[Johnny555]

If these habitats are also housing people, then the atmosphere will likely be very close to Earth pressure and composition.

The atmospheric pressure on the ISS is very close to 1 atmosphere, and it's an oxygen-nitrogen mix close to earth's atmosphere.

[Dylan16807]

A mars mission won't be getting frequent short-hop trips from earth. I would expect the habitats to use reduced-pressure atmospheres with less to no nitrogen.

[joshvm]

What would you expect the habitats to have instead of nitrogen? You wouldn't want a high oxygen atmosphere. Not only is it an extreme fire risk (see Apollo 1), but humans aren't built for it - oxygen toxicity exists.

[Dylan16807]

Those are both hazards from having too much absolute oxygen, not too much relative oxygen. Our current launch craft use an atmospheric mix so they are safe on the launchpad, and the ISS uses an atmospheric mix to easily interface with those launch craft. On mars you don't need to worry about space shuttles coming to visit, so .25atm of mostly-oxygen would be fine.

[Simon_says]

Nothing. He said reduced pressure.

[Tuna-Fish]

Then everything needs to be made of fire-retardant materials. Reducing the nitrogen content of the atmosphere has little deleterious effects for people, but makes fires easier to start, hotter, and harder to put out.

[Dylan16807]

If you reduce the amount of nitrogen, and don't replace it with oxygen, it doesn't have all that big of an effect on fires. And in trade it becomes vastly easier to contain the pressure.

[Tuna-Fish]

Yes, it really does.

The issue is that the inert nitrogen has a really important role in carrying away heat from the flame. Fire is a dynamic reaction where temperature and speed is determined basically by how much heat a set of reagents produce/how much heat is carried away in the waste products. Nitrogen doesn't participate in the chemical reaction, but it gets heated alongside the waste gases, and so it reduces the temperature of the fire.

In a pure 0.2bar oxygen atmosphere, anything that would smoulder or burn slowly in standard atmosphere burn rapidly with a bright flame instead, and some things that you'd normally not consider flammable can sustain flame.

This is the reason the space station maintains a normal atmosphere instead of a thin pure oxygen one.

Conversely, if you double the amount of nitrogen (without reducing the amount of oxygen) in the air, almost nothing burns anymore.

[gozur88]

The atmosphere of Mars contains nitrogen. It could be extracted and concentrated to provide the same percentage as earth's.