Aman Sharma asked:
Will ice melt in dark vacuum if the temperature of the base on which it is kept is equal to its temperature?
Chris - [In our previous question,] you're saying that your body fluids exposed to a very low pressure will boil and therefore, you will lose them as a gas. So if I had some ice in space, what happens to that? Does that boil off as well?”
Dominic - Yes. Boil isn’t actually quite the right word because boiling is going from a liquid state to a gaseous state. In fact, the very low temperatures in the dark coldness of space, there is no liquid state. You go straight from solid ice to vapour. So the word is 'sublime' for going from a solid to a gas, but ice particles will tend to sublime into the gaseous state, yes.
I presume it cannot sublime that rapidly because NASA have found traces of water in a cold dark crater on the Moon... but what does everyone else think? chris, Tue, 17th Jul 2012
Based on the water phase diagram, water cannot exist as liquid below the triple point (0.006 atmospheres of pressure and 0.01 degrees Celsius). A true vacuum would have no pressure, so the ice could not melt into a liquid. It could, however, sublimate into a gas. Indeed, based again on what I can see of the water phase diagram, any temperature above absolute zero should cause ice to eventually sublimate into a gas in a perfect vacuum. That would probably take an extremely long time, since the low internal energy of the ice block would make it difficult for any one water molecule to acquire enough energy to be jostled free from the mass.
For any given temperature, the ice will have a vapour pressure. If the amount of water vapour round the ice is less than the vapour pressure then the ice will evaporate.