Stratification Dynamics of Titan's Lakes via Methane Evaporation

TL;DR

This study uses numerical modeling to explore how methane evaporation and non-ideal interactions influence the stratification, structure, and evolution of Titan's lakes, revealing temperature-dependent layering and potential episodic stratification.

Contribution

It provides new insights into the physical behavior and stratification processes of Titan's lakes considering non-ideal molecular interactions and temperature effects.

Findings

Methane-rich mixtures can be denser than ethane-rich ones at certain temperatures.

Lakes can stratify into ethane-rich upper layers and methane-rich lower layers.

Temperature fluctuations can induce polymictic or meromictic stratification, affecting lake dynamics.

Abstract

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.