Mercury's thermal evolution controlled by an insulating liquid outermost core?

Abstract

The weak intrinsic magnetic field of Mercury is intimately tied to the structure and cooling history of its metallic core. Recent constraints about the planet's internal structure suggest the presence of a FeS layer overlying a silicon-bearing core. We performed 4-electrode resistivity experiments on core analogues up to 10 GPa and over wide temperature ranges in order to investigate the insulating properties of core materials. Our results show that the FeS layer is liquid and insulating, and that the electrical resistivity of a miscible Fe-Si(-S) core is comparable to the one of an immiscible Fe-S, Fe-Si core. The difference in electrical resistivity between the FeS-rich layer and the underlying Fe-Si(-S) core is at least 1 log unit at pressure and temperature conditions relevant to Mercury's interior. Estimates of the lower bound of thermal conductivity for FeS and Fe-Si(-S) materials are calculated using the Wiedemann-Franz law. A thick (>40 km) FeS-rich shell is expected to maintain high temperatures across the core, and if temperature in this layer departs from an adiabat, then this might affect the core cooling rate. The presence of a liquid and insulating shell is not inconsistent with a thermally stratified core in Mercury and is likely to impact the generation and sustainability of a magnetic field.