Lunar Lava Tubes and Water

There have been some recent discoveries on the moon that are very interesting.

The first is that data from the GRAIL gravity mapping mission has added evidence that the moon has large underground voids created by flowing lava which eventually drained, leaving behind hardened empty tubes, or in some cases domes. These may be gargantuan structures - hundreds of meters deep, kilometers across, and hundreds of kilometers long. We’ve identified over 200 ‘skylights’ where the roof has been punched through, exposing the interior.

These things have a constant temperature inside of roughly -20C. Some deeper ones may actually be above zero inside.

The next discovery is that water is very likely being constantly produced on the moon, due to the solar wind interacting with oxygen in the regolith, producing molecules of water. Now, water can’t exist on the surface of the moon for any length of time, but the belief is that for short periods, this water may sublimate into vapor, and then be transported around the moon. When it hits a dark area, it would freeze out solid, then when the sun comes up it would sublimate again and continue moving around, until eventually the hydrogen is stripped from the water molecules and lost to space, and the oxygen winds up back in the soil.

However, if this water lands in a permanently shadowed area, such as a deep crater near the poles, it may stay there permanently. And indeed, we’ve discovered hundreds of millions of tonnes of water ice locked up in permanently dark craters at the north and south poles of the moon. Not long ago it was thought that all of this water was the result of comet strikes, but recent evidence points to water being generated constantly and traveling around the moon in a tenuous atmosphere.

So, after all that, he’s my question: What would happen if that water vapor makes its way into a lava tube? Lava tubes are different than dark craters in that the temperature in the ice of a dark crater would be only about 40 degrees above absolute zero - cold enough that the sublimation rate would be so low that it could last for billions of years. Inside a lava tube, it’s cold enough for ice to form, but that ice would be constantly sublimating away back into water vapor. But being trapped in a cavern, and being protected from solar energy and cosmic rays, what would happen to it?

Sublimation stops when the vapor pressure reaches a point where molecules of water would be added to the ice as fast as they left it. So if there was a high enough vapor pressure in the tube, you could expect ice to begin to collect. But is that what would happen? The opening to the tube is a tiny fraction of the interior volume, so I would think that water vapor that entered the tube might get trapped there. Some molecules would find their way back to the entrance hole and escape. But if the accumulation rate from the surface was faster than the escape rate, you would expect the vapor pressure in the tube to build until some equilibrium was reached.

If that vapor pressure was high enough to stop ice sublimation, then a lava tube could become a giant water collector, and if we explored them we could find a pre-made habitat for humans with a livable temperature, massive amounts of water and oxygen, and protection from cosmic rays and micrometeorites.

If the conditions were perfect, we might be able to just seal the skylight and any other holes, and warm up the tube with heat exchangers from the surface. The warmer tube would begin sublimating the ice, building up pressure and creating a local atmosphere. Nitrogen is a problem, as we’d need a lot of it for a safe, breathable atmosphere, but let’s stop there.

I don’t know how to begin to figure this out, but would anyone who knows chemistry better than I do be willing to throw in their opinions? Is this a feasible scenario? Or am I way out to lunch?

A small exit area would in fact slow down water loss from a tube. But by the same token, it would also slow water entry into the tube. You wouldn’t expect them to be any more full of water than anywhere else on the Moon, unless there were some era in the past when water was more abundant.

Here’s a way to maybe simplify the problem, or at least the thought experiment:

Let’s say I have a cylinder that is open to vacuum at the top, and it’s very deep. At the top, the temperature is very cold - maybe -50C. At the bottom, it’s warm enough for liquid water to exist. Now I start pouring water into the cylinder. What would happen?

I’m thinking that the water would begin to boil, but the water vapor released would form a pressure gradient in the tube.At some depth, the pressure at the bottom would be high enough to stop the water from boiling, and the water would begin to collect. As it filled the tube and it got colder, it would eventually form an ice cap. Given a deep enough tube, the vapor pressure on the ice cap would be enough to sustain the ice, and the ice cap would grow in thickness. Eventually, it would get high enough that the vapor pressure would be at an equilibrium and the ice growth would stop. Above that, you’d just have a tube with water vapor in it, with the pressure decreasing to zero when it got to the vacuum at the surface. Is that about right? The question then becomes how deep would it have to be, how much volume it has compared to the size of the opening, etc.

Really? I’m not sure. Wouldn’t the water, being heavier than air, have a pressure gradient in the tube? I agree that the vapor pressure at the surface of the hole would eventually be the same as the that on the surface, that doesn’t mean it’s the same inside the tube at the bottom. So I would think there would be some combination of tube shape, depth, and hole opening size that could result in water ice collecting in the tube. But maybe the required depths and sizes are orders of magnitude off for this to happen?

The tubes also bend and twist underground, and they’d have to be sloped for lava to flow.

Cometary impacts on the surface would also change the density of the surface water vapor from time to time.