[paul cotter (OP)]

I see talk of an expedition to find water at one of the lunar poles. I would have thought any frozen water would have sublimed in the high vacuum that exists on the moon.

[Janus]

I see talk of an expedition to find water at one of the lunar poles. I would have thought any frozen water would have sublimed in the high vacuum that exists on the moon.

As long as the water is in a crater or other permanently shaded spot, it shouldn't be any more susceptible than ice in a comet far from the Sun. If mere exposure to a vacuum were enough, by now there would not be any icy bodies in the Solar system. 

[alancalverd]

There being no wind to disperse it, I guess it just sublimes a bit when the sun shines and recondenses in situ (in craters at the south pole) at night.

[paul cotter (OP)]

Janus, I accept what you say regarding comets but my confusion remains. Lyophilisation is commonly used to remove water from solutions where heating may degrade a product. I suppose the real question is why do comets and other icy bodies not undergo sublimation?

[alancalverd]

I rather think reply #2 answers that question. There must be a minimum size for a comet to survive, such that molecules evaporated from the surface cannot escape the gravitational pull of the rest of the body, so it acquires a sort of halo (or tail) if it approaches a hot star, then recondenses as it departs.

[Bored chemist]

There being no wind to disperse it, I guess it just sublimes a bit when the sun shines and recondenses in situ (in craters at the south pole) at night.

Gases:
flow and completely fill their container, because their particles can move quickly in all directions
can be compressed, because their particles are far apart and have space to move into
From
https://www.bbc.co.uk/bitesize/guides/z93jfcw/revision/1


You don't need a wind.
The sublimed water molecules are travelling at roughly the local speed of sound.

[Bored chemist]

. I suppose the real question is why do comets and other icy bodies not undergo sublimation?

They do, but it's slow.
In order to pull water molecules away from the ice you have to supply energy (the latent heat of sublimation).
In deep space there's practically no energy supply so evaporative cooling will chill the comet down to a point where it only evaporates very slowly.
When the orbit takes it near the sun, it starts to evaporate because it warms up.
The vapour is seen as a tail

[alancalverd]

You don't need a wind.
The sublimed water molecules are travelling at roughly the local speed of sound.

But where do they go? They are still subject to gravitation and no other force.

Come to think of it, the speed of sound isn't a terribly useful concept as the only sound conductor on the moon is the occasional and rather lonely water molecule. However we know the shade temperature at the south pole is around 20K, from which we can calculate the speed of a water molecule leaving the surface as about 170 m/s.  Escape speed from the moon is about 2400 m/s, so they all end up back at the bottom of the crater.

[Bored chemist]

The average energy corresponds to about 170m/s but it will be (roughly) a Boltzmann distribution.
Some will be fast enough to escape into space from the moon. Also, some will "fall" back onto a warmer bit of the moon and acquire a much higher speed.
Given time they (nearly) all will.
Some might get blown off by the solar wind.
I will leave that as an exercise for the interested reader.

[alancalverd]

It is also left as an exercise to the reader to consider where the moon's water came from, how much there was to begin with, and how long it has been there.

Then to apply the same logic to Mars and wonder whether the planet was once teeming with life until the water all evaporated

Just how old are the icy comets, and where did they originate

And how long it will be before Earth becomes as desiccated and sterile as our neighbors.

[Petrochemicals]

Water on mars is mostly sub surface and more facinating than that is the fact that the water is beneath the co2 icecap.

https://www.nasa.gov/news-release/nasa-confirms-evidence-that-liquid-water-flows-on-todays-mars/

[Bored chemist]

And how long it will be before Earth becomes as desiccated and sterile as our neighbors.

We might be OK for ages- as long as we avoid a runaway greenhouse effect.

[alancalverd]

....which is caused by water vapor in the atmosphere!