An Experimental Investigation of Liquid Hydrocarbons in a Simulated Titan Environment

Saturn’s moon, Titan, has surface conditions (89–94 K, 1.5-bar atmosphere) that permit lakes of methane, ethane, and dissolved atmospheric nitrogen. The effects of atmospheric nitrogen on methane-ethane liquid properties is poorly understood, leading to uncertainty in Titan modeling. I address this question by experimentally investigating the physical properties of methane-ethane liquids under a 1.5-bar nitrogen atmosphere in a simulated Titan environmental chamber. Chapter 1 addresses nitrogen dissolution kinetics in Titan’s liquid hydrocarbons. I found an exponential increase in nitrogen quantity and diffusion coefficients with increasing methane mol%. I find that Titan’s liquids are likely not saturated in nitrogen, with dissolution alone. This would result in strong disequilibria between liquid layers, creating lake dynamics (i.e. overturn). Chapter 2 systematically investigates the conditions necessary for bubble formation under Titan surface conditions. I found that liquid methane and ethane, along with temperature and concentration perturbations, are necessary for bubble formation. Bubbles are likely prevalent in Titan’s lakes and influence the formation of geologic features (i.e. deltas). Chapter 3 investigates methane-ethane freezing points in equilibrium with a 1.5-bar nitrogen atmosphere. I found that liquid ethane-alkane compositions of ~75–100 mol% will freeze above 89 K, and a peritectic point is observed. Brightening events on Titan’s surface may indeed be ethane-rich hydrocarbon ice, while bright features in the seas are likely not floating or suspended ice. Chapter 4 couples Cassini Visual and Infrared Mapping Spectrometer (VIMS) observations with laboratory Fourier Transform Infrared Spectroscopy (FTIR) to find that Cassini VIMS is sensitive to ethane-alkane quantities of 5–75 mol%. Titan’s lakes are likely within this compositional range. The results of this dissertation indicate that dissolved atmospheric nitrogen plays a significant role in Titan’s liquid hydrocarbons. Small perturbations of temperature and concentration will cause explosive bubble exsolution events, and nitrogen increases the likelihood of methane-ethane freezing on Titan’s surface. Nitrogen will also cause changes in methane’s density, leading to lake dynamics, such as overturn, stratification, and bubble formation. By comparing Cassini VIMS and RADAR measurements of Titan’s liquid bodies, we can discern if the liquids are indeed stratified or well mixed

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

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