The thermal state and chemistry of the metallic core of terrestrial planets and moons governs their evolution and in particular, the intrinsic magnetic field. Though it is accepted that Mars has a sulfur-rich metallic core, its chemical and physical state as well as its time-evolution are still unconstrained and debated. Several lines of evidence indicate that an internal magnetic field was once generated on Mars and that this field decayed around 3.7-4.0 Gyrs ago. The standard model assumes that this field was produced by a thermal (and perhaps chemical) dynamo operating in the Martian core. In Davies & Pommier (2018), we use this information to construct parameterized models of the Martian dynamo in order to place constraints on the thermochemical evolution of a Fe-S Martian core, with particular focus on its crystallization regime.  Models that match the available magnetic and geodetic constraints have a sulfur content of ≈ 10% and snow zones that occupy approximately the top 100 km of the present-day Martian core.

In Pommier et al. (2020), we present results from electrical experiments conducted at 10 GPa and up to 1850 K on high‐purity powder mixtures in the Fe‐S‐O(±Mg, ±Si) systems using the multianvil apparatus and the four‐electrode technique. The sample compositions contained 5 wt.% S, up to 3 wt.% O, up to 2 wt.% Mg, and up to 1 wt.% Si. We observe that above the eutectic temperature, electrical resistivity is significantly sensitive to the nature and amount of light elements. For each composition, thermal conductivity‐temperature equations were estimated using the experimental electrical results and a modified Wiedemann‐Franz law. These equations were implemented in a thermochemical core cooling model to study the evolution of the Martian dynamo. Modeling results suggest that bulk chemistry significantly affects the entropy available to power dynamo action during core cooling. The presence of oxygen would delay the dynamo cessation by up to 1 Gyr compared to an O‐free, Fe‐S core. Models with 3 wt% O can be reconciled with the inferred cessation time of the dynamo if the core‐mantle boundary heat flow falls from >2 TW to ~0.1 TW in the first 0.5 Gyr following core formation.

Support: This work has been funded by NSF-GEONERC.