Presentation #402.04 in the session “Titan Atmosphere”.
Titan is a dynamic world with a unique N2-CH4 atmosphere where photochemistry actively converts nitrogen and methane into organic molecules such as C2H6, C2H2, C6H6, HCN, etc . These simple organic molecules could further polymerize into more complex molecules and coagulate to form the refractory aerosols that make up Titan’s haze layers. Titan’s unique temperature profile makes it possible for the simple organic molecules to condense into liquids or solids in its atmosphere, forming liquid or ice clouds. Observations have detected layers of clouds made of the simple organic species in Titan’s atmosphere, such as CH4, C2H6, C6H6, HCN, while clouds such as C4H2, C3H6, C2H4 have been not seen . Many of the cloud species would remain solid when they fall onto Titan’s surface, where they could interact with the surface of Titan’s lakes. Using a combination of laboratory measurements and theoretical tools, we are able to provide a theoretical framework to better understand cloud formation through condensate-haze interactions and cloud-lake interactions on Titan.
We studied 18 possible hydrocarbon and nitrile condensates and their cloud formation efficiency in Titan’s atmosphere, including CH4, C2H2, C2H6, C2H4, C3H4 (allene), C3H4 (propyne), C3H6, C3H8, C4H2, C6H6, HCN, CH3CN, C2N2, C2H3CN, C2H5CN, HC3N, C4N2, and CO2. We assume these condensates would form clouds mainly through heterogeneous nucleation, where they nucleate and grow upon foreign particles. The most likely cloud seeds on Titan are the refractory solid aerosols. Using the laboratory-determined surface energy of Titan’s aerosol analogs “tholins” [3, 4, 5] and the wetting theory, we estimate the contact angles between these condensates and tholins and subsequently the critical supersaturation needed to form clouds. We found species with observed clouds tend to require low supersaturation than the species that have not been observed. In particular, C4H2 requires the highest supersaturation to form clouds. We also found most hydrocarbons clouds should either dissolve or be completely wetted by the lake liquids on Titan, while some nitrile clouds could form small contact angles with ethane-dominated lakes (ethane content > 55%). Given the currently observed lake compositions  and the densities of the cloud species, it is unlikely that any clouds could float on Titan’s lakes unless they are highly porous.
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