Ray system - Definition

A ray system is the term used to describe the radial steaks of fine ejecta thrown out during the formation of an impact crater. Ray systems are only found on planetary bodies that lack an atmosphere. If the material has a different reflectivity, or albedo, than the surface on which it is deposited, the rays form visible patterns. The resulting rays can extend for several multiples of the impact crater's diameter. Rays are often accompanied by small secondary craters formed by larger chunks of ejecta.

The layering of rays across other surface features can be an indicator of the relative age of the impact crater. Over time these rays can also become obliterated due to various processes, providing additional clues for determining the geologic age. Space weathering from exposure to cosmic rays and micrometeorites causes a steady reduction of the differential between the ejecta's albedo and that of the underlying material. The rays can also become covered by lava flows, or by other impact craters or ejecta.

Among the lunar craters on the near side with pronounced ray systems are Aristarchus, Copernicus, Kepler, Proclus, and Tycho. Ray systems have also been identified on the planet Mercury, and some satellites of the outer planets.