Phase diagram of Fe-C-S ternary system under planetary core conditions

High-pressure, high-temperature experiments were conducted to investigate melting relations and phase assemblages in the Fe-C-S ternary system at 5 and 15 GPa, covering a temperature range of 1300–1900 K, conditions directly relevant to the Moon’s and Mercury’s cores. At 1300 K, the system is primarily governed by Fe-S eutectic melting, exhibiting notable complexity in the carbon-rich and sulfur-poor regions. With increasing temperature, the phase diagram simplifies: at 5 GPa and 1700 K, the Fe-Fe₃C-FeS system features three regions (Fe+L, C + L, and L). Similar phase assemblages are observed at 15 GPa, with Fe7C3 and diamond replacing Fe3C and graphite, respectively. Extensive Fe+L, C + L, and L regions are observed at 1900 K.
For a Moon’s core composed of a Fe-C-S alloy, nearly pure Fe is the only viable inner core phase above 1700 K. Below this temperature, both Fe and Fe₃C are potential solid inner core phases, depending on carbon content; a two-phase solid inner core is also theoretically possible. The inferred compositions of the outer core suggest densities of 6200–7300 kg/m³, with tighter constraints for models featuring an Fe₃C core.
At Mercury-relevant pressures, either Fe or Fe₇C₃ may form the solid inner core, again depending on carbon content. If the inner core is nearly pure Fe, the liquid outer core density ranges from 7300 to 7900 kg/m³. In both scenarios, a “snow” regime is plausible, though with distinct settling times. The ternary phase diagram indicates that Mercury is likely to develop a structurally layered inner core during secular cooling.