Craters On The Moon

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. 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. The size, mass, speed, and angle of the falling object determine the size, shape, and complexity of the resulting crater. Small, slow objects have a low energy impact and cause small, simple craters. Large, fast objects release a lot of energy and form large, complex craters. Very large impacts can even cause secondary cratering, as ejected material falls back to the ground, forming new, smaller craters, or a series of craters. Most of the craters on the Moon are circular. The few craters that are not circular, like Messier and Messier A (pictured at the left) in the Mare Fecunditatis, are an enigma. Scientists do not know exactly how these oddly-shaped craters were formed.