What would happen in this cave on the Moon?

Factual Questions
1

I’m thinking about a story idea in which a cave filled with ice is discovered on the Moon.

The scenario I had in mind is that there is an old volcanic magma chamber which drained and left a hollow chamber behind, about 1.5 km high and 5km in diameter. I chose those numbers because Purdue University says that this is about as large as you can go while remaining stable on the moon.

So this thing forms, then over billions of years it is covered by many layers of lava, and now sits under a few kilometers of lava in one of the Mare.

Now, the moon is quite depleted of water in the mantle, but not uniformly. There may be pockets of quite high water content - say from the impact of a Carbonaceous Chondrite meteor when the moon contained large lava oceans. At very high pressures and temperatures, the water vapor and other gases will be in solution in the magma, but at lower pressures the gas will begin to bubble out and travel towards the surface. So my theory is that an overturn in the mantle (perhaps from a collision event) causes a pocket of gas-rich magma to rise under where this chamber is. This causes it to outgas its trapped water and other volatiles, much of which eventually makes its way into this chamber, but at very low pressure.

The temperature in one of these caves should be about a constant -20C, the equilibrium temperatures of space at the Moon’s distance from the Sun. The interior of the Moon is, I believe, uniformly at equilibrium once you get past the first few metres of the surface, down to where the mantle is still hot, which is hundreds of kilometers below the surface. So I am trying to figure out exactly what someone would see when they entered the cave. Here’s my straw man argument:

The water vapor would freeze on various surfaces then re-sublimate. There would be a continual process of sublimation and freezing going on. Eventually, the water vapor would make its way into the pores and cracks in the basalt walls of the dome, and freeze there. This might give us enough of a seal in the cave to hold a few hundred pascals of pressure.

Eventually, the water in the pores and cracks will migrate up through the layers to the surface, but my understanding is that this could be a very slow process involving cold-grain chemistry, which I don’t understand. So I’m assuming that the supply rate from below the chamber is greater than the leakage rate until we hit some equilibrium pressure. I’m trying to keep that low but above the vapor pressure of water, because the lower it is the more likely that it could be sustained for a long period of time.

Eventually, the water vapor would reach saturation, I guess. But you’d still get sublimation and re-freezing.

So the question is, are these assumptions reasonable enough? Am I missing some big show stopper? Also, what would such a chamber look like after say, 100 million years? Would it look like the inside of a freezer, with thick layers of frost on every surface? Or would the water eventually collect at the bottom as a frozen ice surface, with maybe just small grains of ice on the walls and ceiling of the cavern?

Would it make any difference if the water vapor was accompanied by other gases, such as argon or nitrogen?

One issue is energy. Sublimation takes energy, which would be provided by the rock walls being only -20C instead of absolute zero. The surfaces would be cooled by the sublimation, but then would be re-heated by the surrounding rock, I suppose.

Another issue is whether millennia of sublimation and freezing would erode the rocks and change how they look, or destroy them.

Chemistry and geology are not my strong suits, so I’m trying to get a reality check here from people who know more than I do. I know it’s problematic to have a cave on the moon that can be even remotely gas-tight, but I’m trying to put together a chain of flukes or luck that could create such a thing. Also, given that there seem to be fairly recent explosive outgassing events on the moon, gas must be able to be trapped in larger quantities somewhere underground for long periods.

All comments welcome. If you have other ideas for how to achieve this, I’d be happy to hear them.

2

I mostly understand what you’ve written – I’m no expert at any of this. But one thing you mentioned jumps out at me. You said sublimation takes energy, which might be replaced by the heat from the walls. As I understood your description though, the ice would sublimate, but then water vapor would condense and freeze still within the sealed chamber. Condensation and freezing are exothermic, so the energy would be released by those subsequent state changes. (There might be fluctuations in the wall temperatures as part of this process, this goes beyond my very limited knowledge. But the majority at least, of the energy would just be heating/cooling the chamber as the H2O changed state, I think.)

3

Good point. I think your point is valid once an equilibrium is reached - I guess when the vapor pressure of water in the chamber reaches saturation? Until then, the water that sublimates would remain as vapor in the air (or some would), so teh net effect would be cooling. But I do think the energy issue can probably be ignored since there is an entire Moon’s worth of thermal mass out there to force it back to equilibrium eventually.

4

How close to the surface is this chamber supposed to be? For the purposes of your story, does it matter? Because the idea of molten lava (still) being anywhere close to the surface seems implausible to me.

5

The cave is something like 1-5 kilometers from the surface.

I agree with there being no molten lava at the surface, or anywhere near it. This overturn event could have happened in the deep magma (the magma of the moon is only partially melted, and even then only within a couple of hundred km from the core).

The location of this (old, dead) magma chamber would be somewhere in the vicinity of the Aristarchus Plateau, chosen because it has by far the most number of reported events suspected to be outgassing. Now, most of this is probably argon-40, as this area of the moon is rich in radioactive potassium (which might also affect the temperature of the cave). I’m assuming that an uplift of gas-rich magma deep in the moon caused it to bubble out and begin finding its way through the cracks and voids in the mantle to the surface. Not all of it has to wind up in this magma chamber.

The other possible mechanism is the same one that creates ‘mascons’ on the moon. When a large impactor hits the surface, it rings through the moon and the reflection can push up dense magmatic basalts. It could be an event like this which pushed up an especially volatile rich section of magma - not to the surface, but high enough in the mantle that it begins to outgas its volatiles. There is such a mascon just south of the Aristarchus crater.