Magnetic field generation in the lunar core: The role of inner core growth

The source of the magnetic field recorded in the lunar crust remains an unresolved problem. The field was most likely produced by a self-sustaining dynamo in the Moon's electrically conducting metal core, but heat flux across the core-mantle boundary was probably insufficient to power a dynamo for the field's currently known duration from 4.2 to 3.56 Ga. Since seismic measurements indicate the existence of a solid iron inner core in addition to a still-liquid iron alloy outer core, inner core solidification and its associated thermochemically driven convection in the outer core could have been responsible for extending the dynamo's lifetime even in the absence of superadiabatic heat flux. Here we present a coupled mantle-core thermal evolution model of the Moon and show that core solidification could explain the onset and shutoff of the lunar dynamo consistent with the global magnetic field inferred from the paleomagnetic record.