The Rise and Fall of the Martian Mantle: Effects of a Crustal Dichotomy on Mantle Convection

The origin of the crustal dichotomy on Mars remains an outstanding question in Martian evolution. The process that produced the dichotomy may have been endogenic, exogenic or some combination of both. Despite this uncertainty, it appears to have taken place early in Martian history, making the crustal dichotomy a long-lived feature. This implies that the topography of Mars along the crustal dichotomy must be in isostatic equilibrium. The absence of gravity anomalies along the dichotomy may indicate older, thick crust beneath the southern highlands and younger, thinner crust beneath the northern lowlands. Through a series of numerical modeling experiments, we investigate the impact of a large-scale crustal thickness variation on the planform of Martian mantle convection. Geoid and topography calculations are compared to recent Mars data for a constraint on our experiments. Using moderately high Rayleigh number calculations with no internal heating, our preliminary results indicate that an unrealistically large lithospheric thickness variation on the order of 300 km is necessary in order for small-scale convection to develop at the boundary. We are able to see large-scale effects from a dichotomy on mantle convection for a more realistic crustal thickness of 100-200 km. When an upwelling occurs over a thinner lithosphere, small-scale convection begins to develop at the boundary, as along as the crustal thickness variation remains large. Mantle-scale effects from a large dichotomy, as in this model, exist as well. Further investigations will include modifying the viscosity, internal heating and crustal thickness variations towards more realistic Martian values in order to determine the long-term effects of the crustal dichotomy on Martian large and small-scale mantle convection.