Sublimates in one place, condenses in another. It moves from sunny parts of a body to darker parts. And since everything is rotating, the net effect is minimal.
For that to happen, it can't be gravity that keeps a single molecule near long enough for that to happen - dipole forces at a fairly large distance in vacuum over enough time... maybe... at enough distance from the sun. Maybe. Maybe.
Static electrical field from the asteroid, stoked by the sun?