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Atmosphere of Venus consists predominately of Carbon dioxide and Nitrogen. Is it possible to convert the atmospheric Carbon dioxide into Oxygen with the help of bacteria by photosynthesis, and make the planet habitable also for humans, on top of other organisms that would be required to terraform it?

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To some extent, we can answer the question for the tools currently available to life, using basic physical principles, and the answer to that is "no". This needs consideration of the atmosphere of Venus and the nature of photosynthesis. The goal of terraforming would primarily be to sequester (or "lock away") the extra atmosphere. So we mainly need to take the CO2 and put it in compound. Photosynthesis does this in principle but the reaction could not trivially be complete on Venus.

Photosynthesizing organisms on Earth basically operate in an open system. Lots of reactions are involved, but you can "bound" the process to only look at what goes in and what goes out. In the following notation, D is an electron donor, which is basically Oxygen as far as we're concerned.

goes in:

  • 2n CO2
  • 2n DH2 - basically H2O
  • photons

Goes out:

  • 2(CH2O)n
  • 2n DO - basically O2

photosynthesis

Ideally, we would like an environment where all the reactants are abundant, and where there's also a diverse environment of other elements that contains the intermediary species. For instance, on Earth we like to use so-called "PNK" fertilizer, which stands for Phosphorus, Nitrogen, and Potassium.

Now let's look at how well the Venus environment fits that. We will have to limit this to the upper atmosphere, because the surface is too hot for any life as we know it. There are other concerns, like how much light it gets and other toxic species that would be less present in the upper atmosphere. But Earth has some floating organisms itself (although not photosynthesizing).

Venus "score" for plants:

  • CO2 - great, too much even
  • photons - great, better than Earth
  • H2O - fail
  • Nitrogen - fail (the N2 form is not sufficient)
  • Phosphorous - fail
  • Potassium - fail

Here is the actual composition of the Venus atmosphere.

Atmosphere of Venus

The nitrogen might not actually need to be removed. Here is my calculation of the N2 mass:

$$ m_{N2} = (4.8 \times 10^{20} \text{kg}) \times 0.035 = 1.68 × 10^{19} kg $$

In comparison, the mass of Earth's atmosphere is $ 5 \times 10^{18} kg$ with about 2/3rd Nitrogen. It's thinkable that something could live on a planet with 10x as much Nitrogen. Anyway, the primary concern is CO2, which is the reason the surface is at huge pressure and temperature.

The next question is: can't we just do photosynthesis and recycle the water? The real reason you can't do this is the nature of the sugars, which generally have a formula like CxH2xOx. This is the form that photosynthesis gives us to lock away the extra Carbon. But there's a problem here - it's not just Carbon. It also has Oxygen, and more problematically Hydrogen. You will run out of H if you store the product as Sugar. Plants stop being helpful here. You can't engineer a plant to produce pure Carbon structures!

The real process you would want is something that uses photons to turn CO2 into a solid structure, with only a small amount of Oxygen gas produced as a byproduct and no water. How? You could use plants as a part of the process in a closed ecosystem floating in the Venus open atmosphere. But then you would have to refine the plant products to an extraordinary purity so that you won't run out of Hydrogen. But whatever system you use would have to also be floating and self-replicating due to the sheer scale. That's where the discussion gets crazy.

The problem is much more space-age than simply engineering some algae and dropping it into the atmosphere (there's no water). Your options are limited to

  • bringing in material from elsewhere in the solar system
  • engineering artificial life to sequester Carbon from the ground up
  • some kind of self-replicating, autonomous, robotic, floating isolated ecosystem with amazing chemical processing performance

None of these are ideas that we can even intelligently speculate about at the present moment in time.

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  • $\begingroup$ You might need to remove at least some of the nitrogen. At about 3.3bar it'd be a partial pressure at the high end of the 'mild' effects range for nitrogen narcosis: en.wikipedia.org/wiki/Nitrogen_narcosis $\endgroup$ – Dan Neely Feb 10 '14 at 15:16
  • $\begingroup$ @DanNeely Ballpark, we're talking about 3x the Nitrogen mass of Earth. So that's only 2x additional pressure. This would be equivalent of 20 meters depth for Earth diving. Beyond that, the discussion gets subjective. Perhaps this would lead to mild impairment and euphoria, as per your argument and reference. But with the scale of engineering this, perhaps you'd throw up your hands and learn to live with it. But is this narcosis short or long term? The Aquarius lab is under 20 meters, so we can ask its residents how it feels. $\endgroup$ – AlanSE Feb 10 '14 at 16:37
  • $\begingroup$ The WP table is for total pressure not additional pressure so 3.3 bar is the correct value to use. 1 bar/10 meters of depth only works on the table if a depth of 0 meters is 1 bar. $\endgroup$ – Dan Neely Feb 10 '14 at 16:58
  • $\begingroup$ @DanNeely 3 bars (what you said) corresponds to 20 meters (what I said), so I don't think you're correcting anything. $\endgroup$ – AlanSE Feb 10 '14 at 17:12
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    $\begingroup$ I wonder how viable would it be to decompose sugar thermally, leaving solid carbon and freeing up hydrogen (with water vapor) for reuse. $\endgroup$ – SF. Nov 4 '17 at 12:15
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No.

You could use various techniques to convert/fix all the harmful substances in Venus' atmosphere, but photosynthesis is not likely to be one of them.

This is because Venus has an incredibly opaque atmosphere, so all you would be able to photosynthesise (assuming you had bacteria resistant to the acidic sulphur compounds found n the atmosphere) would be the top layer.

In any case, the technology we have now or for the foreseeable future just won't let us do this at a rate which will terraform Venus in time for humans to still be human.

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Kim Stanly Robinson had a few throwaway lines about the people trying to do so in his Mars trilogy; however it was with technology well advanced beyond our own. At this point in the series Mars was well on the way to being terraformed with swarms of von Neumann (autonomous self replicating) robots available to do the mass industrial steps.

The initial stages of the plan were to deploy a sunshade large enough to fully block out all the sunlight from reaching the planet (originally this was the mirror array used to increase lightfall on Mars; the Venus project grew out of the question of what to do with the mirrors after the Martians voted to remove them). Decades later, after the atmosphere cooled enough that all the CO2 was liquified/frozen the plan was to seal off the tops of the CO2 seas with sheets of artificial diamond.

A normal day/night cycle would then be provided by wrapping the planet in superconducting cables to spin it up like the solar systems largest electric motor and then adjusting the sunshade to allow an Earth normal amount of light to reach the planet.

If there was insufficient water in the residual atmosphere the Kuiper belt would've been mined for comets similar to how the Martian hydrosphere was expanded.

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  • $\begingroup$ KSR goes into terraforming Venus in more detail in his more recent novel 2314, which is essentially a sequel to the Mars series. The method employed is to use a sunshade to freeze the CO2, then cap it with foamed rock, aiming to eventually create an artificial day/night cycle using orbital shades and mirrors. The politically divisive alternative was to bombard Venus with comets/asteroids to spin it up to a normal day, which would undo some of the capping, but would blow off some CO2 into space. Saying which they end up doing would be a spoiler. $\endgroup$ – Blake Walsh Feb 4 '15 at 16:05
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Photosynthesis won't work, as AlanSE has explained. A better option would be figuring out how to extract the carbon from the CO2 to make the carbon nanontubes that we're going to need to build the space elevators.

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