[S-CG48] Planetary interiors revealed by exploration, experiments, and simulations

[SCG48-P01] Structure and chemical composition of the core of Mars predicted from the phase relations of Fe-light-elements alloys

*Shunpei Yokoo¹ (1.The University of Tokyo)

The recent InSight mission revealed that Mars has a larger and lighter liquid core than previously estimated (Samuel et al., 2023; Khan et al., 2023). While sulfur is a widely accepted candidate for the light element in the Martian core, its small density suggests the presence of other light element(s), such as oxygen, carbon, and hydrogen in addition to sulfur. High-pressure experiments have revealed that adding these light elements to Fe-S alloy changes its phase relations from those of the Fe-S binary system. In the Fe-S-O system at < ~50 GPa, oxygen content in Fe-Fe₃S(FeS)-FeO eutectic liquid is small, and FeO crystallizes as a liquidus phase from a wide compositional range (Urakawa et al., 1987; Tsuno and Ohtani, 2009; Yokoo et al., 2019; Yokoo et al., 2024). Similarly, Fe₃C crystallizes from a Fe-S-O-C liquid whose carbon content is smaller than the Fe-Fe₃C eutectic carbon content (Yokoo et al., 2024). Fe-S-H liquid separates into S-rich and H-rich liquids depending on pressure-temperature conditions (Yokoo et al., 2022). Such liquid immiscibility in Fe-S-H liquid was observed at pressures higher than the maximum pressures of liquid immiscibility in Fe-S-O liquid and Fe-S-C liquid (Tsuno et al., 2007; Dasgupta et al., 2009).
In this poster, I review phase relations in the ternary or quaternary systems, including Fe and S, at the Martian core conditions. Those phase relations suggest different core structures for O-, C-, and H-rich core compositions from the previously expected Fe-S Martian core structures. A stratified structure formed by the crystallization of a solid inner core or the separation of a liquid core is key to estimating the core composition from seismological observations.