Icy Moons as Probes of Carbon-Rich Conditions During Giant Planet Formation

The densities and moments of inertia of Jovian and Saturnian icy moons, dwarf planets, and other trans-Neptunian objects (TNOs) suggest the presence of a significant low-density carbonaceous component in their rocky cores. In a homogeneous accretion scenario, where these components are mixed in solar proportions, ices differentiate from the carbon-rich refractory core, while silicate hydration may occur. Thermal models that account for the presence of carbonaceous matter indicate that originally hydrated silicates are now largely dehydrated in the refractory cores of large moons and dwarf planets, due to interactions with volatiles released by the metamorphism of carbonaceous matter.Progressive gas release from the slowly warming, carbonaceous matter-rich cores may sustain, up to the present day, the replenishment of ice-ocean layers with organics and volatiles, as well as outgassing to the surface. This process accounts for the observation of nitrogen, light hydrocarbons, and complex organic molecules at the surface, in the atmospheres, or in the plumes emanating from moons and dwarf planets. The formation of large carbon-rich icy bodies in the outer solar system suggests that a carbon-rich environment prevailed during ice giant planet formation—a scenario that could also lead to the formation of carbon-rich planets at the outskirts of extrasolar systems.In the Neptunian system, Triton—presumed to be a captured TNO—shares density and surface composition characteristics with other large TNOs, including Pluto, Eris, Makemake, Gonggong, Quaoar, and others. Notably, the carbon-bearing molecules at the icy surfaces of TNOs shift from CO₂-dominated compositions in smaller objects (Pinilla-Alonso et al., 2024) to CH₄-rich compositions in the largest TNOs, including Triton (Brown, 2012; Emery et al., 2024; Grundy et al., 2024).In the Uranian system, the latest estimates of regular satellite masses (Jacobson, 2014) reveal a power-law relationship between size and density, reflecting varying rock/ice ratios caused by fractionation processes (Reynard and Sotin, 2025). This relationship is explained by mild enrichment of rock relative to ice in the solids that aggregated to form the moons, following Rayleigh's law of distillation (Rayleigh, 1896). In the outer solar nebula, Rayleigh fractionation may account for the separation of a rock-dominated reservoir and an ice-carbon-dominated reservoir, now represented by CI carbonaceous chondrites/type-C asteroids and comets, respectively. Potential consequences for the composition of Uranus’s moons and targets for future exploration are discussed. 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.