Worlds apart: Geophysical modeling of the evolution of crustal plateaus on Venus and the propagation of radar through the Martian polar layered terrains

This thesis is divided into two different geophysical topics. First is the investigation of relaxation of crustal plateaus on Venus as an evolutionary track for these dominant physiographic features. They likely originated through dynamic mantle processes, and a debate currently exists on whether they formed through mantle upwellings or downwellings. Regardless of the mode of formation, several observations led to the hypothesis that viscous relaxation may be the driving force behind the apparent evolutionary sequence from a high-standing plateau to a low-standing plateau with elevated margins. We apply analytic and finite element models to test this hypothesis for isostatically compensated topography, uncompensated topography, and topography supported by subcrustal time-dependent buoyancy. Relaxation can only explain the elevated rims observed at crustal plateaus if initial topography is uncompensated or if buoyant support has timescales that are shorter than that for crustal flow. A low thermal gradient (<10 K/km) in these two cases best reproduces crustal plateau shape but cannot address all of the tectonic features present at crustal plateaus. Relaxation of compensated topography is generally too slow and yields shapes that lack rims. Second, understanding the 3-dimensional picture of the Martian polar caps is fundamental in determining their hydrologic history, past climatic changes, and the properties of lithosphere underneath. We model the dielectric profile of the Martian Polar Layered Deposits (PLD), apply it to a 1-D electromagnetic plane-wave propagation model, and calculate the strength of radar reflections produced by layering within these deposits. Variations in dust fraction derive from brightness profiles of a polar trough. We find that the detection of the fine layering in the PLD is possible if dust fractions are >0.01%, and that the mapping of the boundary between the Upper (dusty ice) and Lower (sand-rich ice) Northern PLD units by SHARAD orbiting radar sounder is likely if they indeed have dramatically different silicate inclusion fractions. A thin veneer of CO2 ice at the surface of the cap enhances the subsurface reflections by a several dB because it has a lower dielectric constant than water ice, and it may be used to enhance weak subsurface returns.