The Role of the Nucleation Barrier on Lunar Geodynamo Evolution

Abstract:

The Moon’s particular magnetic field history solicits a variety of mechanisms that may have powered the long-lasting geodynamo. Thermal and structural evolution modeling are needed to understand the link between metallic core crystallization and the production of magnetic fields, along with the impact of the nucleation energy barrier on this relation. The undercooling (cooling below the melting temperature) for the Moon’s iron core to completely supersaturate is in the order of 20 degrees. However, the critical supercooling to homogeneously nucleate solids nears ~400 K, requiring the presence of metallic substrates to lower the nucleation barrier in order to initiate solidification. Experimental work on the equation-of-state and melting behavior of Fe–S alloys has previously established that depending on core composition, the Moon’s core may thermodynamically favor either top-down or bottom-up solidification. Thus, the combination of the Moon’s relatively small size and range of possible crystallization scenarios presents a crucial opportunity to understand these processes. We use the Burnman Python library to build a self-consistent model of the lunar core for a range of possible structures and compositions. We simulate lunar core crystallization, subject to the nucleation barrier, over a broad range of compositions in order to understand the implication for core evolution. We present the scenarios and basic implications of nucleation for the core’s inner growth, and consequently the Moon’s geodynamo for different nucleation and growth regimes. This work provides a framework to investigate how the nucleation barrier affects the evolution of metallic cores in all rocky bodies.