On the support of the Tharsis Rise on Mars

The most significant long-wavelength geoid and topographic anomalies on Mars are associated with the Tharsis Rise [Smith et al., Science 286 (1999) 94–97; Smith et al., Science 284 (1999) 1495–1503]. However, the origin of the elevated geoid and topography in the Tharsis region remains unresolved between two competing models. In the first model the Tharsis Rise is dynamically supported by a thermal plume in the mantle [Kiefer and Hager, LPI Tech. Rep. 89-04 (1989) 48–50; Harder and Christensen, Nature 380 (1996) 507–509; Breuer et al., J. Geophys. Res. 101 (1996) 7531–7542], while the second model attributes the Tharsis anomalies to volcanic construction and its associated lithospheric flexural effects [Turcotte et al., J. Geophys. Res. 86 (1981) 3951–3959; Phillips et al., Science 291 (2001) 2587–2591]. Here we resolve this ambiguity by modeling the ratio of gravity and topography at long wavelengths with a new loading formulation that simultaneously considers plume buoyancy and surface loads. We demonstrate that a thermal plume contributes no more than 15% to the geoid and 25% to the topography for the Tharsis Rise for the range of lithospheric thicknesses that are constrained by observations. This indicates that the Tharsis Rise is best explained by volcanic construction on lithosphere.