Many studies based on Cassini observations show the spatial and temporal evolution of cloud formation on Titan as a consequence of seasonal variations. Cassini/VIMS data 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 [1]. The location at which species condense depends on their abundance and the temperature profile. For different times and locations, changes in the temperature profile 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 [2,3,4], when a high concentration of gaseous species is observed, which may explain the cooling required to form clouds at these altitudes.
We use a 1D numerical model previously applied to Titan and Pluto [5,6], 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.
I will first demonstrate the validity of the model used at the equator, where more observational constraints for hazes and clouds are available. I will then present the first results of the simulation at the South Pole, during the post-equinox period, where we will compare the evolution of the condensation of gaseous species and the haze and cloud profiles with equatorial conditions. We also validate the model with Cassini/CIRS observations of gas abondance.
