Thermodynamic modeling of metamorphic fluids supports internal source of carbon-bearing molecules at the surface of TNOs

JWST observations of TNOs up to 800 km in diameter show surface ices that include carbon-bearing species such as CO₂, CO, CH₃OH, and complex organic molecules (Pinilla-Alonso et al., 2024). Although surface compositions vary, no systematic trend with object size suggests these variations are dominated by surface processes. The surface compositions of larger TNOs display strong methane bands in addition to H₂O ice and CO₂ (Brown, 2012), and recent hydrogen and carbon isotopic measurements of CH₄ on Eris and Makemake by JWST suggest an internal origin for these species (Grundy et al., 2024). The bulk densities of icy moons and dwarf planets support the idea that their refractory cores contain a mixture of CI chondrite and carbonaceous material, reinforcing the idea that carbon-bearing molecules at their surfaces may originate from internal activity. 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 studied the impact of carbon on the mineralogy of trans-Neptunian object (TNO) interiors. We model phase relations in the MgO–SiO₂–Fe–C–H₂ and MgO–SiO₂–CaO–Fe–C–H₂ systems across the P–T range of TNOs (300–1300 K, 1–7000 bar), using thermodynamic modeling with Perple_X (Connolly, 2005) and assuming CI elemental composition. The results reveal that at high fO₂, carbonates (magnesite, dolomite), water, and CO₂ are stable, whereas at lower fO₂, carbon is progressively reduced to graphite, with methane and hydrogen as the dominant volatiles. The stability fields of major species (CO₂, H₂O, CH₄) in COH fluids are bounded by different oxygen fugacity buffers defined by the stability of various carbon-bearing mineral assemblages. Conversely, if fluid composition is fixed, for example by degradation reactions of carbonaceous matter, it will determine the mineral assemblages. Pressure does not significantly influence these transitions, whereas changes in oxygen fugacity and temperature strongly affect the gas species released from the mineral assemblage into metamorphic fluids. Specifically, at high temperatures, reduced phases such as methane are stable, while at lower temperatures, oxidized species and CO₂ are favored. Thus, temperature and oxygen fugacity play a crucial role in controlling the nature of carbon-bearing phases in solids and in fluids that can reach the surface of TNOs.

The predicted metamorphic evolution of mineral assemblages shows that the internal composition is directly reflected in fluid composition, which may eventually reach the surface and atmosphere of TNOs. Small TNOs (typically <800 km in diameter) have cold cores and therefore high oxygen fugacity, supporting the idea of oxidized interiors where carbonates and CO₂ are stable. In contrast, large TNOs such as Eris, Makemake, and Pluto, with higher core temperatures and/or lower oxygen fugacity, likely host more reducing phases, leading to the release of reduced fluid species such as methane. Overall, these findings suggest that redox-driven transformations have significantly shaped the interiors and volatile emissions of icy, carbon-rich bodies in the outer solar system, influencing their potential habitability. These results are consistent with JWST and earlier spectroscopic observations suggesting that volatiles of internal origin contribute to present-day surface compositions. It is predicted that CH₄ should become increasingly dominant as TNO size increases, a hypothesis that will be tested further by upcoming JWST spectroscopic investigations of TNOs.

Acknowledgement

This work was supported by Institut National des Sciences de l'Univers through Programme National de Planétologie, by the Agence Nationale de la Recherche (ANR, project OSSO BUCO, ANR-23-CE49-0003) and by the European Union (ERC, PROMISES, project #101054470). Views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them.

References:

Brown, M. E. (2012). The Compositions of Kuiper Belt Objects. Annual Review of Earth and Planetary Sciences, 40 (Volume 40, 2012), 467–494. https://doi.org/https://doi.org/10.1146/annurev-earth-042711-105352

Connolly, J. A. D. (2005). Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation. Earth and Planetary Science Letters, 236(1–2), 524–541. https://doi.org/10.1016/j.epsl.2005.04.033

Grundy, W. M., Wong, I., Glein, C. R., Protopapa, S., Holler, B. J., Cook, J. C., Stansberry, J. A., Lunine, J. I., Parker, A. H., Hammel, H. B., Milam, S. N., Brunetto, R., Pinilla-Alonso, N., de Souza Feliciano, A. C., Emery, J. P., & Licandro, J. (2024). Measurement of D/H and 13C/12C ratios in methane ice on Eris and Makemake: Evidence for internal activity. Icarus, 411. https://doi.org/10.1016/j.icarus.2023.115923

Pinilla-Alonso, N., Brunetto, R., De Prá, M. N., Holler, B. J., Hénault, E., Feliciano, A. C. de S., Lorenzi, V., Pendleton, Y. J., Cruikshank, D. P., Müller, T. G., Stansberry, J. A., Emery, J. P., Schambeau, C. A., Licandro, J., Harvison, B., McClure, L., Guilbert-Lepoutre, A., Peixinho, N., Bannister, M. T., & Wong, I. (2024). A JWST/DiSCo-TNOs portrait of the primordial Solar System through its trans-Neptunian objects. Nature Astronomy. https://doi.org/10.1038/s41550-024-02433-2