Mercury’s Early Sub-Adiabatic Core Based on Thermal Conductivity of Fe-8.5wt%Si

Abstract:

Recent theoretical studies have tried to constrain Mercury’s internal structure and composition using thermal evolution models. The presence of a thermally stratified layer of Fe-S at the top of an Fe-Si core has been suggested, which implies a sub-adiabatic heat flow on the core side of the CMB. In this work, the adiabatic heat flow (q_ad) at the top of the core was estimated using the electronic component of thermal conductivity (κ_el), a lower bound for thermal conductivity. Direct measurements of electrical resistivity (ρ) of Fe-8.5wt%Si at core conditions can be related to κ_el using the Wiedemann-Franz law. Measurements were carried out in a 3000 ton multi-anvil press using a 4-wire method. The integrity of the samples at high pressures and temperatures was confirmed with electron-microprobe analysis of quenched samples at various conditions. A possible phase transition occurs at low T between 6-8 GPa, however such transition is not found in the literature. The results suggest the heat flow on the core side of the CMB became, and remained, sub-adiabatic around 0.08-0.22 Gyr after formation.