Like the Moon, Mars, and other rocky bodies in the solar system, asteroids are covered with a layer of pebbles, sand, and dust, called regolith. Given that asteroids formed when large boulders coalesced under their mutual gravity, the presence of regolith requires that surface rock be broken up somehow.
For asteroids 1 km across or smaller, whatever process makes regolith cannot be so energetic that ejected fragments acquire sufficient velocity to escape the asteroids’ feeble gravity. Bombardment by micrometeorites meets that criterion. Observed by astronauts on the Moon, it had been thought to be the only process at play.
Now Marco Delbo of the Côte d’Azur Observatory in Nice, France, and his collaborators have identified a second process: thermal cycling.1 As asteroids spin about their rotation axes, their surfaces plunge in and out of shadow. Provided the rotation rate is neither so fast that solar irradiation is effectively constant...
