Editing Ray system (section)

{{short description|Radial streaks of material thrown out during formation of an impact crater}}
[[File:Fresh impact crater HiRise 2013.jpg|thumb|Fresh [[impact crater]] on [[Mars]] showing a prominent ray system of [[ejecta]]. This {{convert|30|m|ft|abbr=on}} diameter crater formed between July 2010 and May 2012 (19 November 2013; {{coord|3.7|N|53.4|E|globe:Mars}}).<ref>[https://arstechnica.com/science/2014/02/spectacular-new-martian-impact-crater-spotted-from-orbit/ Spectacular new Martian impact crater spotted from orbit], [[Ars Technica]], Feb 6 2014.</ref>]]
<!-- [[Image:AS11-42-6285.jpg|right|thumb|250px|Crater ray system on the [[Far side (Moon)|far side]] of the [[Moon]] ([[Apollo 11]] image).]] Replaced with better image above. PKr 11 Aug 2014 -->

In [[planetary geology]], 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 [[Line (mathematics)#Ray|rays]] may 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]], [[Earth]] ([[Kamil Crater]]), [[Mercury (planet)|Mercury]], and some [[natural satellite|moons]] of the outer planets. Originally it was thought that they existed only on [[planet]]s or moons lacking an [[Celestial body atmosphere|atmosphere]], but more recently they have been identified on [[Mars]] in infrared images taken from orbit by ''[[2001 Mars Odyssey]]''{{'}}s [[Thermal Emission Imaging System|thermal imager]].

[[Image:Gratteri.jpg|right|thumb|250px|Gratteri crater, a rayed crater on [[Mars]] that was imaged by [[Thermal Emission Imaging System|THEMIS]] at night. An individual THEMIS image, of which this mosaic is comprised, is ~32 km across.]]
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 [[basalt]]ic-[[lava]] deposits on the [[lunar mare|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.

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 to [[cosmic ray]]s and [[Meteor|micrometeorite]]s 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 the [[regolith]] that lowers the [[albedo]]. Rays can also become covered by [[lava]] flows (such as those of [[Lichtenberg (crater)|Lichtenberg]] on the moon), or by other impact craters or ejecta.
[[File:Kuiper crater EW1065899876G.jpg|right|thumb|250px|The rays of [[Kuiper (Mercurian crater)|Kuiper]], one of the freshest craters on Mercury]]