The oxygen level throughout the movie remained at 20%. Why?

2 Answers 2


He had a machine called an oxygenator in the Hab. Throughout the book, a lot happens with this machine and it is the reason he has to cut a hole in the top of one of the Rovers to carry it. He brings it with him to the lander (Ares IV) site which explains the oxygen during that long drive and there.

It should be noted that the book focuses a lot on the fact that if you breathe in a sealed environment, you will die of carbon dioxide poisoning long before you can deplete the oxygen. Removal of CO2, therefore, is more important on the whole.

He has a large but limited amount of disposable filters which can trap CO2, and greatly extend the amount of time he can breathe a limited quantity of air. It doesn't replenish oxygen though so he still needs to get oxygen from somewhere.

The oxygenator works by cooling air to very low temperatures and decent pressure. It then separates gases based on their condensation/sublimation temperatures. This is how compressed gases are produced and is probably the most effective way to do it in bulk.

In real-life there are also filtration systems which can separate gasses as gasses (this is often used for putting N2 in bags of chips for example). On the ISS, they perform electrolysis to separate water into oxygen and hydrogen but that would make no sense on Mars as he has very limited water supplies.

  • Hm, but mere cooling and pressurising won't split CO2 into C and O2 Oct 7, 2015 at 15:18
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    @HagenvonEitzen The cooling and Hab pressure (not really that elevated) is used for gas separation. A catalyst with electrochemical process is used as described in the linked question for the actual new oxygen production.
    – kaine
    Oct 8, 2015 at 18:17
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    @HagenvonEitzen It's done via electrolysis. I'm quoting the book here, [The oxygenator] heats CO2 to 900°C, then passes it over a zirconia electrolysis cell to yank the carbon atoms off. If you want to see how NASA plans to do it, see here. (It's not overly technical.)
    – arkon
    Dec 24, 2015 at 9:58

As explained in the book, between the oxygenator and the MAV fuel plant and the thin but definitely present Martian atmosphere, Watney had an abundance of oxygen - as long as all the equipment kept working.

The Martian, Ch. 1:

   In the Hab, we had the Oxygenator, a large piece of equipment that could break CO2 apart and give the oxygen back. But the spacesuits had to be portable, so they used a simple chemical absorption process with expendable filters. I’d been asleep long enough that my filters were useless.

The Martian, Ch. 2:

   I ran a full diagnostic on the Oxygenator. Twice. It’s perfect. If anything goes wrong with it, there is a short-term spare I can use. But it’s solely for emergency use while repairing the main one. The spare doesn’t actually pull CO2 apart and recapture the oxygen. It just absorbs the CO2 the same way the spacesuits do. It’s intended to last 5 days before it saturates the filters, which means 30 days for me (just one person breathing, instead of six). So there’s some insurance there.
   The Water Reclaimer is working fine, too. The bad news is there’s no backup. If it stops working, I’ll be drinking reserve water while I rig up a primitive distillery to boil piss. Also, I’ll lose half a liter of water per day to breathing until the humidity in the Hab reaches its maximum and water starts condensing on every surface. Then I’ll be licking the walls. Yay. Anyway, for now, no problems with the Water Reclaimer.
   So yeah. Food, water, shelter all taken care of.

The Martian, Ch. 3:

   But oxygen’s easier to find on Mars than you might think. The atmosphere is 98% CO2. And I happen to have a machine whose sole purpose is liberating oxygen from CO2. Yay Oxygenator!
   One problem: The atmosphere is very thin. About 1/90th the pressure on Earth. So it’s hard to collect. Getting air from outside to inside is nearly impossible. The whole purpose of the Hab is to keep that sort of thing from happening. The tiny amount of Martian atmosphere that enters when I use an airlock is laughable.
   That’s where the MAV fuel plant comes in.
   My crewmates took the MAV away weeks ago. But the bottom half of it stayed behind. NASA is not in the habit of putting unnecessary shit in to orbit. It left the landing gear, ingress ramp, and fuel plant behind. Remember how the MAV made its own fuel with help from the Martian atmosphere? Step one of that is to collect CO2 and store it in a high pressure vessel. Once I get that hooked up to the Hab’s power, it’ll give me half a liter of liquid CO2 per hour, indefinitely. After 5 days it’ll have made 125L of CO2, which will make 125L of O2 after I feed it through the Oxygenator.

  • Martian atmospheric pressure is 0.6% of Earth's! The movie got it wrong! /pedant
    – Shamshiel
    Oct 7, 2015 at 0:14
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    @Shamshiel: it varies considerably and is twice that in some places. Oct 7, 2015 at 8:25
  • @Shamshiel Usually it's 0.6% but with the added stress of having a human inside it, it was under a lot of additional pressure to keep that human alive.
    – corsiKa
    Oct 7, 2015 at 14:45
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    @Shamshiel, Acidalia plantia, where the Ares three hab is, is low. It is well below the 6.1atm datum level, in fact it is about 2000m below datum, and at that level the pressure is considerably more. He's comparing mean local pressure with the Earth.
    – James K
    Oct 18, 2015 at 11:21
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    @JamesKilfiger Mars atmosphere has a scale height of 11km, so a 2km drop in elevation gets you a 20% change in pressure — not enough. But there's another effect, a seasonal one as dry ice at the poles melts and re-freezes. Mark is on Mars for nearly a year, but in Chapter 3 it's only been a month, so he could be talking about the pressure at that time, which is still within the mission timeframe he would have been briefed on. And my guess is they would have scheduled for the northern summer to get the best sun.
    – hobbs
    Jan 13, 2016 at 5:03

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