Abstract Meteorites suggest the Martian core comprises iron (Fe) or iron‐nickel (Fe‐Ni) alloy with sulfur (S) identified as the primary light element. The InSight data revealed a larger, less dense Martian core than previously estimated, indicating additional light elements. Recent studies have considered hydrogen (H) as a light element candidate for the Martian core. However, the co‐alloying behaviors of S and H in Fe metal at the Martian core conditions are not well understood. This study investigates the influences of H and its amount on phase relations in the Fe‐S systems under the pressure‐temperature conditions relevant to the Mars core in laser‐heated diamond‐anvil cells. We found that hydrogen destabilizes . At low S and H contents upon melting, the Fe–S–H ternary produces H‐rich metallic liquid coexisting with S‐rich solid alloy phase. Stable solidus phase changes from to FeS to with an increase in content in the medium from 0.06 to 0.18 to 0.33 wt%. The cotectic line inferred from our data is located close to the S (13–15 wt%) and H (<2 wt%) contents considered geochemically plausible for Mars. The observation provides a compelling explanation for the possible difference in light element composition between the molten Martian outer core and the recently found solid inner core inferred from geophysical observations. This study sheds light on the interplay of S and H for Fe metal, offering important insights for possible core mineralogy for Mars and sub‐Earth rocky exoplanets across various scenarios for the H and S contents.
Wei et al. (2026) studied this question.