The year-long observations of Mars by Mariner 9, combined with earlier spacecraft and ground-based data, have demonstrated a complex evolution for the Martian crust and a complex interaction between crustal and surficial processes that is still in progress. Volcanic, tectonic, and impact processes are prominent in the development of the crust; eolian, glacial, fluvial, and mass-wasting processes are paramount in surface modification.

Maps of Mars prepared from television pictures can be correlated with the planetary shape from occultation data, pressure elevation mapping, ground-based radar, and gravity measurements. The southern hemisphere and equatorial zone are underlain by high standing "continental" rocks; the northern region by low-lying "oceanic" basins underlain by basaltic rocks.

Based on these data, a history of the crust of Mars can be developed. The major events are (1) early differentiation of the crust and segregation into highlands and lowlands; (2) impact cratering; and (3) continued volcanic activity in the highlands and lowlands involving emplacement of basaltic and silicic rocks. Concurrent or later modification includes (1) formation of chaotic terrain by slumping; (2) formation of three types of fluvial channels in the equatorial belt; (3) formation of glacio-eolian deposits in the polar regions; and (4) regional eolian erosion and deposition.

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