Core and Mantle Evolution of a Reduced Mercury

Mercury’s highly reducing formation environment has profoundly influenced its internal structure and long-term evolution, setting it apart from the other terrestrial planets. Unlike Earth, Venus, or Mars—characterized by olivine-rich mantles and sulfur-enriched metallic cores—Mercury likely possesses a silicon-enriched metallic core and a mantle rich in pyroxenes. These distinctive properties result from early planetary differentiation under extremely low oxygen fugacities, estimated to lie between 2.6 and 7.3 log units below the iron-wüstite (IW) buffer. Such conditions promote silicon partitioning into the core and stabilize pyroxene phases in the silicate mantle. The primary objective of this study is to investigate how mantle composition—through its control on viscosity—affects heat extraction from the deep interior and governs the timing and extent of inner core solidification.

We developed a coupled numerical model that self-consistently simulates the thermal evolution of Mercury’s mantle and core. The mantle model solves conservation equations for mass, momentum, and energy in a two-dimensional Cartesian geometry using a finite-difference method. A non-Newtonian rheology based on dislocation creep is implemented, with flow laws derived from experimental data for dry peridotite and orthopyroxene (as a proxy for pyroxenitic material). We explored a wide parameter space of pyroxenite content (0–40 wt%) and mantle thickness (300–500 km). The core model, based on an Fe–Si alloy (9 wt% Si in the liquid, 3.8 wt% in the solid), follows an analytical formulation describing the core’s thermal, density, and phases’ stability evolution, including latent heat and gravitational energy release during inner core crystallization.

Our results demonstrate that the rheological properties of a pyroxene-rich mantle play a critical role in Mercury’s interior evolution. Increased pyroxenite content reduces mantle viscosity, enabling vigorous and long-lasting convection. This enhances heat extraction from the core and favours the progressive growth of a solid inner core over geological timescales. Conversely, a more viscous, peridotite-dominated mantle suppresses convection earlier in planetary history.

To evaluate the plausibility of the modelled scenarios, we compute the time evolution of Mercury’s normalized moment of inertia, constrained by MESSENGER gravity and rotation data. These data support the presence of a partially molten core, and a mantle mechanically decoupled from a solid inner core.

These findings have implications for the origin and persistence of Mercury’s magnetic field. A long-lived outer liquid core may sustain dynamo activity. Moreover, the model-derived histories of radial contraction and internal temperature evolution are consistent with observed tectonic features.