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Wouldn't the atmosphere on Mars sublimate all of his water supply? He even says that the water reclaimer's tubes burst due to the event.

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    Hello and welcome to SFF! Don't forget to register your account and take the tour! – TheLethalCarrot May 9 '18 at 14:38
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This question on the Space Exploration Stack Exchange about what would happen if you had a pool on Mars and it was exposed to atmosphere has the answer you're looking for.

In summary, the drop in atmospheric pressure would cause some of the water to boil off right away, but boiling causes what is left behind to become colder. The first layer of water to boil off would leave a layer of ice behind, and ice doesn't sublimate nearly as quickly under low atmospheric pressure. With the layer of ice protecting it, any water sealed between a container and the ice layer should either freeze, or remain as is. If Watney acted quickly enough (we're not talking seconds here, more like hours to days or more), all he just needs to do is seal up the hab and repressurize, which will allow all that ice to melt back into water. In the end, he'd lose some water, but not all of it. How much depends on a bunch of factors that I wouldn't know how to calculate even if I did know the exact setup.

Edit: Drew Stephens in a comment below found a great video demonstrating this in a vacuum chamber. In that experiment, it looks like he only lost about 1/3 of the total water to boiling.

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Mark generated his own water, by burning Hydrazine (N2H4). In the film, this initially caused a minor explosion... I can't recall if the same thing happened in the book. Even if all the water he had gathered boiled/sublimated, he could probably generate more, assuming he still had hydrazine and LOX.

Hydrazine is used as rocket fuel, and may have been used in the MDV which remained on Mars.

It's probably worth pointing out that the water he generated wasn't intended as drinking water for him; it was needed to grow his potatoes. With all the plants dead, he wouldn't need as large a supply. He probably could have recovered enough for his own needs as detailed in Cody's answer.

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    I can't recall if the same thing happened in the book. It did. – Renan May 9 '18 at 20:56
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    @Renan Please do not misuse code formatting for non-code text, like quotations. Quotation marks, or if you really must, bold or italics, work just fine. The issue is that code markup doesn’t precisely make text monospace with a gray background—really it marks the text as “code” and the formatting you see is just the standard way to visualize that. For alternative browsing technologies, particularly non-visual technologies (e.g. screen readers for the blind), something else has to be done. That something else may inhibit easy understanding (reading letter-by-letter is not unheard of). – KRyan May 10 '18 at 18:39
  • @KRyan Thanks. I didn't know that. I'll use italics next time. – Renan May 10 '18 at 18:40
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    @Renan It sortof happened in the book. He did burn hydrazine, and there was an explosion, but they were two different events. As a result, the "accounting for his own breath" thing made a lot more sense to me in the book than it did in the film. I don't remember the exact details, though. (The way I recall it, he noticed that the hydrogen trap he built didn't burn nearly all the hydrogen, so he stopped the hydrazine experiment, empty the hab of oxygen, then burn the hydrogen little by little by having a tank of oxygen and a flame, but he forgot to take his breathing apparatus into account.) – Arthur May 11 '18 at 13:07
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    @Renan - Or, you know, quotes. Because we've been using quotes to indicate quoted material for Quite Some Time Now. :-) – T.J. Crowder May 11 '18 at 13:18

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