Carbon-rich icy moons and dwarf planets

Internal structure models of dwarf planets and giant planets’ moons previously assumed essentially Earth-like silicate-metal cores surrounded by ice. Inner density models of the rocky cores of differentiated Ganymede and Titan, the largest icy moons in the solar system indicate the presence of a low-density component in addition to silicates and metal sulfide. Carbonaceous matter akin to coal formed from abundant organic matter in the outer solar system is a likely low-density component. Progressive gas release from coal may sustain up to present-day the replenishment of ice-oceanic layers in organics and volatiles. This accounts for widespread observation of nitrogen as well as light hydrocarbons to complex organic molecules at the surface, in the atmospheres, or in plumes emanating from moons and dwarf planets. Analysis of available density of rocky cores of other icy moons and dwarf planets also suggests the presence of a low-density carbonaceous component. We tested this hypothesis and found that rocky core densities in dwarf planets and icy moons are consistent with a mixture of chondritic silicate-sulfide rocks and a rock-free precursor composed of ices and carbonaceous matter in near-solar proportions. Thermal models taking into account the presence of carbonaceous matter is performed to evaluate its effects on the present-day structure of icy moons and dwarf planets.