Before space flight was possible, the appearance of planetary bodies in the Solar System was largely a secret. But once it began, scientists were excited to discover a large number of craters on planets and satellites, called 'percussion' craters because of the high-speed impacts that formed them. Such impacts are one of the most important factors in the formation and transformation of planetary bodies' surfaces. 

Although all percussion craters share the common features of a hollow space surrounded by a circular 'swell', other details of their structure depend not only on their size, but also on the material from which the crater was formed, the planet's gravitational force, and the crater's age.

The simplest craters are cup-like hollows surrounded by a swell. Larger craters have central hills.

When a crater is formed, most of the rock fragments thrown out fall onto the planet's surface as a series of deposits. The angle of impact has a much greater effect on these deposits and their spread than on the shape of the crater itself. In the case of Martian craters, their shapes are determined by the effects of ice on the lower layers.