On the thermal evolution and magnetic field generation of planet Mercury

Heat transfer through convection in Mercury's large core may be limited to a liquid layer between a solid inner core and a stably stratified outer liquid layer. Convection in the thin mantle may even have entirely stopped. Here, we consider the transition from convective to conductive heat transport in a coupled thermal evolution model of the mantle and core and assess implications for the generation of the magnetic field. We argue that a conductive temperature profile best describes the temperature in regions of the core with a subadiabatic heat flux. Implementing an adiabat in these regions in a model of the evolution of the core, as is often done, implicitly assumes the existence of a mechanism that transports heat downward. Such a mechanism not only consumes power that could otherwise be available for sustaining dynamo action, but is also unlikely to be effective. We show that a thermally convective layer deep in Mercury's liquid core below a thermally stratified layer is more likely to persist until present if light elements depress the liquidus of the core by several hundred degree compared to iron. Substantial partitioning of light elements into the liquid core can drive strong compositional convection in the upper part of Mercury's core, but this may not be in line with dynamo studies that are consistent with the observed magnetic field. Therefore, thermal evolution scenarios with light elements in the core that depress the core liquidus significantly but do not strongly fractionate into the core liquid are the most consistent with the present-day core dynamo. Present-day dynamo action below a thermally stratified layer does not necessarily imply that the mantle is currently convective. If the mantle has a high concentration of radiogenic elements and a low viscosity, it must be convecting, but mantle convection can have ended before the present for a more viscous mantle with low concentration of radiogenic elements.