Simulation of the interplay between haze and clouds at equatorial and polar conditions of Titan's atmosphere

Titan's atmosphere is mainly composed of nitrogen and methane. The solar flux triggers complex photochemical reactions that produce organic compounds, leading to the formation of photochemical aerosols. Further down in the atmosphere, temperatures drop sufficiently to allow the photochemical species to condense on the surface of the aerosols and form clouds. Titan is subject to strong seasonal variations due to its inclination, resulting in a global circulation that generates dynamics from the deep stratosphere to the mesosphere [1].

Many studies based on Cassini observations show the spatial and temporal evolution of cloud formation on Titan as a consequence of seasonal variations [2,3]. Data from the Visible and infrared mapping spectrometer (VIMS) on board Cassini, were used to study the seasonal changes that occur between the two poles. The polar cloud observed in the north during winter gradually disappears, only to reappear in the south during spring in the north [4]. The location at which species condense depends on their abundance and the temperature profile. For different times and locations, changes in the temperature and the transport of photochemical species by the meridional circulation allow some species to condense at different altitudes. The formation of HCN and C6H6 clouds has been observed between 250 and 300 km at the South Pole after the equinox [5,6,7], when a high concentration of gaseous species is observed, which may explain the cooling required to form clouds at these altitudes. Hanson et al. 2023 [8] demonstrated through a 1D simulation the formation of HCN clouds near 300 km and descends to the lower stratosphere followed by precipitation to the surface. Batz de Trenquelléon et al. 2025 [9] obtained results on the formation of winter polar clouds from 60 to 300 km after enrichment with trace compounds using the Titan Planetary Climate Model (3D model).

Here we advance further on the details of the interplay between gases, haze and clouds, by investigating the condensation of all major gases in Titan’s atmosphere. We use a 1D numerical model previously applied to Titan and Pluto [10,11], which combines radiative transfer, photochemistry, microphysical evolution of haze and clouds, condensation and nucleation. The model also takes into account atmospheric mixing, molecular diffusion, particle sedimentation and diffusion. Primary particles form in the upper atmosphere and then coagulate to form aggregates. The growth mode of settling haze particles is controlled by the fractal dimension of the aerosol. Cloud particle formation is initiated by heterogeneous nucleation of gas on a haze particle under supersaturated conditions. We introduce 23 gaseous species into cloud formation. The rates of condensation and evaporation are given by the mass flux of condensing species into and out of the particle surface.

We first demonstrate the validity of the model used at the equator, where more observational constraints for hazes and clouds are available for gas abundances and optical properties. We will then present the results of the simulation at the South Pole, during the post-equinox period, where we compare the evolution of the condensation of gaseous species and the haze and cloud profiles with equatorial conditions. To account for the photochemical and haze particle enrichment occurring  at the South Pole post-equinox, we incorporate a circulation contribution, derived from observations, into our model. This addition contributes significantly to the observed mass of chemical species and haze particles involved in cloud formation. We further validate the model using the Composite Infrared Spectrometer (CIRS) observations of gas abundance and haze abundance & extinction.

 

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[2] R.A. West, A.D. Del Genio, J.M. Barbara, D. Toledo, P. Lavvas, P. Rannou, E.P. Turtle, J. Perry, Cassini Imaging Science Subsystem observations of Titan’s south polar cloud, Icarus, Volume 270, 2016, Pages 399-408, ISSN 0019-1035, https://doi.org/10.1016/j.icarus.2014.11.038.

[3] S. Vinatier, B. Schmitt, B. Bézard, P. Rannou, C. Dauphin, R. de Kok, D.E. Jennings, F.M. Flasar, Study of Titan’s fall southern stratospheric polar cloud composition with Cassini/CIRS: Detection of benzene ice, Icarus, Volume 310, 2018, Pages 89-104, ISSN 0019-1035, https://doi.org/10.1016/j.icarus.2017.12.040.

[4] S. Le Mouélic et al. Mapping polar atmospheric features on Titan with VIMS: From the dissipation of the northern cloud to the onset of a southern polar vortex, Icarus, Volume 311, 2018, Pages 371-383, https://doi.org/10.1016/j.icarus.2018.04.028.

[5] Teanby, N. A., Sylvestre, M., Sharkey, J., Nixon, C. A., Vinatier, S., & Irwin, P. G. J. (2019). Seasonal evolution of Titan's stratosphere during the Cassini mission. Geophysical Research Letters, 46, 3079–3089. https://doi.org/10.1029/2018GL081401

[6] de Kok, R., Teanby, N., Maltagliati, L. et al. HCN ice in Titan’s high-altitude southern polar cloud.Nature 514, 65 - 67 (2014). https://doi.org/10.1038/nature13789

[7] Vinatier, S. et al. Seasonal variations in Titan’s middle atmosphere during the northern spring derived from Cassini/CIRS observation. Icarus, Volume 250 (2015), Pages 95-115, https://doi.org/10.1016/j.icarus.2014.11.019.

[8] Hanson, L. E., Waugh, D., Barth, E., & Anderson, C. M. Investigation of Titan’s South Polar HCN Cloud during Southern Fall Using Microphysical Modeling, PSJ, 4, 237 (2023). https://doi.org//10.3847/PSJ/ad0837

[9] Bruno de Batz de Trenquelléon et al. The new Titan Planetary Climate Model. II. Titan’s Haze and cloud cycles. Planet. Sci. J. 6 79 (2025). https://doi.org/10.3847/PSJ/adbb6c

[10] P. Lavvas, C.A. Griffith, R.V. Yelle, Condensation in Titan’s atmosphere at the Huygens landingsite,Icarus,Volume215,Issue2,2011,Pages732-750. https://doi.org/10.1016/j.icarus.2011.06.040.

[11] Lavvas, P., Lellouch, E., Strobel, D.F. et al. A major ice component in Pluto’s haze. Nat Astron 5,289–297 (2021). https://doi.org/10.1038/s41550-020-01270-3