Aristarchus (crater)

. Aristarchus is located on the Aristarchus plateau, an elevated rocky rise in the midst of the Oceanus Procellarum, a large expanse of lunar mare. This is a tilted crustal block, about 200 km across, that rises to a maximum elevation of 2 km above the mare in the southeastern section. Aristarchus is just to the east of the crater Herodotus and the Vallis Schröteri, and south of a system of narrow sinuous rilles named Rimae Aristarchus. Aristarchus is bright because it is a relatively young formation, approximately 450 million years old, and the solar wind has not yet had time to darken the excavated material by the process of space weathering. The impact occurred following the creation of the ray crater Copernicus, but before the appearance of Tycho. Due to its prominent rays, Aristarchus is mapped as part of the Copernican System. The brightest feature of this crater 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 fractures. The crater has a terraced outer wall, roughly polygonal in shape, and covered in a bright blanket of ejecta. These spread out into bright rays to the south and south-east, suggesting that Aristarchus was most likely formed by an oblique impact from the northeast, and their composition includes material from both the Aristarchus plateau and the lunar mare. NAC image of the central peak, with colors showing variations in the composition ==Remote sensing==

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 alternative name for the Aristarchus plateau. . NASA photo. Spectra taken of this crater during the Clementine mission were used to perform mineral mapping. ==Transient lunar phenomena==

The region of the Aristarchus plateau has been the site of many reported transient lunar phenomena, with a total of 122 such reports by 2007; the highest recorded for any lunar feature. Such events include temporary obscurations and colorations of the surface, and catalogues of these show that more than one-third of the most reliable spottings come from this locale. In 1971 when Apollo 15 passed 110 kilometers above the Aristarchus plateau, a significant rise in alpha particles was detected. These particles are believed to be caused by the decay of radon-222, a radioactive gas with a half-life of only 3.8 days. The Lunar Prospector mission later confirmed radon-222 emissions from this crater. These observations could be explained by either the slow and visually imperceptible diffusion of gas to the surface, or by discrete explosive events. One of the oldest reports of transient lunar phenomena in Aristarchus is an observation made by Heinrich Wilhelm Matthias Olbers on 5 February 1821. Contemporary Henry Kater publicly believed that this and other events were due to volcanic activity on the Moon, a belief not so confidently shared by Olbers, who believed it explainable in a manner "more consistent with what we know of the physical construction of the moon." ==Names==

Aristarchus is named after the Greek astronomer Aristarchus of Samos. Like many of the craters on the Moon's near side, it was given its name by Giovanni Riccioli, whose 1651 nomenclature system became standardized in 1935. Earlier lunar cartographers had given the feature different names. Michael van Langren's 1645 map calls it "Balthasaris Hispa. Pri." after Balthazar Charles, then the heir apparent to the kingdoms of Spain. And Johannes Hevelius called it "Mons Porphyrites" after the mountains near Olbia, Egypt. A particular region of very low ultraviolet albedo is called Wood's Spot after its discoverer, Robert W. Wood. == Satellite craters ==

Surrounding Aristarchus are several smaller craters, many of which are probably secondary craters. Secondary craters form when large blocks ejected from the primary crater reimpact the surface at high velocities. By convention these features are identified on lunar maps by placing a letter on the side of the crater midpoint that is closest to the primary crater. The following craters have been renamed by the IAU. • Aristarchus A — See Väisälä. • Aristarchus C — See Toscanelli. File:Aristarchus F crater 5194 med.jpg|Aristarchus F (Lunar Orbiter 5 image) ==See also==