FRACTAL AGGREGATES IN TITAN ATMOSPHERE

It has been suggested that the haze aerosols in Titan's atmosphere mig ht 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 f ractal concept to numerically generate aggregates, allowed us to suppo rt this idea and to provide constraints on their size and shape by com paring the observed and modelled polarization properties of such parti cles. The building mechanism of these aerosols, when analysed using mi crophysical modelling (Cabane et al., Icarus %, 176, 1992), leads natu rally to aggregates. They are formed of spherical compact monomers, wh ich build up in the region of photochemical synthesis, and whose radiu s depends mainly on the atmospheric pressure at the formation level. T he subsequent growth of aggregates in the settling phase is treated he re by introducing the fractal dimension as a parameter of the model (D (f) almost-equal-to 2 in the case of cluster-cluster aggregation). Usi ng this fractal model, a vertical distribution of size and number dens ity of the aggregates is obtained down to almost-equal-to 80 km for di fferent production altitudes. The previous estimate of the formation a ltitude of photochemical aerosols (almost-equal-to 350-400 km) is conf irmed when comparing the number of monomers per aggregate deduced from the present study with the value proposed by West and Smith. The vert ical 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 measu rements is obtained (almost-equal-to 0.3-0.5 mum), still using a low f ormation altitude. Finally, it must be emphasized that, for the first time, observational and theoretical results about the size and the str ucture of particles are reconciled.