Organic Content of Titan’s Interior Ocean from Accreted Insoluble Organic Matter

Titan is well-known for its organic-rich surface and atmospheric chemistry. The seemingly ubiquitous nature of insoluble organic matter (IOM) in primitive bodies and the high abundance of this material in comets together suggest that such material may have been an important accretionary component in the outer solar system. IOM is refractory at low temperatures, and may remain intact if accretion is gentle. However, laboratory experiments on IOM extracted from chondritic meteorites demonstrate formation of more volatile compounds, including both gases and small organic compounds, starting at moderate temperatures of approximately 200 °C. Such temperatures may be plausible for Titan’s interior.

Here, we consider the implications of such a scenario for the organic content of Titan’s subsurface ocean. We investigate a range of starting organic content from chondritic (4 % of rocky material by mass) to cometary (45 % by mass), as well as a range of water/rock ratios from 0.5 to 10 that may represent different interior environments (e.g. pore spaces in rock versus the water-core interface). We find that for the full range considered, IOM-derived organics are under-saturated in the ocean up to compounds with four or more benzene rings. Under these conditions and assuming temperatures near 25 °C, the predicted dissolved organic concentration of Titan’s ocean ranges from 3 to 530 μM, with a corresponding dissolved carbon content of 30 to 5000 μM. For comparison, typical organic carbon concentrations in Earth seawater are on the order of 10s of μM. Terrestrial dissolved organic matter contributes to microbial metabolism and supplies nutrients for biological reactions; our results suggest that Titan’s water-rock interface may have abundant organic carbon available for such processes.