Geologic map of the Mare Australe area of Mars

The south polar region of Mars is among the most geologically diverse on the planet. The areas is within the heavily cratered hemisphere of Mars but is distinguished by a variety of younger, regionally extensive deposits and an assemblage of unique erosional forms. The spectrum of units varies from the oldest to the youngest on the planet. The oldest unit, typifying the cratered hemisphere, is the material of ancient large craters, most of which have undergone advanced degradation. These ancient crater materials are overlain by a variety of regional deposits, including extensive intercrater debris blankets, ridged volcanic flows, and massive eolian deposits that eroded to form complex cliff-rimmed pits. In the central zone of the polar region, south of roughly 75˚ S., these depositional units are, in turn, unconformably overlain by a tremendous sedimentary complex 1,500-2,000 km in diameter, consisting of 40 or more regularly spaced horizontal starta. Each stratum retains apparently uniform thickness (several tons of meters) over hundreds of kilometers; erosional unconformities between starta have not as yet been identified. Resting on the uppermost surface of this layered terrain is the permanent residual polar ice cap. This complex stratigraphic record may ultimately hold the keys to the initial atmospheric evolution, climatological fluctuations, and the history, budget, and mobility of water on the planet.