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Life in the clouds of Venus

Robert Walker

The phosphine story is just the latest evidence suggesting life in Venusian clouds - there have been several previous observations harder and harder to explain in other ways but so far not conclusive. It is equally hard to explain as life. For details of the new result, see:

Life in the Venusian clouds? Or new chemistry nobody has thought of? What does the new phosphine result tell us?

Also

Sara Seager’s own ideas on “Our crazy finding suggesting life on Venus”

If there is life there, which is very speculative, it couldn't have evolved in the Venusian clouds. Most likely it evolved n the surface of Venus which is believed to have been habitable and Earth like maybe as recently as 715 million years ago. Venus doesn't have plate tectonics and its plates are all stuck, and as a result it can't release heat in the way that Earth can from its core, One theory is that the whole crust turned over in one big overturning event, which destroyed its biosphere but created a new habitat high up in the clouds where life slowly drifts down for a few months before it is incinerated, and updrafts can then blow some of it back up to the top of the clouds to complete the cycle. It is quite harsh - droplets of sulfuric acid in the clouds - but not beyond the survivability limit of terrestrial acidophiles. Apart from the acid then it is similar to Earth's atmosphere in temperature and pressure and humidity at the Venus cloud tops and in the 1960s the Russians had ideas to set up cloud colonies floating in the Venusian clouds -which would be the easiest places for humans to live outside of Earth as you'd only need protection from acid to go outside your floating habitat balloon - and air to breathe - much easier and less cumbersome than a spacesuit. Only Saturn's large moon Titan with its thick methane atmosphere is likely to be an easier place to live than the Venus clouds in terms of the minimal amount of equipment you need to survive.

So if Venus did evolve life like Earth did then it is not too surprising for it to survive in the clouds. It would be only microbial, though just possibly it could include larger creatures, likely still microscopic, but with the ability to drift down slowly in the air slowly enough to circulate around again.

However the conditions there are far more acidic than I realized when I wrote the rest of this page (some of the early papers I relied on for this material were mistaken on this point).

So - in the rest of this page bear in mind, life there is far harder than it seemed to be in the papers I read when I originally wrote this material.

It i s nothing to be scared of.

This is material from my OK to Touch? Mars? Europa? Enceladus? Or a Tale of Missteps? (on kindle)

Acidity of the clouds

The papers I relied on here had an incorrect explanation of the acidity of the clouds.

The very high concentration of H2SO4 and extreme acidity is a unique challenge for life on Venus. Prior speculations about life on Venus often emphasize that terrestrial polyextremophiles can tolerate very low pH, high temperature, and low water activity environments with a “high concentration” of sulfuric acid. The implication is that the Venusian sulfuric acid cloud conditions have similarly low water activity and low pH, and hence are conducive to life (Morowitz and Sagan, 1967; Grinspoon, 1997; Cockell, 1999; Schulze-Makuch and Irwin, 2002, 2006; Schulze-Makuch et al., 2004; Grinspoon and Bullock, 2007; Limaye et al., 2018).

This is an incorrect implication. We cannot emphasize enough that the Venusian sulfuric acid clouds are much more acidic than even the most harshly acidic conditions found on Earth—the Dallol Geothermal Area, within the Danakil Depression in Northern Afar (Ethiopia) (Cavalazzi et al., 2019). The Dallol acidic pools are high temperature (108°C), hypersaline (NaCl supersaturated), and anoxic hydrothermal sites containing up to 150 g/L of iron with the lowest environmental pH recorded to date (pH between −1.7 and 0) (Kotopoulou et al., 2019).

Preliminary studies suggest that even in such harsh polyextreme hydrothermal conditions, life can survive and possibly even thrive; there are examples suggesting that the Dallol acidic pools are inhabited by very small (∼0.2 μm cell size) Nanohaloarchaea and other phyla of archaea (Belilla et al., 2019; Gómez et al., 2019). (Note that there is one Dallol pool where life has not yet been detected; instead of NaCl, the salts are composed of Mg2+ and Ca2+ ions.) The Dallol acidic pools, however, are not a model for Venusian clouds. The hyper-acidic environment of Dallol pools has water with sulfuric acid dissolved in them; they have the properties of water, even though they have a pH <0. In other words, the Dallol pools correspond to “only” ∼5% solution of H2SO4. By contrast, the Venusian cloud droplets are sulfuric acid with water dissolved in them.

The “pH” of Venusian clouds defined in a conventional way (−log10[H+]) is meaningless because the conventional pH scale refers only to dilute aqueous solutions. The Hammett acidity value is a measure of acidity that naturally continues the pH scale up to concentrated acids such as sulfuric acid (Liler, 2012). The Hammett Acidity of 85% sulfuric acid is about −11.5 (Yates et al., 1964). Acidity functions are on a log scale, so the clouds of Venus are >1011 times as acidic as the Dallol geothermal area. This supports our statements that the Venusian cloud drops are an entirely different environment from any found naturally on Earth.

 

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

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

Artist impressions of Venusian clouds, credit ESA
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:

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:

Also the observations that suggest life might be there, though not conclusive, are:

Of all those observations, the three most important lines of evidence which just possibly could indicate the presence of life are the

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:

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.

//www.space.com/6968-oceans-ancient-venus-study-suggests.html#sthash.PwM2gqOC.dpuf
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:

In any of those cases, it has been isolated from Earth for hundreds of millions to billions of years, or possibly evolved independently.

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 and Earth (ESA)
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.

Great plate count anomaly
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 section is even more speculative with the conditions there so acidic, 80% sulfuric acid.

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As a fun speculation that I haven't seen anywhere else, terrestrial spiders can use "ballooning" trailing silk threads to keep them aloft for a long time, so it's not totally impossible that the Venusian astmosphere has higher lifeforms that have adapted to drift down only very slowly in the clouds. Even microbes could do that perhaps evolving very long flagella that trail in the atmosphere like the spider's threads. It might be that microbes could also slow down the descent using hydrogen filled gas vesicles similarly to some cyanobacteria that use these vesicles to float in sea water. Very speculatively maybe with enough hydrogen bubbles attached to them microbes could even float neutrally buoyant in the Venusian atmosphere. I talk about this in the section: Possibilities of indigenous life in the Venusian clouds

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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
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
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:  

In both directions:

In the forwards direction:

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.

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:

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 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.

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 ,..

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?

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.

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?

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:.

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):

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

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.

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 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.

Need for care exploring other worlds - example early science fiction story about Venus

We need to be careful even exploring the Venusian clouds. It's not impossible that some of the microbes we bring to Venus could alter the ecosystem there. They might be polyextremophiles that are able to tolerate acid from an accident of their past history - or they might live inside Venusian lifeforms.

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.

Although this particular scenario is not a realistic possibility today, this poignant sad story shows how important it is to be careful of other lifeforms as we explore our solar system.

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.”

For more about planetary protection see my OK to Touch? Mars? Europa? Enceladus? Or a Tale of Missteps? (on kindle)

Fermi's paradox and sustainability solutions for why our solar system and galaxy doesn't seem to have been colonized by aliens already

Fermi's paradox and sustainability solutions for why our solar system and galaxy doesn't seem to have been colonized by aliens already

If there is life in the Venusian clouds it might well be related to Earth life, through exchange of meteorites in the early solar system when both had Earth-like atmospheres. But it could also be independently evolved. If it is independently evolved then this probably means life is very common in the universe.

This is inspiring blog posts about Fermi's paradox in relation to this announcement.

The paradox here is that our solar system clearly hasn't been colonized by aliens in the past billions of years (and so, probably never will be). So why is that, when you would expect humans to colonize the entire galaxy in as little as a million years traveling at only a tenth of the speed of light? That is just a blink in geological time.

The dramatic solution that often gets shared by bloggers and journalists is that aliens all go extinct when they develop technology. They usually then go on to say that this likely means we will soon make ourselves extinct.

I find that totally unbelievable, we are more rather than less resilient as a result of our technology, and it is hard to see how anything could make humans extinct now. Climate change certainly can't. See my

We are so resilient, that I expect there will be creatures evolved from humans in our solar system a billion and probably a trillion years from now.

The "aliens humans go extinct" solution is an idea that has got popularized because it is so sensationalist, but it is one of dozens of explanations for the paradox that have been given over the years. I favour the sustainability solutions myself, or else that we are amongst the first. It is probably a mixture of several different explanations but I think many aliens who potentially would be expansionist do so in a way that protects the galaxy (and so us) and keep their populations at a galactically sustainable level - as a result of developing respect for biodiversity, understanding the importance of sustainability and gradually taking longer and longer term views and realizing the importance of not just planetary protection but also galaxy protection. By protecting the galaxy they also protect themselves.

Please see my: