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

Abstract

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.

Figure 1

Illustration of the Titan Module, not to scale. A 2 m tall steal cylinder houses the Titan module and maintains a 1.5 bar atmospheric pressure of nitrogen (N₂). In our experiments, the hydrocarbon “raindrop” is dripped from the condenser into the liquid below (i.e., “bulk liquid”). The experimental setup is stationary, and the droplet height (h₂) decreases as the experiment progresses. An endoscope camera allows real-time viewing of the sample from above. The bulk liquid and droplet temperature are measured by a thermocouple submerged in the bulk liquid sample and inside the condenser, respectively.

Figure 2

Plan view images of floating liquid droplets. (a) A floating liquid droplet of ethane on a bulk liquid with a composition of 79–84 mol % methane–alkane ratio. (b) A floating liquid droplet of methane with a bulk liquid composition of ∼95 mol % methane–alkane ratio.

Figure 3

Plan view images of pure ethane liquid droplets floating in a bulk liquid with a composition of ∼95 mol % methane–alkane ratio. Here, two floating liquid ethane droplets coalesce to form a larger daughter droplet. (a) t = 0 s, two ethane droplets are floating on the surface of the bulk liquid. (b) t = 0.13 s, the two droplets are beginning to coalesce. (c) t = 0.23 s, the two ethane droplets have coalesced to form a larger daughter droplet. Notice the ripple in the bulk liquid caused by the droplets merging. The dark object on the left (part of the condenser) and the line at the bottom of the image (atmosphere thermocouple) are located above the liquid surface. The liquid sample thermocouple is out of view. The white color is the bottom of the sample dish.

Figure 4

Plan view images of pure ethane liquid droplets floating in a bulk liquid with a composition of 68–72 mol % methane–alkane ratio. Here, multiple floating liquid ethane droplets conglomerate before coalescing to form a larger daughter droplet. (a) t = 0 s, multiple ethane droplets are stuck together and floating on the surface of the bulk liquid. (b) t = 40 s, the floating liquid droplets have transferred liquid to form one larger droplet and multiple smaller droplets. (c) t = 41 s, the ethane droplets have coalesced to form a single larger daughter droplet. The blue object on the bottom left corner (part of the condenser) and the black object in the left corner (FTIR probe) are located above the liquid surface, while the yellow line near the bottom is a thermocouple submerged in the bulk liquid sample. The white partial circle is the bottom of the sample dish. Notice the shadow from the floating liquid droplet on the bottom of the sample dish.

Figure 5

Plan view images of ethane floating liquid droplets and nitrogen bubbles. (a) Image obtained during a nitrogen exsolution event (described in ref (40)) illustrating the visual difference between floating liquid droplets and nitrogen bubbles. (b) Same image as (a), with labels for reference. (c) Image of ethane floating liquid daughter droplet (merged larger droplet). (d) Same droplet as (c) with the interior chamber light off. The endoscope light (blue) reflects off the floating liquid droplet (inner ring) and illustrates how the droplet alters the liquid surface (outer ring).

Figure 6

Illustrates the impact velocity of the droplet as a function of the bulk liquid composition. Solid points representthe calculated composition at the start and end of each sample pour, while horizontal lines represent the evolution of the bulk liquid over the duration of the sample pour. (a) Ethane floating liquid droplet experiment overview. This plot illustrates the bulk liquid compositional range where floating liquid droplets of ethane were observed. Ethane droplet experiments began with 100 mol % methane in the bulk liquid (100 mol % methane–alkane ratio) and then increased in ethane concentration as the experiment progressed (proceeding from the right to left of the plot). We find that ethane droplets are observed in nearly all bulk liquid compositional ranges tested. (b) Methane floating liquid droplet experiment overview. Floating liquid droplets were observed in the bulk liquid compositional range/impact velocities associated with black data points, and samples that immediately coalesced are red in color. Methane droplet experiments began with 100 mol % ethane in the bulk liquid (0 mol % methane–alkane ratio) and then increased in methane concentration as the experiment progressed (proceeding from the left to right of the plot). We find that floating liquid droplets of methane did not easily form, with an occurrence between 94.8 and 95.4 mol % methane–alkane ratio, and an impact velocity <0.55 m/s. The y-axis has a systematic error from the uncertainty in the original distance from the condenser opening to the bottom of the sample dish, while the x-axis is a random error. The uncertainty discussion can be found in the Supporting Information.

Figure 7

Terminal velocity as a function of raindrop radius on Titan for an ethane droplet (with and without dissolved nitrogen, orange), a pure methane droplet (green), and a methane droplet saturated in atmospheric nitrogen (blue). The impact velocity in our experiments is highlighted by the dashed orange (0.7 m/s for ethane) and blue/green (0.5 m/s for methane) lines.