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A MIT Mars-One Study found that "crop growth, if large enough to provide 100% of the settlement’s food, will produce unsafe oxygen levels in the habitat".

Assuming this to be true, are there any effective ways for removing/decreasing oxygen in a habitat on Mars?

The MIT article came to the conclusion that venting the excess oxygen was necessary, which had the adverse effect of destabilizing the breathable atmosphere.

Since oxygen is "readily reactive with other elements" (Chemistry of Oxygen), it seems like there should be an alternative way to decrease/remove excess oxygen from a habitat's atmosphere. What am I missing?

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    $\begingroup$ There will be more oxygen needed as breathed by the astronauts. Losses by small leaks and by airlock operation should be compensated. There should be enough reserves for emergencies. Pure oxygen will be needed for operation of space suits any way. Venting pure excess oxygen without wasting nitrogen is therefore possible using the oxygen separator. $\endgroup$
    – Uwe
    Dec 22, 2018 at 17:24

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I had come across a blog post on this subject soon after the report came out. That answers your question just as well as anyone else could.

My main take-away was that the problem is totally solvable. What the MIT report accomplished was that it forces anyone looking at the data to not accept the present Mars One architecture. Honestly, I found this confusing because I haven't seen an architecture specific enough to make such claims in the first place, and even the papers reference list doesn't clear this up easily.

In order to solve it, the blog post proposed either a controlled burn or keeping animals. Either way, it's still a drain on the burnable matter, and possibly food, available.

However, the main problem (as far as I can tell) is Nitrogen. See the following graphs from the report, because this is critical:

O2 fraction

N2 stores

Oxygen fraction goes up and down, but clearly it is actively maintained by the station's systems. However, if you look at the N2 stores, it's pretty darn obvious that one of those systems involves venting gas. Oxygen is continually produced by the plants, but the Nitrogen comes from a finite supply.

The story becomes pretty clear with these two components. After you've vented all your Nitrogen out of the habitat, you will not longer be able to maintain both a) low O2 molar fraction and b) sufficient total pressure. To me, this seems pretty obvious from the point that we're venting atmosphere in the first place. You can't just throw away the gas which can only come from Earth.

CO2 is easy to get on Mars but in-situ Nitrogen production is vastly beyond the tech level that we're looking at for the Mars One scenarios. So venting it is fairly flagrantly unsustainable, and whether the colony lasts 60 days or 400 days seems like an unimportant distinction. You won't launch missions with this being a credible scenario.

If the plant matter to burn couldn't be produced easily, it could still be a problem. To whatever extent that is true, then MIT has pointed out a very large planning problem for any mission of this general type.

If you'll accept some solutions that involve heavier machinery and a greater mass penalty, I would tend to think that either gas separation or storage could address this. If you just compressed the air O2/N2 mix then it could hang out long enough for some heterotrophs to get there and draw down the O2 quantity. Alternatively, if you could concentrate the O2 and then vent, that could be sustained as long as you need.

I could be wrong, I hope I've understood the problem correctly. I would sooner call it a "Nitrogen" problem than an "Oxygen" problem.

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    $\begingroup$ That blog post is just wrong. Then again, so was the MIT report. The problem with the MIT report was that it attacked a minor problem. The big problem: Name one long-term closed biosphere project that actually worked for an extended period of time. $\endgroup$
    – David Hammen
    Mar 9, 2015 at 13:19
  • $\begingroup$ @DavidHammen Why would you need a closed biosphere? You simply use machinery to measure and produce the gasses you need in the right amounts. Making a fully functioning eco system is overcomplicating things. You should not rely on having some optimum mix of plants and animals. That would require an unfeasibly large initial habitat. $\endgroup$
    – Erik Engheim
    Aug 17, 2015 at 7:49
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    $\begingroup$ @AdamSmith - We simply don't know how to do that. "Simply" and "just" (as in "you simply do this", "you just do that") are the two of the deadliest words in engineering. $\endgroup$
    – David Hammen
    Aug 17, 2015 at 12:26
  • $\begingroup$ I've added an answer, I agree there's a nitrogen problem but why not just reduce the number of plants and add both mushrooms and pygmy goats which will reduce the oxygen levels. Also not sure why adding N2 and Ar concentrated from the atmosphere to the habitat isn't feasible. $\endgroup$
    – Brooks Nelson
    Dec 21, 2018 at 18:51
  • $\begingroup$ The real problem here is nitrogen. There is too little of it on Mars to concentrate easily (partial pressure looks like about 12 Pa) so it is (at best) a scarce and expensive local resource, and at worst, a fixed resource that you have to import from Earth. That means you can't afford to vent it, which means you lose one convenient tools for managing oxygen levels, so you need another one. You are also going to have to work hard to avoid losing too much nitrogen through airlocks etc. $\endgroup$
    – Steve Linton
    Dec 21, 2018 at 19:49
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The current plan calls for too many plants which will create too much oxygen. Solution: less plants, more mushrooms and add pygmy goats.

Fewer plants will reduce the creation of excess oxygen. Adding mushrooms will reduce the oxygen and add CO2 to the habitat. Adding pygmy goats will also reduce oxygen and add CO2 to the habitat. Also the goats can add milk for part of the year, occasionally meat, and they can eat inedible parts of the plants humans can't eat. The goat waste is also good for fertilizer.

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Oxygen is quite easy to remove from air. An extreme measure would be to light a fire. But there are numerous other chemical reactions that will do it too. A lot of metals oxidize on contact with oxygen for example. And of course there is breathing... A vigorous workout. Raising animals...

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The "MIT Study" is wrong. At least in regards to this "Oxygen problem" it's so completely wrong that it breaks down at several places at once. And sadly it breaks down in places where even high-school physics should be enough to debunk it. That said: I'm only talking about the oxygen part. I don't have any expertise on the other claims. That also said: Mars One was a bonkers idea to start with and still is

So, why is this study so flawed? It makes several claims, that are factually wrong:

1. Oxygen levels will rise because of plants

Since plants can't produce O2 if there is not enough CO2 in the air, I'm wondering how the oxygen levels should rise as mentioned in the "study". Plants do not just "produce" oxygen, they split up CO2 and release the oxygen as "waste product" of that process. And humans use the O2 to burn carbohydrates and exhale CO2 as a result. So it already breaks down here. But for arguments sake let's assume that somehow CO2 from the Martian atmosphere "leaks in" (keep in mind, pressure inside the habitat is 100 times bigger than outside, but let's still assume). So now the plants can produce more O2...

2. There is no easy way to separate O2 and N2

Even without googling I can tell you 2 tried and true ways how this can be done!

  1. cryo separation (uses the fact that O2 condenses into liquid around -180°C while N2 at around -196°C at normal atmospheric pressure. Takes lots of energy but supplies you with O2 and N2 on a nicely storable form)
  2. molecular sieves (basically a membrane with tiny holes where one type of molecule can pass easier than the other one. Systems can be bought basically off the shelf right now and are widely used in the industry) A quick google search would have revealed, that this stuff is available. So again, the argument breaks down here. And there is no recovery this time. But just for the fun of it, let's dig deeper

3. Hypoxia and fire hazard at the same time

I really had to laugh at this one! Had to laugh really hard! While they correctly use "partial pressure" to define the hypoxia level they use "molar fraction" to define the fire hazard, which is completely bonkers in this situation. Molar fraction is only usable at a defined pressure. Otherwise there would have been a huge fire hazard on all Apollo flights which flew with a pure oxygen atmosphere. The Apollo 1 fire was a result of not realizing, that a pure oxygen atmosphere is a bad idea if you pressurize the capsule above sea level pressure. At the reduced pressure the capsules had while in space, it posed no dramatic risk.

Conclusion

The argument about the oxygen breaks down in at least three different places just from the top of my head. And every one of them makes the whole argument completely break down. This "study" does not shed a great light on the physics knowlege of the authors. That's somehow sad...

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