Aristarchus (crater) - Definition

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. 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.

Lunar orbit view of craters Aristarchus and Herodotus from Apollo 15

The Aristarchus crater is located on an elevated rocky plateau in the midst of the Oceanus Procellarum lava plain. It is just to the east of the Herodotus crater and the Vallis Shröteri. 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. 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.

Aristarchus is noted for possible lunar transient phenomena, 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.

Satellite craters

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 — See Väisälä crater.
• Aristarchus C — See Toscanelli crater.