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Mars has a lot of carbon dioxide in its atmosphere. It also gets a lot of sunlight. On Earth, plants use carbon dioxide and sunlight for photosynthesis. So, why can't plants live on Mars?

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    $\begingroup$ If we put them there, they might grow: planetary.org/blogs/guest-blogs/… $\endgroup$ – Rikki-Tikki-Tavi Jan 9 '15 at 3:17
  • $\begingroup$ @Rikki-Tikki-Tavi That article's about cyanobacteria and lichens, not plants. $\endgroup$ – cpast Jan 9 '15 at 6:59
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    $\begingroup$ The next largest barrier (after the atmosphere and composition of the air already mentioned) is soil. You need soil. You can have some success with hyrdroponics and artificial environment, but for more, you need soil: made by other organisms. You will not get an apple tree from regolith. $\endgroup$ – Mikey Jan 10 '15 at 13:04
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    $\begingroup$ Related question: space.stackexchange.com/questions/4089/… $\endgroup$ – HDE 226868 Jan 10 '15 at 15:49
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    $\begingroup$ Be interesting to ask for a laundry list of features a Martian greenhouse would need: thermal insulation, water, oxygen trap? It could slowly release excess oxygen for the solar wind to whisk away. $\endgroup$ – Bob Stein Jan 16 '15 at 18:44
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Plants don't just need carbon dioxide; like most organisms, they need oxygen to survive as well. They can produce oxygen from carbon dioxide, but that doesn't help cells that aren't exposed to sunlight; if you put a plant in a very low-oxygen environment, it will die.

There are organisms that can survive on little to no oxygen (e.g. cyanobacteria, which can directly supply their respiration cycle from their photosynthesis cycle), but as a rule plants aren't among them.

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All life as we know it requires liquid water as a medium (a polar solvent) in which the chemistry of life can take place. Mars surface conditions pretty much preclude the existence of liquid water. The atmospheric pressure is at or below the triple point of water, meaning that water can only exist as ice or gas (vapor). Even though Martian soil probably contains all the elements necessary for plant growth, and it appears as though Mars once had liquid water on its surface (it would have had a thicker atmosphere back then), plants could not survive in current Martian conditions due to the absence of liquid water.

If the Martian atmosphere were a bit thicker, it could be a different situation, but, as pointed out in one of the other answers, plants actually need oxygen as well as carbon dioxide. In the presence of sunlight, plants consume carbon dioxide and produce oxygen as a "waste product". In darkness, plants still have to support their metabolism and consume oxygen from the atmosphere to do so. Apart from that, plants are not islands of life; they have symbiotic relationships with other organisms including bacteria. Some of those organisms require oxygen too.

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Life may once have existed on Mars, which is largely why so much exploration of it is being done now. There is evidence of flowing water, which implies a warmer climate and thicker atmosphere. But that atmosphere was stripped away as the sun grew hotter, and now Mars is frozen. It doesn't have enough gravity or a magnetic field strong enough to hold in an atmosphere thick enough to keep it warm, with the sun the way it is now.

Plants need liquid water. Mars today has none. Well, actually it does, in the summer, for a few hours each day. Because of the presence of perchlorates in the soil, the melting temperature of water is lower, it works like antifreeze. This is enough to allow it to exist briefly in a liquid state. But then it freezes again. That brings up the matter of the extreme swings in temperature between day and night, because there isn't enough atmosphere to even temperatures out. At the equator, temperatures vary by about 100oC each day and in mid-latitudes by about 60oC. Aside from water issues, such big temperature swings present a challenge to any life form, though some can handle it.

As said by cpast, plants also make use of oxygen and will die without it. They only need a little. It is used to process nutrients drawn from the soil. On Earth, the evolution of plants began after cyanobactria began to produce oxygen as a metabolic byproduct, which eventually caused atmospheric oxygen to build up. Cyanobacteria seem able to survive on Mars, so if there weren't other problems, this mechanism could also have occurred there. Alas, the other problems already mentioned made that impossible.

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  • $\begingroup$ According to this article “About 2 percent of the soil on the surface of Mars is made up of water, which is a great resource, and interesting scientifically.” While deserts on Earth have around 1.3% to 6.3% of water (by weight). So water would not necessarily have to be a problem. $\endgroup$ – fibonatic Jan 10 '15 at 3:10
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    $\begingroup$ @fibonatic liquid water is a problem. $\endgroup$ – kim holder Jan 10 '15 at 14:29
  • $\begingroup$ you are right, I jumped to conclusions. Also looked up the triple point of water, which occurs at 611.73 Pa, which just above the average surface pressure of 600 Pa of Mars. So this would make conditions even more difficult/specific for liquid water to exist. $\endgroup$ – fibonatic Jan 10 '15 at 17:35
  • $\begingroup$ @fibonatic perchlorates lower the melting point such that liquid water can exist. I edited that in - beat you to it :) $\endgroup$ – kim holder Jan 10 '15 at 17:37
  • $\begingroup$ Liquid water is what the Kepler program is looking for. We're looking for those planets in the goldilocks zone where liquid water is possible. That's basically the only way for life to happen, as far as we know $\endgroup$ – Premier Bromanov Jan 10 '15 at 19:54
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Things lacking for Mars to grow plants:

Water. Think that's well covered.

Atmosphere. Even if it had a similar composition to earth's, it's simply too thin. Lack/Loss of magnetic field led to Mars's atmosphere being mostly stripped. If you were to give it a full 1 atm pressure today, this too would disappear. So if you plopped an earth plant on Mars, it would be voided of most of it's gases and die.

Loss of Magnetic Field. Except for small pockets, Mars's magnetic field is too small to protect the surface. This means far more radiation gets to the surface. If the above didn't kill plant life, the radiation level would. The plants cancer would get cancer.

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    $\begingroup$ Radiation isn't a big threat to plants. The distributed nature of a plant (i.e. no organs to be compromised) means they are innately resistant to radiation damage. A dose high enough to damage/kill a lot of the cells will kill off a plant, but gradual radiation damage will only reduce vigor, (at least some) plants also have defenses against radiation they can deploy, i.e. see: news.sciencemag.org/2009/05/how-plants-survived-chernobyl $\endgroup$ – Blake Walsh Sep 29 '15 at 9:38
  • $\begingroup$ In a quick comparison it seemed that the dose on Mars is higher than that where the experiments were likely done. Do you have any sources on how many Sv will kill a plant? $\endgroup$ – Nathan Sep 30 '15 at 14:33
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    $\begingroup$ I don't know. What's easy to find is how much dosage a human would receive on Mars and from there kind of extrapolate. Quick research indicates that a yearly dose on the surface of Mars would be associated with increased cancer risks in a human, but is nowhere near certain death. So plants should be okay. More worrisome would be solar flares, because acute radiation poisoning is far more harmful than chronic, a solar flare would seem to have potential to wipe out a crop, especially if it hits when the plants are young and delicate. $\endgroup$ – Blake Walsh Sep 30 '15 at 15:01

protected by Community Sep 28 '15 at 18:04

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