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Preliminary Thermodynamic Modeling of Phase relations and Fluid Composition in Carbon-rich Icy Bodies of the Outer Solar System

Giorgia Confortini, Bruno Reynard

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Giorgia Confortini, Bruno Reynard. Preliminary Thermodynamic Modeling of Phase relations and Fluid Composition in Carbon-rich Icy Bodies of the Outer Solar System. 19th EMPG Conference, Jun 2025, Orléans (45), France. hal-05171436

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https://hal.science/hal-05171436v1

Submitted on 22 Jul 2025

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Distributed under a Creative Commons Attribution 4.0 International License
Preliminary Thermodynamic Modeling of Phase
relations and Fluid Composition in Carbon-rich Icy
Bodies of the Outer Solar System

Giorgia Confortini*¹ and Bruno Reynard

¹Ecole normale supérieure de Lyon - UMR CNRS 5276 Laboratoire de Géologie de Lyon, Terre, Planètes, Environnement (LGLTPE), École Normale Supérieure de Lyon, Université de Lyon, 46 allée d'Italie, 69007 LYON, France - France

Abstract

The bulk densities of icy moons and dwarf planets support the idea that their refractory cores contain a mixture of CI chondrite and carbonaceous matter. Oxygen fugacity (fO2) plays a crucial role in controlling the stability of carbonates, graphite, and associated mineral phases, governing carbon speciation and fluid composition in planetary interiors. We study here the impact of carbon on mineralogy of trans-neptunian object (TNOs) interiors. We model phase relations in a MgO-SiO2-Fe-C-H2 system at the P-T range of TNOs (300-1300 K, 1-7000 bar), using thermodynamic modeling with Perple_X (Connolly, 2005). The results reveal that at high fO2, carbonates, (MgCO3), water and CO2 are stable, whereas at lower fO2, carbon is progressively reduced to graphite or diamond, with methane and hydrogen as the dominant volatiles. Magnetite (Fe3O4) and iron act as a redox buffer, promoting carbonate reduction and graphite formation, a process supported by natural and experimental studies (Frezzotti et al., 2011; Galvez et al., 2013). The stability of mineral phases and the different speciation of COH fluids follow the different buffers: hematite-magnetite (HM) promote the stability of oxides phases, while iron-wustite (IW) promote the stability of reduced phases. Between the two, fayalite-magnetite-quartz (FMQ) promote the transition from carbonates to graphite, along with hydrated phases.

The observation of JWST that TNOs populations have CO2, H2O, CO, CH3OH and complex organic molecules at their surface (Pinilla-Alonso et al., 2024) supports the idea of oxidized interiors where carbonates and CO2 are stable. These findings suggest that redox-driven transformations have significantly influenced the interiors and volatile emissions of icy and carbon-rich bodies in the outer solar system.

Connolly, J. A. D. https://doi.org/10.1016/j.epsl.2005.04.033

Frezzotti, M. L. et al. https://doi.org/10.1038/ngeo1246

Galvez, M. E. et al. https://doi.org/10.1038/ngeo1827

Pinilla-Alonso, N., et al. https://doi.org/10.1038/s41550-024-02433-2

**Keywords:** Small bodies, Volatiles, Carbon, Trans neptunian objects

<footnote>*Speaker</footnote>