Mars North Polar Deposits: Stratigraphy, Age, and Geodynamical Response

The Shallow Radar (SHARAD) on the Mars Reconnaissance Orbiter has imaged the internal stratigraphy of the north polar layered deposits of Mars. Radar reflections within the deposits reveal a laterally continuous deposition of layers, which typically consist of four packets of finely spaced reflectors separated by homogeneous interpacket regions of nearly pure ice. The packet/interpacket structure can be explained by approximately million-year periodicities in Mars ’ obliquity or orbital eccentricity. The observed ~100-meter maximum deflection of the underlying substrate in response to the ice load implies that the present-day thickness of an equilibrium elastic lithosphere is greater than 300 kilometers. Alternatively, the response to the load may be in a transient state controlled by mantle viscosity. Both scenarios probably require that Mars has a subchondritic abundance of heat-producing elements. Thenorth polar region of Mars is part of animmense sedimentary basin that recordsover 3 billion years of deposition and ero-sion of ice, dust, lava, and other lithic material of varying composition and includes the plateau of Planum Boreum, covering an area of ~800,000 km2. Beneath a thin residual cap of bright, nearly pure water ice, the north polar layered deposit