The Flushing of Ligeia : Composition Variations Across Titan’s Seas in a Simple Hydrological Model

Abstract:

I use a simple box model to explore possible differences in the liquid composition of Titan’s seas. Major variations in the abundance of involatile ethane, somewhat analogous to salinity in terrestrial waters, arise from the hydrological cycle, which introduces more “fresh” methane rainfall at the highest latitudes in summer and keeps the system 'off balance'. Comparing the precipitation and evaporation fluxes with likely tidal mixing fluxes, I find the composition of Ligeia Mare to be substantially flushed by methane rainfall which causes it to export its solutes to Kraken via a narrow labyrinth of channels. Ligeia therefore may have a methane-rich (~80%) composition, well out of thermodynamic equilibrium with the atmosphere, whereas the basins of Kraken are relatively well-mixed and will be ethane-dominated (~60%). These composition variations, analogous to Earth’s salinity gradient between the Black Sea and the Mediterranean (or the Baltic and the North Sea) may be detectable with Cassini measurements and are important for future exploration.

Comparing the depth of Ligeia measured at 160m with the annual rainfall predicted by Global Circulation Models of about 10m at the highest latitudes, and assuming a modest catchment area, suggests that Ligeia Mare should undergo only modest seasonal changes in composition, assuming surface 'waters' are mixed into the depths.