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Aristarchus (crater)

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   Crater characteristics
   Aristarchus (centre) and Herodotus (right) from Apollo 15. NASA photo.
   Coordinates 23.7° N, 47.4° W
   Diameter 40 km
   Depth 3.7 km
   Colongitude   48° at sunrise
   Eponym Aristarchus
   of Samos

   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 earthshine.

   The crater is located at the southeastern edge of the Aristarchus
   plateau, an elevated area that contains a number of volcanic features,
   such as sinuous rilles. This area is also noted for the large number of
   reported transient lunar phenomena, as well as recent emissions of
   radon gas as measured by the Lunar Prospector spacecraft.

   Aristarchus was originally named by the Italian map maker Giovanni
   Riccioli. His work Almagestum novum ("New Almaghest"), published in
   1651, gave the spot-shaped telescopic features (later called craters)
   eponyms of noted astronomers and philosophers. Although widely adopted,
   the name only became an official international standard by a vote of
   the IAU General Assembly in 1935.

Selenography

   The Aristarchus crater is located on an elevated rocky rise, known as
   the Aristarchus plateau, 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 altitude of 2 km above the mare in
   the southeastern section. Aristarchus crater is just to the east of the
   Herodotus crater and the Vallis Schröteri.

   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 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.

   The main reason for the crater's brightness is that it is a 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
   Copernicus crater, but before the appearance of Tycho crater. Based on
   the asymmetrical spread of the ejecta, it was formed by an object that
   struck at a low angle to the surface, arriving from the north-east. The
   composition of the ejecta includes material from both the plateau and
   the mare.

   To the north of this crater is a system of narrow sinuous rilles named
   Rimae Aristarchus. These rilles extend for a distance of 121 km, or
   three times the diameter of the Aristarchus crater.

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 alternate name for the Aristarchus
   Plateau.
   Clementine image of Aristarchus crater and surroundings mapped onto
   simulated topography. NASA photo.
   Enlarge
   Clementine image of Aristarchus crater and surroundings mapped onto
   simulated topography. NASA photo.

   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, a titanium oxide mineral that could
   potentially be used in the future by a lunar settlement for extracting
   oxygen.

Transient lunar phenomenon

   Aristarchus is noted for possible lunar transient phenomena (TLP). 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 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.

   On April 23rd, 1994, amateur observers reported another TLP event in
   the vicinity of the "Cobra Head", the name for the bulbous start of
   Vallis Shröteri, a volcanic sinuous rille. The Clementine spacecraft
   took before and after pictures of this region, and colour changes were
   claimed to have been observed.

Satellite craters

   Surrounding the Aristarchus crater are several smaller craters, many of
   which are probably secondaries. 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 mid-point that is closest
   to the primary crater.
   Aristarchus Latitude Longitude Diameter
        B      26.3° N   46.8° W    7 km
        D      23.7° N   42.9° W    5 km
        F      21.7° N   46.5° W   18 km
        H      22.6° N   45.7° W    4 km
        N      22.8° N   42.9° W    3 km
        S      19.3° N   46.2° W    4 km
        T      19.6° N   46.4° W    4 km
        U      19.7° N   48.6° W    4 km
        Z      25.5° N   48.4° W    8 km

   The following craters have been renamed by the IAU.
     * Aristarchus A — See Väisälä crater.
     * Aristarchus C — See Toscanelli crater.

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