Evidence for a Martian D"-like layer

Abstract Seismic recordings made as part of the InSight mission (Banerdt et al., 2020) have shown that Mars’s liquid core is ~27% lighter than pure liquid iron (Irving et al., 2022), implying that it contains considerable quantities of light elements. However, geophysically-constrained core compositions require abundances of the volatile elements H, C, and S that are too high with respect to their cosmochemical availability in the potential building blocks that accreted to form Mars (Khan et al., 2022). Here we show that P-waves diffracted along and traversing the core-mantle boundary of Mars in combination with first principles computations of the thermoelastic properties of liquid Fe-rich alloys (Huang et al., 2022) require the presence of a D’’-like molten silicate layer (Samuel et al., 2021) overlying the liquid core of Mars. To match the elastic properties of the core, this layer must be 145+/-25 km thick, causing a reduction in core radius to 1690+/-50 km and an increase in density to 6.65+/-0.15 g/ccm relative to previous models (Staehler et al., 2021, Duran et al., 2022, Khan et al., 2022, Drilleau et al., 2022, Irving et al., 2022). These new bulk core properties are able to reconcile geophysical- and cosmochemical requirements for the light-element content of the Martian core, which is shown to consist of 86--89 wt% Fe-Ni and 11--14 wt% light elements (S, C, O, and H). The chemical characteristics of such a layer may be revealed by products of Martian magmatism.