Titan is the largest moon of Saturn and the second largest moon in the solar system. It has a thick atmosphere rich in nitrogen and hydrocarbons, analogous to the atmosphere of early, prebiotic Earth. This atmosphere precludes surface observation by traditional optical methods. The Cassini-Huygens spacecraft (a joint endeavor of NASA/ESA/ASI) began orbiting Saturn in 2004, with a flyby of Titan nearly every month. Its RADAR instrument, with a 2.2-cm wavelength, can penetrate the hazy atmosphere and detect the surface. It operates near closest approach on roughly half of the Titan flybys, such that we have 24 collections of Titan radar data to-date.
The RADAR instrument operates in several modes, yielding surface height profiles, emissivity measurements, and also mapping the surface at resolutions as fine as 300 m. These maps reveal a surprisingly Earth-like physical surface, complete with icy mountains, dune fields, cryovolcanoes, flowing liquids, and hydrocarbon lakes. Another operation of the instrument, called scatterometer mode, measures the real-aperture (beam-averaged) backscatter reflectivity as a function of incidence angle. The shape of this backscatter curve reveals much about the surface, such as material composition and roughness structure.
We process the real-aperture scatterometer data and use the same approach on all active modes of the RADAR instrument to obtain detailed scattering behavior at different locations on Titan. We must calibrate the different modes in order to combine the data sets globally. We then correct for incidence angle effects to produce a global backscatter map (93% surface coverage) with real-aperture resolutions between 10 and 150 km. This is the first time Titan has been mapped globally at cm wavelengths.
The backscatter response of specific features on the surface follows from this collective set of backscatter data. The observed backscatter has two components, a surface scattering process that dominates at low incidence angles and a diffuse volume scattering process that dominates at large angles. A superposition of classical facet scattering laws (Hagfors + Exponential generally gives the best fit) describes the quasi-specular scatter of the surface term, and yields the tightest constraints on dielectric constant and surface slopes. We use an empirical cosine-law to model the diffuse volume term. The sum of these laws is the composite backscatter model that we use to compute the surface radar albedo.
We invert the composite backscatter model to estimate surface dielectric and physical conditions for a selection of surface features. For example, the dune fields are best fit with a composite Gaussian + Exponential + Cosine-law that yields an rms surface slope (on the scale of the 2.2-cm wavelength) of 11 degrees, and a dielectric constant with a lower limit of 1.9, suggesting material composed of solid hydrocarbons, such as those that likely rain from the atmosphere. With this modeling approach, we can constrain the composition and structure of specific units on Titan's heterogeneous surface, and thus gain insight into the processes responsible for each feature's formation and evolution.