Coupled tidal tomography and thermal constraints for probing Mars viscosity profile

Abstract

Computing the tidal deformations of Mars, we explored various Mars spherically symmetric internal structures with different types of interface between the mantle and the liquid core. By assessing their compatibility with a diverse set of geophysical observations we show that despite the very short periods of excitation, tidal deformation is very efficient to constrain Mars interior. We calculated densities and thicknesses for Martian lithosphere, mantle, core–mantle boundary layers and core and found them consistent with preexisting results from other methods. We also estimated new viscosities for these layers. We demonstrated that the geodetic records associated with thermal constraints are very sensitive to the presence of a 2-layered interface on the top of the liquid core in deep Martian mantle. This interface is composed by 2 layers of similar densities but very different viscosity and rheology: the layer on the top of the core is liquid (Newtonian, NBL) and the one at the base of the mantle, overlaying the liquid one, is an Andrade layer (ABL) with a viscosity in average 10 orders of magnitude greater than the Newtonian layer. Our results also indicate that the existence of this 2-layered interface significantly impacts the viscosity profiles of the mantle and the lithosphere. More precisely, models including the 2-layered interface do not display significant viscosity contrast between the mantle and the lithosphere, preventing mechanical decoupling between a lithosphere and the mantle immediately below. Such models are in favor of a stagnant lid regime that can be supported by the current absence of an Earth-like plate tectonics on Mars. Finally, in our results, the presence of liquid Newtonian layer at the top of the liquid core is incompatible with the existence of a solid inner core.