The thermo-chemical evolution of Mars with a compositionally stratified mantle

To better constrain the evolution of Mars, the InSight mission has recently landed on its surface to record its seismic activity, surface heat flow, magnetic field, and to refine the current estimates of Mars' response to tidally induced gravitational changes. Combining such data with geodynamic considerations provides a way to better understand and to constrain the thermo-chemical history and the present-day structure of Mars.

Despite our relatively poor knowledge of Mars' early history, several indications suggest that its mantle went through a global magma ocean stage. The crystallization and fractionation of such a magma ocean is likely to have led to the presence of a compositionally distinct material at the bottom of the mantle. Such a layer would have been heavily enriched in iron and Heat-Producing Elements (HPE) - with either a homogeneous or a depth-dependent enrichment - with respect to the overlying mantle. The significant iron enrichment may have led to a strongly stratified mantle with a flat interface.

Using a parameterized convection approach, we modeled the thermochemical evolution of Mars' main envelopes: a liquid convecting core, a dense silicate layer enriched in HPE and sitting atop of the core-mantle boundary, overlaid by a less dense and more depleted silicate mantle, convecting under a stagnant lithospheric lid. The latter includes a crust building up with time and enriched in HPE with respect to the underlying silicate mantle. The dense layer is assumed to be convecting separately if its composition is homogeneous, or purely diffusing heat if its iron enrichment decreases with depth (stable stratification). Our efficient approach allows exploring a wide range of parameter space including the dense layer thickness, mantle rheological parameters, initial thermal state and core size. For each case considered, we predict the obtained present-day thermal structure, heat flux, crustal thickness, degree two Love number k 2 in order to compare with available and upcoming observations from the InSight mission. This will allow interpreting InSight data to place constraints on the thermal evolution of Mars, but also on the initial conditions as a function of planetary accretion and differentiation.