Cloud Formation on Titan

Titan, Saturn’s largest moon, has an atmosphere composed of mainly nitrogen and methane. Photochemistry in Titan’s upper atmosphere produces simple hydrocarbon and nitrile species, which may further react and aggregate to produce the complex organic particles that form Titan’s thick atmospheric haze layers. These aerosol haze particles may serve as cloud condensation nuclei (CCN) in the process of cloud formation for organic ices (CH₄, C₂H₆, C₂H₄, C₂H₂, C₃H₆, C₃H₄, C₄H₂, C₆H₆, HCN, C₂N₂, C₄N₂, HC₃N) and liquids (C₃H₈) (Anderson et al., 2018). The goal of this project is to better understand the cloud formation processes on Titan using wetting and nucleation theories. The surface energy of Titan’s haze analogs, “tholin,” was recently measured by Yu et al., (in revision), which enabled us to evaluate the wetting scheme between the Titan haze particles and possible cloud condensates. The wetting scheme (or contact angle) between haze-cloud condensates is important because it has consequences on whether heterogeneous nucleation can occur efficiently for the condensable species on the haze particles. Using the surface energy values of both the haze and the condensable species, we can calculate the contact angle between them. We found that most organic condensates would form small contact angles on the haze particles (θ < 30°), implying that the haze particles are good CCN for most clouds to nucleate and grow to form visible clouds. Using the estimated contact angle, we also calculated the heterogeneous nucleation rates, the rates at which clouds form, for individual organic condensates. Nucleation rates provide us with a deeper understanding of cloud formation on Titan.

References: 1. Anderson, C. M., Samuelson, R. E., & Nna-Mvondo, D. 2018, SSRv, 214, 125 2. Hörst, S. M.2017, JGR-Planets, 122, 432–482.2. Yu, X., Hörst, S. M., He, C., et al., in revision.