  Saturn's moon Titan is the only extraterrestrial body known to host stable
lakes and a hydrological cycle. Titan's lakes predominantly contain liquid
methane, ethane, and nitrogen, with methane evaporation driving its
hydrological cycle. Molecular interactions between these three species lead to
non-ideal behavior that causes Titan's lakes to behave differently than Earth's
lakes. Here, we numerically investigate how methane evaporation and non-ideal
interactions affect the physical properties, structure, dynamics, and evolution
of shallow lakes on Titan. We find that, under certain temperature regimes,
methane-rich mixtures are denser than relatively ethane-rich mixtures. This
allows methane evaporation to stratify Titan's lakes into ethane-rich upper
layers and methane-rich lower layers, separated by a strong compositional
gradient. At temperatures above 86K, lakes remain well-mixed and unstratified.
Between 84 and 86K, lakes can stratify episodically. Below 84K, lakes
permanently stratify, and develop very methane-depleted epilimnia. Despite
small seasonal and diurnal deviations (<5K) from typical surface temperatures,
Titan's rain-filled ephemeral lakes and "phantom lakes" may nevertheless
experience significantly larger temperature fluctuations, resulting in
polymictic or even meromictic stratification, which may trigger ethane ice
precipitation.
