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Aristarchus (center) and Herodotus (right) from Apollo 15 '' NASA photo''
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237
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N
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474
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W
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40 km
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37 km
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48
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is a prominent
Lunar Impact Crater that lies in the northwest part of the
Moon 's near side, named after the astronomer
Aristarchus Of Samos . 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 Photograph s 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
Ray s 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.
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 .
The Aristarchus crater region was part of a
Hubble Space Telescope study in 2005 that was investigating the presence of
Oxygen -rich glassy soils in the form of the mineral
Ilmenite . Baseline measurements were made of the
Apollo 15 and
Apollo 17 landing sites, where the chemistry is known, and these were compared to Aristarchus. The Hubble Advanced Camera for Surveys was used to photograph the crater is visual and
Ultraviolet light. The crater was determined to have especially rich concentrations of ilmenite.
Aristarchus is noted for possible
Lunar Transient Phenomena (TLP), and there are indications of
Volcanic activity, including
Volcanic domes and
Rille s.
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
Kilometer s above this
Crater , a significant rise in
Alpha Particle s 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 .
