Adiabatic heat flow in Mercury's core from electrical resistivity measurements of liquid Fe-8.5 wt%Si to 24 GPa

Abstract

The effect of the core thermal conductivity on the heat flow along the adiabat is investigated using direct measurements of electrical resistivity of Fe8.5Si at pressures from 5-24 GPa and temperatures above melting. Unexpected behaviour at low temperatures between 6-8 GPa may indicate an undocumented phase transition. Measurements of electrical resistivity at melting seem to remain constant at 127 μΩ⋅cm from 10-24 GPa, on both the solid and liquid side of the melting boundary. The adiabatic heat flow at the core side of Mercury's core-mantle boundary is estimated between 21.8-29.5 mWm−2, considerably higher than most models of an Fe-S or Fe-Si core yet similar to models of an Fe core. Comparing these results with thermal evolution models suggests that Mercury's dynamo remained thermally driven up to 0.08-0.22 Gyr, at which point the core became sub-adiabatic and stimulated a change from dominant thermal convection to dominant chemical convection arising from the growth of an inner core.