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Project Progress

Co-Investigator Roger Yelle and his research group have performed detailed simulations of the aerosol and cloud evolution at the Huygens landing site, validated against the optical properties retrieved by the Huygens DISR instrument. Our results provide a good fit to the observed particle size, density, and phase functions, which put further constraints on the aerosol mass production rate, the atmospheric mixing, the monomer radius and the aerosol charge density. The aerosol mass flux required to match the observations is 3.0 × 10^-14 g cm²s-¹, the charge density of the particles is 15 e/μm, while the atmospheric mixing necessary to reproduced the observed aerosol extinction profiles is larger than the profile retrieved for the gas species in the lower atmosphere. Furthermore, we have retrieved an adjusted imaginary refractive index (k) for the aerosol particles that is able to reproduce the observed wavelength dependence of the single scattering albedo.

The pure aerosol simulation provides a good match to the observations above 80 km, while for lower altitudes the inclusion of cloud formation is necessary in order to reproduce the vertical extinction profiles and the wavelength dependence of the opacity. We include in the aerosol simulation detailed description of nucleation and condensation/evaporation processes that are also coupled with the evolution of the condensing/evaporating gases. We consider three condensates: HCN, C₂H₆ and CH₄, which are able to reproduce the main observed characteristics. Between 80 and 30 km, the HCN-nucleated aerosols are the main opacity source, while below 30 km the contribution of CH₄ clouds is becoming more important. We consider both ice methane and liquid CH₄-N₂ clouds in the calculations with the latter reaching to a sub-millimeter size. Ethane clouds have a minor contribution to the opacity and their size is of a few microns.