Editing Ray system (section)

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

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

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