From Finite Element Mantle Model to Martian Gravity Field - Determining dynamic topography and gravity rates by mantle plume under the Tharsis Rise on Mars.

Abstract

The Tharsis Region is a large volcanic regions spanning a sixth of the Martian sur face. It has a large influence on the long wavelength gravity field and topography of Mars. The leading theory on the origin of the volcanic region is a combination of both isostatic flexure of a thickened crust and a small contribution due to a large superplume residing in the upper mantle. The isostatic balance, on which previous studies have relied, does not adequately explain the long-wave length gravity field spectra. Our current understanding of the Martian mantle structures is insufficient to explain the observed gravity field. Using mantle dynamic models we explore the effect that an actively rising mass anomaly like a superplume has on the gravity field over time and if this signature would be measurable in gravity missions to come. We ran a series of instantaneous axis-symmetric finite element models of Mars with varying plume and structural parameters constrained by Nasa’s InSight mission. We ran the model for a time-step of 50 years, simulating the duration of future satellite data acquisition. The deformation in the model allows us to calculate the change in dynamic topography and the gravity anomaly resulting from the plume. Our results show dynamic topography rates of a few centimetres per year and gravity rates in the order of 0.1-1 µGal per year. We also compared different numerical profiles used to describe the internal structure of Mars (based on both pre- and post Insight constraints) to show the importance of the radial viscosity profile. Our findings agree with the current understanding of dynamic topography for a hot upwelling plume but show the importance of the viscosity distribution when looking at dynamic topography and gravity anomaly rates. The analysis of the three different numerical profiles showed that should the Martian mantle be highly viscous than the gravity anomaly of a mantle plume might not be measurable unless we measure for 10-50 years with high accuracy and precision. This study shows that the prolonged monitoring of the Martian gravity field could supply evidence for ongoing mantle activity in the Thar sis region and could even help us constrain the radial viscosity profile. Further research on the best-fitting plume and tests within elasto-viscous models is needed to give an estimate for the true size of a the gravity anomaly rates we expect for the Tharis region.