Titan’s refractory core evolution: Implications for organics in its subsurface ocean

As an organic-rich world with a subsurface ocean, Titan is an object of great astrobiological interest. However, geological signs of surface-subsurface exchange are limited, and Titan’s thick icy crust may significantly impede delivery of organic-rich surface materials to the subsurface ocean. Heating of accreted complex organic material in Titan’s interior may provide an additional source of organic molecules to serve as building blocks for life and/or chemical energy sources for ocean habitability. Here, we investigate the composition of Titan’s subsurface ocean with a focus on organic materials derived from the rocky interior. We develop organic-rich compositional models, and consider what effects heating of these organics may have on the total noble gas signature in Titan’s atmosphere. We compare this signature to existing constraints to provide a consistent framework for the abundance of organic material derived from the interior. Present day constraints generally allow contributions of both noble gas ices and Phase Q noble gases from heating of abundant organic material, but the Xe signature may be inconsistent with ices. We combine this compositional framework with a thermal profile for Titan’s interior that includes the thermophysical properties of organic material, and perform thermodynamic calculations for extraction of volatiles and formation of the hydrosphere. Our results suggest that formation of Titan’s massive hydrosphere requires a large initial ice mass, including a minimum ice/dust mass ratio of 1.4 for anhydrous cometary dust. Finally, we consider the composition and abundance of organic material produced by such processes and them to dissolved organic carbon in terrestrial oceans. We find that Titan’s accreted complex organic material may be a significant contributor to organic material in the subsurface ocean, producing organic abundances that are comparable to terrestrial oceans.