Ray system

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Crater ray system on the far side of the Moon. NASA photo credit.

A ray system comprises the radial streaks of fine ejecta thrown out during the formation of an impact crater. Ray systems were once thought to be only found on planetary bodies that lack an atmosphere, but have since been discovered on Mars (see Zunil) in infrared images taken from orbit by the Thermal Emission Imaging System (THEMIS). Ray ejecta material have different reflectivities (i.e., albedo), compositions, or thermal properties than the surface on which it is deposited, so that the rays form visible and, in some cases, infrared 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.

Typically visible rays have a higher albedo than the surrounding surface. They stand out as brighter features that can form streaks and patterns across the surface. More rarely an impact will excavate low albedo material, such as basaltic-lava deposits on the lunar maria. These dark streaks can provide a contrast with higher albedo features, resulting in dark rays.

Gratteri crater, a "Thermal" crater ray system on Mars that was imaged by THEMIS at night. Each THEMIS observation is 32 km across (320 pixels across with a nominal pixel resolution of 100 meters per pixel). NASA/JPL/ASU/University of Tennessee photo credit.

Thermal rays, as seen in thermal infrared images and like the ones discovered on Mars with the THEMIS infrared imager, have contrasting temperatures with their surroundings. They are especially apparent at night when slopes and shadows do not influence the infrared energy emitted by the Martian surface.

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. On non-atmosphered bodies, such as the Moon, 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. Micrometeorites in particular produces a glassy melt in the regolith that lowers the albedo. The rays can also become covered by lava flows, or by other impact craters or ejecta.

[edit] Lunar rays

Asymmetrical ray system about Proclus crater on the Moon. NASA photo.

The physical nature of lunar rays has historically been a subject of speculation. Early hypotheses suggested that they were deposits of salt from evaporated water. Later they were thought to be deposits of volcanic ash or streaks of dust. After the impact origin of craters became accepted, Eugene Shoemaker suggested during the 1960s that the rays were the result of fragmented ejecta material.

Recent studies suggest that the relative brightness of a lunar ray system is not always a reliable indicator of the age of a ray system. Instead the albedo also depends on the portion of iron oxide (FeO). Low portions of FeO result in brighter materials, so such a ray system can retain its lighter appearance for longer periods. Thus the material composition needs to be factored into the albedo analysis to determine age.

Among the lunar craters on the near side with pronounced ray systems are Aristarchus, Copernicus, Kepler, Proclus, and Tycho. Similar ray systems also occur on the far side of the Moon, such as the rays radiating from the Giordano Bruno and Ohm craters. Ray systems have also been identified on the planet Mercury, and some satellites of the outer planets.