The role of organics degradation in the thermo-chemical evolution of trans-Neptunian objects

 

Large trans-Neptunian objects (TNOs) such as Pluto, Eris, and Makemake exhibit increasing evidence for complex internal evolution, including differentiation, volatile transport, and potentially ongoing activity. The orbital evolution of the Eris–Dysnomia system may imply unexpectedly efficient internal dissipation (Nimmo and Brown, 2023), suggesting that some large TNOs may still host evolving interiors. Recent JWST observations provided isotopic measurements (Grundy et al., 2024) that constrain the origin of methane on the surface of Eris and Makemake as either primordial (Mousis et al., 2025) or internally produced through hydrothermal or thermogenic processes (Glein et al., 2024). Interestingly, thermal observations of Makemake reveal an anomaly (Kiss et al., 2024), the origin of which may or may not be linked to ongoing outgassing. Indeed, evidence for gaseous CH4 has been reported, making Makemake only the second trans-Neptunian object with confirmed volatile release (Protopapa et al., 2025).

One possible source of internally produced volatiles is the thermal evolution of refractory organic matter inherited from cometary building blocks. Organic molecules, which are abundant in comets (Bardyn et al., 2017), may represent a large fraction of the refractory cores of outer Solar System icy bodies (Reynard and Sotin, 2023). Following accretion, radiogenic heating progressively raises internal temperatures, driving the evolution of initially primitive insoluble organic matter toward more graphite-rich compositions through devolatilization processes (Delarue et al., 2026). Reactions such as decarboxylation, dehydration, and pyrolysis lead to the release of volatile compounds, including H2O, CO2, CH4 and N2, which may contribute to the volatile inventories observed at TNO surfaces. At the same time, the chemical evolution of the refractory material modifies key physical properties such as the H/C ratio, density, heat capacity, and thermal conductivity, thereby affecting the internal structure, cooling efficiency, and long-term evolution of these bodies.

To address this question, we are developing a coupled thermo-chemical evolution model (CRISP, Carbon-Rich Icy-worlds Simulator in Python) to investigate the long-term evolution of organic-rich interiors. The model follows the conductive thermal evolution of a differentiated refractory core composed of hydrated silicates and insoluble organic matter, heated by long-lived radiogenic isotopes. In our study, we explore how the initial proportions of silicates and refractory organics control the timing and magnitude of methane production as well as the radius evolution. For a Pluto-like body containing 30% refractory organic matter in its core, our simulations predict a contraction of the core of about 30 km. The degradation of organics and dehydration of silicates generate enough CH4 and H2O to form the equivalent of a  ∼44 km thick clathrate layer, while leaving a residual free methane reservoir of ∼6×10^19 kg potentially available for long-term volatile release. These results suggest that the thermo-chemical evolution of refractory organics may play a major role in shaping the internal structure, volatile inventories, and present-day activity of large trans-Neptunian objects.