Limited Influence of Hydrogen on the Sound Velocity of the Martian Core: Constraints From First-Principles Molecular Dynamics Simulations of Fe-S-H Liquids

Abstract

Recent seismic observations from the InSight mission have provided new constraints to the structure, density, and sound velocity of the martian core. Despite these advancements, the precise compositional makeup of the martian core remains largely uncertain, partly due to the poorly constrained equations of state for Fe-light element alloying liquids. Here we performed first-principles molecular dynamics simulations of Fe-S and Fe-S-H liquids under pressures of 16–58 GPa and temperatures of 1,700–3,200 K, covering the martian core conditions. The effects of hydrogen on the density and sound velocity of Fe-S liquids were investigated based on the calculated pressure-density-temperature data. Our results show that the calculated density of an Fe-S-H core can match that of the martian core, depending on the core sulfur and hydrogen contents and the seismic model used, but the corresponding sound velocity is always lower than the seismically observed P-wave velocity of the core. This implies that an additional light element, likely carbon, that can elevate the sound velocity of Fe-S liquids, must be present in the martian core.