When an object impacts the Moon's surface and forms a crater, it displaces material. Much of this is banked into the familiar circular wall, whilst most of the rest of the ejecta - the material thrown out by the impact - is generally deposited as bright rays radiating from the impact site. These can extend for hundreds of miles, but are actually an extremely thin layer. Consequently they are obliterated over the millennia, and a ray system is a telltale sign of a more recent impact.

In large impacts, some of the ejecta can be thrown out at more than the Moon's escape velocity. Fragments of this have eventually reached the Earth, to fall as meteorites.