IMPACTS, CRATERS, AND BASINS
A telescope shows an amazing array of lunar features. Most are the result of impacts by meteorites, asteroids, and comets. When a piece ot rock, ice, or anything hits the Moon at several miles per second, it blows up in a cloud ot vapor, blasting a scar on the Moon's surface.
Small impacts make tidy, bowl-shaped craters, such as Aristarchus, which is only 25 miles (40 km) across. These small craters are surrounded by aprons of ejected rock fragments, seen as bright areas around the younger craters.
Bigger impacts make larger craters with a more complex structure. A good example is Copernicus, 57 miles (92 km) across, with central peaks formed from pieces of deep lunar crust thrust upward by the shock of the impact. Landslides have terraced the inside of the crater's walls, and hardened pools of impact-melted rock are littered across its floor.
Craters that are larger still tend to resemble walled plains. They have flat or convex floors and their central peaks may form concentric rings. These big craters rarely have rays, however, because rays disappear in about a billion years through the action of tiny meteorites striking the surface—and all of the largest craters are older than that.
The biggest craters are called basins. Filled by Mare Imbrium, the Imbrium basin spans 720 miles (1,160 km)—about the size of Texas—and was created by a small asteroid about 3.9 billion years ago. The Apennine Mountains form the southeastern part of the Imbrium basin's rim.