They tend to be firmly bound to the regolith until the surface temperatures spike near lunar noon
. These molecules then 'thermally desorb' from the lunar soil to reach a nearby cold location where they can stick or populate Moon's "extremely tenuous atmosphere or exosphere, until temperatures drop and the molecules return to the surface."
A crater may appear to be a deep, dark well at sunrise, turn increasingly saucer-shaped as the Sun climbs high in the lunar sky, become a flat-looking white spot or ring near lunar noon
, then reverse the process as the Sun declines, with shadows now reaching in the opposite direction.