The rings of Saturn are almost entirely little chunks of water ice, and they are stable over many hundreds of millions of years. Cold ice (below about 150K) has a very low vapor pressure.

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?