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?


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.

  • 1
    $\begingroup$ Water is a compound made of oxygen(O) and hydrogen(H). To have no water you would need no hydrogen and hydrogen is the most abundant gases in the universe. $\endgroup$
    – Martamo
    Commented May 15, 2023 at 15:53
  • $\begingroup$ @Martamo thank you very much! However, we know that it is possible to have water without breathable atmosphere. Is the opposite possible though? Let' say that water disappeared from Earth. Would the atmosphere remain breathable or, after a while, this would change? $\endgroup$
    – Haiwas
    Commented May 15, 2023 at 16:04
  • $\begingroup$ If we are talking about Earth, then no. This is because water has the lovely effect of reflecting solar energy and radiation back out of Earth’s atmosphere. If Earth had no water then the planet would become a burning, desolate planet, void of all plant and animal life. But for your world you don’t have to worry about the surface becoming hell, because you can control the temperature of the ring. The people that answered gave much better explanations and advice. $\endgroup$
    – Martamo
    Commented May 15, 2023 at 17:38
  • 2
    $\begingroup$ With extreme lack of water such environment would be very desiccating, This would cause health problems pretty quick for many people. Humidification masks would probably be required for long term operations $\endgroup$ Commented May 15, 2023 at 19:57

2 Answers 2


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.

  • $\begingroup$ that's brilliant. Just to clarify, when you refer to 'void in the middle of the habitat' you obviously mean the 'centre of the ring', i.e. when rotation axis is, right? $\endgroup$
    – Haiwas
    Commented May 15, 2023 at 16:13
  • $\begingroup$ Well, I was imagining a rotating cylinder. But with earth like gravity the air pressure is halved about each 5km up. So at 100km from the edge you'll have a sort of kármán line situation. Ports for entering the habitat will most likely be located near the axis where there are only a few fast ions zooming around. See Wikipedia for Bishop Ring, the big brother of the O'Neill cylinder. $\endgroup$ Commented May 15, 2023 at 16:30

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....

  • 1
    $\begingroup$ A habitat built for long-term stability would definitely use materials that are completely/maximally oxidized, at least for the surface of everything. Yes it's gonna be more expensive but it should last forever or something, let's use the more expensive steel & paint $\endgroup$
    – Hobbamok
    Commented May 16, 2023 at 7:24

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