Thermal and magnetic evolution of Mercury with a layered Fe-Si(-S) core

Abstract

Elucidating the structure and composition of Mercury is important for understanding its interior dynamics and evolution. The planet is characterised by unusual chemical characteristics and a weak magnetic field generated in a large metallic core, and its early evolution was also marked by the presence of a magnetic field, widespread volcanism and global contraction. Here we develop a parameterised model of coupled core-mantle thermal and magnetic evolution considering a layered Fe-Si(-S) core structure with chemical and physical properties of the mantle and the core based on previous laboratory studies. We seek successful solutions that are consistent with observations of Mercury's long-lived dynamo, total global contraction, present-day crustal thickness, and present-day interior structure. Successful solutions have a mantle reference viscosity $>{10}^{21}$ Pa s (corresponding to a present-day bulk mantle viscosity $>2\times {10}^{20}$ Pa s), a silicon concentration in the core >13 wt%, a present inner core radius of $\sim 1000-1200$ km and a thermally stable layer ∼ $500-800$ km thick below the core-mantle boundary. Our results show that if present, a molten FeS layer atop the core has minimal effect on Mercury's long-term thermal and magnetic evolution. Predictions from our models can be tested with upcoming Bepi-Colombo observations.