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

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. Plain Language Summary Understanding the composition of the martian core is important for unraveling the planet's formation and its subsequent evolution processes and for deciphering the mechanisms for the generation and extinction of its early dynamo. Recent seismic observations from the InSight mission provided new information on the density and seismic wave velocity of the martian core. The relatively low density of the core compared to pure liquid iron under martian core conditions suggests that a considerable amount of light elements are present in the molten iron core. These light elements likely include sulfur, oxygen, carbon, and hydrogen. However, the precise composition of the martian core remains elusive, partly due to the poor understanding of the physical properties of molten iron‐alloys under martian core conditions. In this study, we focused on sulfur and hydrogen as the potential light elements in the martian core and numerically computed the density and sound velocity of iron‐sulfur‐hydrogen alloying liquids under high‐pressure and high‐temperature conditions. We found that the sound velocity of an iron‐sulfur‐hydrogen core is always lower than the seismically recorded velocity for the martian core. This implies that an additional light element capable of elevating the sound velocity, likely carbon, is present in the mar