Why are the surfaces of the tiny asteroids Ryugu and Bennu rocky and not smooth? This came as a surprise to planetary scientists when the two small rocks were visited by the spacecraft Hayabusa2 and OSIRIS-REx (respectively): Instead of the surfaces being mostly covered in smooth dust, as has been seen in other rocky bodies like the Moon , Mars, and other asteroids like Eros , they instead looked like the site of a demolition pit , just rocks of all sizes piled up everywhere with no dust at all. The expectation of seeing dusty surfaces is due to the rocks wearing down over time. As an asteroid rotates it brings rocks on its surface into sunlight on the day side and into darkness on the night side. That can be a huge change in temperature, hundreds of degrees. When heated the rocks expand and when cooled they contract, creating stress. Eventually the rocks crack, breaking down into smaller and smaller pieces until you get dust everywhere.…