Are there underground caverns on Moon or Mars? Are they expected to have air (vel: some gas) inside? Do we already know of any?
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$\begingroup$ @AndreiROM Are you sure about the lack-of-caves bit? theverge.com/2014/7/18/5915743/… (And to clarify, it is the Verge, but their source for the statements appears to be NASA) $\endgroup$– RangerCommented Aug 4, 2016 at 21:39
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$\begingroup$ @NexTerren - looks like I need to rethink my comment. $\endgroup$– AndreiROMCommented Aug 4, 2016 at 21:56
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2$\begingroup$ Seems to me like you could easily Google this stuff, my friend. I fail to see how your question has anything to do with worldbuilding. But I'll be nice: Mars will have all the lanscape features of Earth (caves, etc.) but the atmosphere is very, very thin compared to ours. The Moon has been found to have a small, liquid core of its own, and magma moving under the surface is believed to have carved out tunnels, some of which have exits to the surface. As for "having air inside" ... what? Planets are not balloons. Does the Earth have "air" inside? Neither does Mars $\endgroup$– AndreiROMCommented Aug 4, 2016 at 21:58
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2 Answers
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There are lunar caverns. NASA says so here and the National Geographic reports here.
There are also NASA reports about entrances to martian caves here. More work is needed, but Mars looks promising for caves too.
There is evidence of caves on both the Moon and the planet Mars.
answered Aug 5, 2016 at 13:14
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Yes, there may be subsurface features. Lava is known to a have flowed in the past on the moon (there may still be a low level of outgassing). When a fluid lava flows, it can form a lava tube. With relativly little to disturb it, these can last for hundreds of millions of years. In places the roof of the tubes have collapsed forming pits - Space.com video
There is speculation that the largest of such tubes could be exceedingly large. Very large tubes on Earth would collapse under their own weight, but in the moon's gravity they could be much larger.
A Tube would be a good place for a lunar base. It is protected from solar and cosmic radiation and micrometeorites. It would have a stable temperature. However, they would not contain air or gas in appreciable amounts.
As a setting for a story, you might imagine an exceptionally large pit, some cubic km in volume. Tunnels cut into the rock could be pressurised to allow for human activities. Rockets would leave via the centre of the pit to lunar orbit and Earth transfer.
There would be no water: making and controlling water supply would be a major issue, as would maintaining a breathable atmosphere, as sunlight would not be available for plants.
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1$\begingroup$ If it had an opening, it might collect water ice just as the craters where the sun never shines! It would be a perfect trap in fact, so that might add to the appeal! $\endgroup$– JDługoszCommented Aug 5, 2016 at 0:19