Impact craters are formed on a planet or moon when a smaller object collides with the surface at a very high velocity (typically 15 000 ms-1). An impact crater is identifiable by its approximately circular shape, raised rim, pattern of ejected material and crater floor that is lower than the surrounding surface.
Many objects in the Solar System, for example Mars, Earth’s Moon and Mercury, have their surface covered in impact craters. For other objects, including the Earth, visible impact craters are far less common; these objects have active geological processes and the impact craters become weathered over time.
The smallest impact craters on a planet’s surface are called simple craters, because they have simple bowl-like shapes. These craters are excavated when an impactor, i.e. an asteroid or comet hits the surface creating a shock wave that radiates into the crust of the planet.
Download Stage 1, 2 and 3 of simple cratering
Large impactors, i.e. asteroids and comets, produce complex craters with crater walls that are so steep they are prone to collapse and uplifted rock in the centre that forms high central peaks or central peak rings.
Contact Paul Denton, MarsQuake project leader, for further information.