Eons of bombardment pounded the rocks of asteroids into layers of soil called regolith. Later impacts sometimes welded these mixtures of powder and fragments into new rocks called regolith breccias. These rocks contain samples of gas emitted by the Sun solar wind which is trapped only by the uppermost layers of the regolith.
Bununu, Plainview, and Bustee are called regolith breccias. All contain trapped solar wind gas a sign that they formed near the surfaces of their different parent asteroids.
