Nitrogen Exsolution and Bubble Formation in Titan's Lakes

Abstract

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

The Bathymetry of Moray Sinus at Titan's Kraken Mare

“…If instead we ascribe the low values of sea surface backscatter recorded on Moray Sinus to an effect of sea surface roughening (the second scenario), and consider a sea surface permittivity of ε s = 1.7, we would require an increase of effective sea surface RMS roughness σ h from the 0.9 ± 0.3 mm of northern Ligeia Mare (see also [Zebker et al, 2014]) to 2.4 ± 0.1 mm in Moray Sinus. Note that nitrogen exsolution in the form of bubbles at the surface of the sea, caused by outflow from methane-nitrogen rivers mixing with more ethanerich lakes or seas as proposed by Malaska et al (2017) and Farnsworth et al (2019), would potentially have two effects: a decrease in the effective permittivity (because of the presence of the empty space within the bubbles themselves), and an increase the effective roughness of the sea surface. Moray Sinus could be such an area.…”

“…(2017) and Farnsworth et al. (2019), would potentially have two effects: a decrease in the effective permittivity (because of the presence of the empty space within the bubbles themselves), and an increase the effective roughness of the sea surface. Moray Sinus could be such an area.…”

Pressure and temperature dependence of solubility and surface adsorption of nitrogen in the liquid hydrocarbon bodies on Titan

“…Recent experiments, thermodynamics models, and molecular simulations suggest that the liquid hydrocar-bon bodies on the surface are major sinks of nitrogen [8][9][10][11]. Most of these experiments were motivated by the detection of transient bright features by Cassini RADAR system, also known as "Magic Islands" [12,13].…”

“…Various scenarios for the exsolution of nitrogen in the form of bubbles were hypothesized, including superheating, supersaturation, titration of ethane into methane rich regions [8,15]. Explicit experiments on the bubble formation in Titan-like conditions were recently performed and authors found several criteria for the formation of bubbles on Titan [9]. The focus of many of these experiments have solely been the solubility of nitrogen and exsolution of nitrogen from the lakes and seas of Titan [8,9].…”

“…Explicit experiments on the bubble formation in Titan-like conditions were recently performed and authors found several criteria for the formation of bubbles on Titan [9]. The focus of many of these experiments have solely been the solubility of nitrogen and exsolution of nitrogen from the lakes and seas of Titan [8,9]. Theoretical work using thermodynamic models utilizing available experimental data have focused on the same process [14,15,18].…”