The southern hemisphere is more heavily cratered than the northern hemisphere. Winds are the main erosional force on Mars and windblown dust and soil erode craters over time. The structure of some Martian impact craters, such as the one pictured here at left, provide evidence that suggests the presence of water or ice in the surface at the time the impact occurred. More about Martian craters. A steroids are rocky and usually heavily cratered due to a long history of impacts with other asteroids and possibly comets.
Old impact craters on asteroids have beem deformed and erased by newer impact craters.
Alternatively, impact events can disintegrate asteroids into smaller pieces. This asteroid, Mathilde, is interesting because of the large size of the impact craters on its surface. Despite the obvious intensity of the impacts, the asteroid was not destroyed. Scientists believe the asteroid must be uncommonly dense to have withstood such bombardment. Mathilde 35k JPEG Courtesy USGS and the Applied Physics Laboratory NEAR mission team.