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,
Dionysius,
Glushko, and
Tycho. Smaller examples include
Censorinus,
Stella, and
Linné. Similar ray systems also occur on the
far side of the Moon, such as the rays radiating from the craters
Giordano Bruno,
Necho,
Ohm,
Jackson,
King, and the small but prominent
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 and
Tycho craters, launched primary ejecta halfway around the moon.
North Ray and
South Ray craters, each with a clear ray system, were observed from the ground by the astronauts of
Apollo 16 in 1972. File:AS15-94-12836.jpg|Asymmetrical ray system about the lunar crater
Proclus (
Apollo 15 image) File:Pierazzo crater Clementine mosaic.jpg|
Pierazzo crater (Mosaic of
Clementine images) File:Giordano Bruno crater rays AS11-44-6665HR.jpg|The rays of
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 (
Apollo 16 image) File:AS11-42-6285.jpg|Crater ray system on the
far side of the
Moon (
Apollo 11 image) == See also ==