FROST_POINT

Where water ice stops subliming away in vacuum, Kelvin.

Not the melting point. A surface in vacuum loses ice by sublimation, and the rate falls off exponentially with temperature — around 145 K a millimeter a billion years, around 170 K a meter a year. So this is the line between a body that keeps exposed ice for geological time and one that does not, and it is why Mercury's poles hold ice at 1,600 K of subsolar heat: the floors of their polar craters never see the sun.

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