Stratification Dynamics of Titan’s Lakes via Methane Evaporation

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 nonideal behavior that causes Titan’s lakes to behave differently than Earth’s lakes. Here, we numerically investigate how methane evaporation and nonideal 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 86 K, lakes remain well mixed and unstratified. Between 84 and 86 K, lakes can stratify episodically. Below 84 K, lakes permanently stratify and develop very methane-depleted epilimnia. Despite small seasonal and diurnal deviations (<5 K) 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.

Floating Liquid Droplets on the Surface of Cryogenic Liquids: Implications for Titan Rain

“…We use TITANPOOL (see ref 41) to compute the liquid properties of methane and ethane under Titan surface conditions and observe an opposite behavior in our experiments. Liquid ethane has a surface tension value twice that of liquid methane (∼33 and ∼16 mN/m, respectively; at 95 and 92 K; [42][43][44] ) and a dynamic viscosity value an order of magnitude higher than that of liquid methane (1 × 10 −3 Pa•s and 2 × 10 −4 Pa•s, respectively, at 92 K; TITANPOOL 41 ). Moreover, the addition of dissolved nitrogen at Titan temperatures does not significantly affect the surface tension (<2 mN/m) or viscosity (<15% difference in value) of the liquids (at 92 K; TITANPOOL 41 ).…”

“…Liquid ethane has a surface tension value twice that of liquid methane (∼33 and ∼16 mN/m, respectively; at 95 and 92 K; [42][43][44] ) and a dynamic viscosity value an order of magnitude higher than that of liquid methane (1 × 10 −3 Pa•s and 2 × 10 −4 Pa•s, respectively, at 92 K; TITANPOOL 41 ). Moreover, the addition of dissolved nitrogen at Titan temperatures does not significantly affect the surface tension (<2 mN/m) or viscosity (<15% difference in value) of the liquids (at 92 K; TITANPOOL 41 ). Thus, the conclusions proposed by Hazlehurst and Neville, 6 Seth et al, 5 and Jeffreys and Davis, 7 regarding surface tension and viscosity, do not appear to apply to liquid methane and ethane.…”

Air–Sea Interactions on Titan: Effect of Radiative Transfer on the Lake Evaporation and Atmospheric Circulation

“…However, this is not totally straightforward as a higher insolation also triggers a stronger cooling by evaporation, which tends to produce cold, sinking fluid. Additionally, non-pure methane lakes (potentially mixed with ethane and/or nitrogen) and lakes with a composition varying with depth (Steckloff et al, 2020) could have a different equilibrium stratigraphy to cold at the bottom and warm at the top triggering different values for the mixed layer depth (Tan et al, 2015). A more complex lake model with multiple layers could partially address the issues above, but the composition and pycnal behavior as a function of composition and temperature would still need to be known or specified.…”

Phase Diagram for the Methane–Ethane System and Its Implications for Titan’s Lakes

“…This means the methane-ethane phase diagram presented in this paper may have applications aside from lake surfaces. Temperature gradients in the lakes could lead to stratification with layers being delineated by methane-ethane mixing ratios (Steckloff et al, 2020). It could also reveal supersaturation effects within the lakes, possibly providing insight into the composition and texture of the lakebeds.…”