Mercury and exo-Mercury interiors constrained by reduced meteorite analyses and thermodynamic modelling

Mercury is the most chemically distinctive terrestrial planet in the Solar System. Its large metallic core, thin silicate shell, low-FeO surface composition, high sulfur content, and volatile-rich geochemistry are often discussed as separate anomalies (Cartier and Wood, 2019). Here, we test whether these properties are instead linked expressions of a single formation pathway: accretion and differentiation under highly reducing conditions. In this framework, Mercury is not an Earth-like planet with an unusually large core, but rather an end-member reduced planet and the Solar System benchmark for a broader class of rocky worlds, including possible exo-Mercuries.

Because no confirmed Mercury meteorites are available, several reduced precursor materials have been proposed as Mercury analogues, including aubrites and ultramafic reduced achondrites such as diopsidites (e.g., Malavergne et al., 2010; Cartier and Wood, 2019; Anzures et al., 2020; Steenstra and van Westrenen, 2020). In this work, these differentiated reduced meteorites are used as analogues for plausible inner-disk building blocks. Their pyroxene-dominated mineralogy, minor sulfides, and lack of plagioclase provide a natural starting point for testing reduced mantle compositions. Nucleosynthetic systematics further link these reduced planetesimals to an inner-disk reservoir relevant to Mercury, while Mg isotope constraints indicate that ²⁶Al alone was insufficient to drive widespread melting. Instead, extensive differentiation likely required protracted growth, impact heating, and gravitational energy release during the assembly of Moon- to Mercury-scale bodies.

After constraining the mineralogical and isotopic properties of these reduced precursor materials, and having shown through nucleosynthetic fingerprinting that they sample an inner-disk non-carbonaceous reservoir relevant to Mercury, we then used their bulk compositions as inputs for thermodynamic phase-equilibrium modelling. This step anchors the inferred interior mineralogy of Mercury to building blocks that plausibly formed in the innermost regions of the solar nebula, the likely accretion environment of the planet.

Thermodynamic phase-equilibrium modelling with Perple_X (Connolly, 1990) shows that reduced Mercury-like bulk compositions do not produce Earth-like peridotitic mantles. Across the investigated CH-like, aubrite-like, and reduced enstatite-like compositions, orthopyroxene plus clinopyroxene exceed olivine, yielding FeO-poor, sulfur-bearing, pyroxene-rich mantle assemblages, consistent with recent experimental findings (Boujibar et al., 2025). This result is central because it affects the planet’s density structure, solidus, melt productivity, viscosity, and thermal evolution (Cioria et al., 2024). Applying terrestrial olivine-controlled rheology to Mercury therefore introduces first-order inconsistencies: Mercury’s mantle is likely governed by a different compositional and rheological regime. The same framework predicts that melts extracted from reduced, pyroxene-rich mantles crystallize into crustal cumulates ranging from noritic to gabbroic compositions. These crustal products are FeO-poor, Mg-rich, and dominated by pyroxene and plagioclase, rather than resembling typical terrestrial basaltic crust.

Overall, this work supports a nebula-driven, selectively accreted reduced lineage for Mercury, in which the planet’s present-day structure reflects not only post-accretion differentiation, but also the geochemical nature of the materials from which it formed. In this framework, Mercury’s large core, FeO-poor silicate shell, sulfur-bearing mineralogy, and pyroxene-rich mantle are interpreted as inherited consequences of accretion from highly reduced inner-disk building blocks, rather than as features that must be produced exclusively by catastrophic mantle stripping (e.g., Benz et al., 1988). The predicted low-FeO, pyroxene-rich lithologies, systematic Mg/Si and Ca/Si domains, silica-rich components, and density contrasts provide testable targets for BepiColombo. More broadly, if comparable reducing environments existed in the inner regions of other planetary systems, the same meteorite-constrained thermodynamic workflow can be extended to rocky exoplanets that formed from similarly reduced reservoirs. In this sense, Mercury is not only an anomaly of the Solar System, but a local expression of a broader planetary outcome: rocky planet differentiation controlled by formation region, precursor chemistry, and redox state.

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

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