Lécaille, Manon ; Tosi, Nicola ; Rivoldini, Attilio, ; Baumeister, Philipp ; Namur, Olivier ; Charlier, Bernard

Talk presented at Mercury 2026 – An International Meeting on Planet Mercury, Leuven, Belgium on 2026-09-04

Abstract: Reconciling volcanic, tectonic and magnetic observations available for Mercury within a self-consistent thermal evolution model remains a significant challenge. A major source of uncertainty arises from the poorly constrained compositions of the core and mantle. Although there is evidence that silicon is the dominant light element in the core, experimental data is missing to clearly assess its influence on thermophysical parameters at high pressures. While the silicate mantle is thought to mainly contain Mg-endmembers of olivine and pyroxenes, their relative proportions are debated. In this study, we aim at refining Mercury’s thermal evolution model (from 4.5 Ga to present) by running a comprehensive set of Monte-Carlo simulations. These simulations are subsequently filtered using observational constraints on the timing, rate and amplitude of radial contraction, the period of dynamo generation, the crust production rate, and the final crust thickness. Given the uncertainties surrounding these constraints, we first evaluate the influence of each constraint individually before combining them to identify representative evolution models that best align with observations. For the mantle, we use the 1D parameterized thermal evolution code TEMPURA [1], which we expanded by incorporating pressure- and temperature-dependent material properties of potential forming minerals, i.e. forsterite, enstatite and diopside. This model is coupled to a 1D parameterized core evolution model [2]. The silicon concentration of the core is computed to match the bulk planet density, based on the model’s initial state and assumed mantle composition. All core properties are then calculated self-consistently from equations of state. We vary the initial temperature profiles of the planet, the mantle proportions of enstatite and forsterite, its reference viscosity, the crustal enrichment factor of radiogenic elements, and the radius of the core. Fundamental to the core evolution are its melting entropy and thermal conductivity. Since both quantities are not well constrained, we also allow for variations around the values predicted from the equations of state. Evaluating the impact of observational constraints sequentially revealed a clear dichotomy in terms of reference viscosity, between constraints that favor a rapid and efficient cooling of the planet (i.e., rapid contraction, rapid crust production and ancient dynamo generation) and those that necessitate the interior to remain warm (i.e., present-day dynamo generation and final crust thickness). Upon applying the most robust constraints (crust thickness, ancient and current dynamo generation, early contraction and crust production), we found that ~1% of the models satisfy them all at once. They are characterized by low mantle reference viscosities (< 10^20 Pa s), high crustal enrichment factors, and initial CMB temperature between 1900 and 2100K. Successful models have a large inner core (1300 – 1600 km radius), a thin upper core stable layer (< 300 km), a thick crust (30 – 60 km thick), and significant radial contraction (8 – 11 km). The choice of constraints can favor very different core and mantle properties, and lead to markedly different thermal evolution models. Only a small number of models satisfy the most robust constraints, highlighting the difficulty to reconcile Mercury’s magmatic, magnetic and tectonic evolutions. References: [1] Baumeister P, Tosi N, Brachmann C, Grenfell JL, Noack L (2023) Redox state and interior structure control on the long-term habitability of stagnant-lid planets. Astronomy & Astrophysics 675: A122. [2] Davies CJ, Pommier A, Greenwood S, Wilson A (2024) Thermal and magnetic evolution of Mercury with a layered Fe-Si(-S) core. Earth and Planetary Science Letters 641: 118812

Funding: 3PORDPLANINT/3PORDPLANINT/3PORDPLANINT