Abstract Carbon has been proposed as a major component in the Martian core alongside sulfur for its siderophile behavior during core‐mantle segregation. However, the core C content remains poorly constrained, due to uncertainties in both seismically observed core properties and the equation of state of C‐bearing Fe‐rich liquids. Here we conducted first‐principles molecular dynamics simulations to investigate the equation of state and sound velocity of Fe–S–C liquids under pressures of 10–55 GPa and temperatures of 1,700–3,200 K, conditions relevant to the Martian core. Our results show that the presence of C increases the sound velocity of Fe–S liquids, in contrast to what is observed for other light elements such as S, O, and H. Regardless of the particular seismic model used for the Martian core, we find that about 4.3 ± 1.5 wt% C is required to reproduce the velocity of the core, confirming its role as a major light element.

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