At what temperature does ice begin to sublimate in a vacuum?

Answer

The ice sublimes when enough energy is impinged on it to dislodge hydrogen molecules from the ice.

In comets, this happens when they move close to the sun, creating the spectacular tails of comets. Ice exposed directly to sunlight at the distance the moon is from the sun would sublimate quickly. Where ice is suspected on the lunar surface, it is at the bottom of craters close to the poles, in places never exposed to sunlight; this would also occur on Mercury, even closer to the sun.