I like the idea, but at 0.2 bar humans would experience decompression sickness unless they decompressed a lot, and to be honest I don't really know what would happen even then in such a low pressure. Since the point of the story is a rescue mission, I imagine that the characters won't have time to decompress. +1 for the idea, though

@MichaelStachowsky, 0.2 bar atmosphere of oxygen is/was used in space missions.

fair enough, although I expect that the astronauts required time to decompress and be brought down to that pressure beforehand, yes?

The rescue mission would not pop out of the blue, so time to decompress doesn't sound like an issue

they can decompress while on the way

@MichaelStachowsky Bringing along a hyperbaric chamber to keep the rescuee healthy, if not happy, is well within the bounds of possibility. It doesn't even have to be huge an unwieldy. See also: Gamow bag.

"At 0.2 bar humans would experience decompression sickness unless they decompressed a lot": citation clearly needed.

@AlexP ISS astronauts decompress for an hour before spacewalks and a 24 hour period of lower pressure acclimatisation before that, FWIW. I expect that is very conservative, but the risk would appear to be nonzero.

Anyway: +1 for neat answer. Not quite sure how you're get a pure O2 atmosphere though...

the rescuees will have space suits or at least some emergency breathing aparatus i suppose? Having to take a few hours to acclimatize, maybe make-shifting a decompression chamber in their shipwreck could be a nice story point.

Interesting comments, everyone. I am fairly sure that going from a 1atm N2/O2 atmosphere to a 0.2atm O2 atmosphere will require decompression. I like @ths 'ssuggestion, although that's for a different question (namely: how do you make a decompression chamber out of a shipwreck)

@MichaelStachowsky you'll need a spacesuit and a needle...

ISS astronauts prebreathe pure oxygen (at their current ambient pressure) while decompressing. They can get the nitrogen out of their bodies faster that way. You don't even have to decompress if you can prebreathe pure O2 long enough (a couple hours is likely to suffice).

Yeah, Apollo astronauts didn't decompress, only prebreathe. (Well, they decompressed, but it was on top of a rocket ascending through the stratosphere). The rapid drop in pressure was apparently non-problematic as long as nitrogen was thoroughly purged from the blood and tissues beforehand.

@StarfishPrime On how you would get a pure O2 atmosphere. I see two ways. The first one requires an ice-planet so small that N2 can escape into space. As the star heats up, the water ice evaporates and is splitt into H2 and O2. H2 excapes very quickly, O2 less so. If you play around with the values enough you might find a usable setup. The other way would be a terrestrial sized water dominated planet, whose original N2-rich atmosphere got stripped of by a late impact. O2 is generated in the same way as in case one.

@TheDyingOfLight I think with the water world you end up with a significant amount of water vapour in the atmosphere too (and worked examples end up with very high ppO2 in any case... many bars, very dangerous for terrestrial life) and in the latter case you'll require some pretty fine tuning to make sure you don't just end up with a very tenuous O2 atmosphere. That way might be more plausibly human-friendly, though.