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This is the article I've been working on for some time on Venus Cloud Colonies for my Science20 column

Venus Cloud Colonies - Second Home for Humanity?

Yes (at least a likely hypothesis for Tungaska) - but that would be a comet large enough to be worth breaking up before it hits Venus. If you are able to divert comets to hit Venus, would be easy presumably to break them up into tiny chunks say 1 or 2 meters across before they hit the atmosphere. Then you'd hardly notice that anything happened just a nice meteor shower.

I don't know if it is worth while to divert comets to hit Venus, whether that's the best way to deliver water, or if we can somehow make do with the water already there - and I rather think one would do it a bit differently. With advanced technology probably wouldn't be hard to just make lots of parachutes and aeroshells for the fragments of ice, maybe each a few meters across, and parachute them down to wherever they are needed. (Have semi- autonomous machines to make the parachutes, say - or whatever - with only the most minimal of supervision - I think by the time we can divert comets where we like across the solar system and need to make these decisions then will have such devices, 3D printers, etc etc).

But if you did just want to bombard Venus with comets, I think it could be done gently, as gently as you want to, by breaking all the comets into 1 or 2 meter diameter chunks in some way before impact, so it is just experienced as a meteor shower by the hab inhabitants.

Yes indeed, that's what I meant, we can build the cloud colonies (planetary protection permitting) - and then study to see what is the best way to terraform after that.

The cloud colonies would be fine for the gentler ways of terraforming. And if we did want to export the atmosphere, is plenty of atmosphere to export without disturbing the colonies.

I do feel a bit cautious here, that we don't have any experience at all of anything like it, and so, maybe there might be some "gotcha" we can't anticipate even about seeding a Venus type planet with cloud colonies. I suppose the main things are

1. Is there a risk we could make the planet less habitable for humans - for Venus I think NO (For Mars, YES)

2. Is there a risk that colonization could get in the way of finding out new and valuable science - for Venus - I think - possible but at present seems unlikely - need to explore it first (to check to see if there are any life forms in the clouds). For Mars - again I think, strong YES.

3. Is there a risk that early colonization could cause problems with ideas to terraform the planet in the future. For Venus - yes possibly - if the future terraforming involves impacting asteroids into a planet inhabited by billions of people in cloud colonies. Also could be an issue if the terraforming involves a careful progression introducing photosynthesizing plants that create oxygen first in great quantities and to keep aerobes well away. But I think less of an issue than for Mars for both those things. For Mars I think there is a high risk that early colonization, just landing humans on the surface with all their microbes, if they start to reproduce and transform the planet in an unplanned way, could thoroughly mess it up for terraforming.

So - I think Venus is far better than Mars - but not totally clear that it is okay - I think needs exploration and research, quite a bit of it, before we set up our first cloud colony on Venus if it is feasible and seems a good thing to do. And I think terraforming Venus also - though much less likely to go wrong perhaps - but you have the same issue as for Mars - that we don't really know much yet about how planets naturally terraform. Earth may be a rare exception or may be common. Also we would be attempting to do in a few thousand years something that on Earth took billions of years, which is a huge speed up. So may not be so easy to guide it to the desired end state as one might think.

E.g. may be many different stable states it could end up in and perhaps many of them might have atmospheres poisonous to humans so may not be as easy as it seems to steer it to the desired goal. I don't think we can really be confident about that right now but in future as our understanding progresses we may be able to be.

And is a long term thing also and it might be you have to keep it going keep doing things to keep it on target - so you have to commit to 1000s of years of terraforming once you get started, and we don't yet have enough experience to know if we can maintain a single technological society with a single long term project for thousands of years. It might well be possible, but we only have a century or so of reasonably high technology to guide us and have no other similar projects to compare it to in scale.

Okay perhaps ideas like that might work later on - but with present day Venus, even with the long year long night, the night side is almost the same temperature as the day side.

The CO2 near the surface forms a "supercritical fluid" technically, is so dense, is over a tenth of the density of water and transmits heat very efficiently throughout the night side of Venus, same for poles.

https://en.wikipedia.org/wiki/Atmospher … roposphere

In the upper atmosphere above 100 km then you do get a temperature change - the usual thermosphere of Venus on the day side turns into a surprising cryosphere on the night side, extremely low temperatures - but that is well above the cloud habitats level, 50 kms higher.

Another issue with attempts to remove the atmosphere from Venus is that it has so much atmosphere, and the gravity of the planet is strong enough so that unless it is sent into space with enough velocity to leave Venus's orbit and go elsewhere in the solar system, then Venus will just gather all its atmosphere up again, especially heavy gas CO2, over quite short time geologically.

I had the idea - not sure it is original may have read it somewhere else - to freeze the Venus atmosphere and fire it away from Venus and from Venus orbit as dry ice pellets - maybe with some reflective covering added to the pellets to keep the CO2 together as a pellet stop it evaporating - e.g. to Mars if you needed CO2 there or the Moon or wherever in the solar system you needed CO2. Someone else suggested that you could use a Stirling engine, to refrigerate and create dry ice so no need to cool down entire atmosphere and freeze it out. If so that could be a gentle on-going process that gradually removed Venus's atmosphere over thousands of years, lots of these machines, perhaps part of the income of the colony if they can sell the CO2 to other parts of the solar system that need it (at low cost once it is all set up and easy to do).

Okay on extracting water from the sulfuric acid that's the section "Water for Venus". The figures are from 1991 so actual numbers are likely to be wrong. But he says "Sulfuric acid from the clouds will provide water by reactions like H2SO4 + MgSO3 -> MgSO4 + SiO2 + H20 " That's all actually so doesn't say much about it sorry.

Yes the whole thing about the sulfuric acid too concentrated to use wouldn't apply if you are using it within the habitat and diluting it first as you say.

BTW stomach acid has pH 1-2. https://en.wikipedia.org/wiki/Gastric_acid

Acidophiles survive and thrive at PHs below 2. https://en.wikipedia.org/wiki/Acidophile

It says here that the sulfuric acid at the cloud tops would have a pH of 0 and below - similar to battery acid - but that there are some Acidophiles that can live at such low pHs.

http://books.google.co.uk/books?id=b7Xh … ds&f=false

Quite detailed working out of possiblityof life in the Venus Atmosphere by Louis Neal Irwin, and Dirk Schulze-Makuch

I notice says "Buoyancy in air is ultimately a problem for any organism because its density is greater than the air it inhabits. The density of droplets of sulfuric acid, however, is sufficient to keep organisms easily afloat. That means, of course, that organisms are restricted in size to the space available in their airborne aquatic habitat."

Does that mean what it seems to - that sulfuric acid droplets would actually float in 1 bar pressure levels of CO2?? Density of sulfuric acid is 1.84 g/cm³. CO2 has a density of 1.977 g/cm3 at 1 bar and 0C. So yes, seems so, both water and sulfuric acid would float in the Venus atmosphere.

CO2 gas at 1 atm is actually denser than water. If you filled a thin skinned balloon with water it would float in the Venus atmosphere - indeed at the 1 bar cloud colonies level, it would actually float up into the air until it reached the 0.5 bar level.

Yes - crashing comets to give the aerostats an easier time sounds a nice idea .

With that approach if you want to encourage shading once we have cloud colonies on Venus, then there is Hall's Weather Machine - seems reasonably likely we would have technology to do that once we have floating cloud colonies on Venus .

Also remember that even without mega-engineering we have the ability to grow trees and plants which have 90% by weight just from the CO2 plus water in the atmosphere - so long as we can find some process to get the water from the sulfuric acid in a form usable by plants.

Perhaps somehow we can also extract the CO2 to use to make the mirrors for the Hall's weather machine.

Though - all of this is still in the realm of kind of hard sci. fi. I think at present. Nobody has done a properly worked out study of it, while have been a few for Mars (I think those are sketchy also in the extreme though). Would be good if someone were to do a proper engineering type study of Venus cloud colonies.

Anyway here is a link to John Hall's Weather Machine

Youtube videos: http://www.youtube.com/watch?v=Fd63OMosnq0, http://www.youtube.com/watch?v=EOPsczPlzzY

Somewhat critical article about it - http://www.nanotech-now.com/columns/?article=486

Yes that was Carl Sagan's idea to split the CO2 and create carbon and oxygen instead. But he didn't realize how dense the atmosphere was or how hot the surface. As Venus is now, then the carbon would just burn and return to the atmosphere. You need some way to get the carbon out of the atmosphere - and also - turn it into some form of rock that will not burn up and return the Carbon to the atmosphere at the surface temperature of 420C.

There is one idea here - to cool down the entire surface with orbital mirrors to below temperature of dry ice, and then cover it with some covering to prevent it from returning to the atmosphere when it warms up, maybe chemically change it? But that is a big mega-engineering project.

Then you still have the long  243 days long day on Venus - so a terraformed Venus wouldn't be that habitable for higher organisms if adapted to and depending on shorter days. (Perhaps if they hibernated every night and woke up in the day?).

While in the cloud tops the super-rotation means that the atmosphere rotates around the surface with a day of about 4 Earth days - which seems reasonable for life to adapt to.

Certainly true for most life. The thing is though that we have life able to live in environments on Earth that would be just as lethal to nearly all living organisms. For instance well above 100C, well below freezing, very acid, very alkaline, high levels of metals, high levels of ionizing radiation, extremely dry conditions etc. You also get polyextromephiles that can handle several of those extremes at once. See Extremophile Classificiations

And the thing is also, that microbes retain their adaptations when they are in more clement conditions. So a human habitat, and indeed the human body itself, can host extremophiles and polyextremophiles.

So - is not immediately obvious that Earth life would be unable to survive there, might be, but needs study and simulations.

However the 2006 planetary protection report for Venus seems pretty conclusive

So  that means they think the chance that Terrestrial microbes could grow in the Venusian atmosphere is less than one in a million.

So that seems conclusive. But that is back in 2006, and I wonder if it needs re-assessment. At the time, there was a dissenting voice on the subject, see Planetary Protection Study Group Mulls Life On Venus:

I think part of it is that planetary protection studies usually use similar ideas to biohazard protection on Earth where a 1 in million chance is thought adequate. But if you fail at planetary protection, that could be a hazard for Earth or for the planet being protected for all future time. So potentially trillions of people in the future could be affected if it goes wrong.

So, I think one should use lower probabilities than 1 in a million. I think a closer parallel is the creation of XNA based life in the laboratory - which has potential to out compete DNA in the wild. The required probability of escape for that, if we ever do it - in one study - was set at one in 10^20. I.e. you have to be pretty certain it can't escape, not just 1 in a million sure. They would do it by such techniques as making it so that it requires ingredients you only find in the laboratory and not in the wild to grow - and to make sure it can't adapt to use normal materials available on the Earth.

Those are "Existential Risk" calculations and personally I think that sample returns for the first time from outside of Earth should use probabilities based on existential risk assessment, and also I think same for contamination of other planets by Earth, that is a personal view - I would require all missions to Mars to be sterilized to Viking levels myself or better, because I'd make the probabilities far more stringent. But that wasn't done and there is a chance we have contaminated Mars, perhaps 1 in 1000 or so, hard to estimate - but reasonable chance we haven't. I think, personal view, again, that for places we haven't yet sent missions to such as Europa especially then we should do far better than the 1 in 10,000 and interestingly some of the scientists in the Europa planetary protection assessment put forward that same view that the standard 1 in 10000 risk of contaminating a planet during the exploratory phase is not strong enough for Europa.

I'm not sure if we need to use such a strong standard for not contaminating the Venus cloud decks. But I think we should proceed with care, and take into account the possibility that we might not know enough to properly assess probabilities yet, and that it may need more research.

Our knowledge moves on and there is always the possibility of e.g. finding new acidophiles that could survive on Venus for instance. I don't think that they actually simulated a Venus atmosphere for instance and tried putting Earth microbes into it to see if any were able to reproduce - it was a purely theoretical study. And our knowledge of the Venus atmosphere isn't that great either, I think we need to know more before we can assess the probabilities well - know enough to be able to simulate it on Earth - if we can do that - and try seeding it with Earth microbes, lots of times in long running experiments to simulate e.g. humans visiting Venus, and none of them grow - then can be reasonably confident that we are not going to contaminate it.

Yes that's exactly one way that organics from ancient Earth or Venus could end up on the Moon. Yes if you return a sample from Mars to Earth is a big issue to make sure you didn't contaminate it. It's a bit of an issue if you don't return the sample but study it on Mars also, studying for traces of life in situ.

The rover that examines the sample must itself be very clean so as not to contaminate the readings. Clean not just of dormant life, but - especially if you are going to send the miniaturized DNA sequencer, also of dead life as well. That's quite a challenge. The normal ways of sterilizing a spacecraft would leave dead microbes on the surface, even heat sterilized like Viking. There's a technique for Mars using supercritical CO2 snow - CO2 in a semi-liquid state  which can get rid of the organics as well and is rather neat as it uses the CO2 in the Mars atmosphere, you'd do that when the spaceship gets to Mars on top of all the other sterilization procedures.

Yes I've come across flame wars on this topic in other forums, and will tread lightly - if there are signs of a flame war developing I'll shut up. I got in one truly horrid one early last year actually - didn't realise what was happening until I was right in the middle of it. Learnt from that!

One rule I use now is that if I find myself repeating myself, then that normally means both sides have had their say in an argument and there is probably not that much more to be said at this stage. And the other rule I use is - that in an online conversation - it is okay to just drop out of it when things get heated - and you don't need to keep answering back - can be good to let the other person have the last word . And if you have said something that challenges their ideas they need time to think it through anyway, is good to give them some space if there are signs that their views are being challenged in a major way by what you say.

Yes that is a great mystery about abiogenesis. If Venus is the place with XNA then there would probably be very little evidence of how it evolved, much less so than for Earth. I think Mars is by far the most likely planet to have extensive past deposits showing all the details of whatever evolution did happen on Mars. And even if there is no life on Mars and never has been - it had global oceans almost certainly - so will show what happens to a planet with global oceans and organics and the ingredients of life left for a few hundred million years. Should have e.g. protobionts which form easily in the laboratory - things that look like cells but can't reproduce. May have chemical mixtures that are almost life, but not quite, reproduce a bit but imperfectly, or whatever. Anything like that would be fascinating and maybe shed a light on the very distant past.

There is one other source of information though - that could give us insight into not just early Mars but also early Venus and early Earth. That's the debris from the late heavy bombardment. After a big meteorite impact on Earth (say) - then most of the material ends up in the sun, Venus, Mars, also the Moon within a few tens of millions of years. The interesting thing is there - the Moon - this means there should be debris of impacts on early Earth on the Moon. Also there should be debris of impacts on Venus also on the Moon (obviously much less but still should be some). For Mars the best place to look for it is the surface of Phobos. So - may be meteorites, recognizable meteorites or meteorite debris from Earth or Venus on the Moon and especially if buried deep, so not too much damaged by cosmic radiation, then we might find reasonably intact organics, maybe even reasonably intact XNA and DNA in it. Might even find things such as fragments of fossil ammonites on the Moon as someone said in one of the news stories on this subject. But unlike on Earth - would be unaltered by anything since then, any organics for instance might still be there - if buried deep enough so also in conditions of extreme cold.

Is a nice idea anyway and a good motivation for sending rovers and astronauts to the Moon along with other reasons for going back there.

It's not so easy as that. First are different layers in the atmosphere, life may not be everywhere so may be some searching needed. Then - our tests may not detect it at first. If it is DNA based we can find it easily - just a tiny fragment of DNA and we can find it by DNA sequencing. But if it is XNA then not so easy to find. (XNA is like DNA but with a different backbone and so XNA microbes probably can't exchange genetic material with DNA based microbes or be detected by DNA sequencers).

Some tests work for both DNA and XNA such as searching for chirality - probably most forms of life will prefer one isomer rather than another - e.g. the XNA probably always (or almost always) spirals the same way so that the transcription machinery can work more easily.

Another good test for XNA is the labelled release where you try to cultivate the life and see if it takes up nutrients and  has chirality preferences again.

But - only 1% of species are cultivable on Earth with us knowing a lot about how to try to cultivate it. And though it is likely that XNA would have chirality preferences, we can't really say it has to. The thing about alien life is that the more different it is from DNA, the more interesting it is, but also the more possibilities that our tests miss it.

So - I think - more than just doing some simple tests such as you might do if searching for life here on Earth. Same also for Mars. You are talking about a scientific process and these typically take months, years, sometimes decades, and sometimes can't be rushed. I think we shouldn't rush the scientists who want to study life on Mars or Venus. There is no deadline and no need for hurry. Instead give them the resources they need, to find out what they want in as much detail as possible - and bear in mind the possibility that they might find interesting life different from Earth life - and that that is the most interesting possibility that we should celebrate. So, give them a chance to find this.

That is I think the view of xenobiologists generally, for instance eight of them wrote a paper for the last decadal review strongly urging against a Mars sample return for these reasons saying we don't have enough understanding of Mars to know what samples are likely to be biologically interesting enough to return, but need to study it in situ, and surface deposits down to a depth of a few meters are sure to be severely degraded by cosmic radiation as well as other processes that seem to remove organics from the surface as an on-going process - and hard to interpret biologically. Their paper was ignored in the Decadal review summing up as far as I can see.

Planetary geologists I think tend to be a little impatient with the exobiologists at times, you can see all the geology plain to view on Mars and Venus, maybe at times a bit hard to appreciate that the situation is not the same for exobiology, where we can't see XNA or microbes from orbit or even have a clue about whether there is any there at all from those wonderful orbital images the planetary geologists use. And is not enough to just take a few samples - until you know where to look, then samples you return to Earth are likely to be uninformative, and they also pose a planetary protection issues as well for Earth, because although they are unlikely to have life in them when we don't know where to look, on the chance that they do ahve life, biological samples of course can reproduce, which purely geological samples can't do. Have written quite a lot about this also on my science20 column.

XNA particularly, if it exists, could present a particularly tough challenge for planetary protection, because of the possibility that either XNA or DNA may be superior to the other - e.g. XNA microbes are hardier, better at metabolizing - or vice versa. If XNA is better then it could wipe out DNA on Earth in the very worst case, or most of it. If DNA is better, it could wipe out most of the XNA on the target planet we want to study - and if both coexist - then still, the microbes based on XNA may behave differently from the previous DNA based microbes in the same ecological niche and so disturb the balance of nature, as well as lead to extinctions, also the microbes based on DNA could lead some of the interesting XNA species to extinction before we have a chance to study them and disturb the cycles again so we can't study the target planet in its pristine state - and also means of course that it is hard to tell what came from Earth in our probes and what was indigenous.

Lots of other contamination issues both ways - needs a fair bit of thought and there have been several studies which all conclude that great care is needed just in case. I think at least not going to sort this out with a simple sample return as the geologists seem to think, but is a long term at least several decades project - which would go more quickly if we have more resources available to study Mars e.g. by telerobotics from orbit around Mars. And if also the instruments developed by the exobiologists and the interesting missions presented to NASA and other space agencies by exobiologists were given higher priority than they are now.