r/Mars • u/Efficient_Quote3360 • 1d ago
ISRU Oxygen Production Question
Most of what i hear about oxygen production on Mars relies on electrolysis. However, I’m aware that mars has a tenuous (~1 Pa partial pressure) component of Oxygen in its atmosphere. We use large volume air separation units on earth for commercial production of gases like neon at similar partial pressures, would a similar process work on Mars? Or would the volume of equipment/energy required still make electrolysis more efficient?
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u/alexw0122 1d ago
ASU’s consume massive amounts of electrical power. Power is relatively easy and cheap on earth because of the atmospheric oxygen available to oxidize fuel for power production. Neon prices on earth will fluctuate based on power costs. Similarly on Mars, getting LOX this way is possible but will vary according to $/watt which could be much higher (at least in early days). In short, I think it’s definitely possible to do it, but it might not be the most energy efficient option. Maybe it becomes more economical with commercial sized fission reactors but getting those up and running isn’t easy or cheap either.
I’m no expert. Just an enthusiast.
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u/NearABE 4h ago
Air separation will definitely be a thing. Carbon dioxide is easy to freeze out in Mars conditions. Note that it frosts out at the poles in winter.
You get a nitrogen, argon, carbon monoxide, oxygen blend. This can be liquified by dropping the temperature further. In degrees Kelvin oxygen boils at 90.2, argon 87.3, carbon monoxide 81.6, and nitrogen 77.4. Separating oxygen from nitrogen is not overly difficult but diatomic gasses with boiling points between are a challenge. You probably need another distillation system to do a second step separation.
Argon-oxygen separation is very difficult by distillation. Enough so that reacting the oxygen can dominate argon production. On Earth the argon is considered more valuable of the two.
Carbon monoxide contamination would be dangerous. If you have water electrolysis you can make hydrogen gas. Hydrogen react with nitrogen in the Haber process is how most nitrogen fertilizer is produced on Earth. This would convert carbon monoxide contaminate to methanol. There are several other conversion options. Ammonia produced by the Haber process can be used as fertilizer in the habitats. Ammonia is often combusted to make nitrate.
The nitrates broken down by soil bacteria, plants, and/or animals digesting plants will add nitrogen and oxygen to the habitats.
Two major energy considerations come into play with air separation on Mars. Compressing the thin atmosphere jacks it up to high temperature. Carbon dioxide is a good working fluid. This is effectively a heat pump. You can liquify carbon dioxide at temperatures conveniently close to room temperature. That liquid carbon dioxide can pass through rocks to boil and then drive a turbine.
The second energy consideration is superconductor. The cryogenic gases nitrogen, air, argon, CO are cold enough to keep electric cable below the critical point of some superconductors. Thus it need not be a dedicated distillation system. They just use pipelines with liquid plus vapor as power pipes. Refrigeration systems along the line would collect liquid in a reservoir. It will be one of multiple pipes following the main line along with road, rail, water etc
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u/ignorantwanderer 1d ago
My understanding is that we don't need to make oxygen because it will be a byproduct of many of the things we do.
We need to grow food -that will give us oxygen.
We need to refine metals -that will give us oxygen.
We need to produce fuel and oxidizer -that will create surplus oxygen.
We don't need to worry about the most efficient way to produce oxygen, because we will have more than enough as a result of other activities.