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I'm working on a far-future SF story taking place on a terraformed Mars and would like a fact check. Current estimates (see here and here) are that the CO2 on Mars could be released in gas form to warm the planet and raise the atmospheric pressure to about 30% of earth, which is not breathable but is sufficient pressure to not be lethal (about Mt. Everest level).

Assuming the oxygen content can be increased (through some technological means separating CO2 into carbon and oxygen), would a higher proportion of oxygen at low pressure allow a human to breath it? It would certainly be thin, but athletes who climb mountains find their bodies increasing red blood cell count to more effectively capture oxygen. Is Everest-level pressure just too thin even if the percentage of oxygen were higher than Earths (about 21% of air)?

I'm assuming that things like the solar wind stripping away atmosphere due to lack of a magnetic field have been taken care of. The limiting factors are Martian gravity and air pressure and content. I'm also assuming no extra mass (gas or otherwise) has been imported from elsewhere and we are working solely with what Mars has to offer.

I'm aware of this question about giving Mars a survivable atmosphere, but the accepted answer claims that atmospheric pressure would be limited by gravity without explaining why Titan, which has gravity 14% of Earth, is able to sustain a mostly nitrogen atmosphere at higher pressure than Earth.

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  • $\begingroup$ Some have answered with suggestions of ways to convert CO2 to O2. That's not what I'm asking. I'm asking if gas at 30% Earth pressure can be made breathable. I will edit the question for clarity. $\endgroup$
    – hatch22
    Commented Sep 24, 2015 at 15:44
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    $\begingroup$ Upon seeing the title, my thought is that Mars doesn’t fit in my nose. $\endgroup$
    – JDługosz
    Commented Nov 20, 2016 at 5:54

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Yes. You'll need to have a much higher percentage of oxygen in the atmosphere. The key is the O2 pressure being within the acceptable range. Early space craft used environments with lower overall pressure but near pure oxygen.

But, just putting the O2 in there does not make it breathable You still need have acceptable levels of CO2, for example, as too much is deadly. Also, very high levels of oxygen may introduce other problems.

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  • $\begingroup$ Right. I'm aware of the dangers of having too much CO2. Even 3% is really pushing it. If the CO2 could be converted to other forms (graphite, oxygen, water with hydrogen from space...) do you think that the pressure would still be sufficient if the O2 content was high? Does that get me giant insects for free? $\endgroup$
    – hatch22
    Commented Sep 24, 2015 at 18:52
  • $\begingroup$ Yes, I think so, as long as the resultant pressure of O2 is approx that of Earth. So if the atmosphere is 1/3 of Earth's, you'd want 3x the O2. I don't think it would impact insects because the O2 pressure is the same, so no giant buggalo herds roaming the planet. $\endgroup$
    – GrandmasterB
    Commented Sep 24, 2015 at 19:33
  • $\begingroup$ The giant insect comment was mostly a joke, but thanks for the response. At 30% pressure (~30 kPa) that's 70% oxygen (21 kPa = 30 kPa * 70%). Hopefully such a high oxygen content won't cause other problems. $\endgroup$
    – hatch22
    Commented Sep 24, 2015 at 20:19
  • $\begingroup$ Oh, I know it was in jest... it just reminded me of the giant Martian beetles called Buggalo from Futurama :-) $\endgroup$
    – GrandmasterB
    Commented Sep 24, 2015 at 21:02
  • $\begingroup$ Seriously, though, read up on the atmosphere used in earlier space flights, because they did this to lighten the load (air has weight) and prevent the bends. I believe they still do this for space walks for that reason. $\endgroup$
    – GrandmasterB
    Commented Sep 24, 2015 at 21:06
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If this is far-future and the atmospheric stripping has been solved, then you can do whatever you want. Solving the atmospheric stripping will require inducing a stronger magnetic core in Mars involving gigatons of metal or some kind of shield has been placed between Mars and the sun to prevent the solar wind from doing it's thing. Either solution involves engineering skill and power sources far far beyond what we have now.

With this kind of advanced tech, separating CO2 into carbon and oxygen should be easy. Nuclear reactors could be tasked with this kind of processing. If a "lower tech" solution is required, plants able to operate at the atmospheric pressures and CO2 densities cited in those two articles could easily begin the process of converting CO2 to O2.

== Edit ==

The pressure itself is going to be a problem. From this atmospheric pressure calculator at 0.3 of surface pressure, the altitude is just under 30,000 ft. While it's possible that a human could survive at that altitude, they're not going to be comfortable there and extended exposure is going to cause all kinds of oxygen deprivation induced damage. At those altitudes/pressures, there just isn't enough oxygen around for the body to use. Certainly, the FAA recommends using supplemental oxygen for all passengers above 14,000 feet.

There may be the same [ratio] of [nitrogen]/oxygen molecules at 20,000 feet as there are at sea level, but because of reduced partial pressure, those molecules are spaced farther apart. Consequently, the partial pressure of oxygen in the bloodstream is significantly reduced; so there's not enough pressure to allow the oxygen to force its way into the blood, and you can't breathe deeply or fast enough to compensate.

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  • $\begingroup$ Apparently I wasn't clear. I'm not asking how to convert CO2 to O2. I'm asking if a gas at 30% Earth pressure can be made breathable simply by having a higher proportion of O2 in it, or will the pressure itself prevent breathing? Another way to think of it is, would the far-future technology be required to maintain a breathable atmosphere, or would Mars be able to naturally sustain one once terraformed (with no additional extra-planetary material)? $\endgroup$
    – hatch22
    Commented Sep 24, 2015 at 15:50
  • $\begingroup$ @hatch22 Thank you for clarifying. I believe I have an answer to your question. Basically, the amount of oxygen in the air doesn't matter if the oxygen is too thin to force itself across the alveoli walls in your lungs to get to your red blood cells. $\endgroup$
    – Green
    Commented Sep 24, 2015 at 17:01
  • $\begingroup$ @hatch22, without being modified even the thin atmosphere you propose would be impossible - it would be blown into space by the solar winds. $\endgroup$
    – user8827
    Commented Sep 24, 2015 at 17:02
  • $\begingroup$ @SeanBoddy, the OP said that the solar wind problem had been solved (though not how it was solved). $\endgroup$
    – Green
    Commented Sep 24, 2015 at 17:03
  • $\begingroup$ Indeed it did - I really only wanted to address that one specific comment, as I understood it. Mars must be permanently modified, and I don't think the core of Mars could be made to generate a sufficient natural shield without extraplanetary mass. So, big shield generator it is, right? $\endgroup$
    – user8827
    Commented Sep 24, 2015 at 17:08
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The best method to convert CO2 into oxygen is ... by photosynthesis. It costs nothing, is maintenance-free and goes on automatically. But for this to occur, you would need to add lots of carbon in Mars' crust.

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  • $\begingroup$ Apparently I wasn't clear. I'm not asking how to convert CO2 to O2. I'm asking if a gas at 30% Earth pressure can be made breathable simply by having a higher proportion of O2 in it, or will the pressure itself prevent breathing? $\endgroup$
    – hatch22
    Commented Sep 24, 2015 at 15:51
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You would basically have a pure oxygen atmosphere as you have low total pressure and need most of that pressure for the oxygen partial pressure. That would be very dangerous as pure oxygen (even at lower pressures) makes things highly flammable - see the Apollo 1 disaster. You need a buffer gas like nitrogen, and there isn't a lot of nitrogen on Mars.

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