TITAN'S HYDROLOGIC CYCLE: A POST-CASSINI VIEW (Invited Presentation)

Saturn’s largest moon, Titan, is the only extraterrestrial body known to support standing bodies of stable liquid on its surface. Along with Earth and early Mars, Titan is one of three places in the Solar System known to have or have had an active hydrologic cycle. With an atmospheric pressure of 1.5 bar and surface temperatures of 90–95 K, methane and ethane condense out of the nitrogen-based atmosphere and flow as liquids on its surface. Exchange processes between these atmospheric and surface reservoirs produce erosional and depositional landscapes by routing material across their surfaces. These processes produce morphologic forms including dunes, channels, lakes, and seas that are strikingly similar to their terrestrial counterparts.

Discoveries made by the Cassini-Huygens mission during its decade of exploration in the Saturn system demonstrate that Titan’s methane-based hydrologic cycle is dominated by processes that operate over a range of timescales, including geologic (~10⁹ Earth yr.), Milankovitch (~10⁵ Earth yr.), seasonal (~10¹ Earth yr.), and the timescale of a single convective storm (~10^-3 Earth yr.). While one could say this for the Earth as well, this similarity only emphasizes the point that, beyond Earth, Titan’s active hydrologic system is unique.

Over geologic timescales, methane is lost to photolysis in the upper atmosphere while ethane and other higher order hydrocarbons are deposited on the surface. Surface features found around Titan’s polar terrains include abandoned deltas, paleomare basins, drowned river valleys, and embayed coastal features. These imply long-period cycling of liquid between Titan’s poles as Saturn’s orbital eccentricity and longitude of perihelion evolve. Over seasonal timescales, liquid composition varies as methane is both transported between and cycled within the polar terrain. During a single convective storm, methane and ethane rain out of the atmosphere and redistribute materials through both mechanical and chemical processes. In this presentation, we will describe the dominant exchange processes that operate over these timescales and present a post-Cassini view of Titan’s hydrologic system.