Hemispheres Apart: The Crustal Dichotomy on Mars

The hemispheric dichotomy is a fundamental feature of Mars, expressed by a physiographic and geologic divide between the heavily cratered southern highlands and the relatively smooth plains of the northern lowlands. The origin of the dichotomy, which may have set the course for most of the subsequent geologic evolution of Mars, remains unclear. Internally driven models for the dichotomy form the lowlands by mantle convection, plate tectonics, or early mantle overturn. Externally driven models invoke one giant impact or multiple impacts. Areal densities of buried basins, expressed by quasi-circular depressions and subsurface echoes in radar sounding data, suggest that the dichotomy formed early in the geologic evolution of Mars. Tectonic features along the dichotomy boundary suggest late-stage modification by flexure or relaxation of the highlands after volcanic resurfacing of the northern lowlands. Subsequent deposition and erosion by fluvial, aeolian, and glacial processes shaped the present-day dichotomy boundary.