FRACTAL AGGREGATES IN TITAN ATMOSPHERE

It has been suggested that the haze aerosols in Titan's atmosphere might present an irregular structure, rather similar to the morphology of aggregates experimentally synthesized by Bar-Nun et al. (J. geophys. Res. 93, 8383, 1988). The theoretical approach of West (Appl. Opt. 30, 5316, 1991) and West and Smith (Icarus 90, 330, 1991), which uses a fractal concept to numerically generate aggregates, allowed us to support this idea and to provide constraints on their size and shape by comparing the observed and modelled polarization properties of such particles. The building mechanism of these aerosols, when analysed using microphysical modelling (Cabane et al., Icarus %, 176, 1992), leads naturally to aggregates. They are formed of spherical compact monomers, which build up in the region of photochemical synthesis, and whose radius depends mainly on the atmospheric pressure at the formation level. The subsequent growth of aggregates in the settling phase is treated here by introducing the fractal dimension as a parameter of the model (D(f) almost-equal-to 2 in the case of cluster-cluster aggregation). Using this fractal model, a vertical distribution of size and number density of the aggregates is obtained down to almost-equal-to 80 km for different production altitudes. The previous estimate of the formation altitude of photochemical aerosols (almost-equal-to 350-400 km) is confirmed when comparing the number of monomers per aggregate deduced from the present study with the value proposed by West and Smith. The vertical profile of the effective radius of aggregates is calculated as a function of the visible optical depth derived from Voyager imaging. A good fit with the radius derived from Voyager forward-scattering measurements is obtained (almost-equal-to 0.3-0.5 mum), still using a low formation altitude. Finally, it must be emphasized that, for the first time, observational and theoretical results about the size and the structure of particles are reconciled.