to reach boiling point
This is vacuum -- all liquid water will boil immediately, at zero Celsius. Besides I'm sure there will be some sublimation of ice directly to water vapor.
In fact, looking at water's phase diagram, in high vacuum liquid water just doesn't exist so I think ice will simply sublimate without the intermediate liquid stage.
Right, I forgot the effect of pressure. So E will be different, perhaps very different. What will it be?
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