Ice sublimes on the moon? [closed]

Question

I read that the ice sublimes on the moon as the vapour pressure will be really low, but won’t a low vapour pressure increase the boiling point? (Might be wrong, kindly don’t judge.)

Answer 1

Lower vapor pressure lowers the boiling point. Don't you use a pressure cooker to increase the boiling point of water so that food can cook rapidly?

Why go to the moon? Ice sublimes in Canadian weather, where temperatures can go as low as -40 to -50 Celsius in some cities. People claim that ice sublimes on the roads and never melts. I never did this experiment but wet clothes are claimed to become dry in those extremely cold weather conditions.

From NASA's website Ice on the Moon

The Moon has no atmosphere, any substance on the lunar surface is exposed directly to vacuum. For water ice, this means it will rapidly sublime directly into water vapor and escape into space, as the Moon's low gravity cannot hold gas for any appreciable time. Over the course of a lunar day (~29 Earth days), all regions of the Moon are exposed to sunlight, and the temperature on the Moon in direct sunlight reaches about 395 K (395 Kelvin, which is equal to about 250 degrees above zero F). So any ice exposed to sunlight for even a short time would be lost. The only possible way for ice to exist on the Moon would be in a permanently shadowed area.

Answer 2

There is no equilibrium vapor pressure on the moon. That's because the moon does not have a significant atmosphere to speak of, not even on the dark side. Any water that becomes gas is likely to drift off into space. Note even Earth loses water to space (although mostly as other molecules or atomic hydrogen and oxygen).

In a closed container there is at each temperature below the triple point of water a value of the vapor pressure at which solid and vapor can coexist such that without additional input of energy the amounts of both remain constant over time. This is what is meant by equilibrium: their relative amounts, as well as T and P, remaining unchanged. The equilibrium (coexistence) temperature at a given vapor pressure below the triple point is the sublimation temperature $T_{\textrm{sub}}$ at that pressure.

Since the moon's atmosphere is certainly not enclosed and not likely very stable, with pressure negligible, none of this holds, that is, there are no equilibria. In addition there is no feedback mechanism, at least at the low pressures under consideration here: the chemical potential of water in ice is independent of that in the vapor (except at high pressures).