures for a long time to make sure there is nothing viable and remove most of any remaining organics. Then finally, CO₂ snow (see above) to remove any remaining organics as far as possible, because even fragments of DNA could change an early form of life that has never seen DNA, or GTA's. could transfer capabilities if it has some common ancestor billions of years ago.

All of this might be especially easy to do at an early stage for electronics in an impactor / penetrator type design. That's because it would already have to withstand high g force and perhaps high temperatures too. So it would need to use specially hardened electronics already.

A rather more far out idea - 3d printer on Europa plus raw materials for some of the components

Another idea, just for fun for now - but: 3D printers are getting very capable. What about sending a sterile 3D printer + some raw material feedstock for it, also sterile? The surface conditions would be high vacuum, ideal for electronics. The first thing it does is to 3D print a shelter for itself or dig below the surface for protection from the cosmic radiation and Jupiter's ionizing radiation (if on Jupiter's Europa). Then it sets about printing out whatever you need, including a Europa submarine from the sterile components you supplied it with. If it is a nanoscale printer it can do circuit boards as well - I mean just the board, not the chips. So all you need to do is to send it some sterile chips to attach to those circuit boards, and send other hard to print out components pre-sterilized. Most of the rest it does itself.

This is a bit far future perhaps. In particular though we have 3D printers, we don't have automatic ways to assemble the components once made. But perhaps some element of 3D printing could help for an idea of partial in situ construction of devices for helping to study Europa in a sterile way? Especially small chipsat type devices. Sterile electronics plus 3D printing of some extra components to help with mobility or sampling or some such.

If we can’t achieve 100% sterile landers for Europa

If we can’t do it, I think we simply should not send a lander or submarine to Europa until we can. Especially, we just should not risk introducing Earth microbes to a habitable environment on Europa. It is just risking too much to do that. Not just for us, not just for the mission that goes to Europa right now, but for our descendants and indeed all future civilizations on Earth also. It would be just tragic to find some interesting form of exobiology on Europa only to know that we have seeded Europa with microbes that will eventually make it extinct.

It could be very vulnerable to Earth life. The example I like best there is the idea of some primitive early life, for instance RNA based, or even an RNA ocean or autopoetic cells. If Europa was like that, then introduced Earth microbes in a globally connected ocean through exponential growth would surely do short work of converting it all to DNA based life.

Each target as "our only chance" to find something unique

I suggest that until we know more about the targets, and whether they are rare and unusual or commonplace and uninteresting, we should approach each potentially habitable target in our solar system with the view that this may be the only chance we have to find out about something unique.

For instance, let's sketch out one example scenario.

• Mars is the only planet with life which is related to Earth, and has RNA world cells that are precursors of Earth life.
• Ceres has a subsurface muddy ocean with autopoetic cells
• Enceladus has life that is only a million years old but is independently evolved, and not based on RNA or DNA
• Europa has an advanced form of XNA based life with multicellular creatures and even perhaps intelligent beings and a civilization.

You can try many other scenarios. But for all we know these locations may all be precious and we learn different things from them.

For instance, suppose that of those four, Europa is the only one with life - there is no way we can duplicate the billions of years of Europa’s history and the vast oceans larger than Earth’s oceans. If we mess it up, then the nearest “Europa” analogue may be light years away. And even then, chances are that if Europa and some Europa analogue light years away both have life, then most likely it has its own unique lifeforms, and may not even have the same informational biopolymer in the place of whatever Europa has - not at all likely that it has the same lifeforms or proto life that evolved on Europa. And whatever there might be light years away, it's beyond our reach at present for study.

You can try that same argument for any of them. Mars, Ceres, Enceladus. Any of those also could be the only one with indigenous life, or something else as complex and wonderful and unique as that.

Until we study these habitats there is no way to rank them or to decide if one is interesting and another is not. We just can't know how precious or otherwise any of them is until we find out more.

This is primarily an ethical decision, so it can't be decided by pure scientific methods alone. It requires a moral assessment of acceptable levels of risk.

Why not just send Earth life there?

Some enthusiasts suggest we just send life to Europa to seed it with Earth life. The problem with this idea is that then we won't be able to find out about the life that is already there, if there is any - or pre-biotic or non biotic chemistry - or whatever there is there right now.

Suppose that our Earth microbe colonizes an RNA ocean say, or an ocean of autopoetic cells or RNA world cells. The whole thing could be taken over with anything native extinct after just a few years of exponential growth after the first contamination by Earth life throughout the entire ocean, if it is connected and its ocean has food sources for the life to use throughout its ocean, which is not impossible. Going back to Carl Sagan's example suppose we have a single microbe and it doubles its population only once a month (it might have a doubling time of hours depending on the conditions in the Europa ocean). After ten years of unrestricted growth, in theory you could have 2¹²⁰ or 10 ³⁶ microbes. Europa's ocean may be two or three times the volume of Earth's. Supposing Europa's ocean has a volume of 3×10 ¹⁸ cubic meters, nearly three times the over a billion cubic kilometers of Earth.Then with exponential growth, that would mean 333 billion Earth microbes per cubic centimeter of its ocean after a decade.

Of course we wouldn't expect exponential growth all the way through the decade after Europa was first contaminated by an Earth microbe. However, that may help give some idea of how rapidly the oceans could be colonized by invasive microbes from Earth, if the Europan life or proto-life is so naive that it puts up no resistance at all to the Earth microbes. But, however quickly or slowly it happens, there is no way we could reverse something like that once it got started. It would be the worst possible anticlimax to our searches for life in Europa's ocean, to know that Europa was such a biologically fascinating place, until the first probes from Earth landed there, and is no longer like that, or will lose its unique biology in the near future. This is not science fiction. It may be a possible future for us.

Until we know what's in these places, I think we absolutely have to treat every potentially habitable planet or moon or other habitat in our solar system as if it was the only one of its type in the solar system. A lifeform that evolves in Europa's ocean may well not evolve in Enceladus, or Ceres or on Mars or whatever place you study next. Each one could be our only opportunity for light years in every direction, to study such a lifeform, or indeed, each one might be our only chance to study any form of exobiology at all apart from an abiotic ocean of life precursors.

Any of these things, and more could be true:

• All have different unique lifeforms or types of pre-biotic life.
• Life from a previous star seeded them all or most of them, so they are all closely related
• Mars life is closely related to Earth life, none of the rest are.
• Only one of them has life apart from Earth - which could be the one we are about to explore next.
• None of them have life apart from Earth.
• Many other possibilities.

So long as they have organics and habitable conditions, they are sure to have complex chemistry and we can learn from that also. If none have Earth life, maybe we learn that life evolves only with great difficulty and find out what happens when it doesn't evolve in many different conditions. That too could be interesting and significant. We won't know until we find out.

Plenty of places to experiment with sending life to other places in our solar system - Asteroid belt resources, NEO's, caves on the Moon

As for experiments in adding Earth based life to unusual environments in space - we can use closed system habitats to try that out anywhere we like, throughout the solar system.

For instance the Moon may have vast caves kilometers in diameter, so maybe we do it there. Or we use free flying space habitats. There's enough material in the asteroid belt alone to create habitats with a total land area a thousand times that of Earth. Not living on the asteroids, but using the materials from the asteroids to create habitats spinning slowly for artificial gravity, and with access to concentrated sunlight from the Sun with huge mirrors to concentrate the sunlight, and so on.

Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths

Some asteroids consist almost entirely of pure metals including iron, nickel, and heavier metals such as the industrially useful platinum, and gold (a small asteroid 452 by 1011 meters across, to take an example, was estimated to contain 90 million tons of platinum). These metals may also be capable of being extracted without physical mining using carbon monoxide at low temperatures of 50–60 °C to convert metals to gas (second half of the Mond process with no need to extract the pure metal first), and perhaps directly converted back to metal parts again from the extracted metal carbonyls in 3D printers operating at the higher temperatures of 220–250 °C. They would be valuable exports and a reason for being in space, and also useful for constructing the habitats.

Other asteroids have organics, volatiles, and everything that we need.

The conclusions of the 1970s are still valid, that there is enough material in the asteroid belt to eventually build habitats with area the equivalent of a thousand times the surface area of the Earth or more.

This is an observation that goes back to O'Neil in 1969 when he was teaching freshman physics, see Colonies in Space: chapter 2. So there's far more potential for settlement in the asteroid belt than there is on either Earth or Mars, measured according to the available land area. And what's more, you can choose whatever climate and even atmosphere, and gravity level that you like for the habitats.

As Thomas Oppenheimer puts it, summarizing the results of O'Neil's students when they first began to study this, in his Colonies in Space:

"The next question the students considered was how much land area could you build in such habitats, using the material resources of the moon or the asteroids, which may be readily available in space? What are the limits to growth?"

"The first answers they came up with indicated there was more than a thousand times the land area of Earth as the potential room for expansion. They concluded that the surface of a planet was not the best place for a technical civilization. The best places looked like new, artificial bodies in space, or inside-out planets."

"The classical science-fiction idea, of course, is to settle on the surface of the moon or Mars, changing the conditions there as desired. It turned out that there were several things wrong with this, however. First, the solar system doesn't really provide all that much area on the planets—a few times the surface area of Earth, at most. And in almost all cases the conditions on these planets are very hard to work with."

"The idea of using a planet to provide gravity and to hold an atmosphere really represents the hard way to go about doing these things. Really tremendous amounts of material must be collected, enough to make a planet 5000 miles in diameter, before there is enough gravity to hold down an atmosphere and keep it from leaking into space. Even Mars isn't quite big enough—its atmosphere has almost entirely leaked away. Ever since Wernher von Braun published his space-station articles in Collier's over twenty years ago, people have been aware that a few tons of metal will suffice to build such an inside-out world, to give gravity and an atmosphere."

"Also space is not an empty, hostile environment. It is a culture medium, rich in energy and in the resources needed for life. An artificial world in space gets solar energy full time, without the day-night cycles and the atmospheric absorption of a planet. Further, planets have strong gravity fields against which a spacecraft must fight. The earth's gravity is strong enough to have the same effect as a hole, 4000 miles deep, out of which we must climb. If we wish to colonize the surface of another planet, we are just climbing up a deep hole, passing through the sunshine of space—and then going down another hole."

(emphasis mine)

Though that wasn't his motivation, habitats like that are also far more planetary protection friendly too, as the habitats can be made anywhere, from anything, and by choosing where to source the materials to make the habitats, we can protect places like Mars, Europa etc. With sterile telerobotics we could even have exports from places like Mars to these habitats while keeping Mars free of Earth microbes, if that turns out to be a necessary and valuable thing to do.

Let's look a bit closer at this idea of habitats made from the materials of asteroids in the "sunshine of space" in Oppenheimer's words.

Space habitats made from asteroid and comet materials get plenty of sunlight - right out to Pluto (using thin film mirrors to concentrate it)

You might wonder how we could build space habitats in the asteroid belt. Wouldn't they be too cold, that far from the Sun, further even than Mars? They would be outside of the "habitability zone" of our sun after all.

Well first of all they have one big advantage, that the get sunlight predictably, 24/7 - while on Mars of course you get no sunlight at night or in the dust storms. A space habitat at the distance of Mars already gets twice the amount of solar power of a habitat on the Mars surface, and what's more, gets power at night too, directly from the sun, without need for batteries. The ISS has to have batteries to withstand a few minutes of darkness at certain times of the year depending on its orbit. But it never has to endure twelve hours of darkness, without solar power. A habitat in a more polar orbit, or further from the nearest planet or moon, would never be in darkness at all.

But as well as that, you can concentrate the sunlight using large thin-film mirrors. They require a fraction of the mass and technology needed for the habitat itself. So actually, this is not a restriction at all. The space habitats could migrate inwards from the asteroid belt, but they could also migrate outwards. Even right out to Neptune and Pluto and beyond.

As the authors of the Stanford University 1975 publication "Space Settlements: a Design Study" wrote,

"At all distances out to the orbit of Pluto and beyond, it is possible to obtain Earth-normal solar intensity with a concentrating mirror whose mass is small compared to that of the habitat.”

So once we have this capability, we can make habitats not just from materials in the asteroid belt, but also further afield, from Jupiter trojans, for instance, and even trans-Neptunian objects and Kuiper belt objects, the entire solar system out to Pluto and beyond is open to us.

The diameter of the thin film mirror would go up linearly according to the distance from the Sun. Pluto is 39.5 au away - so what might be a one kilometer diameter thin film mirror in Earth orbit would deliver the same amount of sunlight to the habitat if it is 40 km in diameter out at Pluto. That's a seriously large mirror, but probably not beyond our capability. Or more likely, lots of smaller half cylinder mirrors, such as are used for a solar furnace. These are easier to design than the sort of mirror you have for a telescope, and effective, focusing sunlight on a focal line rather than a point, and focusing only approximately (or we can make them with a parabolic cross section to focus on the focal line more exactly)..

Such large areas of thin film mirrors pose a major challenge of course at present, but we are talking here about a future where the technology for space habitats is well developed. Some optimistic ideas for terraforming Mars involve huge mirrors in orbit around the planet with total area for the thin film mirrors similar to that of the planet itself. These huge mirrors to reflect light into a habitat out beyond Neptune are relatively small by comparison.

Closer to home we could build habitats in the Jupiter system, outside of the radiation belt.

We could also colonize the distant Oort cloud too, eventually venturing more than a light year from Earth, but out there, the mirrors would be vast, 63,000 km in diameter for a mirror to generate the same amount of sunlight at a distance of one light year as a one kilometer diameter mirror at Earth's orbit. Such a large mirror would be about five times the diameter of the Earth. So we would probably need some other form of power such as fusion power at that point, if we do ever build human habitats that far from the Sun.

With these prospects available to us, we don't need to have as our first priority to turn everything into the closest possible approximation to Earth we can imagine, especially a very poor imitation of it. Do we really need to create a new ocean with Earth microbes in it which is:

• covered in kilometers of ice
• humans on the surface would face the deadly conditions of Jupiter's radiation belts.
• too far from the sun for most photosynthetic life to be practical
• no photosynthesis at all in its oceans (except for life that uses the heat radiation from hydrothermal vents for photosynthesis).

Meanwhile constructed habitats from asteroid materials can be designed with whatever environment you like. Just by adjusting the mirrors and shades, they can be tropical gardens in climate if you like, depending how much sunlight you reflect into it using space mirrors or solar collectors. Or, for those who wish it, you can simulate conditions on Europa or Mars or other places in our solar system if that's your aim. Or you could simulate some the conditions on an interesting exoplanet.

You can also use spinning habitats with artificial gravity for whatever level of gravity you want, too.

That's looking forward a bit there - but only decades, centuries at most. They concluded already in the 1970s that we could build a Stanford Torus habitat within a decade or two with the funding and political will to do so even with 1970s technology. If we want to explore setting up habitats with Earth life in it outside of Earth, I think things like that would be the way to go - starting on a much smaller scale first probably. You could start with small exovivaria in LEO or on the Moon, and experiments with closed system recycling.

See also:

• Instead of terraforming Mars in a multi-millenium project, why not terraform a lunar cave in a multi-decade project?
• Best places to introduce Earth life right now - while continuing with biologically reversible exploration of Mars

Sending humans to Callisto or Ganymede

In the shorter term, and for those who prefer to live on a planetary surface, we can build habitats on the "moon planet" of Callisto. This has no planetary protection issues as far as we know, and it has resources of value to us too.

Artist's impression of a human exploration base on Callisto, the outermost of Jupiter's four largest moons, and its second largest moon after Ganymede. Image from page 22 of NASA's vision for space exploration, 2004.

For details of the mission design, see this paper. They explain their choice of Callisto:

"Callisto—the second-largest Galilean satellite and the most distant from Jupiter. Scientific interest is prompted by the possibility of subsurface water. Callisto’s distance from Jupiter places it in a significantly less hazardous radiation environment than Europa, potentially permitting human surface operations.

..."Callisto is a desolate, minor planet with more impact craters than any other object in the solar system. Its crust dates back 4 billion years, which was around the time that the solar system was formed. This ancient surface may be able to provide valuable information about the state of the early solar system. In addition to scientific information, Callisto will also be able to provide mission resources; e.g., propellant for the mission lander."

Later on with its abundant ice resources, perhaps Callisto could be a "refueling stop" in the Jupiter system? Elon Musk suggested Europa as a "fueling stop" but that has obvious planetary protection issues and it is also deeper in Jupiter's gravity well.

Elon Musk showed an artist's impression of his Interplanetary Transport System on Europa as part of his visionary speech in 2016. This is a spaceship he plans to build large enough to hold a hundred colonists or explorers at a time. He said that it could use Europa as a refueling stop in the outer solar system. Well Callisto is just as good a refueling stop, perhaps better, and it doesn't have the planetary protection issues of Europa.

Actually his artist's impression here perhaps more closely resembles the surface of Callisto, as Europa's surface is probably very rough on the meter scale (at least on current understanding) and not nearly as smooth as it seems from the distance. Also Callisto has a surface of ice - see the artist's impression for Callisto below, it's not dissimilar.

Callisto orbits just outside Jupiter's hostile radiation belt (the other Galilean satellites all orbit within it) and has a thick surface layer of ice like Europa.

It's dark in colour and clearly has a significant amount of non ice material on its surface as well, which could be useful as resources. According to one study, Callisto should have accumulated a thick surface layer of between 120 and 140 meters of organics and other materials from carbonaceous chondrite asteroids and comets that got captured by Jupiter as irregular moons.

It has a thin "atmosphere" of carbon dioxide, so thin that it's really an exosphere with the molecules seldom colliding. However even such a thin atmosphere is impossible unless it is constantly replenished. This could be the result of ions bombarding organics on its surface and decomposing them. It could also be the result of outgassing of primordial materials - like the outgassing of carbon dioxide which we get from comets. See this paper for details.

It does have a subsurface ocean, scientists believe, but as far as we know so far, the surface is totally stable, unchanged for billions of years, and with no planetary protection issues involved.

It's a "Category II". "Of significant interest relative to chemical evolution and the origin of life, but where there is only a remote chance of contamination" This means it is similar in planetary protection status to our Moon, so there shouldn't be significant planetary protection issues sending humans to its surface.

Constructing facilities for humans on Callisto with "Robonauts" to help. This is their "large scale" concept with two unpressurized "bulldozers" and three robonauts. They envisioned a crew of six, at least three land on Callisto, a round trip of three to five years from Earth to Callisto, and one of the things the crew would do is to teleoperate a Europa submarine remotely from Callisto. They would be useful for teleoperation of any robotic spacecraft in the Jupiter system indeed. They'd also study Callisto.

In this 2003 study, NASA also worked on a spacecraft design for a mission to Callisto, in this paper.

(click to watch on YouTube)

Much of the details of that idea will be dated now of course, fifteen years later on.

This summarizes the reasons for choosing Callisto in the HOPE study.

"The key requirements considered in selecting a worthy exploration destination beyond Mars were

• (1) the opportunities for conducting interesting science and
• (2) the availability of in-situ resources to support a human mission.

The body chosen for the HOPE study was Callisto, the third largest satellite in the Solar System, and the outermost Galilean moon of Jupiter. Orbiting at a distance of ~1.9 million kilometers, Callisto is located beyond Jupiter’s main radiation belts making its local environment more conducive to human exploration. Callisto is an icy, rocky world with a surface gravity of ~0.127 gE and a composition consisting of water-ice and rock in a mixture ratio of 55:45.

Besides having significant quantities of water-ice for propellant production, Callisto’s heavily cratered and ancient landscape (~4 billion years old) has a relatively low albedo indicating that significant quantities of non-ice materials and asteroid dust may reside on its surface."

(bullet points added and paragraphing)

This is Erik Seedhouse's book about a human mission to Callisto.

If we send humans to Callisto, or use habitats built in Jupiter orbit using local materials, then we have an interesting and useful destination for humans in the Jupiter system. It's a "refueling stop" for the journey home or indeed for voyages within the Jupiter system and beyond, a local base for telepresence exploration of Europa, Io and Ganymede, easy of access and protected from Jupiter's ionizing radiation. What's not to like about that?

There's another place we can send humans in Jupiter's system (apart from its numerous smaller moons, many orbiting beyond the radiation belts). Jupiter's largest moon, Ganymede, like Callisto, is "provisional category II" which means it needs more research.

It's more varied than Callisto, with a mixture of terrain that is very old and terrain that is younger. However it seems to be no longer geologically active. It orbits within Jupiter's radiation belt so it has high levels of ionizing radiation/ But it is somewhat shielded because it has a magnetosphere.

Ganymede gets 8 rems a day, meaning that in a single day on Ganymede you receive as much radiation as you do in 333 years on Earth. That's a lot compared to Earth. But it's much less than Europa, with its 540 rems a day (500 rems is considered fatal for humans).

Ganymede has abundant resources of silicates and iron, which might be enough of an advantage over Callisto to make it attractive. Bases on Ganymede could be built underground to protect from ionizing radiation, and as for Callisto, there's abundant ice for water and fuel. Ganymede is provisional category II so more work would be needed to check whether there are environments there where life from Earth could survive and to check that there is no significant risk of contaminating them if there are.

The Lifeboat Foundation did a study: Colonizing Jupiter’s Moons: An Assessment of Our Options and Alternatives and concluded that Ganymede and Callisto were the two main alternatives in the Jupiter system.

Why there's probably no need for planetary protection for humans to Callisto, even with a subsurface ocean

The idea that Callisto could have a subsurface ocean , like many of the ice moons was first announced in Nature in October 1998 .This seems generally accepted now. However, they think there is no connection between its subsurface ocean and the surface. That's mainly because of the appearance of its surface - it seems ancient and cratered with nothing to suggest cryovolcanism or tectonic plate type movement:

Photograph by Galileo of Jupiter's Callisto, showing its ancient cratered surface, the most heavily cratered region of our solar system. It shows no signs of any large scale geological processes on the surface apart from cratering. It's thought to have an ocean deep below the surface but there seems to be no connection with the surface. As a result it is classified as Category II like the Moon. There are no known planetary protection issues with humans setting up a base here.

It's got a cratered surface with no sign of any modification.

If any humans there want to study its subsurface ocean they would have to drill deep. The ocean, if it exists, is at least a few kilometers thick and is less than 300 kilometers below the surface, however, it is not too likely to be habitable.

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The same would be true for any other icy moon or dwarf planet, with its subsurface not connected to the surface, and no surface habitats for present day life.

Its ocean is probably salty. It might perhaps have life, but it's only heated by radioactive materials in Callisto's core. It wouldn't have the energy flux of the Europan ocean which is heated by tidal heating as well.

Interleaved "Ocean Worlds Exploration" to Europa, Enceladus, and Titan - and Ceres?

Jonathan Lunine wrote a paper in Acta Astronautica in February 2017 "Ocean Worlds Exploration". It's a survey of our current state of understanding of the ocean worlds. But he also introduced the idea of a multi-worlds approach to exploring the icy moons.

"In terms of the search for evidence of life within these oceans, the plume of ice and gas emanating from Enceladus makes this the moon of choice for a fast-track program to search for life. If plumes exist on Europa—yet to be confirmed—or places can be located where ocean water is extruded onto the surface, then the search for life on this lunar-sized body can also be accomplished quickly by the standards of outer solar system exploration."

"It will be argued that a robust search for life in the solar system beyond Earth requires a program that targets multiple bodies—but not too many—and a sustained determination to do so given the long (5–10 year) flight times. ..."

"The instrument package has the potential for characterizing the habitability of the Europan ocean to a more detailed extent than Cassini did for Enceladus, particularly if plumes are present. It will also map out where organic deposits may be present, and determine whether oxidants are fed to the Europa ocean as has been speculated since the results of the Galileo mission "

His main points in the discussion section are

• Interleaved missions allow lessons learned to be applied from one object to another: the experience of Cassini at Enceladus has greatly informed the next step in the exploration of Europa ...
• Interleaved missions make better use of fiscal and technical resource ... With the long trip times to the outer solar system, waiting for the results of one mission at one object in order to plan the next mission leaves long gaps in a scientific campaign to look for habitability and life. Interleaving missions among three bodies (at two giant planets) allows a leapfrog approach where new discoveries are always being made at one ocean world while plans for future efforts are being brought to fruition at others. It also allows for a more level funding profile and hence the possibility of a more stable program.
• A program hedges the bet: The disappointing results from Viking with respect to life on Mars had a strongly negative effect on the program because there was no plan B for what to do next. With multiple ocean worlds, strongly negative results on one target simply means that it is time to refocus on the remaining one or ones.

Here I'm going to take this as a starting point and explore a few other issues.

First, it's tempting to put all our resources into one target to try to "crack it". Currently all NASA plans for exploring ocean worlds are focused on Europa. But there's a lot to be said for exploring Enceladus at the same time, using similar spacecraft.

If we explore these ocean worlds one after another, and explore Europa thoroughly first, say, then Enceladus, then Ceres, or whatever, it may be decades before we get to whichever is last in our list. If we do it in an interleaving way, then we can find out about them all in a preliminary way first.

After that preliminary step, which has already been done with Enceladus, but not yet with Europa, we go on to the next step up, which is to do some in situ study, probably to sample plumes. At this stage, if we get null results at one of them, we can focus our efforts at the one that seems most interesting. Or perhaps one of them is easier to study than the others, so we focus our attention wherever it will be easiest to get good results early on.

This may seem likely to be more expensive, but actually it will save a great deal in the long term.

• Saving on spacecraft and instrument design - design once, fly many times.

This is something that's been done in the past with Voyager I and 2, or Opportunity and Spirit, or Viking I and 2. Or with Apollo of course, Apollo 11, 12, 13, 14, 15, 16 and 17 all using essentially the same hardware with some changes to do the same very complex mission.

So, at least two interleaved missions for each stage in the exploration. Launch both within a two or three years of each other. It's best done within a few years so you don't need to do a redesign of a now ancient spacecraft. We can't send Viking now. We won't want to send a copy of the Europa Clipper to Enceladus twenty or thirty years from now, but, say, five years from now it may seem a good deal still.

We could even also send multiple spacecraft on the same launch if we have heavy lift by then, or low cost transport to orbit - the SLS, or Falcon Heavy, or Skylon, or whatever it is. A dual mission to Europa and Enceladus seems very attractive.

There will be some adaptations of course, Particularly, missions to Europa need to be hardened or protected to withstand the ionizing radiation. But we should be able to use essentially the same spaceship and instruments.

Then as we get results back and have experience using them in space, the instruments we develop will carry through to the next stage for all the missions.

• More efficient exploration

One of the main problems with focusing on only one of these worlds, Europa with the current plans, is that we have to wait for results before we can do a truly intelligent design for the next mission.

Even with heavy lift to get our spacecraft there in a couple of years instead of the 5-10 years that Jonathan Lunine assumes,, we have to wait at least a few years to get results before we can make a really good design for the next phase.

That's one of the main problems with launching a lander to Europa so soon, that we don't know enough to design it intelligently or to know if it is the best thing to do, and all the issues I mentioned above are mainly to do with that.

However, as he says, we can keep the pace up by launching to Enceladus instead. We can do that right away. It just needs approval, and funding. We already know what we need to do, to send a mission to study the geysers close up and fly through them. Everyone is agreed on that as the next logical mission for Enceladus - though with varying ideas on details of how to do it and sometimes with additions such as landers.

So instead of a Europa lander launched in 2024, if we do want to keep up the cadence and launch something then, let's send an Enceladus geyser flythrough at that time instead. Launch to Enceladus in 2024. Leave the next Europa mission to the second half of the 2020s.

It can be based on the Europa Clipper. This is another of my speculative suggestions, but we could even use the same solar panels if the idea of hibernating for three days out of four was used based on the approach suggested for the Hera Saturn Entry Probe . See my Suggestion - hibernation with solar power to permit Juno sized solar panels out to the Saturn system and perhaps even further, right out to Neptune? (above)

In this way we will probably never need to wait for results before sending the next mission. If we have an opportunity to send a mission, we design it, or we make another copy of an already designed spacecraft, and send it to whichever of the three destinations is most in need for a new mission.

• Continual flow of discoveries

This way we will be able to maintain a continual flow of new discoveries. It also maintains a continuous or at least frequent spaceship presence in both the Jupiter and Saturn systems.

If we are careful about this and design them with enough fuel, perhaps with ion thrusters which are almost inexhaustible, they can be used for extended missions too, after the primary mission is over. The same spacecraft could do multiple slow flybys of Titan, Dione etc as we saw, on its way to Enceladus, and could do them also in reverse order afterwards, do flybys of Titan, and from there, with the large gravity well of Titan, we can take it anywhere interesting in the Saturn system, and indeed beyond if that is what was desired for an extended mission.

We could have a similar approach in the Jupiter system if the spacecraft is sufficiently shielded from the ionizing radiation to survive for an extended mission though that's more of a challenge especially if we try a Europa orbiter.

• Add Ceres to the list of interleaving missions

This is my own suggestion, I think that Ceres also, is a prime target for astrobiology, now that there's some possibility of a subsurface ocean, and just possibly occasional activity. (see Search for early life on Ceres (above). That's especially so, since it is also closer to Earth.

We have Hohmann transfer orbits to Ceres every 1 year 3.3 months (see table here) and it takes one year 3.5 months for a spaceship to get there. Dawn took so long because it had to use gravity assists, and ion propulsion, but if we have the ability to get to Europa or Enceladus in two years and three years (respectively) with our spacecraft, it's an easy matter to get to Ceres, in a journey of a little over a year.

So it's another one that we could start to study in a preliminary way using a similar spacecraft design to the one used for Europa. Life might have started on Ceres and have remains we can study, or if it had a separate genesis, it may still be a good place to look for early forms of life especially, if it does have subsurface liquid escaping to the surface. Or we could use impacts to create an artificial plume to study there.

• Ceres and Europa still in need of stage 1 orbital reconnaissance. Enceladus and Titan are already read for stage 2.

Right now, the main thing for both Ceres and Europa is to do close up orbital reconnaissance. We've already done enough of that for Enceladus, and identified the only currently active astrobiologically interesting feature, which is the small field of geysers at its south pole. So, that phase is pretty much done for Enceladus. We know where the most interesting astrobiological target is, and know a lot about it. We have excellent preliminary data from Cassini, even though it wasn't designed for this. So for Enceladus we are ready to move on to the next stage of looking at the geysers close up with in situ biology, geochemistry and biochemistry instruments.

We also have close up orbital reconnaissance of Titan from Cassini too. We even have results from a lander on Titan, the Huygens lander. So for Titan too, we've done the first close up reconnaissance, if not as thorough as we'd like.

• Bonus items for Saturn - Titan, Rhea, Dione etc.

Titan is a likely ocean world too, but it's a bit like Callisto, it's ocean is likely to be hard or impossible to access. We don't know for sure, perhaps it has cryovolcanism. However, there's plenty of scope for interesting bonus science studying it using the instruments designed for Enceladus, to find out more about its ethane / methane oceans and atmosphere. Indeed any mission to the Saturn system has Titan as a bonus if the main objective is Enceladus, as you have to do at least some flybys on the way in.

Dione also is a bonus on the way to Enceladus. as it has a possibility of a subsurface ocean and maybe even plumes, though the evidence is rather tenuous. There are many other mysteries of course in the Saturn system, and the flybys of the other satellites on the way to Enceladus are "bonus" missions too, such as the 15 flybys of Rhea in the Enceladus orbiter proposal (above), which may possibly have had a ring system in the past and possibly still has a very tenuous one. Perhaps some of the Enceladus orbiter instruments can be useful for trying to answer this question of whether it still has a very faint ring. That would make it the only known moon with a ring. That's a ring around Rhea not a ring around Saturn.That's of no relevance to astrobiology as far as I know, but it's a science bonus of an Enceladus mission.

Mimas is interesting too as a moon with many mysteries and a possible ocean world too - rather tenuous reasoning, most likely its core just hasn't reached gravitational equilibrium - but it's a mystery to clear up. However it's not on the way to Enceladus so would have to be for an extended mission if the spacecraft was able to leave Enceladus orbit at the end of its science phase there.

I mention this because I'm not sure enough emphasis is made of the interesting science potential on the way to Enceladus. I wonder if an Enceladus orbiter would get more interest if it was made clear quite how much interesting science we can do on the way to Enceladus - and possibly on an extended mission after it? It could study the rings too. Basically a new Cassini except its designed as an Enceladus orbiter. But both before and after, it could study all the same targets as Cassini with more modern instruments if we wanted to do it that way. At any rate, with many flybys of interesting targets on the way in, it's bound to have lots of interesting additional science spinoffs.

• No way to decide which is most interesting for astrobiology

Also, we have no way at present to decide for sure which is of most astrobiological interest. Perhaps we spend several decades exploring Europa then decide that, despite its initial promise, that actually Ceres, or Enceladus was the far more interesting target to study for astrobiology, or vice versa.

We don't know yet that the Europa ocean is habitable. The evidence is reasonable, and it seems a good guess, but we don't know for sure, it is a lot of deduction based on rather little information. We don't know if it is oxygen rich or oxygen poor. We don't know it's pH. And we don't know what decides whether a habitable ocean evolves life, or whether indeed it is just pure chance and some do and some don't.

With the approach of focusing on one destination at a time for decades on end, we could study Europa for years only to find out that the most interesting target is Enceladus, or Ceres, if we are unlucky. While with the interleaving approach, if we get a null result for biology for Europa - well if we have been studying them all at once we can change our focus to whichever other one is our current top priority. We get a null result for Enceladus too, or a disappointing one at least, like the Voyage results, maybe with some controversy,- well we've also been studying Ceres and perhaps counter to expectation Ceres is the one that gives us the most revolutionary insights into astrobiology. Who knows....

• Interleaving hardware and power requirements

The hardware for geyser flythrough probes, landers, ice moles and submarines would work for nearly all of them with mainly minor changes, except of course that Europa needs much more by way of radiation hardening (or shorter missions) and Titan has special advantages as well as disadvantages because of its atmosphere (for aerobraking) and its liquid ethane / methane surface oceans if we were to combine this ocean worlds program with Titan exploration in a single mission or multiple missions.

Even the power requirements are not that dissimilar. They are identical if we use RTG's. If we use solar cells, then it's true that Jupiter mission solar cells won't be sufficient for Saturn "as is" but it's not that hard to modify them.

We could also (my suggestion here) use batteries combined with the same sized solar panels out to Enceladus. Power up the batteries for four days for a one day experiment, and repeat. With solar panels optimized to work at low temperatures this would be fine. In this way we could use identical spacecraft for many of the missions, so saving a lot on development costs. Of course this doesn't matter for RTGs. For more on this see my suggestion based on the proposed 2016 Hera Saturn Entry Probe in Would we use solar power or radioisotopes for missions to Saturn's moon Enceladus? (above)

• This interleaved approach may not seem optimal if you are laser focused on finding out more about Europa

It may seem a distraction from Europa. But even then, the discoveries we make in an Enceladus geyser flythrough may help with the Europa flythrough if we do that. The discoveries by Cassini for Enceladus so far have certainly helped with our understanding of Europa.

• If your aim is to make as much progress in astrobiology as quickly as possible, this is surely the way to go.

It also keeps astrobiology on the agenda, and continues the interest in astrobiology, with new things happening all the time. And it lets us be more daring. It's much easier to accept a null result from an astrobiological experiment if you know you can run it again on several different targets in the near future.

Also it gets the astrobiological instruments flying in space frequently. We can do upgrades of the equipment too, if necessary, minor upgrades of each new version of, e.g. the geyser flythrough or the ice mole or submarine etc based on the previous mission results and experiences, as well as all the astrobiological instruments of course.

• Multidimensional astrobiology

Also, given how different astrobiology could be for these oceans - they could be, early life, pre-biotic chemistry, different genesis, etc etc, then comparing the discoveries, if they all have life, could lead to much more than two or three times the amount of knowledge we get from studying one of them.

It's more like increasing the dimensionality. This is just an analogy and perhaps stretching it a bit, but astrobiology different from Earth is like a step up from one to two dimensions, like being able to explore squares, circles and triangles instead of just line segments. But to have two different forms of astrobiology in our system would be like moving from two dimensions to three dimensions, giving us the equivalent of cubes, torus, spherical shells, and the vast possibilities of 3d geometry.

Or, if they are all uninhabited, to have two, or three uninhabited ocean worlds, again would vastly increase our understanding of uninhabited ocean worlds, over a study of just one uninhabited ocean world.

• Benefit to Mars astrobiology too

This may seem a bit of a surprise. But I think al this astrobiology experience also would directly feed in to missions to Mars, and vice versa - directly for many of the astrobiology experiments. The brine layers there are like miniature subsurface oceans. For microbes even a tiny drop of water is like a swimming pool as Nilton Renno said.

Especially, we have the same problem of keeping the rover sterile and many of the same issues. The one thing we don't have on Mars is any possibility of a geyser sampling flythrough for astrobiology. But once we get to the landers and ice moles - Mars has ice of astrobiological interest at its poles, especially the flow like features in Richardson crater. Even the ice mole could be very useful when studying the flow-like features and other possibilities for life beneath solid state greenhouse clear ice at the Martian poles, if those habitats exist.

And the one thing we also have in common is the need to sterilize the landers, or rovers, for explorations that involve direct contact with liquid brine - and also, possibilities of impact of the spacecraft into ice causing temporary habitats. And the astrobiological instruments would be essentially the same.

In situ instrument capabilities

In this section I'll look at astrobiological instruments generally, for Mars as well as for the icy moon. This will avoid the need to repeat the same material in both the Mars and ocean worlds sections of the book.

The main focus for astrobiological instruments so far has been on Mars, and two of them, UREY and Solid3 actually got selected for missions to Mars but were descoped. So far we haven't had any dedicated astrobiological instruments sent into space since Viking. Some other instruments have been sent, such as mass spectrometers, also have some interest for the search for life, Curiosity has a Chiral and the Trace Gas Orbiter, though its aim is to analyse chemicals in the atmosphere of Mars, I think you could call an astrobiological mission as its particular aim is to search for organic molecules like methane that could possibly be the result of biological activity.

So what have astrobiologists been doing since Viking? With almost no opportunities to send anything into space, you might think they would give up on the idea of actually making instruments to fly. However, though the instruments never flew, there were those two instruments that nearly got to fly and were descoped at the last minute, UREY and SOLID3, both for ExoMars at different stages of the project. Meanwhile many other astrobiologists have continued actively developing many instruments to send to Mars or elsewhere in the search for life outside of Earth, even though there seems no immediate prospect of them flying quite yet. Some of them have exquisite sensitivity, and could find life based on the minutest of traces, even able to detect a single molecule in the sample of biochemical interest. Most of these instruments were developed for Mars.

Whether they can be used "as is" for Europa, or Enceladus, or need more modification, this shows the range of instruments we can send. If the sample is collected into a liquid as we discussed above (see Capturing the ice and dust particles more gently, in an aerogel - for sample return, or in situ analysis), perhaps it's not that different from analysing a sample on Mars, except, of course, that the instrument has to be capable of operating in zero g for an orbiter mission. For a lander it has to be able to operate in low g (for Europa, 13% of Earth gravity) or in almost zero g (for Enceladus, only 1% of Earth gravity).

I think if astrobiologists were asked to submit proposals for astrobiological instruments for a Europa or Enceladus flyby, or a lander, they would rise to the challenge, and you might get some surprises. You would probably get neat new ideas that you didn't expect. A lot of them are "labs on a chip" that weigh hardly anything, and with the huge mass of the Europa missions on SLS, you could fly many of them on a single mission.

Many of the instruments are based on looking for patterns of molecules that are distinctive of life. For instance they may look for chirality or analysing the molecules to look for unusual patterns. This is all related to Chris McKay's "Lego principle", see Strengths and weaknesses of the "lego principle" approach (above).

Some of the instruments attempt to find biological activity directly - to detect metabolism for instance, or even, to see the microbes moving with high resolution microscopes.

Others are based on the idea that we might find our distant cousins on Mars. For instance, could Mars life have DNA? This is worth checking, whether we are related or not. If we don't find DNA, but have something that is clearly life - e.g. metabolizes, and we can see it moving - then an attempt to sequence it is a great way to show that it is not related to us. That's a very interesting "null result". It might also be possible to sequence informational polymers that are related to DNA but not identical, for instance with different base pairs, though this work is at an early stage at present.

Here are some that are already at reasonably developed state, including a couple that were all set to fly to Mars and then descoped:

Rapid non destructive preliminary sampling

• Raman spectrometry - analyses scattered light emitted by a laser on the sample. Most light scatters elastically with no exchange of energy (Raleigh scattering) but one in ten million photons will exchange some energy with the sample, and be scattered at a lower energy than the laser light. This is Raman scattering. The next step is to measure this light repeatedly and build up a spectrum. From the spectrum you can identify the vibrational modes of the molecule which took up the energy. Most molecules have many of these modes, providing a unique fingerprint of the molecule.
  
  It's sensitive, and able to detect organics and some biosignatures. One of its best features is the way it can measure the distribution of the organics and other compounds by pointing the laser at different points on the surface - and is non destructive so it can be applied first before any of the other tests.
  
  Amongst other targets, it could perhaps spot biosignatures that microbes use to protect themselves such as beta-carotene (UV protection) or glycerol (low temperature protection), both of which are targets of Raman spectrometry on Earth.

This thesis by David Hamilton in 2016 looked into the question of how easy it would be to spot biosignatures in organics that are mixed into clay and other minerals altered by water on Mars. They may give us the best chance of finding reasonably undamaged traces of past life, because they get incorporated into the minerals in a way that protects them from deterioration, but this can make it harder to disentangle the organic from the mineral it's embedded in.

He found that the transition metals in the clays particularly can confuse the picture. He used stearic acid as his candidate organic to detect (one of the most common naturally occurring fatty acids), and found that of the various substrates he tested, it could only be clearly identified as stearic acid in gypsum. In other cases such as clays, you might detect that an organic is present, and at high concentrations, that it is a fatty acid, but it would be hard to say for sure which one.

This is mature technology and both Mars 2020 (previously called "Curiosity 2020") and ExoMars will use Raman spectrometry. However it has limitations of sensitivity compared to the other methods. It could detect some biosignatures.

When searching for elusive biosignatures then it may be important to synchronize it with optical microscopy and do Raman microspectrometry. See Optical microscopy (below)

Detection of trace levels of organics and of chirality

• Gas chromatography with chiral analysis - this is the idea used for the MOMA (Mars Organic Molecule Analyser), the largest instrument on ExoMars. It is used to analyse volatiles evolved from the soil samples in twenty small ovens, each of which can be used for a single sample analysis. The ovens are heated to a high temperature and the volatiles then evaporate and can be analysed as a gas.
  
  Some of the ovens are filled with a "derivatisation agent" which can transform the chemical compounds into similar ones suitable for chiral analysis. They are then ionized and analysed with the mass spectrometer. The chiral compounds can then be separated using a chiral gas chromatography column.
  
  The main problem with this approach of heating the samples in an oven that it can degrade the organics, and indeed in presence of perchlorates and other reactive chemistry, it can destroy them altogether before they can be analysed. 
   
• Although Curiosity wasn't designed for searching for chiral biosignatures as a primary goal, like MOMA, it's worth mentioning that it does have one experiment on board, as part of its Sample Analysis at Mars (SAM) that can detect chirality. That's one of the six gas chromatograph columns. It's shown as: "GC4 chirasilDex (Chiral compound separation)". And apparently it's in the mass range to detect amino acids. The Mars Curiosity Rover Can Detect Chiral Compounds. But it is not designed especially to find life signatures.
   
• Planetary In-situ Capillary Electrophoresis System “lab on a chip” – separate the organics by ionic mobility, by electrokinetic methods in sub millimeter capillaries with the fluid manipulations done within the chip itself. Mass 3 kilogram, power consumption 2 watts. Demonstrated it works in an end to end device.
   
  This is a more recent test of the approach, in a test on Mono soda lake they achieved parts per trillion sensitivity. Uses laser induced fluorescence. It's able to work with liquids directly, and by labeling the amino acids it can separate them according to chirality (they used sodium taurocholate and γ cyclodextrin). In their tests they were able to distinguish up to 17 different common biotic and abiotic amino acids simultaneously in the conditions of brine from a highly saline soda lake. Its advantages include small size, low weight, low power consumption, short analysis times, small sample and electrolyte consumption and low waste generation.
  
  
  PISCES - or the Chemical Laptop - also known as the "microfluidic life analyser" - size comparison with a conventional laptop, on a test rover in JPL Mars yard.
  
  It's one of the instruments tested in the Atacama desert in 2017 as part of NASA's Atacama Rover Astrobiology Drilling Studies (ARADS)
   
• LDChip and Solid3, also the Life Marker Chip developed for ExoMars but descoped, using polyclonal antibodies to detect organics. These detect a wide range of organics not specific to DNA based life. An earlier version LDChip300 was tested in the Atacama desert and was able to detect a layer of previously undiscovered life at a depth of 2 meters below the surface in the hyper-arid core of the desert from analysis of less than half a gram of material. The mass for LMC was 4.7 kg, and for Solid3, under 7.5 kg.

• 

  Solid 3 - detects biosignatures using polyclonal antibodies. This version of it has total mass less than 7.5 kg, with a sample preparation unit weighing 5.5 Kg and a sample analysis unit weighing less than 2 kg.
  
  Another earlier version of the same technology, the Life Marker Chip, mass of 4.7 Kg, was approved for ExoMars but later descoped.

  SOLID stands for Signs Of Life Detector. It's one of the instruments tested in the Atacama desert in 2017 as part of NASA's Atacama Rover Astrobiology Drilling Studies (ARADS)
  

• UREY, relies on liquid extraction using high temperature, high pressure, sub critical water as a solvent (in case of Urey, between 100°C and 300°C at 20 MPa, or about 200 atmospheres, for several minutes) - under those conditions water has chemical properties similar to organic solvents. Urey is able to study the organics relatively unmodified, more details here. UREY was approved for ExoMars but was descoped when NASA pulled out of the partnership.
   
• Astrobionibbler - similar idea to UREY, smaller, later development. Able to detect a single amino acid in a gram of soil, end to end system does its own sample collection, uses pressurized super heated water to extract organics which it then labels with fluorescent dyes to identify the amino acids, using microfluidic channels in a "lab on a chip". Mass 2.5 kg.

Dallas Ellman fine tunes a component of the astrobionibbler. It uses ideas from the larger UREY instrument, using high temperature high pressure subcritical water as a solvent for non destructive extraction of organics. Now miniaturized to a "lab on a chip", low power requirements, low mass. During his summer internship at JPL in 2014, he helped discover and replicate the conditions the Astrobionibbler team needed to extract and detect amino acids from Martian regolith, so sensitive that it could detect a single amino acid in the sample. The target goal is a mass of 2.5 kilograms, a quarter of the mass of UREY in an end to end system that drills to do its own sample acquisition, to avoid cross contamination with other instruments. For details of how it works, see Searching for organics in a nibble of soil.

Search for life directly by checking for metabolic reactions

These can detect life even if it doesn't use any recognized form of conventional life chemistry. However it does require the life to metabolize in vitro. It doesn't have to reproduce.

• Microbial fuel cells, where you check for redox reactions directly by measuring the electrons and protons they liberate. This is sensitive to small numbers of microbes and has the advantage it could detect life even if not based on carbon or any form of conventional chemistry we know of.
• Levin’s idea of chiral labeled release, where he has refined it so you feed the medium with a chiral solution with only one isomer of each amino acid. If the CO₂ is given off when you feed it one isomer and not with the other, that would be a reasonably strong indication of life. This has the advantage that the life just needs to metabolize amino acids, and to produce a waste gas that contains carbon (such as methane).

There are many instruments like this we could send, and several of them are already space qualified but never flown.

Electron microscope

• Miniaturized scanning electron microscope. This can’t detect whether it is life or not, but is useful along with the others for examining tiny structures.

Optical microscopy - and Raman microscopy

Most of the optical instruments we've sent into space are low resolution, the equivalent of a "geologist's hand lens". But what about taking much more details photographs, with a diffraction limited optical microscope? With the highest possible resolution for a normal optical microscope of a fifth of a micron, it could let us see micro-organisms directly and tell us things about the behaviour and structure of micro-organisms or protocells we might not be able to find out by other methods.

We probably won't be able to verify life or protolife with a microscope, unless it is actually still viable and active. But it could give interesting data in combination with the other instruments.

One limitation is that it won't let you spot microbes that are beyond the diffraction limit of optical microscopy. DNA life ultramicrobacteria are right at the diffraction limit of 200 nm. RNA world cells could in theory be too small to see at 50 nm.

This is one thing that the Europa lander report has covered in some detail. NASA actually do hope to send very high resolution optical microscopes to Europa on their Europa lander. Amongst other things, they mention the idea of Raman microspectroscopy. The way this works is that an optical microscope focuses on a particle in an optical field, and then analyses it with a laser light focused onto the same visual plane as the optical microscope. The Raman scattered light is analysed as for ordinary Raman spectrometry

This powerful technique lets you do spectroscopy of individual cells (or cell like structures)

Raman microspectroscopy - the spectra on the left correspond to the in focus particles in the microscopic images on the right, letting the experimenter distinguish the spectrum of a protein rich particle, a lipid rich particle and a living cell. This could help work out what it is that the spacecraft sees in its microscopes. Figure 4.1.7 on page 4-35 of the report.

This could help you to work out what kind of a particle it is, including perhaps sometimes a microbe. For more details see the section Using microscopes to search for Europan life (above).

This ability to synchronize Raman microscopy with an optical image could be very important on Mars also, being able to focus right down to the microscopic level. In a study of the microbial colonization of salt (halites) using Raman spectroscopy in 2015, the authors looked at the implications for Mars in their section on "Implications for the search for biomarkers on Mars" (page 3219). They found that focusing on microbial colonies can be difficult and time consuming, needing precise work, even when the microbes were common enough so that you could see them as a "clear greenish zone on the macroscopic scale and recommended:

"Therefore, if biomarkers are present within Martian rocks, the laser spot size and positioning/focusing method of the future Raman instrumentation used for Mars exploration will be crucial for successful detection of such traces of life on that planet."

Atomic force microscopy

The Europa lander report also mentions use of atomic force microscopy to measure the forces between molecules and properties of cell walls such as elasticity, hardness etc (page 4-26 of Europa lander report). This is mature space technology that has flown on the Phoenix mission to Mars and the Rosetta mission, though it's not been used to search for life.

Using an optical microscope to watch microbes swimming

It would be much easier to see the microbes if they are viable and able to move, even if they are very tiny. I'll summarize some of the material from this paper: Microbial Morphology and Motility as Biosignatures for Outer Planet Missions

The smallest microbes are too small to swim because they get jostled about too much by brownian motion (the jostling of molecules hitting them). But microbes can swim if they are larger than 0.6 microns, and we often see microbes larger than 0.8 microns swimming on Earth, far larger than the limit of optical resolution of 0.2 microns. Also, they are often mobile even in very cold dry conditions on Earth.

For an Enceladus or Europa flyby orbiter, you could collect viable spores at flyby speeds of up to 5 kilometers / second (see section 5.1 of this paper, also page 56 of this one). You could collect the spores direct into liquid, and before other tests such as heating it in an oven you can melt the ice to a liquid, and look at it with a microscope to see if there is any mobile life there. Microscopes on the ISS could be used as a starting point for the design. For some ideas of ways to collect microbes directly into a culture medium in a flyby mission see Suggestion - for low relative velocities, capture directly into water, brine, ice, or foamed up water or ice (above).

One problem is that if the magnification is high enough to see a microbe less than a micron across, then the field of view may be narrow with not get many microbes within the field of view. So, you may need to look at a lot of images before you see the moving microbes. He gives a calculation there, for a typical CCD camera used for microbial imaging, and typical microbe concentration of Earth seas, of 100,000 to a million microbes per millilitre, then you would get an average of much less than one cell per image. It would help if you have some way to concentrate the population of microbes.

However there is another approach that may help with the problem of the limited field of view for optical microscopy of swimming microbes. Holographic (interferometric) microscopy. It is still diffraction limited, but you do the focusing after capturing the light, digitally. So there are no mechanical moving parts, and it can be operated without user input to focus the microscope. The authors of this paper: A Submersible, Off-Axis Holographic Microscope have developed such an instrument with 800 nm resolution which can be used underwater. They were able to detect active prokaryotes through their mobility in sea ice imaging samples immediately near Nuuk in Greenland. They suggested that their technique could be useful for space missions to locations such as Europa.

Using an optical microscope to watch fluorescing microbes

This again is from Microbial Morphology and Motility as Biosignatures for Outer Planet Missions. All microbes autofluoresce when they are illuminated with UV light to some extent, though the fluorescence can be hard to spot. Some pigments, especially chlorophyll, absorb UV and emit strongly in visible light.

The other thing you can do is to use fluorescent dyes that attach to nucleic acids, lipids, cell walls, and other biosignatures. This needs to be looked into careful. It could have planetary protection issues, as it brings complex organics to another planet. Also, the dyes may be unstable at high or low temperatures.

However, if you can't do that, you still have the autofluorescence. Even life which is independently evolved may well have pigments that autofluoresce similarly to chlorophyll.

This is their figure 10 from the paper. At top left,cell walls of Bacillus subtilis in soil are labeled with a violet stain. At top right, an arctic biofilm is stained with a green stain for nucleic acids (the yellow is from sulfur minerals), and at bottom left a microbe that lives inside gypsum is labeled with an orange die that ties to esterase which means it ignores dead cells.

The red dots at bottom right show autofluorescent chlorophyll from photosynthetic cyanobacteria (sometimes known loosely as "blue green algae" though they are not strictly speaking algae). In the same photo, the green fluorescent die labels a fungal hyphae (thready strand of a fungus).

Also, deep UV at wavelengths less than 250 nm can make some of the amino acids fluoresce. These are the aromatic amino acids (i.e. the ones that incorporate a ring of six carbons). Similarly chlorophyll fluoresces as well as other biological organics.

Near field scanning optical microscopy

Another technology not mentioned in the Europa lander report (as far as I can tell) is near field scanning optical microscopy.

This technique is mentioned in "An Astrobiology Strategy for the Exploration of Mars ", page 72.

"Other imaging technologies including interferometry, scanning near-field optical microscopy, and electron microscopy techniques should also be developed for spaceflight applications."

Of that list, there is work on electron microscopy already (see Electron microscope above) , but I don't know of anything published on on near field scanning optical microscopy for astrobiology missions. Do say if you know of anything.

This gives optical images that are higher resolution than 0.2 microns, so below the diffraction limit, using evanescent waves. The detector has to be very close to the object being observed, at a distance less than the wavelength of the light. This is an example of its use for fossil microscopy combined with confocal Raman spectroscopy.

Beyond the diffraction limit with expansion microscopy (physically expand a cell to make it easier to study) and other forms of superresolution microscopy

This is another very speculative section, just sharing it in case it is of interest. Are there any other optical methods apart from NSOM that could perhaps work with an RNA world cell, too small to see with conventional methods?

Well there are in theory at least. T here are many other forms of superresolution microscopy - first introduced in 2006, brief history and overview here. It's a very rapidly growing field with numerous new techniques developed all the time. Perhaps some of those could have promise for astrobiology, especially if they can be miniaturized and used in situ.

To take one example, what about expansion microscopy? This was introduced for the first time in 2015 and is a relatively "low tech" way to go beyond the diffraction limit of 0.2 microns. It only works for cells that have been fixed for microscopic examination.

The idea is to embed the microbe in a polyelectrolyte gel, anchor biomolecules such as RNA etc to the gel, and then expand the gel isotopically, for instance just by adding water. This let them expand them up to 4.5 times. You can then study biomolecules in the expanded specimen using conventional fluorescent microscopy, to study it. For more about this, see Nature: Methods - Method to Watch - Expansion microscopy. (sadly behind a paywall). Also short summary in wikipedia.

In 2017 the authors introduced a new version, iterative expansion microscopy which lets you expand a specimen twenty times. They do this by introducing a second gel and using that to expand the first one.

Could this be used for astrobiological "in situ" microscopic observations? Perhaps it could work for RNA world cells and other structures too small to be seen otherwise with an optical microscope. If it could, This could expand the 50 nm RNA world cell to one micron in diameter. Although large, the expanded cell would be largely transparent, being less than 0.01% the original biomaterial. So you'd be able to study it in 3D.

This is just a suggestion. The Europa lander suggestion to use NSOM is the first example I've seen of a suggestion to use superresolution microscopy in the papers on optical microscopy for astrobiology that I have read so far. Do say if you know of more suggestions / proposals of this sort!

Direct search for DNA

A DNA sequencer can detect life if it is DNA based, which would probably mean, related to Earth life. It can also sequence the entire genome of any lifeform found. These are now small enough to send on a spaceship. The focus so far is on DNA but it's possible to sequence RNA based life too- so long as it uses the same bases as on Earth. Sequencing for life based on other non standard bases is also possible but work in progress.

• SETG - miniaturized DNA sequencer could sequence life if we had a common ancestor right back to the very early solar system whenever DNA first evolved. This is in a reasonably advanced state. They say it could be ready to fly by 2018.

Mars is the place we usually think of as a place to search for DNA based life. But what about Europa or Enceladus? Could they also have DNA based life? We would want to know either way and it is possible if there is a shared ancestry even way back in the early solar system or even from our sun's birth nebula.

This shows how it could work for an icy moon DNA search

Figure 1 from Towards In Situ Sequencing for Life Detection

In the same paper they discuss the possibility of using their techniques for non DNA based life. They mention that single stranded RNA polymers can be sequenced, referring to this paper. They also mention various situations in which non standard bases have been detected during standard DNA sequencing, referring to various papers on the topic.

They also give an example themselves, in the paper:

"We provide proof of principle data that strand sequencing can detect the non-standard inosine (I) base (nucleoside). This base is formed when the nucleobase hypoxanthine is attached to a ribose sugar, and is astrobiologically relevant: of the extraterrestrial nucleobases identified on meteorites, hypoxanthine is second in abundance only to guanine (G)"

They studied an artificial polymer with the base sequence …CICICICICI... with only two 3-mers (CIC, ICI) where I is the non standard isonine base and C is cytosine. They expected to detect peaks for CIC and ICI using a library consisting of the C and I bases. They got few good reads, but amongst those reads, some of them did show this dual peak signal that they were looking for. The event rate was more than a hundred times slower than for natural DNA which could be because of the unnatural I base or because of the repetitive nature of the polymer. The polymer they used was

"poly-(dI-dC), a synthetic DNA polymer composed of alternating deoxy-inosine (I) and deoxy-cytosine (C) bases".

Check the paper for the techy details.

One of the issues for gene sequencing in space is the large amount of data generated by the experiment. Depending on the experiment, and how many of the micropores are used, it can generate between 1 GB and 1 TB of data. Luckily, a lot of that can be processed immediately in situ using a flexible programmable gate array. But even with that, they calculated that for a 22 billion base sequence, it generates 700 GB of raw data, and they would need to return 44.2 GB of base and quality data. That's equivalent to 22 images of the Mars Reconnaissance Orbiter's HiRISE camera.

A run that sequenced 2.21 billion base pairs would return a tenth of that data, 4.42 GB, still the equivalent of a couple of HiRISE images. See their table 1 and their comments on it for more.

Mars is the place we usually think of as a place to search for DNA based life. But what about Europa or Enceladus? Could they also have DNA based life?

Could Europa or Enceladus have DNA based life?

We've looked at the idea of life from another star in the sun's birth nebula and related ideas in Distant cousins with last common ancestor from a planet around another star (above)

What about transfer via meteorite impact from one planet to another within our solar system to Europa or Enceladus? Well it's a lot harder for a meteorite from Earth or Mars to get to Europa or Enceladus than to transfer to each other. But is it possible at all?

This was the subject of this paper: Seeding Life on the Moons of the Outer Planets via Lithopanspermia. First, yes, meteorites can get from Earth to Jupiter or Saturn. This table shows the percentage of debris that gets to various destinations, starting from Earth or Mars based on a gravitational simulation using the positions of all the planets in the solar system.

Their figure 1, shows the percentage of the initial population of tens of thousands of test particles that got to the final destination. The simulation starts with them randomly distributed around the "hill sphere" of the originating body with a random outwards velocity.

When you compare the graphs to each other, notice that they are all to a different scale on the vertical axis. Numbers show percentages of the original population of test particles. For instance Earth to Earth has a nearly 30% in the first million years. In the same time period, Earth to Venus has a 5% chance and Earth to Mercury has less than a 0.001% chance.

Anyway the interesting thing is that some particles do get to Jupiter or Saturn but only after 3 million years or 8 million years respectively. That's a long time, however spores are able to survive such long transits if they are deep within a large meteorite protected from cosmic radiation. They could also deliver genetic material and other elements of Earth based biochemistry even if the microbes are too damaged to revive when they reach their destination.

So, how does that translate into numbers of rocks with viable life? They cover that in their table 4 and table 5 which estimate the total number of rocks which could get to various destinations and contain viable microbes on arrival. Those tables covers all the rocks ejected since the end of the Late Heavy Bombardment soon after the formation of the Moon.

First, they find that it's reasonably easy for rocks to transfer between Earth and Mars in both directions in the early solar system. In their table 4 they estimate 360,000±40,000 rocks large enough for viable microbes get from Earth to Mars, and 21,000,000±500,000 for the direction Mars to Earth. This is just for the transfer - they don't take account of the shock of ejection from Earth.

It's much harder to get from Earth to Enceladus or Europa. In their table 5, they estimate around 1±0.7 rocks get from Earth to Enceladus and 6±0.9 get from Earth to Europa. It's a little easier to get from Mars to Europa, 20±4 and <1.4 get from Mars to Enceladus.

So, this makes it rather easier for DNA based life to reach Europa or Enceladus if if there is life on Mars as well as Earth in the early solar system. In that case perhaps 26 or so got to Europa. If there was life only on Earth, perhaps six got there. As for Enceladus then only one rock gets there large enough for viable life from Earth and only one from Mars. So assuming there is life on Mars or was in the early solar system, that suggests that there might be life on Europa from Mars, and just possibly life from Earth there. There's a rather remote chance of life from Earth or Mars on Enceladus.

However, it's not enough to just arrive at their destination. These rocks will hit Europa and Enceladus at great speeds - and it then has to find its way down into its ocean. So is that possible?

First, it seems pretty unlikely for Enceladus in its present state, with most of its surface showing no sign of resurfacing for billions of years except the tiger stripes at the south pole. A small meteorite couldn't find its way through kilometers of ice, the tiger strikes are far too small a target to be easy for that average of one rock to hit, and there is no obvious process cycling material from the surface back down into its ocean.

Even if the life got there, how could it get into its ocean? Also some think that its ocean may be quite young, as the solar system goes, mainly because it is hard to see how it can get enough heat to keep the ocean liquid for billions of years, as we saw above ( Is Enceladus' ocean temporary or long term?). If the ocean is only a billion years old (say), then it gets much harder for Earth life to get there, long after the late heavy bombardment.

For Europa, with its global resurfacing and possibility of liquid water near the surface - the whole thing may be easier. So, all this so far favours Europa over Enceladus for DNA transfer.

However, there's another obstacle in the case of Europa, the impact speed. Another study started with 18,000 particles spherically arranged around Earth with escape velocity, and found that up to 20 reached Titan, but 30 to 100 reached Europa. However the ones impacting on Europa hit it at speeds of 20-30 km / sec. That is a fast impact to survive, even into thin ice overlaying liquid water (as might have been the situation in the early solar system). So perhaps it's not so easy to get a viable population transferred to Europa from Earth either.

Just my own suggestion - with 100 particles reaching Europa, if it was indeed that many, could there be a grazing hit? A bit like aerobraking, for the rock to plough through part of the ocean, enough to slow it down but not stop it immediately, for a later hit? Or through aerobraking into Jupiter's atmosphere, and then hit Europa? But either of those would be quite a coincidence.

There's also the idea which we mentioned, of life that was seeded from a planet orbiting another star that past through our forming nebula. Distant cousins with last common ancestor from a planet around another star (above). We'd still receive rocks from the birth nebula up to 700 million years after our solar system's formation. So, it could still be receiving them up to 650 million or more years after the formation of Earth's Moon.

So - is that a heads up for Europa? Well, I can't find anything yet on this specifically, seeding of Europa and Enceladus' oceans by life from another star in the sun's birth nebula, so let's just take a look at the question and try to come up with some preliminary thoughts. If you know of anything, do say!

The timing is right for Europa at least, since the Galilean moons formed soon after Jupiter. One theory is that they were the last of five generations of moons that formed around Jupiter and spiralled in as the disk around it was depleted from 10% of it's mass to the current 2% of it's mass. (See article in new scientist, and paper). But even then, the whole process only took a few million years so Europa should have formed in plenty of time to receive microbes via panspermia from other stars in the sun's birth nebula.

Those first 0.7 billion years could have been a pretty good time to seed Europa with life. According to one theory then the formation of the Galilean satellites was followed by debris raining down on the moons from carbonaceous chondrite material from the irregular satellites orbiting Jupiter giving thick layers of dark debris - for Europa this comes to 7 - 15 meters over its lifetime, a lot of that in the early solar system, which may have included organics useful for its ocean and for life. Some of it continues to rain down today and would perhaps be responsible for 0.17 to 5 cm of surface material found on its surface (see page 791 of this paper).

If it's true that Enceladus' ocean is rather young then that surely makes it harder to see how life from another star could get into it. It could get to the young Enceladus - but how could it survive into the present, in dormant state, for over four billion years to reseed the ocean? Or, where else could it survive, to seed Enceladus in the recent past However, if the ocean is ancient, which is still a possibility, then Enceladus could be seeded like Europa, in the early solar system.

So, in short, DNA based life in the Enceladus ocean seems a long shot if its ocean is young. If it is ancient, then with the panspermia from other stars hypothesis, it's more likely.

At any rate, whether it's likely or unlikely to have DNA based life on Europa or Enceladus, or even if it seems virtually impossible, it's still a useful instrument to send. A null result could be very important. If we find life in Europa or Enceladus, and we have a DNA sequencer to prove that the life doesn't include DNA, that could be a major discovery, as it would give conclusive proof that it is not related to Earth life.

Also there's another way we could find DNA based life in these oceans. It could also be that DNA is so favoured by evolution that it's likely to come up by convergent evolution. Perhaps even the four bases that were chosen for DNA have some advantage which favour them over the other possibilities?

So, if we do find DNA based life there, I think we'd need to look at the life more closely to see if it seems likely to have a common origin, and to rule out convergent evolution of DNA with the same four bases, however unlikely that may seem.

What if we get ambiguous results from Europa just as we did for Viking?

We have to be prepared for ambiguous results. After all, there are plenty of ambiguous results in planetary geoscience too. Does Mimas have an ocean? We have absolutely no idea. The discovery of physical libration shows that there's something anomalous about Mimas that could be explained by an ocean, or could be explained by a non relaxed interior core.

Saturn's small moon Mimas undergoes physical libration - it rocks back and forth as it orbits Saturn in its slightly elliptical orbit. This means its surface doesn't spin at a constant rate, but is tugged one way then the other by Saturn. This could be because its core is shaped like a football, or it could be because it has a global subsurface ocean like Enceladus. We don't know enough yet to tell.

In the same way the Viking experiments did what they were intended to do, and functioned perfectly. They just detected something strange about Mars, something which they were not designed to study. It's exactly analogous I think. Two hypotheses for the Viking results - complex chemistry, or life - and we can't decide between them yet, not for sure, as the debate still continues as to what Viking found. Just as we can't tell if Mimas has an ocean or an ellipsoidal core.

If we think of the Viking data as scientific evidence, rather than a "mistake", then the natural response was to do a follow up experiment as soon as possible to clear it up. We could send the chiral labeled release which its Principal Investigator Gilbert Levin wants them to fly. If we'd sent this experiment in the 1980s or 1990s then by now we'd almost certainly know for sure if Viking found life or not. We'd be discussing follow up experiments to resolve any remaining ambiguities.

If we are so scared of an ambiguous result in astrobiology that we never fly an astrobiological experiment anywhere unless we are sure it will give a clear cut result - that's going to slow down the pace of discovery.

Europa is only two years travel time from Earth via fast Hohmann transfer, if they use SLS, and by the 2020s we may have more heavy lift capabilities that will make it easy to send follow up missions to answer questions from early missions. Even Enceladus is close enough for a reasonably fast follow up, three years by fast Hohmann transfer.

Also a modern spaceship would be far more capable than Viking. Back then we had nothing else to send with the same level of sensitivity as the labeled release experiment. Now we have many such instruments to detect biosignatures and complement its results. Also, our experiments are now so low mass, and so low in their power requirements, that we can send a suite of several of these on a single mission. They can provide checks on each others' results. So we can expect an astrobiology mission to produce much more robust results than Viking, even when faced with a totally unexpected and baffling situation. The instrument suite should give us with many different lines of investigation to follow right on the spot, without any need for follow up missions.

Ambiguity is something we should cheer at, because it means we are going to learn something. The Viking results were frustrating for the search for life, yes, but they did show that the Mars soil is rather unusual, a mystery that has been partially solved but is still not completely cleared up. We still don't have an explanation of how the unusual chemistry could cause circadian rhythms offset against the temperature variations by two hours. Ordinary chemistry only gives up to twenty minutes of offset. So - if it isn't life, perhaps it is something else. Whatever it is, it is a mystery to clear up, and possibly a sign of new science to discover. See Rhythms from Martian sands - what if Viking detected life? (above)

We should celebrate ambiguous results like the Viking ones. You thought the experiment would be decisive, but it isn't. That shows there is something unexpected going on. In some cases, what you discover when an experiment "goes wrong" like that might even be more interesting than the thing your experiment was designed to test for.

It also gives exobiologist experience in sending their instruments in space. Then, every new instrument that flies adds to the ones space rated for future missions, so adding to our tool box for instruments to use for exobiology. Right now we have no mission tested astrobiological instruments since the Viking instruments and those are far too ancient to fly again. If the DNA sequencer gives a null result for Europa, for instance, it's now a space rated astrobiological instrument with a proven record. Then you can fly it to Ceres, if we find habitats on Ceres, or to Enceladus, and indeed to Mars.

The process also helps inspire a whole generation of astrobiologists. Just to see their instruments fly in space and to operate them can help invigorate the field. At the moment only geological instruments have flown, apart from the very early Viking instrument.

A 57 old astrobiologist would have been twenty when the Viking 2 was finally turned off in April 1980 when its batteries failed. As for the launch of Viking 1 in August 1975, well you have to be 62 by now, to have been a twenty year old when that happened.

You have to start somewhere with the astrobiological in situ searches. And, yes, the safest approach is to search for things you know you know are there. Geology is always a safe bet, because you know there is geology there. We don't, yet, know that there is life there. So for astrobiology there's a risk that your astrobiological instruments don't find anything.

Could this be part of the reason mission planners are reluctant to include astrobiological instruments perhaps? That mission planners feel if they take a "life detection instrument" on a mission, then it is a "failure" if it doesn't detect life or gets ambiguous results?

As I discussed in Requirement that life detection experiments should also provide valuable non biological information (above), in the Europa lander report they lay down as one of their guiding principles:

• "Life-detection experiments should provide valuable information regardless of the biology results"

I think we need to get away from this approach somehow if we are going to have really serious in situ searches in our solar system. Perhaps one thing that might help is if we treat uninhabited habitats as an interesting potential discovery in their own right. They are so rare on Earth. Rocks from volcanoes, soon after they cool down, are inhabitable but uninhabited. Most other things you think about are uninhabitable. Some regions of our harshest deserts don't have life, for instance gypsum pillars in the very driest areas of the Atacama desert, but that's because they are too dry for any Earth life. Just possibly Don Juan lake in Antarctica is uninhabited too - it has microbes but they probably don't grow there, but if so, it's because it is too salty for Earth life.

So, if we do discover a habitat n space, and get a null result from the life detection experiments, this might suggest an uninhabited habitat in space, I think this should be treated as a major discovery in its own right. It would be the first discovery of what may be a common situation in our galaxy.

And as for places that are too dry or too cold or too salty for life, they are of astrobiological interest too, because we don't know of the limits for non Earth life. Is there life on Mars that can live in such habitats though Earth life can't? The answer to that is also interesting both ways, whether we find it, or don't. Earth has so few uninhabitable habitats that perhaps our life has never had to adapt to those conditions. Maybe Mars life has?

So let's send our astrobiological experiments to these places, designed to look for life, or to rule it out, and let's not add any requirement that they also produce valuable geological information. And let's see if there is life there - either way. And if there isn't life in what seem to be habitable brines - especially if it also has organics - well study the heck out of it with our astrobiological and non astrobiological instruments, look for prebiotic chemistry, and try to find out more!

See also (for search for life in icy moon oceans)

Part of this section originated as my answer to If there is a possibility of life on Europa, then why did NASA land a craft on Titan and not Europa? on Quora

• See also my "Super Positive" Outcomes For Search For Life In Hidden Extra Terrestrial Oceans Of Europa And Enceladus (goes into the search for life and possible scenarios in more detail. The life detection instruments section above is from there).
• You might be interested in the facebook group: Search for ExtraTerrestrial Life - Europa & Enceladus + Subsurface Oceans
• See also my As Philae Awakes - Where Might Life Hide In Our Solar System?

Other places in the solar system where potentially life could be found - Titan, Triton, Io, etc

There are many other interesting places in our solar system to search for life .We've already looked at the Moon, Ceres, Europa and Enceladus. Now let's look further afield at some of the other places in the solar system where potentially we might find life, or perhaps, prebiotic chemistry. Some of these have similar planetary protection issues to Mars, Europa and Enceladus. For others, though, the conditions are so different from Earth life that there may be no issue at all even sending human beings to do in situ searches, once we have that capability.

An international committee of scientists, COSPAR, meets every two years to discuss issues to do with planetary protection amongst other things. They publish the results using a classification scheme, described in detail in the wikipedia article on Planetary Protection:. So let's look at those first. I will quote from that article, which is a summary of the guidelines (I've checked it for accuracy - actually I'm one of the authors of that wikipedia article).

Planetary Protection classifications

Category I

“not of direct interest for understanding the process of chemical evolution or the origin of life.” 

• Io, Sun, Mercury, undifferentiated metamorphosed asteroids

Category II

… "where there is only a remote chance that contamination carried by a spacecraft could jeopardize future exploration. In this case we define “remote chance” as “the absence of niches (places where terrestrial microorganisms could proliferate) and/or a very low likelihood of transfer to those places.”

• Callisto, comets, asteroids of category P, D, and C, Venus, Kuiper belt objects (KBO) < 1/2 size of Pluto.

Provisional Category II

Same as Category II, but the classifications in "Provisional Category II" could be changed as we find out more.

• Ganymede, Titan, Triton, the Pluto-Charon system, and other large KBO's (> 1/2 size of Pluto) Ceres,

Category III / IV

“…where there is a significant chance that contamination carried by a spacecraft could jeopardize future exploration. We define “significant chance” as “the presence of niches (places where terrestrial microorganisms could proliferate) and the likelihood of transfer to those places.”

• Mars because of possible surface habitats.
• Europa because of its subsurface ocean.
• Enceladus because of evidence of water plumes.

Category V

Unrestricted Category V: “Earth-return missions from bodies deemed by scientific opinion to have no indigenous life forms.”

Restricted Category V: "Earth-return missions from bodies deemed by scientific opinion to be of significant interest to the process of chemical evolution or the origin of life."

For Category V, sample returns, the conclusions so far are

• Restricted Category V: Mars, Europa, Enceladus.
• Unrestricted Category V: Venus, the Moon.

with others to be decided.

Note on Venus: it was classified as "Category II" by COSPAR in 2006 as for the Moon, so that you only need to document what you did when exploring them. Venus sample return was also classified as for the Moon, so no precautions are needed.

However some astrobiologists challenge these classifications. They agree that there is no risk of contamination either way for the surface, but are less sure about, for the Venus clouds which are high up, with moderate temperatures and pressures, and think it needs to be revised. See Life in the clouds of Venus and following (below).

Its classification at present has nothing provisional about it, it is classified as a simple "category II" and "unrestricted category V" for the sample return . However, I think that could be regarded as possibly subject to change.

So now, what kinds of habitats might we find in these places. Well let's start with one that seems very exotic, but may also be present here too.

Life in liquid ("supercritical") CO₂

This first habitat is unusual because we might have it here on Earth. Yet, if we do, it is so inaccessible, that we have no way yet to know if it exists. If it does exist, it would be a major challenge to study, or it to search for life there. It also may be present in other places in our solar system.

It's based on carbon dioxide. We are used to this as a gas and as a solid, "dry ice", which turns instantly into a gas when it is heated. But, did you know that CO₂ can also be a liquid under high pressures and at normal temperatures? What's more, you can actually find liquid CO₂ in its natural state on Earth, at the high pressures of the ocean depths, Just as volcanic vents often produce carbon dioxide gas, deep sea vents will produce liquid carbon dioxide, if they are deep enough. It's not that deep either. If you explore our oceans around 0.8 kilometers below the ocean surface, CO₂ is a liquid. . This video is taken at a depth of 1.6 kilometers at a white smoker vent system on one side of the small undersea volcano Eifuku off Japan

(click to watch on YouTube)

The "bubbles" in this video are in fact bubbles of liquid CO₂. Also, as you see in that video, there's life there, not actually living in the bubbles but apparently not bothered by it at all. Check it out 38 seconds into that video. See "Life in liquid carbon dioxide". The first discovery of natural liquid CO₂ in the oceans goes back to 1990.

So far this is normal liquid carbon dioxide, which is not especially useful for life. There are fairly large reservoirs of liquid CO₂ beneath the Earth's oceans. For instance a CO₂ lake found off the coast of Taiwan at a depth of 1.4 kilometers.

At that depth, liquid CO₂ is lighter than water. That is how they found it through the rising bubbles of liquid CO₂ rising through the sea water. The whole reservoir would empty, if it weren't kept in place somehow. In this case they think it is probably trapped by an overlying layer of clathrates, because they observed clathrates. They found life there, even in low numbers within the liquid CO₂ layer itself.

So, that's just cool liquid carbon dioxide. But as you raise its temperature, at a reasonably high pressure, liquid CO₂ at around 31.1°C, and 73.8 atmospheres in pressure becomes a supercritical fluid. That's a phase where the distinction between a gas and a liquid disappears. Often the properties change also.

Table from the paper Supercritical Carbon Dioxide and Its Potential as a Life-Sustaining Solvent in a Planetary Environment by Ned Budisa and Dirk Schulze Makuch

This is important for habitability. CO₂ at these temperatures (above 31.1°C) and pressures (above 73.8 atmospheres), is actually a solvent like water. One atmosphere corresponds to 10.3 meters of water. The surface is at one atmosphere already. So for it to be supercritical it needs to have a temperature above 31.1°C, and it needs to be at a depth of at least 10.3*72.8 meters or around 750 meters.

The researchers Ned Budisa and Dirk Schulze Makuch found that this supercritical carbon dioxide actually has advantages over water for life processes. Enzymes are more stable in liquid CO₂ than they are in water, and it also helps makes enzymes more specific about the molecules they bind to.

This supercritical liquid CO₂ is often used for sterilizing, because of its affinity for organics. However, some microbes and their enzymes can tolerate living in it. This was a very surprising recent discovery reported in February of this year. The researchers found six strains of microbes, isolated from three sites targeted for geological carbon dioxide sequestration - that have the astonishing ability to grow on the interface between water and supercritical CO₂. See Microbial growth under supercritical CO₂.

So, does Earth have supercritical carbon dioxide? Below a depth of around 3 kilometers, CO₂ becomes heavier than water. So, there are likely to be large reservoirs of liquid CO₂ below that level in the floor of our oceans, but they would be hard to detect, as no bubbles would escape from them. So - do these reservoirs exist? If they are deep enough, in the sea floor, they'd be warmed by geothermal heating.

Are any of them warm enough for supercritical CO₂? And if so, are they perhaps inhabited by novel forms of life?

That's a lot of "if"s but isn't it fun to think that we could possibly have life as novel as that even on our own Earth? If so, we simply would have no idea that it is there yet. Deep below the ocean floor, at a depth of over three kilometers, we might have exotic lifeforms that live out their entire life in lakes of warm and dense supercritical carbon dioxide under high pressure. Perhaps they would only be able to survive there, and if so, we'd never see them in our biosphere.

You get supercritical CO₂ on other planets too. Indeed the entire Venus atmosphere is supercritical at ground level and for a short way above ground level too. However - it's not a likely place for life. It's too hot for organics to be stable. As well as that, there are different kinds of supercritical carbon dioxide. The Venus version is more of a gas than a liquid at those temperatures. That's because it is below the "Frenkel line" - the line above which a supercritical fluid has more liquid like than gas like properties (for details see page 2786 of this paper).

Diagram from Structural Evolution of Supercritical CO₂ across the Frenkel Line (Text for "supercritical fluid" moved and duplicated) - a supercritical fluid is called a "fluid" whether it is above or below the Frenkel line. But below the line, it behaves more like a gas, and above it, at higher pressures or lower temperatures, it behaves more like a gas.

The Venus atmosphere is dense enough, at 9.8 atmospheres, to be above the critical pressure for a super fluid, but it is not above the pressure needed to behave like a liquid, so it is a supercritical fluid that behaves like a gas. According to some theories, the early Venus atmosphere could have been much higher, high enough so that the supercritical fluid was above the Frenkel line, and if so, early Venus could perhaps have been habitable for exotic lifeforms able to survive in supercritical CO₂. But it would probably have been too hot even so.

However even Venus may have a layer of supercritical liquid CO₂ a short way below its surface, at increased pressure, and this then would be more like a liquid. However it would be at very high temperatures, and so, not so congenial for life right now. Perhaps in the past, when Venus's atmospheric pressure was probably several times higher (getting into the hundreds of bars), and the temperatures cooler, it might have had supercritical liquid CO₂oceans, which might have flowed like a fluid and carved out some of the features found on the surface. Nediljko Budisam and Dirk Schulze-Makuch suggest that if so, and if it still has supercritical liquid CO₂, it might still preserve some traces of that early biochemistry. See the end of section 4 of this paper.

You don't need such a thick atmosphere (as on Venus), nor do you need oceans (as on Earth) to supply the pressure needed for supercritical carbon dioxide. Those high pressures can come from surface ice and rock layers too, and this could happen on Mars. Liquid CO₂ would be stable on Mars beneath about 100 meters of rock, in the cold conditions there. At one point this was the favoured explanation of the Mars gullies as many of them start at a depth of about 100 meters below the top of the cliff.

The temperatures would normally be far far too cold on Mars for supercritical carbon dioxide even at a depth of 100 meters. But if you combine those conditions with high enough temperatures, say from hydrothermal heating, again you would have supercritical CO₂ on Mars. So, that's a potential habitat for an exotic form of life on Mars, and if so, it could exist there right through to today.

Life or prebiotic chemistry in comets

Comets seem rather attractive places to look for life at first. After all, they have ice, and organics, and indeed, all the ingredients for life. Many of them get warm enough for liquid water too. That's especially so for short period comets, which come close to the sun regularly. For instance Comet 67P/Churyumov–Gerasimenko has an orbital period of 6.45 years. That's not so long. Dormant microbes can survive a six or seven years freeze easily.

However other things make it less likely that we'll find life on comets. The problem is, the comet is of course surrounded by a hard vacuum, and any water exposed to a vacuum just evaporates into water vapour. At the temperature of melting ice, which is 0 °C in a vacuum too, then it loses ice at a rate of about 60 meters a day! Ice also sublimates rapidly as it approaches those temperatures

To contain water as a liquid you'd need some impervious surface layer like the ice crusts of Europa and Enceladus. Those work because they are very cold, and so the ice, though it does still sublimate, does so very slowly.

ESA's Rosetta and the Philae lander gave us the closest survey of a comet to date. The exposed layers seem basically unaltered from their time of formation in the early solar system. It seems to be made up of many three meter diameter spherical "goose bumps" piled together, as you see in places where the interior is exposed to view

The roughly spherical bumps on the sides of this pit in the surface of Rosetta are all about 3 meters in diameter. They are remarkably consistent in size wherever they are spotted on Rosetta, suggesting that the entire comet interior may well be made up of these - which may be the "building blocks" that later came together to make comets.

This all points to a picture where the comets are undifferentiated primordial objects, loosely aggregated, and not likely to have liquid water habitats. However, there are some puzzling observations which don't fit neatly into this picture.

Unlike meteorites, we don't have samples on Earth to study. Comet fragments don't survive impact into the Earth's atmosphere. They are too soft and mainly made of ice - so the only way to find out about them is to send our spacecraft to comets to study in situ or return samples. And that's what Stardust did, collecting fragments of dust from comet Wild as it flew past it.

Shows site of a particle impact into the stardust aerogel

Close up of two of the particles detected in the Stardust aerogel, impact was from right to left.

The puzzle is, these samples returned from comet Wild shows clear evidence of alteration by liquid water in some of the smaller particles. The original paper in 2011 looks at various nickel, copper and zinc bearing iron sulfides in the comet and found evidence that they must have formed in wet low temperature hydrothermal environments (below 210°C). Several independent studies since then have confirmed this. For a technical overview of the latest research see this paper under "Aqueous alteration in comets". I'll summarize some of what they say here:

So, the first question was, could it be that the comets originally consisted of liquid water with an impermeable crust? That is a model by Wickramasinghe.

Well, it seems unlikely because they didn't find any of the clay like materials that are common in carbonaceous chrondite meteorites (polysilicates). If the comet was ever altered in a large scale way, they should have found clay, especially since the samples did have silica particles, so it could have formed clays as well as the iron sulfides which they did find. But they found no clays at all. Also, all the particles collected larger than 2 microns were completely dry (anhydrous), at least the part that they sampled in dust ejected from the comet.

They came to a similar conclusion by considering meteorite showers from comets (the showers with a comet as the parent body". The "shooting stars" in these showers disintegrate so easily in our atmosphere that they can't be much stronger than soft snow. If the comet was altered by water in a large scale way, those fragments would be harder, like the carbonaceous chondrites.

From these and other lines of evidence they conclude that "we do not believe that Wild 2, or at least its portions ejected into space, has experienced pervasive aqueous alteration in the parent body"

However, from the evidence of the iron sulfides, there obvious was some aqueous alteration, just not pervasive. The main question then is, did the water form on the comet, or was it brought there from elsewhere in the solar system (incoming meteorites for instance, including debris from other objects in the Kuiper belt when it was in the outer solar system). If it formed on the comet, then it could be debris from areas of the comet made liquid by impacts.

Core of Comet 81P/Wild. It's about 5 km in diameter. This is the only comet we have samples from on Earth, taken from its coma by the Stardust probe. Some of the smaller particles show clear signs that they were altered by water. Yet the sample also has large mineral particles that are very dry with no sign of ever being wet, and most strikingly, no clays were found.

The tentative conclusion is that probably Comet Wild has not been pervasively altered by water. Perhaps it had meteorite impacts that made parts of it liquid, or alternatively, the altered materials could have come from elsewhere.

The evidence of the influence of past liquid water is clear. Where the water was and how it formed is less clear, requiring a lot of extrapolation especially since Stardust only sampled its coma, so, material that was easily liberated from the comet.

Temporary water on comets doesn't mean life

If the water was the results of impacts, then it's not too promising for the search for life. We already have Carbonaceous chondrites, a rare type of meteorite with organics in them. They show clear signs of alteration by water, and they don't have any signs of life (at least not generally accepted), though lots of interesting organic chemistry. Perhaps their parent bodies just didn't have liquid water for long enough for life to evolve? Similarly, temporary impact lakes on comets wouldn't seem to be long lived enough for life to evolve.

There's one dissenting voice here, however, Chandra Wickramasinghe. His model predicts liquid water habitats on comets that could be recurring and long term. This theory is not widely accepted. The summary just mentioned says that his model seems "contrived". But let's have a look at it.

Wickramasinghe's theory

Chandra Wickramasinghe's ideas are interesting then, though often controversial. For a long time he and Fred Hoyle were just about the only ones arguing for panspermia, based on their idea that life originated in comets and spread to planets. Recently the idea of transfer of life between planets became mainstream, although through life spread as a result of meteorite impacts and debris from those impacts, not involving comets (except when they impact on a planet and send debris from that planet into space)..

He has argued for many years, in connection with his comets panspermia idea, that there may be liquid habitats inside comets where life may flourish even to this day. His theory is based on the idea that comets pass through two main stages in their life (from his 2009 paper)

He suggests that to start with, in the early solar system, the comets had radioactively heated liquid cores. He suggests that this may also be when life originated. The Wild results may seem puzzling, as he would expect clays to form in the cores of the comets. However his theory is not yet ruled out by the Wild observations, as the clay would only form in the interior of the comet, in his model. As this clay hardened, it might well have formed boulders and rocks too, which would be unlikely to be ejected into space.

The second stage of his theory follows when the radioactive elements decay, and are no longer a source of heating. Distant comets would just freeze through at that point and the life would evolve no further, but short period comets could take advantage of the heating of the sun at perihelion.

Crust of "sunburnt organics"

In his model, present day comets have a dark crust of "sun burnt" organics about 1 - 2 cm thick, which can frequently break and reseal. The solar heating heats the ice in a layer below this surface crust. The crust then traps the water vapour, at a saturation vapour pressure of 6 millibars. He predicts that these pools could form at depths from 2 cms downwards - and pools that form at depths of 30 to 50 centimeters below the surface could remain liquid throughout the perihelion passage of the comet. As the water vapour pressure increases some of the water would escape through cracks and pores in the organic layer forming the comet tails - but these would then reseal, so keeping most of the water liquid.

If his model was right, it would make it possible to have liquid "puddles" of water just a short way below the surface of the comet. They would be temporary oases that form during every perihelion, as places where microbes could survive.

Comet 67p does seem to have a hard crust, as we see from the "trampoline" effect, from the way that the Philae lander bounced off the surface - and this was a surprise to many who thought Philae would sink into a surface with the consistency of soft snow, or even less. I wonder if that is a point in favour of this model?

Of course you don't need to subscribe to his entire "life started in liquid core of ancient comets" theory to be interested in his model . These recurring perihelion puddles, if they exist, could also be seeded by life from elsewhere (including Earth). Or they might not have life, but could have prebiotic chemistry altered in the presence of liquid water.

So - were the chemicals found in comet Wild the result of processes inside the comet, such as either a temporary impact lake, or these crust covered perihelion puddles? Or were they the result of delivery of material from elsewhere in the solar system?

Perhaps there was a chance of ESA's Philae finding the answer but sadly it didn't get to do much analysis on the surface of Comet 67p. The question of whether there are any water altered chemicals on comets, or indeed whether comets do form liquid water under the surface crust has to stay open for a while.

Life or prebiotic chemistry on the Moon

As we saw in the introduction, the Moon is classified as category II:

"… where there is only a remote chance that contamination carried by a spacecraft could jeopardize future exploration”. In this case we define “remote chance” as “the absence of niches (places where terrestrial microorganisms could proliferate) and/or a very low likelihood of transfer to those places.”

But what about the ice at its poles? Also what about the discovery that the earlier deeper layers of the Moon might have had higher water content? You might wonder if we might learn new things about the Moon that could lead COSPAR to revise their Category II classification. How remote is that "only a remote chance" for the Moon? Could the ice at the poles be habitable?

Well first, ice at the lunar poles is not exposed to sunlight so there is nothing to warm it up enough to be liquid. Ceres might possibly have habitats exposed to he surface, which is why the Dawn mission has to stay in orbit at the end of the mission instead of crash into its surface. But that's because it has a thick icy crust and meteorite impacts expose that ice to the surface- it has ice in Oxo crater exposed to direct sunlight which must be rapidly subliming (see Search for early life on Ceres (above) ).

The ice at the lunar poles would of course melt locally after meteorite impacts, so it must frequently get small amounts of liquid water forming after small impacts, but this would soon freeze over. That water r could be prevented from evaporating by a layer of organics, not too implausible for comet originated ice - but to have the thermal insulation to form long lived impact lakes the ice would have to be very deep and the polar ice is probably only meters deep.

Ice sublimates very quickly in a vacuum at temperatures approaching liquid water. You would lose 60 meters a day thickness of water at 0 °C exposed to a hard vacuum. The Moon has ice in permanently shadowed craters at the poles, but these are extremely cold. Temperatures as low as –272 °C, as measured from orbit, the lowest surface temperatures known in the inner solar system. That's why ice can accumulate there even in a hard vacuum. But ice warm enough to be habitable wouldn't last long, unless it was covered in some layer to insulate it from the vacuum of space. That is possible actually, a thin layer of organics from comets could do the trick, high molecular weight organics.

There are permanently shaded regions up to 58 degrees from the poles (only 32 degrees from the equator). Though these regions are too warm to have ice on the surface, there may be ice there underground. See Ice may lurk in shadows beyond Moon's poles (Nature, 2012). There's evidence of volatiles escaping from the Moon, and Arlin Crotts put forward the hypothesis that the Moon has subsurface ice, a few meters down over much of its surface which could be detected by ground penetrating radar. But this is all ice, not evidence of liquid water, The Moon is hotter, and possibly has more water, deep below the surface, but nobody is suggesting it has a subsurface ocean.

Carl Sagan suggested the Moon could have life below the surface in the early 1960s. According to their theories at the time (before Apollo) then if the Moon started off with the same proportion of water as the Earth, then it could have had open water and an atmosphere for anywhere between ten and a hundred million years or so in the early solar system, long enough for organics to form through processes in the atmosphere (back then they thought of it as forming as a result of lightning as in the early Uri Miller experiments). It could then have been covered over by dust from the meteorites. So then, according to their ideas at the time, it could be covered over to depths of up to tens of meters. and could be present at concentrations of up to 10 grams per square centimeter of the surface mixed to some depth. This couldn't happen in the lava flows but he thought it was possible in the lunar highlands and much of the lunar far side (more about that here). Then, the Moon could also have ice in the permanently shaded craters and below the surface. That's all in pages 14 onwards of his book "Organic Matter and the Moon". (free download from NAP).

He then considers the possibility of life on the Moon, starting the section (pages 24 onwards) as "Because of its great potential importance, the admittedly very speculative possibility must be raised that, at some time in the remote past, life arose on the Moon. ".

They also recognized the possibility of ice in permanently shaded craters on the Moon. The temperature also increases with depth, as on Earth. He finds that the optimal temperature for life of around 300 K is at the same depth that's optimal for organics too, and if there's ice there as well, then so long as it is trapped, isolated from the surface, you have all the essentials for a habitat for anaerobic life such as you find in petroleum fields.

He summarizes it all in a separate paper here (page 92):

"Thus, at a depth of roughly fifty meters, we expect temperatures on the Moon near 300 °K. At that depth the diurnal temperature fluctuations are damped, and temperature should remain quite constant over the lunar diurnal period. It has also been suggested that the Moon today may have a layer of subsurface permafrost beneath which liquid water may be trapped. This layer would be at about the same depth as the layer of moderate temperatures. It is possible that processes in the early history of the Moon led to the format ion of organic matter, just as in the early history of the Earth. If such organic matter were also sequestered at such a depth, we see that the possibility of indigenous life in the Moon's subsurface is not out of the question "

He concludes

"It follows that the possibility of an extant lunar parabiology must not be dismissed in as cavalier a manner as it has been in the past. As we shall see in section VI, it is likely that relics of past lunar organisms , if any, could be pre ­ served indefinitely if sequestered well beneath the protective cover of the upper lunar surface material. Thus, neither should the possibility of lunar paleontology be over looked. It is probably unnecessary to remark that the study of any extraterrestrial organism will have the deepest influence on the fundamental problems of biology. Even if the chances of success are small, attempts should be made to detect lunar subsurface autochthons, both living and dead . "

If there were such habitats they would be insulated from the surface, but there is a risk of contaminating them if large numbers of microbes are introduced to the habitat as a result of an impact on the surface. That's why the first few Ranger probes to the Moon were sterilized in the early 1960s. If there were trillions of microbes deposited on the Moon then perhaps some of them might find their way into these subsurface habitats tens of meters below the surface? See conclusion of this paper.

However that's back in the 1960s. Since then, we now no longer think early Moon could have open water or an atmosphere. On the other hand ice is still possible there below the surface, especially below the permanently shadowed craters and Arlin Crotts thinks it may be present elsewhere as well. The temperature does increase with depth. Could there be ice as deep as tens of meters below the surface though? If you go with Arlin Crotts idea of ice replenished by outgassing from below, then maybe there could. Or, could it be deposited that deep by comet impacts? After a large comet? What about organics? Well organics do get delivered by comets and asteroids and we no longer think of it a needing to be made in an atmosphere using lightning and such like.

If there was water anywhere on the surface or deep below the surface of the Moon, well it would have to be isolated from the surface in some way to remain liquid. If it had any ice heated geothermally at some depth, then theoretically, it could remain liquid if it had an impermeable covering layer of organics or other materials.

Those are a lot of "ifs" though.

Could such habitats be possible even on or near the surface? Ice could stay warm enough for liquid water, heated by sunlight, if covered in an impervious layer similar to the organic comet perihelion puddles idea, see Wickramasinghe's Crust of "sunburnt organics" (above).

So, I have wondered if there is any possibility of someone making a similar case to Carl Sagan's ideas today, for a deep subsurface biosphere on the Moon. But I can't find any papers that suggest this. If any of you reading this know of something do say - either way - either suggesting it's possibility or showing why it can't happen.

At any rate, so far there is no suggestion of any of that, and at the moment the consensus seems to be that the probability of any habitats on the Moon that life could replicate is very low. So, it seems there is no problem at all sending humans to the Moon,, at least as far as introducing Earth life to the Moon.

However the Moon does have great interest for prebiotic chemistry as well as for the search for organics from comets, asteroids, and also meteorites that may contain life from other planets including Earth and from the early solar system.

Life in atmospheres of gas giants

This is an idea suggested by Carl Sagan in his Cosmos series. Here he is talking about it:

(click to watch on YouTube)

His scientific paper on it is Particles, Environments and Possible Ecologies in the Jovian Atmosphere.

The idea was also highlighted more recently by Stephen Hawking in his "Into the Universe"

(click to watch on Discovery)

It's been taken up by astrobiologists since Carl Sagan and elaborated in a fair bit of detail, Louis Irwin and Dirk Schulze-Makuch look at it carefully in In Cosmic Biology: How Life Could Evolve on Other Worlds,

The upper layers of Jupiter, the ones with the clouds we can see from Earth, may perhaps be habitable but would be far too cold for liquid water. Lower down, the temperatures are high enough, but there is little water (see page 166 of the book). So, it is unlikely that Earth life could survive there, not being pre-adapted to the conditions.

That's why Galileo at the end of its mission was sent to crash into Jupiter, for planetary protection reasons, and it's also why Cassini will be sent to crash into Saturn. Astrobiologists are pretty sure that Earth life can't survive in either of the gas giant atmospheres, and by doing this we avoid the possibility of contaminating the interesting moons of these giants with Earth life.

However, that's only for Earth life. What about life based on different principles from Earth life? The main problem in that case, for Carl Sagan's gas creatures, is how the life could evolve there in the first place.

Yes, there are trace amounts of hydrocarbons, nitrogen compounds, and sulfur complexes. But there's a lack of oxygen. Also, there's no mechanism for concentrating the organics into one place. If life starts to form in some region there, it's also liable to be torn apart by the strong winds, and turbulence, and affected by strong radiation. They did think that there is a possibility that the conditions there could lead to complex organics. These could also form larger structures made up of large scale chemical networks of life precursors. But without a hard and fast physical boundary and with no way to achieve exact replication, then it couldn't develop to the next step.

So there might be something that resembles life in some ways, but would not fit the criteria to actually count as life.

They did find one way though for life to get started in the gas giant atmospheres. That's if the atmospheres could be seeded by life from their own planetary moons, such as Io for Jupiter, and Titan for Saturn. Those are both moons with radically different chemistry from Earth life, which may make the lifeforms similar enough in chemistry to the planets for their life to survive there.

• Life on Io, which could seed Jupiter, would be pre-adapted to radiation and very low temperatures and to a sulfur based chemistry, all conditions that could be found in the Jupiter atmosphere.
• Life on Titan, which could seed Saturn, would be adapted to a dense, cold, organic atmosphere at temperatures far below the range for Earth life, again conditions that could be found in pockets in the Saturn atmosphere.

So, their conclusion is that yes, there could perhaps be life in the atmospheres of these gas giants, seeded by their moons Io and Titan. For the details see page 168 of their book, most of which is available online.

Supercritical liquid hydrogen layers in gas giant atmospheres

Another possibility, again for the gas giant atmospheres, is life at a much lower level in the gas giant atmosphere. At high enough pressures, hydrogen becomes supercritical, and, similarly to the warm high pressure supercritical liquid CO₂ (see Life in liquid ("supercritical") CO₂ above) , it becomes a solvent for organics.

The pressure has to be high enough for liquid hydrogen to be supercritical, but the temperatures not so high that it dissociates organics (for discussion the authors choose a temperature < 500 K). Hydrogen first becomes supercritical above 12.8 atmospheres and 33.3 K. Neon and nitrogen also become supercritical at higher pressures of 26.3 and 33.5 atmospheres respectively.

The zone for supercritical liquid hydrogen for some form of life to exploit is very narrow on Jupiter but is quite wide on Saturn. For details of this idea, see page 75 of the The Limits of Organic Life in Planetary Systems which was produced by the Space Studies Board.

Life in liquid nitrogen - Triton

This photograph of Neptune's largest moon Triton shows dark streaks thought to be nitrogen geysers.

Triton's south polar terrain photographed by the Voyager 2 spacecraft. The dark lines are the trails of plumes from volcanoes, thought to be caused by eruptions of liquid nitrogen from below the surface. Some of the plumes were actually observed erupting during the flyby- by anaglyph projection, which made them easy to pick out as they were closer to the spacecraft than the other features.

This was one of the surprise discoveries of Voyager, the Triton geysers, which are thought to be the result of eruptions of liquid nitrogen. So Triton could have thin layers of liquid nitrogen, sandwiched between a surface of solid nitrogen ice and a subsurface of water ice. So could there be life in these layers of liquid nitrogen?

Liquid nitrogen would be a non polar solvent (there is no separation of charge in its molecules), which means it can't dissolve organics (which are polar). So it's no good for life as we know it on Earth, but it might be just the thing for polysilanes, a complex molecule that uses silicon in the place of carbon.

So what about that idea, that Triton and Pluto could have silicon based lifeforms?

It turns out that silanols - a kind of silicon version of alcohol - can dissolve in liquid nitrogen. And - in these conditions, silicon has as diverse a chemistry as carbon, as William Bains, who has been working with Sara Seager at MIT on what life could look like on other worlds, has argued in his hypothesis paper Many Chemistries Could Be Used To Build Living Systems. The silanols have weaker bonds than carbon based organics - but this is just the thing you need in very cold conditions.

It's "silicon based life" but not in the sense that rocks are made of silicon - any more than humans are diamond or graphite or charcoal based life. The silicon of course is combined in long chains with many other atoms. See also Peter Ward's chapter on this idea.

The main problem with silicon taking the place of carbon in biochemistry is that the silicon- hydrogen bond is destroyed rapidly, within hours at mammalian temperatures, in presence of water. It is not nearly as stable as the carbon hydrogen bond.

However if the fluid is not water, and the temperatures are much lower, those weaker bonds and more rapid chemical reactions might be just what you need. He writes:

"As a barrier to using silicon for living systems, however, this is only a problem if we assume that strongly protonating solvents (water, ammonia, hydrogen sulfide, and sulfuric acid in a cosmological context) are the solvent base. Oceans of cold, non-protonating fluids, and specifically liquid nitrogen, would be more favorable for silicon life. In such environments, the inherently greater reactivity of silicon-based chemicals could be an advantage, enabling “living” processes to occur at greater than geological speeds at the relatively lower temperatures where such fluids are stable. Such fluids could also include liquid methane, ethane, neon, or argon."

He suggests that the same approach could also work in oceans of liquid methane, ethane, neon, or argon

"Oceans of cold, non-protonating fluids, and specifically liquid nitrogen, would be more favorable for silicon life. In such environments, the inherently greater reactivity of silicon-based chemicals could be an advantage, enabling “living” processes to occur at greater than geological speeds at the relatively lower temperatures where such fluids are stable. Such fluids could also include liquid methane, ethane, neon, or argon."

If there is life there, then the situation is rather similar to Enceladus and Europa. It would be easy to sample without even landing as we may actually be able to sample the life simply by flying through the nitrogen geysers. If our robots do land there however, there'd be no issues at all of forward contamination by Earth life as there is no way Earth life would be able to survive in these liquid nitrogen "habitats".

However the only close up observations of Triton are the ones by Voyager 2, and we don't yet know enough to say for sure that it doesn't have cryovolcanoes with liquid water or brine or ammonia / water "lava". If it does have liquid water near or on the surface it then becomes a planetary protection issue for a lander.

In any case, as for Europa and Enceladus, then a flythrough of the geysers would have no planetary protection issues for all practical purposes.

(click to watch on YouTube)

If it is okay to land on the surface of Triton according to planetary protection criteria, then there's a rather cool idea for a "Triton Hopper" from a 2015 NASA study.

Triton hopper - Credit NASA / NAC / Steve Oleson

That spherical tank is used to collect nitrogen gas under pressure, from the atmosphere, and from the nitrogen ice on the surface. When it has collected enough, perhaps after a week or two, then it uses it as propellant to hop to another place on the surface of Triton.

(click to watch on YouTube)

The Triton Hopper would use an RTG to heat up frozen nitrogen ice collected from the surface of Triton which it would analyse, and then heat it to use it as a propellant. It could hop about 5 kilometers at a time, reaching a height of one kilometer. It could record the surface from above, fly through geysers, and analyse the points it lands on, traveling 300 kilometers in 60 hops. Abstract here and detailed description of the hopper engine. wikipedia article has several cites for the idea.

Life in liquid nitrogen on Pluto?

Before the New Horizons flyby, researcher Jeff Kargel suggested that Pluto could have rivers of liquid nitrogen or neon. If not on the surface, these could flow below the surface since solid nitrogen is "a fantastic insulator". Before the flyby again, Alan Stern and Kelsi Singer thought that there may well be nitrogen geysers, like Triton, because Pluto should be constantly leaking nitrogen into space and would have to be replenished from within.

They did spot what might be wind streaks

The very light gray smudges in these images may perhaps be wind streaks. If so they could be evidence of wind, or possibly even geysers.

At that point the team thought that Pluto must be losing 500 tons of nitrogen per hour. However now that all the data is returned to Earth, the studies of the results from the flyby revealed a rather different picture. The upper atmosphere of Pluto is very cold and it's now thought to be too cold for nitrogen to escape into space, though methane does. It's lost 28 meters thickness of methane over its lifetime. However it has lost almost no nitrogen because the upper atmosphere is so much colder than they expected.

Haze layers in the Pluto atmosphere, up to a height of 200 kilometers, also with some illumination of the night side of Pluto. In the diagram, SP is the south pole and AS is the Anti Sun position on Pluto. The haze layers were more complex than expected and the higher layers of the atmosphere too cold for nitrogen to escape. The illustration is Figure 4 from this paper.

Pluto is now thought to have a nitrogen cycle, but not to be losing significant amounts of the gas. However it does have nitrogen glaciers.

Sputnik Planum, the left hand side of Pluto's "Heart" region, is a vast nitrogen glacier, or ocean, 1000 km wide. It has no craters, so must be active right now, either it flows easily like a glacier, or it reforms its surface every few million years. Nitrogen ice is denser and also softer than water ice on Pluto, and it has iceberg like mountains of ice floating in the nitrogen.

So, we haven't yet found liquid nitrogen on Pluto but it does have soft nitrogen ice. No confirmed nitrogen geysers yet. There is though good evidence that it had liquid nitrogen in the past. This is a frozen liquid nitrogen lake

Frozen nitrogen lake on Pluto which may originally have consisted of liquid nitrogen. The image shows details as small as 13- meters across and the lake at its widest point is around 30 kilometers across. Alan Stern is quoted as saying “In addition to this possible former lake, we also see evidence of channels that may also have carried liquids in Pluto’s past,”

So, I wonder, even if it doesn't have present day liquid nitrogen, if Pluto could be a place to search for traces of past silicon based life to test for William Bains hypothesis of low temperature silicon based biochemistry mentioned in the last section

Life in a Pluto subsurface ocean?

Pluto should have had plenty of radioactive elements in its core when it formed, enough to melt a liquid ocean below its surface. So, is it still there or has it frozen through completely? Well, New Horizons found indirect evidence that some of its subsurface ocean may still be there. This evidence is in the form of recent geological features that suggest that the surface of the planet has expanded upwards through water in its interior freezing into ice. Also that it hasn't compressed (which would be a sign that the ocean was frozen through completely):

.

The cracks here show where the Pluto crust has expanded. This suggests it had a liquid water ocean that froze into ice. But there are no compression features. A completely frozen ocean should have eventually compressed to Ice II.

The puzzle here is that if its ocean froze through completely, it should have converted to Ice II. This is denser than ordinary ice, so after first expanding, the ice shell should then compress and you'd see compression features as well as expansion features. They don't see that.

It could avoid getting turned into Ice II if its crust is less than 161 miles (260 kilometers) think, but its icy crust is estimated to be 186 miles (300 kilometers) thick.

So there is no obvious way out of this for a frozen ocean. So, the most likely explanation seems to be that it still has liquid water somewhere below the surface.

How could Pluto's ocean have stayed liquid so long? Well the frozen nitrogen on its surface is a good insulator and can help to keep the ocean liquid. Also ammonia in the water acts as an antifreeze, and can also help keep it liquid at lower temperatures. Models that reproduce these observations, using 5% ammonia in the water, suggest that it could still have a subsurface ocean of liquid water. For details, see Does Pluto Have a Subsurface Ocean? New Research Says Probably

So if it has liquid water, what about organics and the ingredients of life? Well there's evidence for organics in its atmospheric hazes, at far higher altitudes than expected. It actually has an atmosphere similar to Titan, though a lot thinner of course. It consists mainly of nitrogen, and with methane as a minor component.

In Titan's atmosphere, UV light breaks down methane, so that it can join together to form longer chain hydrocarbons. The same process probably happens in Pluto's atmosphere, but the resulting particles fall much faster, in its near vacuum atmosphere, and instead of forming spherical shapes as they do in Titan's atmosphere, they seem to form fractal shapes (a bit like snowflakes).

The resulting rain of snowflake-like organics onto the surface turn parts of it red, the colour of "tholins". Michael Summers, planetary scientist with the New Horizons team and specialists in structure and evolution of atmospheres of planets, estimates that it may have 10 to 30 meters thickness of organics in places on the surface.

Combine those thick layers of organics on its surface with radioactive heating and possibly liquid water below the surface, and, it's a long shot perhaps, but Pluto could potentially have life. See Does Pluto Have The Ingredients For Life?

. For more details on the latest ideas about Pluto see New Horizons: News from Pluto.

Other subsurface oceans - Ariel, Triton, Eris, Sedna etc

Though most of the attention has been on Europa, Enceladus,perhaps Ceres, and now Pluto, there are many other places in the solar system that could have subsurface oceans. Dione, Rhea and just possibly Mimas too are all candidates for subsurface oceans in the Saturn system. I mentioned those already in Enceladus geysers as the low hanging fruit right now - with other interesting targets in the Saturn system - could Dione also have a subsurface ocean? (above)

But let's cast our net wider, beyond the Saturn system. Let's look at Uranus. The best candidate there is Ariel, the moon with the youngest looking surface of all of them. Our only close up views of it are from Voyager 2 in 1986

Mosaic of the four highest resolution photographs of Ariel taken by Voyager 2 in 1986. This is the only time we have seen it close up. It's got the youngest surface of any of the moons of Uranus. Is it at all possible that it still has an ocean today, deep below its surface, like Enceladus?

There was a plan at one point to send Cassini to fly through the Uranus system at the end of its mission at Saturn, which would have taken another twenty years, amongst other proposals considered, before the final decision to crash it into Saturn.

There are no missions currently planned for Uranus though there was a proposal for a Uranus orbiter submitted to ESA's call for science themes for its large-class mission program in 2013.

Two scientists put forward a suggestion in Nature in 2012, that Ariel could have a subsurface ocean like Enceladus. Their argument is that heating from radioactive materials in its interior could heat up its interior to the point where the ice, especially if mixed with ammonia, could respond to tidal flexing. If so, they calculate that the tidal heating effects from the other moons of Uranus would give it five times the tidal heating levels for Enceladus.

This paper looks at Ariel in detail. It is not in any resonance at present but in the past it could have been in a 5:3 resonance with Miranda, a 2:1 resonance with Umbrial (4.26 billion years ago) and a 4:1 resonance with Titania (3.8 billion years ago) - see page 12 of this article, at different times, all of which could heat it up.

However it seems to be hotter than those resonances could explain. It seems to have an anomalous amount of heat at present. This is not measured directly but estimated from its surface features. They calculate an estimated temperature at the base of the lithosphere (its rigid outer shell) of 99 to 146 K, so up to -127 °C based on their study of these features.

Titania and Oberon are also possible candidates for subsurface moons in the Uranus system.

So now turning to Neptune, then the only candidate is Triton. But that's an interesting one as it is in a retrograde orbit. It must have had a lot of tidal heating when it was captured by Neptune, but did it retain enough heat to have a liquid ocean today? It could have a thin layer of liquid water mixed with ammonia if it started originally as a moon in a highly eccentric orbit that gradually got circularized.

Montage of Triton and Neptune from Voyager 2 photos. Triton is in a retrograde orbit and it could potentially still have a thin liquid ocean due to tidal heating as its orbit was circularized. Paper here.

Other places where there just possibly might be subsurface oceans of liquid water include the largest Trans Neptune Objects such as Sedna, Eris (2003 UB313) and Orca (2004 DW)

Modeled subsurface oceans assuming ammonia concentrations of 5% for all except Sedna, 1.4% and Titania, 4.3% Figure 7 from this paper

Titania included again for size comparison with ammonia 4.3%. Assumes Oberon 2.9%, Rhea 0.5%. Iapetus and Enceladus don't end up with a liquid layer in their model (though we know Enceladus does have one). Figure 6 from this paper.

Life in the oceans of ethane and methane on Titan

Saturn's moon Titan is, so far, the only known place in the solar system with liquid on its surface apart from Earth. Most of the lakes are around the north pole, with one lake, lake Ontario, at the south pole. But the lakes are of ethane and methane instead of water.

Glint of sunlight on the lake region around the northern pole of Titan.

(click to watch on YouTube)

Titan is the only place in the outer solar system which we have sent a lander to, the Huygens probe.

(click to watch on YouTube)

Sometime maybe we will send some more probes there to explore it further. This was a recent idea for a submarine to explore Titan:

(click to watch on YouTube)

And this is an idea for an aerostat to explore it's atmosphere, VAMP

The Northrup group's VAMP aerostat could also be used for Titan - where it is called Titan Lifting Entry & Atmospheric Flight (T-LEAF)

See also Life on Titan (wikipedia).

This is such an extreme habitat, at temperatures of -180 °C, that it seems impossible that Earth originated life could grow there naturally, except in cryovolcanoes or ice temporarily melted by impacts. The limit for microbes to complete their life cycle is usually given as around - 20 °C (the usual temperature for freezers), around -10 °C for lichens, and around -2 °C for higher lifeforms such as insects, though there can be some metabolic activity down to -26 °C for microbes and down to -50 °C or lower for some cold hardy multicellular life.

Details, lowest temperatures for Earth life: The lowest temperatures that microbes can grow, i.e. complete their lifecycle and reproduce, is usually said to be around - 20 °C in salty brines, for instance, there have been no examples of spoilage of food in freezers kept below that temperature. When cold loving (psychrophile) microbes are cooled, they produce a kind of antifreeze that prevents formation of the ice crystals which normally damage cells as the ice expands. Instead their cells gradually turn into a kind of glass as they cool down (vitrify). As they do this, they can can still do some metabolic activity until they are completely vitrified and it stops completely at - 26 °C which seems to be the limit for any activity at all.

Some invertebrates may continue to have some metabolic activity down to much lower temperatures, below - 50 °C. Insects may not just have metabolic activity but even remain active at low temperatures, for instance a Himalayan insect which is still active down to −16°C. But even these cold tolerant multicellular lifeforms typically need temperatures of around -2 °C to complete their life cycle.

Many microbes and multicellular life also, if the cooling is slow, will revive after vitrification when restored to normal temperatures. More details here: A Low Temperature Limit for Life on Earth, and The thermal limits to life on Earth, for details of how low temperature life is possible, see Psychrophilic microorganisms: challenges for life , also see more cites in wikipedia article Psychrophile

Also there is no liquid water, instead, ethane and methane, which makes it exceptionally interesting for exobiology, if there is life there. There is no way it can be similar to Earth life.

Even the cell walls can't be the same. There are some microbes on Earth that can live in hydrocarbons, by having an extra layer to protect themselves from the oil, including a report of one strain of Pseudomonas able to tolerate up to 90% of a hydrocarbon alpha-pinene. They don't metabolize the hydrocarbons but usually have an outer membrane modified to let them tolerate them. See page 38 of this PhD thesis by Lucy Norman. However Titan life would have to live in 100% hydrocarbons.

The cell membrane structure of Earth life would not work in those conditions as it depends on hydrophilic molecules that are attracted to water to retain its structure. There is water inside and outside the membrane, and then the membrane itself consists of long chain molecules with one end hydrophobic and the other hydrophilic. Then they join together back to back to form a membrane with the hydrophilic heads pointing both outwards and inwards and the hydrophobic tails touching each other in the middle of the membrane.

This shows how it works on Earth:

Earth life cells with the hydrophobic tails inwards and heads outwards

Yellow polar heads separate the grey hydrophobic tails from the surrounding water and the interior fluid of the cell itself. Diagram by Jerome Walker.

This won't work on Titan as it will need to have the hydrophobic tails outwards, like this:

Figure 7 of Lucy Norman's PhD thesis on spontaneous self assembly of reverse vesicles suitable for Titan in hydrocarbon fluids. She credits it as: "Illustration of the reverse vesicle structure taken from Norman & Fortes"

Lucy Norman looks at many ways that reverse vesicles can spontaneously self assemble, from her experimental results. It's very technical. But she has a summary in her chapter 9 which looks at the applications for astrobiology.

I will paraphrase what she says there in the first few paragraphs of her chapter 9 (as it uses rather technical language):

It's unlikely that these reverse vesicles would be used only for cell walls. The hydrocarbon lakes of Titan are likely to contain more than one alkane liquid, with methane-ethane-propane in equilibrium with the atmosphere. The vesicles could be used to create a cell with a "cytoplasm" that differs from its medium, and also, pockets of unique solutions for specific purposes within the cell.

These pockets then could be used to create the equivalent of the organelles of Earth microbes, which could also trap different alkanes to help with whatever process that organelle is devoted to facilitating. Multiple vesicles joined together then have the potential to carry out long chemical processes with different chemical environments for each step.

Some of them could also trap ammonia water in bilayers which could decrease its freezing temperature to the point where perhaps it could be used for nano-reaction centeres for bological processes that depend on polar solvents. Then there could be nano-channels of liquid ammonia water to transport these throughout the cell. The microbes on Titan could get the water from short lived liquid water from cryovolcanoes (cryolava) or maybe from the icy bed of the hydrocarbon lakes using antifreeze proteins.

They could be used as reservoirs for polar solutes (like the ammonia rich water) and to transport proteins and enzymes.

(paraphrase of the first few paras of Chapter 9, page 331 of Lucy Norman's 2015 thesis)

Here is Chris McKay talking about prospects of life in the Titan lakes

(click to watch on YouTube)

He mentions there that Huygens made provisional observations of hydrogen and acetylene and ethane depletion near the surface of Titan, which they'd predicted as a possible sign of life on Titan. Also he mentions that oxygen would be in short supply, in a hydrocarbon ocean without water, and need to be extracted from water ice "rock" by microbes.

This is related, a possible way that life on Titan could make cell membranes without use of oxygen atoms in an environment like Titan where it is in short supply.

Possible oxygen free cell structure made of organic nitrogen compounds that could function at the low temperatures of Titan's ocean.

Such oxygen free cell walls are exotic biochemistry but still carbon based.

Titan's ethane / methane ocean is also ideal for William Bains' silicon based biochemistry (with silicon's weak bonds permitting much faster chemical reactions in such cold conditions). See page 160 of this paper and earlier pages.

You can read about Chris McKay's ideas about Titan in detail in his 2016 paper Titan as the abode of life.

Amongst other evidence, the photochemical models predict a layer of ethane enough to cover the surface to a depth of many meters, but Cassini didn't find it. The Huygens lander didn't find any acetylene in the gases released from the surface. And models suggest that hydrogen is being transported towards the surface suggesting something there is removing hydrogen from the atmosphere.

Also, there is more hydrogen above 50˝N than the global average. Could this be because the southern hemisphere, with more ethane, is more hospitable to life than the northern hemisphere dominated by methane?

He suggests four possible conclusions (see page 11)

1. The result about a strong flux of hydrogen onto the surface may be mistaken
2. There is some physical process leading hydrogen to fall towards the ground maybe absorbed onto organic haze particles. But this would just be a hydrogen flow, and it wouldn't take hydrogen out of the atmosphere
3. That there is some chemical catalyst that causes a hydrogenation reaction at 95 K - which would be quite interesting and a startling find, if not as startling as life
4. That it's due to a liquid methane based life form.

Titan as potentially the easiest place for humans to live outside Earth

If that's right, then we may be able to land humans on the surface of Titan with no planetary protection issues, at least in the forward direction from Earth to Titan. That is, unless there is any cryovolcanism or other connection with its subsurface ocean. But if there is no such connection, then there would be no way to contaminate its deep subsurface ocean from the surface. You'd also have to consider whether there is a chance that Earth biochemistry could give native Titan life "new ideas" I suppose.

If Titan does turn out to be okay for planetary protection, then in some ways it might be one of the easiest places for us to live outside of Earth, as suggested by Charles Wohlforth and Amanda Hendrix, authors of Beyond Earth: Our Path to a New Home in the Planets. Their idea is described in brief in Let's Colonize Titan in the Scientific American. Some of the advantages are:

• Atmosphere at the same pressure as our Earth's atmosphere. Your spacesuits wouldn't need to be pressurized. They would of course have to be insulated from the cold, but they would be much more flexible, without the enormous outwards pressure of tons per square meter needed in vacuum conditions.
• You could build conventional houses too, equal pressure inside and out. They would only need to be air tight.
• Protection from cosmic radiation by the atmosphere
• Plenty of ice for water.
• Earth life would probably have no impact on any Titan life
• Abundant resources for making plastic which could be used to make housing. Habitats would be so easy to construct, with just a thin covering to keep out its atmosphere, equal pressure inside and out. This means like the Venus cloud colonies, you could have huge internal spaces filled with oxygen and nitrogen for very little by way of mass
• Terminal velocity a tenth that on Earth so if you fall out of a plane in the Titan atmosphere, no problem. You would gently fall to the surface, at most a few bruises
• Human powered flight - could fly in the light gravity

The gravity levels are lower even than for the Moon. Would humans be healthy at such low levels of gravity? Well, if not, we may be able to augment it with artificial gravity. Considerations are similar to the Moon. See my sections in Case for Moon First:

• What about gravity - isn't that a big advantage for Mars over the Moon?
• Artificial gravity on the Moon to augment lunar gravity

You have to be design the habitats to be well insulated and they have to be airtight to prevent the methane in the atmosphere mixing with the oxygen rich atmosphere of the habitat. On the plus side, it doesn't have the risk of rapid decompression of a normal space habitat, and any leaks will be very slow.

The main remaining issue would be power, as Saturn gets very little sunlight and Titan with its nearly opaque atmosphere, even less. We need it for heat, or some way to generate heat. We also need electricity etc. Perhaps by then we have fusion power, or we use a conventional nuclear reactor with imported fuel. But it has native sources of power too, from this paper Energy Options for Future Humans on Titan

• Hydropower - just as on Earth if it has any lakes at higher altitudes they could produce power by cutting a channel to a lower level. Sadly the seas discovered so far seem to occupy the lowest levels in the landscape, at the lunar poles. Still, the topography isn’t very high resolution and there is plenty of scope for finding lower levels close to the seas. It would only take 145 meters of difference to generate a very useful 9 MW assuming flow rates typical of hydropower installations on Earth adjusted to Titan conditions. The Kraken ocean could supply power at these levels for thousands of years - and it could be replenished too by natural processes refilling the seas just as happens for hydropower on Earth. However this depends on more detailed topography for Titan.
• Wind power - with such a dense atmosphere then wind power is feasible (unlike the case for Mars) - but surface winds are slow so you could use tethered blimp or balloon mounted windmills at a height of 3 km where the speed is 2 meters per second (4.4 miles per hour) or even better at 40 km where the wind speeds are 20 meters per second (44 miles per hour). These could generate hundreds of terrawatts
• Solar power. Surprisingly, with modern efficient solar cells, covering 10% of the surface would give you enough power for the population of the US (which has a surface area 10.8% that of Titan). I.e. at typical population densities of the US you’d need roughly equal areas for the solar panels and for the human populations.
• Chemical - for instance hydrogen and acetelyne can be extracted from its atmosphere and react together to produce energy. There are several other reactions including hydrogenation of nitrogen to produce ammonia using chemicals in its atmosphere which is quite a bit out of chemical equilibrium.
• Nuclear power - likely to be useful for early misisons there using RTGs as a source of heat and power. Mining for it locally may be hard as radioactive materials are likely to be deep below the surface, under hundreds of kilometers thickness of ice, so these materials would probably be imported.

The solar power calculation there may surprise you, so here is the relevant part of their paper where they explain the assumptions they use that lead to this conclusion:

"We estimate the amount of solar energy available at Titan’s surface by scaling down from Earth. At the top of Earth’s atmosphere, the average solar energy is 1400 J/m2 -s. At the top of Titan’s atmosphere, this scales to 14-17 J/m2 -s. Titan’s atmospheric transmission depends on wavelength: the red and nearinfrared are transmitted (minus methane absorption) whereas blue light is absorbed; here we assume that 10% of the solar flux makes it to Titan’s surface. In reviewing the response function of various photovoltaic materials, we estimate that Titan’s transmitted spectrum is best matched by the response of amorphous silicon or perhaps cadmium telluride (CdTe) photovoltaic material. Нe efficiency of these material is in the ~13-20% range but the performance at Titan temperatures is unknown. To be conservative in this initial, simplified exercise, we estimate the efficiency at 10%. We also consider that for any low-mid latitude location on the surface, the sun is up for ~1/3 of the day. (This does not consider seasonal variations or eclipses by Saturn.)"

The solar power doesn't have to be solar panels actually. Thin film mirrors to concentrate the sunlight could make a big difference and would be easier to construct with the low gravity and benign conditions with no winds to speak of. Maybe large solar furnaces would be the future there. Or thin film mirrors focusing the sunlight on smaller solar panels.

Thin film mirrors would still be only a small proportion of the weight of a habitat that depends on solar power right out to Pluto and beyond.

It still needs more in situ research first though, I think, to establish that there are no planetary protection issues at all.

Could Earth life harm Titan life?

Even if there are no habitats for Earth life there, as seems likely except for the possibility of cryovolcanism - you still need to think about molecules from Earth life such as RNA and DNA. Could they somehow be incorporated by Titan life? Also, if any of the life forms there exist in small quantities, hard to study, could organics from a human occupied habitat confuse the search for life in the region close to the habitat?

Could Titan life harm Earth life?

Also, what about contamination in the backwards direction from Titan to Earth. We can show by studying Earth life that it is impossible for Earth life to survive there in its natural conditions.

But we need to have a reasonable understanding of Titan life (if it exists) how can we be sure the other way around, that Titan life can't survive on Earth or in human occupied habitats?

It may seems unlikely that life adjusted to those habitats could survive here. But you can make a plausibility argument for it even so.

Here is how it could happen.

What if the Titan life evolved from previous life that first originated in its deep subsurface oceans? That's one suggestion for how it could have originated. Is there any chance it could retain capabilities that would let it survive as spores, and then maybe reproduce in oil deposits or in some other way survive in the much hotter conditions of Earth?

Or indeed, is there any chance that there could be a lifeform that on Titan can survive not just on the surface but also in occasional cryovolcanoes, or lakes formed from meteorite impacts? I.e. a life that has already evolved the capability to live in both those radically different environments? It seems unlikely, but can we rule it out before we know what is there?

Both directions - cryovolcanism

Also, as a planetary protection issue in both directions: Titan probably has a subsurface ocean. If so, if there is communication between the subsurface and the surface, e.g. cryovolcanoes with liquid water in place of lava, then that's an obvious contamination issue for the subsurface oceans.

Titan is currently categorized as "Provisional category II" with "only a remote chance that contamination from Earth could jeopardize future exploration". There category II is the same as our Moon. But it is provisional because they say that more research is needed. Other places categorized as provisional category II are Ganymede, Triton, the Pluto Charon system, Ceres and the larger Kepler belt objects (down to half the size of Pluto).

Likely to know a lot about it before humans get there

Titan would surely be studied robotically first. There are no serious proposals to send humans there right away, unlike Mars.

So, unless there is some drive to send humans there quickly, we'd probably answer all those planetary protection questions already before we land humans there, just in the natural course of solar system exploration. If so, and if the results turn out favourable, it may well turn out to be one of the best place for a human colony outside of Earth. It would be a fun place to live in some ways, with the human powered flight, for instance, and the intriguing seas. And it might potentially be a biologists paradise if there is indeed native Titan life to study.

On the other hand it’s not such a great place for astronomy, at least, naked eye astronomy. Despite its spectacular location close to Saturn you’d have to go into orbit or use wavelengths able to penetrate its atmosphere to spot Saturn or indeed, stars or planets at all. But it might well be a base for astronomers and exobiologists studying Enceladus and the Saturnian rings etc - with their work done outside of Titan, maybe some of it robotically - but their main base on Titan itself.

See also

• Colonization of Titan - Wikipedia
• Beyond Earth: Our Path to a New Home in the Planets. Summarized in Let's Colonize Titan in the Scientific American.

Life in neon seas of Kuiper Belt Objects and beyond

They haven't found any liquid neon on Pluto yet (one of the other liquids suggested as a possible habitat for life by William Bain). Jeff Kargel's idea of neon rivers on Pluto may seem a rather exotic suggestion, but though neon is very rare on Earth, it is actually similar in cosmic abundance to nitrogen, behind hydrogen, helium, oxygen and carbon, and also similar in solar abundance too (neon's solar abundance is not very well determined with many papers revisiting this question).

It's a light gas, lighter than air, and very volatile which is why there isn't much left in the inner solar system (it is a heavier atom than nitrogen, atomic mass 14, but nitrogen comes as a molecule N₂, so that's 28 atomic mass units compared to Neon's 20). Pluto is a little warm for neon to be liquid. REX estimated a temperature next to the surface of 45±3 K on occultation exit, and 37±3 °K on occultation entry. The lower temperature was measured above Sputnik Planum, an area of nitrogen ice, and is close to the saturation temperature of nitrogen. So that suggests a cold layer hugging the nitrogen ice. At any rate, it's far too warm for liquid neon right now.

So, Pluto is rather warm for neon rivers and seas, but it might easily be abundant in cooler locations further out in the Kuiper belt. The triple point of neon is at 24.5 K and 0.4 bars.

Stacy McGaugh wrote an intriguing speculative piece about the possibility that Sedna could have a neon atmosphere when close to the Sun,

He writes:

"The following is a speculative idea about the possibility of a thin, hazy neon atmosphere on the remote planetoid Sedna. It is almost certainly wrong. I post it as an intriguing (if remote) possibility, and to stimulate thought more generally on the possible role of the cosmically abundant element neon in frigid places like the outer solar system (well beyond the Kuiper belt) and dusty star formation cocoons."

He talks about how it's possible it could have a neon atmosphere when it is closest to the sun, and it could have crossed the solid / gas boundary for neon when it got closer than 120 au and that if the atmosphere gets thick enough it could even have liquid neon. It's not so easy for neon to become part of a forming object though, because it is so volatile, and it doesn't form bonds with anything else, being a noble gas. So as the condensing gas warms up the neon would evaporate. However we do have some neon on Earth, and it's also found in meteorites and in comets. He concludes that these "all combine to suggest that it is worthwhile to consider the possibility of a neon atmosphere on Sedna even if it may seem farfetched".

So, though they didn't find neon rivers on Pluto - I wonder if we will find any KBO's with a neon atmosphere? If so, what about a world in the outer solar system with a neon atmosphere, and liquid neon lakes and rivers and a neon precipitation cycle? If so that may be another place to look for William Bains hypothesis of low temperature silicon based biochemistry mentioned in Life in liquid nitrogen (above)

Life in liquid hydrogen or liquid helium as far away as the Oort cloud

This is another rather speculative section. The idea of life in liquid hydrogen or even liquid helium is touched on by William Bains. The most speculative part of this section is whether or not there can be liquid hydrogen oceans out in the distant Kuiper belt and Oort cloud, or potentially, significant amounts of liquid helium. Eventually as you get further out in the Kuiper belt and into the Oort cloud then the temperatures are so low only hydrogen and helium are available as liquids.

William Bains was interested in objects of 1,000 km diameter or larger with a potential for an atmosphere. As we saw above, his idea was to use chemical reactions that are far too rapid to be useful for life at our temperatures. William Bains suggests silanols. The silicon - hydrogen bond is destroyed within hours at mammalian temperatures in presence of water, and so is not nearly as stable as the carbon - hydrogen bond. However, it turns out to be just what you want at such low temperatures and in non polar solvents. As he wrote in one of his papers:

"... Oceans of cold, non-protonating fluids, and specifically liquid nitrogen, would be more favorable for silicon life. In such environments, the inherently greater reactivity of silicon-based chemicals could be an advantage, enabling “living” processes to occur at greater than geological speeds at the relatively lower temperatures where such fluids are stable."

So first, what about liquid helium? He works out that in those conditions, with the background radiation of the Big Bang at 2.7 °K and Helium boiling at 4.2 °K that (page 147 of this paper).

" Indeed, the only obvious liquid that is missing from the list is helium. This is because, with a boiling point of 4.2 °K (4He) at 1 atmosphere pressure, there's no reasonable set of circumstances where a planet's surface is likely to be cold enough to maintain a temperature of < 4.2 °K and where the planet is massive enough to retain a significant atmosphere."

What about smaller cooler bodies with no internal heat? I can't find much about this idea, and William Baxter doesn't discuss it, but the science fiction writer Stephen Baxter populates a fictional airless distant tiny Kuiper belt object just a hundred miles across (160 km) with helium condensing on its surface, and with creatures with helium instead of blood in his short story "Sun people". He describes it as "so cold that helium condenses on the surface - superfluid pools, sliding over a water crust". However I think he may have used a bit of poetic license there.

Thermal velocity of Helium on an 80 km radius asteroid compared to escape velocity: it's escape velocity would be around 1.89 meters per second (assuming density same as ice, (4/3)*pi*80000^3 / 1000 tons and escape velocity calculator) or 5.3 meters per second (assuming density of iron, 7.874*(4/3)*pi*80000^3/1000 tons). Meanwhile the mean velocity of helium at 2.7 K is 158.9 meters per second (using molecular weight of 4 in this thermal velocity calculator).

So the escape velocity of 5.3 meters per second, even assuming solid iron instead of ice, is far less than the thermal velocity of 158.9 meters per second. To retain an atmosphere of Helium it should be far more.

To achieve an escape velocity of six times the 158.9 meters per second, or a little under 1 km / sec, so a planet a bit smaller than Pluto would do, but would be likely to have significant internal heating.

William Bains also mentions the idea of liquid hydrogen oceans at the beginning of the paper. So what about that idea? He calculates that hydrogen would be stable in a region extending from 100 au upwards, peaking at around 1000 au. But he writes

"The surface temperature of such cold worlds as predicted here is also most likely incorrect, as heating from cosmic rays and internal radioactivity will be a significant part of their heat budget. Thus the frequency calculations for liquid hydrogen or neon must be regarded as even more speculative than those for other liquids. The uncertainties in all aspects of this calculation as applied to hydrogen are too large to consider liquid hydrogen further in this paper."

With his biochemistry based on silanols, then hydrogen could support life in such an ocean (doesn't have to be supercritical as for the gas giant habitable liquid hydrogen). But he considers the uncertainties too great to discuss it further.

However he doesn't look into this in any detail in that paper. Let's take a closer look. As a speculation only, might a liquid hydrogen ocean be possible in the far reaches of the Kuiper belt and Oort cloud?

Liquid hydrogen is rather easier to form than Helium however, with a freezing point of 14 °K, and a boiling point at Earth's atmospheric pressure of 20°K (Hydrogen properties). It's boiling point depends on the pressure, so more generally, liquid hydrogen can only exist at temperatures between its triple point of 13.8 °K and its critical point of 33 °K (I'm talking here about normal rather than supercritical liquid hydrogen). That's not far below the 37±3 °K measured on Pluto above the nitrogen ice of Sputnik Planum. Meanwhile, Sedna has an estimated surface temperature of around 33 K, and that's when close to the sun.

Techy details: The triple point of hydrogen - the point where the gas, liquid and solid phases all meet - is fixed by definition at -259.3467 C = 13.8033 K according to the International Temperature Scale of 1990. The pressure for that triple point is 0.0704 atmospheres. In that definition, the deuterium content is assumed to be 0.15 micromoles per mole of hydrogen typical of hydrogen on Earth (in experiments on Earth it actually can vary by microkelvins depending on the exact deuterium content of the hydrogen)

If you vary the deuterium content, the figures change. The triple point of deuterium is 18.7 K at atmospheric pressure and its critical point is 38.34 K. So if there is more deuterium it can be liquid at slightly higher temperatures. It also depends, but in only a very minor way, on the spin states - hydrogen occurs in two spin isomers depending on the spin states of the protons, whether aligned (ortho) or anti aligned (para). The temperatures can vary for the two isomers, depressed by about 0.1 K for the para hydrogen compared to normal (mixed spin state) hydrogen.

Those are temperatures that might be normal in the remoter reaches of the Kuiper belt and in the Oort cloud (depending on whether there are other sources of heating). So it seems they could perhaps have liquid hydrogen oceans. Also, if they are cold enough they can retain an atmosphere of hydrogen. So how low does that temperature have to be? It depends on the escape velocity, and so, on the size and mass of the body.

Figure 3 of this paper. This shows the gases that are prone to escape from the atmosphere of worlds of various sizes through "Jeans' escape" - that is to say just through the loss of individual molecules whose velocity due to temperature is enough to reach escape velocity. The lines are based on the 'rule of thumb' that Jeans escape velocity is important if the velocity at the "exobase" is more than a sixth of its escape velocity. There the exobase refers to the height in the atmosphere where collisions between the molecules become rare. For Earth, this is at 500 - 600 km.

There if you draw a line up to the Hydrogen line through the Pluto dot and then across, it passes through the Mars dot (note, this is from 2009, before the New Horizons results). Though the exobase could be warmer than the planet surface, still, it looks as if Mars sized objects or larger, further out than Pluto could hold onto a hydrogen atmosphere. That's making some assumptions though, that they can get one in the first place, and that they lose it only (or mainly) by Jeans' escape.

Gases can also escape via thermodynamic escape, a heating up of gas through absorption of UV from sunlight, leading to bulk flows of the gas into space starting from below the exobase, which makes it more complex. However Pluto is losing nitrogen at the Jeans' escape rate, not the thermodynamic escape rate, corresponding to a loss of about 6 cm of nitrogen ice over the age of the solar system. See the section on Escape towards the end of this paper.

So, if other small worlds out there lose hydrogen through Jeans escape, it seems at least possible that worlds out there as small as Earth and smaller, even as small as Mars, could hold onto a hydrogen atmosphere. If so, potentially, if the temperatures are right, they might have a hydrogen ocean. So, could there be any objects out there with enough gravity to form surface oceans of liquid hydrogen?

Mike Brown (one of the discoverers of Sedna and Eris and numerous other objects in the outer solar system) in one of his talks predicts that we are likely to find planets beyond Neptune that are Mercury, Mars or even Earth sized. The video is here:

(click to watch on YouTube)

Paraphrase: "Sedna has a twelve thousand year orbit around the sun. And we happened to find this one almost at the closest point it ever gets to the sun. Not by coincidence, because there is only about a 200 year period, where we could have seen it."

"So, 200 years out of 12,000 years means a 1 in 60 chance of finding it. So either we are very lucky, or since we only had a 1 in 60 chance of finding it, probably there are 60 of them and we just found the one that happened to be close. 

"Maybe it's not 60. Maybe it's 30 and we got a little bit lucky.  Maybe there are 90 and we got a little bit unlucky. But there are a lot of objects in this very distant region where we never knew of anything before. 

"Now the fun thing to think about is, if there are 60 of these, about three quarters the size of Pluto, like Sedna, there are probably 

•  30 objects the size of Pluto. 
• 10 objects that are twice the size of Pluto,
• Two or three objects that are three or four, and maybe even five times the size of Pluto in this region here.

"Our big goal now, and one of my current graduate student's PhD thesis is to find these objects. If there are some big objects, two or three or four times the size of Pluto, these things are the size of Mercury, these things are the size of Mars, these things are the size of the Earth. 

"I am willing to go out on a limb and say that we will find something like that the size of Mars somewhere in this region of space.

"... I think if you find something the size of Mars, something the size of the Earth, I think most people are going to want to call it a planet.If we are really lucky we will find them in two or three years. They have to be a little bit close. Otherwise it is going to take another ten or fifteen and some bigger telescopes. But that's where we are headed and that's where this whole field is I think going next. "

We haven't discovered any of these yet, but here for instance is a paper from 2017, by Kathryn Volk and Renu Malhotra, building on earlier work by Lykawka&Mukai in 2008, suggesting the possibility of a Mars sized object at a distance of 65–80 au to explain the “Curiously warped mean plane of the Kuiper belt”. That’s based on the observation that the Kuiper belt bodies are in a plane offset by about eight degrees from the “Invariable plane” of our planets. For non technical account of their research by the astrophysicist Brian Koberlein, see Goodbye Planet Nine, Hello Planet Ten

There's also the possibility that "Planet 9" consists of not just one but several objects: Worlds Beyond Pluto --"There's Not Just Planet 9, But Several Planets Beyond" and then you also have the possibility that while searching for it, they turn up smaller objects as happened for Pluto with Percival Lowell and Clyde Tombaugh's search.

So, though we haven't found them yet, such objects are certainly possible and it's maybe even rather likely that we find them, probably in the next decade or two.

If these objects exist they could be the most common type of rounded object in our solar system. Especially if there are more of those objects even further out in the distant Kuiper belt and Oort cloud. I happen to think that we should follow the geophysical definition and call them all planets, including Pluto, Eris, Ceres, Haumea our Moon and so on. See my article on the subject here. However, one way to be neutral about that debate is to just call them "worlds". If you include the smaller worlds, then the small worlds in our solar system already outnumber the better known planets.

So now, if any of those worlds, the larger ones, have surface oceans, not of water, but of liquid hydrogen, then they could even be examples of the most common type of world in our galaxy to have surface oceans. In our solar system then the only examples would be Earth itself, Titan, and then any distant worlds with neon or hydrogen oceans. So is such a world with a liquid hydrogen ocean possible?

I've found one mention of the idea so far, in a 1980 book "Life Beyond Earth" by Gerald Feinberg (theoretical physicist) and Robert Shapiro (biochemist). On page 386 they describes a hypothetical world Cryobus similar to our Neptune but orbiting at several times its distance from the sun, or orbiting a cooler star or even perhaps a white dwarf, with a liquid hydrogen ocean. They describe hypothetical "H-bits" that would use the two spin isomers of hydrogen, ortho and para hydrogen, as part of their biochemistry and use either solar thermal power as a source of energy, or the conversion of ortho to para hydrogen. They would be made up of the two types of hydrogen in solid form floating in the liquid hydrogen with trapped helium for buoyancy.

Also - I've put this up as a question on space.stackexchange.com

Could there be life at such very low temperatures as liquid hydrogen? Well it's still a fair way above absolute zero. I'm reporting what William Bains says here. He suggests that though life would probably get off to a slower start in such conditions, in the early stages of evolution, that once it has evolved it should be no harder to imagine than at our temperatures. He says that (I leave out his inline cites for easy reading):

"Can chemistry happen fast enough in such cold environments for life to start, and to be maintained? Reaction rates for the initiation of life will be limiting—simple reactions such as polymerization of HCN, believed to be important in some scenarios of terrestrial biogenesis, happen on time scales of the same order of magnitude as the age of the Solar System at Titan’s surface temperature. However, catalysis— even very weak general acid/base catalysis—can speed this up dramatically. If such chemistry was possible, then selection of chemistry that was self catalyzing would ensure the development of more efficient catalysts. This is the route by which peptides and proteins are created in Russell’s scenario. "

"Terrestrial enzymes can catalyze reactions extremely efficiently, such that some apparently abolish the activation energy entirely so that the enzyme-catalyzed reaction rate approaches the diffusion-limited maximum possible rate, as do other processes such as protein folding in some circumstances as well as the more obviously diffusional protein:protein binding interactions. Diffusion-limited chemistry is not unfeasibly slow in liquid ethane, nitrogen, or hydrogen, because values of the diffusion-limited reaction rate constant kx are linearly proportional to temperature (T) and inversely proportional to viscosity."

"Terrestrial enzymes have been shown to be catalytically active down to temperatures of -100°C (170 °K), using a variety of mixed polar solvents; below 170 °K these solvents become so viscous that diffusion limits effectively stop chemistry from happening, a limit that would not apply to the liquids in Table 5 [table of liquids such as neon, hydrogen etc]."

"Thus the origin of life in such circumstances would be slower and more dependent on the presence of endogenous catalysts. However, maintenance of life after that would be no harder to imagine than maintenance on Earth. It is notable that the “rate of living” of terrestrial organisms is not usually dependent on ambient temperature; the natural rate of growth of bacteria in the Antarctic seas and in black smoker volcanic vents is determined by competition for nutrients, not on how fast their chemistry can run. "

(from page 151 of his "Many Chemistries Could Be Used to Build Living Systems")

We also saw the possibility of Supercritical liquid hydrogen layers in gas giant atmospheres (above), which first becomes possible above 12.8 atmospheres and 33.3 °K. So could we get supercritical liquid hydrogen in these oceans as well? This could only happen rather deep in a surface ocean of liquid hydrogen as with a density of only 70 kg / cubic meter it is 14 times less dense than water. With a mass of 10.8 meters depth of water equivalent to one atmosphere in pressure, it would take 10.8*(1000/70.8)*12.8/1000 = 1.95 km depth of a liquid hydrogen ocean to have a supercritical layer at the bottom. On the other hand, liquid hydrogen trapped under low density amorphous ice with density 0.94 that of water would be supercritical at 10.8*(1/0.94)*12.8 = 147 meters.

Going back to the idea of a liquid hydrogen ocean - David Stevenson has looked at this same idea of small Earth sized planets with a hydrogen atmosphere. He found another solution, if the hydrogen atmosphere has an atmospheric pressure of as much as a thousand atmospheres - that it could also have liquid water surface oceans!

Rogue planets and very distant planets - liquid water or liquid hydrogen surface oceans

David Stevenson, professor of planetary science at CalTech has suggested another way surface liquid water oceans could form. He envisions an Earth mass planet with thick layers of hydrogen. The effective temperature would be 30 K because it's kept warm by long lived radionucleotides in its core (assuming it's the same age as Earth). However, assuming a thick atmosphere of hydrogen, then it could have temperatures high enough for a liquid water ocean at the bottom of the atmosphere (because hydrogen is opaque to infrared at high pressures). The surface temperatures get warm enough for liquid water at a pressure of about 1,000 atmospheres. He describes the idea in a letter to Nature here, and in more detail in this longer paper "Possibility of Life-Sustaining Planets in Interstellar Space".

He concludes his letter to Nature by saying that (depending on assumptions), "These could conceivably be the most common sites of life in the Universe.":

"It seems, then, that bodies with water oceans are possible in interstellar space. The ideal conditions are plausibly at an Earth mass or slightly less, similar to the expected masses of embryos ejected during the formation of giant planets. Bodies with Earthlike water reservoirs may have an ocean underlain with a rock core. Either way, these bodies are expected to have volcanism in the rocky component and a dynamo generated magnetic field leading to a well developed (very large) magnetosphere. Despite thermal radiation at microwave frequencies that corresponds to the temperatures deep within their atmospheres (analogous to Uranus), and despite the possibility of non-thermal radio emission, they will be very difficult to detect."

" If life can develop and be sustained without sunlight (but with other energy sources, plausibly volcanism or lightning in this instance), these bodies may provide a long-lived, stable environment for life (albeit one where the temperatures slowly decline on a billion-year timescale). The complexity and biomass may be low because the energy source will be small, but it is conceivable that these are the most common sites of life in the Universe. "

He mentions on page 6 that

"For sufficiently low masses, an alternative (collapsed atmosphere) solution exists with a molecular hydrogen ocean overlain by a thin vapor pressure-equilibrium atmosphere."

So that suggests there's a chance also of a liquid hydrogen ocean on these very cold planets, as in the last section.

Sulfur based life on Io

Io is rich in sulfur, and has constantly erupting volcanoes of silicate lava, along with liquid sulfur and sulfur dioxide, erupting into a vacuum. When the volcanoes were first discovered they were thought to be entirely sulfur based, because the maximum temperatures were 700 K, but it's now known to have hotter silicate lavas too.

This is a harsh environment which might seem totally inhospitable to life, and it certainly would be to Earth originated life. It has no water at all and is contniually bathed in intense ionizing radiation from Jupiter's radiation belts.

However it does have some liquid, in the form of pools of liquid sulfur dioxide, but only below the surface, under pressure. It also has another liquid there too, hydrogen sulfide. It's less abundant, but it forms hydrogen bonds easily, and so is more suitable for a solvent within cells. So life might seek out the liquid H₂S and use this as its solvent for the biochemistry within cells, while living in pools of the more abundant liquid SO₂. The life would flourish underground and it could live and reproduce only within a very narrow temperature range.

Volcanoes erupting on Jupiter's inner most moon Io, photographed by the Galileo spacecraft. The surface is rich in sulfur, and one suggestion is that Io could be host to an exotic sulfur based biochemistry.

Louis Irwin and Dirk Schulze-Makuch explore this in some detail in Cosmic Biology: How Life Could Evolve on Other Worlds, This Io life would have biochemical molecules withbackbones of sulfur, nitrogen, phosphorus and other elements and would take advantage of the complex chemistry of sulfur compounds. Sulfur has a wide variety of oxidation states, and even has fractional oxidation states such as -0.4 or -2/3. It also forms a variety of ring compounds, and polymers.

Their suggestion for life on Io requires both H₂S and SO₂ to be liquid, so it would only be able to live in a rather narrow temperature range of 15°C, from -75°C to -60°C. These temperatures are rather lower than for Earth based life, so the reaction rates are likely to be slower than they would be on Earth.

A plume ejected from same general region as one of the regions imaged by Voyager, called Masubi, it erupts 100 km into space from Io. The eruption comes from different places in this region and leaves plumes of SO2 on the surface.

The chemistry of Io is very complex as this diagram from their paper suggests:

Diagram from their 2009 article, showing the major environmental conditions relevant to possible life on Io.

Louis Irwin and Dirk Schulze-Makuch say in their concluding section of this chapter:

"It would be all underground and able to thrive only when temperatures reached an appropriate narrow range. But that would happen, for at least a brief period, in local pools or over short stretches of ground, every time a hot spot erupted or a sheet of lava advanced."
in Cosmic Biology: How Life Could Evolve on Other Worlds - Chapter 9, Fire and Ice (not available online).

For planetary protection, Io is classified as category I, the same category as the Sun, Mercury and undifferentiated asteroids, so there are no planetary protection requirements at all. You don't even need to document your visit there. However, it would be a hazardous place for humans to land because of the high levels of ionizing radiation from Jupiter.

This is such a different environment from Earth, that it seems extremely unlikely that Earth microbes could be harmful to any Io life, if present. Also, in the other direction it doesn't seem likely that any putative Io life could harm Earth

What about life in lava flows on Venus?

Venus has lava flow channels that travel for thousands of kilometers across its surface. These are empty now but did host rivers of lava in the past.

The arrows show the two ends of this section of Baltis Vallis on Venus - the longest known channel of any kind in the solar system, total length of 7,000 kms. Though there is no liquid in it now, it may have been carved out by rivers of liquid lava, which leads some astrobiolgists to ask whether,, if not too hot, it might have been a suitable place for life that relies on silicones.

Venus has no continental drift so doesn't have volcanoes all the time as Earth does - instead its entire surface may "turn over" every few hundred million years. But it shows signs of "young" features and may still be geologically active.

Recent observations show that some parts of the Venus surface may still be active today, with hot spots that appear and disappear, perhaps pools of lava, superheated rocks, or plumes of hot gas.

So, it's a bit of a long shot, but could there be life in liquid lava? Well Louis Neal Iwrin and Dirk Shulze Makuch discuss this briefly in their book Cosmic Biology. Perhaps it could use silicones - organosilicon polymers with a silicon-oxygen backbone. These are stable at temperatures so high they would destroy any organics. But - would they remain stable at the temperatures of any lava, even cooling magma pools on Venus? That's the big question. Unlike water chemistry which is easy for us to explore in our laboratories - it's not so easy to explore the chemistry of silicones in magma pools.

They say that if life needs some form of liquid, as they suspect, then the only likely solvent on Venus would be silicates in the form of liquid magma, and conclude

"and there is a real question of whether even polymeric silicones could remain stable at the temperature of magma. If some form of silicon based biochemistry had evolved on Venus, its features would likely be so exotic that they would be hard to recognize. Nothing resembling the form of a living organism has ever been found among the extensive deposits of lava on Earth. We therefore regard the prospect of silicon-based life on Venus so speculative that informed consideration of its possible features are not possible at this time."

I thought I should cover this, as "silicon life" is such a staple of science fiction. It seems that exobiologists don't totally rule it out. But as they say, if life is possible in magma pools - then why hasn't it evolved on the Earth also? Or if it has, why have we never found silicon based life fossils in lava flows on the Earth? Is it because we are unable to recognize it for some reason, or is it just not here?

They don't actually say it is impossible. Rather that it is so speculative that "informed consideration of its possible features are not possible at this time."

So, that argument seems fair enough from our perspective as an organics based lifeform living on planet Earth which has no life in its lava lakes.

What would silicon based life forms living in molten magma think about water based life?

However, approaching this from a rather different direction, in theoretical philosophical way, as someone who trained as a mathematician and philosopher - you can also ask another question here. Might it just be a a low probability lifeform? Might the reason we don't have it on Earth be just because the chance of it evolving is tiny?

Perhaps amongst thousands, or even millions of planets with lava flows, only one or two ever evolve silicon based life? That could explain why we don't have it on Earth. If so, what about organics in water evolving to life, how low probability is our own form of organic based life? We don't seem to have enough information to assess this either.

Maybe, just as we have no silicon based life in our lava, as far as we know - maybe, on other planets, there's no carbon based life in their water. Organics but no life.

So, approaching this philosophically and as a mathematical probability question, maybe right now, far away around a distant star, there is a similar discussion going on between silicon based life forms living in molten magma on a planet with lifeless seas of water.

To them, water might well seem too cold to sustain life, which they think can only occur at the temperatures of molten lava :). For those silicon based life forms, if they exist, the idea of carbon based life living in a liquid as cold as water, under one bar atmospheric pressure would seem very speculative too. By looking at their own chemistry, they might work out that it would take geological timescales for anything to happen if cooled down to the temperatures of liquid water, and so deduce that our form of life is probably impossible. Experiments with cold water might be very hard for them because they live at such high temperatures themselves and perhaps the only water on their world is in the form of sulfuric acid rain, and hot steam.

Or if they do have oceans of cold water, they might be lifeless places - which are as hard for them to study as molten lava is to us - just as we have lifeless flows of lava (as far as we know anyway).

If that was right, then we might be as likely, or unlikely (as the case may be) to find independently evolved life in the Venus lava flows as in the Europan or Enceladus oceans :).

I supposed there that life in lava and life in water were both very unlikely. But they don't even need to be particularly low probability.

Just to take a speculative figure to show how this works, suppose the chance of a planet evolving life in lava is 50 / 50 and the chance of it evolving in water is 50 / 50. Then there would be a 50 / 50 chance of life in lava on Earth. It would be no great surprise that we don't have it. But there would then still be a 50 / 50 chance of finding it in lava on Venus. Meanwhile, with those speculative figures, about half of the planets with lava based life also wouldn't have any life in their water and would speculate about life in water just as we speculate about life in lava. Meanwhile, a quarter of the planets with both potential "habitats" would have both types of life, and a quarter would have neither.

That's just fun speculation, not to be taken too seriously. Something to think about though, perhaps :).

Life in the clouds of Venus

This may seem an unlikely habitat at first, life in clouds of sulfuric acid high above the surface of Venus. Yes, it is a bit of a long shot. But the case is stronger than you might think. If Venus was always like that, the clouds seem an unlikely place for life to evolve. However it seems likely that Venus started off similar to Earth in the early solar system, with plenty of water, oceans and an atmosphere.

If this is right, then at some point it dried up, and lost its ocean due to a runaway greenhouse effect. This didn't affect Earth in the same way because continental drift on Earth continually buries and circulates the carbonates. At any time most of our carbon dioxide is locked up in limestone, chalk and other carbonates and in coal, oil, and other forms of organics in the slow carbon cycle.

Venus is of course totally inhospitable to life on its surface now, but some scientists think it could perhaps have remained habitable to life on its surface until as recently as 715 million years ago. The surface is all around the same age, and it was resurfaced by volcanic processes around 300 hundred million years ago. It's rather like Earth'ssuperplumes, huge but very very slow motions deep below the surface such as the one that drives the volcanic activity around the Pacific "ring of fire". Venus may have had superplumes so large they resurfaced the entire planet. So it doesn't have continental drift any more. The surface is just static, waiting for the next global superplume. But it may well have had continental drift in the past and been rather similar to Earth.

It is hard to settle this question about how long ago Venus became hostiel to surface life, because it depends on details of its terrain before the global resurfacing event, and its spin rate (how that changed over time) and other variables such as whether it had a sink for carbon dioxide. The researchers concluded that if it had

• a low spin rate already back then,
• a surface with the seas that are small and deep
• Much more dry land, especially in the tropics

Then there'd be less water surface to evaporate and so less of a greenhouse effect. The slow rotation would lead to more clouds, and the result of their climate modeling was a cooler more habitable world. Actually early Venus this model would actually be a few degrees cooler than Earth is today, in this model with all those assumptions, even though the earlier spinning Venus would be closer to the Sun than Earth.

For more on this, see the NASA press release about their research, and techy details in their paper here.

So if that is true, could Venus have had life on its surface and in its oceans, similar to Earth life, even as recently as 715 million years ago? If so, given the tenacity of life, the way it tends to cling on somewhere, even in very extreme environments, could it still be there? And if so, where?

The surface of Venus is totally hostile to Earth life, a dim, hot furnace, with temperatures well over 400°C. But conditions are different at the Venus cloud tops. Temperatures are ideal, with plenty of light. It is almost Earth like in

• Temperature,
• Pressure
• Atmospheric composition (without the oxygen of course) - the atmosphere has all the main ingredients needed for life, as we'll see.

Artist impressions of Venusian clouds, credit ESA. The surface of Venus is utterly hostile to Earth like life, at temperatures of well over 400°C is. It is also dim, not much light filters through the clouds. It's surface actually gets less light than the Earth, even though it is closer to the sun, because of its highly reflective clouds. The reason that it is so hot today is because of its runaway greenhouse effect. Earth has similar amounts of carbon dioxide locked up in limestone, and could look the same in the future as the sun heats up further, though it can't enter into a state like that right now.

But high in the atmosphere above the cloud tops, then conditions are far more conducive to life, at temperatures around 0°C. The cloud droplets themselves are the main challenge, concentrated sulfuric acid, with acidity similar to battery acid. There are intriguing signs that just might indicate life, in the upper atmosphere though they can also have other interpretations.

The main drawbacks are:

• Droplets of concentrated sulfuric acid. However we do have microbes on Earth that can survive in concentrated sulfuric acid outflows from mines, in conditions not far off the acidity of Venus clouds. Indeed the higher layers of the Venus clouds have a pH 0.3 to 0.5 which is within the tolerance range of some Earth microbes. As for the lower layers, the Venus pH goes as low as -1.3, but there are acid streams that acid on Earth, including naturally occuring hot springs near Ebeko volcano . It's hard to say if there is life in these conditions on Earth as it's hard to study life in very acid conditions. For details, see Possibility of Earth life able to survive in the Venusian clouds (below)
  
  For Earth originated life, the main problem with acid inside a cell is that protons inside the cell cause proteins to unfold. Acidophiles deal with this by keeping the interior of the cell neutral in pH and pumping the protons out of the cell. Some have acid stable proteins with a higher abundance of the two acidic amino acids aspartic and glutamic acid, and they can relocate acid-sensitive ionic bonds away from exposed regions. Summary by Charles Cockell.
  
  If the cells had acid inside them, the acidic environment would also change the shape of the bases used for DNA so that they no longer match each other.
  
  The plus side for all that sulfuric acid is that sulfur is one of the ingredients needed by life. Also, sulfuric acid also contains water (when dissociated) so it's a source of water too for hardy microbes. Indeed the Venus clouds seem likely to have all the elements needed for life, as we'll see.
   
• The UV light at the cloud tops could be hazardous also - however, life in the Venus clouds could be protected by pigments like the ones used by UV tolerant microbes on Earth.
  Another possibility is that it could be protected from UV light in a novel way by an external layer of solid sulfur (the allotrope S8). (see also page 11 of Astrobiology and Venus Exploration)
   
• No solid surfaces.

The sulfuric acid is a major challenge but it seems not impossible that life evolved on early Venus could adjust to it. The UV again seems something that life could evolve to protect against. So - if we accept that the cloud tops could be habitable to some forms of life, and that it is possible that life could have migrated there as the surface of Venus got hotter, drier and less habitable, the main remaining question is, could the life find some way to stay aloft with no solid surfaces?

The residence time of particles in the Venus atmosphere is months rather than days. That is a good start as it makes it easier than it would be for Earth's atmosphere. Also, there's a fair bit of turbulence, which could return some of the life to the tops of the habitable layer after it reproduces.

It is still quite a challenge, but the Venus clouds seem interesting enough for some astrobiologists to look carefully to see if there is any possibility of present day life there. The results of that search is rather intriguing, though we can't say that life has been discovered there yet. They have found several factors that suggest life may be possible there - and there is a chance that it may even have been detected already, indirectly:

First, conditions that favour life:

• The atmosphere has chemical imbalances useful for life, out of equilibrium - it has H₂S and SO₂ present together, also oxygen and hydrogen, which life could use as a source of energy.
   
• Shorter nights - the upper layers of the atmosphere super rotate giving a day night cycle totalling four days. This makes photosynthetic life far easier because the night is shorter than the 117 day cycle on the planetary surface
   
• Continuous presence of thick clouds more stable than those of Earth, with water present in the form of sulfuric acid in the clouds - the clouds are the region of the Venus atmosphere with most water present.
   
• It takes more than 200 days for a 1.1 micron particle to fall out of the clouds , which is more than enough time for several generations of microbes to reproduce and form spores even if they are rather slow growing ( page 10 of Charles Cockell's Life on Venus. ) ,
   
• Most of the ingredients of life are easily accessible in the atmosphere. Five of the six main elements life needs, hydrogen, oxygen, carbon, nitrogen and sulfur are present in large quantities . The main limitation for life similar to Earth life may be the requirement for phosphorus which is present in trace quantities. Chlorine is present in low quantities. Calcium, potassium, sodium, magnesium and iron are not yet detected. However, they are likely be present in the form of dust since surface rocks include MnO, MgO, CaO, Na₂O,K₂O,FeO, TiO₂ and SiO₂ . So everything life needs is there. See Geoffrey Landis's Astrobiology: The Case for Venus. Also see section 4 of Charles Cockell's Life on Venus (he's not saying that he thinks there is life there, instead, he approaches it as an astrobiological theoretical exercise).

Also the observations that suggest life might be there, though not conclusive, are:

• Unusual chemistry that could be caused by life - Detection of carbonyl sulphide (OCS) - That's a clear sign of life here on Earth, though it could be created inorganically on Venus, by volcanoes. The levels of H₂S are also higher in the upper atmosphere, a surprising result which might mean it is produced by some process in the upper atmosphere rather than from volcanoes. If so, again it could possibly be produced by life.
   
• Doesn't have the expected concentrations of carbon monoxide. Something seems to be removing it. That could be the result of inorganic processes, but it could also be a sign that life is using this gas for its metabolism. On Earth, 450 strains of photosynthetic bacteria rely on carbon monoxide for their carbon. Schulze-Makuch, building on earlier work by David Grinspoon, suggests that Venus life in the clouds could be combining carbon monoxide with sulfur dioxide and possibly hydrogen, to produce hyrogen sulfide or carbonyl suflide. He draws an analogy between the anoxic acidic environments in the Venus clouds and the similarly anoxic, acidic and hot hydrothermal vents where the ancestors of the carbon monoxide eating microbes lived. See Venusian cloud colonies in the NASA Astrobiology online magazine.
  
  David Grinspoon makes an interesting point there, as interviewed by Astrobiology Magazine:
  

  “Confronting Venus makes us realize that it is not enough to simply state that life creates disequilibrium, This is true, but there are many non-biological processes that also destroy equilibrium, such as ultraviolet light and lightning. As I pointed out in ‘Venus Revealed,’ not all life increases disequilibrium. You and I, for example, breathe in oxygen and exhale carbon dioxide, thereby eating up some of the disequilibrium provided for us by green plants. So an atmosphere mysteriously close to equilibrium might also be a sign of life.”

  Carbon monoxide has also been suggested as a potential energy source for Martian microbes (it's common in Mars type conditions in dry salty deserts). Carbon monoxide in the atmosphere might also have been an energy source for early Earth life.

• Detection of non spherical particles similar in size and shape to microbes. This was a rather surprising discovery, as there is no obvious vapour source that would condense as non spherical particles of this size.
  
  Some scientists have suggested that these large particles may be a result of miscalibration of the equipment, with the conclusion: , "This allows a simple understanding of the source of all the cloud particles, but at the cost of disbelieving some of the measurements.". However others say that there is good evidence that the equipment was operating under nearly optimal health conditions.
  
  This is the "Mode 3 particle controversy", for details of the controversy see section 4.2 of this paper. These "Mode 3" particles are quite large with an average diameter of 7 microns (see first page of this paper). By comparison most bacteria on Earth range from 0.2 to 10 microns in size.
   
• Detection of dark streaks in the atmosphere of unknown composition which absorb UV. The absorption spectrum is broad without distinct peaks which suggests it is due to particles or droplets rather than gas, Could this be due to absorption of the UV by life, either to protect itself, or for photosynthesis, or both?

Of all those observations, the three most important lines of evidence which just possibly could indicate the presence of life are the

• UV absorption (possible photosynthesis or UV protection)
• Chemical disequilibrium, especially the presence of OCS
• Presence of large non spherical particles.

All of these could also be due to non life processes but are not easy to explain in that way. Here are some useful sources, if you want to find out more about this:

• Venusian cloud colonies (NASA astrobiology magazine),
• Astrobiology: The Case for Venus
• Life on Venus
• Could dark streaks in Venus' clouds be microbial life
• Could there be life in the clouds of Venus?
• Acidic clouds of Venus could harbour life

It's an interesting idea, and the evidence is intriguing enough to be interesting, but I don't want to give the impression that it is thought to be a likely thing to find. I think many would say it is somewhat of a long shot. Here is what Charles Cockell writes (2003):

"As one rises into the Venus atmosphere the temperatures drop and become more conducive to life. Between altitudes of 48 and 57 kilometers the temperatures lie between 0 and 60 °C, quite pleasant for microbial growth. The likelihood that there is life in the clouds of Venus is very low, though. Although there is some water in these regions, it is tightly bound into sulfuric acid droplets that have a concentration of up to 98%. It would be very difficult for a microbe to extract water in sufficient abundance from these droplets and the sulfuric acid would be very damaging to organic matter. Organisms that could use sulfate and hydrogen to make a living and that like oxygen-free conditions would stand the best chance of making a living in the Venusian clouds, but even for them the conditions on Venus are probably too extreme."

Still, it's not ruled out, and interesting enough to look into it in some more detail.

If there is life there, well it would be of special interest as the only remaining relic of a now vanished biosphere - all that is left of surface life on Venus in the early solar system, which migrated to its upper atmosphere as the conditions became harsher. (See also section on Venusian clouds in "Cosmic Biology - How Life could Evolve on Other Worlds").

As we saw, most scientists think that Venus was a near twin of Earth in the early solar system, with oceans like ours. They don't have quite the same confidence about this that they have for Mars because its entire surface was resurfaced a few hundred million years ago. This would erase any clear signs of the ancient oceans.

We are not too likely to find ancient deltas and coastlines on Venus like the ones we found on Mars. However, there are still hints in its present day geology that suggest it did have ceans originally. We can't confirm it yet, but there are some indications that it may have granite on its surface.

Granite only forms when water combines with basalt. So if we confirm granite there in the future, it will prove that it had ancient oceans at some point in the past. The basalt would be the reamins of its ancient continents. The observations so far are not conclusive, and are result of a map at only one wavelength of infrared, but are suggestive.

Evidence for early oceans on Venus is indirect. This map plots infrared light of 1 micron wavelength emitted by the Venus southern hemisphere, with all the recordings done during the Venusian night and combined together to make this map.

The lower regions emit more infrared at this wavelength. The higher regions are darker in infared - which would suggest that visually they are lighter coloured rocks such as granite. These observations from orbit are consistent with the idea that Venus had earlier oceans, with suggestions that it might still have granite land masses right now "floating" on top of the basalt as they do on :Earth. If so, these may be the remains of ancient continents

Sadly all the eight Russian landers of the 1970s and 1980s touched down away from the highlands and found only basalt-like rock beneath their landing pads. However, the new map shows that the rocks on the Phoebe and Alpha Regio plateaus have infrared emissivity similar to granite. On Earth, then the rocks are granite and form continents. The granite itself is the result of basalt rocks subducted through plate tectonics, when water combines with the basalt to form granite.

Nils Muller: "If there is granite on Venus, there must have been an ocean and plate tectonics in the past,"
See New map hints at Venus' volcanic wet past (ESA)

In more detail: it's only measuring one wavelength, and there's no way to distinguish between heat anomalies and thermal anomalies but such large heat anomalies from volcanism seem unlikely. If there were volcanic eruptions they'd be more localized, or would be over quickly. So their preliminary conclusion is that it's actually due to a different composition of rock. The higher regions would be made of granite as on Earth. Their readings are consistent with this but don't prove it. The paper is here. Another study using Galileo observations when it flew past Venus on its way to Jupiter in 1990 came to a similar conclusion, this time measured at 1.38 microns, that the highlands are made of rocks which emit less infrared at night. This suggests they have lower concentrations of the mafic minerals - minerals rich in iron and magnesium.

This research was reported in some sources as saying that they saw lighter coloured rocks in the highland areas. But it was a bit more indirect than that, as they weren't observing the surface in visual light (you can't see it beneath the dense clouds). Rather, they saw rocks with lower thermal emissivity, which on Earth is correlated with mafic poor rocks which in turn are lighter in colour. It also makes sense geologically for the highlands to be lower density granite, and the long lava channels in the lowlands are consistent with a basaltic composition. So it's rather indirect, but reasonably convincing all the same.

Other rocks that scientists have suggested we could look for once we are able to land on the surface and drill include Tremolite - which is unstable in the current Venus environment, but only marginally so. It would take 3.8 billion years for 50% of the Tremolite to decompose. at the temperature of the Venusian lowlands today. See page 304 of this paper. Tremolite is interesting because it normally forms from Dolomite which in turn is a result of life or life products, so if we find tremolite it would suggest that life in the past may have producted dolomite and then in turn that turned to tremolite. Another suggestion is to look for Hornblende, another mineral that requires water for its formation, which on Venus could be turned into fluorohornblende.

If Venus did have life in the past, in its oceans, with a much thinner atmosphere than today, this is something we might be able to find out from meteorites from Venus that landed on the Moon hundreds of millions or billions of years ago, see Search for life from Mars, Venus, or the Earth - on the Moon in Meteorites! (above)

So, if there is life on Venus, how could it relate to Earth or Venus life? Well, the main possibilities would be:

• Venus life evolved independently from anywhere else
• Or Venus was seeded by life from Mars or Earth, (or perhaps Ceres etc)
• Or Venus life seeded Earth or Mars, or both.

In any of those cases, it has been isolated from Earth for hundreds of millions to billions of years, or possibly evolved independently.

As we saw, in What about Zubrin's meteorites argument? and following , there's some chance of life transferred from Mars to Earth in the last few hundred million years, and after a very large impact, just possibly, from Earth to Mars (though much harder that way). There is a chance that Mars life is closely related to Earth life, depending on whether any life did get transfered between the planets recently and how much of it made the journey..

However, there is almost no possibility of life getting from Earth to Venus at least in the last 715 million years or so, and no chance at all from Venus to Earth since it developed this thick atmosphere (its atmosphere is too thick for even the largest asteroid impacts possible today to send debris with escape velocity all the way to Earth). And again, similarly to Mars, the most likely time for the planets to exchange life is in the early solar system, when there were much larger impacts, of objects as large as 100 km or larger and more of them too.

So, though life there may seem a bit of a long shot, still, if it does exist, it is a really exciting possibility for biology. We may make amazing discoveries from studying life that's been isolated from Earth life for so long, or perhaps evolved independently.

Venus (left) may have had oceans like Earth (right) in the early solar system, and life could have evolved there, or been seeded by Mars or Earth. If so it might still exist in the clouds.

What about planetary protection for Venus? Even if the possibility of life there is very remote, we still have to consider the implications.

An international team of scientists for COSPAR (Committee on Space Research) examined it carefully back in 2005. Their report is here: "Assessment of Planetary Protection for Venus Missions" (you might find that the easiest way to read this report online is to get free membership of NAP and then use the download button and read it as a pdf). They came to the conclusion that conditions on Venus are so different from Earth, even in the more hospitable cloud tops, that there is no need for planetary protection.

As a result, Venus is currently classified as Category II, and sample return is classified as unrestricted Category V. So, you can go to Venus and do anything you like there with no need for sterilization, so long as you document what you do. You can also return a sample of the Venus atmosphere to Earth for study, with no need to contain it or act in any way to protect the Earth environment. The only requirement in both directions is that you have to keep detailed documentation of whatever you do.

However, there was a dissenting voice at the time, from Dirk Schulze-Makuch who was not part of the team. See Planetary Protection Study Group Mulls Life On Venus. As you might expect, everyone agrees that there are no planetary protection issues for the Venus surface, with temperatures well over 400 °C.

The dispute here is of course, about the cloud tops. Should the Venus upper atmosphere perhaps be re-categorized as category III, meaning that you have to sterilize spacecrafts that visit it? Should sample return from Venus clouds be re-categorized as restricted Category V, meaning that you have to take precautions to protect Earth?

Our knowledge of the Venus cloud tops so far is rather minimal and it's not as though we have any actual experience in studying astrobiology from another planet yet. We have a dissenting voice from an expert astrobiologist, and so it seems reasonable to ask if he might be onto something.

I think we should consider this as a classification that may need to be revisited and might not be as straightforward as it seemed to them at the time. Not provisional in the sense of COSPAR classifications. But subject to change in the future, so provisional in that sense, that perhaps future discussions might lead to a change in the classification.

Some of the material here comes from my article If there is Life in Venus Cloud Tops - Do we Need to Protect Earth - or Venus - Could Returned XNA mean Goodbye DNA for Instance?

So, why were there any dissenting voices at all? Why was there any dispute about this? Let's have a look at planetary protection for the Venus clouds in a bit more detail.

Possibility of Earth life able to survive in the Venusian clouds

First lets look at planetary protection issues in the forward direction. Could any Earth life survive and reproduce in the Venus clouds? At first sight this seems most unlikely. Surely no Earth life could survive in concentrated sulfuric acid droplets. The Venus pH goes as low as -1.3 in the lower clouds, more acidic than battery acid (pH 0.8). This is the main reason why COSPAR concluded that no planetary protection is needed in the forward direction.

However, the situation is not as clear cut as you might think. In 1991 researchers found Earth microbes able to survive sulfuric acid with pH 0 or lower, close to the Venus cloud conditions. These researchers also wrote that it is possible that we might find organisms able to tolerate even lower pH levels. Their most acidophilic (acid loving) microbe was Picrophyilus, which grows optimally in sulfuric acid at pH 0.7 and is capable not just of survival, but growth, down to pH -0.06 (1.2 M sulfuric acid). This is a microbe which you can find living naturally in highly concentrated sulfuric acid in the wild, in acid mine drainage and in solfataras (sulfur emitting fumaroles). Picrophilus oshimae and P. torridus are now known able to survive down to pH -0.2

Then, it turns out that the clouds are not all equally acid. The comparatively water rich upper clouds of Venus have pH 0.3 to 0.5 (page 9 of this paper). So the upper clouds are already within the tolerance range of Earth acidophiles.

The Venus pH goes as low as -1.3 in the lower clouds. So could any Earth life survive there? The Iron Mountain pyrite mining operations have created conditions with a pH as low as -3.6, and naturally occurring hot springs near Ebeko volcano have a pH of about -1.7. So is there any life in these conditions on Earth? It's not easy to check up on this. David Grinspoon and Mark Bullock, writing about the Iron Mountain outflow in 2007 put it like this (page 9 of their Astrobiology and Venus Exploration):

"However, it is not easy to search for life in the more acidic waters in the negative pH zone of this stream, as ordinary culture mediums would simply dissolve in this water (Nordstrum, 2005). New, acid-resistant, culture mediums will have to be created in order to test for life in the most acidic waters. Thus, the low pH limit of terrestrial life is currently not known."

So perhaps some Earth micro-organisms could live there after all. Now you might think - "Ah but those are microbes in acidic hot springs - how are they going to get onto the spacecraft?". Well the problem is that extremophiles can get anywhere. There are many extremophiles that are perfectly happy also living in other much less extreme conditions. An ordinary seeming microbe can have extraordinary capabilities when you put them in an extreme environment.

Only a tiny percentage of all species have been studied in any detail. So it is hard to say for sure what the capabilities are of the micro-organisms we haven't yet studied, such as the majority of the archaea. This is the issue of "Microbial dark matter". For instance a recent study found that - "Of the 100 major branches, or phyla, of microbes, less than one-third have any described species", see How Many Microbes Are Hiding Among Us?

Then, it's a major issue actually trying to work out what a microbe can do - how can you do that if you can't cultivate it? Only 1% of the bacteria on Earth can be readily cultivated in culture media. There are various reasons why this might be the case. See Strategies for culture of ‘unculturable’ bacteria for an overview.

The "Great Plate Count Anomaly" - if you cultivate cells in a medium and count the number of Colony Forming Units (CFU's), and if you then take the same sample and count individual cells in a high powered optical microscope, typically you find that there are about 100 times as many cells as you detected with the CFU method. This is because biologists currently can only cultivate 1% of living cells, typically.

The microbes carried by humans can have hidden extremophile capabilities - because microbes do not lose their capabilities, usually, when they move to a different environment. Some are polyextremophiles able to live in a variety of extreme environments as well as in much more ordinary ones (ordinary for humans). A typical human has 100 trillion microbes in 10,000 species - and the species mix varies from one person to another. Many of these will be unknown to science, and some may well have extremophile capabilities. For example a recent study of microbial populations of human belly buttons found a couple of species able to thrive in extreme cold and extreme heat. Another example is the discovery of a microbe on a human tongue able to thrive in conditions of very low pressure. There is no way to do a complete census of the species in a human occupied spacecraft to check there are no microbes there with extremophile capabilities. The same is true of robotic spacecraft too.

I'd also like to share another one of my speculative questions to stimulate thought. How uniform are the conditions in the Venus clouds? We already know that there are non spherical particles that are multiple microns in size. A habitat doesn't need to be large to be useful to a microbe. Could there be microhabitats of some sort in the Venusian clouds in which Earth life or Venus life could survive?

Indeed, if there is indigenous life there, maybe it could itself create microhabitats in the clouds in some way, that Earth life could survive in somehow? Even if Earth life can't survive directly in the clouds, could it survive in a microhabitat created by microbes in those clouds, including perhaps inside the microbes themselves? Especially if the microbes there haven't evolved defenses against Earth life.

Then there's another thought too, again my own suggestion. Lateral gene transfer works across completely unreleated forms of Earth life. So, if Venus life is related from the distant past, in the early solar system, might it be able to swap genes with Venus life via lateral gene transfer using GTAs? The Earth life wouldn't need to be able to survive in the droplets for this to happen. Could this happen between Earth life and these microbes, perhaps between an acidophile Earth microbe and a Venusian microbe?

Before we discuss planetary protection in the backwards direction, I'd like to touch on a rather fun idea for ways that microbes could stay aloft in the Venusian clouds for longer.

Possibilities of indigenous life in the Venusian clouds

This is another of my fun speculative sections. My question here is - the Venus atmosphere is so thick that microbes and other particles would stay suspended for months, rather than the days for Earth. Still, they will eventually fall to the lower layers; which makes it an issue, how do the microbes stay aloft, and reproduce? Perhaps microbes in one droplet, descending, could send out spores (explosively perhaps) that land in other droplets that ascend, and so continue the reproduction. But it would seem to be an evolutionary advantage for microbes to stay aloft for as long as possible and sink through the atmosphere as slowly as possible. So - might they have evolved techniques to stay aloft for even longer than the months calculated for ordinary microbes in the Venus atmosphere?

Well here are a few suggestions to think over.

First, one idea is that the microbes could trail long threads, rather like the spider webs for parachuting spiders or spider ballooning. I expect most of you know about the way spiders can throw out a line of thread which gets caught in the wind and can transport them for many miles through the atmosphere. Here is a rather charming silent documentary film from 1909 "To Demonstrate how Spiders Fly" using an animated spider, surprisingly advanced for its time.

(click to watch on YouTube)

Many microbes have long flagella already. So it's not hard to imagine these evolving to be longer and longer to help keep them aloft for longer in the Venus atmosphere. They could even use them for navigation too by changing their position a bit like a free diver changing the positions of their arms and legs. Here are some examples of how microbes use flagella already.

(click to watch on YouTube)

Peter Gorham suggested that spiders might also levitate using electrical charge, taking up charge into the webs as they spin them through "flow electrification". This could explan how it is that they are found at altitudes of up to 4 kilometer. It's hard to see how they could have got their using just thermals from hot air. The idea is that the spider's web picks up negative charges from the air - which is always somewhat charged even without lightning - and the other negative charges in the air then repel the spider silk, causing the spider web to levitate. This would also explain how the strands fan out, through like charges repeling. This is a summary in the National Geographic voices column, and see also summary in physics arxiv blog, and his paper Ballooning Spiders: The Case for Electrostatic Flight

Whether or not that's how spiders are able to fly to such high altitudes as four kilometers - could something like that work in the Venus atmosphere? There's good evidence for lightning in the Venusian atmosphere. So - I know this is speculation on top of speculation, but could microbes in the upper atmosphere use a similar technique to stay aloft - trail a microscopic equivalent of spider silk or a long flagella - which picks up electrostatic charges - and use it to stay levitated in the atmosphere?

Lightning storms on Venus, detail of artist's impression courtesy ESA. High resolution complete image here. Venus Express confirmed earlier tentative detection of lightning in the Venus atmosphere, similar in strength to Earth lightning. They occur most often on the sunny side and at lower altitudes. See Lightning Storms on Venus similar to those on Earth.

So there should be plenty of electrostatic charge around which could help with electrostatic "spider ballooning" type levitation, if the microbes there were to evolve this technique somehow, perhaps using modified flagella in place of spider's web. Just a speculative idea to think about..

Then, there's another way they could stay aloft longer than they would otherwise, perhaps even indefinitely, and that's to use gas filled vesicles or (for multicellular life, if there is any), bladders. This idea goes back to Carl Sagan, who suggested that life in the Venus atmosphere could use gas bladders filled with hydrogen to float in the atmosphere. This is in a paper from 1967 published before the discovery of sulfuric acid in the Venus clouds, so at a time when it seemed more habitable than it does now. He speculated about multicellular life there, which could take the form of a ballloon filled with hydrogen a few centimeters in diameter. In his later "The Trouble with Venus", he writes

"The only serious problem that immediately comes to mind is the possibility that downdrafts will carry our hypothetical organisms down to the hot deeper atmosphere and fry them faster than they reproduce. To circumvent this difficulty, and to show that organisms might exist in the Venus clouds based purely on terrestrial biochemical principles, Harold Morowitz and I (1967) devised a purely hypothetical Venus organism in the form of an isopycnic balloon, which filled itself with photosynthetic hydrogen and maintained a constant pressure level to avoid downdrafts. We calculated that, if the organism had a wall thickness comparable to the unit membrane thickness of terrestrial organisms, its minimum diameter would be a few centimeters."

Here isopycnic means that it has a surface of constant density.

This is not nearly as bizarre as it might seem. Seaweeds use just this method, with gas bladders with oxygen, nitrogen or carbon dioxide inside to float in the sea. Carbon dioxide of course wouldn't work in the Venus atmosphere, but oxygen and nitrogen would both float. However, differences in density at the same pressure could make a huge difference when floating in an atmosphere rather than in water, and hydrogen has much more buoyancy in a carbon dioxide atmosphere.

The huge bladder of bull kelp, with the smaller bladders of giant kelp in the background. Examples of pneumatocysts. They can include oxygen, nitrogen, carbon dioxide or carbon monoxide, produced by the seaweed to keep it floating in the sea.

So his idea is an organism a bit like kelp floating in the Venus upper atmosphere, with bladders a few centimeters in diameter, but filled with hydrogen instead of the terrestrial bladders of oxygen, nitrogen or carbon dioxide.

Nowadays astrobiologists are thinking more in terms of microbes in the Venus atmosphere. What, though, about the same idea as Carl Sagan's but used by microbes rather than the large multicellular organism of his vision? This extrapolation of his idea is my own suggestion (do say if you know of someone who has suggested it in a scientific paper).

So first, do microbes use gas for buoyancy on Earth? Well yes, turns out they do. Some microbes form gas vacuoles on Earth, much like seaweeds. They are used by cyanobacteria to regulate buoyancy in water, which is not that far off the idea of using hydrogen vacuoles to regulate buoyancy in CO₂.

So, that seems promising so far. Is it possible I wonder? If Venus had similar microbes in its oceans, could their descendants in the Venusian clouds evolve over billions of years to use hydrogen to regulate buoyancy in a thick atmosphere of carbon dioxide?

The main difference from Carl Sagan's hypothetical Venus organism and this idea is that normally gas vacuoles in cyanobacteria take up only a small part of their bodies (and are made up of smaller, rigid, gas vesicles). For example, Anabaema has gas spaces occupying up to 9.8% of their volume (see page 124 of the paper "Gas vesicles"). This is far below the levels needed for a microbe to float upwards in the Venus atmosphere.

Gas vesicles. These are filled with ordinary air, and are used by cyanobacteria to regulate buoyancy in water, several of these cluster together to make a gas vacuole. The gas can occupy up to 9.8% of the volume of the microbe.

To get this work in the Venus clouds, first, the vesicles would need to be filled with hydrogen instead of air. Then with the density of CO₂ of 0.001977 (and hydrogen, 0.000089) compared with water, at 0°C, they still need to have so much hydrogen in the vesicles that the vesicles occupy approximately 98% of the volume of the microbe.

I'm not sure if this is possible. However, life solutions are often surprising.

How could the microbes evolve such an adaptation? Well the first step forward here is the idea that gas vesicles in the Venusian microbes don't need to make the microbes float to confer a survival advantage. There would be selection pressure towards any microbes that don't fall through the air so quickly. A microbe that generates enough hydrogen to make it a bit lighter and so, to slow down its descent, even if it only gives it a few more days floating in the atmosphere, might have an advantage over microbes that don't. That would give evolutionary pressure to evolve more and more tiny hydrogen filled vesicles, and larger and larger ones too. Cyanobacteria don't need much of their body taken up by vesicles to float in water, so they didn't have this evolutionary pressure in our oceans and ponds. How much of their body could they devote to them if they really needed them?

That 98% of the body volume as hydrogen is a big ask though. Is there any other way they could do it? Well there is another idea, also suggested by nature. Perhaps instead or as well as internal vesicles, they might produce something more like external hydrogen filled bubbles, or external vesicles filled with gas, attached to their bodies, somewhat like the bubble nests created by some insects, and use those to float in the Venus atmosphere?

I.e. they blow bubbles of hydrogen to stay afloat. Or, very speculatively, indeed might there even be higher plants, some kind of lichen perhaps, or animals, that do this in the Venus atmosphere?

Froth of Spittle Bug, or Frog Hopper - Larval form - could a similar technique be used in the Venus cloud tops, using bubbles filled with hydrogen, attached to the microbe or higher life form as a type of froth or foam, for buoyancy? To float endlessly at the one atmosphere level on Venus, it would need to have less than 2% of the volume for the bubble walls and the body of the creature, with the rest of the interior filled with hydrogen

Or indeed, perhaps this could be combined with the spider ballooning idea. Have these bubbles attached to them by threads, a little like miniature hydrogen balloons as in "gas ballooning"? Perhaps a sticky thread with a string of hydrogen filled bubbles along it. Microbes don't have to do this on Earth AFAIK, but on Venus, perhaps they would? Again this could be an option for higher plants and animals too. Could it have its equivalent of airborn lichens or spiders?

This is just a fun suggestion, and it is my own idea. I know that when Carl Sagan suggested it for higher organisms (such as plants, say), he had in mind a much more clement idea of Venus than the one we have today, long before the discovery of sulfuric acid in the clouds. However we now have those acidophiles that are rather pushing the limits of what might be possible for life in such acidic environments..

Is it possible for microbes? Do say if you know of anyone who has published a paper exploring any of these ideas, or any research into it.

The main questions still unanswered for planetary protection for the Venus clouds

In the backwards direction: Could indigenous life from Venus colonize earth after a Venus sample return - the COSPAR study came to the conclusion that due to the high acidity then these life forms if they exist are unlikely to be able to colonize Earth. But Dirk Schulze-Makuch was not convinced by this conclusion - so that suggests there is room for discussion here.
 
What can we know in advance about the limitations of life adapted to the Venus clouds, when we don't know yet anything about its biochemistry, if it exists? A couple of thoughts here:  

• Acid environments on Earth - if they can't adapt to Earth generally, could the lifeforms, if acidophiles, colonize some of the most acidic environments on Earth? After achieving this, once established perhaps in acidic springs - or maybe even in lead acid batteries, or animal stomachs - could they then gradually evolve to survive in less acidic environments elsewhere on Earth?
   
• Retained capabilities - also - microbes often retain capabilities that they no longer need. If Venus was indeed as habitable as Earth 715 million years ago, how much of the capability to live in Earth like environments has the modern Venus life retained (if it exists)? Perhaps some of the life still retains the capability to survive in less acidic environments, or has dormant genes that let it do this that could be "switched on" as it adapts to Earth.  

In both directions:

• Could there be less acidic microhabitats in the Venus clouds created by its own indigenous life? We haven't studied the clouds in detail close up, only from orbit. The Venera atmospheric balloons had limited capabilities. We know that life can transform habitats and form micro-habitats.
  
  If that's so, perhaps some of those habitats could be inhabited by symbionts which would perhaps need the capability to live in non acidic environments. If so, then these habitats could be colonized by Earth life, and also, n the reverse direction, the life in those habitats could be pre-adapted to less acidic environments, so more likely to be able to survive on Earth.

• Could gene transfer agents transfer capabilities? If Venus and Earth shared life late enough in the solar system, after the evolution of modern DNA life, you have the possibility of Gene Transfer Agents, those tiny virus like particles, well below optical resolution, able to transfer DNA from one micro-organism to another, very readily, as we've seen already, in that striking experiment where 47% of the culturable microbes in sea water had taken up antibiotic resistance from GTA's. after they were left overnight in bags filled with sea water and the GTA's.
  
  So, even if Venusian cells were unable to survive on Earth, their dead cells could still transfer some of their genetic material to Earth life so creating hybrid lifeforms. In the other direction, it is also a potential issue for forward contamination of Venus clouds as well. Earth archaea could transfer genetic material to microbes in the clouds in the same way even if they can't survive there themselves.

In the forwards direction:

• Might there be Earth microbes capable of surviving in the acid conditions in the Venus clouds "as is"?

Of course none of this is to suggest that we should have a moratorium on materials returned from the Venus clouds for all time, at least, not based on our knowledge so far. It's just to say, let's look carefully and find out a little more about the clouds before we conclude that it is safe to return an unsterilized sample from the Venus clouds to Earth. Also, in the forward direction, it's to suggest that perhaps it might be a good idea also to sterilize the first few probes to look at the atmosphere close up, just until we know what is there and are sure that there are no microhabitats for Earth life and no chance of Earth life surviving there.

It would be such a major discovery to find Venusian life still surviving in the clouds. We need to be careful not to mess it up I would say, even if the chance of it existing seems rather slim.

Does it matter if life from the Venus clouds gets established on Earth

You might wonder, okay we are required by the Outer Space Treaty to protect Earth from harmful contamination from Venus. But does it really matter if life from Venus gets established on Earth, or genetic material gets transferred to Earth archaea via GTA's.? Would it indeed be harmful if this happens? If we can show that it is not harmful, there is no cause for concern, and also, we don't need to worry about the OST either (as the clause refers to "harmful contamination").

So, just to go over it quickly, as some of the things that could happen are similar to those for Mars.

• Pathogen of humans or our crops or animals, sea creatures, plants, trees. It doesn't need to be adapted to us. A disease of microbes for instance can directly infect humans without any adaptations, as happens with Legionnaire's disease, a disease of amoeba that happens to be able to reproduce also in human lungs. Indeed pathogens most usually adapt to keep their host alive for longer, not to kill their host. If what we have spotted in the Venus clouds are indeed large microbes up to 10 microns in size, they probably have pathogens.
   
• Is able to live on our skin (e.g. fungal infection), in our lungs, in our sinuses, or perhaps in our stomachs.This is another of my speculative suggestions. Our stomachs particularly seem an obvious microhabitat for an acidophile from Venus. It's true that it would be more used to sulfuric acid rather than the hydrochloric acid of gastric acid (though the Venus atmosphere does have trace amounts of HCl). Perhaps though, just possibly, it would be able to reproduce in the stomachs of animals, and ourselves, and interfere with digestion, or create byproducts poisonous to us or the animal?
   
• Takes the place of some other organism in an ecosystem but behaves differently so disrupting natural cycles, for instance a photosynthetic lifeform in our oceans. Probably unlikely to do that right away (who knows for sure though) but microbial life can adapt quickly, so if it gets a foothold on Earth somewhere, maybe eventually it finds a way to survive in our oceans.
   
• Is an allergen or creates allergens as a byproduct
   
• Creates a poisonous byproduct that interferes with human or other animal biological processes. with examples such as botulism, ergot disease, BMAA etc, see Many microbes harmful to humans are not "keyed to their hosts" It is of no evolutionary benefit to a green algae to kill cows or dogs, or to cause Alzheimer's in humans.
   
  A microbe from another planet might well create chemicals that resemble ones in our body but are not identical and get substituted for them, and so disrupt the way our cells work or the way cells of other organisms work, or produce poisons that only incidentally harm Earth life, not targeted at us at all.

Life returned to Earth from another planet may well be harmless, but there are many ways that it could cause harm, also. We can't know with reasonable certainty until we know something about the form of life and how it works.

XNA based life in the Venus clouds

Finally, there is the possibility that Venusian life is not based on DNA but some other basis such as XNA (change of backbone) or something more radical than that. If so then we can't really generalize from DNA to capabilities of XNA.

(click to watch on YouTube)

Rotating DNA animation. Could life on Venus have a different backbone from DNA , using PNA, HNA, TNA, GNA or other XNA?

Here XNA is a general term for nucleic acid analogues - with the same bases as DNA but a different "backbone", in place of the Deoxyribose of DNA. These include HNA, PNA, TNA or GNA (Hextose, Peptide, Therose or Glycol NA).

The PNA world hypothesis for instance suggests that life on Earth went through an earlier stage where it used PNA (peptide nucleic "acid") before it started to use RNA or DNA. That's because DNA and RNA are so complex it is a little hard to see how they arose from non living chemicals alone.

Life on Venus could have done the same, but maybe didn't end up as DNA. It may still use PNA or perhaps it evolved to use some different form of XNA.

That raises the possibility that XNA based life could be better at coping with Earth conditions than DNA itself. This could be possible, if it is really a completely different form of life with different metabolism, cell machinery, etc. and has never had any previous contact with the Earth environment.

Venus cloud life might give the best chance for XNA in our solar system

The Venusian clouds indeed might give us one of our best chances of finding XNA in our solar system - in the remote case where there is life there. That's because for hundreds of millions of years, and possibly for billions of years it has been almost impossible for Earth life to be transferred to Venus. The surface of Venus is so hot that Earth life would be destroyed soon after it got there, if it made it all the way to the surface of Venus. So, it's hard to see Earth life reaching the upper Venus atmosphere. Any particles light enough to be captured without damage would surely be thoroughly sterilized by UV and solar storms and cosmic radiation on the voyage from Earth to Venus.

The other way around also, then it is almost impossible for the cloud top life of Venus, if it exists, to be ejected through the thick atmosphere as the result of meteorite impacts on the surface of Venus. A huge asteroid impact on Venus would disturb the cloud deck for sure, but could even a giant impact send significant amounts of the high Venusian atmosphere into space? And if it did, again you have the issue - what could protect the life so that it survives the journey all the way to Earth?

Chandra has put forward a controversial theory that the solar wind could transfer microbes from the upper Venus atmosphere (high above the cloud decks) to Earth at times when the planets are aligned. See Microbes Could Travel from Venus to Earth However other scientists find his research unconvincing, so far, with many details to be filled in. For instance, it doesn't seem that the solar wind would have enough energy to remove a microbe from the Venus gravity well, since it is far heavier than the ions it can transport. Also, any dormant microbes that did get ejected from Venus would also be vulnerable to cosmic radiation and high levels of UV, which they might not be adapted to.

So, it seems at least possible that life could have evolved independently on Venus, and has been there ever since. If so, it would probably be a form of XNA. In that case all bets are off as far as planetary protection of the Earth.

If Venus life is based on XNA, We can't say much by analogy with DNA life even about its size, or its properties or its adaptability to different environments. There are other places that could have XNA, including Mars, or comets.

If Venus was habitable recently, then it's easier to have shared life. But if that hypothesis is wrong, and it wasn't habitable for the last several billion years, Venus has been more isolated from Earth than any of those. Even the Europan oceans could potentially share DNA with Earth through impacts on Earth sending debris all the way to Europa. This probably was only be possible for Venus in the very early solar system. The Venusian surface might also have been too hostile for Earth life already by the time Earth was habitable.

COSPAR study of the Venus atmosphere didn't consider XNA or gene transfer agents

Here the situation is similar to the studies of risk for Mars sample return. Often new planetary protection studies bring up the possibility of new risks not considered in previous studies. The 2009 Mars sample return study by the US National Research Council brought up the new possibility that Mars life forms might be smaller than previously thought and added a new recommendation to contain ultramicrobacteria at 0.2 microns across. The 2012 Mars sample return study by the European Space Foundation added another new recommendation, this time to contain Gene Transfer Agents only 0.01 microns across if possible - it was published just after the discovery of easy transmission of GTA's. between unrelated species of microbes in sea water.

Both studies of Mars sample return mention XNA but they do not go into it in any depth, particularly, they don't mention the researches into safety considerations for XNA in Earth laboratories. Also neither study considered the possibility that the life forms to be contained are smaller than the smallest known Earth microbes. This seems at least possible since, though 0.2 microns seems to be the smallest organism that could contain all the cell machinery of modern life, early cells on Earth must have been smaller than the ultramicrobacteria of the order of tens of nanometers across. Also, we have no way to be sure of the size of XNA lifeforms.

The Venus planetary protection study "Assessment of Planetary Protection for Venus Missions" didn't consider GTA's. or XNA. It is rather short. This is all that it says on protection of Earth from Venus life

"The cloud layers in the atmosphere of Venus provide an environment in which the temperature and pressure are similar to surface conditions on Earth. However, the chemical environment in the clouds, and specifically in the cloud droplets, is extremely hostile. The droplets are composed of concentrated (82 to 98 percent) sulfuric acid formed by condensation from the vapor phase. As a result, free water is not available, and organic compounds would rapidly be destroyed by dehydration and oxidation. Therefore any terrestrial organism having survived the trip to Venus on a spacecraft would be quickly destroyed. It is not possible to demonstrate conclusively that a spacecraft returning to Earth after collecting samples of Venus's surface and atmosphere will not come into contact with hypothetical aerial life forms and inadvertently carry them back to Earth; however, this has to be considered an extremely unlikely scenario. At any rate, any life forms that had adapted to living in the extremely acidic environment of Venus's cloud layer would not be able to survive in the environmental conditions found on Earth."

But as we've seen, doubts were raised about their conclusions about the possibility of Earth originated acidophiles to survive in the Venus atmosphere. Also the study was not based on experimentation and we have limited knowledge of the Venus upper atmosphere. We don't know enough yet to make an accurate simulation of it in a laboratory on Earth for testing.

Schultz Makuch is quoted by Space.com as saying:

"As the task force explained, there shouldn't be any significant interaction between putative Venusian cloud microbes and Earth organisms. However, there is some uncertainty because most Earth microbes are still unknown and there are some known organisms that come close to living in Venus-like conditions.We do not know and thus cannot estimate capabilities of any alien organism. Perhaps, if they originated in an earlier Venus ocean they may have still retained the capability to quickly adapt to their earlier environment. Thus, they might be capable of competing in selected, rare niches on Earth, such as volcanic vents... The chances of an indigenous microbial community floating around in the Venusian atmosphere are not remote but are significant in my view"

On the other hand Jim Rummel and David Grinspoon in the same article are quoted as saying they are satisfied with the report.

The clouds may well turn out to be so utterly hostile to Earth life that there is no chance it could survive there. It may well have no Venusian life in it either. But I'm not sure we can conclude this for certain yet, when faced with a diversity of views like this amongst experts. I think it is possible that a new study, taking account of these ideas, would change the provisional classification of the Venus atmosphere for both forward and backward contamination.

Hazards of XNA from Venus

We've looked at this in a general way in Does it matter if life from the Venus clouds gets established on Earth. However there may be other hazards too, because it is potentially so radically different from DNA based life. Here I thought I might try a different take on this. What might happen if we introduce a new biochemistry to Earth, one not even based on DNA?

We have no experience at all of a planet with two radically different types of biochemistry on it. So what typically happens? The main possibilities seem to involve one or more of:

• They co-exist, but with a separation of habitats - e.g. we end up with Mars life in extremely cold, arid, conditions with high levels of UV, Venus life in hot acidic conditions and Earth life everywhere else.
• They co-exist in the same habitats, maybe with some level of symbiosis, potentially mutually beneficial, but also potentially harmful to each other
• They borrow capabilities from each other to create a new lifeforms with features of both
• One makes the other extinct.

The hope would be that the last of these happens if we return XNA from Mars or Venus - with of course Earth life being the one that makes the XNA based life returned from Mars or Venus extinct.

Any other outcome would lead to some of our ecosystems on Earth being transformed in one way or another, even if it is just a new lifeform existing in niches, which could be anything from a nuisancy lifeform in freezers, a pathogen of animals, or a microbe that has toxic effects. In the worst case of co-existence, the organism could replace key organisms in an ecosystem, such as the photobionts in the oceans, but behave differently. It's poisonous, or inedible, or doesn't produce oxygen, or whatever.

Or - perhaps it is beneficial. Maybe it's a microbe that aids digestion, counteracts diseases, reverses the effects of aging in humans (like Larry Niven's fictional "boosterspice"), ... Or maybe it is good to eat, or a form of medicine, or it's an ornamental organism that we use to decorate our homes or gardens.

An extraterrestrial lifeform doesn't have to have a harmful effect on us when it enters our ecology. Some of those effects might perhaps be beneficial, but we'd want to be really sure before giving a new type of organism a pass to colonize niches on Earth.

Anyway the new thing for this section is that thought, maybe we can get an interesting perspective here by looking at the precautions suggested for experimentation with XNA based life in the laboratory.

It's a similar problem, except that XNA based life in a laboratory is under our control. We create it by modifying Earth life, and we can design it to be safe. Our XNA life from laboratories is also bound to be closely modeled on Earth life, because at present we have no way to create new lifeforms from scratch from basic chemistry.

So, XNA life in a laboratory, with the techniques we have so far, can only explore a tiny fraction of the possibilities for XNA life separately evolved on another planet. Still, any issues that turn up are ones that could also happen with closely related XNA based life returned to Earth - though there may be other issues we don't have the imagination or experience to describe. At any rate, surely all of these will be potential issues for life returned to Earth. So let's look and see what XNA researchers say about it.

In the XNA specifications section of this paper: Xenobiology: A new form of life as the ultimate biosafety tool, the authors talk about a road map that could lead to the creation of XNA based life in a laboratory and discuss biosafety requirements for this procedure

"The ultimate goal would be a safety device with a probability to fail below 10^-40, which equals approximately the number of cells that ever lived on earth (and never produced a non-DNA non-RNA life forms). Of course, 10^-40 sounds utterly dystopic (and we could never test it in a life time), maybe 10^-20 is more than enough. The probability also needs to reflect the potential impact, in our case the establishment of an XNA ecosystem in the environment, and how threatening we believe this is."

So, the idea is that the experiments need to be designed so that there is less than a 1 in 10²⁰ chance of the XNA reproducing in the wild outside the laboratory (most likely by making it dependent on some substance not available "in the wild" outside of the laboratory).

Of course this is just the assessment of one group of scientists. Others could come to other conclusions. Still it's a high bar they have set. If they can achieve this standard, could it be one we achieve for a sample returned to Earth from another planet?

So how do they propose to do this? The issues they identify for artificial life created in a laboratory might perhaps also arise for life returned from Venus or Mars.

• The xeno-organisms must not and cannot produce certain essential biochemical building blocks, i.e., their own nucleotides. These biochemicals will have to be supplied externally.

The name for this is auxotrophy. There has to be some chemical the XNA based life depends on which it can only find in the laboratory and which it can't create for itself. They say "To avoid natural supply of xeno nucleotides, the XNA building blocks should at least be two synthetic steps away from any natural molecule."

So - first - it could be that once we know more about Mars or Venus life, that we are able to provide this level of assurance. We might find that there is some element of its biochemistry that is dependent on chemicals in the Venus atmosphere that are just not present in the natural environment on Earth for instance. But we would have to be very sure there.

• Natural organisms must also not be able to produce these essential biochemicals, to avoid a symbiotic relationship with XNA.

There are so many Earth organisms - if there is something they need from the Venus clouds that isn't available on Earth - could any of them enter into a symbiotic relationship with XNA life from Venus or Mars, supplying biochemicals they can't produce for themselves here?

Well, so far we haven't got as far as this. The only reason they give for supposing it can't survive here is because they assume that it would be dependent on an acid environment. But how can we be sure of that? That's the main question with the Mars and Venus originated XNA. Can we guarantee auxotrophy - that Venus life, when transferred to Earth, can only survive in special conditions available in a laboratory and can't survive in the wild?

If it is a polyextremophile that hasn't lost it's ability to survive in ordinary Earth environments from hundreds of millions of years ago, it might be able to just blend right in and already be adapted to Earth life. Or it might need to enter into a symbiotic relationship with some Earth microbes that provide whatever it is missing in an environment without those high levels of sulfuric acid. Or maybe it finds an organism that provides the acid conditions it needs (such as the stomach of an animal as I suggested above?).

So long as it has the capability to make hardy spores or dormant states, then it won't need to encounter an optimal environment right away on release from the sample container ,..

• Xeno-organisms must not lose their auxotrophic character.

So for instance if Venus based life is only able to survive in the laboratory to start with - could it evolve to survive on Earth? If it can only survive in acid to start with - could it evolve to survive in our rivers and seas, perhaps just by switching on genes from its ancestors hundreds of millions of years ago?

• Gene flow – whether horizontal or via sexual reproduction – cannot occur between the two realms of life. DNA cannot be interpreted by the XNA replication machinery and vice versa. A piece of XNA cannot, therefore, escape to wild-type organisms and be incorporated into their DNA genomes.

This is of course a major issue with XNA from Mars or Venus, or anywhere else. It might have a last common ancestor long back. Perhaps it's a minor modification of DNA with different bases? Could there be enough in common between the two forms of biochemistry for DNA based replication machinery to interpret the XNA? Could the XNA based biochemistry interpret DNA based life and so lead to a flow of genetic information? Well in the case of Venus or Mars life it could even be a distant cousin based on DNA already, maybe with extra bases - for life like that then especially if it separated after the last common ancestor to all Earth life, then the answer might well be "yes", since lateral gene transfer seems to be a very ancient mechanism common to all Earth life.

• Symbiogenesis (the merging of two separate organisms to form a single new organism) between XNA and DNA should not take place.

Could Venus life actually merge with Earth microbes to form a new organism? E.g. the Earth life eats it, and the Venus life becomes a component of an Earth microbe cell, or vice versa?

• XNA must not be a recalcitrant chemical, but should act as food for natural organisms after its death/destruction.

This relates to the issue that it could replace some essential part of the food chain in some ecosystem - but be inedible by the other creatures there. For instance, replace the photobionts but be inedible - or actually toxic - to the rest of the chain of life in the sea. Of course this is most significant if it is able to reproduce in the wild so it goes along with the other requirements.

They also add various requirements to make sure that the XNA based life can't have direct access to the 4+ billion years of evolutionary experience of Earth based life via lateral gene transfer. But it's not really necessary to go into that as it is reasonably clear already that there is no way we can guarantee this level of isolation for an exobiology from Mars or Venus.

Now, there is a plus side there too. All these things they add in as safety features - well if we are very lucky, they are actually present already, all, or most of these:.

• The XNA based life can't live on Earth, it's dependent on chemicals not available here
• There is no way for it to form a symbiotic relationship with Earth life to live here
• There is no possibility of XNA from Venus merging with an Earth microbe
• There is no possibility of lateral gene transfer of any sort or of XNA based life making any sense of DNA or RNA, or Earth microbes making any sense of XNA.
• Single strand XNA can't interfere with the transcription process in natural cells
• When it dies, the XNA based life is also edible by Earth life.

It's possible. Life in the liquid sulfur dioxide pools of Io for instance, if there is any, probably ticks all those boxes, except perhaps the last one, that it might not be edible by Earth life. The same would be true of life based on silanols in the ethane / methane pools of Titan, or in liquid neon or hydrogen. Also it may well be the same life with hydrogen peroxide and perchlorates inside the cells, in their cytoplasm, adapted well sub zero conditions on Mars, or indeed, life in supercritical CO₂. Could the same be true of Venus cloud life once we find out more? (All supposing it exists at all of course). Well at that point, it's less clear, and that's the reason their category II and unrestricted category V classification was disputed.

This is how the biologists studying XNA state it in the article (see the section XNA specification):

• "Xeno-organisms must not loose their auxotrophic character.
• Natural organisms must also not be able to produce these essential biochemicals, to avoid a symbiotic relationship with XNA.
• Natural DNA polymerase should not be able to transcribe XNA to DNA.
• Natural RNA polymerase should not be able to transcribe XNA to RNA.
• Artificial polymerase must not be able to transcribe DNA to XNA, or otherwise the XNA would have direct access to 4?billion years of evolutionary experience.
• XNA genes be taken up by DNA organisms should not be recognized by natural transcription factors.
• Preferably, single-strand XNA should not interfere with the transcription process in natural cells (like iRNA).
• Symbiogenesis (the merging of two separate organisms to form a single new organism) between XNA and DNA should not take place.
• XNA must not be a recalcitrant chemical, but should act as food for natural organisms after its death/destruction.
• Preferably, additional layers of orthogonality such as non-canonical base pairs, rearranged codon assignment, etc. should be used to increase the safety mechanism even further."

I think looking at their specification may help clarify thought about what is needed to ensure safe sample return of exobiology to Earth. Or indeed, safe in situ exploration of that biology by humans.

Impossibility of containing XNA at sufficient probability levels

So could we ensure safety by just containing the XNA?

Well, XNA returned from Venus could not be contained at those sort of probability levels (1 in 10²⁰ ). It would more likely be a one in a million type containment such as is suggested for the Mars sample return proposals. One in a million containment is already potentially a major engineering challenge if the particles to be contained are small, such as 0.01 microns across in the case of the GTAs considered for the Mars sample receiving laboratory. Then there are also the issues of natural disasters (hurricanes, meteorite strikes) and acts of terrorism, forgetfulness, etc.

Of course there are going to be many differing ideas about this. But it might be interesting to note that exobiologists come up with figures like that when considering the similar but probably less risky situation of XNA based life created in a laboratory. Why use lesser standards for life returned from another planet? Something to think about.

For more on this see Is this true: "We cannot take even a small risk with a billion lives"? The bold, cautious and intermediate above

For Venus cloud life, as with Mars based life, then my suggestion for sample returns would be to return the first samples to above GEO until we can study them remotely to find out what is in them - or else - to sterilize them.

Suggestion, to sterilize our Venus cloud explorers for now

This is my personal view for discussion. I feel personally that we should sterilize spacecraft and instruments designed to study the cloud tops of Venus, until we know a bit more about it, even with the current classification as Category II. That it is disputed by a well regarded astrobiologist enough reason to do it this way. The classification is not certain enough for us to be sure that it won't change in light of future discoveries.

The Venus clouds might well turn out like the Moon. The first few robotic missions to the Moon in the early 1960s were sterilized (long before the human landings). Looking back at it now, we can see that it was unnecessary. But at the time, though it seemed very unlikely that Earth life could contaminate the Moon, they didn't know for sure. NASA sterilized the first few Block II Rangers, with dry heat heat sterilization similar to the later Viking landers. They stopped when they came to the conclusion that the mission failures may have been due to its sterilization, also given that the Moon was unlikely to harbour any indigenous life.

NASA's Block II ranger. The upper part is its hard penetrator, which separated and was sent to impact into the Moon, and the sphere is the "impact limiter" to help cushion impact on the surface, made of balsa wood. Its components were baked for 24 hours at 125 °C, and cleaned with alcohol, then assembled. They then saturated the spacecraft with ethylene oxide gas for 24 hours while in its launch faring, to protect any possible indigenous lunar life from Earth microbes.

We now know that there was no need to sterilize it. But that is with hindsight.

In the same way, it might well be that in the future we know for sure that there is no need to sterilize missions to the Venus clouds. It's quite a close parallel. But we can't guarantee that the conclusion this time will be the same as it was for the Moon. So let's sterilize the first few missions there until we are sure.

With hindsight they didn't need to sterilize the Ranger spacecraft. But at the time it was 100% the right thing to do.

We have found life in many surprising places on Earth. Perhaps from time to time we may find life in our solar system in places where we thought originally that it was unlikely. It is just too soon to tell, we didn't find life on the Moon but will we find it in the Venus clouds? For as long as we don't know what is going to happen, we have to go by what we know right now.

Okay this may add 10% to the cost of the mission (sterilizing Viking added an estimated 10% to the mission cost). That is a big increase when margins are tight, I understand. But that is well worth it to be totally sure that e.g. if you do detect apparent signs of life in the clouds, such as DNA or amino acids, that it comes from Venus and not your spaceship.

Also in the forward direction it means we exclude the probably remote chances of some archaea with pH 0 acidophile capabilities getting transferred to Venus on our spacecraft, or some of our archaea managing to share their DNA with Venusian organisms via GTA's (if related), or in the reverse direction, Venusian XNA able to out-compete DNA. It prevents us from either

• Discovering life in the clouds only to find we brought it there ourselves in a previous mission, and that it managed to colonize a widespread microniche we didn't even know existed in the complex chemistry of the cloud droplets and particles, or had a capability we never suspected, or
• Returning life from the clouds which is hazardous to Earth's biosphere.

I think also that we shouldn't think about sending humans there until we've studied it a bit in situ to see if there is life there, remote though the possibility probably is.

So how do we handle the disappointment if we find that there is XNA based life in the Venus clouds, and that therefore, we should hold back a bit on sending humans there?

In my own view again, if there is life in the Venus clouds, especially interestingly different, or XNA based life, this is such a wonderful and interesting result for biology and science and evolution - and in the long run for humanity generally - that it far outweighs the disappointment that we need to postpone direct human exploration of the Venusian clouds for a later date. And we have many other places we can send humans without any exobiological impact.

We should celebrate the discovery of other forms of life anywhere in the solar system. What do you think?

Need to send robotic explorers like VAMP to the Venus clouds to study in situ first

So, how can we do this planetary protection friendly exploration of the clouds? One way is to study them "in situ" to start with, using sterilized robots.

We may get this in situ search soon. Some scientists working on designs for the next Russian mission to Venus, Venera D which hopefully will launch some time in the 2020s. Provisionally 2026. The original plan was for a balloon (as well as a lander and orbiter). They are interested to include ideas for an unmanned aerial vehicle from the Northrup group VAMP project. This would actually deploy outside of the Venus atmosphere and do a hypersonic entry. Because it is so large and light, it decelerates very high in the Venus atmosphere, and so does not need an aeroshell as it decelerates more slowly and the skin is not raised to a high temperature

.

It inflates before it enters the atmosphere (see Patent). Because it is so low in density (low ballistic coefficient), it decelerates slowly in the very thin upper atmosphere, so generating much less heat. So it doesn't need an aeroshell, though, its outer envelope is reinforced to withstand up to 1200 °C along leading edges

They hope it can be used for Venus, and also Titan, possibly Mars.

(click to watch on YouTube)

Eventually we can send humans to the clouds. The HAVOC idea is to do this. Their airship expands after it enters the Venus atmosphere, but the rest of the design is very similar. This is a video showing how it would work.

(click to watch on YouTube)

However I think we should do in situ searches first before sending humans there just in case.

Humans in habitats at the Venus cloud tops

The cloud tops of Venus may eventually be one of the best places to send humans to in the solar system, at least in terms of habitability. They are not just at the right temperature and pressure for Earth life. Just about everything you need for life is in the atmosphere of Venus as it turns out. So you can grow plants there using the atmosphere much in the way epiphytes do on Earth.

Epiphyte from Costa Rica. This survives just on the water and nutrients from the atmosphere. Although they grow on trees, they use them just as a support and are not parasites, don't take any nutrients from the trees.

Humans in the Venus cloud settlements could use the Venus atmosphere in the same way to grow plants and trees. Nearly all the mass of a tree can be got from our atmosphere.

The main hazard of course (apart from a non breathable atmosphere for humans) is Venus' sulfuric acid, which would make it impossible for most, maybe all Earth life to live there "as is". But it's also an asset, is a source for water and sulfur. Indeed, compared to other places where we could have humans in space, it's far easier to protect against sulfuric acid than the vacuum of space.

The trick here is to site your habitat at the cloud tops, where Earth's atmosphere is a lifting gas, so you can use zero pressure balloons. This means the habitats are the same pressure inside and out. If they do get holes in them, it's not a big deal, since the pressure is the same inside and out, they can't burst. They will just lose air slowly as it percolates out. It's a bit like living inside the lifting bags of an airship.

Since the pressure is the same inside and out, you don't need pressurized spacesuits, either. Everything is much lighter weight and in some ways it is easier to set up home there than anywhere we know of outside of Earth, except perhaps Titan (see Titan as potentially the easiest place for humans to live outside Earth ). There is far less launch mass from Earth per settler because the habitats are so lightweight.

It just so happens that the temperatures and pressures at the cloud tops on Venus match the conditions on Earth. Abundant sunlight. The four day superrotation of the Venus upper atmosphere means you can have days and nights both 48 hours long. Gravity levels are similar to Earth, slightly less. You have the equivalent of ten meters thickness of water by weight above you just as we do on Earth, so lots of protection from ionizing radiation. It has rather more UV than Earth but UV light is easy to protect against.

The Russians were interested in setting up cloud colonies in the Venus atmosphere, in the 1970s, and this is some of their artwork:

Russian idea for a cloud colony in the upper atmosphere of Venus, proposed in 1970s
 original article (in Russian) - and forum discussion of the article - includes rough translation (I think anyway), probably by non native English speaker.

This illustration is from Aerostatical Manned Platforms in the Venus atmosphere - Technica Molodezhi TM - 9 1971

Geoffrey Landis is especially keen on ideas of Venus cloud colonies. The main ISRU (In Situ Resource Utilization) comes from the atmosphere itself. So you have to look at things a bit differently on Venus. Some of the things you need ISRU for on Mars or the Moon aren't needed in the Venus clouds. Especially, it doesn't need strong structural materials to contain the habitat atmosphere. It doesn't need heavy pressurized seals. It doesn't need windows able to withstand tons per square meter outwards pressure. All of the construction is much lighter and easier to do than anywhere in space.

You surely need some metals, but with such lightweight construction, it's most likely kilograms per settler rather than tons. Though the papers do discuss ideas for mining metals from the surface, with its thick atmosphere, it is easy to "land" materials using aerobraking, and it may be easier to import materials from the asteroid belt or from the Moon for metals and such like than to get them from the surface.

You can argue a surprisingly good case for it. For more about this see Geoffrey Landis's recent guest appearance on The Space Show, and my article: Will we Build Colonies that Float Over Venus like Buckminster Fuller's "Cloud Nine"? which has many other links and cites.

Travel to other stars

Travel to other stars

So far we've looked at planetary protection for the solar system. What about protecting planets around other stars? Soon we might be able to send our robotic explorers to other stars, and perhaps, eventually, ourselves too. I'll talk about these planetary protection issues in the sections Planetary protection for other stars and exoplanets and Galaxy protection - what about colonizing other star systems? (below). But first, let's see if it is something we could even do, to send humans, or robots, to other stars.

We do have one technology that could in principle achieve human interstellar flight already, though at great expense, and with risks that would be considered unacceptable today. I'm talking here about the Project Orion to use nuclear explosions to propel a spaceship. It seems to be possible even with 1960s technology. It's not such a daft as it might seem at first. It was originally a project for interplanetary rather than interstellar flight. It began in the 1950s through to 1960s as a secret US military project, that few knew about. At one point it was considered as a serious alternative to chemical rockets for human spaceflight. You can read the history in more detail in Nuclear Propulsion: Orion and Beyond. But in brief, this is what happened:

Theodore Taylor got the idea underway in 1958, when he studied the idea of using nuclear explosions to propel a rocket into space. Freeman Dyson joined in the study during the academic year 1958-9.

It would have used nuclear explosions all through the flight, including the take off. The design had a huge pusher plate, and above that, shock absorbers, so the inhabitants of the spacecraft experience smooth acceleration. Unusually for a spaceship, weight was not a problem, with a payload of thousands of tons. So it could be constructed with the robustness of an ocean going ship. In their designs, a hundred and fifty people could live aboard comfortably, with a payload of thousands of tons into LEO.

I can imagine you throw up your hands at this point:

"Nuclear bombs - are they crazy?"

Well, we are used to the dirty nuclear weapons developed by the military. If we could achieve a pure fusion bomb, then it would be clean, with no radiation side effects at all, as it all comes from the fission trigger - the fusion part of the explosion doesn't generate any radioactivity. Sadly that's not yet possible, but fusion bombs for non military use can be designed to minimize the amount of radioactivity. Back then the Russians even explored the idea of using these "clean" nuclear weapons with minimal radioactivity for excavating the Pechora-Kama canal - and as a test they excavated 600 meters with three fifteen kiloton underground explosions, creating the "atomic lake" in Taiga. So, you need to see it in context - it comes from a time when there was less awareness and public concern about nuclear fallout than there is today.

The estimated cost for the interplanetary project would have been similar to Apollo or possibly less. They made small models, the "Putt Putt" models which flew for 100 meters in 1959, using chemical explosives, which showed that the basic idea of using bombs for propulsion worked.

The military lost interest in the idea, but NASA briefly considered a much reduced version which would have been launched into orbit by a Saturn V and then the nuclear bombs would have been used for fast trips to Mars and beyond, getting to Mars in 125 days. They never took that any further.

However, in 1968, Freeman Dyson looked into it some more, and worked out details for a way to use nuclear fusion bombs for an interstellar mission. His design could take thousands of people to Alpha Centauri in a single massive spacecraft, in a little over a century of travel. That's for his ablation spaceship - which relied on ablation of the pusher plate and the brevity of explosions to deal with thermal damage. In his original design, the empty ship would mass 100,000 tons. It would carry as fuel 300,000 tons of nuclear weapons and could take a payload of a further 100,000 tons, and it would accelerate to 10,000 km per second in ten days, with an acceleration of 1 g. That's about 3.333% of the speed of light, and so I make it that the time to get to Alpha Centauri, at a distance of 4.367 light years, is about 131 years.

For details see his 1968 paper. He didn't expect it to be built any time soon, as he expected it to cost around $100 billion - that's in 1968 dollars so about $700 billion in 2017 dollars. He thought that perhaps it would be affordable around two centuries into his future, using an argument based on the yearly increasing US GDP.

So yes, perhaps it was possible to send humans to the stars already in the 1960s at huge expense, and also if one accepted the risk of using nuclear bombs to propel a starship. For more about this, see also the wikipedia article on Project Orion which has many links to follow up further.

Robotic spacecraft to the stars

Though the idea of Orion eventually was dropped, it lead on to other ideas such as Project Daedalus, a design by members of the British Interplanetary Society for an unmanned craft, using microfusion - small pellets of deuterium / helium 3 which would be compressed using electron beams to generate plasma for thrust. The helium 3 would have been mined from Jupiter's atmosphere (using hot atmosphere balloons which work in a gas giant atmosphere) or the Moon. They thought it could reach Barnard's star in 50 years, traveling at 12% of the speed of light, with a payload of 500 tons.

Robots make things easier of course, and especially so if they are lightweight. We can consider spacecraft nowadays with weights of grams rather than tons, which brings the whole thing into the realm of something that we might actually accomplish in the relatively near future.

One of the latest ideas is Breakthrough Starshot, to send tiny single chip spacecraft to our nearest star Proxima Centauri, 4.2 light years away, and fly past the newly discovered planet Proxima Centauri b and send back photographs of it. The idea is to accelerate a tiny single chip spacecraft at close to the speed of light. We can accelerate particles in our laboratories easily. Could we accelerate small spacecraft at similar speeds to other planets, and star systems?

The original idea goes back to Robert Forward in a paper published in 1962 as "Pluto - the Gateway to the Stars" not long after the invention of the laser (see his paper from 1984 for the history) followed by several other papers. Robert Lubin did a detailed study in 2015, for the NASA Innovative Advanced Concepts (NIAC) program, see short description and introductory video. Amongst other things, he worked out that the amount of power needed to launch one gram to a fifth of the speed of light is about the same as is needed to launch the space shuttle (35 minutes into this video by Avi Loeb) and it might take perhaps two minutes or so, accelerating at 60,000 g, and an array of phased laser beams could focus down to a spot of radius about 2 meters at a distance of two million kilometers. That's a level of power that we could achieve with an array of microwave lasers. Talking about it in 2016, in this video, then Avi Loeb reckoned that it could be about 15 mm by 25 mm in size, and could have power supply, lasers for communication, magnetometer, cameras and several other components and it would still only be a fifth of a gram, using the technology available right away.

This proposal was then taken up by the Breakthrough Starshot project funded by the billionaire venture capitalist and physicist Yuri Milner. It's not yet an actual mission, but it's a much more extensive $100 million feasibility study.

Most of the details here are from Patricia Daukantas' overview:Breakthrough Starshot. As it is now, each nanocraft consists of a single chip, weighing about one gram or less, with four digital cameras, four processors, four photon thrusters, an RTG battery of 150 mg powered by plutonium 238 americium 241 and a protective coating to protect it from collisions with dust and atoms in flight. It's attached to a lightsail which would be four meters by four meters (13 by 13 feet).

The idea is to use a ground based array to focus laser light on the light sail as it accelerates rapidly to 20% of the speed of light in only three minutes. This would let it get to the planet Proxima Centauri b in twenty one years. There are many challenges. for instance, 99.999% of the light would have to be reflected by the solar sail to prevent it from just burning up. Here is a list of the challenges they have identified along with ideas of how to tackle them.

One suggested wavelength for the light is 1060 nm, in the near infrared, because Earth's atmosphere is reasonably transparent at that wavelength. Preliminary calculations suggest a 100-GW phased laser array 10 km on a side

It would pass through the star system so quickly it would take only 42 minutes to traverse the Earth to Sun distance and it would traverse the Earth Moon distance in six seconds. Many of the chip sats would be destroyed in transit, but a few would survive and send back their images. The idea is to make many low cost nanocraft and send them towards the distant exoplanet in their thousands. It would only take a few percent of them to survive and beam back images to get our first ever close up images of a distant exoplanet.

It would be a large scale scientific project, like the Large Hadron Collider, but one that could be completed well within a human lifespan.

(click to watch on YouTube)

And a longer talk

(click to watch on YouTube)

(the presentation starts around 14 minutes in)

The nanocraft might even be able to slow down into orbit around Proxima Centauri, and potentially also, to orbit the planet too, by using gravity assist of the other two stars in the Centauri system like this:

Illustration by Phil Saunders / Adapted from PHL, University of Puerto Rico at Arecibo, source Patricia Daukantas' overview:Breakthrough Starshot

There are many other ideas on the go, apart from this one. Icarus Interstellar have a number of projects as steps towards interstellar flight . They also host the Starship Congress. Videos from previous ones are here and details for the latest one here.

Humans to the stars

As we saw in Travel to other stars then Freeman Dyson's interstellar Orion gave a way to send humans to Alpha Centauri in a journey that would take around 131 years. However, apart from the issues of safety and radioactive fallout, there's another reason why we aren't likely to go any time soon, and that's the "incentive trap" as it is sometimes called.

Suppose someone had set out to Alpha Centauri in 1978 (after a $100 billion 1970s crash development program, ten years after Dyson's paper) in a generation starship. Then they, or rather their descendants, would get there in 2109. However as time goes on, we will be able to travel faster and faster. Indeed, later designs of Dyson's Orion could achieve up to perhaps 10% of the speed of light, fast enough to get to Alpha Centauri in 42 years, Even if they set out 80 years later, in 2058, they would still beat the first slow coach star ship by nine years, arriving in 2100. But now suppose they can travel at 20% of the speed of light. They can still beat both those other travelers by setting out before 2079, with a journey time of only 21 years. What's more they would arrive there after a shorter more comfortable transit, and with more modern technology.

So when should you set out for the stars, if you have the funding for the journey? Well, it depends on future developments. But let's suppose for instance that the speed we can achieve continues to double every 15 years, as it did in the period from the first steam-driven vehicles to Voyager 1 according to this paper by Rene Heller,. Then he worked out that there is no point in setting out until you can reach the object with a 20 year travel time.

He isn't predicting that speeds will increase like this, but supposing they did, that's how it would work out. He also found that the Breakthrough Starshot program, if it succeeds, will be way ahead of the curve for that doubling every 15 years historical record.

"Although this exponential growth captures the development of historic top speeds, we do not claim in this report that it will continue as such. Instead, we investigate the implications for interstellar travel if it does continue. Moreover, note the substantial offset of the yellow symbol referring to Starshot. In Section 4 we demonstrate that this jump in velocity in the year 2040 would save about 150 yr of speed growth according to the historic record. "

(see Figure 1 of his paper).

Writing in 2017 he concluded:

"If Starshot would go on line within the next 45 yr and if the kinetic energy transferred into the probes can be increased at a rate consistent with the historical speed record of the last 211 yr, then humans can reach the ten most nearby stars within 100 yr from today."

Of course that's based on a lot of assumptions.

So, a human mission might be technologically possible even today, and there are people working on ways to do it. But the expense would be vast. If we were more cavalier about radioactive fallout, we could use the Orion starship right now, at least if we had a few hundred billion dollars for the project. If we can improve the design so that it reaches 20% of the speed of light, like the Breakthrough Starshot design, then we'd be ready to set off, at least as far as the incentive trap is concerned.

When it comes to robotic interstellar spacecraft, yes, we may see those in the next few decades, thousands of thin film spacecraft flying to other star on laser beams. That's hard enough. For humans, it's a vast and expensive project. A century from now though, who knows? In the next few centuries we may well have the ability to visit other stars in person. Whether we'll be able to do it in decades or centuries, however, it's an interesting question to look at. There are many people looking in great detail at interstellar flight. What about interstellar planetary protection?

So with that background,what then are the planetary protection issues, and for that matter, galaxy protection issues, for robotic and for human interstellar flight?

Planetary protection for other stars and exoplanets

I don't know of any paper on planetary protection for robotic probes to exoplanets. Would Breakthrough Starshot need planetary protection? Well maybe. The probes certainly could hit habitable regions in the destination star system - moons, or small undetected planets, or the destination planet itself, in case of some mistake in the course correction, as it would fly as close to the planet as possible to take the photographs. With a journey time of decades, and tiny chip sats as our first robotic emissaries, then you'd think they would be easy to sterilize. Still - you are talking here about a design that ends up with functioning electronics at the destination. So, could microbes survive as well? The easiest way to be sure would be heat sterilization, using hardy electronics - after all it does have an RTG on board, which could perhaps be used to heat the chip to temperatures Earth life can't survive.

The knottier questions arise if we send humans there, or indeed anything from our biosphere, perhaps in an attempt to "prepare the planet" for humans. First, we could have similar problems for exoplanet biospheres to the ones we discussed for a Mars sample return in the introduction in Should we return samples from Mars right now? (above). The quotes I gave there bear repeating I think.

Joshua Lederberg put it like this for a Mars sample return:

"If Martian microorganisms ever make it here, will they be totally mystified and defeated by terrestrial metabolism, perhaps even before they challenge immune defenses? Or will they have a field day in light of our own total naivete in dealing with their “aggressins”?

from: "Paradoxes of the Host-Parasite Relationship"

So what happens when you have, not just a sample return but a clash of two complex biospheres? Then you get many more possibilities as the interaction is two way.

Earth's biosphere and the exoplanet biosphere, in the best case, might ignore each other. Or who knows, perhaps they might even be compatible and mutually beneficial, developing forms of symbiosis?

Or it could be asymmetrical - Earth life is able to defend itself from the native exoplanet life, while the exoplanet life is totally naive in response to the Earth life, or vice versa. Or in the worst case, there could be clashes that destroy both biospheres in their entirety, perhaps even all the way down to the point where there are only microbes left.

The physicist Claudius Gros looks at a clash of interpenetrating biospheres in his paper on a "Genesis project" to develop ecospheres on transiently habitable planets, and he puts it like this:

"Let us digress for a moment and ask the question: What may happen, if humanity’s dream of a spaceship load of human settlers setting foot on a second earth would come true? In this case we would bring terrestrial life, microbes included, to a planet with a biosphere as rich as the one of our home planet. Both the alien biosphere and the invading fragment of the terrestrial biosphere would interpenetrate each other and humanity would have started a non-reversible experiment for which the outcome will most probably be determined by how universal the immune system of the respective multicellular organisms are.

"The reason is that all multicellular organisms, plants and animals alike are vitally dependent on a performing immune system for their defense against pathogenic microbes. Key to the functioning of an immune reaction is the recognition of ‘non-self’, which is achieved in turn by the ability of the immune systems, at least on earth, to recognize certain products of microbial metabolism that are unique to microbiota. How likely is it then, that ‘non-self’ recognition will work also for alien microbes?"

"Here we presume, that general evolutionary principles hold. Namely, that biological defense mechanisms evolve only when the threat is actually present and not just a theoretical possibility. Under this assumption the outlook for two clashing complex biospheres becomes quite dire."

"In the best case scenario the microbes of one of the biospheres will eat at first through the higher multicellular organism of the other biosphere. Primitive multicellular organism may however survive the onslaught through a strategy involving rapid reproduction and adapt ion. The overall extinction rates could then be kept, together with the respective recovery times, 1–10 Ma, to levels comparable to that of terrestrial mass extinction events."

"In the worst case scenario more or less all multicellular organism of the planet targeted for human settlement would be eradicated. The host planet would then be reduced to a microbial slush in a pre-cambrian state, with considerably prolonged recovery times. The leftovers of the terrestrial and the indigenous biospheres may coexist in the end in terms of ‘shadow biospheres’ "

In his paper he suggests sending microbes to planets that will be habitable only briefly. From our experience on Earth, then it took billions of years for the first eukaryotes (cells with a nucleus) and multicellular organisms to arise. So what if we were to try to kickstart all that? To do this, first, we find a planet that has only a few hundred million or a billion years of habitability left - still plenty of time for higher lifeforms, with future prospects not unlike our Earth - but it's got no life there yet. Then the idea is to seed it with a few eukaryotes and other higher lifeforms from Earth. Hopefully that then saves its ecosystem billions of years of evolution and means that new complex life will arise there that would never have done so without our intervention.

He would look for a planet with no life at all, to avoid issues of clashing biospheres and incompatible biochemistry. It would be a very long term experiment, indeed, unusually, it wouldn't matter how long it takes to get there, so long as the process can be automated. The aim is to make the planet into one with complex life perhaps hundreds of millions of years into the future, so a few extra centuries now will make no difference.

This raises many ethical questions, as well as practical ones

• How do we know that there is no life there yet? What if it is cryptic, hidden from view unless you examine the planet close up, or has only just started to evolve?
• Is it true that life always takes billions of years to evolve multicellular life as it did on Earth?
• It's bound to have life precursor complex molecules, as even meteorites and comets have those. Perhaps it will have protobionts too, almost alive life precursors. Might we be interfering in the early stages of development of a form of biology that we don't understand yet?
• If we introduce new life to a planet, do we not have some responsibility for the creatures that will arise there billions of years from now? Maybe what we think will be a stable biosphere turns out to be a disaster for them, just as they evolve to complex intelligent life like ourselves?
• What if the planet then evolves intelligent creatures that turn out to be a harmful "invasive weed" of our galaxy?
• Is it always a good thing to increase the amount of complex life, and intelligence, in our galaxy and universe?

On the other hand, if we knew what we were doing, perhaps it is a beneficial thing to do, bringing life to an entire biosphere which wouldn't be there otherwise. In that case, it's not unlike bringing a desert to life. What difference does it make that it is a distant planet around another star, or a dry barren denuded desert on Earth? The idea of bringing new life to a desert certainly has it's appeal and it seems a worthwhile thing to do, somehow, maybe even irrespective of whether any humans or other intelligent beings enjoy it? What do you think?

Galaxy protection - what about colonizing other star systems?

If we have got as far as considering a human mission to another star, presumably we have found a way to colonize space peacefully in our own solar system by then. I talked about that in the introduction, under Alternative visions for the future. If we take our violence, terrorism and extremism of all sorts with us as we colonize space, how can we remain in space for long? Any habitats in space will be so fragile to violent actions, that even lobbing a rock at them at a few kilometers per second would destroy it. Any group of millions of people with space technology would find that an easy thing to do. How could anyone survive a war in space like that?. With no air to breathe, there'd be no possibility of survivors hiding out in caves. If their environment control, hull integrity, or spacesuits are destroyed by the blast, they no longer have any way to survive. Is there any reason to suppose we will be more peaceful in space than we are on Earth?

I suggested that this is mainly an issue if we have millions or even billions in space. If we are in less of a hurry about this, and don't rush to colonize as rapidly as possible, we can find our way through this, to a sustainable future? Perhaps, just as we have learnt to cope with technology that would have lead to chaos in the nineteenth century, we can also learn to cope with life in space colonies as fragile as an eggshell, easily destroyed completely by someone "lobbing a rock at them".

We could start small, with bases on the Moon like the ones in Antarctica. Eventually, as we find out more, and if we find it useful to do this, then we can have larger populations on the Moon - tourists first and explorers, and "cruise ship" spaceships like the tourist boats that visit Antarctica to start with, then maybe permanent residents. Starting with the Moon as our gateway to the solar system, eventually we will have the ability to set up research stations, and perhaps eventually colonies too, throughout the solar system.

Perhaps, if we do it slowly enough, we will find the solutions we need as we go along. Just as we have found ways to cope with powerful technology on Earth, we may also find ways to have larger and larger numbers of people in space in a way that is consistent with the fragility of spaceships and space habitats. See:

• Further into the future, if we have millions in space - can we be one of the "wise ET's"? (above)
• Signs of optimism, that we can be one of the wise ETs (above)

So, let's suppose we have reached that stage. We are one of the "wise ETs" and have successfully set up space colonies and found ways to deal with the powerful technology without wars in space tearing our civilization apart. What happens after that? We may set our sights on colonizing the stars.

This might seem great at first, awesome even, to think of a "civilization" like ours spreading to fill the galaxy. Our minds may turn to Star Trek for instance. But - is that so desirable long term? I would like to suggest that, it depends very much on how and why we do it, just as it is for colonizing our solar system. If so, even though this is a far future problem that we are not likely to meet for decades or even centuries, perhaps it is not too soon to start thinking through the issues. The sooner we start to think about it, the more likely, perhaps, that we can find the answers we will need when the time comes.

We may need to protect both ourselves, and the galaxy, and other beings in it also, intelligent or otherwise, from possible long term harmful consequences of our own future actions. It's a form of planetary protection I think, or rather, galactic protection. It's not unlike protection of Mars from microbes, but written large over the entire galaxy, and so I think this is an appropriate topic to cover in this book in some detail.

At this point, it might be wise to look into Fermi's paradox, "Where is everyone?" Why haven't extra terrestrials filled the galaxy already through exponential population growth? They should have done that in a blink of time, on geological timescales, within a million years of when they first developed the ability to set up interstellar colonies.

Do they give up? Or do they destroy themselves before they get very far? Or is there some other reason? Perhaps we have pitfalls to avoid here, and executive decisions to make? Do we need to be a "Wise ET" to explore the galaxy, and if so what do we need to do?

There is a massive literature on Fermi's paradox, and a bewildering variety of proposed solutions. Here are a few sources which present an overview of some of the many solutions that have been proposed.

• Stephen Webb presents seventy five solutions to the paradox in his "If the Universe is Teeming with Aliens... Where is Everybody?"
• Io9 article on "Eleven of the weirdest solutions to the Fermi paradox" - I think it might be fair to add the idea that they are hibernating until the universe cools down and is more to their liking, paper here
• List in Wikipedia- hypothetical explanations for the paradox.
• David Brin's survey article from 1983
• Fermi’s Paradox – The Last Challenge for Copernicanism? by Milan Ćirković

Milan Ćirković lists nine recent developments which have made Fermi's paradox "significantly more serious, even disturbing" (see page 5 of his paper above).

"Fermi’s Paradox has become significantly more serious, even disturbing, of late. This is due to several independent lines of scientific and technological advance occurring during the last two decades:

1. The discovery of more than 350 extrasolar planets so far, on an almost weekly basis [the count as of writing this, nine years later, is ten times as many: 3,502 confirmed exoplanets, and thousands of exoplanet candidates, see the NASA Exoplanet Archive summary counts)]
   
   ...It seems that only the selection effects and the capacities of present-day instruments stand between us and the discovery of Earth-like extrasolar planets, envisioned by the new generation of orbital observatories [he wrote this in 2008 - , Earth like planets in the habitable zone are now commonplace amongst the exoplanet discoveries]
    
2. Improved understanding of the details of the chemical and dynamical structure of the Milky Way and its Galactic Habitable Zone ...show that Earth-like planets began forming more than 9 billion years ago, and that their median age is (6.4±0.7) billion years - significantly more than the age of the Earth.
    
3. Confirmation of the rapid origination of life on early Earth ... offers strong probabilistic support to the idea of many planets in the Milky Way inhabited by at least the simplest lifeforms
    
4. Discovery of extremophiles and the general resistance of simple lifeforms to much more severe environmental stresses than had been thought possible earlier ...
    
5. Our improved understanding of molecular biology and biochemistry leading to heightened confidence in the theories of the naturalistic origin
   of life or biogenesis ...
    
6. Exponential growth of the technological civilization on Earth, especially manifested through Moore’s Law and other advances in information technologies . This is closely related to the issue of astroengineering: the energy limitations will soon cease to constrain human activities, just as memory limitations constrain our computations less than they once did. We have no reason to expect the development of technological civilization elsewhere to avoid this basic trend.
    
7. Improved understanding of the feasibility of interstellar travel in both the classical sense, and in the more efficient form of sending inscribed matter packages over interstellar distances. The latter result is particularly important since it shows that, contrary to the conventional skeptical wisdom, it makes good sense to send (presumably extremely miniaturized) interstellar probes even if only for the sake of communication.
    
8. Theoretical grounding for various astroengineering/macroengineering projects potentially detectable over interstellar distances ...
    
9. Our improved understanding of the extragalactic universe has brought a wealth of information about other galaxies, many of them similar to the Milky Way, while not a single civilization of Kardashev’s Type III has been found, in spite of the huge volume of space surveyed."

I'm taking a "sustainability solution" approach to Fermi's paradox here. Could we colonize our galaxy in a sustainable way? Could ETIs do that? And could this perhaps help solve Fermi's paradox? This is an approach suggested by Seth Baum, a mathematician and electrical engineer who did a PhD in geography, and co-founder and director of the Global Catastrophic Risk Institute.

His approach is to tackle the usual assumption that an extra terrestrial would be bound to colonize a galaxy with exponential population growth, as soon as they develop that capability. What if they don't? I think it provides interesting insights into the whole debate. Most of the solutions are based on this assumption. What happens if we question it?

Actually that's not a new idea. Carl Sagan studied spread of life through the galaxy as a diffusion process and he came up with much longer timescales for galactic colonization in this paper from 1995. Some of the timescales were so long that a galactic civilization wouldn't have got to us yet. His idea is that when we colonize, we explore and then colonize new territory much more slowly than we can travel. We build up infrastructure. We explore first, before deciding to settle down. Rome wasn't built in a day and though you can walk across it in hours, you can't build a city in a few hours. He thinks that a galactic civilization could spread across the galaxy only slowly as it builds up the infrastructure, even though some explorers may go well ahead of the traveling wave. While a faster expanding civilization might collapse even after filling a galaxy, due to the inability to sustain the infrastructure to keep it together.

Seth Baum's idea was to look for sustainability solutions to the Fermi paradox. He uses He writes in his conclusion to The Sustainability Solution to the Fermi Paradox

"Thus, the Paradox can only conclude that other intelligent civilizations have not sustained exponential growth patterns throughout the galaxy. It is still possible that slower-growth ETI civilizations exist but have not expanded rapidly enough to be easily detectable by the searches humans have yet made. It is also possible that faster-growth ETI civilizations previously expanded throughout the galaxy but could not sustain this state, collapsing in a way that whatever artifacts they might have left have also remained undetected. Both of these growth patterns can be observed in human civilization, suggesting that they may be possible for ETI civilizations as well."

He writes in Is Humanity Doomed? Insights from Astrobiology

"When a population expands exponentially, it typically experiences one of two fates. First, it could overshoot the carrying capacity of the ecosystem supporting it, depleting key resources. In this scenario, the population quickly suffers a dramatic crash. The damage to the population or to the ecosystem is often permanent such that the population never regrows its numbers. Perhaps extraterrestrial civilizations that continue to expand exponentially suffer similar crashes before they can expand throughout the galaxy, or before they would be observed by us or any other civilization. While civilizations sufficiently intelligent for space travel might understand the dangers of unsustainable expansion, they might not act on this understanding, just as human civilization does not always act on its own understanding of these dangers. This would explain why we do not observe other civilizations."

" The other fate of an exponentially expanding population avoids the crash. In this scenario, the population slows and possibly ceases its growth early enough that it remains within the carrying capacity of its ecosystem. This growth pattern approximates a logistic curve and is also commonly observed in populations on Earth. This growth pattern is fundamentally sustainable. We may be experiencing this growth pattern with human populations, since global human population growth has been slowing in recent decades. In this case, the reason we do not observe extraterrestrial civilizations is because they do not expand rapidly enough to fill the galaxy. These civilizations could be those that understood the dangers of unsustainable expansion and successfully acted on this understanding. They are out there, but they are hard to find."

"Note that some extraterrestrial civilizations might not expand at all. They might be highly intelligent but simply not desire to expand. Indeed, there are human populations that do not pursue expansion but instead favor other objectives. The existence of intelligent, non-expansive extraterrestrial civilizations is fully compatible with the sustainability solution to the Fermi Paradox because these civilizations do not expand rapidly—indeed, they do not expand at all. If the sustainability solution explains the absence of observation of extraterrestrial civilizations, then non-expansive civilizations and slowly-expanding civilizations could exist. Both of these civilization types would be sustainable, but they would also be hard for us to find."

Although I base my ideas here on the sustainability solution, I think I come to it with what may be a somewhat new perspective, based on my interest in planetary protection.

Suggested solution to Fermi's paradox - ETIs practice galaxy protection just as we practice planetary protection

I'm coming to this with three new(ish) ideas. Surely they aren't totally new in such a thoroughly debated field, but I can't find them discussed in quite this form yet. The ideas, when combined together, lead to a new way of looking at the paradox. It leads to this conclusion that galaxy protection will be a top priority or ETIs that get to the point where galactic colonization is something they could realistically do.

So, this is another of my speculative sections. Do say if you know of any new cites on these ideas, thanks!

First idea - galaxy protection as a motivation for ETIs and ourselves

The first new idea is that the main reason that ETI civilizations haven't colonized the galaxy, including Earth, at least, not in an exploitative fashion, is for galaxy protection, as an extension of the planetary protection we already do. I suggest that they do this to protect themselves as well as others, just as we do planetary protection to protect both Earth, and other lifeforms that may or may not exist on other planets with our current planetary protection policy.

Any ETIs, if they exist, and if they have advanced, stable and technologically capable civilizations, I think may well be highly motivated by a wish for galaxy protection. We already have planetary protection discussions of ideas to send robotic probes with Earth life to other star systems, see Planetary protection for other stars and exoplanets (above).

When we come to apply ideas of planetary protection to colonization of the galaxy, our main focus might be protection of rare forms of biology, and especially, intelligent non technological lifeforms. Even in our own solar system, we may have a similar motivation to prevent forward contamination to protect intelligent lifeforms in the Europan ocean, as we have no way, so far, to rule out intelligent non technological life there.

What do you think? If we came to discover that there is indeed an ancient billions of years old civilization of non technological Europan intelligent sentient beings, in their ocean, should we risk destroying their biosphere by introducing Earth microbes? I think most would say "No", in no uncertain terms. I discuss this possibility in If life in a Europan ocean can have the intelligence level of squids, basking sharks, octopuses and cuttlefish - what about a non technological civilization? (above). Such ideas may be just the start of a tendency to extrapolate it to the whole galaxy. In the same way, as we become aware of the presence of intelligent non technological lifeforms throughout the galaxy, we may wish to protect them, too from harmful effects of exploitation or accidental or thoughtless biosphere contamination throughout our galaxy.

That clash of biospheres could happen through forward contamination. It also could happen through backward contamination, and not just through the biosphere of an exoplanet harming colonists who attempt to set up home there. There's also the possible scenario that someone returns a pet or interesting species from another planet to Earth, not realizing the consequences as it then adapts and evolves and becomes an invasive species on Earth, or even, that they just return microbes with the capability of some more advanced form of photosynthesis, more efficient metabolism etc. Also it could happen between other exoplanets that the colonists accidentally or intentionally introduce species from one exoplanet biosphere to another, where again it becomes an invasive species.

This is similar to the "Zoo hypothesis" but with an extra strong exobiology focus, based on the potential of clashes of biospheres. Indeed it's a little different, with the focus on avoiding clashes of biospheres, rather than transfer of technology or ideas particularly. So - especially if they had actual practical experience or object lessons from other ETIs of the clashes of biospheres - that might lead to a fair bit of caution and a focus on galaxy protection in the biological sense.

So, it would be based on awareness of vulnerability of other non technological ETIs. However they would also develop a strong awareness of their own mutability as well. Having seen the rapid evolution of their technology, just as we have for the last century, they realize that they can't predict where they are heading, and so, what kind of creatures would fill the galaxy if they expand into it. Would those future creatures be safe for the galaxy, or themselves? Is this a future they really want to head for, and if so, how can they ensure this?

They will also realize that they are close to being able to create new lifeforms (through genetic manipulation or other methods) or already able to do so, and that it may soon be possible to create self replicating machines, at first large, then smaller. So that would lead to an awareness that the end result of an attempt to colonize a galaxy might be a galaxy filled with these creations, as either lifeforms or even dumb self replicating machines. So they will become acutely aware of what might happen to the galaxy if they expand into it and unleash such technology on it in an unplanned way. They may also at this point start to have extended lifespans. Maybe, a few centuries from now, future generations of humans have lifespans of thousands or millions of years - if so this would strongly favour forward thinking like this. That galactic chaos might be a future they would live to see themselves, and they will be strongly motivated to make sure it is a galactic civilization instead and to set it off on the right tracks to ensure that as much as possible.

In this way, they see in the far future the possible prospect of a future galactic chaos, as we'll see in Boom and bust - a nightmare future (below). ETIs, and our future selves, might then be strongly motivated to find a way forward that ensures a galactic civilization rather than chaos.

That might be especially so, again, if they become aware of an "object lesson" of a distance galaxy where it has gone wrong, with its inhabitants beaming signals warning against what happened to them.

What we see as an apparently untouched galaxy may be intentionally so, just as an untouched Europan ocean may be by intention in our future solar system centuries in the future. Maybe only carefully sterilized probes enter it.

In the same way, our galaxy may look like as untouched as it does for a similar reason. It could be the result of foresight, planning and directed colonization by extra terrestrials with a strong sustainability and galaxy protection focus. It might be policed, or perhaps it just comes natural to a civilization, at a certain level of maturity, that it becomes unthinkable to do anything else. So it might be a single policy imposed by one advanced ET or a collaboration of them, maybe each new member building on work of previous ones, or many policies agreed on separately by many ETIs that take part in the galaxy.

There may be other reasons why it is like this, based on galaxy protection. I go into this in Could humans achieve slower than exponential growth like this? (below)

Or we may be the first in our galaxy. If so, it might be our responsibility to address these issues of galaxy protection for ourselves and for all future civilizations that may emerge in our galaxy.

Before we get onto that though, let's look at the other two new(ish) ideas that combine together to make this new approach I wish to suggest here.

Second idea - no need to colonize planets or search for habitable zones - we could set up colonies almost anywhere in habitats around almost any kind of star

The second idea is that we have many places we could live, and we are not necessarily restricted to planets in the habitable zone of a star. Small colonies that may be capable of surviving independently without much of the infrastructure Carl Sagan imagined. Instead of a need for large galaxy spanning empire with supply routes to keep a civilization going, and the need to terraform a new Earth like planet before they can expand into a new region, as he suggested in his diffusion based analysis, I think that without that much by way of future technology development, at most a century or so, perhaps less, we may be in a situation where it is easy to set up lots of small colonies that could be self sustaining almost anywhere in the galaxy.

This is not really a new idea, but most discussions of the Fermi paradox seem to assume that we would target habitable Earth like planets as we colonize the galaxy, and that ETIs would arise in similar planets to ours would target them too, including our own of course. I suggest that if the biology is incompatible with Earth biology, in either direction, then we might want to avoid trying to colonize Earth like worlds, to avoid a clash of biospheres, especially if there is life there already.

If that is typical of what happens if you try to introduce the Earth biosphere to a habitable exoplanet - it's entirely possible that we avoid all life bearing planets, especially as there may be many better alternatives available to us by then.

Amongst other things, then in our own solar system, we have the possibility of the cloud tops of Venus type habitats, of icy moons like Callisto or Ganymede, and even moons or planets with methane / ethane for the liquid instead of water, and dense atmospheres like Titan. See the sections above:

• Sending humans to Callisto or Ganymede
• Titan as potentially the easiest place for humans to live outside Earth
• Humans in habitats at the Venus cloud tops

Then, we can also combine this sustainability approach with the possibility of Stanford torus type free floating colonies.

We may be planetary chauvinists because of our familiarity with living on a planet. Isaac Asimov explains here that he got the term "Planetary Chauvinism" from Carl Sagan. He can't say for sure that Carl Sagan invented it, but that was the first he heard of it. He talks about this 35 minutes into this video:

(click to watch on YouTube)

Settlements in space provide much more living area than planets can, for much less investment of effort and much less use of resources. As he says in that interview, our future, for most of humanity, is likely to lie in space habitats rather than on the surface of planets.

"I'm convinced that we will build space settlements in space, we will live inside small worlds, and we will eventually recognize that as the natural way to live. It is economical. You have just a relatively small amount of mass, and it is all used. In the case of the Earth, you've got an enormous mass, and almost all of it is not used. It's down deep where we can't get at it, and the only purpose is to supply enough mass to produce enough gravitational intensity to hold stuff onto the outside. And that's a waste! With the same mass you can build a trillion space stations carrying incredible numbers of people inside. And this is what we will eventually come to. I'm sure we will use the asteroid belt to build any number, thousands upon thousands, hundreds of thousands of space stations, which will eventually flee the solar system altogether."

The only way to get more habitable living space from the materials in a star system is through taking apart the planets themselves to make either more free floating colonies or mega structures such as Dyson spheres or swarms. Or indeed we or ETIs could eventually find ways to extract the matter of the star itself to construct space stations or Dyson spheres.

The free floating colonies are far easier to construct and more flexible and easier to adapt than Dyson spheres.

Then, as we saw, once we make habitats like that we are no longer limited to the narrow habitable zone. These free flying colonies could be placed anywhere, from inside of Mercury's orbit, with enough shielding from the sunlight, all the way out to beyond Pluto.If you have the ability to make a Stanford Torus type habitat, then all that is needed to set up home out as far away as Pluto is to add a lightweight and very large thin film mirror to collect enough light for the habitat. That shouldn't be much of a problem if you can build a Stanford Torus habitat. See Space habitats made from asteroid and comet materials get plenty of sunlight - right out to Pluto (using thin film mirrors to concentrate the light) (above)

One way or another, once we have peaceful colonization of our own solar system, at least, peaceful enough to be tolerable, then we may well see our next new frontier as colonies all the way out to the Kuiper belt and beyond. Also the Oort cloud has everything we need in it all the volatiles, also metals, and rocks too. Smaller in percentage than in the inner solar system but there would be no trouble finding enough for a civilization out there.

Once we achieve that - then we have got beyond the need for Carl Sagan's infrastructure with Earth like planets as the nodes and hubs. Now colonies are no longer tied to any particular type of star system. We could survive just about anywhere in our galaxy that has rock, ice, methane ice, nitrogen, and trace elements, and with enough light for solar power and photosynthesis, concentrated using thin film mirrors if necessary.

We also have the possibility of nuclear fusion in the near future. It's always 20 years away, but maybe this time it's twenty years away for real? There are many ideas for ways to do it, and it might not necessarily be the big ITER that wins the race to the first commercial fusion reactor, or if it does, perhaps it will be followed up by smaller scale ones based on other ideas. The small scale ideas of Polywell fusion reactors for instance look interesting, small, and clean, about the size of a two story house. If we can achieve nuclear fusion, with common elements such as hydrogen, deuterium, boron etc as fuel, then almost anywhere in the galaxy would be available to us as a place to set up home.

Also by then, we may have self replicating machines to do a lot of the work for us, of building these habitats. This also gives us vast amounts of living space inside our own solar system, as much as we could get by colonizing thousands of habitable worlds around other stars, and much more quickly also, for less expense.

So, I'm talking here about probably a century, or several centuries even, into the future, when we might seriously start to think about interstellar colonization. Despite all the enthusiasm for it amongst geeks, I don't really see interstellar colonization in then near future, by humans, though robotic exploration, yes, for sure.

If we do this, it's unlikely to be for reasons of survival of the species, because once we have these free floating habitats, there is nothing that could make us extinct in our own solar system. We could survive all the way through the red giant phase, then the helium flash and collapse to a white dwarf and beyond for trillions of years, just by moving our habitats further away or closer to the sun, and adding more or less shielding and adjusting the sizes of the thin film mirrors that bring sunlight into our habitats.

The future is hard to predict, I suppose we could get this capability much sooner than that if we had some major development that made it much easier to do. But that doesn't change the situation. If we go through this stage where we are no longer tied to any particular planet, before we consider galaxy colonization, then our main motivation is going to be other than survival of humanity.

So, it seems that our technology is headed towards a future where humans can live almost anywhere in our galaxy. Also, habitats may be very easy to make with 3D printers, and self replicating machines, and a vast apparently uncolonized galaxy. This seems a major challenge to the sustainability solution, so what is the resolution of it?

Third idea - the trap of exponential growth - galaxy protection as a requirement for sustainable growth into an uncolonized galaxy

The third new idea I bring to this sustainability question is that, without planning, and careful thought to ensure galaxy protection, exponential growth would be highly favoured. In the unplanned race to fill a galaxy, any group of ETIs (or ourselves) who figure out a way to achieve an exponential population growth will be able to colonize the galaxy far faster than anyone else. At least, that's what would happen if it's true that the galaxy is wide open for colonization, and that for our descendants, colonization itself will be an easy process.

This is a corollary of the previous assumption. If humans can live in small colonies anywhere, then we are no longer in a diffusion type situation.

So how would fast exponential growth actually happen? Probably you still have Carl Sagan's steadily growing diffusion wave. The infrastructure in a densely populated region will make things easier. But beyond that you would get the more rapidly traveling explorers who can set up home anywhere, and start up seed colonies anywhere in the galaxy. They then would only be limited to whatever is a safe travel speed, which could be as much as 10% or 20% of the speed of light.

So then - if you have explorers who have set out from a small colony that's self sufficient in the Oort cloud - maybe even that entire colony "ups sticks" and moves en masse in a fleet of spaceships to a new location. In a situation like this, they might just leave a seed colony anywhere, and not need to do anything much to get it started, as it would all be done quickly and almost automatically with their self replicating nanobots.

It would be rather like a cuckoo leaving its egg in another bird's nest - except - that it doesn't even need a constructed nest. All it needs is a source of ice, rock, sunlight, nitrogen, carbon (for instance as methane) and then it leaves a seed module to create the first self replicators, and a small habitat for the humans to live in while the rest of the colony is made for them. You might even get generation starships cruising around the galaxy, stopping for a few weeks or months around each star and planet, to pick up supplies of ice, etc, and leaving multiple colonies wherever they go. Given that the galaxy would be open to whoever is able to colonize it fastest, then if something like that is possible, it would probably happen.

There's also the idea of seeder replicating probes. It's a bit hard to say how much of this is genuine future tech and how much is speculative, but potentially, it seems that they could rebuild themselves as they fly and make new copies by taking up materials from the interstellar medium, and they could use slingshot effects around the stars of the galaxy, exploiting their orbits around the center of the galaxy to travel faster and faster through the galaxy seeding it with copies of themselves. These would be very fast indeed as they never even have to slow down to less than their traveling speed of 10% of the speed of light (or whatever it is) even to replicate.

These seeder probes then, with probably not that far future nanotechnology, be able to create human embryos from templates in their memory. They have to create a single cell from scratch, which is a vastly complex undertaking for us now. We are nowhere near able to do this at present - but a few decades or perhaps centuries from now? They would then have to "give birth" in an artificial womb, and then have helper robots to help the first young colonists as they grow up. Tipler summarizes the idea in this paper (page 268).

"As to getting the inhabitants for the colony, it should be recalled that all the information needed to manufacture a human being is contained in the genes of a single human cell. Thus, if an extraterrestrial intelligent species possessed the knowledge to synthesize a living cell - and some experts assert the human race could develop such knowledge within 30 years - they could program a von Neumann machine to synthesize a fertilized egg cell of their species. If they also possessed artificial womb technology - and such technology is in the beginning stages of being developed on Earth - then they could program the von Neumann machine to synthesize members of their species in the other stellar system. As suggested by Eisley, these beings could be raised to adulthood by robots in the O'Neill colony, after which they would be free to develop their own civilization in the other stellar system."

I don't know how realistic that is. It's like a miniaturization of the more "clanking" replicator of a colony seed ship staffed by humans. But if such is possible, it would enable even faster exponential growth.

The main point here is that the most rapid exponential growth would be favoured in an unplanned expansion, as whichever group practices it even if other colonists frown on them and find their approach dangerous and warn them of the long term outcome - so long as there are people who will ignore those warnings - those would be the ones that fill most of the galaxy most quickly.

This then returns us to the trap of exponential growth. There seem to be many ways we could achieve exponential growth, indeed faster than exponential growth into our galaxy - at least at the early stages of colonization. However, that can't be sustainable for any length of time. There is no way to manage a sustainable steady exponential growth for more than a few thousand years (depending how slow the growth is). This makes it almost certain that any ETI we encounter is no longer expanding exponentially.

It's not just a matter of more technology. It's simply impossible within the laws of physics as we know them. Even with warp drive, if that were ever possible, even if you could cross back and forth through the entire observable galaxy with your star ships, you still can't keep up exponential growth for long.

So what happens after that, after an unplanned exponential expansion into a galaxy?

I think that though we have potential for a sustainable growth, I think it has to be directed in some way. By this, I don't mean it has to be "policed" in some authoritarian way. That might be one way it happens but there could be many other solutions. However, it would be, in some way, the result of the direction of the thinking components of biospheres, intelligent creatures such as ourselves, or our creations if we find ways to create new forms of intelligences. They provide the technology and the ability for the biosphere to spread through the galaxy, but they also provide the intelligent direction that could permit it to do this in a sustainable way.

However, let's look at the problem with exponential growth closely first, then we'll return to the question, how then can we achieve sustainable growth, when exponential growth seems both a natural consequence of the situation we find ourselves in, and unsustainable? How can we avoid a process that seems likely to lead us to galactic chaos? Or can it somehow resolve itself, and if so, how?

The problem with exponential population growth into an uncolonized galaxy

So, first, what's the problem with exponential growth? Well, from where we are standing now, to start with, there's no problem. As far as we can tell, we have a vacant galaxy out there. At any rate no signs of any technological society. If anyone valued its planets and suns, as we would, then surely they would have colonized our solar system long ago. At least, so it seems, from our viewpoint here on Earth with what we know so far.

As far as we can tell, we could expand into it, probably for centuries, no problem. But if we expand because of rapid population growth, and we need to find somewhere for all our descendants to live, then this approach will hit a crunch rather quickly on timescales of only thousands of years. Even with just a doubling of our population every century, if we continued population growth at that rate for ten thousand years, then we would need to turn as much mass as we have in our sun into new humans every century to keep going with the exponential growth of the population itself, never mind whatever necessities they need to survive. For the calculation, see my Why ET Populations Can't Continue To Expand For More Than A Few Millennia.

We seem to be the intelligences in our biosphere, and so, we can help direct it towards a better future. We can foresee future problems before they arise, as our biosphere starts to spread through our solar system and our galaxy, and perhaps, with foresight, we can do something about them.

Luckily we reached "Peak child" in 2005. The number of children in the world has remained steady for over a decade now. Our population will continue to increase rapidly for a while, because people world wide are living longer (on average). It seems at least possible that our population will level off naturally. Even the “middle of the road”projections now show the population trending towards 11 billion by 2100 while lower projections have it level off at ten billion or even start to decline towards the end of the century.

In the graph above, the red dotted lines show the upper and lower limits for the 95% prediction interval. The blue lines are for +- 0.5 children per couple average. You can look up the data here, projections for the whole world, or for individual countries and regions, on the graphs page for the UN population division.

If the future pans out like that, we should be okay on our planet, through to 2100, and if we can achieve a stable population by then, or soon after, as those graphs suggest, we'd be okay through into the indefinite future.

However, what about longer term? What happens if we start to set up interstellar colonies? If it triggers a new exponential growth process, then ugly problems soon rear their head again, even for very slow growth to start with, as we'll see.

To see how this works, let's suppose we set up only ten colonies around nearby stars in the next thousand years, with one new colony on average each century. Let's suppose that each of those set up another ten in the next thousand years

(techy note on these calculations: in practice some of them would set up new colonies even before the first thousand years is over, but to simplify the calculation, let's ignore those - the actual numbers would be a bit larger than for our calculation).

So now, after two thousand years, we have a hundred space colonies. So far, fine. Now each of those set up another ten colonies, and after three thousand years we have a thousand colonies, and so on. Still there seems no problem. Exponentials are like that. For a long time nothing seems to be happening much. But then, rather suddenly, it gets you.

After only twelve thousand years, we have a trillion colonies. Our galaxy has around 100 billion stars for them to colonize, so we have run out of stars in our galaxy already. But it's worse than that. Our galaxy is around 100,000 light years in diameter. Unless the colonists have warp drive, or some other form of Faster Than Light (FTL) travel -, then in those twelve thousand years, they can't travel further than 12,000 light years. The limit is 1,200 light years if they travel at a less hazardous speed of a tenth of the speed of light. That's only 1.2% of the diameter of the galaxy. Clearly our region of the galaxy would get pretty crowded, within 12,000 years, even at this slow rate of a ten fold growth per thousand years.

When you hit an exponential there isn't much you can do except to delay the effects slightly. Even tiny two gram colonists as small as the Etruscan shrews, are no help.

Etruscan Shrew - the smallest known mammal, only 2 grams, much less than a ten thousandth of the mass of an average adult human.

If you think humans can evolve to get as small as this, and evolve this far in the near future too, within a few thousand years, through genetic manipulation, you can add an extra four or five thousand years to the time it would take to run out of matter to make colonists (still assuming a ten fold increase every thousand years).

Even if we go all the way to a science fiction future of. massless colonists, in a civilization with total conversion of matter into energy, and faster than light travel (e.g. warp drive), it only delays the inevitable by thousands of years. Zero rest mass colonists still need energy. Even photons, though they have zero rest mass, still have energy, which has to come from somewhere, for instance, from nuclear fusion or some other form of conversion of matter to energy.

To take this to the limit, suppose we have minimal energy "colonist photons". The lowest energy photons we could fit into our observable universe would be vast ultra ultra low frequency feeble pulses of light with wavelengths of order of the diameter of the observable universe. I'm not saying that it is possible to make such photons, or for a colonist to consist only of a single photon like that. However, any other form of colonist will surely require at least as much energy as this massless minimal energy colonist photon, so this is surely pretty much the ultimate limit of what we could do by way of fitting the maximum possible number of colonists into the observable universe.

Even this does no more than to delay the inevitable by a few more thousand years. Soon, your exponential growth spurt has to stop. As usual, I'll indent the calculations so that they are easy to skip:

Even if you can reduce individual humans to tiny creatures of a few grams like a pigmy shrew, you'll soon run out of matter for all the colonists, The mass of the universe is about 3 x 10⁵⁵ grams according to one estimate. If we can reduce the mass of a colonist to one gram, starting with one billion humans (say), and increasing the population ten fold every thousand years, you'd run out of matter in the observable universe to make colonists within 46,000 years. That's just for the mass of the colonists themselves, not taking account of any necessities like food, water, power, life support, air to breathe etc.

You can try a science fiction scenario of massless colonists - could we have "colonists" in the future that have similar mass to a photon somehow? If that was possible, you are still limited because the energy of a photon depends on its wavelength.

The energy of a photon in electron volts is 1.2398/λ where λ is its wavelength in microns. There are about 10³⁶ electron volts to a kilogram (if you can directly convert matter to energy as an advanced ETI might be able to do). So, now suppose a photon has a wavelength of 93 billion light years (the diameter of the visible universe of 93 billion light years according to one estimate) or around 8.8x10³² microns. Then its energy will be around 1.4x10³³ electron volts (1.2398/(8.8x10³²)).

If we can have massless colonists, each consisting of just one photon, and use the feeblest possible low energy photons, so that each one has a wavelength so vast it spans the entire observable universe , then the total number of colonists we could make from the available matter in the observable universe is at most 3 x 10⁵⁵/ (1.4x10^-33) or about 2*10⁸⁸ . So, with our assumption of a start population of a billion colonists, multiplying the population by ten every thousand years - then even with warp drive, total conversion of mass to energy, and massless colonists with wavelengths that span the entire observable universe, we run out of matter in our observable universe to make these massless minimal energy single photon colonists within 77,000 years.

So, even if the population increases only ten fold every thousand years, you will run out of matter to make new colonists in the entire observable universe, well before 46,000 years if the colonists have a mass of only one gram each. You run out of matter to make colonists before 77,000 years even if each colonist consists of only a single photon with the least amount of energy compatible with fitting that photon physically into the size of the observable universe and you turn all the matter in the observable universe into these minimal energy single photon colonists.

If the exponential growth is very slow, say, a ten fold increase every million years, the calculation is the same. It's now 76 million years until we get to the single photon colonist end point, even with warp drive and total conversion of matter to energy. Whatever the timescale is for a ten fold increase, just multiply that by 76 and that's the absolute limit of exponential growth within our observable universe. That's going to happen no matter how those colonists evolve and what technology they have, at least so long as they remain within the laws of physics as we understand them today.

What about populations that plateau or crash?

Typically in nature populations expand exponentially for a while, but they can't keep it up as they run out of resources. They may plateau.

Logistics curve for populations that plateau (public domain image from wikipedia)

Or you get boom and bust cycles, as happens with lemmings for instance. Every few years the numbers increase a thousand fold, then suddenly crash. Lemming populations don't crash through mass suicide - that's a myth.

(click to watch on YouTube)

Instead it happens through predators and starvation.

See the Amazing Lemming. For some reason there have been no population peaks in recent years, possibly a side effect of global warming and less snow cover.

There are many ideas about why these population explosions happen, not well understood. E.g. that it is due to abundance or scarcity of moss, their main food source. Or that it is due to reductions in populations of snowy owls and other predators. The predators are inversely correlated with the lemmings - that's well established - but it's hard to disentangle what is cause and what is effect.

A dead lemming on a stone in the river Revåa in Norway. After a lemming boom and bust, so many die, that drinking the water becomes a health hazard for hikers. Photo by Bjørn Christian Tørrissen

If we expanded into our galaxy without any planning, we could expect something similar. Either our population plateaus or it booms and busts. You might think then - so what is the problem? It's obviously absurd to suppose any population increase continues exponentially endlessly.

But now stop to think what it would be like to live in such a galaxy.

Boom and bust - a nightmare future

In this future, from time to time parts of the galaxy might seem to be free of humans, or some part of it, for a while. But like lemmings, they'd never go extinct. A new wave appears and suddenly that region is full of them again, within a few thousand years. Or they are there all the time, but constantly at war and dying in trillions of starvation. The only respite from humans is when they are destroyed by some successor that is even more aggressive. We could go extinct but only as a result of something else taking over from us in these boom and bust cycles.

I think a boom and bust future like this would be the worst nightmare, not just for us but for any beings in this galaxy and eventually the entire observable universe

In this boom and bust future, if people learn to co-operate in one part of the universe - that’s only going to work in a small region, perhaps a few tens of light years in diameter. Beyond that, the chaos would just start up again or rather continue without check. If someone somewhere establishes a peaceful spot in the galaxy - they would never know when some horde of beings with strange ideas would suddenly appear having developed for a thousand years, at a distance of over a thousand light years away. Then they arrive at close to the speed of light using unfamiliar technology. They would be our distant cousins, or the creations of our cousins, but that wouldn’t help. And once started, how can this ever stop?

The plateau future could be just as bad. It depends how the plateau happens. If the underlying pattern is still a process of exponential growth, then it might be that the population of the galaxy is steady as a whole, but underlying that is constant starvation, or warfare, and local booms and busts. It's not an appealing or enticing future, is it?

There's one other possible outcome of exponential growth, though it's hardly much better, which Seth Baum suggests in his conclusion to The Sustainability Solution to the Fermi Paradox

" It is also possible that faster-growth ETI civilizations previously expanded throughout the galaxy but could not sustain this state, collapsing in a way that whatever artifacts they might have left have also remained undetected. Both of these growth patterns can be observed in human civilization, suggesting that they may be possible for ETI civilizations as well."

If this is possible, it doesn't seem to have happened in our past. Whatever may happen around other stars, our own solar system shows that there would have been pristine refuges for the ETIs to retire to, to evade the hazard, whatever it is. It can't be that they ran out of resources, because there is no sign that they even tried to utilize resources in our solar system. So, what about other possibilities? A self replicating carefully designed weapon, targeted only to destroy their own kind, or sterilize them to make them unable to reproduce, or a meme that leads to them choosing voluntary extinction by having no more children? Bearing in mind that it takes 100,000 years for a signal to cross the galaxy, after tens of thousands of years of independent development, knowing only of each other's discoveries tens of thousands of years ago, how could they all be so similar in technology and social structures that they all succumb to the same weapon or crisis galaxy wide?

It's hard to see how anything could make a galaxy spanning self replicating creature extinct, except, a successor, another self replicator that out competes them. That is, unless they, or some other extra terrestrial, have FTL travel. In that case, maybe, but given the vastness of the galaxy, I still find it implausible that, even with Faster Than Light (FTL) travel, that they are able to destroy their own kind, or all choose voluntary extinction, or for someone else to do it for them, over all the 100 billion stars in our galaxy... With FTL, you can't rule this out, all I can say here is that it seems a bit far fetched.

Is there anything we can do to avoid these nightmare futures?

Slower growth, for instance cube or square law growth rates

Could we expand into our galaxy in a way that avoids this exponential growth disaster? Well yes, that's possible. One way is to simply fill the nearby region with space to a comfortable capacity and no more. The back story here would be that we expand in a star system until we reach a comfortable capacity similar to the 11 billion population prediction for future Earth (if restricted to a single moon or planet). Maybe with Stanford Torus style space colonies we can comfortably have ten or a hundred trillion or more colonists per star system. But at some point we reach the comfortable carrying limit for that star system. It's no problem for us however. Just as for Earth now, we reach peak child and our population within the solar system stops increasing. So the colonization is not driven by a population explosion.

In this slower than exponential growth scenario, then at this point the population is perfectly content to stay in that star system with a self sustaining population and habitats. It has no need to colonize. However, if there is another nearby star, then they will colonize it too, and indeed ten such stars or more every thousand years. But if all the nearby star systems are already colonized, they stop. They might even stop before that. Perhaps they like to colonize only one star in a hundred. If they see no way to add a new colony without "crowding the neighbourhood" according to their ideas, they stop, content. This gives other extra terrestrials and non intelligent species their own space to live in, and maybe this is something they value highly.

If we could have a civilization like that, then the population expands just depending on how far they have got so far (assuming no warp drive). For the first few centuries, all the colonies are surrounded by unpopulated stars, and so perhaps it expands exponentially, perhaps far faster than that ten fold increase every thousand years to start with. Here is a visualization of the trip to one of the nearby exoplanets in the habitable zone of its star, one the ones that seems most potential to be habitable to date, at a distance of 41 light years, LHS 1140 b.

(click to watch on YouTube)

At a tenth of the speed of light, it would take 410 years to get there, either to live on the planet, or to live in its solar system and peacefully coexist with any residents the planet has already. With a population steadily filling our galaxy at a tenth of the speed of light, we could get there within a thousand years, easily. I don't think there is any need to factor in a time to "get established" at each colony before they start up a new one. At this stage surely they can make self sustaining habitats in the asteroid belts, or from moons, or other debris in the solar system. Indeed, they might be able to manage just fine in the distant Oort clouds using mini sun fusion devices. They might just hop from one Oort cloud to another.

If they continue to expand at a tenth of the speed of light, giving time to travel between the stars, and set up new colonies as they do so, then within a thousand years they fill a region 100 light years in radius from their starting point, in a way they find comfortable. After ten thousand years they fill a sphere radius a thousand light years, but by now they have filled the entire "thick disk" of the galaxy, (we are in the "thin disk" which has gas and dust as well as stars, only 400 light years in thickness, while the "thick disk" is devoid of dust and gas). So far it's been a cube law growth of population into 3D space, with the population proportional to the volume occupied. After that, they can only spread in two dimensions through the disk of the galaxy, apart from a few explorations to scattered stars in intergalactic space, and maybe nearby dwarf galaxies. So now they fill an expanding disk shaped region 1000 light years in thickness, so their growth rate has gone down to square law growth. This then continues until they fill the entire galaxy, less than a million years later. At that point they occupy a billion of the 100 billion stars in our galaxy, and see no need to colonize the remaining 99% of the star systems in our galaxy. Or they occupy every star system, but with sustainable moderately sized populations.

At this point, they see nowhere else to colonize except dwarf galaxies, globular clusters, and intergalactic straggler stars, and maybe they send off expeditions to other large galaxies too like the Andromeda galaxy, taking millions of years to get there. However, most of them won't be colonizing any more, not in this physical sense, but just remain happily content in their own star systems in a vast galactic empire. They have other frontiers to explore. After all even on Earth today many of us feel no need to colonize physically.

That would work if they were peaceful and all think alike on matters that influence how they colonize the galaxy. They can peacefully colonize a galaxy like ours in a million years, and if their civilization is advanced, perhaps there is no need even for wars.

Perhaps some ETIs have already done this? If only we could find some way to guarantee such a reasonably peaceful future for ourselves and for our galaxy.

Could humans achieve slower than exponential growth like this?

Yes some of us could perhaps do this. Maybe even entire colonies might have this as their philosophy for expansion into the galaxy. But if this was a human future with beings like us, as we are now, then of the first thousand colonies, one of them is bound to be a more expansionist than the others. Maybe it develops the technology to spawn a hundred colonies every thousand years, and fosters a rapidly expanding population. If one of their colonies in turn is able to spawn a new colony every year, maybe through manufacturing clones of themselves, and with the help of self replicating machines, then they would be the ones that fill most star systems of all. If there is no intelligent oversight, the ones able to make the most colonies in the least amount of time would win the race to fill the galaxy with their kind. It doesn't need to be through genetic evolution. Just through memes, ideas and culture.

So now, imagine us as we are now, as aggressive, as expansionist as we are now, but culturally evolved to be even more aggressive and expansionist, with strange future ideologies we can't even begin to imagine, developed by humans tens of light years from Earth. Our future distant cousins and descendants have the ability to modify their own genes, make clones, self replicating machines, cyborgs etc. They are also separated from us by the light speed barrier (assuming no warp drive). By, say, a thousand years from now, there may be colonies already a hundred light years away. After ten thousand years, some may be a thousand light years away.

By the time we've reached this stage, how could humans ever go extinct? Or rather - our cyborgs or genetically engineered super-humans, or "uplifted" animals, or self replicating machines, or whatever it is we have given rise to by then? Our civilization as a whole can't "self destruct" because if any parts of their civilization does this, well there will be others hundreds, or thousands or even 100,000 light years away that aren't affected. So wave after wave of their descendants, or cyborgs, or uplifted other creatures, or self replicating machines will wash across our galaxy from then on for all future time. With exponential growth highly favoured, then if we are unfortunate, this could become a boom and population crash future of short lives, of people (or their descendants or creations) dying in their countless trillions every year.

How can we make sure that such a future is reasonably peaceful? How can we stop it from turning into never ending waves of destruction in a future galaxy filled with remote cousins many times removed, with bizarre ideas and unfathomable technology approaching at close to light speed from thousands of light years away? This might be a significant and important future challenge that we have to find a way through. Perhaps all Extra Terrestrial Intelligences (ETIs) that develop space travel encounter these issues eventually.

So what might a sustainability solution look like?

Perhaps the solution may be one that surprises us. Malthus wrote in the early nineteenth century about the inevitability of population growth to out pace production. As it gets easier to grow food, he reasoned, then population grows rapidly and exponentially. This outpaces the production of food and then you get mass famine and starvation, and the population levels out again, in what has been called a "Malthusian catastrophe". Then labour is cheap, because there are so many people, and production of food increases, for a time things are easier, but the population then increases until the poorest are barely able to subsist, and the cycle repeats. He looked for solutions, but they weren't very satisfactory, and none of his solutions match what actually happened.

We should have had such a catastrophe in the 1960s onwards, but then we got the "Green revolution", a dramatic technological improvement in our methods for producing food which had been in development from the 1930s onwards, prevented it. Without that, we'd have had mass starvation, with billions dying. Since then, we've had plenty of food world wide. We've had famines too, but they are due to things like war, political and social issues, and distribution of food, not a shortage of food world wide. So far we seem to have no difficulty keeping up with the increasing population and feeding everyone. Also there's no immediate near term prospect of us being technically unable to grow all our food - at worst it could become more expensive to do so, as I covered in Human settlement and exploration - hugely positive or hugely negative - it all depends how it is done (above).

We also have a population that is naturally leveling off. It seems to be a universal rule, amongst countries of all political persuasions and religious and ethnic backgrounds, that as wealth increases, then we have fewer children. In many countries, the number of children is actually below replacement levels. This is happening without famine or starvation - the countries with growing populations tend to be the poor ones. The wealthy ones have leveling populations. It looks as if we aren't headed for a future Malthusian catastrophe after all.

Malthus did predict the possibility of increasing crop yields in his "Essay on the Principle of Population", but saw that as a short term solution:.

"If the progress were really unlimited it might be increased ad infinitum, but this is so gross an absurdity that we may be quite sure that among plants, as well as among animals, there is a limit to improvement, though we do not exactly know where it is. It is probable that the gardeners who contend for flower prizes have often applied stronger dressing without success. At the same time, it would be highly presumptuous in any man to say, that he had seen the finest carnation or anemone that could ever be made to grow. He might however assert without the smallest chance of being contradicted by a future fact, that no carnation or anemone could ever by cultivation be increased to the size of a large cabbage; and yet there are assignable quantities much greater than a cabbage. No man can say that he has seen the largest ear of wheat, or the largest oak that could ever grow; but he might easily, and with perfect certainty, name a point of magnitude, at which they would not arrive. In all these cases therefore, a careful distinction should be made, between an unlimited progress, and a progress where the limit is merely undefined."

I think he'd be surprised by how much the green revolution was able to improve crop yields. Also, he didn't predict a natural leveling off of population associated with wealth. Surely he'd be surprised to learn what actually happened in his future.

So perhaps we also are unlikely to predict the sustainability solution, whatever it is. Not unless, perhaps, we encounter ETIs that tell us how they did it.

Still, let's have a go. First, so long as we stay in our own home solar system, then there's no problem. If we colonize just a few nearby stars, again there is no problem, but it's going to be hard to stop at just a few stars colonized.

So, delaying interstellar colonization altogether will give us more time to find a solution, if we don't have one yet by then. Here are a few ideas about that:

• Because interstellar colonization is difficult and expensive. Perhaps we are lucky that it is so hard to get to the nearest stars. This may give us the breathing space to find a solution.
   
• Our descendants become more and more aware of the issue. It becomes unthinkable, to them, to set off an exponential population explosion in our own galaxy, like letting off a nuclear weapon in your home city - until we find a solution. This is surely just a delay, and amongst our billions or trillions, there will be a few who think we should colonize the galaxy. But social pressure can be strong. And for a while at least, only governments and very wealthy individuals, trillionaires (if we have them by then) will be able to contemplate interstellar colonization. So, if it's agreed that this is an issue, it may be that we need to raise awareness of the issue amongst those who would make decisions to embark on a multi trillion dollar program to colonize another star.
   
• Our descendants have immensely long lives and as a result, they take it personally - this future potential chaos is something that will happen to them, or to their children, in what seems to them the near future, only a few tens of thousands of years into their long lifetime.
   
• We make contact with extra terrestrials that have solved the problems and learn from them
   
• An object lesson - we spot a distant galaxy that is in a state of galactic chaos, with messages beamed in all directions from some of its inhabitants warning us not to do the same in our galaxy.
   
• We are stopped by intergalactic "police" of some sort. Advanced robotic sentinels activate when we leave our solar system and warn us that we should not attempt colonization anywhere - and they are present around every inhabitable solar system to prevent colonization.

Meanwhile we explore the galaxy robotically with lots of identical robots, all launched from Earth, perhaps miniature laser propelled chipsats. Later on we can send carefully designed self replicators to explore the entire galaxy for the cost of just a few initial self replicating robots sent off from Earth. They make the rest of the robots locally from in situ materials, as they spread through the galaxy.

I don't think that we need to worry about these self replicating robots evolving, not unless we design them to. There are various ways to prevent that, to ensure exact replication, and to have a limited number of generations built into the self replicators, a kind of "artificial telomere". For organic life, then there is an advantage to have imperfect replication, as that is how it is able to evolve. Perhaps there never was any advantage for life that replicated exactly.

However for robots, we can design in exact replication. We can design it so that flaws are unlikely in the first place. Then we can also design them to check themselves and each other, to start again using rejects as raw materials if they detect a flaw, to self destruct, or be unable to replicate, if there is any flaw in what develops. The design can be robust with multiple safeguards, to detect damage from external influences and again, self destruct if any such is detected. We can use a telomere to prevent them from replicating beyond, say, 20 generations with at most ten copies per generation (plenty to fill our galaxy). Then we can have a "keep alive" signal you have to send out every year, say, relayed from one to another. If that signal stops, all the robots self destruct. Also if any extra terrestrial approaches and tries to change their programming, they self destruct. We can test all this on a small scale in our own solar system, before we try it for interstellar exploration. One way or another, if we want to, I think it will be no problem to ensure 100% accurate reproduction of our robotic emissaries into the galaxy. For details see my: Self Replicating Robots- Safer For Galaxy (and Earth) Than Human Colonists- Is This Why ETs Didn't Colonize Earth?

We can also have human explorers if we are confident they won't start up any colonies around other stars.

Longer term, though, what kind of sustainability solution could we have? Planning for this is the easy part, the hard part is making sure the plans are followed.

As an example of a way to plan it, green belt areas in the UK protect rural areas from over development. We could use that as a model, which works here in the UK, where I live. There are occasional grumbles, and sometimes someone will attempt to evade the rules, for instance, build a house hidden behind straw bales so that nobody can see it until it is finished (it didn't work, he had to demolish it). But on the whole it works pretty well and most people understand the need for the rules though they may grumble and protest at particular examples.

Similarly, perhaps, we could have a rule that only one in 10 stars can be colonized, for instance, establishing a "green belt" of the ten nearest stars around every colonized star. It would need some careful thought and tweaking, and this is surely just one of many possible ideas. One way or another, we can work out rules for colonization that would permit sustainable colonization of the galaxy, and avert any chance of an exponential growth catastrophe.

The problem is, how can we ensure that future humans keep to the plans, especially once they have disappeared away from us beyond the light speed communications horizon? Planning regulations have to be enforced. So do we have to enforce sustainability planning regulations for interstellar colonization, and if so, how?

Then, there's a further problem - even if we reach a point where we can see that our society as it is now can fill the galaxy in a sustainable way - how can we ensure that all our future descendants also will do this? What if they become stupid, child like, develop short lives, lose their direction and moral compass, etc? In a galaxy of several trillion stars each with its interstellar colony - how can we be sure that none of them, amongst all of those colonists, will start off an exponential expansion into the galaxy?

Here are a few thoughts:

• Social change, and perhaps genetic changes, our ideas, memes, things to do with our physical being too possibly, evolve to the point where the idea of non sustainable exponential growth becomes unthinkable . Hard to imagine this at present, but maybe it is possible?
   
• Object lesson in another galaxy (again). Could this be so compelling as to make exponential growth unthinkable?
   
• Policed by ourselves - we develop robots or some other way of policing ourselves, so capable that we can't outwit them. Super intelligent "caretakers" developed for their job, through genetic manipulation, or artificial intelligence (if that is possible) that we respect. They play a role in society and we find a way to give them the authority and power to prevent over population of the galaxy, yet to prevent them also from overstepping their authority.
   

• Education programs - we find ways to educate ourselves, throughout the galaxy, whenever we fall away from the path of sustainable growth.
  
• Policed by ETs who over billions of years, and many mistakes, have developed multifaceted ways of stopping both themselves and other ETs such as ourselves from unsustainable exponential growth patterns.
   
• Contact from ETs that show us the way ahead (whatever it is, may be none of these solutions)

Could any of those work?

There's another possibility, but a sad one

• Perhaps it is inevitable and the best we can do is to postpone the inevitable as much as we can. Every century before we set up interstellar colonies is an extra century before the onset of galactic chaos.

Maybe though, we are in a position similar to Malthus in the early nineteenth century. Perhaps the ideas, technology, or surprising developments that will lead to a solution just haven't happened yet.

Meanwhile, if I can make a suggestion for discussion, I think the best thing right now is just not to be in a rush to start up our first interstellar colonies. Let's focus on protecting Earth as our top priority, as it surely has to be. Then for interstellar missions, use robots first. They are also much safer and far cheaper. Let's send them in the vanguard, as explorers rather than colonizers. Then when we send our first humans on interstellar missions, if we do, let's do these again as exploration missions rather than colonization missions. If nothing else, if we haven't found a solution yet, at least this approach delays eventual onset of galactic chaos for our descendants. Also, the longer it takes, the more time we have to find a solution, before the colonies we spawn disappear beyond the light speed communications horizon.

Do you have any ideas for solutions? Do let me know if you do, and I'll add it to this list. Also if you spot any mistakes in any of this, be sure to say.

Optimism - especially if we are not first

Actually I'm optimistic there, especially if we are not the first extra terrestrial space capable species in our galaxy. Our predecessors have to have found a way through this, as otherwise the chaos in our galaxy from battling ETIs would be plain to view, and it is hard to see how it could end as it seems that they could never go extinct.

Once an aggressively expanding civilization has reached its nearest stars, then - it seems pretty much inevitable that it will fill the galaxy within a million years. And I mean totally fill it, in a population explosion. They'd have taken over Earth long ago, as they would need to use all the resources they could find to cope with their constant wars, exponentially increasing populations and resource crises. How could our Earth and solar system remain untouched?

Yet it is. We haven't found any extra terrestrial footprints or tracks on the Moon or Mars, and we can spot our rover tracks there easily.

Curiosity's tracks photographed from low Mars orbit by HiRISE on NASA's Mars Reconnaissance Orbiter. This instrument has a resolution of 30 cm. We have found no signs at all of any extra terrestrial tracks or footprints yet, anywhere in our solar system. Our solar system, to all appearances, is pristine.

There are no signs of mining operations, or any kind of exploitation, anywhere in our solar system. If any extra terrestrials have been this way in the past, their impact on our solar system has been minimal. Either that, or they are great at erasing all traces of their presence when they leave.

It's not just our galaxy either. There's no sign of distant galaxies being modified in large scale radical ways, as they would in a future in which intelligent creatures like us, with technology, fill a galaxy. .A search of 100,000 nearby galaxies has turned up no clear signs of galaxy spanning civilizations. They would expect to spot any widespread use of technology on a galactic scale.

“In some sense it doesn’t matter how a galactic civilization gets or uses its power because the second law of thermodynamics makes energy use hard to hide. They could construct Dyson spheres, they could get power from rotating black holes, they could build giant computer networks in the cold outskirts of galaxies, and all of that would produce waste heat. Wright’s team went right to the peak of the curve for where you’d expect to see any sort of waste heat, and they’re just not seeing anything obvious.”

Any expansionist interstellar ET must have minimal impact on the galaxy - or else - we are the first such, at least in our galaxy

From this, it would seem to follow that there are three main possibilities.

1. We are the first - and as the intelligent component of our expanding biosphere, it's our responsibility to make sure we take good care of our galaxy, and leave it in a decent state for future generations and civilizations just as we would want earlier civilizations to leave it for us.
   
   If so, we must be exceptionally far sighted and careful as our decisions can reshape the entire galaxy for ourselves and other intelligent creatures here for all future time. If we get this wrong, we can turn it into a never ending chaos. We could lead our galaxy into a state that no-one will ever be able to calm down, neither ourselves nor any other species in our galaxy, nor any of our creations or their creations (short of warp drive and incredible technology).
    
2. Or, if there are other technologically advanced ETs, they have all found some way to explore the galaxy without major impacts on it, and without doing this uncontrolled galaxy cramming exponential expansion.
   
   This might happen automatically indeed. The far sighted ones show restraint, eventually, maybe only after centuries or millennia of social development, constantly destroying their space capabilities until they "get it", or maybe quite quickly. The ones who can't do that simply never get that far. If so, this is rather optimistic, and maybe we can learn from other more mature ETs in our galaxy once we make contact with them.
   
   However, it may take a while. Because they have had minimal impact then the nearest ones may be some distance away, even thousands of light years if they are non expansionist. They might have sentinel type robots around every star though, if so we might contact those sooner.
    
3. Or, we are already in that future and our universe periodically fills with waves of lemming like exponentially growing colonies - we don't see ETIs because we are currently in a "bust" phase in our part of the galaxy and they recently made themselves locally almost extinct.

I think we can rule out the third possibility though, because our solar system has remained untouched by them. If they had exponential growth they'd be desperate for resources periodically as they reach the highest point of the boom phase of their population, which they'd do frequently.

So long as a few are left with space technology they would fill the galaxy again well within a million years (with the competition from their own kind gone). There would be boom and bust cycles at least every million years and perhaps more often, perhaps even every few years like lemmings. How could Earth and our solar system remain so pristine and untouched through all that?.

Will we be an invasive weed, or a flower in the galaxy?

The anthropologist Mary Dora Russell says:

'Anthropologists used to say that Homo sapiens was a unique and special species because we were the only ones who used tools, or who were self-aware, or had language, or passed culture to our offspring… Then we started finding out that chimps and dolphins and crows and African grey parrots and snow monkeys were making a mockery of our pretensions to uniqueness, so we’ve kind of shut up about all that in recent years.

If you want a nice reductive definition of our species, I could defend this: “Human beings are bipedal tailless primates who tell stories.”

That’s probably just as stupid as earlier definitions, but it’s catchier than my other version, which is

“Human beings are a dangerous, invasive weed species that has invented central heating, air conditioning, and food that can be stored for up to ten years, so not even a direct hit by an asteroid would likely make us extinct.”'

When nothing else matters, by Mary Dora Russell

I think that’s rather how I see the future of us in the galaxy if we just expand into it without foresight. But far worse than a weed on Earth. We’ve unnaturally made ourselves almost impossible to go extinct already by our technology, and if we expand through the universe without evolving social breakthroughs of some sort, to catch up with our technological breakthroughs, I think we’ll become the ineradicable weed of the galaxy. But harmful to ourselves as much as to everyone else, and able to create even more dangerous replicators through our technology.

It's not just us. Our galaxy may well contain many non technological species, for instance intelligent fish-like or octopus-like creatures, living in the oceans of icy moons, or ocean planets, where they have no chance to develop control of fire. Or creatures that are just not very strong, and don't have good "hands" like us for manipulation, like parrots or crows. Even an elephant would have a lot of trouble building a fire and smelting metal. These could include millions or even billions of years old ancient civilizations, perhaps advanced in mathematics, art, poetry, music, perhaps socially very advanced. Yet without technology they would be especially vulnerable to a new technological species spreading out of control like an ineradicable weed through the galaxy. Out of all the intelligent creatures on Earth, I think only humans (and the other great apes) had a decent chance of developing technology based on fire, even with intelligence. I can't see parrots, crows, dolphins, whales, octopuses, elephants, dogs, cats, developing technology like that, with tools, fire, metal working, and so on, however clever they became and however skilled at communication. They could develop writing, as that just requires them to scratch marks in mud or wood etc, but metal working and fire? So if we can use the most intelligent species on Earth as a good basis for generalizing to the possibilities for extra terrestrial intelligences,, then the non technological civilizations universe wide may well outnumber the technological ones many to one.Even a billions of years old civilization of that type could still be highly vulnerable to an immature few centuries old civilization of technological ETs such as ourselves,

I think any sensible ET will look at that possible future for themselves and the galaxy, and find a way to become a flower of the galaxy instead of a weed that will eventually choke all the species in the galaxy, including themselves. If they can’t see a way to a future like that, then if they have any sense, they just stay at home until they can. And if they haven’t the sense to do that, I think, perhaps, that they either make themselves extinct, or they keep destroying their own spaceflight capabilities, and get nowhere, until they develop some sense.

We tend to focus so much on negative traits of our civilization. But there is much in it that is very positive indeed. We have learnt so much in just one century, since the nineteenth century. Our present technology, if we dumped it into the nineteenth century, would cause so much chaos, it's hard to see how we could even survive. Yet we did, we are still here, and have worked together, pulled together in many ways to find a way forward. Although social change happens at a painfully slow snail's pace on the year to year level, if you project into the future and see where that may be headed centuries from now, the situation is hopeful, as I covered above in Signs of optimism, that we can be one of the wise ETs.

As I said in the introduction, let’s be one of the civilizations in our galaxy and universe that flowers like a beautiful flower!

(click to watch on YouTube) - Amazonian giant water lily.

See also these sections of my Case for Moon First:

• Earth is the best place for a backup
• As for self created problems, again we can't escape from them in space
• A future with large numbers of humans in space with high technology is not necessarily the best thing to aim for
• Generally, sending humans into space is something new, that humans have never done before.

See also my articles:

• Why ET Populations Can't Continue To Expand For More Than A Few Millennia.
• Self Replicating Robots- Safer For Galaxy (and Earth) Than Human Colonists- Is This Why ETs Didn't Colonize Earth?
• Wait, Let's Not Rush To Be Multiplanetary Or Interstellar- A Comment On Elon Musk's Vision (the interstellar section is towards the end)
• Why Didn't ETs, Or Self Replicating Machines, Colonize Our Solar System Millions Of Years Ago?

What should our objective be for humans to Mars?

If we decide we shouldn't send humans to the Mars surface quite yet, but want to go one step further than robotic exploration from Earth, and send humans there, there are many exciting missions we can do to the Mars system. We can explore Mars by telepresence from orbit, or from its moons Phobos and Deimos, or in flyby missions (using Mars as a gravity assist to get back to Earth)..

Telerobotics as a fast way for humans to explore Mars from orbit

Telerobotics lets us explore Mars much more quickly with humans in the loop. The early stages of telerobotic exploration of Mars would use an exciting and spectacular orbit if we follow the HERRO plans. Every day the Mars space station would come in close to the poles of Mars, swing around over the sunny side in the equatorial regions and then out again close to the other pole, until Mars dwindles again into a small distant planet - and not only once. It does this twice every day. This "sun synchronous" orbit always approaches Mars on its sunny side so you get to see both sides of Mars in daylight from close up, every single day.

Imagine the view! From space Mars looks quite home-like, and the telerobotics will let you experience the Martian surface more directly than you could with spacecraft. You'll be able to touch and see things on the surface without the spacesuit in your way and with enhanced vision, and adjust the colours to show a blue sky also if you like. It's like being in the ISS, but orbiting another planet.

12th April 2011: International Space Station astronaut Cady Coleman takes pictures of the Earth from inside the cupola viewing window.- I've "photoshopped" in Hubble's photograph of Mars from 2003 to give an impression of the view of an astronaut exploring Mars from orbit.

This is a video I did which simulates the orbit they would use. I use a futuristic spacecraft as that was the easiest way to do it in the program I used to make the video. Apart from that, it is the same as the orbit suggested for HERRO.

(click to watch on YouTube)

It would be a spectacular orbit and a tremendously humanly interesting and exciting mission to explore Mars this way. The study for HERRO found that a single mission to explore Mars by telepresence from orbit would achieve more science return than three missions by the same number of crew to the surface - which of course would cost vastly more. Here is a powerpoint presentation from the HERRO team, with details of the comparison.

Then, if you have humans orbiting for Mars, then for sure, you'd also have broadband streaming of everything back to Earth from Mars. As well as being very safe, also comfortable for the crew, you'd also have wide-field 3D binocular vision, which we can all share at home back on Earth. It's amazing what a difference this makes, I recently tried out the HT Vive 3D recreation of Apollo 11. We'd have similar 3D virtual reality experience of the Mars surface. It would actually be a much clearer vision than you'd have from the surface in spacesuits, digitally enhanced to make it easier to distinguish colours (without white balancing the Mars surface is an almost uniform reddish grayish brown to human eyes), and so that we can see bright colours even as it gets dark, and indeed, with false colour you could see ultraviolet, infrared etc as well if you want to.

Here is this hololens vision again, which though it's not telepresence, I think gives a good idea of what it might be like for those operating rovers on Mars in real time from orbit, some time in the future with this vision.

(click to watch on YouTube)

No need to suit up for an EVA when exploring by telepresence

The HERRO study mentions this briefly, but I think it is worth going into it in detail as it is a major advantage of telepresence exploration. There is no need to suit up, which currently is a long procedure. On the ISS, then you start preparing for your EVA at least one day before, by checking out the airlock and your spacesuit.

You sleep in the airlock the night before, as the pressure is gradually reduced. That's because the EVA suits would be so stiff if you filled them with Earth normal pressure atmosphere, that it would be almost impossible to move your fingers. So they use pure oxygen, which lets humans breathe and be comfortable at a much lower pressure of 30% of air pressure at sea level on Earth. However, they don't want to keep the entire ISS at such a low pressure, which also requires a pure oxygen atmosphere which is a fire risk too, so the only solution is for the crew who are doing the EVA to adjust to the lower pressure for every EVA, which they do by this procedure which they call "camping out" or sleeping overnight in the airlock.

Piers Sellers (left) and David Wolf using pre-breathe exercises to purge their blood of nitrogen to prevent "bends" as they adjust to a third of Earth's atmosphere and a pure oxygen environment. This is done the day before the EVA and they "camp out" overnight in the airlock ready to exit for their EVA the next day. This photo was taken during the STS-112 mission on 10 October 2002. (Image: NASA). For details see page 4 of this article.

That's the main difference between the ISS and Apollo. The Apollo mission used a pure oxygen atmosphere pressurized to five pounds per square inch, so 34% of Earth normal. This was a fire risk, as they knew from the Apollo 1 fire, and originally NASA awarded North American Aviation a contract for a mixed gas environment, but they then changed that back to pure oxygen, because a mixed gas would add to the mass. NAA wanted to use the mixed gas, but NASA argued for oxygen which they had already used successfully with four Mercury missions, on the basis that at a third of Earth pressure, the risk from fire is much less and within the capability of a well trained crew to manage it, if they took care also with construction of the spacecraft with few flammable materials.See Why Did NASA Still Use Pure Oxygen After the Apollo 1 Fire? and see Stack Exchange discussion here for some more interesting material and links. TildalWave makes some interesting points, mentions as other advantages that the ventilation system works much better at higher pressures, with no chance for pockets of carbon dioxide or carbon monoxide to build up. It also helps them to shed excess heat while exercising, it makes air cooling more difficult - equipment including electronics that rely on air cooling would need to be redesigned, and it would complicate biology experiments.

You might wonder why it's an increased fire risk when the partial pressure of oxygen is the same as on Earth. However the level of oxygen we have on Earth is already high enough so that, for instance, forest fires start quite easily. We'd have many more fires if it wasn't that nitrogen is a natural flame retardant, because it absorbs heat. A thinner atmosphere with the same amount of oxygen but no nitrogen is much slower at taking heat away from a flame or smoldering ember or a wire that is heating up too much etc, and so there is an increased fire risk. Not so bad as for Apollo 1 of course which had oxygen at Earth normal pressures, but high enough so that they took care with Apollo to reduce the amount of flammable material inside the spaceships as much as possible and still recognized that it was an increased risk that they countered by having well trained crew.

Though NASA back at the time of Apollo took the view that the increased risk of fire was acceptable and could be worked around for Apollo, a fire is one of the most dangerous things that can happen in space, more dangerous than on Earth (there is nowhere to go to escape from it, and it might not be easy to purge the atmosphere or vent smoke and carbon dioxide if the spacecraft itself is on fire). So, it doesn't seem likely that a long duration space missions would use pure oxygen at low pressure in the future, especially for weeks or months long missions.

Donning the spacesuit itself takes hours, as it is complex with many parts to it. The pressure has to be reduced further, you prebreathe inside the spacesuit for fifty minutes, do leak tests etc. The whole process of putting on a spacesuit is a matter of hours, not minutes. What I just described is an overview for the general public. If you are an astronaut, there is far more to it than this. Your detailed checklist runs to hundreds of steps which they have to do in sequence, many of them timed. You can read their detailed EVA check list for the ISS here. The crew leave the crewlock on page 379 of those instructions under "Crewlock Egress". More documents for ISS EVA's here.

Surely this will get speeded up somewhat as the technology improves, but suiting up seems bound to be a slow procedure for a long time into the future, and it is one you won't want to rush, you wouldn't want to skip on the airlock checks or suit checks for instance, as a mistake there could mean you die, or that your emergency gear doesn't work if you get into trouble.

Also because of this need to use a higher pressure inside the habitat for safety reasons, it seems likely that for a fair while into the future, astronauts will pre-breathe oxygen at gradually reduced pressure in the airlock the day before they do an EVA, unless they find a way to make astronaut spacesuits flexible enough to be practical with a full Earth pressure oxygen / nitrogen atmosphere inside.

Dava Newman's mechanical counterpressure biosuit still has normal gloves at present. Also she designed it around an internal pressure of 4.3 psi, a third of normal atmospheric sea level pressure. Photo from this article. Could a similar approach work with full Earth pressure?

Here are a few ideas just to think about:

• Flexible gloves with lots of bearings inside - for each joint in each finger. Much like the bearings currently used to let astronauts move their head, arms and legs. Or any other way to somehow ensure that the volume stays the same as you move your finger. What makes the gloves stiff when there is a high pressure difference between the inside and outside, is that as you bend your fingers the volume has to change. That's also why the spacesuit bearings at the neck, elbows etc work, because bearings let you move without changing the internal volume of your suit. It's also why suits balloon outwards when astronauts wear them because a more spherical shape has the greatest volume for the surface area. So,, if you can make your gloves also constant volume somehow, that would make a huge difference.
  
  Dava Newman does talk about the idea of a biosuit approach to the hands here, but it's a much more complex problem than for the body and arms and I can't find anything detailed about the idea of counterpressure biosuit style gloves yet. The biosuit prototypes that she demos have normal gloves.
  
• Actuators like an exoskeleton suit just for the hands - to enhance strength of astronauts fingers - suggested by Dava Newman here.
  
• Telepresence hands - so your real hand is kept inside a spherical "helmet" and attached to it, just beyond the tips of your fingers, a tel-operated hand that responds to the movements of your fingers as you move them. So it works like telerobotics but just your fingers and hand are telerobotically operated. Positions of fingers and hand sensed using infrared or whatever.
• Something like the JPL biosleeve, same idea but the hand instead of responding directly to movement of your fingers, responds to the messages your nerves are sending to your hand. They could work like the iLimb which is already used by amputees, a hand that responds to the nerve signals picked up by electrodes taped to the skin.

Or, perhaps they could replace the nitrogen with helium. I can't find much mention of this idea, though it is mentioned briefly in this Stack Exchange conversation. Helium is also a flame retardant. As  supercat says in that discussion, its specific heat is greater than that for nitrogen, nearly five times as much, 5.19 kJ/(kg K, compared with 1.04 kJ/kg K but its density is a lot less, 4.02 kg / cubic meter compared to 28.02 kg / m3 for nitrogen, so its density is a seventh that of nitrogen. They could reduce the time needed to adjust to the lower pressure of a spacesuit hugely if they started with helium in the atmosphere of the space station, or the ISS, in place of nitrogen. Because of its lower molecular weight, it enters and leaves the blood more quickly and blood and tissues also can't take in as much helium as nitrogen. Or they could use a mix of oxygen, nitrogen and helium in the space habitat, like the breathing gas Trimix used by deep sea divers. I don't know of any scholarly articles on the idea of using helium in space habitats or rovers. Do say if any of you reading this know of one.

They could also have rovers that have low pressure inside, or have helium inside, to take the place of the "Camping out" - so instead of going into an airlock for a day before a one day EVA, the astronauts go into a rover, initially at full atmosphere, and "Camp out" in the rover for a day, driving over the Moon in the meantime if they want to, gradually reducing the pressure inside, or replacing the nitrogen by helium, and to their first EVA on the next day. The increased fire risk of a pure oxygen environment without the nitrogen flame retardant would be offset by designing it to be very fireproof, similarly to the command module and lunar lander for Apollo. Or in the case of helium, the inconvenience of squeaky helium voices is something they could just put up with for the duration of the EVA in the rover.

Artist's concept of NASA Multi Mission Space Exploration Vehicle exploring the Moon. Perhaps astronauts future could "camp out" for the day before an EVA in a rover with either reduced atmosphere or helium instead of nitrogen as a flame retardant and for Earth normal pressure inside, and then do an EVA of several days or a week or more before returning to the habitat. They would still need to spend a fair bit of time preparing for each EVA, but more on the timescale of the Apollo astronauts on the Moon than of ISS astronauts doing an EVA.

If they do that, they may reduce the time needed to prepare for an EVA from the previous day to just hours but it doesn't seem likely that we'll have astronauts just putting on a spacesuit and heading out onto the surface as they do so easily in the TV series and movies. Not in the near future.

If you explore the surface via telepresence, you have no need to do any of this. A huge time saver.

Incidentally, I can't think of a single science fiction story, or future science "documentary" like the National Geographic Mars series, even the hardest of hard science fiction, that has the astronauts spending hours preparing for an EVA, or that explains why it is that they don't have to do that any more. Anyone reading this know of anything like that?

Much safer

It's much safer too. You have no risk from solar storms which can lead to high doses of radiation. At worst you have to take off your VR goggles, and go into a storm shelter in your spaceship, not rush back to your habitat as fast as you can from some remote location, to get out of the storm in time. There is no risk of falling over and damaging your spacesuit. Dust storms aren't a problem either, not for you, though they may delay operations for the robots on the surface. You are orbiting in comfort far above such things. Also when you need to take a break, to have your lunch or sleep, or whatever, no problem. Just leave your avatar on the surface, wherever it is, or leave it doing some routine task, and take it up again where you left off after your lunch or tea break or snooze.

This is an artist's impression from ESA showing a telerobotically operated rover on Mars

The ESA are currently testing rovers controlled from orbit - which will eventually use binocular vision and haptic feedback. in a program called IMPACT. This is their artist's rendering of such a rover on Mars. Tim Peake's recent test of controlling another rover on Earth from the ISS was part of this program. Previously, Andreas Morgeson drove the rover shown in this rendering, on Earth from the ISS, using binocular vision and haptic feedback to remove and plug a pin into a small hole with a tolerance of 0.15 mm, less than a sixth of a millimeter. He did this successfully. You can watch up a video of the actual event here.

Robotic and telerobotic drilling versus human drilling

Yes, it's true that drilling is likely to be especially important for the search for past life. It may also be important for present day life that may live deep underground in geological hot spots, or kilometers below the surface, or beneath ice sheets. However, in Mars conditions, robots can drill as easily as humans in spacesuits, and probably more so. The Apollo astronauts had a lot of trouble drilling by hand by just two meters, sometimes falling over. So humans have problems too. Here is a video of the Apollo 17 astronauts trying to drill into the lunar surface by hand.

(click to watch on YouTube)

If you want a deeper hole, you can't use water as a lubricant on Mars, so you can't use a conventional drilling rig. There are several ways to do it, but the best technology for deep drilling is is likely to be the robotic self hammering mole, which has the potential to far below the ten meters depth needed to find organics not degraded by cosmic radiation and solar storms. These moles may eventually drill for tens and hundreds of meters, even for kilometers in Mars conditions at ten to twenty meters a day.

These moles don't need an astronaut to operate them. ExoMars will be able to drill down two meters, using a different approach. The Insight lander was going to drill three to five meters using a robotic mole. The regolith thickness varies, here is an estimate for the site of the Insight lander - they estimate that ~90% of the Insight landing region is covered by a regolith that is at least ~2 to 3 meters thick. ... and that the regolith is 5 to 6 m thick over ~50% of the region. Honeybee robotic are working on a drill that will be able to drill through gypsum and hard ice to a depth of hundreds of meters. They also say that their inchworm mole will be capable of drilling up to tens of kilometers through soil, ice and rock without need for a tether and then return to the surface.

Drilling is very slow at present - it takes a week on Curiosity to just drill a small sample and analyse it. However that is mainly because it has such a huge amount of latency. Not just minutes, as you'd think from the light speed delay to Mars from Earth. The team send commands to Curiosity typically the next day after they get the images back. That's because they only get the opportunity to communicate with Curiosity around once a day, and that's because they don't have a dedicated communications satellite in orbit around Mars. They can use direct communication, but the bandwidth is very low that way. And they get only occasional opportunities to get data back by relaying through the orbiters around Mars which also have to send back their own data. At present Curiosity communicates for about eight minutes, per sol, transmitting around 200 - 250 megabytes during that window of opportunity.

Also it's not really fair to compare that one week with the time it takes to actually drill out a sample on Earth because it also includes the time needed to take it back to the laboratory and analyse it, and all that is done in situ on Mars, which is a time saver. So it's not as bad as it might seem, but it could be done a lot faster via telerobotics. For more on this see this presentation by Marc Boucher (slides and audio).

Mobility for humans and robots

Our rovers travel very slowly on Mars but that's mainly because of the turn around time of 24 hours between getting the images and sending the next day's instructions, and limited autonomy. The Apollo lunar roving vehicle had a rated top speed of 8 km / hour (though it could go faster), weighed 210 kg, for the entire vehicle, and had a range of 92 km, nearly twice the total distance Opportunity covered in ten years. So it's not lack of power that limits our rovers - they could go much faster, and further, if there was the need to design them to do so.

We can use the same methods to power robots on the surface as you can use for human driven rovers. So, for instance, we can use Zubrin's ideas to generate fuel in situ using hydrogen feedstock from Earth. Or we can use solar power and batteries, adapting the Mars One idea to spread a large area of thin film solar photovoltaics over the Mars surface for power. Perhaps we could use batteries with enough charge to last for a few hours at 8 km / hour, as for the lunar rover, and then when the charge runs out, then it leaves the battery to charge up, and uses power direct from its own solar panels until it is ready to do another tens of kilometers long journey again.

In this way, the methods designed for human missions to the surface can be used for our robots also, so that they can travel faster, and explore more in each day. The robots don't have to go back to a base, are not limited by the amount of oxygen and food they can take with them, can travel right up to habitable regions if sufficiently sterilized. Humans can start telerobotic control within seconds, in a shirt sleeve environment, "teleport" from one rover to another from orbit. And of course when not controlled by humans in orbit, they can be controlled by teams of scientists on Earth as before. So it's not at all clear even that the robots are less mobile than humans. Once the technology is well developed they might even be more mobile.

Need for adventurous experiments in life support and artificial gravity in LEO first

Before we start adventurous human exploration further afield, I think that just as with the early Gemini missions, we need to start being adventurous closer to Earth first. I think that we need to start working on human factors research. And there, I don't mean a re-run of the ISS. The ISS is not really a human factors research facility.It has only limited objectives for humans, to work on effects of zero gravity on humans and ways to ameliorate those effects. For life support the top priority there is mechanical life support based on splitting water into hydrogen and oxygen to get oxygen for them to breathe, combined with scrubbing by mechanical methods of carbon dioxide and various poisonous gases that can build up in a human habitat.

As a result of medical research onboard the ISS, we know a lot about how we can support humans in space, at a cost of a billion dollars per astronaut per year, and several progress missions to the ISS every year with supplies. We know about how to reduce the effects of zero gravity on the human body through two hours of exercise every day, to between 0.5% and 1% of bone loss every month. By comparison, older people, especially post menopausal women, who experience accelerated bone loss, will lose perhaps 1% of bone mass per year .

Though this is bone loss only from load bearing bones, and although it seems that most of it is recovered within a few years of return to Earth, even this reduced level of bone loss doesn't seem promising for very long term multi-year missions. It could be deadly dangerous indeed. If an astronaut fractures a hip in a multi-year deep space mission, far from help on Earth, due to fragile hip bones, it is unlikely that it would be possible to save his or her life.

There are many other issues with zero gravity. As soon as an astronaut enters zero gravity then their blood rushes to their head causing swelling, which they all experience. The heart rate goes up (you might expect it to go down, but it increases). The blood count goes down. So the heart is rapidly sending thinner blood through your circulation system. There are many other physiological changes which leads to them having reduced appetite, and less able to tolerate some medicines, which may have to be taken intravenously. Also, when they exercise, without gravity, there is no heat convection to take the heat away. The warm air wraps around their body like a blanket, so they sweat a lot more, which itself causes problems, especially since they have to exercise for two hours every day. Their eyes also get affected by the change of gravity which quite often leads to vision changes by the end of the mission. This is just part of a long list of changes to the body in zero g. All the research on human factors in the ISS is to do with finding ways to reduce these effects as much as possible. They have found no way to prevent these things from happening. All they can do is to ameliorate them. We have not yet had anyone spend two years in zero gravity.

Until we have multi-year missions close to Earth and ones that require no resupply from Earth for life support throughout the mission - how can we even start to consider deep space missions that would last that long, but with the crew at the distance of Venus or Mars or even Jupiter? It seems premature even to consider multi-year missions to the EVA village on the Moon until we have that sorted out in LEO. We could do it but the costs would be immense, and our astronauts would continue to be harmed physiologically and require years of adjustment back on Earth before their bones return to their original strength. Based on our experiences on the ISS we can do six month or one year missions, and we can perhaps do two year missions, but that would be ten months over the longest time ever spent in space continuously (fourteen months by Valeri Polyakov). As for three year or longer more missions, nobody can say from experience what the effects of so much continuous zero gravity would be, as it has never been attempted. NASA's current plan is to do a one year mission to their "Deep Space Gateway" near the Moon at the end of the 2020s. Their next stage is a spacecraft that can fly for 1000 days. without resupply. They can do that with ISS style life support if they take plenty of provisions, but the zero gravity issue seems a major one. Would the crew be healthy after 1000 days of zero g, and loss of at least 16% of bone mass (if they achieve the lowest bone loss rate of 0.5% per month)? And it's not very scaleable if they want to do later missions further afield, say, to Jupiter, without first cracking longer duration life support.

Astronauts don't get nauseous when spun rapidly in zero g - so could a device as simple as a spinning hammock be all that is needed to keep us healthy in space?

There may be a very simple way to deal with this. Astronauts can tolerate spinning motions in space much more than on Earth. Indeed they don't seem to be affected at all, or hardly at all, up to the fastest spin rate tested of 30 rpm, though the tests have only been for minutes at a time so far.

Many animals don't experience nausea when spinning at all, for instance rats don't. Humans do, but in space, this reaction seems to go away. This is something scientists first discovered with the Skylab astronauts. They could tolerate tumbling motions that would make them feel nauseous both before and after the mission. I have no idea why this wasn't followed up vigorously at the time. This is open research, so we don't know where it is headed. But we should surely at least try small scale centrifuges at some point. It could make such a huge difference to the health of our astronauts if it worked. The MIT researchers recommended it in this paper.

"In order to truly address the operational aspects of short-radius AG, a centrifuge must be made available on orbit. It's time to start truly answering the questions of "how long", "how strong", "how often", and "under what limitations" artificial gravity can be provided by a short radius device."

Artificial gravity was a priority for the ISS up until the loss of Columbia in 2003, first in NASA Ames, then later on the project was passed on to the Japanese space agency, then called NASDA, now called JAXA, who built a Centrifuge Accommodations Module which however never flew because the Space Shuttle was needed to get it into orbit. See page 55 of this paper. Then in 2010 there were proposals to send a centrifuge to the ISS, but it never happened.

Perhaps short arm centrifuges turn out to be a vital capability for human health for deep space missions, or perhaps it's the key to health for lunar habitats, or maybe it makes exercise in space much more effective, or maybe it just makes it more comfortable for astronauts in zero g to eat their meals or use a toilet with brief periods of artificial gravity. Perhaps it is only useful for plants. As basic research, we don't know where it is going until we try it.

However the little data we do have so far is very promising. It seems that we can tolerate faster spins in zero g than on Earth. Here is Tim Peake spinning at about 60 rpm in the ISS. for a couple of minutes, no nausea, only momentary dizziness when he stops.

(Click to watch on YouTube)

He says he is pretty sure he couldn't tolerate that on Earth. So anecdotally it suggests that we can tolerate very high spin rates in zero g. Taking the radius as 0.25 meters at a guess, his head and feet will be both under full g, his torso around zero g as he spins (you can use the online SpinCalc for calculations like this). Could he spin like this indefinitely? If so, it's very promising I think for the use of a short arm centrifuge to counteract health issues of humans in zero g.

He says that it's his vestibular system that's deactivated, but the Skylab researchers concluded that it's probably the otoliths that sense linear acceleration that are de-activated. It's much the same effect if the dizziness and nausea is due to conflicts between the two.

So first, background information. Our vestibular system is a system of canals in our ear that help us to tell whether we are spinning or stationary. For some reason spinning motions make many humans nauseous. Rats don't get nauseous when they spin so it is to do with human physiology. On page 95 of Packing for Mars, by Mary Roach she mentions that NASA Ames researcher Bill Toscano has a defective vestibular system. He only realised this when they put him on the spinning chair and he experienced no nauseous effects at all from the spinning. So, he at least, could spend 24/7 at 30 rpm for full gravity with no ill effects. The same is also true for some deaf people.

But of course we don't want our space stations to be only usable by some deaf people and others with a defective vestibular system. The vestibular system is not the whole story though. If it was, then conditions while spinning in space would be identical to those on the Earth. But to complicate things we also have the otoliths.

The otoliths are small particles of calcium carbonate in the viscous fluid of the inner ear. They sense linear accelerations, as distinct from the vestibular system which senses spinning motions. So technically, no, your vestibular system doesn't "switch off" in orbit. You can still sense whether you are spinning, but for some reason you don't get sick. That seems to be because of the otoliths.

This is something we can say pretty much for certain as a result of Skylab tests. So now I'll summarize some of the results from Chapter 11, Experiment M131. Human Vestibular Function in Biomedical results from Skylab.

All of this was done to study space sickness, and not to study artificial gravity. Still, it's the only real experimental data we have on artificial gravity too. In the rotating litter char they tested the astronauts with both back and forth movements and with spinning motions but only briefly for a few minutes at a time, long enough for the astronauts to do 30 - 60 head movements. Many of the astronauts experienced normal zero g space sickness for the first few days of the mission - so these experiments were done after those symptoms dissipated.

The rotating litter chair used for tests of effects of spinning and back and forth movements on the astronauts in Skylab. It's purpose was to try to understand space sickness, but it also gave our only data so far on effects of spinning motions in space on humans fast enough to generate artificial gravity. There haven't been any follow up experiments on the ISS or MIR or the Chinese space station. So, so far, this is the only data we have on effects of artificial gravity on humans in space..

First the astronauts observations, some quotes from the report, all from (Sec.2,Ch.11)- the M II A end point here is defined as "moderate malaise":

"Preflight, on three widely separated occasions, the M II A endpoint was consistently elicited after 30 to 60 head movements while those astronauts were being rotated at 12.5 r/min (Scientist Pilot) or 15 r/min (Pilot). When rotation tests were carried out in the workshop, both of these astronauts were virtually symptom free; their minimal responses, which were transient, did not even qualify for a score of one point. This was true even when the angular velocities were increased (in two steps) to 30 r/min." (Skylab 2)

It is also noteworthy that both the Commander and Scientist Pilot reported that while engaged in spinning rapidly about their long axes or "running" around the inside of the workshop, they experienced immediate reflex vestibular side effects, mainly "false sensations" of rotation. Based on past experience, both astronauts expected that motion sickness would follow the reflex effects and were surprised by their immunity." (Skylab 2)

- Skylab 2

"Under experimental conditions in the workshop the virtual failure to elicit symptoms of motion sickness in any of the five astronauts who were exposed to a stressful type of accelerative stimuli in a rotating chair (on or after mission day 8) implies that, under the stimulus conditions, susceptibility was lower aloft than on the ground, where symptoms were elicited preflight and postflight. The amount of this decrease in susceptibility could not be measured because the "ceiling" on the test (30 r/min) was so quickly reached." (conclusion)

- conclusions

So - they tested them up to 30 rpm with no symptoms and couldn't test them at any higher spin rate. They were symptom free in space.

Interestingly the lack of susceptibility to nausea actually persisted for a day or two after the flight. The commander of skylab 3 did experience a "vague malaise" in one experiment at 30 rpm, on mission day 52 which persisted for around 30 minutes, but it was not typical of acute motion sickness and so might have had other causes.

"The Commander was tested in the rotating litter chair on two widely separated occasions preflight and demonstrated similar susceptibility levels each time. On mission days 26 and 41 he was symptom free when rotated clockwise, respectively, at 20 and 30 r/min. On mission day 52 he was rotated counterclockwise at 30 r/min and experienced what he described as a slight vague "malaise" that persisted for approximately 30 minutes following the test. The question arises whether secondary etiological factors accounted for both the appearance and nature of this symptom, which is not typical of acute motion sickness, or whether the astronaut was not quite adapted to counterclockwise rotation. Postflight, the Commander was symptom free on the day after recovery when he executed head movements with the rotating litter chair stationary and on the second day postflight when it was rotating clock-wise at 15 r/min. On the fifth day postflight an endpoint was reached that approximated his pre-flight susceptibility level."

(etiological here just means"causal")

The data in their tables is very striking. Here are the results for Skylab 4 for instance.

There, MIIA is a level of mild malaise. During tests at 30 rpm then the astronauts experienced no symptoms at all in zero g, in experiments that evoked symptoms of mild malaise both before and after. They don't say how long the experiments were in minutes, as the results are expressed in terms of the numbers of head movements made.

So why did this happen? Our otoliths are separate from the vestibular system. Instead of sensing turning motions, they sense linear acceleration. In any spin on Earth you have these two things at once - the spin sensed by the vestibular system, and the linear acceleration due to gravity along the axis sensed by the otolith system. They concluded that the reason the astronauts did not get nauseous while spinning was because the otolith system was abnormally stimulated in space and had almost no influence on the vestibular canals. You might think that would make the astronauts more sick, but actually it turns out that the otolith system is part of what makes us feel nauseous and with it almost completely disabled in zero g, we no longer experience nausea in conditions that would make us nauseous in space. They conclude this by a process of elimination, since the vestibular system is stimulated in the same way but the otolith system is not, so it's the only thing that has changed in zero g.

So, let's look at this in a bit more detail For those interested in the techy details I'll include quotes from the report, but I'll explain them in less technical language, so you can also just skip the quotes and go to the non techy summary after each one. So first, on, the otoliths

"The rotating litter chair was used in the stationary as well as the rotating mode. In the stationary mode when head movements were executed aloft, the canals were stimulated in the same way as on the ground, but the otolith organs were stimulated in an abnormal manner because the impulse linear accelerations generated were not combined with a gravity vector as they would have been on the ground. These impulse linear accelerations were transient but well above threshold for stimulation of the otolith receptors. When the rotating litter chair was rotating, the intensity of the stimuli generated by head movement was a function of the rotational velocity, and although the angular and crosscoupled angular accelerations stimulating the semicircular canals aloft were the same as on the ground, the impulse and Coriolis accelerative forces generated aloft were not combined with a gravitational vector. These forces, nevertheless, were substantial at all levels of angular velocity used, and at 30 r/min the centripetal force was, respectively, 0.3 g and 0.6 g at radii of 1 and 2 feet."

So, the canals were stimulated exactly as on the ground but the otoliths were stimulated in an abnormal manner in zero g. The main difference in space is that there was no gravity acting along the spin axis towards what on Earth would be the ground - this is what they mean by the "gravitational vector"

..."Loss of the g-load would affect the "modulating influence" of the otolithic system. If the otolithic influence was inhibitory the responses elicited by stimulation of the canals are said to be "exaggerated" (ref. 30). The observations bearing on this point in parabolic flight, however, indicated reduced responses to canalicular stimulation (refs. 31, 32, 33) during the weight-less phase." (page 58)

So - the "moderating influence" of the otolithic system is different in zero g because of the abnormal way it is stimulated. If in normal use the otolithic system had a "modulating influence" then you'd expect this to make things worse. But instead it seems that the effect of disengaging the otolith in this way actually lead to much less motion sickness while spinning, not more of it.

..."The difference in susceptibility between workshop and terrestrial conditions is readily traced to gravireceptors (mainly in the otolith organs; touch, pressure, and kinesthetic receptor systems possibly contributing) for the reason that stimulation of the canals was the same aloft as on the ground, and visual inputs were always excluded. If it is assumed that the otolith system is responsible, then the absence of stimulation to the otolithic receptors due to gravity must have a greater influence (tending to reduce the vestibular disturbance) than the disturbing influences of the transient centrifugal linear and Coriolis accelerations generated when head and trunk movements were executed in the rotating litter chair. Al-though these transient accelerative forces, as pointed out in the section on Procedure, are substantial their effectiveness as stimuli are virtually unknown. The otolithic zonal membrane has considerable mass, and transient accelerations lasting fractions of a second might have little or no effect. The absence of gravity, causing what has been termed "physiological deafferentation" of the otolith receptor system, would be expected to reduce not only the indirect modulating influence of the otolithic system on the canalicular system but also its opportunity to interact directly with this system"

Because the stimulation of the canals is the same in space and on the ground, then by a process of elimination, it must be the difference in stimulation of the otoliths that made the astronauts less susceptible to motion sickness. The membrane around the otolith has a lot of mass which reduces the effect of sudden short accelerations. Because of the absence of gravity,which disables most of the influence of the otolith system, what they call "physiological deafferentation", then the otoliths not only have less effect on the vestibular canals, they also are less able to interact with them.

I suggest it should be a top priority to research this further, and find out what is going on, test it for longer periods of time, test it for higher spin rates. It shouldn't just be something that astronauts do out of curiosity and fun. Someone should be measuring and testing them, and finding out why they have such high spin tolerance in space and what its limits are (if any)!

And then based on those results, the next step would be to see if it can be used to generate artificial gravity and perhaps prevent deterioration of health in space. We need to find out what effects these spin motions have on health. I think this seems very promising for the possibility of short arm centrifuges dealing with issues of human health in zero g.

From the data so far, it seems almost certain that astronauts could withstand a few minutes of Artificial Gravity (AG) in a non zero g centrifugal toilet for instance. Zero gravity toilets are tricky to use. Also what about while eating, another thing that becomes much harder in zero g - but if they don't get nauseous then why not eat while spinning too? And what about sleeping? Could they tolerate eight hours of AG while asleep? And during exercise? And if they had AG during all those activities, then what effect would that have on their bodies, would they avoid most of the effects of zero g? Might it actually be healthier even to change between zero and AG back and forth every day? What level of AG is needed etc?

There are bound to be some effects. The cells of plants change gene expression of numerous genes within two minutes of the centrifuge starting. To check effects of artificial gravity on plants they have to preserve the cells within minutes of stopping the centrifuge - or actually in the centrifuge. They are also extraordinarily sensitive to minute levels of gravity. In an experiment with lentil seedlings, they responded to two thousandths of a g, and interpolation suggests they are sensitive to levels of less than a ten thousandth of a g.

Humans have many physiological changes right away when they enter zero g. As with the plant cells, these effects are almost instantaneous. You get them even in hyperbolic zero g test flights. Their average blood pressure in the arteries (mean arterial pressure) decreases right away to below what it is even when lying down (though later it increases in a long duration zero g flight, then adjusts in the opposite direction with higher heart rate than normal on return to Earth for up to 15 days).

So, there are bound to be easily measurable changes in the astronauts' physiology immediately when they start spinning in zero g. Tim Peake's body must have responded in numerous ways to those spinning motions. But we don't seem to have any experimental data on this yet.

These "short arm centrifuges" don't need to be big heavy machines as they are on Earth. Here they have to be strong enough to hold up the weight of a human along the spin axis ("downwards" on Earth) as well as exerting a similar force towards the spin axis for the artificial gravity. But in space, they can be as simple as a hammock in structure, because they only need to hold the weight of a human in one direction only - away from the spin axis. They don't need to be rigid structures either. Like a small version of the fairground swing rides - except that with no gravity along the rotation axis, the swings would just hang out horizontally.

Sketch to show two possible orientations for a spinning hammock inside a large space station module. To prevent this from spinning up the station, then there'd be a counter rotating weight automatically spun in the opposite direction to the astronaut, perhaps attached to the "floor", so that the net angular momentum is always zero. You could just leave it to the ISS's own momentum exchange gyroscopes to take up the spinning motion,but by adding a counter rotating weight, then the experiment would compensate for itself.

The motors would not need to be powerful in zero g. It's like spinning a cycle wheel - easy to spin up, and you could stop it just by putting out your hand. Basically it is like the other astronaut spinning Tim Peake around by hand, except that you motorize it.

For a very small one meter diameter centrifuge like this, you can achieve full g at 30 rpm and with the astronauts moving at only a little over 3 meters per second so it is very safe. That's around seven miles per hour - faster than a jog, but easy running speed so it's not that fast (a little faster than the average speed for the London marathon). For more about it see my Astronauts don't get nauseous when spun rapidly in zero g - so could a device as simple as a spinning hammock be all that is needed to keep us healthy in space?

Here is a 2011 idea for a 1.4 meter radius centrifuge to be located in the permanent multipurpose module:

Sketch of the AGREE centrifuge for the ISS. From page 15 of Design and Validation of a Compact Radius Centrifuge Artificial Gravity Test Platform. It would have replaced the four racks at the end of the Permanent Multipurpose Module. Astronauts would cycle in a seated position. This is one exercise excellent for health which you can do with an extremely compact radius centrifuge like this. Chris Trigg concludes: "Given the compact design, subject positioning, available sensors, tested accuracies, and validated operations, the MIT Compact Radius Centrifuge represents one of the most unique yet realistic centrifuges currently in available for artificial gravity research. It is hoped that through these future studies the MIT CRC will provide a better understanding of the effects and capabilities of an inflight-centrifuge, and perhaps contribute in some small way to progressing towards the inevitable trip to Mars. "

a sketch for a human powered artificial gravity in the ISS .

Depending on the results, you could do longer missions later on. But ground experiments can't even simulate AG for a few minutes in parabolic flight. To simulate it for hours or a couple of days in space would be a huge step forward for the field.

Artificial gravity was a priority for the ISS up until the loss of Columbia in 2003, first in NASA Ames, then later on the project was passed on to the Japanese space agency, then called NASDA, now called JAXA, who built a Centrifuge Accommodations Module which however never flew because the Space Shuttle was needed to get it into orbit. See page 55 of this paper. Then in 2010 there were proposals to send a centrifuge to the ISS, but it never happened.

So all of that, we could start testing right away just asking astronauts to spin each other in the ISS and make measurements. It would take very little by way of extra apparatus - but need to re-organize the interior somehow to find space to put it - to add in a proper centrifuge inside the ISS to test it. So - I think there's a lot of potential for artificial gravity even inside the ISS and for sure inside a habitat as large as, say, the Bigelow B330.

With NASA, something like this, even asking astronauts to spin each other and make measurements of their heart rates etc, and ask them if they get nauseous - that has to be a decision that is submitted, goes through the proper channels, gets approved, and then takes up some of the very expensive time of the ISS crew.

For a commercial space company - if they are sending humans into space and are paid for it, they can just go ahead and do it on their own initiative and don't need to ask anyone for permission to do the experiment, so long as it is safe. So I rather expect that this will happen with commercial flight first, because they have obvious reasons to want to do it for the comfort of their tourists, and because they can take the initiative more easily. That is, unless such experiments somehow become a top priority for NASA, ESA etc, but at present there doesn't seem much sign of that happening. They are very much focused on zero gravity health issues as their top priority for humans in space right now.

Any of the larger Bigelow modules if placed in LEO would have plenty of space for an artificial gravity centrifuge.

Cutaway image of the much larger B330 credit Bigelow aerospace. There's plenty of space for a large centrifuge here.

Indeed, though I hope the main aerospace agencies will do these experiments in the near future as they move towards deep space missions and the return to the Moon, if they don't, I wouldn't be surprised if private entrepreneurs explore it instead. For instance artificial gravity for toilet facilities or for eating and drinking could make space hotels much more convenient for tourists, so surely they will explore that if we have space hotels in the future. And if artificial gravity helps to keep the tourists healthy on return to Earth, able to stand immediately on return to Earth for instance, again that's a strong incentive for artificial gravity sleeping or exercise artificial gravity in space for medium duration tourist visits. Also artificial gravity could help prevent space sickness in hotels for tourists too.

NASA has recently been tasked by Congress with spending at least $55 million by 2018 on a "habitation augmentation module" that could be used in cislunar space and eventually for journeys to Mars. So I wonder if there is any possibility of a centrifuge for that?

 

If it doesn't happen before, I expect that companies that build "hotels" in LEO will experiment with AG toilets at least. It's just too inconvenient to have to use a zero g toilet. It's never been tested of course. But Tim Peake's experience together with the anecdotal and experimental results from Skylab I think strongly suggests it would work just fine.

To test to see if this would work, any astronaut on the ISS could get another crew member to spin them for, say, fifteen minutes as part of their recreational time there, say at 60 rpm in zero g. That then would make it a "go" for an AG toilet. It would probably work even in a small space, with head and feet at opposite sides of the spin axis. The tourist companies would be strongly motivated to find a solution here as zero gravity toilets are cumbersome and messy to use.

Spaceship tethered to final stage and then spun up - experiment first proposed for a Voskhod in 1965-6 but never flown - could this solve all the zero g health problems?

There is another way also that we can explore artificial gravity in space. This is another thing we could test, pretty much right away. We could have the experiment running probably within a year if there was the political will to do it. This is Joe Carroll's idea to develop an artificial gravity research gravity in LEO to let us test multiple levels of gravity at multiple spin rates.t can start off as simple as just one space module with a counterweight. And indeed the first experiments are even simpler than that. He has been advocating it for years. He's an expert on space tethers and several of his tethers have flown in space.

The idea actually dates back to the 1960s. We now know that Sergey Korolev had a plan to tether a Voskhod with its spent final stage which he put forward in 1965-6. It was going to be a 20 day flight to upstage the Americans. It would have included a pilot, and a physician and the artificial gravity experiments would have lasted for 3-4 days during the flight. He died unexpectedly in January 1966 and the mission was postponed to February 1966 then cancelled outright. So we came very close to doing this experiment way back in 1966. (See page 17 of this thesis).

Joe Caroll's idea similarly is to start with a tether spin experiment with a module tethered to its final stage, as this goes into orbit anyway. He keeps it attached to the spaceship by a tether. Then he spins it up with a series of thrusts during the perigee, when closest to Earth to spin it up (spinning in the plane of the orbit). Those thrusts boosts its apogee while at the same time setting the combination spinning for AG. After staying in that elliptical orbit for a while to test AG (which has much less drag than a final stage normally has), then he cuts the tether at apogee, so circularizing the orbit of the spaceship at a higher altitude and meanwhile sending the final stage down to a targeted re-entry in the southern Pacific (at present its re-entry is uncontrolled). It's a really neat idea!

The nice thing is that all the delta v put into spinning up the assembly get released at the end of the experiment. It uses no extra fuel unless the tether is severed by space debris, which from his experience in improving tether design is now a very low probability event. The Soyuz always carries extra fuel in event that more is needed than expected during the launch. So he would use this, and only use it if it hasn't been used up during the launch (usual situation|). As a result, the Soyuz would still get to the ISS even in that worst case scenario where the tether is cut at the worst possible time by space debris. Similarly, they could also cut the tether at any time in an emergency and simply continue with the normal approach to the ISS. It would also be done in such a way that even if the tether is cut by orbital debris, there is no chance of it going near the ISS without an extra burn. So there are no safety issues.

It is worked out in detail and could be done right away, as quickly as the Gemini tether mission was put together, for a near future crew mission to the ISS. They'd use the longer phasing approach of several days, so you could test several days of artificial gravity. It could be done with the Soyuz TMA or any other crewed mission to the ISS. The cost wouldn't be much as human spaceflight experiments go, just to add a tether to a Soyuz TMA mission that is going to the ISS anyway.

Though this would be a short experiment, there are many things you can test in a short mission. It would of course test things such as tether dynamics and tether spin up. Also radio communications during tether spins, and orientation of the panels to achieve adequate solar power throughout the orbit.

Also, in particular, it would give us the first real data on spin tolerances of humans in artificial gravity long term. It's a different experiment from the short arm centrifuges in the ISS, because these can be much longer tethers, far too long to fit inside the ISS, so with slower spin rates

This video shows a 600 meter tether at 1 rpm joining a Soyuz TMA to its final stage to achieve lunar gravity. Even the most highly susceptible people have no problems with 1 rpm in rotating room experiments on the Earth long term. So probably this would be fine for everyone in space also - that is if the Earth experiments are a reasonable guideline, which nobody knows of course (that's why we need to do the experiment). There are some indications that in space, with spins around a horizontal axis (above your head) and no gravity pulling sideways along the rotation axis, that we can tolerate spins better than on Earth. Though the data is very limited so far.

(click to watch on YouTube)

All these videos are done in Orbiter, a remarkable space mission simulator by Dr. Martin Schweiger with lots of add ons contributed by enthusiasts.

Thanks to Gattispilot, for making the tethers, and for techy advice about how to attach everything together.

Note that the video shows an "eyeballs out" configuration. The tests would only go from low g up to full g, but still, this is not the most comfortable orientation for the crew. Joe Carroll's plan is for an "eyeballs in" configuration. It's just that for techy reasons I found it much easier to position the Soyuz in the simulator in this "eyeballs out" orientation . The tether would be brighter than this, and you may notice a cube at the center of gravity of the tether - this is just to indicate where the center of gravity is and would not be there in reality.

NB, there's a detail to be sorted out here - do you deploy the solar panels before or after the tether experiment? If deploy before the experiment, their supports need to be strong enough to withstand the artificial gravity - this is probably easier if the solar arrays are orientated radially to the axis of rotation during the tether spin. This is best if possible as then you have no power limitations for the experiment.

If you leave deployment of the arrays until later - it is a case of how long you can manage without external power for the experiment. You'd be dependent on the storage batteries for power. Early Soyuz spacecraft before the solar power systems had 2 days of battery power. Not sure about the TMA.

Based on these very early tether spin experiments, we can answer basic questions such as, can humans tolerate spinning for two days, and if so what tether length and spin rate is tolerated? (The experiment is designed so it is easy to abort from it at any time - you just cut the tether, and then continue to the ISS). And what are the immediate effects on the human body of artificial gravity? What is the gravity prescription for health (what g level, how many hours a day or do we need it full time) and how easy is it to apply the desired levels using AG?

Another experiment you could do in the near future is a tether experiment launched from the ISS. The crew would take the crew module to the ISS with the final stage still attached. Then to do an experiment, they leave the ISS, fly far enough away from the ISS so that a severed tether won't endanger the station, spin up, do the experiment for several days, then spin down and return to the ISS. This idea was suggested by Tim Cole in 2012.

Based on those preliminary results from the Soyuz TMA, or any other crewed capsule that goes to orbit with a third stage which you can use as a counterweight, you'd work towards designing a larger AG research lab in the future for longer duration studies. It might be based around using the newer modules from the ISS when it is decommissioned, for a hub for spacecraft to dock to and for zero g research, and then tethered habitats for the crew going round it. If it gets more elaborate, perhaps it would also use spent final stages, fitted out in advance as "wet workshops" like the early ideas for Skylab.

This then would create a small facility in orbit. It doesn't need to be a big hundred billion dollar facility like the ISS, just a small space station to start with, which can also be a basis for a staging post in LEO later on. It could also be a facility for research into closed systems, growing plants and so on. It would have a science component of course, like the ISS, but the main objective would be human factors. It would be forward looking, helping us to find out what role humans can play in space in the future. Which of course would have science benefits in turn. Once we know more about what humans can do and how best to support them, we can then send them on science expeditions further and further afield into our solar system.

It could have a zero gravity module attached to the hub. So, there's no reason why you can't combine zero gravity with artificial gravity in the same station.

It would start small, based on this idea that we are still experimenting, and are not yet very experienced in space travel. At this stage, I think we need to try out ideas, and lots of them, to see what works. This could lead to advances that we would never get if we proceed in a linear planned out way with some grand plan for the future, based only on the knowledge we have so far.

The same small space station in low Earth orbit would be an ideal place to test methods for life support for longer duration missions on the Moon or further afield. Again if it is in LEO you have easy access to it from Earth. Because it is a small space station, you could do this at the same time as human missions further afield, say, to the Moon. It could even be useful right away as a place for astronauts to go to first, to disembark from rockets launched from Earth and to transfer to spaceships traveling from LEO to the Mon.

It doesn't have to have a continual human presence. That is one of the things that makes the ISS so expensive. Make it largely automated and controlled from Earth, so that it can be left in orbit unoccupied for months on end in between experiments. If astronauts stay healthy in some level of artificial gravity, and if we can perfect biologically closed systems so that they produce all their own food and oxygen, then you could send them supplies only once a year, perhaps, or less, and then you could start to occupy it continuously. So then the costs would go right down, and typically at least some of the astronauts would stay up there for several years at a time. It is not a commitment to billions of dollars a year, but rather, you occupy it and use it as needed as you find out more.

If you can achieve this much, continuous presence of healthy humans in a small space station in LEO for years on end with no need for resupply from Earth - then you would get a lot of confidence and experience for long duration missions further afield, not just to Mars or Venus, perhaps eventually also to Jupiter's Callisto, and beyond. Of course meanwhile by then you also have longer duration stations on the Moon, with the ESA village, and eventually further afield too.

See also my Science 2.0 blog posts on this:

• Crew Tether Spin - With Final Stage - On Routine Mission To ISS - First Human Test Of Artificial Gravity?
• Ingenious Idea: Soyuz Crew in Tether Spin On Way to ISS - For Artificial Gravity - Almost No Extra Fuel -
• Could Spinning Hammocks Keep Astronauts Healthy in Zero g?
• Can Spinning Habitats Solve Zero g Problem? And Answer Low g Questions?
• Crew Tether Spin For Artificial Gravity On Way To ISS- Stunning New Videos- Space Show Webinar- Sunday
• Can Astronauts Spend Years In Space- And How Quickly Can They Recover On Return To Earth?

So, how would they grow their own food, and create their own oxygen? Well there's a natural starting point. You don't need to do it all at once. You can start by using algae.

Starting point - using 18 liters (so 18 kg) of spirulina algae per astronaut to supply all their oxygen - and some food

Historically the Russians started with green algae in 1965 in their successful BIOS-1 which sustained one human with oxygen using only 18 liters of algae and eight square meters of surface of the culture exposed to light. Why not start by growing algae in space to duplicate this approach? As it turns out, ESA are exploring just this very idea.

So, why not start with a hybrid mission, with some biological and some mechanical components. The ISS type system is the one most thoroughly tested to date, but it could be worth adding some easily testable biological systems. A simple form of redundancy would be to add algae to a mechanical system like the ISS. Algae are extremely low mass for the amount of recycling they do.

By the figures for Bios-1, a Russian experiment carried out in 1965, you require 18 liters of algal culture per person to scrub the atmosphere and provide all the oxygen they need. So around 18 kg per person, or just 54 kg for the entire crew of six. You also need containers, and solar collectors. It needs to have a surface of eight square meters exposed to light per person which you achieve by filling the container with light pipes to pipe light into its interior. So you also need light tubes to pipe light into the middle of the mixture. For the algae, they used chlorella which is a fresh water algae cultivated in Asia. It does a great job of producing oxygen. It's used as a food but needs processing to be edible with a tough cellulose wall.

This shows the chlorella cultivar used in the originally rather secretive Russian BIOS-1 experiment. Image from this paper I can't find much by way of details of its construction so far and how it worked. But the principle was simple - supply lots of light to Chlorella algae and it photosynthesizes, absorbs carbon dioxide and produces oxygen.

In the first experiment it was in a separate room, tended from outside, and supplied all the oxygen for a single volunteer. This recycled 20% of the air, water and food needed by a single human. By recycling water as well they got this up to 80-85% in 1968. But they couldn't produce all the food a human needs from algae (the balance of proteins, to carbohydrates is too high and there are issues with trace elements etc).

In BIOS-2 they put the cultivar inside the habitat, recycled other wastes as well, and produced some crops. In BIOS-3 they made the habitat larger, with a crew of 3, and changed to growing crops as their sole source of oxygen.

I can't find a total mass for the equipment as the BIOS-1 summary doesn't go into a great deal of detail, but it doesn't require much space to install it and it can't be that massive.

So how long does a mission have to be, for it to be worthwhile to include green algae as a way to reduce the amount of mass supplied to the mission? You are saving 0.84 kg of oxygen a day per astronaut over a system with no recycling (from table 2 of this paper). This makes, the time for payback of the mass of the algae mixture alone, through oxygen savings, 18 / 0.84 or 21 days. So, in terms of payload mass, there is no point in doing this for missions less than three weeks. The exact timescale for payback would depend on how much mass of extra equipment you need. For instance if it is roughly half in half algae and equipment, the payback period is 42 days.

When you take account of the solar collectors to collect sunlight from outside the spacecraft and pipe it in, and the light pipes that must pervade the solution, it is probably going to be a lot more mass. I don't know of any detailed figures for the mass of those, I've searched but can't find any figures. If anyone knows of any do let me know. But the payback time is surely going to be of the order of months rather than years.

Hamiwari sun tracking solar collector - the light is collected, focused and sent through fiber optics to the interior of the spacecraft or habitat, where it can be used as a light source for algae or growing crops, or to help keep it warm. Details of how this would work for spacecraft, see page 319 of Peter Eckart's book: Spacecraft Life Support and Biospherics. A spaceship might well use these to supply light for the algae and crops.

They could alternatively use LED lighting. This High Efficiency Full Spectrum LED Grow Light - uses 20 watts of power to illuminate 0.2 square meters. So that's 100 watts of supplied power needed per square meter, so those eight square meters would need around 800 watts per person to supply all their oxygen and scrub the carbon dioxide. Those are much lower power levels than for the Russian experiments with their 1960s technology of xenon lights.

This approach really scores long term, as a low mass system. It would only be a modest amount of mass compared to the payload of a three year mission to Mars orbit and back, say, and the algae give you a way to buy extra time in a situation where something goes wrong with the carbon dioxide scrubbing and oxygen recovery. Even if the mechanical life support system fails when you leave Earth, with the prospect of having to come back to Earth via Mars before you can repair it, even then, you have a chance that the algae could save the life of the crew.

I'd say it is more reliable too. There is very little by way of equipment to go wrong, and if anything happens to the algae, then so long as you have a few viable cells left, you can just grow a new batch. Try doing that with machines. Also, it would last indefinitely, with no mass supply per day needed. The algae grow on the carbon dioxide exhaled by the astronauts. All that is necessary is for the astronauts to have enough food to eat, taken along as provisions. So long as the astronauts eat well, they will produce more than enough carbon dioxide for the algae to grow, and so it will continue to produce more than enough oxygen, with no need for any other supplies.

This idea of using algae for oxygen is being explored on the ISS with occasional experiments there. It's part of the MELiSSA project.

(click to watch on YouTube)

They use Spirulina which is more nutritious than the Chlorella used in BIOS-3. It contains 60% protein and has all the amino acids though with less methionine, cysteine and lysine compared with meat and milk. It's a decent source of protein but has to be supplemented with vitamin B12. It's not such a good source of carbohydrates however, so though it can supply all the oxygen needed in a small space and with not much mass, and can provide some of the protein the crew need, it still can't supply all of their food.

Still, spirulina could replace part of the mechanical systems, or double up with it. Chlorella worked fine as a way to generate oxygen for BIOS-1 and the only reason they went on to crops later, was because they wanted to see if they could grow all the food as well. The spirulina algae used in modern algae recycling systems are also edible, though they can't provide a complete diet, so some of it would be used to supplement food.

Some worry about reliability of these biological systems. But they don't have soil borne pests, and as for airborne pests, only whatever you might have in the habitat (e.g. mold). The BIOS-3 system was very reliable. And as for algae, then if you get some disaster that kills nearly all the culture, then as long as you have a few cells left, then it will recover quickly. It's more reliable than a mechanical system in some ways - you don't need spare parts for the vital components, can just grow more of them in the presence of light and water.

So, algae look as if they are well worth taking with you even for short missions of a few months though I can't find exact figures. There is no point in terms of savings of payload mass of using algae for a mission of less than two months. The payback time, which will depend on the mass of the incubator and light collectors, and perhaps LED lights. It seems rather attractive even as an extra safety measure for a deep space mission with a relatively small mass of 54 kilograms for a crew of three for the algae mixture together with the mass of the infrastructure.

How long does an expedition have to be to make it worth growing crops in space - or indeed - any recycling at all, even of the ISS type?

The Apollo missions of course didn't do any oxygen recycling. They scrubbed the carbon dioxide and that was about it. So, you could copy that approach, and just send lots of mass with all the provisions the astronauts need. That might well be the best way for a short duration deep space mission. It is surely the best approach for a very short duration mission such as for Apollo.As missions get longer, it may then be worthwhile to use the ISS system and various other forms of recycling. So when does the break-even point come, where it's worth the extra mass to send a system with better recycling into space?

Maria Johansson worked it out for us in her thesis from 2006, which I summarize in Could Astronauts Get All Their Oxygen From Algae Or Plants? And Their Food Also?

The area you need to grow all the food for the astronauts is far less than you might expect from field agriculture which typically requires about an acre (4,000 square meters) per person.

The Russians needed only 30 square meters of growing area per crew in the BIOS-3 experiments in the early 1970s onwards. They did that using hydroponics, aeroponics and a "conveyer belt system" of rapidly growing crops that crop within a few weeks. The crops were grown on a culture conveyor with 2 to 10 plantings of different ages simultaneously.

"Wheat plants of various ages showing the "conveyor" approach that was used in the BIOS experiments, Young wheat plants are in the foreground, with more mature plants toward the back. The aisle between benches is narrow (to leave as much space as possible for the crops). The post, with some environmental sensors attached, further obstructs the aisle. Crew members planted various herbs and other special plants in the corner and next to the wall to the left, space that would otherwise be wasted." photo from here

They grew wheat, sedge-nut, beet, carrots, and other plants, for a total of ten crops. The aim of course is to grow a lot of food quickly in a small area. So they focused on plants that crop quickly, typically after about a month. They also used aeroponics (roots that are misted with water vapour together with the nutrients necessary for plant growth) and hydroponics (roots in solutions of nutrients).

With just 30 square meters growing area per person, they produced 95% of the daily requirement for oxygen, water, food etc (by weight), reducing the supplies needed per day for the crew of three from 13.01 kg to 0.5982 kg. That remaining 0.6 kg per day, around 5% of their daily mass requirements, consisted of animal products, salt for the humans, nutrients for plants and personal hygiene supplies. They could produce 45% of the food and nearly all the oxygen and water with only 13 square meters.

Maria Johansson did her calculations for a crew of four. She found that break even for BIOS-3 over a mission with no recycling would happen after 67 days and for the ISS type system at 76 days. So those forms of recycling are worth doing for missions of more than a couple of months or so. There isn't much in it for shorter flights and the ISS system of course is better tested in space. Where biological recycling such as BIOS-3 really shines is for long duration missions. It then works much better than the current ISS system. Using her figures for BIOS-3 then it uses 0.5 kg per day for four people, with a rather large startup mass of 6.25 tons. But that compared to 10.252 tons and 5.12 kg per day for the ISS system. If we had a fully developed BIOS-3 system, then at least by payload, it would seem to make sense to use it right away. These figures don't take into account the actual habitat growing area for BIOS-3 however.

The MELiSSA system is not in the picture for the shorter duration missions, with a startup mass of 15,711 kg for four people - it is a hydroponics system and the water takes up most of the mass at 8.89 tons, and much of the rest is taken by the centrifuges at 2.8 tons. But for longer duration missions it's only 1.4 kg per person.

Her figures come from before the the new approach of recovering the oxygen from exhaled carbon dioxide using the Sabatier reaction which reduces the daily mass requirement but increases the startup mass. It also omits some the mass required for resupply of parts, and spares. A more recent calculation for the ISS (or as a pdf) makes the startup mass (upper bound) 2,563 kg/person or 10,252 kg startup mass for a crew of four and supplies of 467.1 kg/person/year or 5.12 kg per day for a crew of four.

For a 730 day mission (three years approx) with no redundancy, and a crew of four, these figures make the total mass:

• 730×1.4+15,711 kg = 16.7 tons for MELiSSA
• 730×5.12 +10,252 kg = 14 tons for the ISS type system
• 730×0.5+9,320 kg = 9.7 tons for BIOS-3
• 730×140 kg = 102.2 tons with no recycling (using larger estimate of 140 kg / day for a crew of four)
• Or, 730×38.1 kg = 27.8 tons with no recycling (using lower estimate of 38.1 kg / day for a crew of four).

So, MELiSSA is most useful for very long duration missions. By these figures it would score over the ISS + Sabatier for n = (15,711-10,252) / (5.12 -1.4) or 1,467 days, or about four years. If the ISS had used MELLiSSA then by these figures it would have paid off the excess mass long ago.

This doesn't however take account of the need for multiple redundancy. But that's a bit hard to calculate. ISS needs triple redundancy of key systems. BIOS-3 - what do you need redundancy of? With MELiSSA then, you surely don't need redundancy of the water for instance, which is much of its mass. So I'm not sure how to take account of the need for redundancy unless someone does a detailed study taking that into account.

At any rate, for a three year mission, then the ISS system seems good enough - we could send astronauts around Mars using the ISS system, except for questions of reliability. It does go wrong quite often, so we'd need to be very sure of the reliability of the life support of our spaceship before sending it so far afield. . BIOS-3 would score over it, saving nearly three tons of payload, and it would seem to have less to go wrong, but it hasn't yet been tested in space. MELiSSA seems a good system for long duration missions of over four years, but again hasn't been tested in space. It is surely a high priority to test these systems for future use, especially if we have missions that are longer than three years. For a mission to Jupiter for instance, then BIOS-3 would clearly score highly over all the others in terms of mass.

Let's try a ten year mission

• 3,650×1.4+15,711 kg = 20.8 tons for MELiSSA
• 3,650×5.12 +10,252 kg = 28.9 tons for the ISS type system
• 3,650×0.5+9320 kg = 11.1 tons for BIOS-3
• 3,650×38.1 kg = 139 tons with no recycling (using lower estimate of 38.1 kg / day for a crew of four).

And a twenty year mission.

• 7300×1.4+15,711 kg = 25.9 tons for MELiSSA
• 7300×5.12 +10,252 kg = 47.6 tons for the ISS type system
• 7300×0.5+9320 kg = 13 tons for BIOS-3
• 7300×38.1 kg = 278 tons with no recycling (using lower estimate of 38.1 kg / day for a crew of four).

Extra mass per decade for each system (rounded to nearest tenth of a ton), and let's express in terms of the number of Progress rockets - with payload of 2.4 tons to the ISS:

• 3,650×1.4 = 5.1 tons for MELiSSA, or 2.125 Progresses
• 3,650×5.12 = 18.7 tons for the ISS type system, or 7.8 Progresses
• 3,650×0.5 = 1.8 tons for BIOS-3, or 0.75 of a Progresses
• 3,650×38.1 kg = 139 tons with no recycling (using lower estimate of 38.1 kg / day for a crew of four) or 58 Progresses

So after they have saved the overhead, then every ten years, the Bios-3 saves seven Progress flights, and Melissa saves more than five over the ISS system, for a crew of 4. While the idea of doing no recycling is out of the picture for missions this long.

The ten year mission is still feasible with the ISS system, though it requires more than twice the payload than for BIOS-3. The twenty year mission is much much more efficient with BIOS-3 and MELiSSA also does quite well but requires twice the payload for BIOS-3 while the ISS system requires nearly four times the payload for BIOS-3.

Biological closed systems research in LEO

We can also do biological closed systems research in LEO. See if we can duplicate the BIOS-3 results in space. The experiments in growing plants on the ISS seem to lack an overall direction and focus. I think it is more a case of "we have a facility in zero g, and astronauts in it, so let's see if we can grow food there for the astronauts?"

It's interesting research but it's not so clear that it will help with future deep space missions. Even a very slow spin can give you a hundredth of a g for the plants, which is enough to change the gene expression of every cell in the plant, turning many genes on or off. Within 2 minutes of starting a centrifuge in the ISS, the cells in the plants change gene expression of numerous genes. So when checking effects of various levels of gravity - they have to preserve the cells within minutes of stopping the centrifuge - or actually in the centrifuge. Plants are also sensitive to tiny amounts of gravity. For instance in an experiment with lentil seedlings, they responded to two thousandths of a g, and interpolation suggests they are sensitive to levels of less than a ten thousandth of a g.

In future interplanetary missions, we may spin the habitats and greenhouses with a counter balance using tethers to achieve whatever g we want for the plants. Or we might just spin the growing plants themselves in centrifuges.

A proposal from 2010 explored the idea of a "Farm module" which combined artificial gravity with fish and plants in space.

From 2010, this module idea has a rotating cylinder in the center if I understand right. The fish tanks and tomatoes are suspended from it, and then further out you have lettuces shown below and dwarf wheat above.

For the Moon we are particularly interested in how well plants grow at lunar levels of gravity. And also of course, how humans manage at lunar g. Again, there is very little data, just the few days of astronaut EVAs on the Moon, with 1960s and 1970s monitoring equipment. Also, there has been no attempt to achieve almost biologically closed systems in space, where nearly all the food and oxygen for humans comes from plants, even though it has been achieved on the Earth already by the Russians with BIOS-3.

For a lunar base, I think we need experiments in growing plants at lunar g. The obvious place to do that is on the Moon of course, but we can speed things up by testing this right away in LEO, testing long term adaptation of plants and humans in lunar gravity. We will need some kind of base in LEO anyway at some point, if we have frequent missions to the Moon.

All this could make a big difference in cost for long stay habitats such as the EVA village, for instance.

There's one other thing we can find out about on the Moon before venturing further afield. That is, how to deal with the vexing issue of trash in long term space missions?

Trash on the Moon - testing ground for planetary protection measures for a human base

How do you deal with the trash on the Moon? This is not an exciting or glamorous subject I know, but it is part of the reality of space travel. This is something that's easy to forget about, how much trash astronauts generate every year. We don't notice it so much on the ISS because it is easy for the astronauts to dump it, and it all burns up cleanly in the atmosphere. The Apollo crew didn't spend enough time on the Moon for it to be that noticeable.You may be surprised at how much trash the ISS generates. And what about footprints on the Moon, and rocket exhausts, and all the dust thrown up by spaceships as they land and take off (two tons of dust in the case of the Apollo misisons)?

Well, the Moon is also a good place to study the effects of humans and their wastes and trash on their surroundings in space. In such challenging conditions we are bound to have wastes of all sorts pile up around the base. Think how much refuse the Progress rockets return to Earth, to burn up in the atmosphere:

(click to watch on YouTube)

See all that trash which you can see filling up those white bags in this video? The ISS discards that much trash into our atmosphere every few months. Now imagine all of that piling up around a base on tiny Phobos or Deimos? 

The Moon would be a great place to see what happens to all that trash, and find out what are the best ways to deal with it before it becomes an issue further away. How much can we recycle? Or do we just bury it all? Some of it is "smelly stuff". For instance on the ISS the astronauts don't have facilities to wash their clothes. As soon as their socks, smalls, shirts etc begin to get a bit smelly or dirty they just put them into a bag and eventually it ends up burning up in the Earth's atmosphere at a very high temperature, along with the Progress vehicle itself.

On the Moon, all those bags will pile up around their base unless they find another way to deal with the trash. They would recycle what they can of course, but that might not be easy in such challenging conditions. How do you recycle a spacesuit that doesn't work any more for instance? They will have quite an incentive to find a way to do some recycling. Washing their clothes makes sense, but that's not the easiest of things to do in a spaceship. 

Washing machine for the ISS designed for the UMPQUA research group but I don't think it ever flew. Currently astronauts wear their clothes once and then throw them in the trash and they get burnt up in the atmosphere 

Incinerate most of it in an oven??

A base on the Moon surely isn't going to send all its trash back to Earth. It's not like the Progress attached to the ISS. You need 1.6 km / sec delta v to launch it as a payload from the Moon into orbit, and if you were to send it back to Earth to burn up in our atmosphere, you'd need another kilometer per second for a Trans Earth Injection. You would use up a lot of good fuel to burn your trash in Earth's atmosphere. They surely won't do that. So your base would have piles of trash build up around the base.

Then the thing is that, like the Apollo 11 footprints, any trash you leave on the surface of the Moon will still be there thousands, and probably millions of years into the future. It won't degrade , rust away, and mix into the landscape.

Then you have the astronaut footprints too. Maybe it's just aesthetic - but the landscape around any Moon base would soon get covered in overlapping footprints in all directions towards the nearby horizon.

Buzz Aldrin standing near a leg of the lunar module - notice how many footprints they left on the Moon? These will still be there a million years from now.

It will be the same for the area around any lunar base. It will soon be totally covered in footprints - unless they either turn the dust around the base to glass or they rake the soil. In the same way, all the trash they leave on the surface of the Moon will also still be there a million years from now too.

It would be the same for a base on tiny Phobos or Deimos. Also any other small region such as the most favoured "peaks of almost eternal sunlight" at the lunar poles.

It's the same for any geographically small region of the Moon, for instance the region around a lunar lava tube cave entrance (unless they put all the trash into the cave?). It will soon be covered in footprints and trash after humans have been there for a few months / years. The Moon would be a great place to get a first idea of what the scale of impact of this will be, as we explore the solar system, and to make our first experiments in dealing with this issue and learning what we can do about it.

I can't find much on the problem of trash on the Moon. It's often mentioned in passing but not discussed in detail. But here is an abstract from 1988 which gives a succinct summary of the main issues. 

After describing the main sources of trash including the descent platforms - which remain on the surface when the modules return to orbit, and they estimated as 4.9 tons each, paper, cloth, wood, plastics, ceramics and glass, aluminium and steel, they then go on to describe possible disposal methods, considering landfill in caves, craters, and areas of permanent shade.

The primary disposal process on the lunar surface of the by-products of shredding, wet oxidation or solar furnaces, will be burial. The initial temptation of filling nearby craters will be unavoidable for the initial settlement, but cannot continue for any extended time. As lunar mining becomes a reality, ample landfill opportunities should arise. An alternative to crater filling or direct burial would make use of areas of perpetual shade if available near the colony. These areas, if large enough, exhibit absolute darkness and temperatures near 150 K that would make for excellent limited disposal sites.

Most of that surely still applies, though I'm not sure if piling trash into areas of perpetual shade would be favoured so much nowadays given the interest in any ice or volatiles that might have accumulated there. Perhaps in caves though? Would some of the lava tube caves, or sections of them, be earmarked for trash disposal?

Or perhaps they just dig pits in the regolith and cover them over?

Also much of the trash is a resource. The trash on the Moon will have many materials in it that would be almost impossible to make locally and that would cost a lot to send up from Earth. Recycling would be hard to start with but get easier as they develop more technology and industry on the Moon. Perhaps in the not so distant future our descendants or even our older selves may return to those trash pits from the early twenty first century and mine them for valuable resources. If so it might be worthwhile segregating the trash before burying it to make things a bit easier for the future astronauts on the Moon who wish to re-use it.

Also, though it may seem the best thing to do is to compact it to take up as little space as possible, crushing, and burning it, is that really the best solution? Maybe in the not so distant future, some of the materials may be useful if not incinerated or compacted? 

For instance tanks, or a spacesuit, perhaps equipment that has some small thing wrong with it that the astronauts couldn't fix on the spot with the materials they had - either in the future the astronauts may find a way to recondition them, or to re-use the materials or components for some less demanding task.

These are just a few thoughts on the subject. If anyone reading this knows of good material on trash on the Moon do say!

Rocket exhausts, microbial spores and organics mixing with levitating lunar dust

The Moon is also an ideal place for us to learn lessons to help with protection from Earth microbes in places further afield than the Moon. But on the Moon also, even though our microbes can't survive and reproduce there, the organics from dead microbes can confuse scientific studies of organics on the Moon.

We can study survival of microbial spores in space conditions. For one thing, we can study the spacecraft that have been there for decades, crashed or landed there, to see if there is any viable life on them. This could give us good ground truth on planetary protection. On the same subject see the slides in this powerpoint presentation: Organic Measurements on the Lunar Surface: Planned and Unplanned Experiments

There were some early studies with returned Apollo 11 samples that suggest that it's rare for microbes to remain viable in the lunar surface conditions, however this is with very limited data. First, they estimated that the lunar surface where the Apollo 11 astronauts collected their samples had only 10,000 to 100,000 viable microbial spores from Earth per square meter (between one and ten spores per square centimeter). They did do a few analyses of selected samples from other missions up to Apollo 14 for colony forming spores, inoculating them into with different media, and no colonies formed. 

Here is a detailed description of the process of examining a sample from Apollo 11, no colony forming life was found after inoculating 3,000 petri dishes each with 4 mg of the sample (total of 12 grams). In another paper, one of the four samples they tested was actively biocidal (perhaps heavy metal toxicity) but other soil samples were able to support colony forming spores, but didn't. However, those are only a few analyses of selected samples and with early 1970s technology.

The returned Apollo samples actually highlight the need to take care about contamination of the Moon if we are interested to find out about native organics. A NASA study in 2015 re-analysed some Apollo samples, using modern methods searching for amino acids.

The analysis was a tricky one because there were several ways the amino acids could form (native to the Moon, produced by reactions with rocket fuel contamination in the lab, or from Earth organics).

In the end they decided that most of the organics came from Earth microbes (because of preference of the "left hand" form and the carbon 12 / 13 isotope ratios) but some probably came from meteorites. The samples are now known to have up to 70 parts per billion of organics from life. However much of that could be contamination of the samples after return to Earth (see this paper).

Sadly this is a bit inconclusive, but it does suggest that, even with the Moon we may need to take care to avoid contamination of the samples by Earth organics, either in situ on the Moon, or after return of the samples to Earth, when exploring areas where the amounts of organics on the surface are of scientific interest.

The microbes, rocket fuel, and trash left on the Moon by the Apollo astronauts in a stay of up to three days is nothing to the trash that would accumulate around a human base. So, another thing we can do on the Moon is to find out how much by way of organic contamination and other types of contamination accompanies human explorations, and how far it spreads.

This could be important for a small site, for instance Phobos or Deimos and especially if there is a scientifically interesting site close to the human base where they wish to study organics in material that got there from early Mars (say).

In the low gravity the dust and organics could spread far just by  being kicked off the surface by an astronaut. Indeed you could kick a ball into orbit around Deimos easily, anyone could. and as you walk or hop around Deimos, some of the dust you kick up would go into orbit around it.

For instance the Deimos gravity is only 0.003 m/s² compared to 9.807 m/s² for Earth, so 3269 times weaker. So a throw that would last one second on Earth would last 3,269 seconds on Earth. The distance thrown goes up in the same proportion by s = 0.5 at². So whereas on Earth a bit of dust scuffed by your foot will travel maybe one meter, on Deimos it will travel 3.27 km, or even further given that the surface is curved. Indeed kick a bit faster and the dust will go into orbit.

It's radius is only 6 km. Indeed the orbital velocity for Deimos is 4 meters per second (use 1.4762e15 kg for the mass and 6 km for radius in this orbital velocity equation online - derivation of the equation here for those interested). That's about 9 miles per hour. So you could kick a ball into orbit around Deimos very easily and trash from the base will easily spread to cover the entire moon if we aren't careful.

The dust won't get kicked so far on the Moon. Just six times further than on Earth, or a little further because of the lack of air to slow it down. You could shoot a golf ball 2.5 miles on the Moon, though Alan Shepherd's shot went less than 200 meters, as Robert Pearlman of collect space pointed out - the location of both golf balls is known, with one of them photographed from the lunar module window.

So you might think that organics on the Moon would only spread as far as the humans themselves travel, away from the base, and a bit further to allow for dust being kicked up by human feet? Most of it just localized around the base?

So, yes, of course the Moon doesn't have any weather as we know it, and it doesn't have global dust storms like Mars either. So, the organics won't spread as easily and as far as they do on Mars. However, it does have electrostatically levitated dust, which surprisingly can levitate even particles as large as 140 microns in diameter (line 215 of this paper). The dust is levitated through UV radiation and plasmas.

A microbe in a 140 nm particle would be protected from UV (though of course affected by the electrostatic discharges that levitate the particles in the first place). So, if you introduce foreign material to the Moon, it could spread some distance in this levitated dust. Perhaps even viable microbes.

But it can travel further and faster as a result of the rocket launches. The rocket exhausts for Apollo 12 disrupted about two tons of dust around the landing site that lead to localized dust storms observed during the next few sunrises as measured from the Apollo 12 landing site levitated to a height of one meter above the ground.

So how far can the dust spread? The finest dust might have gone right up to orbital altitudes. At least, the Apollo astronauts sketched what seem to be rays of sunlight hitting the dust at sunrise and sunset from orbit. However recent observations by LADEE show that there is no dust at altitudes between three kilometres and 250 kilometres. At least not any more, if there was back then.

Lunar horizon glow as photographed from orbit by Clementine spacecraft. The bright dot at the top is Venus and the Sun is behind the Moon. The Moon has an exosphere - an atmosphere so thin that the molecules rarely encounter each other. However LADDEE proved that there is no dust above 3 kilometers.

Since the Moon has a surface area larger than Africa, a few spores spreading out through dust levitation aren't going to do much to confuse science results kilometers away, unless there are habitats for life there. Every indication is that there are no habitats that Earth life could colonize on the Moon. Even ice exposed to the surface temperatures, if it ever sees sunlight or even light reflected from nearby boulders or the landscape, will vaporize quickly, at tens of meters a day at 0 °C in a vacuum (the Apollo astronauts used this to cool themselves down with their ice sublimators). It's only because of the extreme cold conditions, colder than liquid nitrogen, that ice can get trapped in the permanently shadowed craters at the poles.

Some areas of the Moon could be particularly sensitive to organics from Earth. In a review paper from 2007, the authors suggest that perhaps the Moon's categorization for planetary protection may need to be revisited depending on what we find out about the lunar ice. If this ice and the other volatiles there are of especial interest for study of prebiotic chemistry for instance, perhaps we might need to set up "organic special regions" on the Moon that need to be kept free of organics.

"Other locations, like the permanently shadowed craters at the Moon’s south pole, may contain water ice or hydrated minerals and other valuable scientific and physical resources. If, for instance, these sites contain ice with signs of prebiotic chemistry, one can envision the establishment of organic special regions to protect these native lunar organics for careful scientific study."

So, unlike Mars, it's not an issue with replicating life. Just the organics confusing searches. But that may be enough so that we need to set aside special regions to preserve the science interest of some of the ice there. Perhaps they would need to be studied only by sterilized robots if you wanted to do ultra senstiive measurements of the organics in the surface layers of the ice, and especially if it is "fluffy ice" as is one possibility for the lunar ice deposits.

Although the human bases at the poles would be right next to permanently shadowed craters, perhaps the organics wouldn't spread far in the cold desiccated conditions there.

Two tons of dust per landing is quite a lot of dust to send off into the nearby craters, if you are doing ultra sensitive scientific measurements of the organics. On the other hand however, there is no UV light to levitate the dust once it gets into the craters so it may mainly contaminate areas around the edges of the permanently shadowed areas. And much of the surface will be shadowed anyway as the sun is so low close to the horizon. Maybe that will help with the problem of levitated lunar dust at the poles.

Then in addition, there are ideas to turn the dust into glass for a landing pad, so perhaps that would help with the dust too. You could also glaze the areas immediately around the haitat, and perhaps you have glazed boundaries around the special regions to warn astronauts from approaching too closely. Just suggesting a few ideas here.

So, it may not be a major issue, but this is certainly something to study and monitor. It could be an issue for regions that we want to keep free of organics and other contaminants close to a human base.

Luckily there does seem to be plenty of space there to have regions for ice extraction for the base and other separate regions that are kept intact for careful scientific study. And the process of ice extraction itself would be of scientific interest as it would be expected to turn up many meteorites from early Earth, Mars and maybe Venus and Ceres that may have organics preserved all those billions of years at cryogenic temperatures.

If we follow that suggestion to set up regions of special scientific interest to keep free of contamination - there are hundreds of square kilometers of permanently shadowed ice at the poles, an estimated 1,850 square km of ice at each pole .

That's just an estimate though. And the ice might be patchy.

However if the organics do spread beyond the base - well the Moon is an ideal place to study such things. It has minimal planetary protection issues compared to anywhere else. If it does still have some contamination of the polar ice by organics from the human base, well this is a chance to study the situation.

The historical lunar landing sites are another area that may need protection from contamination by Earth microbes. They are valuable for planetary protection scientists,as places to study the effects of a brief human presence on the Moon several decades later. They are a:

"valuable and limited resource for conducting studies on the effects of humankind’s initial contact with the Moon" (quote from page 774 of this paper).

These sites, with microbes exposed to cosmic radiation, UV etc are also decades long unplanned experiments in the interplanetary cruise stage of panspermia - the ability of microbes to remain viable in the conditions of interplanetary space for transfer from one body to another. (see page 771)

Another planetary protection question for the Moon (in its broadest sense) is whether our landers would change the Moon's very tenuous "atmosphere" or exosphere with rocket exhausts. Each Apollo lunar landing added 10 tons of exhausts to the atmosphere with a persistence half life of approximately one month. That might not seem a lot but it is a noticeable amount for the lunar atmosphere which has a total mass of approximately 25 tons.

We have a golden opportunity right now to observe its atmosphere "as is". The rocket exhausts from Apollo should have dissipated long ago. Amongst other things we can study the movement of water vapour in the lunar atmosphere and see where it comes from. Well it will be a while probably before we get humans landing on the Moon again, but we do have man robots due to land there in the near future, including (if they keep to schedule) the remaining Google Lunar X Prize contenders in 2017, and Astrobiotic, in 2018 (they dropped out of the Lunar X-Prize saying the 2017 timetable was unachievable). So what effect will they have on the lunar atmosphere?

Well when the Chinese Chang'e 3 landed on the Moon on 14th December 2013, we had an excellent chance to find out as NASA's LADEE is still in orbit analysing the lunar atmosphere.

Artist's concept of NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) Image Credit: NASA Ames / Dana Berry

They came to the surprising conclusion that the Chinese lander hadn't modified the lunar atmosphere at all, or at any rate, if there were any changes, they were beyond their detection limits They studied it from a distance of 1,300 km so they couldn't observe the dust it kicked up (which only lasted for about 15 seconds in the Chinese descent video). No exhaust products were detected and the lunar atmosphere wasn't changed in any way.

This is good news for robotic missions to the Moon at least. This was a particularly large lunar lander for a robotic mission, so it seems that any effects from rocket exhausts are only local and don't have any global effects on the atmosphere, and the exhaust products don't travel large distances either.

Here is what they said in detail:

"Surprisingly, the LADEE science teams' preliminary evaluation of the data has not revealed any effects that can be attributed to Chang'e 3. No increase in dust was observed by LDEX, no change was seen by UVS, no propulsion products were measured by NMS. Evidently, the normal native lunar atmospheric species seen by UVS and NMS were unaffected as well. It is actually an important and useful result for LADEE not to have detected the descent and landing. It indicates that exhaust products from a large robotic lander do not overwhelm the native lunar exosphere. As the descent video shows, the interval of time that dust was launched by the lander is very short, perhaps less than 15 seconds. LADEE would probably have had to be in just the right place at the right time to intercept it. Also, significant amounts exhaust products apparently cannot migrate to large distances (hundreds and thousands of miles) and linger with sufficient density to be measured. "

What, though, about more local effects on the lunar surface, especially the lunar ice? We don't have any ground data on that yet, but we have some theoretical modeling. This shows the modeled effect of the Apollo 17 landing exhausts on the lunar surface near their landing site:

Figure 28 from this paper showing their modeled rocket exhaust contamination of the lunar surface from Apollo 17 superimposed over Google Moon. The contaminated area spans 522 kilometers of the lunar surface. The red range rings contain 50% and 67% of the total contamination respectively.

There would surely be scientific interest in the organics on the surface of the ice at the lunar poles. This modeling suggests we may need to take care about the effects of rocket exhausts from spacecraft landing in the vicinity of the lunar village, especially once the larger rockets start landing with astronauts on board. So what can we do about this?

The authors of the paper looked at future missions "where contamination by exhaust gases is not desired" and recommended that:

• Most of the braking is done with the engine pointed over the horizon and with the exhaust gas velocity much greater than the lunar escape velocity.
• Once the lander has lost enough momentum, its braking engine and excess fuel could be dumped to reduce the landing mass
• With the lunar lander now lower in mass, it can make the final descent to the surface using less fuel.

Perhaps I can make another suggestion of a way to reduce the exhaust problem even more, once we have frequent travel to the Moon. Hoyt's Cislunar tether transport system. uses counter rotating momentum exchange tethers. At the Moon end of the transport system, the tether can be stationary relative to the Moon's surface whenever it is closest to the Moon, so that you can pick up payloads from the surface and deliver them to the surface with no need to use any rocket fuel to do this.

His lunar tether masses only seventeen times the payload mass, so you don't need that much traffic per year for this to be worth doing, just for economic rather than for planetary protection reasons. Once you get that much mass in orbit around the Moon, from then on, landing on the Moon and taking off again is essentially fuel free, and what's more, you get an automatic boost from the tether to take your spaceship down to LEO. Or indeed you could use the same method to go all the way down to Earth's atmosphere on an Earth return trajectory. This would not only reduce the amount of fuel needed to land on the Moon and take off to essentially zero, and make it more economical to travel to and from the Moon. It would also protect the lunar surface and exosphere from the effects of rocket exhausts. For details see the Exporting materials from the Moon section of my Case for Moon First.

Then, there's another potential benefit for Hoyt's cislunar tether. Perhaps you could dispose of lunar trash in the Earth's atmosphere as for the ISS, after all as there is essentially no cost of fuel to do that. That is, if you wanted to. Perhaps the trash may come to be very valuable on the Moon with much to recycle in it, once they had the capability. They might value the elements and components even, things like screws, sheets of metal etc, even paper, cloth and so on, that would take a lot of work or import costs from Earth to make on the Moon. But if they so wished, with Hoyt's tether, they could dispose of trash from the Moon in the Earth's atmosphere easily. There's a lot to be thought over and worked out here.

In conclusion, the Moon is an ideal place to look into such issues. Before we send humans to Phobos or Deimos or wherever they go next, we'd better know what humans will do to a celestial body when they set up a settlement there. This is especially important if the aim is to study ice deposits in craters, or other sensitive locations. In the case of Phobos, its regolith should have records of organics and even spores of life possibly, from pretty much the entire history of Mars mixed up with it, brought there by asteroid impacts into early Mars. So, it might well be important to keep the Phobos regolith, or parts of it, clean from organics and other contaminants from Earth. Our experiences on the Moon may help to give us the experience we need to design suitable precautions for missions like this.

Protection of the historical Apollo landing sites

Also - the lunar X Prize will have many smaller missions go to the Moon, commercial ones. We may see the first of them towards the end of this year. There are five teams in the competition, SpaceIL plans to use the SpaceX Falcon 9, but it’s had trouble fitting its mission into their faring. Hakuto has made a pact to land with Team Indus and both will use the Indian rockets - it’s proven technology but they are having trouble raising the funding. Moon Express will launch on Rocket Lab’s Electron, and Synergy Moon on Interorbital System’s Neptune. All are sharing the nose cone with other payloads on their rockets.

Will Anyone Win the Google Lunar XPRIZE?

Astrobiotics, the top favourite for a long time, have pulled out saying they can’t be ready to launch until 2018. But they are planning a “FedEx service to the Moon” - their Griffin lander will carry other lunar missions to the lunar surface.

In the future we may see tiny rovers image the landing sites.

Artist’s impression of a Google Lunar X-Prize rover at an Apollo landing site. Image Credit: Google Lunar X Prize

(I actually got the image from: Team Indus joins Google Lunar X-Prize finalists, Astrobotic drops out)

That was actually one of the challenges for a bonus prize for the Lunar X prize rather controversially, some think the area immediately around the historic landing sites should be kept pristine and the rovers not permitted to drive over them.

That’s particularly because of the prospect of them landing by accident on top of the flag or some such:

““I’d like to see them demonstrate their ability to do a precision landing someplace else before they try it next to the Apollo 11 site,” Logsdon says. “You wouldn’t have to be very far off to come down on top of the flag or something dramatic like that.” “

Preserving Tranquility

The Lunar Legacy Project say in their Introduction

“Our goal is to preserve the archaeological information and the historic record of Apollo 11. We also hope one day to preserve Tranquility Base for our planet as a World Heritage Site. We need to prepare for the future because in 50 years many travelers may go to the moon. If the site is not protected, what will be left?”

Another group with a similar mission aim is "For All Moonkind".

However none of the finalists plan to take up this part of the challenge as far as I know. So this is not an issue for a while.

Perhaps in the future then we will get Lunar Parks set up, recognized world wide, and rovers and humans will only be able to approach within some set distance of the landing site. The lunar landing sites occupy only a tiny part of the moon’s surface. It’s sometimes called our The Eighth Continent, the second largest after Asia, at 37.9 million square kilometers, it’s larger than Africa, and five times the size of Australia.

So, it's only protecting a very small area, a bit like protecting Stonehenge on Earth. It seems reasonable enough to me.

Though there would need to be some way for scientists to access them if we are going to study the effects of that brief human presence on the Moon, as we discussed in the last section: Trash, rocket exhausts and microbes on the Moon - testing ground for planetary protection measures for a human base . I wonder if that is best done using remote controlled robots for minimal impact? Especially for the biological surveys?

There are no internationally agreed treaties or guidelines yet. But NASA has published a set of guidelines. From: NASA's Recommendations to Space-Faring Entities: How to Protect and Preserve the Historic and Scientific Value of U.S. Government Lunar Artifacts, the requirements include, amongst other things:

• No overflight. A landed spacecraft mustn't fly over a region of 2 km radius centered on the landing site, or half a kilometer for impact sites like the Ranger probes. That helps avoid risk of accidentally crashing on the sites or of contaminating them with exhaust or debris.
   
• The landing ellipse also must be outside of that region. The distance of 2 km places them over the lunar horizon of 1.8 km from the landing site. This 2 km distance is ideal also for dealing with the sandblasting effects of rocks and dust thrown up by the landing. These can have velocities from 300 to 2000 meters per second. The larger ones wouldn't reach the landing site. The smaller ones would fly right over it and miss. There would be some that hit it but they would be small in number.
   
• Use natural barriers where possible like hills, crater rims, ridges, terrain slopes to block the spray of the landing spacecraft from the heritage sites.
   
• Exclusion zone for rovers etc after landing - they can approach the sites from their landing site 2 km away, but must come no closer than 75 meters for Apollo 11, and 225 meters for Apollo 17. For Apollo 12, 14 - 16 then they recognize that they may need to come closer for scientific investigation, so footprints are not protected, but there's a 3 meter exclusion distance for the descent stage, and 1 meter for the experiments, sampling sites and other artifacts.

For details see the NASA Recommendations.

Imagining a telepresence mission in the HERRO Molniya type orbit

Imagine yourself in orbit around Mars - in a Molniya orbit - comes round to the sunny side of Mars twice a Martian day - you go really close to the surface - and spend some time there controlling rovers on the surface - driving them around - with reality headsets like the Occulus Rift -

(click to watch on YouTube)

- the Mars astronauts in orbit could explore the surface with headsets like this - and haptic feedback gloves so you can feel what you are doing, and omni directional treadmills like the Vrtuix Omni

(click to watch on YouTube)

and automatically enhanced vision with everything you see on Mars streamed back to Earth so everyone back here can join in and see what you see exactly as you see it whenever you explore the surface of Mars.
Then after a few hours of that you see that Mars is now getting further away, becomes smaller, and then 12 hours later you come in again for another close approach and real time exploring - you can continue to explore all the time - but when you are really close you can control things on the surface in real time as if you were there.

Small planes and entomopters etc

All our missions to Mars so far have used either landers, or slow moving rovers that travel only a hundred meters or so a day. The rovers can be much faster with greater bandwidth and with a reasonable power source on Mars - such as the ones considered for human missions. However there are also many other innovative ideas for ways to explore Mars creatively. Most of these have never been tried anywhere outside of Earth so it is far too soon to know what their potential may be.

NASA is already considering a Mars copter for Mars 2020. If it does fly, perhaps it will be the first of many such innovations. Perhaps we might see some of these ideas tested on the Moon first too, the ones that don't need an atmosphere to fly in. The Moon is far closer and it is accessible to smaller private companies who can perhaps go out on a limb more easily and try out ideas, for instance for the Google lunar X prize, that the larger agencies are unwilling to risk on a once in a decade type billion dollar mission to Mars.

Also, when it comes to Mars, we now have India which already sent a mission to Mars orbit and is interested to do follow up missions, ESA is hoping to have its first lander with ExoMars in 2020, the United Arabic Emirates is interested in a mission there too, and of course Elon Musk hopes to send his "red dragon" to Mars. Perhaps this may lead to more experimentation for Mars too, similarly to those innovative ideas for lunar landers?

If we have humans in orbit around Mars at a later date, that would attract a lot of attention. They'd be able to operate rovers and landers from orbit, so we might get many other countries joining in, sending robots to the surface with telepresence capabilities that could be operated by the humans in orbit. Any rover that can be operated from Earth could also be operated from orbit, of course, by sending the correct commands. However, if the rovers are equipped with binocular vision, haptic feedback etc, they could be much more useful to human operators in orbit around Mars. They'd also be able to deploy small balloons, gliders, and planes from orbit to glide down to the surface amongst other possibilities.

So, let's start with the "entomopter" a type of flying machine that's tailor made for Mars (though very small lightweight versions with a wingspan of cms do fly on Earth). In the Mars thin atmosphere, then bumble bee flight is efficient even with large wings a meter or more in diameter, and they have advantages over both planes and helicopters in terms of efficiency. These are lightweight, so you could carry many of these along with the humans in a human mission to Mars orbit or the Mars moons to send on to the Mars surface.This is from a 2002 report. This is an example of an entomopter, which flies like a bumble bee but on Mars can be far larger because of the thin air. 

(click to watch on YouTube)

It's lightweight, so doesn't add much to the payload, and can take off from a surface lander, return and fly again and again. It could be an "add on" scout for a larger mission or it could be a stand alone mission with a small rover or lander as its base.

Video of Robert Michelson's entomopter.On Mars it might be easier for machines to fly with insect type flight with rapidly beating wings, using the bumble bee wings vortex effect for lift. On Mars that can work scaled up to wings a meter across because of the thin atmosphere also assisted by the low gravity. That's the idea of the entomopter.

Then, they could try this idea - make the light weight construction into an asset. It can travel over areas of Mars covered with large rocks, without the problems that Curiosity and Opportunity have of threading a way through them or round them:

NASA's Space Place. See also Inflatable Robotics

This is another fun and innovative idea, which again need not be particularly heavy. They would need some way to get to the cliff to descend of course, because of the landing ellipse issue - so need a rover - which is a challenge in itself. But maybe it can be an inflatable rover, that detaches part of itself to descend down the cliff?

AXEL Rover - Mars Cliffbots

Or insect type rovers with whegs (wheel like legs instead of wheels)

(click to watch on YouTube)

With really small light rovers, then the landing would be simpler. But they could go one better, and omit the landing altogether Here the idea is to use solar powered balloon rovers which don't land at all - so missing out one of the most dangerous steps in the landing sequence.

This is project Archimedes - a 2009 mission by the German Mars society which never flew. Or they could try the Solar Montgolfiere Balloons for Mars

Or they could try planes or microgliders. In the early days of the space shuttle, the plan was to fly a space shuttle every week. One of the things they could do with all those launches is to send lots of miniature airplanes to Mars for close up exploration of the surface. It has the advantage again - that though you do get there eventually, you don't land on Mars right away and don't have the complexities of a soft landing on the surface. In some of the ideas. it is a single flight to a crash landing on Mars, and that's it - a long glide, still worth doing with images sent back all the way. If the plane has power, it could extend its flight. Or you could have dozens of small microgliders to get a lot of information in a short period of time.  NASA has explored many different flying vehicles of this type for Mars, but none of them have actually flown.

The problem with large scale conventional planes on the surface of Mars is that the take off speed is well over 200 mph, so it is usually impractical to launch from the surface. You can deal with that by launching the plane from orbit, but though useful, it has to be a one off flight. When you run out of fuel, you land (or crash) on the surface, and your flight is over for good.

It is much more interesting if your plane can take off and land again. NASA have explored that as well, smaller, model airplane sized planes with a camera and a radio. 

NASA's Langley Research Center Artist's concept of the Mars Airplane - one of many ideas - this is a tiny plane with five foot wingspan which folds to fit into an aeroshell for entry into the Mars atmosphere.

So how could they do that? With one of these planes they found a way to put it into a stall at a 70° angle, They found that in this configuration, it falls reasonably slowly, rather like a parachute. The plan was to add thrusters for a vertical soft landing on the Martian surface.  After a vertical tail first landing like this, your airplane could take off again, using the same thrusters.  This is a plane that could land on the surface of Mars and then fly again repeatedly. Essentially, it's a design for a miniaturized Mars version of VTOL flight.

All this does have planetary protection issues also, of course. If you have a way to land hundreds of microgliders on Mars, say, you have lots of opportunities to accidentally introduce Earth microbes to potential Mars habitats. So that would need to be looked at carefully too. Does the 300,000 spores per rover figure for current Mars missions that don't target areas that are thought to have potential habitats still apply if you have hundreds of rovers on Mars? I'd have thought, surely not. But on the other hand such small gliders and rovers can be sterilized more easily perhaps, if carefully designed. I won't go into this in any more detail here as it's already discussed above in the section on Can we achieve 100% sterile electronics for an Europa, Enceladus, Ceres, or Mars lander? Just to say, that it might be a percentage increase of cost of each glider to design it to be carefully sterilized, even ideally, 100% sterile, with electronics capable of heat sterilization. But once you have that worked out, the cost per glider wouldn't be that huge, and anyway it is just a cost you have to bear if you value pristine Mars. If it means you send say 80 microgliders instead of 100 for reasons of expense, well that's just the price of doing responsible exploration.

Thistledown light planes with low take-off speeds for the thin atmosphere of Mars

his is not a worked out study but just an idea I came up with a friend. But it might suggest some interesting approaches to explore.  Again - it is an idea for really light machines, easy to send in a small package to Mars, so seems relevant here.  These would be launched from a surface lander, I'd imagine. It is to do with the 200 mph take off speed for aircraft on Mars and whether you can do anything about it. Even the tiny mini Mars gliders of NASA are still robustly built, not exactly featherweight.

I was talking to Billy Stiltner, he flies model planes able to fly slowly, slowly, almost float through the air like thistledown.  

In case you haven't seen this before, you can find videos of these featherlight planes on YouTube - this is one of them showing an ultra slow collision of two of the planes with each other.

 

(click to watch on YouTube)

He said he doesn't think that there is a minimum airflight speed for an aircraft, if you can make it of light weight enough materials. So even though it is usually said a plane needs to be fast to fly on Mars, perhaps that's because they design them around the idea of a heavy plane with a heavy payload. You don't need strong materials, since the atmosphere is so thin. Especially if you avoid flight during the dust storms.  Even during the strongest winds on Mars, they are fast winds in a near vacuum, in a thin atmosphere corresponding to 30 or 40 km up in the Earth's atmosphere.  Our rovers have never had any problems with wind damage (the only wind related problems are to do with dust spread by the winds when  they cover the solar panels). They have many delicate appendages (cameras, solar panels etc).

So a plane on Mars only needs to be able to withstand winds so gentle they could barely shift an autumn leaf. Not unlike indoor flying on Earth. So, we wondered about the idea, could you have a Mars aircraft made of light materials which would be flimsy on Earth, yet robust enough for the thin atmosphere of Mars? It could fly slowly - at least = obviously not as slowly as in the thick atmosphere of the Earth, but a lot slower than any of the other ideas.  These would be planes with almost transparent wings of mylar and the like, and their structures could be built of reasonably strong ultralightweight materials such as aerographite. They would be featherlight, impossibly fragile for use on Earth, but strong enough for Mars.

Billy also talked about "round the pole flying". Again many videos of this on YouTube if you haven't seen it, here is one:

(click to watch on YouTube)

We came up with the idea - on Mars, what if you hang your planes from a tall pole, on a line short enough so they don't hit the ground, and set them flying around.  You could probably set the thing 100 meters into the atmosphere or more and with the thin atmosphere, there isn't much chance of it blowing over.  If necessarily you can add guy-lines to stabilize the pole. This would be especially useful if you want to carry your planes and the launching pole on a smaller mission or rover. A tiny rover like pathfinder could peg in the guys or loop them over or around boulders. 

Once it is all set up, you  set the planes flying round and round like the round the pole flying for model aircraft. This simulates an enormously long runway on the surface of Mars. 
Eventually they fly fast enough to reach their take off velocity. Then they release their leads and fly off on their missions. When they come back again after their flights, you set the guys spinning fast around the pole, until they reach the flight speed of the airplanes. The planes then dock onto the hooks at the end of the leads. With modern technology - it shouldn't be too hard to find a way for the planes to dock with the leads when they come back again. 

Then the plane flies round and round the pole, as it gradually slows down, like landing on an enormously long runway - and then when it "lands" it goes into a box on the rover, or some kind of shelter for the planes in the dust storms.  If it has a shelter, it only needs to fly in good weather on Mars, permitting yet lighter construction.

Since the planes are so light, and especially if the camera and transmitter can also be miniaturized and light as well, you might be able to have dozens of them stored on a single rover. They could be designed to fold up when not in use, and for transport to Mars. Or they could b be designed to stack on top of each other for compact storage (or both). That is just a back of the envelope type idea, maybe someone would be interested in exploring it a bit further. As far as I know it is an innovative idea, never heard of any study of the idea of either round the pole flying on Mars as a way to launch aircraft - or the idea of featherlight thistledown light aircraft on Mars.

It's unlike the Marscopter or Entomopter because the planes can't land or take off from the surface of Mars, but have to return to the pole. But they are also very light weight so it could carry spares in case for some reason any of them don't make it back to the pole. About the only hazard would be the dust devils, which are rare. Apart from that, just malfunctions and operator errors. Mentioning this in case it suggests ideas for someone.

Exploring caves, steep slopes and other inaccessible places

Proponents of humans on Mars often talk about how we can explore in ways that robots can't. But actually, well robots can drill at least as well as we can, and they can be any size, can be light enough to float in the Mars atmosphere or to fly on helicopters or entomopters - and small enough to fit into tiny gaps. And yes, they can also climb cliffs and go into caves, indeed much smaller caves than humans can.

So let's look at some of those ideas for exploring caves and steep slopes. So first, these are really tiny robots, suggested by Penelope Boston as a way to explore caves. They can hop:

Small, spherical microbots filled with miniature fuel cells, instruments and an artificial muscle for hopping
Microbot Madness: Hopping Toward Planetary Exploration

They could fit a thousand of these into 174 kilograms. Penelope Boston also talks about them here - for exploring caves on Mars.

Her basic idea is to send robots first to scout it out before you send humans - for science reasons and for safety reasons. First, a lander flies over the cave entrance before it lands, to image it close up. It then drops a line into the cave entrance which is used to supply power to the bots underground, and for them to communicate to the surface. The bots themselves can be lowered down on tethers, or just be dropped in, in the low lunar gravity and drive, hop or crawl. or move like a snake, or like a spider crawling over the walls - or they can have little rocket engines and fly about inside.

But why not just use them to explore caves, with humans in orbit? That idea has been looked into in a great deal of detail in some papers for exploration of lunar caves from Earth, a rather similar situation. See the various cave bots on page 21 of the paper. The bots discussed include:

• Spherical hopping microbots - the idea just mentioned, from Penelope Boston - launch many into the cave, each with a small instrument package. Many will not survive but the ones that do can be used to explore, for communications links to the surface and data return.
• Multi-segment tethered robot - the tether is used to lower it down, and then for communication with the surface and recharging.
• Legged tethered robot - the same idea but with legs so it can navigate rough terrain more easily.
• Snake tethered robot - the same idea but it moves like a snake
• Cave hopper - combines hopping with wheels
• Climbot - future robot able to climb walls with a high level of autonomy
• Elevator - large tethered platform used to lower multiple robots at once into the cave, with wheels to allow it to maneuver over debris once down.
• Propulsive flying bot - uses small thrusters to navigate, so it can get to parts that may be unreachable by the other bots
• Telescoping ball robot - has two modes: enclosed in a ball, for launching into the cave and rolling down slopes inside the cave, and then the two halves of the sphere can separate to be used as wheels, with extending tail for balance.
• Prismbot - the bot can move by tipping from one side to another.
• Rope climbing - a fixed tether and the robots can climb up and down it.

See also the sections of my Case for Moon First booklet: Lunar caves and Example lunar cave skylights - Lacus Mortis, Marius pit and the King-y natural bridge

We already know of many lunar cave entrances, and they are so accessible from Earth, and they may be so interesting too, potentially vast inside, could be multiple kilometers in diameter. So we may well start with those, before Mars. But they are useful for Mars too. It also has lava tube caves, and it almost certainly has many other kinds of caves due to weathering, caused by water erosion (clay, salt and soft rock eroded in massive flooding events and perhaps caves deep below the surface from hydrothermal processes), caves carved out by the wind, caves at geological strike / slip faults, forming due to collapse of debris, ice caves in the polar regions, and even dry ice effects which may lead to types of caves we don't have on Earth. Most of these would be almost impossible to see from orbit, so it's not surprising that we have only found collapsed lava tube caves and similar so far.

Team Hakuto plan to send a simple tethered bot to the Moon, with two wheels, to explore the Lacus Mortis pit, in 2017. This first exploratory robotic phase looks likely to be an international effort, with missions from different countries and from private enterprises adding together to give us a complete picture. And the exiting thing is, that it seems it's really going to happen this time. Starting in 2017, we really are going to get lots of new robotic missions to the Moon. Not just China, though China plan to send missions to the lunar north and south poles in, 2017 and to get a sample return from the far side in 2018.

Then, astrobiotic, one of the X-prize contenders, offered to carry payloads from other Lunar x contenders on their first mission , offering a price of 1.2 million dollars per kilogram to the lunar surface on their Griffin spacecraft. They have partnered with DHL for the logistics on Earth. They plan to send their first mission to the Moon’s Lacus Mortis region which seems to be the location for a skylight entrance to a lunar cave. Their plan was to run a Formula 1-like race on the Moon between their own Andy rover and the other rovers they carry with them to the Moon (Team Hakuto and Team AngelicvM from Chile) to see who can travel 500 meters fastest to win the lunar X prize (they will share the prize money, whoever wins). However Astrobiotic dropped out of the Lunar X-Prize saying the 2017 timetable was unachievable and now plan to go to the Moon in 2018

With astrobiotic out of the race, Team Hakuto from Japan have negotiated a new ride to the Moon with TeamIndus. I'm not sure what their current plans are but originally their plan was to explore a possible lunar cave skylight with their Moonraker lander (named after legend of English smugglers in Wiltshire, not the James Bond film). This mission is of especial scientific interest and of interest to human exploration of the Moon for the future, whether they do it or not, so let's look at it in a bit of detail:

(click to show on YouTube)

Moonraker, pulling the two wheeled Tetris. After competing for the 500 meters travel time on the Moon for the Lunar X prize with the other rovers brought there by astrobiotics (assuming it's not won already by another lunar X mission), its mission goal is to go on to lower Tetris into the skylight of a lunar cave. The rover has four independently driven wheels (it can also use them to "turn on the spot"). It has four instead of six wheels to give space for larger wheels as this helps reduce slip on steep slope. (Six wheels have advantages for travel over rugged terrain, but for their particular challenge of approaching a lunar cave over possibly steep terrain, preventing slip was more important to them). When it gets to the cave entrance it will use itself as an anchor as it lowers Tetris down into the cave, to explore it.

They plan to target the partially collapsed skylight in the Lacus Mortis region (originally their target was the Marius pit). For details of those features, see the section of "Case for Moon First":: Example lunar cave skylights - Lacus Mortis, Marius pit and the King-y natural bridge. For details about why the caves are interesting, see Lunar caves and Lunar caves as a site for a lunar base. The team is led by Professor Kazuya Yoshida, designer of the Minerva II asteroid hopping robots for the Hayabusa 2 mission to an asteroid (currently on its way to an encounter in 2018).

The Andy rover (for Team astrobiotics itself) may also include the VESA gravimeter which could explore variation in gravity around the pit and use that to map out any underground voids, which should show clear gravitational signatures if a large lava tube cave exists underground.

They may also include their innovative tetramorph cube rover - a fast and lightweight semi-autonomous folding rover small enough to pack inside the 30 by 30 by 30 cms of a cubesat - and can then unfold and do pioneer exploration to scout out difficult territory that you can't risk exploring for the first time with the main rover. Early prototype of tetramorph unfolding:

(click to watch on YouTube)

Visualization of tetramorph in folded and unfolded position. See also Tetramorph Avionics: My Experience of Building a Lunar Rover

The same techniques of tether bots, gravimeters, and high speed high risk exploration pioneer rovers could be used to explore the similar Mars cave "skylights" robotically, after testing first on the Moon.

The lunar flashlight cubesat will launch piggyback on the first SLS "Exploration Mission 1" which has been delayed several times. The latest projection is that it will launch perhaps early 2019. This cubesat will use laser light to look at the permanently dark shadows at the lunar poles to search for volatiles.

Lunar flashlight - artist's impression

Other confirmed cubesats for the SLS launch include the Lunar Polar Hydrogen Mapper shown above - cubesat from Arizona university to map water resources on the moon - see also the section Cubesat explorers and rep rap printing in my "Case for Moon First".

NASA is also partnering with the commercial lunar companies like astrobiotic in its lunar CATALYST program. India also plan a second lunar probe, Chandrayaan 2 at the end of 2017 or early 2018 , following on from its successful lunar orbiter Chandrayaan 1. This time it will be a combined orbiter and lander-rover. The rover will be 29 - 95 kg and operate for at least 14 days under solar power. Japan has a plan, not yet approved, to send a lunar rover landing by 2018. Then there's the ESA and Russia's Luna 27 to the south pole Aitken basin, some time in the early 2020s.

There are Many other ideas like that - surely much more fun, to operate those from orbit around Mars, in a shirt sleeves environment than living a troglodyte existence on the surface under meters thick layers of soil, going out only rarely to keep down your lifetime radiation dosage - and knowing all the time that just by being there you have contaminated Mars and made it far harder for scientists to find out interesting things about biology and alternative forms of biology and the early history of evolution.

Also all this would be great for collaboration - probably need a big international expedition to send the humans out to orbit around Mars. But as well as that, anyone who can send a spacecraft to Mars (probably many countries by then) can send landers, for them to operate telerobotically. The more the better really. So it is something that all countries with interest in space could work on together, each contributing different things depending on their expertise.

Safe ways to get humans to Mars orbit or its moons to avoid any risk of crashes on the surface

You couldn't do aerocapture in the Mars atmosphere as a way to get into orbit. It would be far too risky. Also Hohmann transfer with insertion burns are too risky also, as the insertion burn is done as close to Mars as possible to reduce the amount of fuel needed due to the Oberth effect. So you would need to be very sure that the insertion burn can't go on too long and end up on an impact trajectory with Mars.

I suggest ballistic capture is a far better method for human missions to Mars. The idea is that you launch the spacecraft to arrive ahead of Mars at just the right point for it to capture you as a temporary satellite. Once you leave Earth, you are already on a trajectory that ends up with your spaceship getting captured temporarily in a distant Mars orbit when it gets there, with no need for an insertion burn. Then once you are in that orbit, you use ion thrusters to spiral down to lower permanent orbits around Mars.

This is surely the safest of all the ways proposed to get into a Mars orbit, and the best way to prevent a crash of a human occupied spaceship on Mars.

Then you also have the flybys. Flybys are safe because although they involve precision targeting, you have months to set the target up. Also, the ones that are of most interest for Mars are free return, so even if your rocket fails, you are still on an orbit that will take you back to Earth again. You would use trajectory biasing of course, so that as you leave Earth you are biased away from Mars rather than towards it and use fine adjustment then to target the flyby orbit.

We have done many flybys, delicate ones, repeatedly for Saturn's moons with Cassini, and get them right every time, so it is obviously one thing we know how to do reliably. This has no time critical insertion burns. Just gentle thrusts nudging until you are in the right trajectory, which you set up long in advance of the actual flyby.

Interesting flyby and orbital missions for Mars

Especially Robert Zubrin's double Athena flyby - a very interesting mission - is safe for humans to Mars. This has two flybys of Mars. The first diverts you into an orbit that closely parallels Mars for half of its year, so a full Earth year. The second flyby takes you back to Earth 700 days after the launch. It's free return - once you leave Earth you are already on a trajectory that will take you back to Earth 700 days later even if your rocket motors fail completely.

It's a great orbit for telerobotics as you spend several hours close enough to Mars for direct telepresence with each flyby, and days close enough for significant advantages relative to Earth, and over the entire one year period when you are almost paralleling Mars in its orbit, your crew are much closer to it for controlling robots on the surface than anyone on Earth. Advantages of this are

• Least delta v of all the proposals, so also the cheapest
• Astronauts are close enough to Mars for direct telepresence control for several hours for each fly by, still remain close for days after the first flyby and before the second fly by, and close enough to be a major asset over Earth based control for the entire year.
• It's a "free return" mission - you don't need to do any extra delta v to get back to Earth. The second flyby, an Earth year later, takes you back to Earth even if you don't do anything.

Robert Zubrin's plan has several advantages over Denis Tito's Inspiration Mars, which is the other free return mission proposal.

• You can do it more often, roughly every two years,
• It doesn't have the high speed return of Inspiration Mars so less issues with aerobraking into the Earth atmosphere on return
• You have far more time for near to Mars telerobotics - with Denis Tito's mission you just have the one, very fast flyby of Mars and then it is over.
• But is a longer mission by 200 days.

I'm not sure that the Inspiration Mars 500 days is such an advantage over 700 days. I think if we can manage 500 days safely (surely not for a while yet) - then we can probably do 700 days also. (it is an extremely poor safety margin for Inspiration Mars if most astronauts are close to death at the end of 500 days). Also the double Athena can be done every two years, doesn't need any special alignment with Venus, and it has a much slower return trajectory relative to Earth - can just do normal aerobraking much like the return from the Moon, which Inspiration Mars can't do because it has a far faster return velocity delta v relative to Earth. Anyway its great advantage is that you are close enough to Mars for hours during each fly by - and in between you spend days really close, close enough to do things like drive rovers in real time and supervise experiments.

The HERRO mission suggested a near sun synchronous Molniya orbit, which is elongated, similar to a Mars capture orbit. This is the easiest orbit to get to in terms of delta v, needing a similar amount of rocket fuel to a mission to the Moon for the same payload. This has the advantages that

• Less fuel to get there
• Approaches the sunny side of Mars twice a day, each time visiting the opposite hemisphere,
  
  So the entire surface of Mars is available for close up direct telepresence control of the landers, in full sunlight, every single day, by just the one crew.
  See: HERRO mission to mars using telerobotic surface exploration from orbit

Turns out, if you choose just the right orbit, it's ideal for studying Mars by telepresence. You use a slowly precessing sun synchronous Molniya type half sol orbit. This is a precessing orbit that automatically keeps your spacecraft approaching Mars on the sunny side twice a day, all through the Martian year.

This shows how you get into this orbit - just directly from the Earth-Mars transfer orbit.

With this orbit, you have several hours of close up telepresence every 12 hours over opposite sides of Mars each time, also always on the sunny side of Mars. The delta v is the same, to all intents and purposes as a Mars surface using aerobraking. But without the dangerous descent to Mars and without the expense of developing human rated landing equipment -I think pretty clear it would cost less than a surface mission.

All of these missions have the disadvantage that you have to take all your shielding for the entire mission with you. You'd need "storm shelters" for solar storms also. This would be worked out with the lunar precursor missions to L2 etc - I don't think we should do a mission like this without experience of similar missions close to Earth. And any Mars mission needs to spend around a year combined (there and back) fully exposed to solar storms and the cosmic radiation anyway.

Its moons are actually, rather interesting places for a human to visit. In some ways they are more exciting than the surface, as we'll see, especially when combined with telerobotic exploration of Mars. Also, they are far easier places to land on safely and return. And there is almost no risk of introducing Earth life to Mars accidentally, so long as you get there without use of aerobraking. I think it may be some time myself before we can send humans even this far safely. There are many more challenges than there are for a lunar landing. The task of physically getting to Mars orbit is perhaps the least of many issues. But when we do get there, then these could be really exciting places to visit,

So which of the two moons should we visit? The most detailed proposal is by Lockheed Martin in their "Red Rocks project" as part of their "Stepping Stones to Mars" program. And in their comparison study, Deimos beats Phobos on almost every score - though Phobos does have a couple of major advantages.

Advantages of Deimos

So - the advantages of Deimos first

• It has a South pole almost permanently shadowed - which may be one of the coldest places in the Mars system (that's including the Mars surface) . It gets a little sunlight in the southern summer, but hardly any. Also it's a place where you are almost completely sheltered from the dangerous radiation in solar flares (which emanate from the direction of the sun). And it has natural cooling for any rocket fuel that needs to be kept at cryogenic temperatures (it is also ideal for infra red observation).
  
  
  Map of lighting of Deimos during its southern summer, with yellow for continuous sunlight and black for no sunlight. (Figure 4 of this paper). Some craters in the southern region get no almost no light year round.
  
  Shows the same map wrapped around Deimos. Cyan arrow points south, red arrow points towards Mars (Deimos is tidally locked to Mars).
  
• The almost permanently shadowed craters are a good place to site any resources you need to keep cool, e.g. rocket fuel
• Close to these places of permanent shadow, you have a site of continuous summer sunlight in the southern summer to place your solar panels.
• You also have permanent line of sight communication with Earth 24/7 for nearly all the year except when Mars is behind the sun.
• Deimos is almost synchronous with the surface of Mars - and tidally locked - so you can have continuous line of sight access of Mars for telerobotic communication.
• Any site on the surface of Mars is accessible for 60 hours at a time from Deimos. By comparison, from Phobos, any site is only accessible for four hours at a time.
• A typical site on Mars is visible from Deimos for 45% of the time, and 97.5% of Mars is directly accessible via telepresence at some point or another.
• The required delta v to get there is somewhat less than for Phobos.
• Surface materials can be used for resource utilization and to cover the habitat to protect from cosmic radiation.
• It's an interesting place to study in its own right.

Lockheed Martin suggested a mission that arrives at Mars during the Deimos summer, and the astronauts set up their base in the region of continuous summer sunlight. As equinox approaches four months later they leave Deimos and loiter in the Mars orbit for 50 days (leaving Deimos requires a trivial amount of delta v) and then return to Deimos to the northern hemisphere for the remaining ten months of their visit setting up their base in a region of continuous northern summer sunlight. In both cases they choose a place which has the whole of the Mars surface visible and with near continuous line of sight communications with Earth. Details here.

Northern hemisphere of Deimos and the yellow region experiences permanent sunlight during the northern summer. This is where the astronauts would spend the remaining ten months of their stay before returning to Earth, studying Mars from orbit and also studying Deimos itself. To give an idea of the scale, Deimos is 12.6 km in diameter/

Advantages of Phobos

There are two major advantages of Phobos,

• because it is closer to Mars, it has a two way time delay for telerobotic exploration of only 40 ms, while Deimos has a two way time delay of 134 ms.
• It has a crater, Stickney crater, on the Mars facing side. A party in Stickney crater would be protected from about 90% of cosmic and solar radiation, possibly more, at least according to this article (if they got it right). That's because it is protected by the crater walls, Phobos, and Mars itself which is overhead, and for solar storms it would be out of the direct line of the storms for most of the time. It is one of the places in the inner solar system most protected from cosmic radiation, even more so than the poles of the Moon. Only the lunar caves and Martian caves are more protected from cosmic radiation - or the high Venus cloud decks (and of course, Earth itself).

Stickney crater on Phobos. This large crater faces towards Mars. A base sited here would be protected from solar storms, and also from cosmic radiation. It's blocked by Mars overhead, Phobos below and the crater rim to all sides, and so gets only 10% of the cosmic radiation of an unprotected base.

Phobos also probably has a high percentage of material from Mars in its regolith, and larger meteorites surely as well. So you can study the geology of Mars by looking at fragments of rocks on the surface of Phobos. Possibly the biology of Mars also.

Phobos was the target for the Planetary Society workshop in 2015. They base it around the SLS (Space Launch System). We send cargo to the Mars system in advance before any humans get there:

• Two spacecraft are waiting in Mars, with chemical propulsion. One to deliver the crew to Phobos and back, and the other, to return the crew to Earth.
• A habitat waiting for them on Phobos.

These are put in place using two SLS launches and then solar electric propulsion once in the Mars system. Solar electric propulsion is slow but it means they can deliver the cargo there outside of the normal launch windows to Mars (using ballistic transfer ideas).

Then two more SLS launches put all the equipment the human crew need for the journey to Mars into Earth orbit. Then crew then launch from Earth in a two and a half year mission which includes a 300 day stay on Phobos. When they get to Mars orbit they dock with the Phobos transfer stage which is already there and leave their deep space habitation module and dock with the Phobos habitat for their mission. They use the same transfer stage to get back again. Details on page 19 following of their report

Mining Phobos and Deimos

Deimos is also an asset for mining - it's not known for sure yet, but is similar in composition to meteorites that can have a large ice content. So it may have large deposits of water ice. If so that's of great value for fuel - and the delta v budget is such that it's actually one of the easier places to mine ice to return to LEO. As well as use in Mars orbit.

David Kuck in 1997 suggested starting up a Deimos Water Company to supply Earth orbit with water from Deimos. The Kuck mosquitoes are small unmanned craft that drill into Deimos and extract water from below the surface, use part of it as fuel to transport the rest back to Earth.

See also Mining Phobos and Deimos

This makes it a project that could be commercially viable, unlike Mars surface, through sale of ice to LEO.

Lack of gravity on the Martian moons

There is one major issue of course - the gravity, almost none. Can humans stay healthy with such low levels of gravity? Or - can we create artificial gravity on the surface (spinning habs)?

But perhaps this can be done with either a carousel type approach or with centrifugal sleeping quarters for the crew. I think this all depends on what turns out to be the requirement for artificial gravity to keep a human body healthy, and on what our tolerances are for spinning motions in AG environments - which we don't know anything about as yet. See also my sections in Case for Moon First:

• What about gravity - isn't that a big advantage for Mars over the Moon?
• Artificial gravity on the Moon to augment lunar gravity

Free flying habitats

Artificial gravity is probably an easy matter to arrange in a free flying settlement in orbit. You can use a tether system to generate gravity for the earlier settlements. You can use materials from the Martian moons for shielding. Long term, you could use materials from the Martian moons eventually to build Stanford Torus type settlements in Mars orbit. Deimos has a mass of 1.48 * 10^12 metric tons, which at 15 metric tons per square meter is enough to make Stanford Tori with about 100,000 square kilometers of living area.

So, if you were to mine the whole of Deimos - not suggesting we do - but if we do - that's cosmic radiation shielding for roughly the size of Iceland, larger than Scotland, or Norway, more than twice the size of Switzerland, which could be useful for Mars orbital colonies. In terms of US states, that's about the size of Oregon or Colorado That's just the outermost hull which you can build on top of - and larger habitats might well have multiple "shells" within it - so that's a lower bound on the total land are you could create from Deimos using it for cosmic radiation shielding.

Anyway that's obviously for some way down the road, if we ever do that.

Planetary protection issues for Phobos and Deimos

When Russia proposed a sample return from Phobos, then it was classified as an"unrestricted category V" mission, meaning that no special precautions are needed. Despite that, Russia did plan to take some precautions on return of the sample, though not required for planetary protection according to the classification their mission received.

The Russian Fobos-Grunt spacecraft, which was designed to return a sample from Phobos to Earth. The spacecraft never got there, it failed to separate and fell back to Earth and was burnt up. This sample return mission got an "unrestricted category V" classification for planetary protection on the basis that Phobos couldn't be habitable for Earth life - but the Russians did plan to take some precautions even so.

There is no chance of present day life surviving on Phobos or Deimos to the best of our knowledge, no chance of habitability on the surface or below the surface. These moons are just too small to have any chance of liquid water even below the surface, so it's thought. But - there could be a very minute chance of dormant life, because Phobos receives material sent into orbit from Mars after meteorite impacts. Any life at least on the surface of Phobos would be sterilized over long time periods such as millions of years. But the most radioresistant life on Earth can resist the equivalent of four hundred thousand years of Mars surface cosmic radiation, although not evolved in the presence of ionizing radiation (as a byproduct of heat and desiccation resistance probably). For more on this: UV&Cosmic Radiation On Mars - Why They Aren't Lethal For The "Swimming Pools For Bacteria" Life on Mars, evolved in presence of cosmic radiation could conceivably resist that much and possibly more. And Stickney crater gets just a tenth of those levels of radiation, so dormant spores there there could survive ten times longer, and more so if somewhat buried.

So given that material is sent into orbit from Mars every one or two million years, with the last known impact large enough to send material as far as Earth a little over 700,000 years ago, there would seem to be a small chance of dormant life on Phobos - that is if life from Mars can be transferred on meteorites at all, which we don't know yet. But theoretically it would be possible. I think myself that this suggests it would be worth while doing robotic in situ exploration of Phobos and Deimos first before we send humans there. And if signs of life are found there, dormant life, to re-evaluate the situation and take appropriate precautions depending on what is found.

In the forward direction also - then though to best of our knowledge, there is no chance of life from Earth surviving on Phobos or Deimos, there could be regions there of especial interest for the study of organics in the early solar system and dormant life. For instance what if there are ice deposits at the South pole of Deimos - including samples of organics from early Mars itself? Then we would need to keep humans well away from these until we have a chance to study them in situ and work out what effect a human base would have on them.

Could a human occupied base there be kept so clean that there is minimal impact on the study of ancient organics in the ice surrounding the base? If not, what can we do? Deimos is rather tiny, only 12.4 km in diameter. After a year or two, then the entire region around the base would probably be covered in footprints unless you take special precautions to minimize the impact of humans on the Moon. And what about refuse piles, human wastes etc? If Deimos is interesting in its pristine state - well that's something we need to think through before we send humans there. Anyway, by then we probably have experience already of bases on the Moon so may be able to use that to assess what impact a human base has on the area immediately surrounding a human habitat.

More ideas for these early orbital or flyby missions

With of HERRO, and indeed for the other missions also, you could send supplies to Mars in advance in separate duplicate spaceships before the human mission gets there. Most of the cost of an innovative mission is in the design, so it often adds little to the costs, percentage wise, to make several duplicates of the spaceship.

So, you have a habitat there already, in orbit around Mars,and with all the systems functioning including life support. Preferably, have two such ships filled with extra supplies, before you send the first humans there.

They would be fully fueled lifeboat ships able to get the crew back to Earth, or for them to survive in if systems in the main ship fail. You can also use them as extra living space at Mars during the mission, and as long term assets in Mars orbit.

Since these lifeboat ships don't need crew or provisions for the journey out - they could be filled with extra supplies, fuel and spare parts instead. These supplies could then be transferred to the main ship and used as extra shielding for the stay at Mars. In the worst case you can cannibalize the other ships themselves, for repairs, or if the main ship fails, transfer the mission to another ship.

And - if we were looking forwards towards such an expedition - all rovers to the surface of Mars could be fitted with binocular vision and hands with haptic feedback by default. Anyone who sent a spacecraft to Mars would be sure to set it up so that it can be controlled easily by telepresence whenever there are astronauts in close orbit around Mars.

Suppose we had a lead time, say of a decade in the run up to the first human missions to Mars orbit (during which we have human missions to L2 etc). Then by the time humans get there, we'd have a decade worth of Mars rovers and landers, all equipped to be controlled via telepresence, ready for use when the first human missions get to Mars orbit.

Later orbital missions could mine Deimos for materials, using the likes of the Kuck mosquitoes - dedicated small spacecraft to shuttle materials back and forth from the moons to the settlements. If there is ice in Deimos; you could use this as rocket fuel to export this extra shielding to the habitat,

Artificial real time

But there is another thing we can do - and that's to do autonomous exploration from Earth, using "artificial real time" which lets you drive a rover around on Mars even with a huge time delay of minutes. At the moment the way we control our rovers on Mars is hugely time inefficient. We could as easily control rovers on Pluto, because they download the data for one day, and use that to direct the rover's operations for the next day.

There's no point in trying to speed that up though, because it is hard to get a data link from Mars to Earth. Once a day is about all we can manage easily, because our orbiters have their own work to do.

If we have a dedicated link though between Mars and Earth, satellites in orbit around Mars just to relay signals to Earth - our rovers could be hugely speeded up.

And - in a situation like that, we could also speed them up so much that using this idea of "artificial real time" from computer games, we could control them almost as easily as a rover on the Moon (say).

(click to watch on YouTube)

Telerobotics with humans in orbit compared to robots controlled from Earth

That's not to say that humans to orbit controlling robots on the surface would be better than robots controlled from Earth, bearing in mind the costs of the two types of mission. I don't know if anyone has done a comparison study there.

You might be able to compensate for the advantage of humans in orbit by having many more robots on the surface for the same cost, especially if broadband communication is possible, better robotic autonomy, and techniques from gaming such as artificial real time (building up a copy of the Mars surface explored by your robot in your computer on Earth and navigating that to help speed up movement from a to b on Mars).

But a human expedition might well capture the public imagination and so permit a much faster exploration of Mars from orbit. And would be an exciting and fun expedition to follow, and interesting for the crew too.

As a later mission you could then go on to explore Phobos and Deimos. They have many advantages for exploration. For instance Phobos has meteorites and micrometeorites throughout its surface layer of regolith, from the entire history of Mars, back to when Phobos first formed or was captured. This probably includes meteorites from the time when Mars had global oceans and then later on, lakes. Our Mars meteorites on Earth all left Mars no more than twenty million years ago (because the terrestrial planets clear their orbits so NEO's have to be replenished over a twenty million year time period).

Deimos also has a Mars facing crater which helps protect it from cosmic radiation, and solar storms - Mars obscures it from the sun in its local daytime, except for a few hours a day. Deimos may well have ice too, as it is related to a type of asteroid that often does have ice in its constitution.

There are many other advantages and points of interest of Mars' two moons.

For more on this, see my:

Exploring Mars By Telepresence From Orbit Or Phobos And Deimos

So, how soon can we do such a mission? I suggested that while we explore the Moon robotically, we work on closed systems research, and also artificial gravity in LEO. That makes sense for a Moon base which you plan to keep occupied for years on end. But what about a first flyby of Mars? When could we try that?

Need for new comparison studies of the various ways of exploring Mars

The HERRO comparison was just a small scale study, done several years ago. But I don't know of any other. It's surely high time that we had a much more thorough and detailed comparison study of the various possible ways of exploring Mars. We may get practical experience of telerobotics in space with lunar missions in the near future. When that happens I think we'll find that machines are far more capable than they were in the days of lunakhod, operated from Earth most of the time, semi-autonomous, route finding on their own, able to do many things just by themselves with occasional help from Earth.

In a situation like that - operated remotely from Earth, or semi-autonomous, doing a lot of their own driving from place to place and then the crew in orbit around Mars step in to control robots that need particular help. I think that it would be much more than a 3 to 1 ratio compared with them working directly on the surface in spacesuits. Also, everything they saw would be streamed back to Earth in HD meaning that after an astronaut has just walked past a place and maybe glanced at a rock via telerobotics, amateurs and experts back on Earth can explore that footage with the same direct telepresence, binocular vision etc. experience, and maybe alert them to something they missed.

Also, yes humans are great at "on the spot" decision making. But they are also able to make very rapid bad decisions, or uninformed decisions.

If the mission is a scientific one, then you need on the spot scientific expertise. To really take advantage of the low latency on Mars, you need astrobiologists in orbit or on the surface, and astrogeologists. They are the ones with the expertise that will let them make fast on the spot decisions. An astronaut without that expertise is still dependent, much like a rover, on instructions from their team of experts back on Earth. That's the same for both surface and telepresence missions from orbit. But the orbital missions do have the advantage that just through their nature they require everything the astronaut sees to be streamed back to Earth. So if the "on the spot" mission specialists miss something, it's not a big deal because some expert back on Earth will spot it instead. The orbital missions also are safer missions to send the astrobiologists and astrogeologists to, once we get to the point where we can send missions safely as far as Mars. You don't have to ask an expert in astrobiology to take the huge risks of a landing on Mars and exploring the surface with spacesuits. If the mission is extremely risky as a surface mission is likely to be, that then restricts it to those who are willing to take huge risks with their lives, such as test pilots and those who like taking part in risky activities. But if it is reasonably safe then you can send scientist who wish to spend a couple of years of their life on the mission.

I think a proper comparison study has to take all of this into account. Enthusiasts for one or the other approach are bound to be biased somewhat to what they consider to be the best method to explore Mars. So,. I think a proper comparison study is probably best done by neutral parties or best perhaps, a workshop / panel that includes proponents of both sides in the debate as well as neutral parties. The cost of such a panel or workshop would be peanuts compared to the costs of the missions that we might commit to in the future for the exploration of Mars.

Compared with Mars surface missions

First of all, whatever the cost, I don't think that COSPAR should pass a humans to the Mars surface mission for planetary protection reasons.

Artist's impression of a human astronaut on the Mars surface holding Oskar Pernefeldt's proposed International Flag of the Earth - the linked rings symbolize how the different parts of Earth are linked together. (This is the latest of several proposed "Flags of the Earth").

Before a mission like that could be approved, a COSPAR workshop would need to show that it is consistent with planetary protection requirements, and would not risk introducing Earth life to Mars surface habitats.

Either that or there would need to be international agreement that Mars no longer needs to be protected from Earth microbes. To my mind, seems unlikely that either could happen before the 2020s or 2030s. As for the idea of a compromise based on humans contaminating only part of Mars, I find it hard to see how that could be approved by COSPAR either. How could the experts in the COSPAR panels sign their name to a statement that they know could lead to Earth life being irreversibly introduced to Mars? I don't really get it, how that could happen.

Meanwhile we could use telerobots to plant flags on Mars if that is the main aim of the mission or to touch Mars. Or if humans touching somewhere else other than Earth and the Moon are considered vital to this mission, we can plant flags on Phobos or Deimos and touch those moons instead.

In more detail there - the Outer Space Treaty is the only treaty we have to prevent siting weapons of mass destruction in orbit, or nations laying military claim to the Moon, etc - it's the main reason that we are able to do peaceful co-operative exploration of space. As well as the outcry from space scientists, the international upheavals resulting from something like this would be enormous. There is no way that the US or NASA could do this.

So, it's the same for planetary protection provisions based on the Outer Space Treaty. They are like quarantine laws; it doesn't matter how you get into space, you are still bound by them as a citizen of your country, which in turn is a signatory of the OST. The US has agreed to make sure that any US citizen or anyone using US hardware will keep to the provisions of the Outer Space Treaty. and the same applies to any other signatory of the OST which includes just about all nations either space faring or with space faring ambitions. The United Arabic Emirates hasn't yet ratified the OST but they will still keep to the provisions.

Cost savings compared with surface missions

It's interesting to notice that these orbital missions would cost less than a surface mission. Especially HERRO and the double Athena which Robert Zubrin proposed as a lower cost precursor mission. In a comparison study, for HERRO, completed in 2013, a single orbital mission for a crew of six does more science than three similar missions on the surface, for far less infrastructure and only a little over a third of the total number of launches (you don't have to land the large human rated habitats on the surface of Mars) Here is a powerpoint presentation from the HERRO team, with details of the comparison. This is their 2011 paper and this is their 2013 paper on the topic.

That's just one study, but it surely needs to be followed up with more detailed studies to check it. Also with the stimulus from 3D virtual reality computer games, the technology for telepresence has moved on hugely since then, so a new study would probably find it is even more of an advantage.

The reason the orbital missions can do so much more in the same time period compared with a surface mission is that

• You can control rovers anywhere on the surface of Mars, so can explore multiple sites at once.
• There is no need to suit up and travel to the area of interest, you can do it all with shirt sleeves environment within the spacecraft.
• When you take account of the reduced mobility of a human in a spacesuit, with clumsy pressurized gloves, then there's no great advantage of humans over telerobotically controlled rovers on the surface as far as mobility is concerned. Spacesuit technology of course will advance, but so also does telerobotic technology.

Then as well as that, there is no need at all to develop technology to land a human mission on the surface of Mars. That's not just a matter of delta v. You can land on Phobos or Deimos with a gentle use of delta v over a long period of time, and right up to the last minute, as for the Moon, if anything is wrong with your trajectory, you just abort and move away from the moon a bit, figure out what went wrong and try again, with hardly any waste of delta v due to the low gravity of these moons.

With a landing on Mars surface, everything has to go exactly right during the "eight minutes of terror" of the Curiosity landing. There's also almost no chance of humans intervening to save the mission if something goes wrong, as everything happens so quickly.

So - that's a whole new technology needed for a Mars surface landing that isn't needed at all for a Mars moon landing. And major human safety issues with a Mars surface landing that again are not issues at all for a Mars moon landing. You can study the Mars surface in a Molniya style Mars capture orbit, which requires less delta v than a surface mission. Even if it weren't for the planetary protection issues then telerobotic missions would seem to be the way to go for more science return and indeed a more immersive way to explore Mars than a surface mission.

Telepresence would be immersive, not using primitive mouse and keyboard control, and needs to be compared to humans in a pressurized spacesuit

First the robots controlled by humans would have some autonomy of course our rovers already have. For instance they may be able to drive autonomously or avoid collisions. On the spot decisions would be done by the humans. I don't think it is at all established that humans would have a significant advantage in a spacesuit over telepresence. It depends how you do the telepresence. Also, it's hard to simulate spacesuits on Earth - they need to be pressurized to the extent that the gloves are stiff and it requires significant effort to close your hands or move your fingers, for instance. Described as like wearing a garden hose over your fingers.

Also, if you navigate by using mouse clicks and keyboard presses and a computer screen it is very different from doing it with binocular vision, an omnidirectional treadmill, and haptic feedback. There are many more programmers working on computer games than on space systems and I think a way forward here will probably involve a fair bit of use of software developed for computer games. We've had several experiments in telepresence from orbit operating robots on the ground and each one is more advanced than the previous one - most recent one was a rover controlled from orbit by Tim Peake. But if we had all out effort to get this working then we could do much better.

The habitats themselves are almost the same in space or on the surface. The Mars atmosphere is not enough to make a difference. We have no idea what the effect is of Mars gravity - you can't just draw a line between zero g and full g. It could even be worse than zero g or better than full g and you can't join them with a straight line based on two data points. But even if Mars gravity is better for human health than full g, not impossible, we could generate it in orbit using artificial gravity. Yes you could have mobile habitats on the surface in a pressurized rover, but that is just adding to the complexity and remember those habitats still can't go anywhere near spots on Mars that are potentially habitable, again the question is, why all that extra difficulty, danger and expense to get those habitats to the surface when they could be in orbit, if the actual search for life has to be done remotely to avoid contaminating the habitats they search for and study?.

Summary of advantages of telerobotic exploration

Of course this needs a proper comparison study but there are many advantages to consider

• Best solution for planetary protection. It is hard to see how you could send humans to the surface of Mars without a risk of a hard landing which would contaminate a random area of Mars with all the hundreds of trillions of microbes in tens of thousands of species that accompany humans. If you introduce Earth life to Mars there is a major risk that you will detect life on Mars only to find that you brought it there yourself.
• Costs far less for more science return - one mission does as much as three to the surface, no need to develop the technology for humans to get to surface, or return from surface to orbit, no need to design for the Mars dust, and to keep the perchlorates out of the habitat.
• Safer . The landing on Mars is the most risky landing almost anywhere in the inner solar system.
• Crew don't endanger their lives in spacesuit EVA's. Crew at all times remain in shirt sleeves environment in the orbiting spacecraft. All they can endanger is the "avatar" rover they control on the surface. And that, if damaged, can be repaired potentially. While if e.g. you damage your air supply to a spacesuit you die.
• A rover can spend days, weeks, even months just at one spot on Mars using only electricity from sunlight while a human explorer has to return to base for provisions, oxygen etc frequently. For safety reasons a human astronaut would need to be close enough to a habitat or pressurized rover at any time to get back there on foot before their oxygen runs out. Even then they risk dying as a result of a sprained ankle or similar impairing their mobility.
• A rover doesn't have to put on a spacesuit every day, which takes up an hour or two of every day.
• Easily given capabilities beyond any human - easily given enhanced vision, can be made any desired size, can be given super-human powers such as great strength or dexterity or the ability to fly in the thin Mars atmosphere, because they can be made far lighter than humans.
• On the surface crew are exposed to the Mars dust, and perchlorates - they can use the suitport to keep them out of the habitat but this is another thing you have to design for which you don't need to think about in space
• Surface habitat and equipment has to be designed to cope with huge fluctuations of temperature. The Mars surface at night also often gets so cold that the carbon dioxide freezes out at night as frosts even for 100 days of the year in equatorial regions and the habitat has to cope with huge fluctuations of external temperature between day and night.
• Solar power is reliable in space - on the surface then every two years there's a possibility of Mars dust storms blocking out the sun, turning night to day
• Crew can explore several parts of Mars simultaneously, and "teleport" instantly from one experiment to another - leave one rover doing routine analysis while they drive another, or direct sampling for another - so the crew do all the interesting stuff and the rovers do all the dull stuff by themselves.
• Mars from orbit looks quite Earth like, an interesting planet and the elongated HERRO Molniya orbit is especially stunning with close flybys of the spectacular landscape and the polar caps every twelve hours, with the landscape skimming past below your spaceship followed by a long fly out so far that Mars becomes quite small. Every day you have that experience, twice, and each time coming in over a slightly different part of Mars. On the surface you'd be stuck in a single spot from then on and probably not see that much, and in the dust storms, nothing at all.
• When you drive the rovers on the surface with telepresence and haptic feedback, and virtual reality goggles to see the Mars landscape in 3D - you'd experience the surface vividly, far more so than if you were really there. Our eyes are not adapted to the Mars light and everything would seem dim and reddish brown, with colours hard to discern and a dull butterscotch sky. Exploring via avatars we can colour adjust automatically to resemble Earth lighting conditions, indeed with a blue sky if you like.
• Whatever you see is digital streamed (haptic feedback also), can be recorded and streamed back to Earth. Anyone on Earth can experience it as you did, and examine the images to see if we spot anything you missed.
• If anything goes wrong on the surface, you have everything recorded and streamed, so we can figure out what happened. There is no possibility of an accident where someone falls and dies and nobody is sure why it happened.

The main thing they lack is dexterity but this is rapidly improving with advancing technology. They are used for telerobotic surgery and deep drilling and many applications. You couldn't do surgery while wearing a spacesuit which I think helps highlight how telepresence could give you more precision of control and accuracy than hands on study of Mars in a spacesuit.

NASA organized a recent telerobotics symposium in 2012 and the conclusion of the conference was that telerobotics has great potential value for Mars exploration. The conference recommended their use during early orbital missions to Mars by humans, saying that a great opportunity would be missed if telerobotics was not used. But - why not just use telerobots from now on until we are sure of what we want to do on Mars?

Artist's impression of telerobotic exploration of Mars for the 2012 Exploration Telerobotics Symposium

Humans on the surface could start a new geological era on Mars - if they introduce Earth life irreversibly.

The eras so far, as we saw, are the earliest Noachian period of high meteorite bombardment and seas, the Hesperian period of volcanic activity and huge floods, the Amazonian period, as it is now, dry with some liquid water, a bit volcanic activity and occasional floods. On Earth geological epochs are named according to the prominent biology on the planet. So why not on Mars?

If some Earth microbes survive on Mars, and spread, irreversibly, this could potentially create a new geological era, with Earth life dominating the biology and any processes that involve habitable liquid water throughout the planet. So, if we introduce Earth life irreversibly to Mars, we'd need a new name, maybe call it the Anthropocene again, as on Earth - of a Mars with Earth life on it, introduced by humans. And the future biology would depend on whatever mix of species were introduced, accidentally or intentionally, and the way they interacted and evolved from then on. There may be many different such futures, many ways that Mars could develop biologically depending on which species get there first.

From then on for all future time, our civilization and all future civilizations on Earth would never have the opportunity to study the Amazonian period, with whatever unique lifeforms it might have. As we've seen in this book, it is well possible that Earth life could make Amazonian period Mars life extinct. After all, according to most theories of the origins of life, DNA life made its precursors on Earth, whatever they were, extinct. And later forms of DNA life made many earlier forms extinct. So it's certainly possible for one form of life to make another extinct over an entire planet.

Land humans quickly - to explore nearby regions kept free of Earth life - NASA's approach

NASA's planetary protection office say that their job is to work out how best to protect Mars in the event of a successful human landing. In their list of knowledge gaps for human extraterrestrial missions, they cover such things as leaks of microbes from spacesuits in EVA, and transport of microbes in the dust storms. But there is no mention at all of the effects of a crash anywhere in the list. They have to assume a 100% success rate for humans landing on Mars as without that assumption they would not be able to recommend any measures that could protect Mars from Earth life, even temporarily. They leave the effects of a crash on the search for life on Mars. for NASA and mission planners to consider on a per mission basis later on.

Also, they no longer aim for biologically reversible exploration of Mars in the case of a human landing. They see contamination of Mars by Earth life as an inevitable consequence of a human landing there. The aim is to control and understand the effects of this before we land there.

From this 2015 report:

• "No habitat or EVA system will be fully closed. Therefore missions carrying humans to Mars or other bodies will contaminate the planet."
• "The increased capabilities that human explorers can contribute to astrobiological exploration only exist if human associated contamination is controlled and understood. It is therefore critical that attempts be made to seek evidence of putative past and/or present extraterrestrial life, especially on Mars, well before these missions occur and to identify, characterize, minimize, and control contamination sources and pathways that would threaten such evidence."

If I understand it right, then their aim is to limit the rate of spread of that contamination over the surface of Mars so that they can do valuable science there before the contamination by Earth microbes can get to habitats of interest to science such as the RSLs.

Cassie Conley, NASA's planetary protection officer was David Livingston's guest on the SpaceShow in March 2016, and gave a very interesting talk about planetary protection. I posted some questions to her via email, which David Livingston read out during the show, so you can listen to her answers to them and get NASA's perspective on these issues in more detail. You can listen to her talk here.

Cassie Conley has also said she thinks Elon Musks' ideas have planetary protection issues, in an interview with National Geographic just before his big announcement here: Cassie Conley. Going to Mars Could Mess Up the Hunt for Alien Life

“The excitement over this pending announcement overshadows a worrisome dilemma: The special regions where Earth life could take hold are also the areas where we would most likely find indigenous Martian life. That means—unless we are very, very careful—we could ruin our chances of discovering extraterrestrial organisms just by going to look for them."

“It’s like looking for stars when the sun's out. If you want to find Mars life, you have to get rid of the signals of Earth life so that you can see it.

There are no detailed guidelines yet for humans to Mars. These would be made by the international COSPAR committee which meets every two year. All of their discussions to date have ended without any firm recommendations, saying that more information is needed before they can do that.

Humvee journey through the Arctic - contamination study

I should mention this as it is often cited, a study by Andrew Schuerger and Pascal Lee from 2010. Enthusiasts for Mars colonization often mention this study, saying it shows you can explore Antarctica with humans in situ with little or no contamination. But of course there was. You can't have a mission with humans traveling through the Arctic without a lot of contamination by microbes wherever they walk.

Their result was more limited than that. They found very little contamination at a distance of ten meters from the human activities. It fits in with NASA's idea of containing the contamination as much as possible around the landing site. Their aim was to show that after even long journeys with a contaminated rover over pristine terrain, that areas of the surface just ten meters from the rover would not experience much contamination. They also end with an optimistic note that after the humans return to Earth, that the levels of contamination they left on Mars would gradually reduce as a result of the many biocidal factors of the Mars surface, especially the UV light

First, the background. These researchers drove a humvee through the Arctic, and looked at how the microbes spread out from their vehicle, which they deliberately did not attempt to sterilize in any way. They found very low levels of contamination at a distance of ten meters from the rover. They took this as a "worst case" analogue of a mission on Mars in a crewed rover.

I think it's important to realize what this study was, and what it wasn't. They were not saying that the ground around a human base would be free of microbes. As we've seen, in Risks of invasive microbes in lake Vostok and Antarctica (above) after a ten day camp in Antarctica following the corral system, you'd expect a hundred thousand microbes per cubic centimeter around the camp. Spacesuits might reduce those numbers, but not to zero, because they leak, and anyway they weren't simulating the use of spacesuits. And indeed the pressure differences between the spacesuit and the surroundings, or between a habitat and its surroundings might actually act to increase the dispersal of spores.

They weren't trying to prove zero contamination by microbes. Instead, they were interested in the amount of contamination at a distance of ten meters from the rover itself and from human activities around it. They were interested in how the microbes disperse in extreme environments like that.

"The snow sampling was designed to measure the aerial dispersal of microbial cells and spores falling upon undisturbed snow within ~10m of the rover and not in its immediate vicinity. Snow and ice surfaces upon which the crew walked, prepared food, conducted science experiments, eliminated wastes, and worked on vehicle maintenance were not directly sampled. Thus, the sample design was specifically constrained to measure aerial dispersal from human activities around a contaminated rover within a 10 m radius of the crewed vehicle but not the direct footsteps of the crew."

So yes, they accept that a rover and base would of course leave a microbial footprint on Mars, but perhaps the contamination would not spread much further from the base. In particular, if astronauts set off on rover journeys across Mars, it might have less microbial impact than you'd expect. They were studying specifically "early sortie missions in pristine environments"

"Based on our results here, and the literature cited above, we propose that the dispersal of human-associated microorganisms is likely to be low during early sortie missions in pristine environments, is likely to increase slightly over time with continued exploration of a specific site, and will decrease over time once the exploration of a given site is halted. Further study is warranted for human exploration activities at temporary field sites on pristine terrains in order to characterize the temporal changes in dispersed bioloads and microbial diversity."

They suggest that in the harsh conditions on Mars, it may be possible to tolerate low to moderate levels of contamination at sites knowing that the harsh environmental conditions present will constrain long-term survival or colonization of nonindigenous species.

They said it should be seen as a first order approximation. Also the rover was unpressurized, and a pressurized rover might be more of a contamination risk. They didn't study archaea or other uncultivable microbes for reasons of time and budget.

They suggest that UV light would sterilize the external surfaces of the spacesuits, rovers and habitats.

"Numerous studies have demonstrated extremely short survival times of viable terrestrial microbes under martian conditions and suggest that microbial contamination on external surfaces of spacesuits, equipment, rovers, and habitats will survive only a few hours to a few sols on Sun-exposed surfaces ... Although dust settling onto vehicles or spacesuits may attenuate a portion of the UV irradiation incident on surfaces, the biocidal activity of UV photons on the viable contamination is likely to be significant and accumulative because the dust does not cover the entire surface and scattering of UV photons around dust particles is likely to occur "

However, this is extrapolation and not something they tested of course. unless I'm missing something here, they don't seem to consider the effect of shadows, or the dust storms in this part of their discussion in the paper. At any moment, half of the rover at least is in shadow, and the same is true for astronauts and for the habitats. Sometimes the entire rover may be in a shadow, and at night of course, there is complete darkness.

UV light is blocked out by a shadow, apart from any scattered light. Also there are many shadows around the rocks. A microbial spore that falls from a habitat, astronaut or rover on its shaded side, or in the shadow of a hill (especially in the evening or early morning), or at night, and lands in a shadow can survive on Mars not just for minutes or hours, but indefinitely. The DLR experiments showed that some cyanobacteria can even "wake up" and photosynthesize and metabolize in semishade on Mars.

It would just depend on where the microbe ends up. A microbe deep in a crack in a rock, beneath an overhang,, even embedded in a crack in a larger dust grain, will be protected even from most of the scattered UV light. Then, when the dust storm season comes, then they can be lifted up and moved almost anywhere on Mars.

Also (my own observation here), it was also a short term study. They just looked at how microbes were dispersed immediately after they arrived there. They didn't look to see if any of the microbes dispersed from the rover found habitats there and survived longer term.

Anyway - they recognize that a mission on Mars is like a mission in Antarctica, and that there would be microbial contamination around the base. It's more about how far that contamination would spread. Also the end remarks discuss whether the Mars environment could be so biocidal that it reverses most of the effects of the contamination quickly.

Explore Mars from orbit by telepresence first - but still land on Mars towards the end of the 2030s - The Planetary Society's approach

Emily Lakdawalla, planetary geologist who often reports for The Planetary Society, expresses a similar sentiment in this article

"NASA recognizes that the potential for contamination is a problem, so there is a Planetary Protection Office that is specifically charged with overseeing how missions avoid contaminating Mars with Earth biota. There are two main approaches. One approach is to sterilize the heck out of anything that will actually be touching Mars. That's why Curiosity's wheels were specially wrapped throughout its final assembly, and why it was such a scandal that the drill bits were handled after sterilization. The other approach is to avoid landing in any location where you might encounter -- or accidentally create, should you crash -- a present-day habitable environment where Earth microbes could thrive. For instance, current rules prohibit NASA from targeting a mission containing a hot radioisotope thermoelectric generator (such as Mars 2020) anywhere near a place where a failed landing might place that generator close enough to subsurface ice that the heat of the decaying plutonium could melt it.

"But all bets are off once you send humans to Mars. There is absolutely no way to make a human clean of microbes. We are filthy with microbes, thousands and thousands of different species. We continuously shed them through every pore, every orifice, with every exhalation, and from every surface. True, almost all of our microscopic friends would fail to thrive in the radiation-baked, intensely cold and arid Martian environment. But life is incredibly tenacious. Sooner or later, humans will get to Mars; even if they die in the attempt, some of their microbial passengers will survive even the worst crash. Once we've put humans on the surface, alive or dead, it becomes much, much harder to identify native Martian life.

"This is one of many reasons I'm glad that The Planetary Society is advocating an orbit-first approach to human exploration. If we keep our filthy meatbag bodies in space and tele-operate sterile robots on the surface, we'll avoid irreversible contamination of Mars -- and obfuscation of the answer to the question of whether we're alone in the solar system -- for a little while longer. Maybe just long enough for robots to taste Martian water or discover Martian life."

The Planetary Society organized a workshop Humans Orbiting Mars in 2015 to explore the idea of exploring Mars from orbit first. Their report proposes a stepping stones approach to Mars, with missions to the Moon and asteroids, followed by missions to the moons of Mars and then boots on Mars towards the end of the 2030s.

Stepping stones approach proposed by the Planetary Society. See page 18 of their summary. Lockheed Martin proposed a similar approach previously.

That's great as far as the exploration by telepresence. But what about the next step of sending humans to the surface as soon as the 2030s, or indeed within any fixed timeframe? Let's try dramatizing this to show the idea:

Prestige or dishonour, first footsteps on Mars

Yes it would be an exciting day when the first human steps on the surface of Mars, and a matter of prestige for those concerned. This is something I have read about since a child, and I would be as excited as anyone. But all that praise and excitement might quickly change to dishonour if we find that they introduced reproducing Earth micro-organisms to Mars. 

The first humans to land on Mars - or to crash there - might easily enter the history books as the people who contaminated Mars irreversibly, making it almost impossible for scientists to study it properly. That's a big deal given that our main scientific reason to go to Mars is to search for life there. To dramatize the idea, here are a series of "Future Possible News" stories in a (hopefully) "alternative future" in which humans accidentally introduce Earth life to Mars, then regret what they did.

(photo shows Artist's impression of a human astronaut on the Mars surface holding Oskar Pernefeldt's proposed International Flag of the Earth )

(Photo is actually of a slope with RSLs from this paper)

#

(Uses artwork by Dmytr0 from wikipedia)

(Photo is of nanobes from "New life form may be a great find of the century" (1999), at one time thought to be relic RNA world life here on Earth)

(Photograph is Hubble's photograph of a Global Mars dust storm from 2001 )

Actually, if the Earth life they found was salt loving haloarchaea, they don't seem to produce spores (archaea generally probably don't). Instead they have other ways of coping with desiccation including dwarfing of the cells, protective capsules and probably forming dormant states. That's why the article says "spores and other dormant states".

The Lascaux cave painting photo is by Prof Saxx.

I made these “Future Possible News” stories with this online spoof newspaper generator. I invented the name of the astrobiology mission specialist using this online fake name generator.

This short story describes a mission similar to NASA's plan for safe zones - based on finding Mars life easily (above), according to which they would land the humans inside a region which they judge "safe to contaminate" with Earth microbes. Outside of that, not too far away, they'd have a potentially habitable and biologically interesting site for them to explore using sterile rovers, such as one of the sites with Recurring Slope Lineae. So what happens if the party get caught up in a major Mars dust storm, and then the dust carries microbes from their base to the RSL? That's the basis for this story. My first draft of it had a crash of the human occupied ship, as in Why do spacecraft crash so easily on Mars (above), but I removed it to keep the storyline as simple as possible.

Why not let those first steps be taken by a telerobot instead, operated by a human in orbit around Mars?

Inspiration value of telerobots on Mars

Advocates for Mars surface colonization often talk about the inspiration of a human surface mission. But we have seen from the rover missions what fondness the general public have in their hearts for our robotic emissaries on Mars such as Pathfinder, Spirit, Opportunity and now Curiosity. Also look at how much interest and excitement there was for the Dawn mission to Ceres and the New Horizons mission to Pluto, and indeed, the Philae Lander.

Incredible news! My lander Philae is awake!

ESA Rosetta Mission tweet after first successful landing on a comet.

I am sure this would apply even more so to telerobots on the Mars surface operated by humans in orbit around Mars. There would be the human stories of the astronauts orbiting Mars, their spectacular views of Mars like the Earth views from the ISS but the planet is Mars, and then this story of the avatars they are driving on the surface of Mars, all the photos of the planet as seen by them from orbit, and the terabytes of streaming video from the Mars surface returned to Earth. For many I think that telerobots on Mars would be more exciting and interesting than a surface mission, especially if the reason for doing it this way is also well understood. And if this is the best way to find life there, and they do find life, then that by itself would make it of tremendous public interest.

Also for Mars enthusiasts a telerobotic mission, with live immersive 3D landscapes you can follow every step of the way - that would be a very exciting mission to follow from Earth. You could even go back and look at rocks and other features close up in 3D which the team had passed earlier in the day, even weeks ago, and perhaps spot something of interest there that they never noticed. We'd probably get citizen science projects just looking for features that the team had missed as they drove across the landscape via telepresence.

Biologically reversible exploration of Mars

The astrogeophysicist Christopher McKay has talked about the need for all exploration of Mars to be biologically reversible in the sense that, if necessary, we can remove all the microbes we brought to Mars, at least until we have a better understanding of Mars. He has also suggested that if we find a second genesis of life on Mars, biologically unique and different from Earth life, we might want to adapt Mars, to make it more habitable for native Martian life even if the result does not make it into a planet suitable for Earth life.

(click to watch on YouTube)

The idea is that if we find interesting life on Mars, we can remove all our contamination from the planet and leave Mars for the Martians instead, even if they are just microbes - so that we can study the biology there. Maybe even get to restore the early Mars climate, if we can find a way to do it.

Here is the same idea in his paper from 2007, Planetary Ecosynthesis on Mars: Restoration Ecology and Environmental Ethics. At the time he wrote this article, back in 2007, he thought it was possible for not just robots, but also humans to explore Mars in a biologically reversible way. Here is how he put it:

"Scientifically, having another example of life expands the scope of biology from one to two. There may well be significant advances in medicine, agriculture, pest control, and many other fields of biological inquiry, from having a second type of life to study. I would argue that if there is a second genesis of life on Mars, its enormous potential for practical benefit to humans in terms of knowledge should motivate us to preserve it and to enhance conditions for its growth.

Observations of Mars show that currently there is no global biosphere on that planet and if life is present it is in isolated refugia or dormant. It is possible that life present on Mars today is at risk of extinction if we do not alter the Martian environment so as to enhance its global habitability.The utilitarian arguments presented above indicate that we should alter Mars to allow any indigenous life to expand and form a global biosphere even if the resulting biosphere is never a natural home for life from Earth or humans.

"...This discussion has implications for near-term exploration of Mars by robots and humans. Until we know the nature life on Mars and its relationship – if any – to life on Earth, we must explore Mars in a way that keeps our options open with respect to future life. I have argued elsewhere that this means that we must explore Mars in a way that is biologically reversible. Exploration is biologically reversible if it is possible and practical to remove all life forms carried to Mars by that exploration. Because of the high UV and oxidizing conditions on Mars, biological reversibility is achievable.

"... Previous missions to Mars, such as the Pathfinder mission and the two MER rovers, have carried microorganisms to the martian surface where they remain dormant as long as shielded from ultraviolet radiation. To reverse this contamination already present on Mars, it would be necessary to collect all metal objects within which microbes could remain viable. Furthermore, the soil at crash sites and in the vicinity of landers that had come into contact with the spacecraft would have to be thrown up into the atmosphere where it would be exposed to sterilizing ultraviolet radiation. A similar approach can be used to reverse the contamination from human bases."

Quote from his Planetary Ecosynthesis on Mars: Restoration Ecology and Environmental Ethics

This was written before the Phoenix lander's discoveries, and the new extremophiles found in many Mars simulation experiments on Earth. At the time he wrote that, then it was reasonable to suppose that all you have to do to reverse contamination after a human landing there is to collect all the metal objects on Mars and also throw up the soil around the base into the air so that it gets sterilized by the UV radiation from the Sun.

However since then we have found that many microbes far from being sterilized in seconds, are capable of withstanding hours of direct Martian surface UV before they die (and indefinitely in partial shade or protected by a thin layer of dust, gypsum or rock), see Ultraviolet radiation (above). Also, we now know of many more potential habitats for life on Mars than we knew about back then, including several types actually on the surface, such as the RSLs and Nilton Renno's droplets on ice / salt interfaces. So far there is no way to tell if any of those are habitable but some or even most of them might be.

This makes it very challenging to do human missions to Mars such as the ones proposed by NASA to explore Mars in a biologically reversible way. See 35 minutes into this video by Cassie Conley (answer to an email question from me). She says that this is discussed, sounds like a really good idea, but when you try to figure out how to do it, then it becomes really challenging and she wasn't aware of a mission with humans to the surface that proposes to be able to do that.

It may still be possible for the robotic missions to be reversible, indeed most think they have been so far, as they can be sterilized far more thoroughly than a human lander. So I'm not sure what his latest thinking is on this, if he has dropped the idea of biologically reversible exploration of Mars. Most of his main papers on the subject are from before the Phoenix lander results about Mars.

Incidentally, though I am right with him about the need to protect native Mars life if there, I'd like to look a bit closer at one of his other points. He doesn't think we need to protect a lifeless Mars. I'm not sure what his latest thinking is on this but in his 2007 paper he writes:

"If there is no indigenous life, these utilitarian arguments indicate that we should alter Mars to support life from Earth even if this never results in a biosphere that can be a natural home for humans.".

So first, yes, if we had warp drive and had many Mars like planets were within easy reach of Earth, Mars with uninhabited habitats might well be of little science value. There must be so many of them in our galaxy, so we can just warp to another solar system to find another one if we want to find out what happens on a pristine Mars without any introduced Earth life.

Star Ship Enterprise flying past a planet resembling Mars, from opening credits of remastered Star Trek original series. If we had warp drive and there were many planets like this in our galaxy within easy reach of Earth, then Mars in its pristine state might not be of much scientific value.

However as we are now, we don't have warp drive. It does us no good that our galaxy probably has numerous Mars like planets as we don't yet have any realistic way to explore any of these other planets close up. We are so lucky to have a Mars like planet, and a Venus like planet as well as Earth in our own solar system. If they weren't there we'd have no way to study such planets at all.

How far did the chemistry on Mars develop in the direction of evolution of life? What has happened to the habitats on Mars that actually have organics in them, from comets, asteroids, or made locally through abiotic processes? Do we have "almost alive" complex chemistry, or just simple chemistry and tarry organics like the tholins? We could learn a lot from this about how difficult or easy evolution is, and what typically happens on Mars like exoplanets. We could also learn a lot about how to disentangle effects of biology and abiotic processes. Not just study what happened in the past, but study the abiotic processes such as the RSLs ,the flow-like features, effect of humidity in the atmosphere and the frosts on the surface, deep subsurface hydrosphere, in the present day, many slow processes all working without biology over an entire planet.

Once you start to think this way, to think of Mars as an example of a typical Mars like exoplanet, in our own solar system, perhaps it puts these ethical and utility value discussions in a new light. For as long as there are habitats for Earth life on Mars, inhabited or uninhabited, almost any scenario where present day pristine Mars would be of great scientific interest I think, "as is" as an exoplanet analogue or for its value for exobiology. Just because of its uniqueness, for us, as the only such planet in our solar system.

Best places to introduce Earth life right now - while continuing with biologically reversible exploration of Mars

If you think about it this way, of Mars as like an exoplanet in our own solar system, of great science value just "as it is" then the best places to introduce Earth life right now are places where no life can survive without our help. Such as the possibly vast lunar caves, see my An astronaut gardener on the Moon - summits of sunlight and vast lunar caves in low gravity in MOON FIRST Why Humans on Mars Right Now Are Bad for Science . The moon is the closest and safest place to try this.Or in free space - if we use the asteroid belt to make large colonies turning slowly for artificial gravity, see my Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths.

The idea of colonizing asteroids goes back at least as far as Dandridge MacFarlan Cole and his 1963 book ‘Exploring the Secrets of Space: Astronautics for the Layman’ written more than a decade before Gerard O'Neill's pioneering work. He suggested hollowing up an asteroid, by controlled use of nuclear devices, which melts it from inside. It's filled with water first which would turn into steam and expand making it into a "bubbleworld" as such a construct has come to be called. You then spin it for gravity. This rather charming 1960s illustration shows the concept.

This shows his concept from outside and inside

The asteroid, once expanded, would be 20 miles long and 10 miles in diameter, and have an internal surface area of 600 square miles (60 by 10 miles) and would support a colony of 10,000 to a million people. The artwork is by the space artist Roy Scarfo.

More on him in the article in Centauri Dreams, and

Nowadays the idea is rather to use the asteroids as a construction yard to get materials which we'd use to build space colonies, but it's much the same idea. You might think the asteroids are too far from the sun, but first, we can start with NEO's and we can move the materials from the asteroid belt to construct stations that orbit Earth or anywhere else. But as well as that, it's easy to make a thin film mirror to concentrate sunlight as much as you like. We could build such colonies, with plenty of sunlight, right out to Pluto and beyond, still using our sun as a source of illumination.

Or we could use the lunar caves. The cave entrances on the Moon probably open out into vast lava tube caves which in the lowe luna gravity can be up to kilometers in diameter - and the Grail data suggests some of them are over 100 kilometers long. Potentially these could be as vast inside as an O'Neil colony. See Instead of terraforming Mars in a multi-millenium project, why not terraform a lunar cave in a multi-decade project?

See also my answer to "Why not send Earth life to Europa" in Plenty of places to experiment with sending life to other places in our solar system - Asteroid belt resources, NEO's, caves on the Moon

Also we can work on making Earth more habitable, which could support many times its current population of humans if we used the technology suggested for habitats in space, for instance with floating sea cities, or habitats in deserts, self enclosed like a space colony so having minimal impact on the rest of the Earth, even, for instance, bringing water to deserts that have recently been denuded by human activities. I cover those in What about Earth deserts? and Seasteading in my MOON FIRST Why Humans on Mars Right Now Are Bad for Science.

Meanwhile, why not continue to explore Mars in a biologically reversible way, even if all its habitats are uninhabited? Especially, why embark on non biologically reversible exploration with humans so quickly, as early as the 2030s or 2040s? Mars "as is" is of great scientific interest as a planet that used to be as habitable as Earth. Even if it doesn't have life and never had life - it gives us a precious opportunity to study a terrestrial planet of that sort, small, cold, habitable early on. The nearest Mars-like planet outside our solar system is likely to be many light years away.

Then there's another consideration too. If we do decide to introduce Earth life to Mars, there may be many possible final states Mars could end up in.

Example end states of introducing Earth life to Mars, as covered in Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?

How do we direct it or choose intelligently between those end states at present?

We have often found on Earth that our introductions of new lifeforms don't go exactly to plan even they are worked out in advance, and with a fair bit of experience of introducing Earth life to new places on Earth before. It's even more unpredictable when it's an accidental introduction. So we can expect this unpredictability even more so when it is an accidental introduction of whatever microbes sneak on board the first human ship to land on Mars.

For instance there's Cassie Conley's idea of a lifeform that accidentally turns most of the water on Mars to cement. There must be many ways in which unintended microbes on Mars could mess up our plans to transform Mars into a new form of biosphere. See Closing off future options - hard enough to roll back introduction of feral camels to Australia, never mind of microbes to Mars (above).

Could we send humans to the Mars surface in a biologically reversible way? Humans in a metal sphere?

As a result of our new understanding of Mars it seems really tricky to send humans to the Mars surface in a biologically reversible way. Apart from anything else, our spacecraft must not crash. How can we guarantee that? See Why do spacecraft crash so easily on Mars (above).

Well actually there might be one way that we could do that. Not so much guarantee that our spaceship won't crash, but guarantee that no life will escape if it does. However the result is a severe limitation as we have to use a featureless metal sphere with probably no airlocks or windows. That's because featureless spheres are really strong and could potentially be guaranteed not to be breached. Even a hollow spheres can withstand just about anything, and perhaps the capsule could be guaranteed to remain intact, or at least, crushed slightly but not breached, after a crash on the surface?

If so, arrange for your sphere to enter the Mars atmosphere at a shallow angle, to ensure that it slows down to terminal velocity of a few hundred miles per hour, with a human being inside. It then decelerates to the surface, via skycrane,or whatever system is used. Because it is bound to slow down at least to terminal velocity then even if all systems fail, it would hit the surface at at most a few hundred miles per hour. If it crashes, the human would not survive, but a sphere can survive such a crash if it is made of strong materials. Perhaps make it of solid tungsten, which has the highest melting point of all metals, at 3,410 °C (6,170 °F).

Hollow spheres (rocket parts) often re-enter the much thicker Earth atmosphere and survive intact to the surface. This hydrazine propellant tank from a rocket:

survives to the surface looking like this

(Space ball found in Namibia)

If all goes well, the human inside the sphere would land on Mars and then could be lifted off again - but of course can never leave the sphere or even look outside directly (as it is opaque), so I'm not sure if this would be thought worth doing. Could you make a transparent sphere that's as robust as this? How could a transparent sphere be this strong?

Perhaps one way to make this into a potentially worthwhile mission is to arrange it so that the human inside communicates via telepresence. They control a rover on the surface, and the advantage is that this mission puts the rover so close to the sphere that you have no light speed delay at all. Perhaps the rover carries the sphere containing the human or humans as cargo. Could that be worth doing?

If:

• The exterior of the sphere is sterilized thoroughly before it enters the Mars atmosphere
• It can't be breached, even if all the landing systems fail, or there's an explosion etc
• The only communication is via telepresence
• With the human supplied with everything they need for the entire mission inside the sphere

- then it would surely be biologically reversible. Just lift the sphere off Mars somehow at the end of the mission, to reverse the contamination of Mars by the Earth microbes inside the sphere.

Another suggestion for a potentially biologically reversible human mission is to send humans to Olympus Mons.

Olympus Mons Caldera Region This might be the area on Mars most biologically isolated from the rest of Mars of anywhere, due to the thin air, high altitude, and the caldera walls. But it is a difficult place to land technically, and also - would even this be a biologically reversible place for a human base on Mars?

The value of the Olympus Mons Caldera region is that it is so high above the surface that the air is very thin and there is almost no dust. That's why it was suggested as a destination to help with planetary protection in an article in The Space Review. It's a major challenge to do this with present day technology however. See Rob Manning's talk on the Space Show where he talks about the challenges associated with landing a spaceship at a high altitude on Mars, where the air is so thin, so the aerobraking much less.

In the space settlement article, they are suggesting a future with new technology with human bases already on Deimos. Some of the technologies Rob Manning mentions could be relevant such as deployable extending heat shields, or using larger parachutes than any of the ones tested supersonically to date. In any case, there's the same problem as with other human landings - we are unlikely to have 100% reliable landing systems.

Yes, if there is anywhere on Mars, where humans can land in a biologically reversible way, then this crater at the summit of Olympus Mons might be it. Or if not biologically reversible completely, perhaps in a way that keeps the landing site separate from the rest of Mars.

However, even with your target the caldera at the summit of Olympus Mons, a failure during approach to Mars, entry, descent or landing could easily land you somewhere else, hundreds of kilometers away. And would a human party - say inside the Olympus Mons caldera - really be biologically insulated from the rest of Mars? Also after a crash there? And would such a landing be biologically reversible in the future, if we need to remove the spores from Mars?

I think it would take a lot of research, including missions to find out more about Mars, to be sure of this. If the mission is not completely biologically reversible, you always have the possibility of an "Oops!" moment where you realize you have introduced Earth life to Mars, can't remove it, and have found a lifeform there you want to preserve or a biology such as ancient RNA based life, and can't do anything to prevent its eventual extinction. I.e. the scenario in the Prestige or dishonour, first footsteps on Mars (above).

Plants on Pristine Mars

You can put plant seeds on the surface, as these can be sterilized. You can grow plants with hydroponics and aeroponics. The difference here is that with hydroponics the plants grow directly in water, and in Aeroponics you use a mist, so it needs less water (which may be useful on Mars). There are different versions of these technologies. Some depend on micro-organisms but some do not. Instead of having micro-organisms you supply whatever the plant needs in chemical form in either water, or if it is aeroponics, in the mist.

The only life you have is the plant seed. You may use the Mars soil, or just use mist, depending on what you want to do. This microbe free version of hydroponics / aeroponics  introduces no risk of contaminating Mars so long as you do it with great care. The only thing  that will grow on Mars as a result of this experiment are these plants that you introduced to Mars. Here is a rather charming designer's concept for plants on Mars, called the "Little Prince Rover".

"Little Prince" rover designed to support a single plant on Mars. Since seeds can be sterilized (unlike humans or animals), these could be grown without any risk of contaminating Mars with Earth micro-organisms.

Named after the "Little Prince" who looked after a single rose on his asteroid in the fictional book by Antoine de Saint-Exupéry

It's possible that plants may be the first living Earth colonists of another planet.

(click to watch on YouTube)

So that introduces the possibility that you could have greenhouses on the surface of Mars and these could grow food for the colonists in orbit. They may have plenty to eat in their habitats anyway by then, but you could grow food on Mars too, maybe delicacies or things that grow particularly well on Mars or medicinal plants or whatever. Also you could have large plants, maybe trees (perhaps growing far larger in the Mars light gravity) or whatever else grows best on the surface of Mars.

Sadly, we can't apply these same principles to humans. If only we were like plants and could be grown from sterilized seeds. Well in principle, in the future, it could be possible by adapting the far future idea of "Embryo space colonization". If you could grow a human from an embryo in a sterile environment, and somehow supply them with all the nutrients they need without the symbiotic microbes we all have. Or else, engineer the microbes to make sure they can't survive on Mars. That is just science fiction at present. Supposing it were possible, it has drawbacks, as the Martian colonists would be vulnerable to Earth microbes if they ever left the planet, not being adapted to them, and anyone who left would not be able to return.

Perhaps there may be other future ways to do it. E.g. an impervious spacesuit or rover that can't be damaged even in a crash on Mars, that is also a biobarrier so that no microbes can get in or out and with the outside 100% sterile. This is way beyond anything we have at present however.

Biologically reversible exploration gives us a breathing space and leaves our future options open

The idea of biologically reversible exploration is to give us some breathing space, of a few decades, hopefully, to find out about Mars on a scientific level. To find out if there are habitats there for Earth life and search for exobiology. Meanwhile, you are also building up an infrastructure on Mars and in Mars orbit that would be useful if we did ever decide to send humans to Mars. Or indeed, it could be useful for other things too, anything we might do on Mars.

Perhaps you decide to try ecopoiesis (duplicate the biological transformations of early Earth on much faster timescales), or you follow Chris McKay's proposal to turn the clock back to early Mars, or transform it in some other way, or even grow plants there (plants could be grown on Mars using sterile hydroponics without impacting on any native Mars life, since seeds can be sterilized).

You also continue to have the option to leave Mars as it is, and not try to transform it into a different planet from what it is now. There would be many possible futures still open to you at that point. The future remains wide open.

Also meanwhile we can work on space habitats, closed systems, eventually build city domes on the Moon and large closed systems in the lunar caves, continue to explore ideas for creating larger and larger self sustaining habitats. Whether we eventually get to the point of terraforming entire planets, I think can be left to later, until we have much more understanding than we have now, with these early experiments. So, then it becomes an open path, where instead of closing off futures, we open out to more and more possible futures, and wider vistas at every step. These vistas don't just include Mars either but many destinations for humans in the solar system.

What if the decision is to keep Mars biologically pristine for ever?

What if we find independently originated life on Mars, or amazingly interesting evidence of early stages that almost reached life but not quite? Should we leave the planet pristine to avoid contaminating it? That would be an exciting prospect seems to me. It's like having our own exoplanet, with its distinct biology, in our own solar system. The nearest terrestrial planet like that, other than Mars, may be light years away. Depending on future technological progress, it might be centuries before we have a similar opportunity - or if life is rare in our galaxy, maybe even millions of years.

I would say why not? Let's go all the way to Mars, and set up colonies in orbit around the planet, but never set foot on it at all, to avoid contaminating it. It is a bit like mountains that are left unclimbed out of respect for the mountain or local beliefs. Not too many of those but the mountains in Bhutan over 6000m are unclimbed.

This is possibly the highest unclimbed mountain in the world Gangkhar Puensum with an elevation of 7,570 m. All mountaineering is prohibited in Bhutan since 2004 out of respect for local religious beliefs.

This would be a future where you have agile rovers on the surface. and increasingly sophisticated humanoid avatars on the surface as well, directed and teleoperated by colonists in orbital colonies. It is a future where the Martian past and present turn out to be amazingly interesting, so much so, that humans never land on the surface in person, in their physical bodies, to preserve a biologically pristine Mars. I, for one, would find that an inspiring future to live in.

Collective sense organs for humankind on Mars

The idea that humans on the surface of Mars would contaminate it with Earth life is not much mentioned in the news. Out of dozens of news stories about ideas for human missions to Mars, perhaps only one or two will ever even mention it even as a topic for discussion. But it's frequently mentioned in the academic literature on spaceflight, with many publications debating the issue, and several planetary protection workshops on human missions to Mars. It's just that their deliberations rarely get into the news.

Generally those discussions are focused on the idea that we need to minimize the impact of humans on Mars. It is much rarer to suggest that it is a serious and significant issue which could be a reason to delay sending humans to the surface of Mars, or as a reason to go somewhere else first such as the Moon or the moons of Mars first. I think myself that it is just not good enough to do our best to exploit the brief window of opportunity of a few years before the first human landings or colonization attempts. We simply shouldn't risk destroying such a precious opportunity to make scientific discoveries, on the basis of ignorance, and there is no way we can do it on a basis of knowledge if the humans land on Mars in the 2020s or the 2030s. It is rare to suggest that, depending on what we find, and our decisions, we have a possible future where humans never land on Mars at all but explore it via telepresence instead.

In 1964, George Simpson wrote:

"There is even increasing recognition of a new science of extraterrestrial life, sometimes called exobiology-a curious development in view of the fact that this "science" has yet to demonstrate that its subject matter exists!"

That was before the Apollo landings. But though we have learnt so much by way of astrogeology, and have made great strides in understanding many things in the field of astrobiology, we are yet to shake off that major central issue. Andrea Rinalid in 2007 writes about astrobiology:

"Despite growing attention, the field is still haunted by the curse that evolutionary biologist George Gaylord Simpson voiced more than 40 years ago: “[T]his ‘science' has yet to demonstrate that its subject matter exists!” 

Now, 52 years after George Simpson wrote his paper, this could be our first opportunity to show that the subject matter of astrobiology exists. It could also possibly be the most major discovery in biology of the 21st century. How can we pass up on such an amazing opportunity and potential "super positive outcome" and ignore the potential issues of Earth microbes introduced to Mars?

Perhaps what we need is a change in the public's perception of how we explore space? Here is a quote from "When Biospheres Collide":

"One of the most reliable ways to reduce the risk of forward contamination during visits to extraterrestrial bodies is to make those visits only with robotic spacecraft. Sending a person to Mars would be, for some observers, more exciting. But in the view of much of the space science community, robotic missions are the way to accomplish the maximum amount of scientific inquiry since valuable fuel and shipboard power do not have to be expended in transporting and operating the equipment to keep a human crew alive and healthy. And very important to planetary protection goals, robotic craft can be thoroughly sterilized, while humans cannot. Such a difference can be critical in protecting sensitive targets, such as the special regions of Mars, from forward contamination.

Perhaps a change in the public's perspective as to just what today's robotic missions really are would be helpful in deciding what types of missions are important to implement. In the opinion of Terence Johnson, who has played a major role in many of NASA's robotic missions, including serving as the project scientist for the Galileo mission and the planned Europa Orbiter mission, the term "robotic exploration" misses the point. NASA is actually conducting human exploration on these projects. The mission crews that sit in the control panel at JPL, "as well as everyone else who can log on to the Internet" can observe in near real-time what is going on. The spacecraft instruments, in other words, are becoming more like collective sense organs for humankind. Thus, according to Johnson, when NASA conducts it's so-called robotic missions, people all around the world are really "all standing on the bridge of Starship Enterprise". The question must thus be asked, when, if ever, is it necessary for the good of humankind to send people rather than increasingly sophisticated robots to explore other worlds"

See When Biospheres Collide

What do you think?

My Moon First books

This book has focused on the question of what happens if humans touch Mars and I have only briefly touched on what humans do if they don't land on Mars. But I go into this in a great deal of detail in my other books. I cover things such as the astronaut gardener on the Moon, the possibly vast kilometers wide lunar caves, ice at the poles with sunlight 24/7 nearly year round, and the value of the Moon for Earth amongst many other things. I see the Moon as a gateway and natural starting point for exploration of the solar system. That exploration can be open ended and lead eventually to human outposts throughout the solar system, in a collaboration where robots go to places where robots do things better, and humans play their part in ways where humans do it better, and the robots, as in "Where Biospheres Collide" are the mobile collective sense organs of humankind.

I've also been guest on David Livingston's The Space Show several times to talk about these ideas as well as the planetary protection issues for humans exploring Mars. See the list of my guest appearances on the show here.

If you are interested to learn more about this, please see my books:

"MOON FIRST Why Humans on Mars Right Now Are Bad for Science", available on kindle, and also to read for free online.

Case For Moon First: Gateway to Entire Solar System - Open Ended Exploration, Planetary Protection at its Heart - kindle edition or Read it online on my website (free).

Facebook groups

I've made a new facebook group which you can join to discuss the ideas in this book, and exciting visions for human exploration with planetary protection and biological reversibility as core principles.

• OK to Touch Mars? Europa? Enceladus? Or a Tale of Missteps?

Also for the particular idea of this approach with the Moon first, as well as general discussions of Moon first, you can join:

• Case for Moon for Humans - Open Ended with Planetary Protection at its Core

For exploration of oceans in icy moons like Europa and Enceladus, and life on Titan etc you may be interested in

• Search for ExtraTerrestrial Life - Europa & Enceladus + Subsurface Oceans

(I'm joint admin with William Brooks who started the group and also organized a conference on the topic in Oxford)

You can get notifications of new posts on my Science 2.0 blog by 'liking' my page:

• Robert Walker
  - Science20 Blog Alerts

See also

Robert Walker

's posts - on Quora

And on Science20

Robert Walker

's posts on Science20

Kindle bookshelf - for my author's page

And I have many other booklets on my kindle bookshelf

My kindle books author's page on amazon

Change log

6th January

• Added photographs of Solid 3 and of Dallas Ellman fine tuning a component of the astrobionibbler to In situ instrument capabilities
• Added mention of Let's Colonize Titan to Life in the oceans of ethane and methane on Titan n
• Work on Life in the clouds of Venus
• Added the VAMP aerostat to Need to send robotic explorers like VAMP to the Venus clouds to study in situ first and Life in the oceans of ethane and methane on Titan
• Copy editing and typos.
• Changed title from "If Humans Touch Mars : Like the Lascaux Story - Another Tale of Human Missteps?"
  to "Touch Mars ? Like Moldy Lascaux - Tale of Missteps?" with fewer words so larger text and I think more impact. The word human is redundant really, especially with the cover picture.

7th January

• More copy editing
• Added images of one of Mary Anning's pterodactyls and one of her plesiosaurs to Searching for fossils on Mars in the popular imagination
• Added map of Mars magnetic field to Oxygen rich atmosphere for early Mars
• Added image for simulated Gale crater lake three billion years ago to Mars geological periods - Noachian, Hesperian and Amazonian
• Expanded the section Tougher conditions for life on Mars - sometimes - and at other times much easier with lots more information about how Mars varied in habitability, and probably had liquid water seas only for a few thousand years at a time, with seas also that froze up every two years.

8th January

• More copy editing - reached the end of the section Microbe stowaways in a human occupied ship
• Examples of invasive Earth life
• Invasive diatoms in Earth inland seas, lakes and rivers
• Risks of invasive microbes in lake Vostok and Antarctica
• Added more information to Demolishing Zubrin's arguments including details of NASA'S plans for 800 gigs per day communication with Mars in the 2020s and exploring its implications
• Added photo of Meteor Crater Arizona to Microbe stowaways on meteorites traveling from Earth to Mars

14th January

• Mixing perchlorates with other salts lets them take up water from drier air than either separately
• After taking up water, perchlorates retain it for a long time even when very dry, or very cold
• Vulnerable to photosynthetic life
• Plausible microbial metabolisms for present day Mars - what else apart from photosynthesis?
• Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells

18th January

• Added examples of perchlorate based life and life in liquid CO₂ as possible "sharks in the Savannah" to the Demolishing Zubrin's argument section
• Using an optical microscope to watch microbes swimming
• Using an optical microscope to watch fluorescing microbes
• Added example of DNA sequencing miniaturization to "Safe on Mars" - could a sample return tell us if Mars is safe for astronauts?
• Why we can't prove yet that Mars life is safe for Earth
• Biosphere, crops, sea, environment of Earth
• Example of Apollo sample return - learning from our mistakes in the past
• Zubrin's arguments in: "Contamination from Mars: No Threat"
• Zubrin's meteorite argument for sample return
• But we unearth ancient life all the time
• Why Mars life would be expected to survive the warmth, and oxidative stress of Earth
• Many microbes harmful to humans are not "keyed to their hosts"
• Why sterilizing a sample from Mars is not like sterilizing a dinosaur egg

20th January

• New shorter Preface (a lot of it was repeated in the next few sections)

22nd January

• Minor editing of Preface - added a screenshot from Star Trek of "beaming down" to a planet plus added final sentence "However ,before we make any decisions, the first thing we need to do is to understand what the possible implications would be of touching Mars."
• Planetary protection - researches by Sagan and Lederberg onwards - and Zubrin's arguments - added to the introductory section, now retitled to Introduction - How often have you seen this?
• Expanded the science fiction section of Introduction - How often have you seen this? with another paragraph and the cover of the first edition of Arthur C. Clarke's "A Fall of Moondust" as an example of a bad fail by one of our most predictive hard science fiction writers.
• Added Landis's "Oh my God, Mars smells like shit." story to Myth of automatic terraforming

23rd January

• Some highlights from the rebuttals of Zubrin by astrobiologists in "No Threat? No Way"
• Added sentence to the end of "What are Zubrin's arguments? ": "Zubrin is such an accomplished debater that even when he has a one on one debate with planetary protection experts, as he does frequently at Mars Society conferences, he is generally seen as winning the debate by his target audience. They come away remembering all of his vivid points, and none of their responses to them. "
• Why don't explorers in science fiction have these problems when exploring other worlds? - split off as a new section, added example from the Martian . Added example of Arthur C. Clarke's story "Before Eden".
• Renamed Myth of automatic terraforming to Trope of automatic terraforming and added speculative suggestion that photosynthetic life on Mars would cool the planet down, so making it less habitable, and create reactive oxygen that would react with its crust, and could keep it in a minimally habitable state, as a kind of "anti Gaia".
• Added map of debris field of Columbia to Introduction - How often have you seen this?
• Edited the captions in the graphs of the varying tilt on Mars in Tougher conditions for life on Mars - sometimes - and at other times much easier to use less technical terms (tilt instead of obliquity, and "billions of years" instead of GYr).

24th January

New sections, subdividing the section What are Zubrin's arguments?

• Sharks in the savannah or rabbits in Australia?
• How do we use a DNA sequencer to prove that a microbe is from Mars, with only 0.00001% of microbial species sequenced?
• With spacesuits on Mars, or shirt sleeves exploration from orbit and Earth, with a hundred new multi gigabyte 3D landscapes streamed back each day - and added images from four gigabyte Curiosity panorama as an example of a multi gigabyte landscape (though not in 3D).
• Renamed: Why do spacecraft crash so easily on Mars
• Divided it up with new subsection: Supersonic retropropulsion - or huge parachutes and added figure showing details of landing sequence for supersonic retropropulsion on Mars.
• New section Humans in habitats at the Venus Cloud Tops - added a bit more there just to briefly introduce the idea of cloud colonies, for those who haven't come across the idea yet.
• New section Need to send robotic explorers like VAMP to the Venus clouds to study in situ first

25th January

• New section (reorganized) Astrobiologists arguing strongly for an in situ search on Mars first
• Subdivided into new section: Decadal white paper by Bada et al - sample return of "pristine" Mars meteorites is not worth the price of millions of dollars per gram - we need to search in situ

Lots of new sections - mainly about sample return. I decided to include more of the material from my many sample return articles in this book. Plus wrote some new sections.

• Paper by David Paige: "Mars Exploration Strategies: Forget About Sample Return!"
• Difficulty of keeping returned sample free of contamination from Earth - Mars 2020 will have a permitted 1 part per billion of Earth originated biosignatures
• Mars samples would cost as much per gram as the world's most expensive diamonds
• Risk assessment survey of microbiologists
• Is this true: "We cannot take even a small risk with a billion lives"? The bold, cautious and intermediate
• Bold
• Cautious
• Can't resolve this
• Nick Bostrom existential risk
• Example calculation: a 1 in 10 billion probability of permanent degradation of Earth could harm an expected one million future people
• Legal liability in case of damages
• Not our "only precious window of opportunity" for space exploration
• World war III
• AI Apocalypse scenarios
• Natural disasters
• Risks from self replicators
• Need for public debate - decision for all humanity
• Legal liability in case of damages
• This is based on zero data about extraterrestrial biology - so doesn't that mean we can ignore the risk?
• Could the risk from Mars life be higher than expected, even as high as 1 in 1,000
• Risks from Mars life if it is at a similar level of development as us

26th January

• Reworked the probability approach along the lines of Drake's equation and renamed to: Drake Equation approach - could risk from Mars life be as high as 1 in 1,000?
• More work on copy editing, from the beginning, got up to What are the effects of these frequent ups and downs in habitability? and also added in mention of the idea of multicellular life evolving before photosynthesis on Mars in that section.
• Added mention of the Mars 96 probe chiral labeled release experiment which was descoped to Why it's likely to be hard to search for present day life on Mars

28th January

Split this into two sections:

• Land humans quickly - to explore nearby regions kept free of Earth life - NASA's approach
• Explore Mars from orbit by telepresence first - but still land on Mars towards the end of the 2030s - The Planetary Society's approach

New sections:

• Worst case, almost certainly not an extinction event - rather - debilitating to habitability of Earth
• Zero data - anything from 0% to 100% risk of sample return degrading the habitability of Earth
• Why it's likely to be hard to search for present day life on Mars
• Organics from meteorites on Mars may boost a molecule over its mirror image, mimicking biosignatures (chirality)

Added new material to

• Comparison of spacesuits on Mars with shirt sleeves exploration from Mars orbit and Earth - with a hundred new multigigabyte 3D landscapes streamed back each day - added example gigapixel zoom into a geological rock core to show what we could "see" on Mars with a bandwidth of hundreds of gigapixels a day.

Lots of copy editing and small scale re-organizing of the text. Got to For a clear signal of past life - just started on that section.

30th January

• Expanded Microbial ethics and biorespect with quotes from Charles Cockell's Planetary protection—A microbial ethics approach
• Added remained habitable to life on its surface until at least 715 million years ago to Life in the clouds of Venus
• Added Annabel Cartwright's interesting "Venus Hypothesis" to Search for early life from Earth, Mars, Venus, etc on Ceres, our Moon, or the moons of Mars
• Added more images to (renamed) What if Mars has really tiny cells - like the structures in the Mars meteorite ALH84001?
• Added codon table to Life on Mars dancing to a different tune
• Finished copy editing and proof reading up to Life on Mars dancing to a different tune

31st January

• Added Mohmann's heat / time / distance below surface plot to Creates potential for flowing fresh liquid water on Mars!
• Added tardigrade video to Search for early life from Earth, Mars, Venus, etc on Ceres, our Moon, or the moons of Mars plus information about hypervelocity impact tests on tardigrades
• Added nannochloropsis oculata to Case study - can photosynthetic life be transferred from Earth to Mars or vice versa? plus question of minimum shock for material ejected from Chicxulub meteorite impact.
• Added results from Seeding Life on the Moons of the Outer Planets via Lithopanspermia to Direct search for DNA - and could the oceans of Europa or Enceladus have DNA based distant cousins of Earth life? (renamed)

New sections

• Joyce's ribozyme which joins together strands of opposite "handedness" - could early life be ambidextrous?
• Surprising distant cousins
• What else could have come before modern life? Alphabet soup of "XNA", Ostwald ripening organic crystals, "naked genes", or almost alive "autopoetic" cells

1st February

New sections:

• How could our cousins differ? Skins of rust, freezer life, different amino acids translation table, ... .
• Could we peek into these missing pages of the book of evolution on Mars? (split off as second half of previous section Half of the pages of the book of evolution have been torn out )

Added material:

• Added material about possibility of cretaceous period sequenceable DNA preserved in the lunar poles to Search for early life from Earth, Mars, Venus, etc on Ceres, our Moon, or the moons of Mars
• Added Chris Chyba's example of cyanobacteria killing cows to Many microbes harmful to humans are not "keyed to their hosts"
• Added Edward Belbruno and Amaya Moro Martin's idea of "weak transfer" leading to all the stars in a cluster seeding each other with life plus the idea of a wandering star from a previous cluster seeding a new one to Half of the pages of the book of evolution have been torn out

2nd February

New sections

• Distant cousins with last common ancestor from a planet around another star and added section about the Pillars of Creation to it
• Ice covered lakes habitable for thousands of years after large impacts
• Ice covered lakes from volcanic activity
• Possibility of geological hot spots in present day Mars
• Life in ice towers hiding volcanic vents

3rd February

New sections

• Habitability of the Mars surface (top few centimeters)
• Temperature and salinity limits, and more generally, water activity limits for Earth life
• Ways that the methane spikes could come from close to the surface (centimeters deep at most)
• Sources of nitrogen - essential for life on Mars

Rearranged to move James Lovelock's argument for a lifeless Mars to after the two top level sections: Habitats for life on the surface of Mars - warm seasonal flows and What we could learn - some examples

4th February

• Added photo of Lujendra Ojha to Habitats for life on the surface of Mars - warm seasonal flows
• How did they form?
• Details of what they found
• Perhaps a hundred thousand metric tons of flowing water per year in the Valles Marineres
• Are the RSLs (Recurring Slope Lineae) habitable?
• Mixing perchlorates with other salts lets them take up water from drier air than either separately
• After taking up water, perchlorates retain it for a long time even when very dry, or very cold
• So deliquescing water is liquid in a wider range of conditions than you'd expect
• Challenges for life in these liquid layers of deliquescing salts
• Microbes could find tiny oases of water inside micropores in salt pillars

5th February

• Added more information to Nilton Renno's droplets that form where salt touches ice - why did he call a droplet of salty water on Mars "a swimming pool for a bacteria"? and renamed
• Added section about rootless cones to Life in ice towers hiding volcanic vents
• Added Leprechaun story to Follow the nitrogen, dig deep and look for biosignatures
• Renamed section to Analogy of ET microbes on Mars with microbes from a planet orbiting another star and removed some duplicated text from it
• Added brief description of Chris McKay's "Lego principle" to In situ instrument capabilities
• Added lots of details to What we could learn - some examples - including Graham Cairns-Smith's Clay model of evolution leading in to the speculations about extraterrestrial life based on a 2D replicating sheets instead of 1 D helixes etc. Also added mention of Z DNA with reverse helix, and triple and quadruple helix structures of DNA as a way to stimulate ideas plus Rodley's "Side by side DNA" which is not coiled around itself.

Added section:

• Porous basalt

10th February

New material mainly to do with conditions for habitability of Mars, possibility of Earth life seeded by life from an older star passing through its birth star nebula, greenhouse gases as a way to warm up Early Mars and comparison of sample return with in situ study of Mars. Also updated How much oxygen would surface photosynthetic life produce on Mars? I'd made a mistake in my calculation, and updated it with new figures from one of Charles Cockell's papers.

• What we could learn - some examples - added image of Ulexite + a bit more about borates.
• Spilt off new section Other requirements for life on Mars: major and trace elements, organics, and pH (alkaline or acid)
• Repositioned and renamed Life that uses hydrogen peroxide, or perchlorates, or both, INSIDE the cells - habitable to Mars life and not to Earth life placed before renamed: Early life not so versatile as present day life - habitable to modern Earth life and not to Mars life
• Divided Distant cousins with last common ancestor from a planet around another star into two sections: How life could transfer between sibling stars in a stellar nursery by "weak transfer" of meteorites and Infecting an entire star cluster with life from planets from an earlier, perhaps billions of years older star and added artist's impression of planets around orange dwarf Tau Ceti
• Updated How much oxygen would surface photosynthetic life produce on Mars? I'd made a mistake in my calculation, and updated it with new figures from one of Charles Cockell's papers.
• Moved Microbes that are almost dormant - but not quite to after Searching for surface life from orbit with ESA's Trace Gas Orbiter where it fits better.

New sections

• Uninhabited habitats on an inhabited Mars - potential habitats that life can't reach or hasn't reached yet
• Idea of returning samples from Mars to Earth - how does it compare with an in situ search?
• In Idea of returning samples from Mars to Earth - how does it compare with an in situ search? new section 2007 report favoured a sample return -what has changed since then?
• Will the samples be out of date already when they are returned? also expanded: Tissint meteorite - a great example of what we might get in a sample return from Mars
• Added to Tougher conditions for life on Mars - sometimes - and at other times much easier new section Greenhouse gases on early Mars and separated out earlier material as Oceans that are only liquid part time as Mars' tilt and orbital eccentricity change

Lots more copy editing. Just about to start Mars samples would cost as much per gram as the world's most expensive diamonds, which makes it about 52% of the way through the book, but it's probably more than that as I've covered many of the most tricky parts of the book to write now. Did a rewrite of the preface.

February 15th

More copy editing of intro. Added NASA quarantine facility photo to Example of Apollo sample return - learning from our mistakes in the past , and divided it with two extra sections Why Carl Sagan and others thought that planetary protection was necessary for Apollo 11 and Were we just lucky? Do extraterrestrials sometimes become extinct after their first Apollo style mission to their nearest Moon or planet?

Subdivided Zubrin's meteorite argument for safety of a sample return into Can life get into the ejecta from an asteroid impact on Mars and Do we know if Mars life has caused extinctions on Earth? - Example of Great Oxidation Event

Split up Search for early life from Earth, Mars, Venus, etc on Ceres, our Moon, or the moons of Mars into

• Search for life from Mars, Venus, or the Earth - on the Moon in Meteorites!
• Search for past life from Venus on the Moon
• Search for past life from Mars - on Phobos and on the Moon
• Search for past life from Earth on the Moon
• Search for early life on Ceres

• Renamed: If it is just an astrobiology "technology demo" - we could sterilize the entire sample returned to Earth and added image of cobalt 60 irradiator.
• Renamed: If likely to be of greater astrobiological interest - return samples to above GEO and added orbital debris video and image.
• Renamed Two ways Curiosity's methane spikes could be generated in the shallow subsurface (centimeters deep at most)
• Made into new section, moved to later in the book and added new material to Subsurface ice in equatorial regions - it didn't really belong in the Viking data section as it's not a likely source of ice / water for the Viking observations.

Added: Published planetary protection guidelines for the Europa lander - analogy of a Ming vase

Re-organized the Europa and Enceladus section and added lots more material about Enceladus and other new sections (mid edit)

March 8th

Added quote from article in Scientific American, Feb. 17, 2017: to So then what about landing on Europa?

Added video of glacier calving in Greenland to Published planetary protection guidelines for the Europa lander - analogy of a Ming vase and evidence of calving on present day Europa in Thera Macula. More editing of it. Renamed to How clumsy can a Europa Lander be? Should we risk making all the life in its ocean extinct?

New sections:

• Redesign of spacecraft before launch - leave the launch to one or two years after the multiple flyby gets to Europa
• Sampling geysers - particle sizes
  • Studying dust and ice by analysing the plasma created during impact of particles - method used by Cassini
  • Capturing the ice and dust particles more gently, in an aerogel - for sample return, or in situ analysis
  • Suggestion - in situ analysis of the particles within a transparent silica aerogel using, e.g. Raman microspectroscopy
  • Suggestion - for low relative velocities, capture directly into water, brine, ice, or foamed up water or ice
• Why Enceladus is so interesting
• Mystery of the Enceladus heat sources
• Is Enceladus' ocean temporary or long term?
• What could we find in the Europa or Enceladus oceans?
• Enceladus orbiter proposal
• Each target as "our only chance" to find something unique
• Ice moles or submarines for exploring Enceladus or Europa
• How clumsy can a Europa Lander be? Should we risk making all the life in its ocean extinct?
• Where will it land? Crumbling and tumbling ice blocks - or smooth ice moving as slowly as continental drift?
• Design change suggestions that could make a lander close to 100% sterile
• RTGs instead of batteries
• Post launch heat sterilization
• Deep cleaning with supercritical CO₂and CO₂ snow
• Would a Europa lander be likely to spot life?
• Using microscopes to search for Europan life
• Should we prioritize dual purpose geochemical / biochemical instruments over dedicated biology instruments?
• Requirement that life detection experiments should also provide valuable non biological information
• Strengths and weaknesses of the "lego principle" approach
• Cholesterol - an example of a molecule with a unique stereoisomer signature, one out of 256 possibilities
• How diagnostic is the carbon 12 to carbon 13 ratio?
• Mixtures with other organics from comets, meteorites, and produced locally on Europa - and degraded organics - it could do a nice study of tholins but could it find life?
• Abiotic organics that mimic biosignatures
• Degraded organics from life
• Would we be able to detect life using sub-millimeter precision photography - or could it be confused with abiotic organics?
• How easy is it for Europan life to reach the surface?
• Could there be life which uses the surface ice of Europa or Enceladus itself as a habitat?
• Could a Europa lander help find geysers for a geyser sampling mission?
• Interleaved "Ocean Worlds Exploration" to Europa, Enceladus, and Titan - and Ceres?
• Would we use solar power or radioisotopes for missions to Saturn's moon Enceladus?
• Evidence for a global ocean for Enceladus
• Planetary protection - researches by Sagan and Lederberg onwards - and Zubrin's arguments
• Searching for present day life on Mars in the popular imagination

March 10th

Copy editing, especially of preface. Rewrote the titles of many of the sections

Split off into new section: Closing off future options - hard enough to roll back introduction of feral camels to Australia, never mind of microbes to Mars

Added series of "Future Possible News stories" about astronauts landing on Mars then contaminating it with Earth life to Prestige or dishonour, first footsteps on Mars

March 11th

Split off with new section heading: Could oxygen generating photosynthetic life set up an "anti Gaia" feedback on Mars?

New section: Chris McKay's view - just grab a sample of dirt as a technology demo, and return it - one day on the surface, no rover - or SCIM - sample a dust storm in a flyby

Also added quote from his interview to Why did the decadal survey choose a sample return?

Split off with new section headings:

• Instead of terraforming Mars in a multi-millenium project, why not terraform a lunar cave in a multi-decade project?
• Best places to introduce Earth life right now - while continuing with biologically reversible exploration of Mars - and added new material on Dandridge MacFarlan Cole's 1963idea of a "bubbleworld" made from an asteroid.
• Plenty of places to experiment with sending life to other places in our solar system - Asteroid belt resources, NEO's, caves on the Moon
• Why a Europa lander is likely to draw a blank - it looks so smooth and those brown lines seem biologically interesting, but what is it really like?
• Enceladus geysers as the low hanging fruit right now - with other interesting targets in the Saturn system - could Dione also have a subsurface ocean?- added details about Dione

Added photograph of The Martian dust cover to Introduction - How often have you seen this? and also added example of Yoghurt as an analogy for "Good Mars".

March 13th

Moved Yoghurt to new section: Why bringing Earth life to Mars could be like making wonderful yoghurt - or bad smelling gone off milk with just a short mention in the intro. Also split off new section: Myth of automatic terraforming and the strong Gaia hypothesis. A lot of copy editing of the preface and introduction. Added Blurb

Removed the section "Pristine Mars" (repeats material already there).

Added: Sun warmed dust grains embedded in ice

Added diagram to Sulfur based life on Io

Lot's more copy editing.

March 19th

Lots more copy editing.

Added material about advantages of Moon to Human settlement and exploration - hugely positive or hugely negative - it all depends how it is done

Space habitats made from asteroid and comet materials get plenty of sunlight - right out to Pluto (using thin film mirrors to concentrate it)

Quote from Thomas Oppenheimer in Plenty of places to experiment with sending life to other places in our solar system - Asteroid belt resources, NEO's, caves on the Moon

Screenshot of Star Ship Enterprise from Star Trek with Earth and Moon for Only one Mars - no warp drive yet

New section Sending humans to Callisto or Ganymede

March 22nd

Expanded Value of humans in space - and fossil hunters on the Moon? to discuss planetary protection issues for the Moon briefly.

New sections:

• Life or prebiotic chemistry in comets
• Temporary water on comets doesn't mean life
• Wickramasinghe's theory
• Crust of "sunburnt organics"
• Life or prebiotic chemistry on the Moon
• Trash, rocket exhausts and microbes on the Moon - testing ground for planetary protection measures for a human base
• Two dimensional planetary surface landers
• Origami rover

Added lighting maps to Advantages of Deimos and more details of the mission, also fixed some errors in this section about the shadowed south pole regions. They are almost permanently shadowed, and probably the coldest regions in the Mars system. I'd been under the impression they were permanently shadowed like the Moon, but no, on checking the sources, they are very cold and almost permanently shadowed, because of those two lobes in the southern hemisphere, but not like the Moon (because of the axial tilt of Deimos relative to the Sun).

Separated out as new section: Could Europa or Enceladus have DNA based life?

April 24th

New sections:

• Near field scanning optical microscopy
• Beyond the diffraction limit with expansion microscopy (physically expand a cell to make it easier to study) and other forms of superresolution microscopy

Also added section on 3D holographic microscopy to Using an optical microscope to watch microbes swimming

Added to blurb paragraph starting "This may be only the second popular book with planetary protection as its main theme"

Expanded Human settlement and exploration - hugely positive or hugely negative - it all depends how it is done with more about the challenges of "colonizing Mars".

Added Triton hopper to Life in liquid nitrogen - Triton

Life in liquid nitrogen or liquid neon on Pluto or KBO's?

Added some more details about Mars 2020 including its Mars Helicopter Scout to Idea of returning samples from Mars to Earth - how does it compare with an in situ search?

New subsections (split off and copy editing)

Could we see green on Mars? .

Would photosynthetic life on Mars be green - or could it be other colours such as red, purple, orange, yellow, brown or black?

Why is it that, decades after the Viking landers, we still know so little about life on Mars?

Added quote from to Optical microscopy

New section: Why there was controversy back in the 1970s

Split off into new section: Titan as potentially the easiest place for humans to live outside Earth

Enceladus geysers as the low hanging fruit right now - with other interesting targets in the Saturn system - could Dione also have a subsurface ocean? - renamed and also added a note about Rhea's possible ocean.

New section: Other subsurface oceans - Ariel, Triton, Eris, Sedna etc

Added video of Apollo 17 astronauts attempting to drill on the Moon to Robotic and telerobotic drilling versus human drilling

Added details of ISS EVA procedures, including checklists, to Telerobotics as a fast way for humans to explore Mars from orbit

New sections: No need to suit up for an EVA when exploring by telepresence and Much safer

Added Brian Wilcox is working on a 100% sterile probe to descend into the Europan ocean.to High temperature sterilization

Added more examples to Small planes and entomopters etc and new section: Thistledown light planes with low take-off speeds for the thin atmosphere of Mars

Divided Human settlement and exploration - hugely positive or hugely negative - it all depends how it is done into subsections:

• Mars fantasies and realities - a sobering dose of common sense
• Earth best for a backup - maybe with a small knowledge and seeds library on the Moon with caretakers
• Why the Moon could have a commercial case
• Further into the future, if we have millions in space - can we be one of the "wise ET's"?

Split off and expanded If life in a Europan ocean can have the intelligence level of squids, basking sharks, octopuses and cuttlefish - what about a non technological civilization?

Added

• Need for adventurous experiments in life support and artificial gravity in LEO first
• Astronauts don't get nauseous when spun rapidly in zero g - so could a device as simple as a spinning hammock be all that is needed to keep us healthy in space?
• Spaceship tethered to final stage and then spun up - experiment first proposed for a Voskhod in 1965-6 but never flown - could this solve all the zero g health problems?
• Starting point - using 18 liters (so 18 kg) of spirulina algae per astronaut to supply all their oxygen - and some food
• How long does an expedition have to be to make it worth growing crops in space - or indeed - any recycling at all, even of the ISS type?

Added Why the Moon is best for humans right now

Added Hubble's confirmation of a repeating plume

Renamed How clumsy can a Europa Lander be? Should we risk making all the life in its ocean extinct? to Analogy of a Ming vase - what is an acceptable level of risk for making all the life in Europa's ocean extinct?

Added the new Enceladus hydrogen results to Why Enceladus is so interesting.

July 6th

Mainly work on the Blurb and the Introduction, copy editing, and moved the section on Trash, rocket exhausts and microbes on the Moon - testing ground for planetary protection measures for a human base to later in the book, and subdivided with new subsection: Rocket exhausts, microbial spores and organics mixing with levitating lunar dust . Then added summary of this, Trash, on the Moon, to the introduction.

July 15th

Added the examples of early life still present on Mars, life evolved further on Mars than on Earth, and either no photosynthetic life there, or a novel form of photosynthesis, as examples of synthesis that I don't know anyone to cite for, to This is an op ed, not an encyclopedia or literature survey

New sections

• Dark streaks in Russell Crater, 55 degrees South
• Could Mars life in the polar ice exploit thin film monolayers in the ice, wrapped around individual microbes?
• Why Lovelock's method wouldn't have detected life on Earth for most of its history!
• What if the most habitable spots on the Mars surface are similar to the driest parts of the McMurdo dry valleys?

Split into subsection: More background on whether photosynthetic life could be transferred from Earth to Mars - other photosynthetic species, shock, and spall zone

Divided up This is an op-ed, not an encyclopedia or literature survey with two subsections:Attribution for the research, opinions, and fun speculations and About me. and added more material to it.

Subdivided Perchlorates on Mars with two new subsections: Perchlorates irradiated with UV light to create sterilizing chlorites and hypochlorites (new material) and Perchlorates as a way to scavenge water from the atmosphere

In Mixing perchlorates with other salts lets them take up water from drier air than either separately - fixed a confusion of "eutonic" (which is for relative humidity) with "eutectic" (which is for temperature).

Moved, and split into a new section, as a conclusion rather than introduction to the section: Summary of ways present day Mars could have life - with its atmosphere close to equilibrium

Added Levin's idea of a ground hugging cold layer and briefly melted morning frosts from Can Liquid Water Exist on Present-Day Mars? to Equatorial frosts - and a source of water for the deliquescing salts

Lots more copy editing and fixing of minor things - got as far as Microbes that are almost dormant - but not quite

July 26th

Added the idea of a free return flyby to return a sample from a dust storm in the upper atmosphere (SCIM) to Chris McKay's view - just grab a sample of dirt as a technology demo, and return it - one day on the surface, no rover - or SCIM - sample a dust storm in a flyby

New section What can we do about the possibility of life in the Mars 2020 cached sample - or Chris McKay's"Grab a sample of Dust" or the SCIM sample from a dust storm? - Sterilize! (split out from existing section:Why sterilizing a sample from Mars is not like sterilizing a dinosaur egg and expanded).

Renamed section to Do we know if Mars life has caused extinctions on Earth? - Example of Great Oxidation Event and added more material to it about the Great Oxidation Event extinction from Mars idea.

Added the more recent papers on cluster analysis and on the possibility of 30% methane in exhaled gases from methanogens to Joseph Miller's analysis of rhythms in the Viking data

New sections

• Global warming and climate change
• Human error - harder part
• How martian surface material can get to Phobos
• Other things we could find out from the mission to Phobos
• Exploration of Phobos (with many new subsections)
• Planetary protection for samples returned from Phobos
• Could Mars 2020 pivot to support astrobiology?
• What about humans on Mars later on?

Rewrite of the blurb (mainly changing the order of the sections and copy editing)

Added some more photos and links to Why don't explorers in science fiction have these problems when exploring other worlds?

1st August 2017

expanded section slightly: Further into the future, if we have millions in space - can we be one of the "wise ET's"?

New sections

• Galaxy protection - what about colonizing other star systems?
• The problem with exponential population growth into an uncolonized galaxy
• What about populations that plateau or crash?
• Boom and bust - a nightmare future
• Slower growth, for instance cube or square law growth rates
• Could humans achieve slower than exponential growth like this?
• So what might a sustainability solution look like?
• Optimism - especially if we are not first
• Any expansionist interstellar ET must have minimal impact on the galaxy - or else - we are the first such, at least in our galaxy
• Will we be an invasive weed, or a flower in the galaxy?

Update of AI Apocalypse scenarios

A little more work on the blurb, copy editing.

4th August 2017

Added quote from Claudius Gros to Should we return samples from Mars right now?

New sections:

• Travel to other stars
• Robotic spacecraft to the stars
• Humans to the star
• Planetary protection for other stars and exoplanets
• Galaxy protection - what about colonizing other star systems?
• Could the solution to Fermi's paradox be galaxy protection as an extension of planetary protection?
• First idea - galaxy protection as a motivation for ETIs and ourselves
• Second idea - no need to colonize planets or search for habitable zones - we could set up colonies almost anywhere in habitats around almost any kind of star
• Third idea - the trap of exponential growth - and the need for galaxy planning of some sort for sustainable growth into an uncolonized galaxy

14th August

New sections:

• Could a Europa lander help find geysers for a future geyser fly through mission?
• Does Europa have thousands of smaller plumes only ten kilometers high?
• How this could change the prospects for a Europa lander
• What we know about the geysers so far - and prospects for life

Divided into new sections to make easier to find:

• Suggestion - hibernation with solar power to permit Juno sized solar panels out to the Saturn system and perhaps even further, right out to Neptune?
• Other radioisotopes - what about Curium 242

How easy is it for Europan life to reach the surface? split into subsections:

• Why top down mission planning may not be the best way to organize a scientific search for life in the Europan ocean
• Sample returns from Europa

Retitled: Why there's probably no need for planetary protection for humans to Callisto, even with a subsurface ocean

Lot's of copy editing, fact checking, some new information especially in the Europa lander sections. Got up to the start of Other places in the solar system where potentially life could be found - Titan, Triton, Io, etc. So that's about 85% through now.

Added to blurb: "Amazon shows the book as over 1800 pages long. I expect you to jump to the sections that interest you rather than read the book from start to end".

18th August

Some more work on the Blurb.

After: Precautionary principle and super positive outcomes

added:

• Benefits to humanity from astrobiology
  • New dimension to biology
  • New biological materials
  • Genetic firewall
  • Industrial applications of astrobiology already- enzymes from extremophiles
  • Example enzymes from cold adapted extremophiles used in industry today - detergents, for fruit juice, bread, textiles, cosmetics, cement, research
  • Enzymes from heat adapted extremophiles used in industry today - manufacture of syrups from starch, baking, brewing, paper making industry

28th August

New section Earth or the Moon as a backup - not Mars

New section What about a comet as large as Hale Bopp (60 km in diameter)?

29th August

More work on the blurb and copy editing.

Divided off into new section: Lifeboats on the Moon with quote from Chris Hadfield.

30th August

New section Cell counts in the most inhospitable environments on Earth - and application to astrobiology

18th February 2018

Work on The knotty problem of human quarantine - and what about exposure of humans during a robotic sample return? (above) mainly copy editing and added quotes from Carl Sagan's 1973 book "Cosmic Connection".

Protection of the historical Apollo landing sites

Added material on Carl Sagan's papers on the possibility of life on the Moon based on the early 1960s understanding of it as starting off like Earth with oceans and an atmosphere, from the early 1960s, before Apollo, to Life or prebiotic chemistry on the Moon. Also ask if there is any possibility of such a theory with the Moon as we understand it today.

Added Pluto possible penitentes to Rugged unknown surface of Europa - ice knives, crevasses, and upturned icebergs

Added New Horizons measured surface temperatures to Life in liquid nitrogen on Pluto (section title changed)

Separated off into new section: Life in neon seas of Kuiper Belt Objects and beyond

New section: Life in liquid hydrogen or liquid helium as far away as the Oort Cloud

Placed the sections Hazards of XNA from Venus and Impossibility of containing XNA at sufficient probability levels after COSPAR study of the Venus atmosphere didn't consider XNA or gene transfer agents

In Hazards of XNA from Venus - wrote it instead as an extended discussion of the XNA life precautions suggested in Xenobiology: A new form of life as the ultimate biosafety tool

More work on Life in liquid hydrogen or liquid helium as far away as the Oort Cloud

New section: Rogue planets and very distant planets - liquid water or liquid hydrogen surface oceans

=========

Titan as potentially the easiest place for humans to live outside Earth

Added section about sources of energy for Titan and made more divisions into subsections and new final section "Likely to know a lot about it before humans get there"

Still need to convert the subsections into links in the contents.

5th July 2018

New section

What about native lunar life?

Split off into small new section:

Search for past life from Mars - on Phobos

 

Added Biological closed systems research in LEO

(october 2018)

Added new information about possible penitentes 15 meters high to section on Rugged unknown surface of Europa - ice knives, crevasses, and upturned icebergs