Metamorphic and density evolution of carbonaceous matter: A potential cometary composition of Titan

Composition models of icy bodies should include a large carbonaceous organic matter (COM) component to account for bulk density and moment of inertia measurements. In previous models, the effects of temperature and pressure on COM were simplified due to a lack of experimental data. Samples, here, from high-pressure experiments using coals as analogs for COM were analyzed to quantify this transformation during the thermal evolution of icy bodies. A kinetic model was fitted to the measured compositions of samples experimentally heated at 473 to 723 K, 0.2 to 2.5 GPa, for durations up to hundreds of days. COM composition is described by the loss of O- and H-compounds resulting in a density variation from ∼1350±100 kg/m³ to that of graphite near 2240 kg/m³. The pressure dependence of COM density at ambient temperature was measured using diamond anvil cell experiments and integrated into the kinetic model. The resulting equation was implemented in numerical models to describe the evolution of refractory core densities in icy bodies, and the nature and proportions of released volatiles (H₂O, CO₂, and CH₄). Applications to Titan suggest that the amount of COM required to match gravitational constraints is higher than previously estimated, reaching values of 45 wt% comparable to those estimated for the refractory part of comets. This result supports a cometary composition for the whole Saturnian disk from which satellites formed, and possibly for Saturn itself. C/Si ratio in Titan suggests chemical zoning in the outer solar system during the formation of gas giants.