Lunar Craters: How Are They Formed?
Cosmic bodies like asteroids, comets and meteors, which move freely in outer space, collide with the Moon, leaving behind evidence of their collisions. The moon is a victim of more frequent collisions, as it lacks an atmosphere, which would protect it from the smaller cosmic bodies. Besides that, volcanic eruptions have created craters on its surface. Even our planet Earth has impact craters on its surface. But unlike the Moon, where weather is absent, erosion, precipitation and tectonic activity cause these terrestrial impact craters to erode, and hence become difficult to detect.
Many of the caters on the lunar surface are named after eminent scientists: Copernicus, Archimedes, Moltke, Schrodinger and Tycho are a few of the craters on the Moon so named. The image above shows lunar crater Peary, taken by NASA's Lunar Reconnaissance Orbiter.
Objects of varying size and speeds collide with the Moon and cause craters of different sizes. As a fast moving object plunges into the lunar surface it compresses the surface creating a depression. Also, the compressed area is accelerated by this impact, while at the same time the compressed lunar surface opposes this action and this resistance decelerates the cosmic impactor. This compression, and subsequent resistance creates a shock wave which propagates through different layers of the lunar surface and the impactor. Due to this shock wave the compressed material is thrown out of the depression formed. The material ejected falls far away from the point of impact. The pressure wave created by this impact is so large that it creates a crater much bigger than the size of the impacting object. The rims of the craters will undergo changes under gravitational impact, where materials on the rim fall into the craters, thus covering part of it. The crater rays, which are formed by the materials ejected during the collision, gives an indication of the age of crater formed. Crater rays appear as bright lines starting from the center of crater. For younger craters the crater rays are more prominent and hence more visible.