New Moon Rising

Boiling requires a source of energy.

Here on Earth ambient heat is quite adequate to boil water under vacuum conditions. On the polar regions of the Moon, there has been no sunlight in the bottoms of deeper craters for thousands, perhaps even millions of years. I don't recall if I've seen actual temperatures for these places, but it's really cold - not enough energy available at the surface to break the crystal bonds, so it stays frozen.

At least one of the articles I read recently indicated an actual water cycle on the Moon. This would imply melting and/or sublimation of some of the ice and movement of the resulting liquid or vapor to another location. I did not see whether this movement was above or below the surface, or both.

Sorry this is so long but I hope it is clear . . .

I work with production Freeze Dryers and daily see the behaviour of ice under very low absolute pressure. (Freeze Dryers remove the frozen water from a frozen solution of product without allowing the ice to thaw, as this can destroy things like proteins and vitamins for example. The solid is then left behind as a fine powder.)

The conditions in the plant go down to -70°c or more - which I have seen quoted as being about the temperature of 'space'. The pressure in process is typically 10^-4 bar and can reach nearly 10^-6 bar under test. (I don't know if this compares to the pressure in 'space'.) We have to have an ice trap betwen the frozen product and the vacuum pumps or they cannot work. It is there that we see the ice under something like 'space' conditions and it just sits there at -70°c, slowly building on the refrigerated condenser pipes as the water vapour sublimes out of the product 'cake'. If we leave the plant under this vacuum after the process is finished and stop cooling the condenser pipes, it takes several days for enough heat to leak in through the insulation to thaw the ice. Without that heat leaking in, it will just sit there. Even if we admit atmospheric air it takes a day and night to thaw with the better heat transfer between walls and ice that is provided by the air.

The other thing is that to get the ice in the product to move out towards the ice trap, we have to supply considerable amounts of heat otherwise nothing happens to the ice in the product - it just sits there even though the pressure is so low. (The pressure has to be lower than the vapour pressure of ice at the temperature that the product solution will start to thaw at, or the product breaks down.) This means we can have product kept at -30°c by the rate of evaporation into the 'vacuum' but sitting on shelves at 0°c. (Even with that temperature difference the process is so excruciatingly slow that it can take a week to evaporate a cube of 30 grams of ice!)

I hope this helps you to see that ice does nothing without a supply of energy. It is only the inherent amounts of energy in our environment that make it hard to keep ice solid and therefore difficult to imagine that it won't just 'fly off' into a vacuum as soon as it finds one. Give it a bit of energy though and it does just that - without even becoming liquid in the process.

I have seen a 'high vacuum' pump suck on a bottle of water and it boils vigorously until the remaining 4/5ths. suddenly freeze. After that it only evaporates as fast as heat leaks in through the bottle walls. That means that one fifth of the water has removed so much energy in evaporating that the other four fifths just solidifies. It can do nothing else. It takes a lot of energy to melt ice!

To get back to the moon. The ice will vapourise as soon as it gets some energy but as the resulting water vapour reaches another cold surface, where there is no energy source, it will freeze onto it and thereby 'travel' across the surface and even through the dust. In that way most of it will be below the surface dust layer. The impact of the rocket hitting obviously provided masses of energy that would have vaporised all local ice, only for it to freeze again somewhere else. Whilst it was doing that the local pressure was minutely higher than usual owing to the vapour pressure of the water molecules and this small pressure would give the water vapour the ability to move away to where it would re-freeze.

I hope this helps to explain the permanence and the moveability of water in a vacuum.