Saturn's largest moon, Titan, has an atmosphere and surface that are rich in organic compounds. Liquid hydrocarbons exist on the surface, most famously as lakes. Photochemical reactions produce solid organics in Titan's atmosphere, and these materials settle or snow onto the surface. At the surface, liquids can interact with solids, and geochemical processes can occur. The consequences of these processes can be explored using a thermodynamic model to calculate the solubilities of gases and solids in liquid hydrocarbons at cryogenic temperatures. The van Laar model developed in this study was parameterized using experimental phase equilibrium data, and accurately represents the data for the CH4-C2H6-C3H8-N2-C2H2 chemical system from 90 to 110K. The model generally gives more accurate results than existing models. The model also features a suitable balance between accuracy and simplicity, and can serve as a foundation for studies of fluvial geochemistry on Titan because it can be extended to any number of components while maintaining thermodynamic consistency. Application of the model to Titan reveals that the equilibrium composition of surface liquids depends on the abundance of methane gas in the local atmosphere, consistent with prior studies. The concentration of molecular nitrogen in Titan's lakes varies inversely with the ethane content of the lakes. The model indicates that solid acetylene should be quite soluble in surface liquids, which implies that acetylene-rich sedimentary rocks would be susceptible to chemical erosion, and acetylene evaporites may form on Titan. The geochemical character of acetylene in liquid hydrocarbons on Titan appears to be intermediate to those of calcite and gypsum in surface waters on Earth. Specific recommendations are given of observational, experimental, and theoretical work that will lead to significant advancements in our knowledge of geochemical processes on Titan. This paper represents the beginning of a new kind of geochemistry, called cryogenic fluvial geochemistry, with Titan starring as the first example.
ASJC Scopus subject areas
- Geochemistry and Petrology