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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• Introduction

• I. Location of features on the surface of a planet: latitude and longitude

• II. Crater formation, modification, and removal
  • A. How are craters made?

  • B. Where have all the craters gone?

  • C. Focus on the Galilean satellites

• III. Crater Image Interpretation
  • A. Crater size

  • B. Crater depth

  • C. Crater size distribution and surface ages

• Conclusion