Simulating clouds formation in Titan&#8217;s South polar region during post equinox
Authors: Damiens Antoine , Lavvas Panayotis 
Published: July 2024

ContextIn Titan&#8217;s lower stratosphere/high troposphere, temperature drops sufficiently for photochemical species to condense over the aerosol surface and form clouds. Condensation is triggered when the gaseous abundance crosses the saturation limit for each gas. Multiple studies, based on Cassini observations have explored the temporal and spatial variability of Titan&#8217;s clouds. Le Mou&#233;lic et al. 2018 [2] showed the progressive disappearance of the north polar cloud during the northern winter and its appearance at the south pole during the northern spring. VIMS data were used to study the rapid temporal changes that occur between the two poles before and after the equinox. Kok et al. 2014 [1] observed the formation of an HCN cloud in 2012 over the South pole in the upper part of the stratosphere, at an altitude of 300 km. The temperature required for HCN condensation is about 125 K. After the equinox (in 2009), a high concentration of tracer gases is observed at the South Pole [3], which may explain the significant cooling required to form the cloud at 300 km (assuming it&#8217;s greater than the heating due to the downward movement of air). Hanson et al. 2023 [4] discussed that the HCN cloud forms near 300 km and descends to the lower stratosphere followed by precipitation to the surface. Here we expand on previous studies by studying cloud formation with simultaneous condensation of multiple species in the south polar region.Model DescriptionWe use a 1D numerical model previously applied to Titan [5] that combines radiative transfer, photochemistry, microphysical evolution of haze and cloud size distributions, tracking of cloud condensation and nucleation, and accounts for atmospheric mixing, molecular diffusion, particle sedimentation and mixing. Primary particles are formed in the upper atmosphere and then coagulate to form aggregates. The growth mode of the falling haze particles is controlled by the fractal dimension of the aerosol. Cloud particle formation is initiated by heterogeneous nucleation of HCN on a haze particle under supersaturation conditions. We introduce 22 gas condensing species into the model that contribute to cloud formation. The rates of condensation and evaporation are given by the mass flux of condensing species entering and leaving the particle surface. This method has already been used for the study of Triton and Pluton [6], based on the model description of Smolarkiewicz 1982 [7].ResultsWe explore the cloud / haze properties by showing simulation results at different latitudes, assuming temperature profiles corresponding to CIRS observations. We vary these temperature profiles as a function of solar longitude. After a benchmark of the simulation in equatorial conditions, we focus on south polar conditions to follow the cloud formation with different gas species. We study the evolution of the condensation of gas species at the South Pole after the equinox by comparing the simulated results with the observations from 2012 to 2017. We provide results on the optical properties, the simultaneous condensation of other gas species, and the microphysics of haze and clouds under the conditions described above.References[1] de Kok, R., Teanby, N., Maltagliati, L. et al. HCN ice in Titan&#8217;s high-altitude southern polar cloud. Nature 514, 65 - 67 (2014). https://doi.org/10.1038/nature13789[2] S. Le Mou&#233;lic 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.[3] Vinatier, S. et al. Seasonal variations in Titan&#8217;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.[4] Lavender E. Hanson et al. , Investigation of Titan&#8217;s South Polar HCN Cloud during Southern Fall Using Microphysical Modeling, Planet. Sci. J. 4 237 (2023), https://doi.org/10.3847/PSJ/ad0837[5] P. Lavvas, C.A. Griffith, R.V. Yelle, Condensation in Titan&#8217;s atmosphere at the Huygens landing site, Icarus, Volume 215, Issue 2, 2011, Pages 732-750, https://doi.org/10.1016/j.icarus.2011.06.040.[6] Lavvas, P., Lellouch, E., Strobel, D.F. et al. A major ice component in Pluto&#8217;s haze. Nat Astron 5, 289&#8211;297 (2021). https://doi.org/10.1038/s41550-020-01270-3[7] Piotr K. Smolarkiewicz. A Simple Positive Definite Advection Scheme with Small Implicit Diffusion, Monthly Weather Review, 479&#8211;486 (1983), https://doi.org/10.1175/1520-0493(1983)1112.0.CO;2