Thermodynamic, thermal and kinetic modeling of carbon fate in TNO interiors

Trans-Neptunian Objects (TNOs) hold valuable clues about planetary formation processes. Recent spectroscopic observations of their surfaces indicate that they host C–O–H species, including CO2, H2O, CH4, and complex organic molecules, suggesting the possibility of substantial internal carbon reservoirs. 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 that 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 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.In this study, we use thermodynamic modeling with Perple_X (Connolly, 2005) to investigate carbon speciation and fluid–mineral equilibria within TNO interiors under hydrated conditions. Models were computed over a wide range of oxygen fugacity (logfO2= −10 to −50) and P–T conditions representative of TNO interiors (300–1300 K, 1–7000 bar), assuming a CI elemental composition with carbon content varying from a saturated to a less carbon-enriched system. The internal temperature of TNOs is derived from a thermal model of mid- and large-sized TNOs, Charon and Pluto, assuming 23% leaching of radioactive elements at a differentiation time of 500 Myr. These results are coupled with those from a kinetic model of carbonaceous matter evolution KIMCAM-E (Delarue et al., 2026) to constrain the evolution of metamorphic fluid composition with temperature and size.The results for the core composition of carbon-saturated systems reveal that at high fO2 and low temperatures, carbonates and hydrated minerals are stable, whereas at lower fO2 and higher temperatures, hydrated minerals are no longer stable and carbon is progressively reduced to graphite. 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, the results for COH fluid composition reveal that at high temperatures and in carbon-saturated systems, reduced phases such as methane are stable, while at lower temperatures, oxidized species and CO2 are favored, consistent with the kinetic model. If the carbon content is decreased, methane is replaced by hydrogen-rich fluids, while carbon dioxide is replaced by water-rich fluids.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 form the ice observed on TNOs. By constraining the internal core temperature from thermal models, conditions for TNO interiors can be projected onto phase diagrams derived from the Perple_X thermodynamic model in order to derive their mineralogy, and the fluids generated (figure). The temperature of the core is a function of object size; the larger the TNO, the higher the temperature it can reach for a given composition. As shown by the line of evolution, as temperature—and thus core size—increases, the core composition and fluids become more reduced.Small TNOs (typically