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When he first tries to light up hydrazine in the greenhouse in order to obtain water, The Martian Mark Watney blows up the contraption. Later, he reasons this was due to the extra oxygen he exhaled.

But the oxygen level of the air in the dome must've been around 20%, and the oxygen in human exhaled air is around 15%. Why would there be significantly more oxygen around the ignition apparatus (which was at an arm's length above Mark)?

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Disclaimer: Answer according to the book, not sure if it is the same in the movie.

The whole process happens in two steps:

  1. Burning the hydrogen directly as it is produced from the hydrazine in the normal 20% oxygen atmosphere. Mark does this until he notices that not all hydrogen is burned and the concentration in the hab atmosphere gets up to 60% hydrogen.
  2. Then he tricks the hab into lowering the oxygen level to 1% so the hydrogen can't ignite and uses a small oxygen tank to burn small amounts of hydrogen at a time. During this process his exhaled air increases the oxygen level until all remaining hydrogen ignites at once. Luckily this happens while the resulting explosion is small enough for Mark and the hab to survive it.
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    This is definitely how it goes in the book. But I wonder if we should be answering questions about movies based on the books they are based on? Anyhow, the film version just doesn't go into any of these details. He initially tries the burn, it explodes, then he tries a second time after "some adjustments" and things work. The film, naturally, had to gloss over a lot of the minute details that were so enjoyable in the book for time's sake.
    – DA.
    Oct 5, 2015 at 16:44
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    Funny enough, the movie had oxygen levels in his recorded videos. When making the fertilizer, it's 20.9%, down to 17.8% after the explosion, back to 20.8% once the moisture collection is happening the next sol. The level drops .5 points with every flare up to 19.1% in that scene. So they did account for some change in o2 due to the hydrazine burn.
    – cde
    Jan 4, 2016 at 5:04
  • Just as a clarification, does he put on a breather mask or something when lowering the oxygen concentration to 1% in the book? In the film he doesn't, but I feel like he would be left gasping for air if the concentration was only 1%.
    – SGR
    May 22, 2017 at 12:34
  • Yes, in the book he does.
    – Raidri
    May 22, 2017 at 12:48
  • @SGR The movie glosses over these details, and that makes that part nonsense in the movie. But the 1% oxygen explains the explosion.
    – cst1992
    Jul 26, 2019 at 20:01

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