Geodynamic Interpretations of Global Topography and Gravity on Venus and Mars.

摘要

The topography and gravity measurements from missions to Venus and Mars are among a few data available to constrain models of the interior structure and dynamics of these planets. These measurements reveal various intriguing features whose origins have generated debates in the planetary science community. On Venus, these include a high correlation between the long-wavelength topography and the geoid. A common explanation is that the Venusian topography is a result of dynamic uplift caused by mantle convection (dynamic topography). Our analysis of convection models shows that in strongly temperature-dependent viscosity fluids, such as planetary mantles, the dynamic topography is actually small. The lithosphere is close to a state of hydrostatic equilibrium (thermal isostasy) and the largest contributions to topography and geoid anomalies come from the lithospheric thickness variations caused by sublithospheric convection. The Venusian topography and geoid can be fully explained by the thermal isostasy associated with lithospheric thickness variations and the compositional isostasy associated with crustal thickness variations. A prominent feature on Mars is the hemispherical dichotomy where the southern highlands stand several kilometers elevated above the northern lowlands. One of the most common explanations for its formation is that the dichotomy formed as a result of a giant impact in the northern lowlands. We show that the impact could have been on the opposite side of the planet. A sufficiently large impact can melt the mantle to such extent that upon isostatic adjustment and crystallization of the melted part of the mantle it forms a region of high standing topography (a megadome) instead of a crater. A topographic low forms antipodal to the impact (a megabasin).