Modeling quasi-specular scattering from the surface of Titan

Abstract

The Cassini spacecraft obtained several scattering observations of the surface of Titan. Four data sets, TA-Inbound, TA-Outbound, T3-Outbound, and T8-Inbound, include scatterometer data for small angles of incidence. Analysis of these data sets provides an example use of a new generalized fractal-based quasi-specular scattering law that gives the surface RMS slope as a function of horizontal scale and covers a wide range of scattering behavior using a small number of parameters. The data have a significant diffuse scattering component which is modeled with the empirical cosine law, A cos ⁿ $\theta$. The reflectivity and RMS surface parameters are retrieved from the quasi-specular scattering model. The average RMS slope for the four data sets is about 4.2° at a horizontal scale of 5λ λ ~ 2.18 cm. The Fresnel power reflection coefficient ρ is approximately equal to 0.085 (corresponding to a dielectric constant $\epsilon$ _r ~ 3.3) for TA-Outbound and T3-Outbound and 0.034 ($\epsilon$ _r ~ 2.1) for TA-Inbound and T8-Inbound. Inadequate modeling of diffuse scattering by the cosine law results in increased uncertainties in the inferred surface parameters, especially ρ, and, consequently, in inferences of the surface composition.