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.

Titan's surface liquids are composed primarily of methane, ethane, and dissolved atmospheric nitrogen. The nitrogen content depends on the alkane composition and temperature, and exsolves as bubbles when these parameters are sufficiently perturbed. Herein, we present an experimental study of nitrogen bubbles in methane-ethane liquids, and propose that both methane and ethane are required for bubbles to form under Titan conditions. Bubbles occur when methane composes 40-95 mol% of the alkanes within the liquid. We identify two mechanisms that produce bubbles: ethane mediated titration and temperature-induced stratification. Both of these mechanisms produce a metastable nitrogen supersaturation within the liquid; equilibration triggers rapid nitrogen exsolution in the form of bubbles. Such equilibration could cause bubble events in Titan's lakes, possibly explaining the transient "Magic Island" features seen by Cassini RADAR (bubbles within the liquid column), and the presence of deltas in Ontario Lacus.Plain Language Summary Saturn's largest moon, Titan, has stable lakes on its surface composed of liquid methane, ethane, and dissolved atmospheric nitrogen. While nitrogen can dissolve into the liquid (dissolution), it can also be forced out of the liquid (exsolution). Nitrogen bubbles in Titan's lakes have been hypothesized, but not experimentally measured under Titan surface conditions. Here we create bubbles in a laboratory under Titan conditions using two methods: ethane slowly added to methane (titration) and temperature-induced methane-ethane liquid layering (cooling then warming). Both mechanisms create a metastable supersaturation of nitrogen. Once the metastable limit is reached, bubble nucleation occurs, triggering rapid bubble formation. Transient bright structures observed in Titan's lakes by the Cassini RADAR, known as "Magic Islands," may in fact be bubbles within the liquid column. Bubbles can also form in rivers as they flow into lakes and seas, potentially influencing geologic structures, such as the delta seen in one of Titan's largest lakes, Ontario Lacus. Key Points:• Nitrogen bubbles were created in the laboratory under Titan surface conditions • Liquid methane and ethane must be present for bubbles to form (40-95 mol% methane-alkane ratio) • Identified two mechanisms for bubble formation: ethane titration (>86 K) and temperature-induced stratification (<86 K)Supporting Information:• Supporting information S1• Movie S1• Movie S2• Movie S3• Movie S4

Saturn's moon, Titan, has a hydrocarbon-based hydrologic cycle with methane and ethane rainfall. Because of Titan's low gravity, "floating liquid droplets" (coherent droplets of liquid hydrocarbons that float upon a liquid surface) may form on the surface of Titan's hydrocarbon lakes and seas during rainfall. Floating liquid droplets, however, have not been investigated in the laboratory under conditions appropriate for the surface of Titan (cryogenic, hydrocarbon, liquids). We conducted a set of experiments to simulate methane and ethane rainfall under Titan surface conditions (89−94 K, 1.5 bar nitrogen atmosphere) and find that floating ethane droplets form in a wide range of bulk liquid compositions, yet floating methane droplets only form in a narrow compositional range and impact velocity. We find droplet formation is independent of the liquid density and hypothesize that dissolved atmospheric nitrogen in the bulk liquid may repel liquid ethane droplets at the surface. We propose that liquid droplets will form in Titan's methane-rich lakes and seas during ethane rainfall with a droplet radius of ≤3 mm and an impact velocity of ≤0.7 m/s. The presence of these droplets on Titan's lakes may result in a liquid surface layer that is dominated in rainfall composition.

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