What about the forward direction, Earth life in the Venus cloud tops? At first sight it seems most unlikely that Earth life could survive in the concentrated sulfuric acid droplets in the clouds. These droplets have pH less than 0, similar to battery acid. This is the main reason the COSPAR team gave for their conclusion that no Earth life could survive in them. However, in 1991 researchers found some Earth microbes able to survive sulfuric acid with pH 0 or lower, close to the Venus cloud top 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 of growth (not just 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
The comparatively water rich upper clouds of Venus have pH 0.3 to 0.5. So this is already within the tolerance range of Earth acidophiles.
The Venus pH goes as low as -1.3 in the lower clouds. 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, writing about the Iron Mountain outflow, David Grinspoon and Mark Bullock write (page 9 of their Astrobiology and Venus Exploration), in 2007:
"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. 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?
Also it would be impossible to screen a spaceship to check whether it has any microbes with those capabilities. 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 (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. Extremophiles get everywhere and an ordinary seeming microbe can have extraordinary capabilities when you put them in an extreme environment.
Also, this is my own thought here, 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?
Might Earth life, if related from the early solar system, even be able to swap genes with Venus life via horizontal gene transfer using GTAs?
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. 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. Horizontal 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 horizontal 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.
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. 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. Horizontal 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 horizontal 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.
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.
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.
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.
