Cloud Formation on Titan through Heterogenous Nucleation

Titan is a unique planetary body that allows us to test our understanding of the physics of cloud formation beyond Earth. The Cassini-Huygens mission has identified various organic clouds in Titan's atmosphere (Anderson et al. 2018), many of which form under conditions vastly different from those on Earth. Contrary to Earth, where the condensate species is predominantly water, Titan’s clouds are composed of methane (CH4), ethane (C2H6), acetylene (C2H2), and hydrogen cyanide (HCN), many of which are photochemically formed and then condense in Titan’s cold atmosphere (e.g., Yu et al 2023). On Earth, aerosol particles made of a range of compositions could serve as the cloud condensation nuclei (CCN) to initiate heterogeneous nucleation. While on Titan, the CCN is dominantly the photochemically-formed refractory haze particles due to their ubiquity in the atmosphere. Titan thus serves as the perfect testbed to test our classical nucleation theory, where the materials involved in the cloud formation process are completely different from Earth. 

In this work, we apply classical nucleation theory, using the experimentally determined surface properties of both potential condensates and haze particles to evaluate the viability of cloud formation in different regions of Titan’s atmosphere. By computing the critical supersaturation required for nucleation, we assess whether observed and hypothesized clouds can realistically form via vapor deposition onto haze particles with varying surface energies.

Our results indicate that classical nucleation theory can explain the formation of many observed clouds in Titan’s atmosphere, including CH4 and C2H6 (in both liquid and solid phases), C2H2, acrylonitrile (C2H3CN), propionitrile (C2H5CN), and HCN. These species are able to nucleate efficiently on haze particles with moderate critical supersaturation (see Figure 1). However, challenges arise for high-altitude clouds, including benzene (C6H6), cyanoacetylene (HC3N), and dicyanoacetylene (C4N2) clouds. We find that clouds of C6H6 and HC3N can form through vapor deposition only if the haze particles have relatively high surface energy, consistent with haze analogs produced using cold plasma in laboratory experiments (Li et al. 2022). This supports that hazes made with cold plasma are better analogs of actual hazes on Titan, which is consistent with previous works (Coll et al., 2013). In contrast, C4N2 clouds require unrealistically high supersaturation levels to form via heterogeneous nucleation, regardless of the haze surface energy, suggesting instead a solid-state photochemical origin (Anderson et al. 2016). This is also consistent with the observations that C4N2 is only observed in the solid phase and is deficient in the gas phase.

We further explore nucleation behavior across Titan’s vertical structure. In the upper stratosphere (80–140 km), haze particles serve as primary ice nuclei for several species. The detection of C6H6 and HC3N clouds in this region provides valuable constraints on haze surface properties. In the lower stratosphere (50–80 km), although multiple species are thermodynamically expected to condense, only C2H2 clouds have been detected. We attribute this discrepancy to compositional changes in the CCN population, likely involving condensation of HCN onto haze particles, consistent with the retrieved DISR/Huygens aerosol property that showed an increase in single scattering albedo of the aerosols in this region (Tomasko et al. 2008). These HCN-coated particles likely inhibit nucleation for other species, such as propyne (C3H4) and propane (C3H8). In the troposphere, cloud formation mechanisms become more complex, involving both deposition and freezing nucleation. Our results suggest that HCN-coated haze particles would still act as efficient CCN and ice nuclei for CH4 and C2H6 cloud formation, depending on whether supercooling occurs before freezing.

Altogether, this work highlights the critical role of particle surface properties in controlling nucleation efficiency. Thus, in future cloud formation studies, solely assessing condensation curves to determine whether a type of condensate can form a cloud is not enough. We need to consider the condensate's properties and the potential CCN to assess whether an atmosphere can provide realistic supersaturation levels to form clouds.

Figure 1: The critical supersaturation ratio (Scric) that is needed when the timescale of the nucleation and settling equals each other for each type of condensate in Titan's atmosphere, where photochemical haze particles are the main INs. Here we use a range of possible surface energies of Titan haze analogs (“tholins") from Li et al. (2022) to perform the calculation, considering the uncertainty of the properties of tholins.

References:

Anderson, C. M., Samuelson, R. E., Yung, Y. L., & McLain, J. L. (2016). Solid‐state photochemistry as a formation mechanism for Titan's stratospheric C4N2 ice clouds. Geophysical Research Letters, 43(7), 3088-3094.

Anderson, C. M., Samuelson, R. E., & Nna-Mvondo, D. (2018). Organic ices in Titan’s stratosphere. Space Science Reviews, 214, 1-36.

Coll, P., Navarro-González, R., Szopa, C., Poch, O., Ramírez, S. I., Coscia, D., ... & Israël, G. (2013). Can laboratory tholins mimic the chemistry producing Titan's aerosols? A review in light of ACP experimental results. Planetary and Space Science, 77, 91-103.

Li, J., Yu, X., Sciamma-O’Brien, E., He, C., Sebree, J. A., Salama, F., ... & Zhang, X. (2022). A cross-laboratory comparison study of Titan haze analogs: Surface energy. The Planetary Science Journal, 3(1), 2.

Tomasko, M. G., Doose, L., Engel, S., Dafoe, L. E., West, R., Lemmon, M., ... & See, C. (2008). A model of Titan's aerosols based on measurements made inside the atmosphere. Planetary and Space Science, 56(5), 669-707.

Yu, X., Yu, Y., Garver, J., Li, J., Hawthorn, A., Sciamma-O’Brien, E., ... & Barth, E. (2023). Material properties of organic liquids, ices, and hazes on Titan. The Astrophysical Journal Supplement Series, 266(2), 30.