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