Abstract | The discoveries of the MESSENGER mission to Mercury have revealed puzzling evidence regarding Mercury’s thermal evolution. The highly reduced mantle chemistry (suggested by the surface composition) combined with geodetic data point to a thin mantle, a large core, and a core composition only ~5-15% less dense than pure liquid iron, indicating a light-element-poor core alloy. These characteristics should have led to rapid cooling, an early transition to sluggish or absent mantle convection, and substantial core solidification. Instead, estimates of global contraction suggest low to moderate secular cooling; relatively young pyroclastic deposits suggest long-lived mantle convection and (locally) supersolidus mantle temperatures; the low-order shape of Mercury’s magnetic field suggests a small inner core. High mantle radiogenic heating can explain these features to some extent but must be reconciled with evidence that the planet and core began cooling early (e.g., the onset of global contraction in the Calorian Era, and crustal remanent magnetization indicating a long-lived early dynamo).
We evaluate the possibility that Mercury’s highly reducing chemistry led to a stratified and unevenly heated mantle, which influenced its thermal evolution. Mercury’s surface composition suggests a sulfide-bearing, reduced mantle. These sulfides should be (1) abundant, up to 20 wt.% of the mantle, (2) low-density, ~15-20% lighter than the silicates, and (3) strongly heated, if radiogenic elements are chalcophile at Mercury magma ocean conditions. The combination of these features implies that the distribution of radiogenic heating would differ from what is predicted for a pure silicate mantle. Furthermore, initial heterogeneity could be stabilized against convective mixing by compositional stratification. Therefore, predictions from models that assume a homogenous mantle may not be capturing important thermal consequences of mantle heterogeneity. We model the thermal evolution of Mercury with a mantle that is layered in density and/or radiogenic heating, exploring a range of plausible initial mantle configurations. We evaluate the implications of that layering for Mercury’s history of volcanism, magnetic field generation, and secular cooling. |
