Core-Mantle Evolution of a Reduced Mercury

Mercury likely formed under highly reducing conditions. In such an environment, silicon behaves as a siderophile element and partitions into the metallic phase during differentiation, leading to the formation of an Fe–Si-rich core (Steenstra and Van Westrenen, 2020). At the same time, the silicate mantle is expected to differ substantially from the olivine-dominated mantles of Earth, Venus, and Mars. Experimental petrology and geochemical studies suggest that Mercury’s mantle may contain significant pyroxene-rich rock assemblages (Cioria et al., 2024; Boujibar et al., 2025), with major implications for mantle viscosity, heat transport, and the long-term thermal evolution and coupling of the mantle-core system.

Here, we investigate the coupled thermal and structural evolution of a chemically reduced Mercury, focusing on the combined influence of an Fe-Si core, pyroxene-rich mantle rheology, radiogenic heat production, and present-day geodetic constraints. To this end, we developed a coupled mantle-core evolution model in which the thermal state of the planet is governed by the time evolution of the core-mantle boundary temperature through a global energy balance including mantle radiogenic heating, surface heat loss, core sensible heat, and the latent and gravitational energy released during inner-core crystallization. The mantle is modelled as a conductive or stagnant-lid convective shell depending on the convective regime, with heat transport controlled by a temperature-dependent, non-Newtonian rheology for an olivine-pyroxene assemblage.

The core is modelled as an adiabatic Fe-Si alloy, with Fe–9Si adopted for the liquid outer core and Fe-3.8Si for the solid inner core, consistent with experimentally inferred silicon partitioning under reducing conditions (Edmund et al., 2022). At each timestep, the evolving core-mantle boundary temperature is used to reconstruct self-consistently the radial temperature, density, pressure, and gravity structure of the core. The equilibrium inner-core radius is determined from the intersection between the core adiabat and the Fe-Si melting curve, allowing the progressive crystallization history of the core to be tracked through time. The resulting internal density structure is then used to compute Mercury’s total mass, normalized polar moment of inertia, and forced longitudinal libration amplitude, enabling direct comparison with present-day geodetic observations.

To identify physically plausible evolutionary histories, we explored the parameter space through a Monte Carlo analysis of coupled mantle-core evolution models. The investigated parameters include the thickness of the silicate shell, the pyroxene abundance within the mantle, the scaling of mantle radiogenic heating, perturbations to mantle and core densities, and uncertainties in the Fe-Si melting relation governing inner-core crystallization. Each realization was integrated over 4.6 Ga and evaluated against present-day geodetic observables, including Mercury’s mass, normalized polar moment of inertia, and forced longitudinal libration amplitude.

Our simulations indicate that the developed coupled mantle–core evolution model can place quantitative constraints not only on the present-day internal structure of Mercury, including the thickness of the silicate shell and the size of the inner core, but also on the planet’s long-term thermal evolution. In particular, by combining coupled thermal evolution modelling with present-day geodetic constraints, the model provides insight into the timing of inner-core nucleation, the persistence of a long-lived liquid outer core, and the coupled thermochemical evolution of Mercury as a chemically reduced terrestrial planet.

 

Acknowledgements
G.M. and C.C. acknowledge support from the Italian Space Agency (2022-16-HH.1-2024).

 

References

Boujibar, A., Righter, K., Fontaine, E., Collinet, M., Lambart, S., Nittler, L. R., & Pando, K. M. (2025).  A Pyroxenite mantle on Mercury? Experimental insights from enstatite chondrite melting at pressures up to 5 GPa. Icarus, 116602.

Cioria, C., Mitri, G., Connolly, J. A. D., Perrillat, J.-P., & Saracino, F. (2024).  Mantle mineralogy of reduced sub‐Earths exoplanets and exo‐Mercuries.  JGR:Planets, 129.

Edmund, E., Morard, G., Baron, M. A., Rivoldini, A., Yokoo, S., Boccato, S., et al. (2022). The Fe‐FeSi phase diagram at Mercury’s core conditions. Nature Communications, 13(1), 387. https://doi.org/10.1038/s41467‐022‐27991‐9

Steenstra, E. S., & van Westrenen, W. (2020). Geochemical constraints on core‐mantle differentiation in Mercury and the aubrite parent body. Icarus, 340, 113621.