Ray system

A ray system comprises radial streaks of fine ejecta thrown out during the formation of an impact crater, looking somewhat like many thin spokes coming from the hub of a wheel. The rays can extend for lengths up to several times the diameter of their originating crater, and are often accompanied by small secondary craters formed by larger chunks of ejecta. Ray systems have been identified on the Moon, Mercury, and some moons of the outer planets. Originally it was thought that they existed only on planets or moons lacking an atmosphere, but more recently they've been found on Mars in infrared images taken from orbit by Mars Odyssey's thermal imager.

Rays appear at visible, and in some cases infrared wavelengths, when ejecta are made of material with different reflectivity (i.e., albedo) or thermal properties from the surface on which they are deposited. Typically, visible rays have a higher albedo than the surrounding surface. More rarely an impact will excavate low albedo material, for example basaltic-lava deposits on the lunar maria. Thermal rays, as seen on Mars, are especially apparent at night when slopes and shadows do not influence the infrared energy emitted by the Martian surface.