Electrical Resistivity of Fe and Fe‐3 wt%P at 5 GPa With Implications for the Moon's Core Conductivity and Dynamo

Abstract

AbstractA high‐magnetic field once existed in the early history of the Moon, suggesting the core once possessed a thermally driven dynamo. The thermal conductivity of core materials is a significant parameter of the dynamo. The lunar core composition is thought to be iron or iron‐alloyed with some light elements (e.g., S, P, Si, and C), but its transport properties remain uncertain. We measured the electrical resistivity of iron and Fe‐3 wt%P alloys at 5 GPa and high temperatures. Apart from the quasi four‐point technique, the four‐probe van der Pauw technique was also employed to measure the resistivity of pure iron. Adding ∼3 wt% phosphorus to iron slightly increases the resistivity at 5 GPa and 1000–1500 K due to the impurity effect. The resistivity of Fe‐3 wt%P alloys increases at the onset of melting. Via the Wiedemann‐Franz law, the thermal conductivity at the lunar core‐mantle boundary (CMB) is estimated to be 28.6–34.2 Wm−1K−1 for a light‐element free core and 31.5 ± 1.9 Wm−1K−1 for a phosphorus‐bearing (∼3 wt% P) core. Therefore, small amounts of phosphorus in the lunar core slightly impact its thermal conductivity. The estimated conductive heat flow across the lunar CMB varies from 4.5 to 5.7 GW, and the adiabatic heat flux varies from 3.3 to 4.2 mW/m2, depending on the core's composition (Fe or Fe‐3 wt%P). Integrating our results with previous lunar core evolution models, we suggest that a thermally driven dynamo persisted until 3.63–3.88 Ga ago.