Describing the Martian gravity field through lithospheric flexure and deep mantle flow modelling

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.

Analyzing the topography and gravity data, we found that a thin shell model of can model the lithosphere of Mars that 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 predicts an average crustal thickness of 55 km, crustal density of 3050 kg/m³, average mantle density of 3750 kg/m³, and an elastic thickness (T_e) of 100 km.

The remaining mismatch between modeled and observed gravity field for the long-wavelengths (between n=2-8 degrees) correlates with the Tharsis Region, suggesting active large-scale dynamic support of the volcanic region. We devise a fast, parameterised modelling approach to calculate the mantle flow and resulting gravity effect of such a mass anomaly. We have explored the multi-variable parameters space and run approximately 500.000 models in which we compare to the geophysical parameters.The optimal models show that there is a substantial negative mass anomaly (hot buoyant mantle material) in the mid mantle underneath the Tharsis Rise. The anomaly seems to have a flat disk shape, with a flatness of 0.2 (thickness over radius). Due to the ambiguity of the gravity data, we computed different sets of depth, size, and anomalous mass (all lighter density) for the mass anomaly underneath Mars.

We used remaining short-scale gravity residuals to derive an 3D Martian crustal density distribution. We are currently correlating these density distribution with geological events in Mars histiory. 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.