Aristarchus is a prominent lunar impact crater that lies in the northwest part of the Moon's near side. It is considered the brightest of the large formations on the lunar surface, with an albedo nearly double that of most lunar features. The feature is bright enough to be visible to the naked eye, and is dazzling in a large telescope. It is also readily identified when most of the lunar surface is illuminated by earth-shine.
The Aristarchus crater is located on an elevated rocky plateau, known as the Aristarchus plateau, in the midst of the Oceanus Procellarum lava plain. It is just to the east of the Herodotus crater and the Vallis Schröteri.
Its brightest feature is the steep central peak. Sections of the interior floor appear relatively level, but Lunar Orbiter photographs reveal the surface is covered in many small hills, streaky gouges, and some minor cracks and rifts. The crater has a terraced outer wall covered in a bright blanket of ejecta, which spreads out into bright rays to the south and south-east. (These suggest that Aristarchus was most likely formed by an oblique impact from the northeast.) Observers have noted that the wall is roughly circular but has a somewhat polygonal shape.
Lunar orbit view of craters Aristarchus (center) and Herodotus (right) from Apollo 15. NASA photo.
The reason for the crater's brightness is that it is a young formation, approximately 450 million years old, which means that the solar wind has not yet had time to darken the excavated material. Based on the spread of the ejecta, it was formed by an object that struck at a low angle to the surface, arriving from the north-east.
In 1911, Professor Robert W. Wood used ultraviolet photography to take images of the crater area. He discovered the plateau had an anomalous appearance in the ultraviolet, and an area to the north appeared to give indications of a sulfur deposit. This colorful area is sometimes referred to as "Wood's Spot", an alternate name for the Aristarchus Plateau.
Spectra taken of this crater during the Clementine mission was used to perform mineral mapping. The data indicated that the central peak is a type of rock called anorthosite, which is a slow-cooling form of igneous rock composed of plagioclase feldspar. By contrast the outer wall is troctolite, a rock composed of equal parts plagioclase and olivine.
Transient lunar phenomenon
Aristarchus is noted for possible lunar transient phenomena (TLP), and there are indications of volcanic activity, including volcanic domes and rilles. William Herschel mistook the crater for an erupting volcano, an error most likely due to the brightness of the structure.
In 1971 when Apollo 15 passed 110 kilometers above this crater, a significant rise in alpha particles was detected. These particles are believed to be emitted by the decay of radon-222, a radioactive gas with a half-life of only 3.8 days.
On April 23rd, 1999, amateur observers reported another TLP event in the vicinity of the "Cobra Head", the name for the bulbous start of the Vallis Shröteri. The Clementine spacecraft took before and after pictures of this region, and definite color changes were observed.
By convention these features are identified on lunar maps by placing the letter on the side of the crater mid-point that is closest to Aristarchus crater.
The following craters have been renamed by the IAU.
Aristarchus A - Väisälä crater.
Aristarchus C - Toscanelli crater.
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