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The Ars Technica article NASA seeks industry help with lunar landings, potentially sample return discusses the potential value of ice on the moon as a source for fuel in future space missions.
Ice sublimates. Even if it is very cold hiding in permanent shadows in craters near the Moon's poles, I'd expect it to have a non-zero vapor pressure.
So what would keep ice from slowly sublimating over millions of years? Or is it being replenished somehow?
Ice sublimates.
So does rock. Yet the planet Mercury is still there.
The reason Mercury still exists is because even though rock does indeed sublimate, the rate at which rock sublimates is extremely low, even at temperatures at the surface of Mercury. The same applies to water ice at the very low temperatures in those permanently shadowed craters on the Moon. Water ice those very low temperatures is essentially rock.
The extremely low sublimation rate of an exposed block of water ice at ~100 kelvins would result in that ice thinning by a millimeter per billion years. The temperatures in the permanently shadowed portions of the Shackleton Crater are ten kelvins lower than that. Even exposed water ice can easily survive for billions of years at those low temperatures. Water ice covered by material can survive even longer.
You overestimate vapor pressure and underestimate dust power
source
0.05 Pa it equivalent to a layer 0.000018382 m of dust which would (if it being a sealant) to prevent the ice from further sublimations if we assume average density of 1700kg/m3 and gravity 1.6 m/s2
There are other factors like water being polar solvent and thus adhesion to the dust particles as an example(same way as of why there are gasses He,H, N in regolith), slowing or stopping their travel trough the dust and helping for better sealing on top of the ice.
The average velocity of ice molecule at 200K is about 16 m/s, which gives us that there is not enough energy(for most of them) to fly out of the 80m deep crater.
Essentially it is the same thing as with icy asteroids and comets.
But overall everything boils down to there was enough water and speed of escaping of the water was not fast enough, so we observe it presence.
It is of course sublimating. And not only is it cold, but the sublimation cools the remaining ice even further.
Since it's in lunar dust, chances are additionally that a sublimated water molecule will hit a speck of dust and re-freeze. This effectively slows the speed at which water moves from deep down to the lunar atmosphere.
I just ran across this reference again while writing this comment (same one as I mentioned here so I thought I would add it to the mix explicitly.
The paper is quite thorough and interesting, and I think deserves a careful read.
New estimates for the sublimation rate for ice on the Moon Edgar L. Andreas, Icarus 186 (2007) 24–30:
This is pretty amazing, the vapor pressure has been experimentally measured over a range of ten orders of magnitude!
Fig. 1. Measurements or reference data for the saturation vapor pressure over a planar surface of pure water ice from Hilsenrath et al. (1960), Jancso et al. (1970), Bryson et al. (1974), Buck (1981), and Marti and Mauersberger (1993). The functional expressions for esat,i are from Buck (1981), Wagner et al. (1994), and Murphy and Koop (2005) and are given in Eqs. (2)–(4). The Murphy and Koop curve is under the Buck and Wagner et al. curves in the region where they all overlap.
Here is the punch line, and it packs quite a punch! Note the annotation of 1 molecule per square centimeter per hour, and the fact that every major tick mark on the y axis represents ten orders of magnitude!
Fig. 2. The sublimation rate for a planar surface of pure ice calculated using the expressions from Buck (1981), Wagner et al. (1994), and Murphy and Koop (2005) for esat,i in Eq. (1). The left axis gives the sublimation rate as a mass flux; its units are µg cm−2 h−1. The right axis gives the sublimation rate as the number of molecules of water vapor leaving a square centimeter of the ice surface per hour. The arrow shows where the sublimation rate is only 1 molecule cm−2 h−1.
