The geologic cycle of a planet depicts the effects of energy from various internal and external sources interacting on the planetary surface and subsurface and thus provides an important tool in comparative planetology. The geologic cycle allows one not only to list and categorize the geologic processes acting on planets but also to suggest inferences about their history and current stage of development.

The Earth presently has a closed-loop geologic cycle in which source rocks are eroded and continuously recycled. In contrast, the Moon apparently has an open-loop geologic cycle in which the crystalline primitive crust is irreversibly destroyed. Preliminary interpretations of the geologic cycle of Mars suggest an intermediate stage of development between a “primitive,” static planet (the Moon) and a “mature,” active planet (the Earth). A preliminary “geologic cycle map” shows that Mars is almost equally divided between primitive cratered terrains and modified smooth terrains, suggesting that Mars may be part-way through an episode of crustal modification. Indeed, Mars apparently is entering a stage of Earth-like tectonic activity. If so, the study of the structural features of Mars offers an exciting opportunity to learn a great deal about the early tectonic development of the Earth.

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