TITANS GEOMETRIC ALBEDO - ROLE OF THE FRACTAL STRUCTURE OF THE AEROSOLS

The collisional mechanisms of Titan's aerosols may lead to a fractal s tructure in which the aerosols are built by the aggregation of spheric al submicrometer particles (monomers). In this initial study of the pr oblem, the optical behavior of these aggregates is modeled assuming th at each monomer radiates a dipole field in response to the incident ra diation including the radiated fields of all the other elements in the aggregate. This dipole approximation, valid if the monomer radius is smaller than the wavelength, is used to calculate the scattering and e xtinction efficiencies of such aerosol particles, which are assumed to be composed of tholins. By applying the two-stream approximation for radiative transfer to the vertical distribution of aerosols obtained b y microphysical modeling, we compute the geometric albedo of Titan. Co mputed values and observational values of the albedo are compared for wavelengths from 0.22 to 1.0 mu m, and the effects of parameters, such as the fractal dimension of aerosols, their formation altitude or mas s production rate, and, in addition, the methane abundance, are invest igated. The hypothesized fractal structure of particles can explain bo th the visible and the UV albedos. In previous models these measuremen ts could only be matched simultaneously under the assumption of a bimo dal population. For a fractal dimension D-f approximate to 2 in the se ttling region, corresponding to a growth governed by cluster-cluster a ggregation, the computed albedo in the near-UV range matches the obser vations. A good fit between measurement and calculated albedo is obtai ned, for a formation altitude z(0) = 535 km, over the whole wavelength range by adjusting the absorption coefficient of the particles within a factor of two from that of tholins and a production rate between 0. 2 and 1.5 times 3.5 x 10(-13) kg m(-2) sec(-1). Lower formation altitu des,like our preferred case, z(0) = 385 km, cannot be investigated in the UV range due to limitations of the dipolar approximation, but we e xpect this case to give the same behavior. (C) 1995 Academic Press, In c.