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Most plants require carbon dioxide for their photosynthesis, which Mars has in overabundance.

Would atmosphere composition (let's ignore temperatures for the purpose of this question) of Mars allow vegetation to grow?

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I'll stick to considering the atmospheric composition, as referred to in the original question: Although there may be some rare exceptions that I can't recall, under normal circumstances all green plants use aerobic respiration with O2 as final electron acceptor for energy production. This means that they require oxygen, which is essentially absent from the Martian atmosphere (0.13%). Even though our hypothetical Martian crop could produce oxygen as a byproduct of photosynthesis, that oxygen would be lost into the atmosphere, and would not be available for respiration.

I believe that under waterlogged conditions some plants can switch temporarily to some sort of anaerobic metabolism as a stress response, but I don't know if this is just to survive or whether the plants can grow in this way - I suspect it is the former.

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  • $\begingroup$ +1 I said it wasn't my field :). Would the oxygen present in Mars's atmosphere be enough if combined with that produced by the plants themselves? $\endgroup$
    – terdon
    Commented Dec 11, 2012 at 19:21
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    $\begingroup$ It took (anaerobic) cyanobacteria one billion years to raise the atmospheric oxygen concentration on earth up to a level that supported aerobes. $\endgroup$
    – Alan Boyd
    Commented Dec 11, 2012 at 20:09
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This is not my field by a long shot, so take what I say with a grain of salt. However, this question is very hard to answer because whether or not a plant will grow depends on a great variety of factors. Even if we ignore the temperature as you say, there are other considerations. These include, but are not limited to:

  • Soil composition, I doubt that Martian soil can support earth vegetation even if its atmosphere could. Plants need various nutrients, and specific pH ranges among other things.

  • Atmospheric pressure, I am not at all sure that the Martian atmosphere (though it is, indeed rich in CO2) would be enough to drive an earth plant's photosynthesis. Bear in mind that the atmospheric pressure on Mars averages 600 pascals (0.087 psi), about 0.6% of Earth's mean sea level pressure (source). This makes it highly unlikely that unmodified earth plants would be able to thrive there.

  • Water water water...

  • Pollinating species. Many many plants depend on other species (e.g. bees or hummingbirds) for their propagation. These would be hard to find on Mars.

  • Sunlight I don't know if Mars receives enough sunlight at its distance from the sun to drive an unmodified plant's photosynthesis.

Now, that said, it should theoretically be possible to start with some extremophile archaea or bacteria that would over the course of many many many years (at least hundreds, thousands more probably) terraform Mars to make it suitable for human habitation. Specially engineered plants could play a role then but I find it very hard to believe that any existing, unmodified, multicellular plant life of earth origin could survive on Mars.

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  • $\begingroup$ i know mars soil will not be up to the task as we know it is highly acidic, i'm just wondering about the atmospheric composition alone $\endgroup$
    – lurscher
    Commented Dec 11, 2012 at 18:16
  • $\begingroup$ @lurscher I'm just pointing out that the atmosphere alone is meaningless. In any case, even if everything else were OK, there is probably not enough atmosphere on Mars to provide sufficient CO2 to unmodified plants. $\endgroup$
    – terdon
    Commented Dec 11, 2012 at 18:19
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I work on photosynthesis in algae, blue-green algae and anoxygenic photosynthetic bacteria. All would more or less be able to live on Mars in a blow up tent equipped with a simple blow compressor (like some tennis courts in cold climates). This could be used to raise the air pressure up enough (say 100 millibars) for water to exist in liquid form over a reasonable range of temperatures (Mars atmosphere is about 10 mB, Earth about 1000 mB). The tent material could be made of UV opague material. Besides many algae are quite resistant to UV anyway. The light intensity on Mars is no problem. Most earth plants saturate photosynthesis at about 30% of full sunlight anyway. Keeping the inside warm enough would not be a huge problem because the Mars atmosphere is essentially a near vacuum and so is a very poor conductor of heat. I do not think it would be all that hard. Low O2 is no problem for many algae. You could grow many higher plants quite easily if the O2 level was resonable.

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