Measurements of Liquid FeS Density Under Martian Core Conditions: Constraint on Core Density Profile

Abstract

The observed large density deficit of the Martian core indicates the presence of large amounts of light elements, and sulfur has been believed to be the dominant one. However, the equation of state (EoS) for liquid Fe‐S has not been well constrained. Earlier experiments examined the liquid Fe‐S density below 8 GPa, much lower than the Mars' core pressure range. Here, the density of liquid FeS was determined based on synchrotron X‐ray diffraction (XRD) measurements at 25–55 GPa and 2,320–3,260 K in a laser‐heated diamond‐anvil cell. We employed a recently developed analytical method that enables density derivation from diffuse scattering signals in XRD data observed in a limited range of diffraction angles. The EoS of liquid FeS obtained in the present experiments exhibits higher compressibility and smaller thermal expansivity under Martian core conditions than the extrapolations of earlier experimental results. We calculated the density of liquid Fe‐S with a composition between Fe and FeS from those of these end‐member liquids considering non‐ideal mixing. When sulfur is a single light element, the observed mean core density requires 17–24 wt% S and 23–34 wt% S for two distinct thermal structures, hot‐ and cold‐core models, respectively.