Role and fate of organics in the thermo chemical evolution of icy moons and dwarf planets

Organic molecules are a major compound of icy moons and dwarf planets (Reynard & Sotin, 2023). During the accretion and differentiation of these bodies, the organics are first altered by water. The residual organics are denser than water and mix with the rocky fraction (silicates and sulfides), eventually forming a refractory core. During this step, the interior temperature is buffered by the melting temperature of ice since water is very efficient in removing interior heat. Once water and refractory compounds have been differentiated, the increase in temperature is controlled by the amount of internal heating (tidal and radioactive) and the thermal properties of the mixture of minerals and organics. Radioactive elements are present in the silicate fraction only, which implies that the heating rate decreases with increasing fraction of organics. As the temperature increases, organics are transformed into denser phases, releasing volatile species. This evolution modifies the volume and the moment of inertia (MoI) of the body. Thermo-chemical evolution models are coupled with equations of state of the different compounds, allowing the calculation of the size and the MoI as a function of time. These models are compared with observations for several icy moons (Titan, Ganymede, Europa) and dwarf planet Ceres. The initial organic fraction can then be retrieved to match the present values of size and MoI.