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Impact Craters on the Moon

The lunar crater Aristarchus ( on the NW edge of the Oceanus Procellarum). This huge, circular crater is 25 miles (40 km) in diameter and 2.2 miles (3.6 km) deep (from rim to floor). There is a lot of ejecta (material thrown from the crater at impact) surrounding the crater. Impact craters are the remains of collisions between an asteroid, comet, or meteorite and the Moon. These objects hit the Moon at a wide range of speeds, but average about 12 miles per second (20 kilometers per second).

The surface of the moon is scarred with millions of impact craters. There is no atmosphere on the moon to help protect it from bombardment from potential impactors (most objects from space burn up in the Earth's atmosphere). Also, there is no erosion (wind or water) and little geologic activity to wear away these craters, so they remain unchanged until another new impact changes it.

Ptolemaeus is the large (diameter=164 km ) flat-floored crater at the top. Alphonsus (diameter = 108 km) is at the lower left and Albategnius crater (diameter = 114 km) is at lower right. These craters range in size up to many hundreds of kilometers, but the most enormous craters have been flooded by lava, and only parts of the outline are visible. The low elevation maria (seas) have fewer craters than other areas. This is because these areas formed more recently, and have had less time to be hit.

There are many rupes (lines of mountainous cliffs or scarps) on the lunar surface. These are remnants of the rims of ancient craters. The Rupes Altai is a mountain range about 315 miles (50 km) long on the moon.