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?
  • 1
    $\begingroup$ Interesting question! Looking for some nice answers. $\endgroup$ Commented Oct 12, 2020 at 2:10
  • $\begingroup$ 1. Yes, 2. 100%. One needs to include in the calculation photo ejection and cosmic ray ejection. But, the ice cube should last a long time in the Oort Cloud, I would think. $\endgroup$
    – eshaya
    Commented Feb 12, 2021 at 18:16
  • $\begingroup$ 1. Yes, but... Ice grains would of course evaporate in any condition where their vapor pressure is less than the atmospheric pressure, but it sounds like you already know this based on your question. The problem is finding a "true" vacuum. As you indicated, the atmospheric pressure estimate of intergalactic space is an order of magnitude higher than the vapor pressure of ice, so you'd either have to find extremely thin area of intergalactic space OR construct a vacuum of your own. $\endgroup$ Commented Dec 21, 2021 at 21:10
  • $\begingroup$ 2. Given the density of material in space, one could reasonably argue that it is at 100% humidity near the ice cube if sufficient amounts of water have vaporized off the ice cube, however...the ice on the surface of your ice cube wouldn't actually evaporate or condense, it would vaporize. So, space would, quite literally, rip entire H2O molecules off the surface of the ice cube. $\endgroup$ Commented Dec 21, 2021 at 21:10
  • $\begingroup$ Another detail is that vapor pressure equilibrium depends not on the total pressure in the gas, but the partial pressure of the volatile species. So in space, most of that 10^-16 torr is hydrogen, not water vapor. The partial pressure of water vapor is going to be much lower than the total pressure. $\endgroup$
    – giardia
    Commented Feb 3, 2022 at 6:06


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