Constraints on Mercury's Si, C, S-bearing Core from Impedance Spectroscopy and Partitioning Experiments at High Pressure

The chemical and physical properties of Mercury's metallic core are fundamental to understanding the thermal and magnetic history of the planet. For instance, the sustainability of the dynamo depends on the core chemistry, the growth of the inner core, and the physical properties of the liquid outer core. Here, we report results from high-pressure, high-temperature experiments on analogs of Mercury's deep interior. The composition of the outer core was investigated by focusing on chemical transport between core and mantle analogs, specifically, reduced silicates and metals containing varying amounts of Fe, Si, Ni, C, and S (Pommier, GRL, 2025). These experiments employed samples with a layered structure in a multi-anvil press at 5 GPa and up to 1973 K, and were monitored using impedance spectroscopy. Electrical results and chemical (electron microscopy) analyses of the retrieved metal+silicate couples support an outer core that incorporates significant amounts of alloying agents (>20 at.% Si, Ni, C, S). Electrical measurements were then performed on relevant Fe-Ni-Si-C/S liquids at 5 GPa and up to 2123 K. The electrical resistivity values were used to estimate the thermal conductivity of the outer core, ranging from 17 to 31 W.m^-1.K^-1. These low values in comparison with the thermal conductivities of Fe-Si liquids could increase the power available to the dynamo during core cooling. More recently, chemical constraints determined from the preceding experiments were used to synthesize new core analogs. These materials are used in phase equilibria experiments to probe partitioning of light elements between the outer and inner core, performed in the multi-anvil press at 5–15 GPa and up to 1973 K. Taken together, these results provide new constraints on the compositional and density differences between the outer and inner core, as well as the power available to the dynamo. The connection of the findings with various scenarios for planetary cooling will be discussed.