Structure and transport properties of FeS at planetary core conditions

The thermal conductivity of iron and its alloys are critically important to understand conductive heat flow and dynamo action within planetary cores, however the effect of sulfur alloying is poorly understood. We have measured and computed the thermal conductivity of FeS at high pressures and temperatures using experimental techniques and first-principles calculations. Experimental conditions range from 19-116 GPa and up to 3000 K. Computations ranged from 20-150 GPa and up to 4000 K. Over this range of conditions, theory shows that FeS is in a low to intermediate spin state with finite moments at least up to 40 GPa. We obtain thermal conductivity κ from 15 W m⁻¹ K⁻¹ at 1000 K to 69 W m⁻¹ K⁻¹ at 4000 K from first-principles calculations, and values of 14(5)-20(10) W/m/K from experimental measurements at temperatures above 1500 K and high pressures. In both cases the effect of structure and pressure is small. We find that FeS is metallic, but a poor metal at the conditions investigated. As a result, sulfur-rich core compositions are compatible with available observational constraints on the cessation time of the Martian dynamo.