Influence of intrinsic thermal conductivity of a stably stratified molten silicate layer above Mars's core : insights from mantle convection simulations

The presence of a molten Basal (BML) enriched in iron and in heat-producing elements (HPE) has been suggested just above the Martian core (Samuel et al., 2023; Khan et al., 2023). Such a BML largely affects interior thermal evolution in multiple ways, through the redistribution of HPE between the BML and the mantle, and the likely suppression of core convection. The mode of heat transport from and across the BML itself is also crucial to Mars's thermo-chemical evolution. This, however, is linked to the convective state within the BML, which is yet to be further constrained. In the case of a compositional stratification within the layer, the large amount of heat generated by the HPE-enriched BML can be transferred to the mantle above and the core below via conduction. If compositional stratification is weak or absent, vigorous convection of the liquid-state BML (compared to the timescale of solid-state mantle convection) would allow additional heat loss from this layer.

 

Here, we consider the scenario where the BML is the product of end-member fractional crystallisation of the initial global magma ocean, followed by the subsequent overturn of the iron- and HPE-enriched component (as described by e.g. Elkins-Tanton et al., 2003). Contrary to the less extreme equilibrium and intermediate crystallisation modes (e.g. Ballmer et al., 2017), this scenario results in a very strong and stable density stratification, strictly preventing the BML to convect (Samuel et al., 2021, 2023). Using the mantle convection code StagYY, we therefore assume in our models that conduction is the only mode of heat transport across the BML; as such, the intrinsic thermal conductivities of the BML and of the mantle are key parameters that may impact the long-term thermal evolution of Mars, while their influence has not yet been thoroughly explored. Varying the intrinsic thermal conductivity as a function of depth, temperature and composition, we report on its effect on observational diagnostics including, but not limited to, mantle temperature and crustal growth history. We further investigate the thermal exchange and feedback between the BML and the core, considering different thermal structures within the core. Our model results assuming a conductive BML and adiabatic core temperature profile are compared with those obtained in Samuel et al. (2023).