PREDICTING CAVE FORMATIONS IN SATURN’S MOON TITAN

Saturn’s moon Titan presents several geomorphological features strongly suggestive of karstic dissolution processes. These include polygonal karst-like terrains, closed valleys systems, and steep-sided lakes with morphologies similar to terrestrial karstic lakes. From both theoretical studies and laboratory measurements, it is reasonable to expect that Titan’s hydrocarbon rains and methane-based weather cycle can dissolve some of the thick layer of organic materials that covers much of the surface. It is thus likely that liquid hydrocarbons could create dissolved conduits and voids in Titan’s subsurface, particularly in areas where the alkanofer base-level lowering is evident. In a Titan cave, what types of formations could occur?

Similar to terrestrial systems, circulating subsurface liquids would saturate in dissolved materials, and on entering a vadose cave environment in partial exchange with the outside atmosphere, could either dissolve or precipitate out materials following either an increase, or decrease in equilibrium saturation concentrations. As an example, on Earth, the exsolution of CO₂ in limestone-saturated water decreases the saturation concentration of CaCO₃, and the excess precipitates out as stalactites, stalagmites, and cave draperies. Other terrestrial aqueous systems include those dissolving/depositing halite (NaCl), gypsum (CaSO₄∙H₂O), or mixed materials.

On Titan, the overall concepts are similar, but the materials and liquids are different. Hydrocarbon liquids have varying abilities to dissolve organic materials. In general, liquid methane is a poor solvent, while liquid ethane and liquid propane (both produced by Titan’s atmospheric photochemistry) are significantly better, with up to two orders of magnitude higher saturation concentrations for many organics. On mixing with ethane, a saturated solution in methane will become undersaturated and become aggressive. In contrast, on mixing a saturated ethane solution with methane, the solution will become oversaturated, and the excess dissolved organic molecules will be deposited. By examining and comparing the properties of liquids, materials, and formation processes of both Earth and Titan, we will present predictions of what types of extraterrestrial organic cave formations could exist on Titan.