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This question is inspired by, but different from, What happens if an ice cube is left in space?

Mark's accepted answer says "…if you stick your ice cube out in the Oort Cloud, it'll grow: the mean surface temperature is 40K or below,…".

To which the obvious response is Everything has a vapour pressure, so everything in the vacuum of space will evaporate eventually. Accordingly I thought it might be worth investigating why ice grains do exist. For which we need some numbers.

Vapour pressure of water at 40K

Most charts do not go that low. The best figure I have come up with was by looking at the pictures in Water Ice Films in Cryogenic Vacuum Chambers, a doctoral thesis by Jesse Michael Labello. Figure 4 on page 4 has graphs which, extrapolated by guesswork, suggest that the vapour pressure of water at 40K is less than $10^{-26}$ torr.

Atmospheric pressure in space

The Moon's atmospheric pressure is said to be $10^{-11}$ torr, and the New World Encyclopedia says interstellar space is $10^{-16}$ torr, and some answers on Physics Stack Exchange suggest about $10^{-25}$ for intergalactic space.


It appears, therefore, that ice grains at 40K do not evaporate because their vapour pressure is not high enough. Does it follow, therefore:

  1. That ice grains would evaporate, even at 40K, if they were in a vacuum?
  2. That, if the ice is at equilibrium with its vapour, the relative humidity of interstellar space is 100%? Or is the process of evaporation/condensation kinetically so slow that the universe is too young for it to have reached equilibrium?
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