View source for Astronomy:Ray system

{{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|Ars Technica]], Feb 6 2014.</ref>]]
<!-- [[Image:AS11-42-6285.jpg|right|thumb|250px|Crater ray system on the far side of the Moon (Apollo 11 image).]] Replaced with better image above. PKr 11 Aug 2014 -->

In [[Earth:Planetary geology|planetary geology]], a '''ray system''' comprises radial streaks of fine ''[[Astronomy:Ejecta|ejecta]]'' thrown out during the formation of an [[Astronomy:Impact crater|impact crater]], looking somewhat like many thin spokes coming from the hub of a wheel. The rays may extend for lengths up to several times the [[Diameter|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 [[Astronomy:Moon|Moon]], [[Astronomy:Earth|Earth]] (Kamil Crater), [[Astronomy:Mercury (planet)|Mercury]], and some [[Astronomy:Natural satellite|moons]] of the outer planets. Originally it was thought that they existed only on [[Astronomy:Planet|planet]]s or moons lacking an atmosphere, but more recently they have been identified on [[Astronomy:Mars|Mars]] in infrared images taken from orbit by ''[[Astronomy:2001 Mars Odyssey|2001 Mars Odyssey]]''{{'}}s [[Engineering:Thermal Emission Imaging System|thermal imager]].

[[Image:Gratteri.jpg|right|thumb|250px|Gratteri crater, a rayed crater on Mars that was imaged by THEMIS at night. Image covers an area 32 km across.]]
Rays appear at visible, and in some cases [[Physics:Infrared|infrared]] wavelengths, when ejecta are made of material with different reflectivity (i.e., [[Earth:Albedo|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 [[Earth:Basalt|basalt]]ic-[[Earth:Lava|lava]] deposits on the [[Astronomy: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, [[Astronomy:Space weathering|space weathering]] from exposure to [[Astronomy:Cosmic ray|cosmic ray]]s and micrometeorites 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 [[Earth:Albedo|albedo]]. Rays can also become covered by [[Earth:Lava|lava]] flows (such as those of [[Astronomy: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, one of the freshest craters on Mercury]]

== Lunar rays ==
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 [[Chemistry:Iron oxide|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 [[Astronomy:Aristarchus (crater)|Aristarchus]], [[Astronomy:Copernicus (lunar crater)|Copernicus]], [[Astronomy:Kepler (lunar crater)|Kepler]], [[Astronomy:Proclus (crater)|Proclus]], [[Astronomy:Dionysius (crater)|Dionysius]], [[Astronomy:Glushko (crater)|Glushko]], and Tycho. Smaller examples include [[Astronomy:Censorinus (crater)|Censorinus]], [[Astronomy:Stella (crater)|Stella]], and [[Astronomy:Linné (crater)|Linné]].  Similar ray systems also occur on the far side of the Moon, such as the rays radiating from the craters [[Astronomy:Giordano Bruno (crater)|Giordano Bruno]], [[Astronomy:Necho (crater)|Necho]], [[Astronomy:Ohm (crater)|Ohm]], [[Astronomy:Jackson (crater)|Jackson]], [[Astronomy:King (crater)|King]], and the small but prominent [[Astronomy:Pierazzo (crater)|Pierazzo]].

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 the [[Astronomy:Copernicus (lunar crater)|Copernicus]] and Tycho craters, launched primary ejecta halfway around the moon. <ref>{{cite book |last=French |first=Bevan |date=1991 |title=Lunar SourceBook: A Users Guide to the Moon |url=https://archive.org/details/lunarsourcebooku00heik |url-access=limited |location=Cambridge |publisher=Cambridge University Press |page=[https://archive.org/details/lunarsourcebooku00heik/page/n309 287]}}</ref>

[[Astronomy:North Ray (crater)|North Ray]] and [[Astronomy:South Ray (crater)|South Ray]] craters, each with a clear ray system, were observed from the ground by the astronauts of [[Engineering:Apollo 16|Apollo 16]] in 1972.

<gallery heights="140px" mode="packed">
Image:AS15-94-12836.jpg|Asymmetrical ray system about the lunar crater [[Astronomy:Proclus (crater)|Proclus]] ([[Engineering:Apollo 15|Apollo 15]] image)
Image:Pierazzo crater Clementine mosaic.jpg|[[Astronomy:Pierazzo (crater)|Pierazzo]] crater (Mosaic of [[Engineering:Clementine (spacecraft)|Clementine]] images)
File:Giordano Bruno crater rays AS11-44-6665HR.jpg|The rays of [[Astronomy:Giordano Bruno (crater)|Giordano Bruno]] extend for hundreds of kilometers from the small crater ([[Engineering:Apollo 11|Apollo 11]] image)
File:South Ray crater AS16-P-4618 ASU.jpg|[[Astronomy:South Ray (crater)|South Ray]] ([[Engineering:Apollo 16|Apollo 16]] image)
</gallery>

== See also ==
* [[Astronomy:List of craters with ray systems|List of craters with ray systems]]
* [[Astronomy:Reiner Gamma|Reiner Gamma]]

== References ==
=== Citations ===
{{Reflist}}

=== Sources ===
{{refbegin}}
* Martel, L.M.V. (Sept., 2004) ''[http://www.psrd.hawaii.edu/Sept04/LunarRays.html Lunar Crater Rays Point to a New Lunar Time Scale]'', Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/Sept04/LunarRays.html. (Accessed 9/15/2005)
* Burnham, R. (March, 2005) ''[http://www.astronomy.com/asy/default.aspx?c=a&id=2980 Chipping pieces off Mars: Martian craters with rays may be the long-sought sources for the Mars meteorites found on Earth]'', Astronomy Magazine Online. http://www.astronomy.com/asy/default.aspx?c=a&id=2980 (Accessed 8/07/2006)
{{refend}}

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{{The Moon|state=collapsed}}

[[Category:Impact craters]]

{{Sourceattribution|Ray system}}