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. The atmosphere is out of equilibrium, with H2S and SO2 present together, which life could use as a source of energy. Our orbiters have detected Carbonyl Sulphide - a clear sign of life here on Earth (though it could be created inorganically on Venus); and particles which are non spherical like microbes and the right size for them.

If we do find life there, it probably didn't originate in the cloud tops. Instead, it's probably a relict of surface life 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").

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. 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.

Venus probably started off similar to Earth. It's surface actually gets less light than the Earth, because though closer to the sun, it has highly reflective clouds. It is so hot, not so much because it is closer to the sun, but because of a 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.

Most scientists think that Venus was a near twin of Earth in the early solar system, with oceans like Earth. We don't have quite the same confidence about this that we have for Mars because its entire surface was resurfaced a few hundred million years ago which would erases any clear signs of the ancient oceans such as the deltas and shore lines of Mars. But there are still hints that suggest it did have them.

Evidence for early oceans on Venus is indirect.

This is a temperature map of Venus. Observations from orbit are consistent with the idea that Venus had earlier oceans, with suggestions that it might have granite land masses. If so these may be the remains of ancient continents

"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.

"The new map shows that the rocks on the Phoebe and Alpha Regio plateaus are lighter in color and look old compared to the majority of the planet. On Earth, such light-colored rocks are usually granite and form continents.

"Granite is formed when ancient rocks, made of basalt, are driven down into the planet by shifting continents, a process known as plate tectonics. The water combines with the basalt to form granite and the mixture is reborn through volcanic eruptions.

"If there is granite on Venus, there must have been an ocean and plate tectonics in the past," Nils Muller said.
See Oceans on Ancient Venus - Study suggests (

If Venus did have oceans in the early solar system, life could have evolved independently from Earth. Or, it's possible that Venus was seeded by life from Mars or Earth, billions of years ago. Or the other way around, it could have seeded Earth or Mars, or both.

If so then this is a really exciting possibility for biology. We may make amazing discoveries from studying life that's been isolated from Earth for billions of years, or possibly 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.

However this may also have planetary protection implications. Some time back, in 2006, an international team of scientists for COSPAR (Committee on Space Research) examined the situation for Venus, in "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 even in the Venusian cloud tops, conditions are so different from Earth conditions that there is no need for planetary protection.

As a result of this report, Venus is currently classified as Category II, and sample return as unrestricted Category V. This means that you simply need to document whatever it is you do.  (For the current planetary protection categories, and policies, see Planetary Protection (Wikipedia) )

This also means that you can 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 is that you have to keep detailed documentation of whatever you do.

However, there was a dissenting voice at the time, by Dirk Schulze-Makuch who was not part of the team. See Planetary Protection Study Group Mulls Life On Venus

Everyone seems agreed that there are no planetary protection issues for the Venus surface, with temperatures well over 400C. But should the Venus atmosphere perhaps be re-categorized as category III, meaning that you have to sterilize spacecrafts that visit it? Should sample return from Venus be re-categorized as restricted Category V, meaning that you have to take precautions to protect Earth in event of a sample return?.


At first sight it certainly seems unlikely that Earth life could survive in the concentrated sulfuric acid droplets in the clouds. These droplets have pH less than 0, similar to battery acid. This is the main reason the COSPAR team gave for their conclusion that no Earth life could survive in them.

However, in 1991 researchers found some Earth microbes able to survive sulfuric acid with pH 0 or lower, close to the Venus cloud top conditions. These researchers also wrote that it is  possible that we might find organisms able to tolerate even lower pH levels.

Their most acidophilic (acid loving) microbe was  Picrophyilus, which grows optimally in sulfuric acid at pH 0.7 and is capable of growth (not just survival but growth) down to pH -0.06 (1.2 M sulfuric acid). This is a microbe which you can find living naturally in highly concentrated sulfuric acid in the wild, in acid mine drainage and in solfataras (sulfur emitting fumaroles).

So perhaps some Earth micro-organisms could live there after all. 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 (CFUs), 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.

Also, 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?

The microbes carried by humans can have hidden extremophile capabilities - because microbes do not lose their capabilities, usually, when they move to a different environment. Some are polyextremophiles able to live in a variety of extreme environments as well as in much more ordinary ones (for humans).

A typical human has 100 trillion microbes in 10,000 species - and the species mix varies from one person to another. Many of these will be unknown to science, and some may well have extremophile capabilities.

For example a recent study of microbial populations of human belly buttons found a couple of species able to thrive in extreme cold and extreme heat. Another example is the discovery of a microbe on a human tongue able to thrive in conditions of very low pressure.


The other way then there have been suggestions of possibilities for life in the Venus clouds, indigenous life. There are one or two interesting hints, observations that could be interpreted as evidence of indigenous life.

The most intriguing of these are, the presence of OCS which on Earth would be strong evidence for life. On Venus however it is just suggestive, not conclusive. There are processes that could create the observed levels of OCS without life, and detailed models of these processes are compatible with the observed levels.

The atmosphere is also not in equilibrium, as it has both H2S and SO2. This disequilibrium is something that life could exploit. The upper atmosphere of Venus also has been shown to contain particles that are microbe sized, and non spherical, which might be an indication of life in the clouds.

Particles in the Venus atmosphere stay suspended for months, rather than the days for Earth. Still, they will eventually fall to the lower layers; so that makes it an issue, how do the microbes stay aloft? 

I haven't yet seen a worked out answer to this, so here are a couple of suggestions to explore.

First, perhaps microbes in one droplet, descending, could send out spores (explosively perhaps) that land in other droplets that ascend, and so continue the reproduction?

Another idea is based on the observation that some microbes form gas vacuoles on Earth more or less permanentThey are used by cyanobacteria to regulate buoyancy in water, not that far off the idea of using hydrogen vacuoles to regulate buoyancy in CO2, evolved over billions of years. Is it possible I wonder? The main difference is, that the gas vacuoles in cyanobacteria take up only a small part of their bodies (and are made up of smaller, rigid, gas vesicles). Apparently Anabaema has gas spaces occupying up to 9.8% of their volume (see page 124 of the paper "Gas vesicles"). But this is far below the levels needed for 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 to 9.8% of the volume of the microbe.

For microbes to float in the Venus upper atmosphere, if the vesicles were 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, then they would need to have so much hydrogen in the vesicles that they occupy approximately 98% of the volume of the microbe.

I'm not sure if this is possible. However, life solutions are often surprising. If we do find life in the Venus upper atmosphere, then we would need to find out next, how it managed to stay there and reproduce. For instance, could the cells provide hydrogen filled bubbles, or external vesicles filled with gas, which remain attached to their bodies, somewhat like the bubble nests created by some insects, and use those to float in the Venus atmosphere? Or indeed might there even be higher plants or animals that do this?

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?

It would need to have less than 98% of the volume for the bubbles and the body of the creature, with the rest all hydrogen, to float at the 1 atm level on Venus.

(This is my own suggestion, not seen it published anywhere)


The 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. I am not either.


First, the lifeforms, if acidophiles, could colonize some of the most acidic environments on Earth.


Also - microbes often retain capabilities that they no longer need. Even though it would be billions of years since these microbes lived in Earth like environments, still there is a possibility that some might retain capabilities from those times to live in less acidic environments than for the Venus clouds.


Another thought, this is my own idea, is that we haven't studied the clouds in detail close up, only from orbit, or with instruments with limited capabilities. Especially if there is life there, then we know that life can transform habitats and form micro-habitats.

If that's so, there could be micro-habitats in the clouds caused by life processes that are not so acidic inhabited by symbionts which would perhaps need the capability to live in non acidic environments.


Another potential hazard for contamination, both ways is through GTAs. This was found to be a potential hazard of a Mars sample return. The report by the European Space Foundation made a special mention of it.

This assumes that the DNA is similar to Earth DNA with the same cell mechanisms, and the microbes similar to archaea and there is a common origin.

All of that is possible if Earth microbes colonized Venus in our Hadean period, or indeed the other way, if Venus life colonized Earth. this could also happen if Mars or some other place in the solar system was the origin of life for both.

Then you have the possibility of Gene Transfer Agents, tiny virus like particles able to transfer DNA from one micro-organism to another.

Many archaea have an extraordinary ability to share tiny fragments of DNA in this way with other totally unrelated species. In one experiment, researchers added GTAs able to confer antibiotic resistance into a sample of ordinary sea water and left it overnight. By the next day 47% of the culturable microbes in the sea water had taken up this antibiotic resistance capabilities from the GTAs

The GTAs are also small, of order of size of a few tens of nanometers so hard to contain during a sample return.

GTAs produced by Reugeria mobilis 45A6 These GTAs can transfer material from one micro-organism to another, and can transfer genetic material between unrelated species of archaea.

They are tiny, tens of nanometers across, far smaller than the smallest known cells at 200 nm across. So hard to contain, and could potentially transfer genetic material even to Venusian lifeforms, or from them, if they and Earth have a common genetic origin.

See also, Horizontal gene transfer in microbes much more frequent than previously thought

In this way, Venusian cells even unable to survive on Earth; even if they are dead, might be able to transfer some of their genetic material to Earth archaea - so transforming them and adding new capabilities.

This is also a potential issue for forward contamination of Venus clouds as well. Earth archaea could transfer genetic material to microbes in the clouds in the same way.


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 GTAs? 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, some of the things that could happen are

  • Pathogen of humans or our crops or animals, sea creatures, plants, trees. Doesn't need to be adapted to us. A disease of microbes for instance can directly infect humans without any adaptations, as happens with Legionaire's disease, a disease of amoeba that happens to be able to reproduce also in human lungs. Indeed pathogens most usually adapt to keep their host alive for longer, not to kill their host.
  • Is able to live on our skin (e.g. fungal infection), in our lungs, in our sinuses, or perhaps in our stomachs. That last possibility is an obvious microhabitat for an acidophile from Venus - though it would be used to sulfuric acid rather than hydrochloric acid of gastric acid (the Venus atmosphere does have HCl as well however).

     Perhaps just possibly, it would be able to reproduce in the stomachs of animals, and ourselves, and interfere with digestion, or create byproducts poisonous to us or the animal?
  • Takes the place of some other organism in an ecosystem but behaves differently so disrupting natural cycles
  • Is an allergen or creates allergens as a byproduct
  • Creates a poisonous byproduct that interferes with human or other animal biological processes. For a simple example, green algae produce a chemical that it's thought, may cause Alzheimer's disease in humans. The way it works is interesting also, it creates a non protein amino acid, β-N-methylamino-l-alanine (BMAA), which substitutes for the protein amino acid, L-serine, and this leads to cells to cluster together and die. A microbe from another planet might well create chemicals that resemble ones in our body but are not identical and get substituted for them, and so disrupt the way our cells work or the way cells of other organisms work.

    It is of no benefit at all to the green algae to cause Alzheimer's and is not part of its natural cycle. It doesn't even colonize humans. Just creates a chemical which gets concentrated in shellfish and the like, and may possibly (not totally confirmed yet), when eaten by humans, cause Alzheimer's

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.

I talk about this some more in Need for Caution for an Early Mars Sample Return.


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.

Rotating DNA animation. Could life on Venus have a different backbone from DNA , using PNA, HNA, TNA, GNA or other XNA?
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, may still uses PNA or evolved to 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.

If there does turn out to be life in the Venus clouds, then, the situation is not that different from the situation for Mars.

The microbiologist Joshua Lederberg said

Whether a microorganism from Mars exists and could attack us is more conjectural. If so, it might be a zoonosis to beat all others.

On the one hand, how could microbes from Mars be pathogenic for hosts on Earth when so many subtle adaptations are needed for any new organisms to come into a host and cause disease? On the other hand, microorganisms make little besides proteins and carbohydrates, and the human or other mammalian immune systems typically respond to peptides or carbohydrates produced by invading pathogens. Thus, although the hypothetical parasite from Mars is not adapted to live in a host from Earth, our immune systems are not equipped to cope with totally alien parasites: a conceptual impasse.

Some of these possibilities may seem unlikely, but how do you assess the probabilities of them? Can we design a mission return to Earth able to cope with these possibilities?

Carl Sagan wrote in Cosmos:

“ If we wish on Earth to examine samples of Martian soil for microbes, we must, of course, not sterilize the samples beforehand. The point of the expedition is to bring them back alive. But what then? Might Martian microorganisms returned to Earth pose a public health hazard? The Martians of H. G. Wells and Orson Welles, preoccupied with the suppression of Bournemouth and Jersey City, never noticed until too late that their immunological defenses were unavailing against the microbes of Earth. Is the converse possible? This is a serious and difficult issue. There may be no micromartians. If they exist, perhaps we can eat a kilogram of them with no ill effects. But we are not sure, and the stakes are high. If we wish to return unsterilized Martian samples to Earth, we must have a containment procedure that is stupefyingly reliable. There are nations that develop and stockpile bacteriological weapons. They seem to have an occasional accident, but they have not yet, so far as I know, produced global pandemics. Perhaps Martian samples can be safely returned to Earth. But I would want to be very sure before considering a returned-sample mission.”


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 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.

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, with enough velocity to leave the strong Venus gravity and get transferred to Earth or Mars.

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? 

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. 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, the subsurface ocean of Jupiter's moon Europa, or comets. But 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.


If it turns out that Venusian life is based on XNA, this does not make it safe for Earth life. Yes, as some say, the XNA would not be adapted to Earth life. But the other way around, Earth life might not recognize XNA as potentially harmful (as in the Ledeberg quote above). And adaptations of microbes are usually in the direction of keeping the host alive for longer; it is of no benefit to a microbe that infects a human for the human to die.

It might also be able to out compete Earth microbes in their own habitats, and yet behave differently from them, transforming ecosystems. It could damage crops or animals, or change the balance in the seas. In the worst case, XNA based life might prove to be better than DNA based life all round. For instance it might be more efficient at metabolizing and reproducing. The very worst case is goodbye DNA.

For these and other reasons, then researchers in the field of synthetic biology, who are actually contemplating the possibility of creating new life based on XNA instead of DNA (by substituting XNA for DNA in a cell, complex process but most of it is now worked out) - they are exceedingly cautious about the research.

In the XNA specifications section of this paper: Xenobiology: A new form of life as the ultimate biosafety tool The authors talk about 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).


XNA returned from Venus could not be contained at those sort of probability levels. 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 Mars sample receiving laboratory.

(Incidentally, my own view, I also think the plans for one in a million level containment of a Mars sample are totally inadequate levels of probability for Mars, if the sample happens to contain XNA or substantially different life forms).

For the issues for a Mars sample return see Could Microbes Transferred On Spacecraft Harm Mars Or Earth - Zubrin's Argument Revisted and  Need For Caution For An Early Mars Sample Return - Opinion Piece


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 GTAs 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 didn't consider GTAs or XNA, and 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.

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.

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.


For all these reasons, putting my own personal view here for discussion, I think the wisest approach in the case of the Venusian clouds is to study them "in situ" to start with. Perhaps we shouldn't bring unsterilized samples back to Earth quite yet.

There are many instruments we can send, most originally developed for Mars. 

These include 

  •  SETG, miniaturized DNA sequencer on a chip The SETG instrument 
    This has already been tested to see if it is space hardy enough to send to Mars and passed the tests so far. It could as easily be sent to Venus. Of course it only works if there is Venus cloud life closely related to Earth, but that is a possibility if the planets shared life in the early solar system
  • Biosignature searches, for instance tests for chirality - for molecules which are asymmetrical, they can occur in two mirror image forms. ExoMars, probably in 2018, will be the first mission to Mars to search directly for life since Viking and includes MOMA which is able to do chiral analysis. Chiral analysis is especially useful as it could detect life that is not based on DNA. If the life does transcription and reproduction and construction of complex structures using methods in any way similar to Earth life, it seems likely to favour only one isomer over the other.
  • Raman spectrometer able to do highly sensitive non destructive analysis able to detect molecular structure.
    Field testing of the Exomars Raman spectrometer
  •  Levin's new version of his labeled release experiment from Viking updated to detect chirality. This is of especial interest since it could detect venusian life based on novel chemistry - something that most of the other instruments can't do. Though it depends on the life to be cultivable, which the others don't require.
  • On going NASA project to develop a miniaturized scanning electron microscope for Mars, again could be sent to Venus

  • High resolution optical microscopes

New instruments continue to be developed. We can now do a huge amount more by way of in-situ studies than we could in the Viking era.

I think the chance that it is hazardous is tiny - but, much as Carl Sagan said for Mars sample return, we shouldn't take even a tiny possibility of existential risk, such as extinction of humans, or long term diminishment of our life prospects, with a billion lives.

Carl Woese, who first classified the Archaea, the third domain of life said in an interview:

When the entire biosphere hangs in the balance, it is adventuristic to the extreme to bring Martian life here. Sure, there is a chance it would do no harm; but that is not the point. Unless you can rule out the chance that it might do harm, you should not embark on such a course.

Carl Sagan wrote in his book Cosmic Connection:

…Precisely because Mars is an environment of great potential biological interest, it is possible that on Mars there are pathogens, organisms which, if transported to the terrestrial environment, might do enormous biological damage - a Martian plague, the twist in the plot of H. G. Wells' War of the Worlds, but in reverse. This is an extremely grave point. On the one hand, we can argue that Martian organisms cannot cause any serious problems to terrestrial organisms, because there has been no biological contact for 4.5 billion years between Martian and terrestrial organisms. On the other hand, we can argue equally well that terrestrial organisms have evolved no defenses against potential Martian pathogens, precisely because there has been no such contact for 4.5 billion years. The chance of such an infection may be very small, but the hazards, if it occurs, are certainly very high.

If there is clear evidence of life there, then we should proceed with extreme caution, and treat the cloud tops as category III, until we know what it is that we are dealing with in some detail.


In the quote given above, Carl Sagan wrote in Cosmos:

“ If we wish on Earth to examine samples of Martian soil for microbes, we must, of course, not sterilize the samples beforehand"

But that was way back in 1980. before easy gene sequencing and many of the other tools we have to analyse samples.

Nowadays, we could do non destructive sterilization of the sample, and still learn a huge amount from analysis of it. Perhaps this deserves study as a possible solution for the first Venus sample returns - if it is indeed a potential back contamination risk? Also for Mars sample returns?

Heat sterilization would damage the sample perhaps too much to get a good idea of its original state.

However a gentler approach could be to subject the sample to the equivalent of a few million years of cosmic radiation, highly ionizing radiation before it is opened. Ideally you would do that before the sample is returned to Earth. Either that, or else, you need to be totally sure that the sample container can't be breached during return to Earth.

If there is a possibility of existential risk, such as XNA, with potential for long term diminishing of life prospects or extinction of humanity then I suggest that we should be sure of containment of the first sample returns to the 1 in 1020 level or similar. For more about this see Need for Caution for an Early Mars sample return, where there is a section discussing existential risks.

I've wondered if it might be enough to put the capsule into an orbit that takes it repeatedly through the Van Allen belts. Typical satellites that pass through the Van Allen belt receive thousands of REM a year, but still that would mean many centuries of passing through the Van Allen belts to build up the equivalent of a million years worth of cosmic radiation. So might need to rely on extremely reliable containment of the sample return and do more radiation exposure on return to Earth.

Ionizing radiation would break up any DNA or XNA into short fragments and would also damage GTAs beyond repair. But yet there would be plenty to analyze with modern tools, to get a good idea of what the sample was like before sterilization.

That might just be needed for the first sample return or first few returns. It would depend on what we find out about the Venus atmosphere as a result.

This is just a thought for discussion, would this approach be an adequate way to make totally sure at the 1 in 1020 level that the sample is unable to contaminate the Earth irreversibly? And would it leave the sample in a reasonably intact state for study and analysis?


I am not sure whether or not the upper Venus atmosphere is correctly categorized, presently, as Category II (no restrictions only required to provide documentation describing any experiments), because of the, probably remote but not impossible, chance of contamination both ways. I think, personally, that the case deserves a review in light of more recent research and ideas.

Also, apart from classification issues, 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, as the classification is not certain enough that it might not change in light of future discoveries.

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 to make sure you do not contaminate Venus samples or the clouds themselves with reproducing life, including the probably remote chances of some archaea with pH 0 acidophile capabilities getting transferred to Venus on our spacecraft, or some of the archaea able to share their DNA with Venusian organisms via GTAs, or Venusian XNA able to out-compete DNA.

If we go into this with our eyes open, and debate all the possibilities, however extreme, we will be able to explore the solar system safely.

This should be regarded as an exciting possibility. In my own view again, then 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 colonization of the cloud colonies for a later date

We should celebrate the discovery of other forms of life anywhere in the solar system. Also, if discovered, proper study of exoplanet life should take priority over colonization, in my view, if there are any conflicts of interest. What do you think?


Will we Build Colonies that Float Over Venus like Buckminster Fuller's "Cloud Nine"?

On forward and back contamination issues, for Mars

Could Microbes Transferred On Spacecraft Harm Mars Or Earth - Zubrin's Argument Revisted - though it is unlikely that Venus and Earth shared microbes for the last few billion years, still, they might have shared life in the early solar system - and indeed, Mars and Earth also are most likely to share microbes in the early solar system during the late heavy bombardment.

There are many practical, ethical and legal considerations for a Mars sample return. The same considerations would apply for a Venus sample return, if there is any chance that it contains indigenous extra-terrestrial life. See:

Need For Caution For An Early Mars Sample Return - Opinion Piece

Mars Sample Return - Legal Issues and Need for International Public Debate

Mars Sample Receiving Facility and sample containment