This does not make sense to me.

Mark Watney needs water. He uses an iridium dish to decompose hydrazine into a gaseous mix of hydrogen and nitrogen (2:1 ratio) that he feeds into a hand made oven. Exact quote: "Soon I had short bursts of flame sputtering from the chimney." For some reason, hydrogen does not fully combust and contaminates the air. In the end there is seven times more the hydrogen in the air than oxygen (64% volume). It should have exploded when reached 4% concentration.

Also, there is a lot of nitrogen generated at the iridium dish, yet nitrogen is only at 22% in the end, even though part of hydrogen is utilized to produce water

One more thing, he produces hundreds of liters of water from CO2 (via regenerated O2), but it should have produced about 100kg of carbon - where did it go to? How does his oxidizer work? I haven't read beyond this point so I wonder if he had an idea to feed that carbon to his plants.

  • Dupe of scifi.stackexchange.com/questions/81274/…?
    – Valorum
    Jul 11 '15 at 21:03
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    Richard, no, this is very different. This one is almost in the beginning, and it's about water, not fuel Jul 11 '15 at 21:05
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    He doesn't need to do anything with the carbon, as the scrubber handles it and presumably discards it in some way (vents to the outside?) - the job of the scrubber is to remove excess CO2 from the air and extract the O2 from it - Watney uses it to remove excess O2 from the air as well at some point in the book, so it also can manage the exact atmosphere mix.
    – Moo
    Jul 11 '15 at 21:12
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    Moo, are you trying to say that the oxidizer vents solid carbon out? I have read to the point where he collects O2 from the air, but we can be sure that this is a gradual process. So it can't just jump from zero to 60% Jul 11 '15 at 21:16
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    I'm sure he does, but I don't see how it is relevant Jul 11 '15 at 21:32

You have confused "explosive mixture" with "exploding mixture". The explosive mixture range for a hydrogen/air mixture is 4% to 75%. However, just because a substance is explosive does not mean that it will spontaneously explode. In the case of hydrogen, a spark or flame is required, and presumably Watney is producing neither. Apparently, high-pressure hydrogen jets can self-ignite, but that does not apply. Likewise, there are reports that hydrogen-air mixtures can self-ignite in the presence of a catalyst such as platinum, but that does not seem appropriate either.

The autoignition temperature for hydrogen is 500 C, so as long as Watney does not provide an open flame or produce sparks, and has no local temperatures in excess of 500 C, he is theoretically safe.

That said, I'd be very surprised if there were not spark sources such as ventilation motors in his habitat.

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    If you read the book you would know that he was burning his hydrogen using an open fire. Jul 13 '15 at 0:34

Just because the hydrogen has the capability to combust, does not mean it will do so without provocation.

Hydrogen needs a spark or an open flame to be ignited, unless it reaches 500 C, which, as Watney was still alive, we can assume it didn't.

The hydrogen that was in the air seperated from the group before reaching the flame provided by Watney, and there was a constant flow of O2 there preventing the Hydrogen from combusting on it.

The reason that the hydrogen content is so low is because of the Atmospheric Regulator, a wonderful piece of technology that has the capability to freeze separate the air in the Hab.

The regulator uses freezeseparation to sort out the gasses. When it decides there’s too much oxygen, it starts collecting air in a tank and cooling it to 90 kelvin. That makes the oxygen turn to liquid, but leaves the nitrogen (condensation point: 77K) still gaseous. Then it stores the O2 - Mark Watney - The Martian .

So this is why the Hydrogen content was so high whilst the O2 and N2 concentrations remained within normality. Note that he could not do this with the hydrogen as it's freeze temperature is much too low.

The way he turned the CO2 into pure O2 is a bit more complicated, however.

The AR (Atmospheric Regulator) stores the CO2 into tanks, which then go to the Oxygenator, a fancy device which takes CO2, liberates the O2, and vents the remainder (carbon) outside.

  • 1. "hydrogen that was in the air seperated from the group before reaching the flame" - can you elaborate on that? 2. "vents the remainder (carbon) outside" by carbon you mean graphite? Feb 13 '17 at 10:05
  • By that quote I'd meant to say that the hydrogen that was left over was from the Iridium plate, not from being pumped (gravity fed, really) into the flame with the O2 pouring, since the flame was sort of protected by the flow provided by the liquid O2, the hydrogen didn't combust, and there is no canon explanation, but graphite would seem to be a likely byproduct which could easily be dealt with via crushing it into a powder, or if it were already a powder just simply fanning it out.
    – Benjamin
    Feb 13 '17 at 14:03
  • Sorry, I can't understand you here. Mar 2 '17 at 17:54

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