Thermodynamics of Fe-S-O-C-H liquids: Implications for the Martian core

Abstract

We present a comprehensive investigation into the composition of the Martian core. We use new density functional theory (DFT) calculations, which consider in detail the magnetic properties of the Fe alloys, essential for obtaining the correct density and velocities. We then fit our results to a new Gaussian Process Regression (GPR) equation of state (EOS). Using this GPR-EOS we search for all compositions of the Martian core in the system Fe-S-O-C-H that match both the density and compressional wave speeds of a liquid Martian core throughout its entire depth range. We consider different models for the interior of Mars – in particular, those with and without a deep melt layer at the base of the Martian mantle (Irving et al., 2023; Samuel et al., 2023; Khan et al., 2023). The existence of a deep melt layer is important as it revises previous estimates for the core's size and density. We consider a range of core-mantle boundary temperatures from 1900 K to 2850 K, although we find that temperature has a relatively small effect on the possible compositions. As with previous studies, and also for the Earth, we find many different compositions that are able to match the geophysical observations of Mars' core density (ρ) and velocity ($V_{\varphi }$), regardless of which set of geophysical observations are used. All models require very high concentrations of light-elements of ≈ 50 mol% in line with previous work. The compositional variation can be reduced considerably by considering cosmochemical constraints on the maximum amount of sulphur, together with geochemical constraints on the partitioning. In this case, all solutions require very high hydrogen content of at least 0.5 wt% (27 mol%) and practically no oxygen.