Thermo chemical models for organic-rich icy moons: Applications to Europa, Ganymede, and Callisto

Using Equation of state for ices, silicates, and organics, the value of the moment of inertia (MoI) of icy moons and dwarf planet Ceres suggest that organic molecules are a major compound of their refractory core (Reynard & Sotin, 2023; Delarue et al., 2026). During the thermal evolution of the refractory core, the composition evolves and the density of the remaining carbonaceous compound increases as H and heteroatoms are released. It eventually reaches the density of graphite. The space observations provide the present state of the refractory core, suggesting that the organic fraction could be a larger fraction of the initial body. In an effort to retrieve the initial fractions of ice, silicates, and organics, two models were developed. First a kinetic model (KINCAM-E) was developed to describe the transformation of the carbonaceous compound with time and temperature. This model is based on experimental data with long duration pyrolysis. Second, a thermo-chemical model was developed to describe the feedback of the carbonaceous compound evolution on the thermal evolution. This model includes the migration of volatiles produced by the degradation of the organic compound to the hydrosphere. It also includes the dehydration of the hydrated silicates as the temperature increases. In these models, the mass of the body is fixed and partitioned between silicates, ice, and organics. The evolution of the radius and other parameters is followed. Only models consistent with the present value of the radius and the MoI are retained. First applied to Titan, this thermo-chemical evolution model shows that the initial fraction of organics composing Titan is similar to cometary amount (Delarue et al., 2026). It is now applied to the icy Galilean satellites. Implications for future observations by JUICE and Europa Clipper will be discussed.