Though fairly rare on the Earth, impact craters are one of the most common, and therefore important, types of surface features in the solar system. Craters are found on almost all the solid planets of the solar system, but not on gas giants like Jupiter and Saturn since there is no solid surface to preserve a record of the impact. All such impacts are governed by a set of physical principles based on properties of the impacting body, the target body, and the speed and angle of the impact. Craters are also affected by the presence (or absence) of an atmosphere on a planetary body. A thick atmosphere can cause smaller impacting bodies to burn up before impact, thus screening the surface of craters caused by these smaller impactors.

A surface which is completely covered with craters is called saturated. New craters on a saturated surface tend to cover older craters, so once a surface becomes saturated with craters, the number of craters remains approximately the same. Saturated surfaces are very old. Only geologically inactive planetary bodies can become saturated, since on an active planet such as the Earth, craters are quickly erased by agents of change such as tectonics, volcanism, and erosion. Thus a saturated surface such as the Moon's is a sign that the Moon is no longer geologically active, and regions with a lower crater density are younger than those with a higher crater density. The study of craters can provide much information about the history of planetary bodies in our solar system.

This section will discuss in greater detail how craters are made, how they are removed, and what can be learned from images of craters on the Earth and on other planets.

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This module was written by Cynthia Phillips, Dept. of Planetary Sciences, University of Arizona, Tucson AZ, and funded in part by the NASA Spacegrant program.

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