Breathable air on large space habitat with no water?

Question

I am theorising a scenario in which humans land on a continent-size space habitat of alien origin (probably a rotating habitat). The gravity is similar to Earth's (96/97% of Earth's) and the air is breathable. But, the environment looks dry and dead, with no water.

My question is, 'is this scenario plausible' or, is it impossible to have breathable air without:

1. Water
2. Microorganisms capable of producing oxygen?

Alternatively,

would it be possible to hypothesise that such environment used to have water, but then the water disappeared (for unknown reasons) but leaving a breathable atmosphere, at least for a limited period? In that case, how long can an atmosphere remain breathable without water?

Thank you very much.

Answer 1

It is possible.

The mechanisms for loosing hydrogen and oxygen are very different. So conditions need to be as such to prefer loss of the former and not the latter.

Earth looses hydrogen at about 3 kg/s or 100000 ton/year. Simply put, it evaporates from the upper atmosphere into space. (See wiki on Atmospheric_escape) In the higher layers of the atmosphere water is split by radiation into H2 and O, the O recombines with something else and the hydrogen floats up. (Oversimplified)

A continent size rotating habitat has an inner atmosphere that goes from thick near the walls, to near vacuum in the middle. If there are any (broken) ports on the ends of the habitat they will mostly leak out the hydrogen and not the heavier air molecules. (This required the habitat to still rotating and separating the atmosphere by gravity.)

Oxygen is mostly lost due to absorption by materials it can react with, like iron. If the interior of the habitat is mostly build of rust free, maintenance free, super materials or inert materials like sand, the oxygen has nothing to bind with and stays as it was. Plastics and organic materials will oxidize during the many many thousands of years or even millions of years this thing has been floating in space.

It needs some number crunching to set the initial conditions favorable for this scenario and the right floating-in-space-time to get rid of all the hydrogen. But I'm just saying: there is a possibility.

Answer 2

Yup you can have oxygen/breathable air, without water

Meet Perchlorates

You can simply get oxygen by heating up perchlorates at high temperatures, which causes them to heat up and release oxygen. Perchlorates are the same stuff that comprise matchstick heads, Martian soil and in some rockets as oxidizers.

In fact, perchlorates have been suggested as a source of oxygen on Mars. Martian soil has a b***load of perchlorates loaded in it, and it makes plants grown in it nasty and poisonous is pretty much a bummer for cultivation for crops, as they are toxic to humans. So a method has been suggested for (future) Martian colonists, that is, to filter the perchlorates from the soil and then heat them up to high temperatures, after which they decompose and release oxygen, which can be used by the colonists for breathing and stuff.

Similarly, your ancient alien habitat could also have been using a ton of perchlorates for oxygen generation, so that they would have virtually have no need for water/plants to generate oxygen. However, due to unknown reasons, it got abandoned, and due to reasons unspecified, the oxygen did not react with the walls of the habitat, remaining in the air of the habitat for a long time....