The Thermo-Chemical Evolution of Mars With a Strongly Stratified Mantle

Kurzfassung

The Martian mantle probably experienced an early global magma ocean stage. The crystallization and the fractionation and overturn of such a magma ocean likely led to the formation of a compositionally distinct layer at the bottom of the mantle. This layer would have been heavily enriched in iron and in heat-producing elements (HPE). The significant iron enrichment can lead to long-term stability with little mixing between the layer and the overlying mantle. We studied the influence of such an enriched basal layer on the thermal and chemical evolution of the Martian mantle using both 2-D finite-volume modeling at mantle scale, and a parameterized convection approach at the entire planetary scale. The basal layer is most likely stably stratified because of its moderate thickness and/or its gradual enrichment in iron with depth that prevents the development of convection in this region. We explored a wide parameter space in our parameterized models, including the layer thickness and the mantle rheology. We show that the presence of an enriched basal layer has a dramatic influence on the thermo-chemical evolution of Mars, strongly delaying deep cooling, and significantly affecting nearly all present-day characteristics of the planet (heat flux, thermal state, crustal and lithospheric thickness, Love number and tidal dissipation). In particular, the enrichment of the layer in iron and HPE generates large volumes of stable melt near the core-mantle boundary. Due to their intrinsic low viscosity and seismic velocities, these regions of silicate melt could be erroneously interpreted as core material.