Fresh lunar rock is pale grey. Left exposed, the solar wind and micrometeorites darken and redden it over tens of millions of years, so brightness is a rough clock.

Tiny impacts melt and vaporise the surface, coating grains in iron-rich glass; the solar wind implants hydrogen. The result is that any bright spot on the Moon is young, and the rays around a crater fade not because the material moves but because it ages. That is how you can date a surface from a photograph.