Describing the Global Gravity Field of Mars With Lithospheric Flexure and Deep Mantle Flow

Abstract

The volcanic complex Tharsis Region on Mars is known for its numerous volcanoes on top of the crust, elevated topography (doming), and a long-wavelength gravity anomaly correlated with the region. Flexural modeling of the lithosphere has commonly been used to understand the relationship between observed topography, crustal structure, and gravity, but no conclusive answers have been obtained due to the ambiguity of these models. NASA's InSight mission has brought new information about the Martian lithosphere, which warrants a reanalysis of the support of the Tharsis Region. After analyzing the topography and gravity data, we found that a thin shell model of Mars matches both the observed gravity field for spherical harmonic degrees higher than 8 and the crustal thickness at Elysium determined by the InSight mission. Our thin shell flexure model uses an average crustal thickness of 55 km, crustal density of 3,050 kg/, average mantle density of 3,750 kg/, and an elastic thickness () of 100 km. The mismatch between modeled and observed gravity field for the long-wavelengths (between degrees) correlates with the Tharsis Region, suggesting active large-scale dynamic support of the volcanic region. After modeling this dynamic support, we concluded that a substantial negative mass anomaly (hot buoyant mantle material, or depleted mantle region) in the mid mantle underneath the Tharsis Rise can explain the long-wavelength gravity residual. The remaining short-scale gravity residual gives insight to the Martian crustal density distribution and seems to correlate with geological structures of Mars. Buried mass anomalies in the subsurface of the northern polar plains seem not to be related to any geological or surface expressions, suggesting a more complex geology of the northern Martian crust than is suggested by the surface topography.