Abstract DGP2026-100

The bulk composition of Mercury is unknown. Yet, most evidence points towards an enstatite-chondrite-like composition. Silicon was strongly partitioned into the core during its formation in highly reducing conditions. Depending on the concentration of silicon left in the mantle, the latter could be forsterite- or enstatite-dominated [1]. Since these minerals have markedly different lattice thermal conductivities, they can drive substantially different planetary evolutions.

To assess how variations in thermal conductivity affect Mercury’s global evolution, and to evaluate whether such effects can be used to constrain the composition of the mantle, we used our 1-D parameterized mantle convection model TEMPURA [2]. We coupled this model to a core evolution model that allows for inner core nucleation and for the formation of a stable, thermally stratified layer [3]. 

We considered a mantle with a fixed diopside content, and varied the relative proportions of forsterite and enstatite. For each mantle composition, we used depth dependent elastic and transport properties deduced from equations of state and recent experimental studies [4,5]. We assumed an Fe-Si core. Given the initial temperature profile and mantle composition, we used appropriate equations of state for mantle minerals and liquid Fe-Si alloy to solve for the interior structure of the planet and determine the amount of silicon in the core required to match the planet mass. The core properties are then calculated accordingly during the evolution. We ran a large set of Monte Carlo simulations, varying the mantle reference viscosity, the forsterite-to-enstatite ratio, the enrichment factor of heat-producing elements in the crust, and the initial temperatures of the mantle and the core-mantle boundary. 

To be considered successful, the simulations must satisfy three main observational constraints: 1) a final crust thickness between 15 and 60 km [6]; 2) an early start of global contraction (before 500 Myr) [7]; 3) and a core able to drive a dynamo at 900 Myrs [8] and today. We deliberately leave out constraints on the magnitude of radial contraction given current uncertainties surrounding its exact value. 

Most successful models have a high enstatite fraction and a high crustal enrichment factor. Additionally, models with high mantle reference viscosity (10^22 Pa s at 1600 K) are clearly favored, which is consistent with an enstatite-dominated mineralogy. Overall, these results indicate that Mercury’s thermal and magnetic evolution is sensitive to mantle mineralogy and may provide independent constraints on the forsterite–enstatite ratio of its mantle.

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