Titan's Geometric Albedo: Role of the Fractal Structure of the Aerosols

The collisional mechanisms of Titan's aerosols may lead to a fractal structure in which the aerosols are built by the aggregation of spherical submicrometer particles (monomers). In this initial study of the problem, the optical behavior of these aggregates is modeled assuming that each monomer radiates a dipole field in response to the incident radiation 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 extinction 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 by microphysical modeling, we compute the geometric albedo of Titan. Computed values and observational values of the albedo are compared for wavelengths from 0.22 to 1.0 μm, and the effects of parameters, such as the fractal dimension of aerosols, their formation altitude or mass production rate, and, in addition, the methane abundance, are investigated. The hypothesized fractal structure of particles can explain both the visible and the UV albedos. In previous models these measurements could only be matched simultaneously under the assumption of a bimodal population. For a fractal dimension Df ≈ 2 in the settling region, corresponding to a growth governed by cluster-cluster aggregation, the computed albedo in the near-UV range matches the observations. A good fit between measurement and calculated albedo is obtained, for a formation altitude Z0 = 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 × 10-13 kg m-2 sec-1. Lower formation altitudes, like our preferred case, Z0 = 385 km, cannot be investigated in the UV range due to limitations of the dipolar approximation, but we expect this case to give the same behavior.