Inplanetary geology, aray system comprises radial streaks of fineejecta thrown out during the formation of animpact crater, looking somewhat like many thin spokes coming from the hub of a wheel. Therays may extend for lengths up to several times thediameter of their originating crater, and are often accompanied by small secondary craters formed by larger chunks of ejecta. Ray systems have been identified on theMoon,Earth (Kamil Crater),Mercury, and somemoons of the outer planets. Originally it was thought that they existed only onplanets or moons lacking anatmosphere, but more recently they have been identified onMars in infrared images taken from orbit by2001 Mars Odyssey'sthermal imager.
Rays appear at visible, and in some casesinfrared 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 examplebasaltic-lava deposits on thelunar 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.
The layering of rays across other surface features can be useful as an indicator of the relative age of the impact crater, because over time various processes obliterate the rays. On non-atmosphered bodies such as the Moon,space weathering from exposure tocosmic rays andmicrometeorites causes a steady reduction of the differential between the ejecta's albedo and that of the underlying material. Micrometeorites in particular produce a glassy melt in theregolith that lowers thealbedo. Rays can also become covered bylava flows (such as those ofLichtenberg on the moon), or by other impact craters or ejecta.
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 ofiron 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 areAristarchus,Copernicus,Kepler,Proclus,Dionysius,Glushko, andTycho. Smaller examples includeCensorinus,Stella, andLinné. Similar ray systems also occur on thefar side of the Moon, such as the rays radiating from the cratersGiordano Bruno,Necho,Ohm,Jackson,King, and the small but prominentPierazzo.
Most lateral transport of primary ejecta from impact craters is limited to a distance of a few crater radii, but some larger impacts, such as the impacts that made theCopernicus andTycho craters, launched primary ejecta halfway around the moon.[2]
North Ray andSouth Ray craters, each with a clear ray system, were observed from the ground by the astronauts ofApollo 16 in 1972.
