Thermal and Dynamo Evolution of the Lunar Core Based on the Transport Properties of Fe‐S‐P Alloys.

Paleomagnetic analyses have suggested that the lunar magnetic field underwent a significant change from 4.25 to 3.19 Ga, indicating the rapid transition of the lunar dynamo mechanism. We used the van der Pauw (vdP) method to measure the electrical resistivity of Fe‐S‐P alloys under conditions relevant to the lunar core and estimated the thermal conductivity of the Fe‐S‐P lunar core. These values were incorporated into thermal and dynamo models to investigate the evolution of the lunar core. Our model indicates that the inner core began to grow as early as 4.35 Ga, the solidification regime switched at 3.50 Ga, and the thermal dynamo ceased between 3.78 and 3.51 Ga. The cessation of the dynamo could be due to a low buoyancy flux and insufficient entropy dissipation. Thermal and compositional dynamos cannot sustain the ancient strength of the Moon's magnetic field, and require other energy sources. Plain Language Summary: The transport properties of planetary core materials dominate heat transfer through thermal convection, which is closely linked to the geodynamo of the planetary core. The van der Pauw (vdP) method was used to measure the electrical resistivity of Fe‐S‐P alloy, and we assessed the effects of pressure, temperature, and light elements (sulfur and phosphorus) on the electrical resistivity of these alloys. We estimated the thermal conductivity of the Fe‐S‐P lunar core and modeled the growth of the lunar core. We found that the inner core nucleated at 4.35 Ga in the center and then solidified at the core‐mantle boundary (CMB) at 3.50 Ga. The adiabatic heat conduction at the lunar CMB indicated that a thermally driven dynamo persisted until 3.78–3.51 Ga in the Moon's history. We discuss the cessation of the lunar core dynamo in relation to the buoyancy flux, entropy, Reynolds number and Elsasser number and explain the variation in the magnetic field in conjunction with the previous dynamo mechanism. Key Points: The electrical resistivity and thermal conductivity of Fe‐S‐P alloys were determined at 3 and 5 GPaA low thermal conductivity Fe‐S‐P core nucleated earlier and the bottom‐up growth regime switched to a top‐down regime at 3.50 GaMultiple dynamo mechanisms worked together to generate a high magnetic field, low buoyancy flux and entropy dissipation causing the dynamo to cease [ABSTRACT FROM AUTHOR]