Condensation in Titan's atmosphere at the Huygens landing site

We present a self-consistent description of Titan's aerosols-clouds-gases system and compare our results with the optical properties retrieved from measurements made by the Descent Imager/Spectral Radiometer (DISR) experiment on the Huygens probe (Tomasko, M.G. et al. [2008]. Planet. Space Sci. 56, 669-707). Our calculations include the condensation of methane, ethane and hydrogen cyanide on photochemical aerosols produced in the thermosphere. Our results suggest that the two distinct extinction layers observed by DISR below 80. km are produced by HCN and methane condensation, while for the Huygens' equatorial conditions simulated here, the contribution of ethane clouds to the total opacity is negligible. The HCN mass flux is comparable to the mass flux of aerosols, thus the majority of the HCN cloud particles have a similar size with the aerosol particles, they are HCN-coated aerosols. Ethane cloud particles have sizes between 2 and 10μm depending on altitude, and methane clouds grow to an average size of ~100μm before starting to evaporate below 10. km. The reproduction by the simulation of the main features observed by DISR suggests that the atmospheric snapshot acquired by the Huygens instruments corresponds to a condition very close to the steady state simulated here. This points to the stability of the equatorial atmospheric conditions at the time of the descent. Moreover, we investigate the resulting abundance of ethane in the lower part of the atmosphere and its impact on the flux of condensates to the surface. Our results suggest that under the steady state conditions investigated, ethane condensates evaporate before reaching the surface and that the ethane gas abundance close to the surface is well below its saturation limit.