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I was reading Why does the ISS atmosphere contain nitrogen, and I wondered if some of the explanations there would also apply to a base on the Moon. There are considerations on the Moon that don't apply (so much) to the ISS.

A lower pressure atmosphere is easier to contain. It doesn't require that as much stuff be shipped up from Earth - especially Nitrogen which is not available there, while at some point Oxygen can be obtained from lunar soil once the infrastructure exists. The larger a base gets, the more this becomes a factor. Also the larger it gets, the easier it is to install the proper airlocks so that people arriving and departing can transition between the atmosphere on the ships and that in the base.

Isn't there wiggle room where the oxygen fraction can be raised, the nitrogen fraction can be lowered, and the pressure can be some substantial fraction of Earth sea-level so that transitions between base and ships aren't difficult? Can it be said what the best mix would likely be?

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    $\begingroup$ Full 1 atmosphere is very nice for underground fascilities. 1 atmosphere can support about 10 meters or 30 foot thickness of Lunar regolith by air pressure alone. Add support and some hard sealed ceiling material and you get maybe 12 or 15 meters of protection in very large areas (scale up domed stadiums by a factor of 6 then link a bunch of them together to get an idea) and nearly free in terms of structural support. Higher than normal oxygen levels allow fires to get ridiculous. He and Ar are OK to add if you can get it easily. Nitrogen will be part of any agriculture. $\endgroup$ – C. Towne Springer Mar 17 '15 at 7:35
  • $\begingroup$ @C.TowneSpringer your comment highlights how much this is an 'it depends' question. It is a sound argument if the base is underground, but maybe that will not turn out to be the best solution. How are you going to get enough light to grow plants, for instance. I keep asking questions like this hoping a brief answer can be given for each probable variant, but they don't go far. $\endgroup$ – kim holder Mar 17 '15 at 23:22
  • $\begingroup$ I think underground is the only safe way to protect from radiation and probably a lot deeper than what I described when the base material is stable. Light can be piped in during the 2 weeks of daylight and artificial the other 2. The bottom picture here quadibloc.com/science/spaint.htm show a version of the mirrored chevron radiation blocking window. I first saw them proposed in the 1970's for space habitats but I'm sure someone thought them up before that. The biggest disadvantage of nitrogen is the problem it presents for people undergoing rapid pressure changes. $\endgroup$ – C. Towne Springer Mar 18 '15 at 5:06
  • $\begingroup$ A (fireproof) ship or habitat in free-fall can run 3 psi with high oxygen percentage, which makes the structure easier to build and seal. Moving from 3 psi to a 1 atmosphere region is easy. Going back from 1 atm saturation will take a couple days, though at low pressure you can go onto 100% O2 and out-gas faster. Running a major facility at 1 atmosphere also means you can loose 1/2 of the air and still be at only a 17,000' altitude equivalent and most people can still function at normal 18% O2. With supplemental O2 you can easily loose 2/3. Loose 2/3 in a 3 psi system and not so good. $\endgroup$ – C. Towne Springer Mar 18 '15 at 5:19
  • $\begingroup$ @C.TowneSpringer An interesting variation on this I've seen proposed: The "ceiling" isn't actually load-bearing, it's there to keep things from shifting. There is a CLEAR membrane, a layer of water and another clear membrane. You build this in a crater so the ceiling is flat. It's supported by the air pressure, it's thick enough to provide radiation shielding and it's nearly transparent besides. $\endgroup$ – Loren Pechtel Mar 21 '15 at 2:33
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Once you have a substantial refining operation on the moon you probably will go with an oxygen/argon atmosphere. Argon is suitably inert and will be obtained when you melt lunar rock. (It's from the decay of potassium, it's not primordial.)

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  • $\begingroup$ There is some VHK basalt where potassium might be as much as 12%. But common substances such as plagioclase feldspar, pyroxene, olivine, ilmenite don't have potassium. I would imagine argon is far less common than oxygen. $\endgroup$ – HopDavid Mar 22 '15 at 6:10
  • $\begingroup$ @HopDavid Of course there's a ton of oxygen--but you don't want a pure oxygen environment. Argon is the most prevalent filler gas on the moon. $\endgroup$ – Loren Pechtel Mar 22 '15 at 17:33
  • $\begingroup$ There may be nitrogen compounds in the cold traps. If argon from potassium decay is the most abundant lunar filler gas, it's too rare to help much. In that case I'd expect filler gas to be imported from earth. $\endgroup$ – HopDavid Mar 22 '15 at 23:58

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