North-south geological differences between the residual polar caps on Mars

Polar processes can be sensitive indicators of global climate, and the geol ogical features associated with polar ice caps can therefore indicate evolu tion of climate with time. The polar regions on Mars have distinctive morph ologic and climatologic features: thick layered deposits, seasonal CO2 fros t caps extending to mid latitudes, and near-polar residual frost deposits t hat survive the summer(1,2). The relationship of the seasonal and residual frost caps to the layered deposits has been poorly constrained(3,4), mainly by the limited spatial resolution of the available data. In particular, it has not been known if the residual caps represent simple thin frost cover or substantial geologic features. Here we show that the residual cap on the south pole is a distinct geologic unit with striking collapse and erosiona l topography; this is very different from the residual cap on the north pol e, which grades into the underlying layered materials. These findings indic ate that the differences between the caps are substantial (rather than refl ecting short-lived differences in frost cover), and so support the idea of long-term asymmetry in the polar climates of Mars.