Backscatter Modeling of Titan's Surface Features Using a Comprehensive Collection of Cassini Radar Data
Résumé
The Cassini RADAR cycles between four modes (radiometry, scatterometry, altimetry, and synthetic aperture imaging) as it swings by Titan on a given orbit. Scatterometry uses the central antenna beam at 13.78-GHz (2.17-cm) in a real-aperture mode to produce regional-scale backscatter images across large areas of the surface. Raster scanning also achieves large angular coverage to properly sample the average backscatter response over the region. A backscatter function over a wide range of angles reveals much about the dielectric composition, surface and subsurface scattering properties. By modeling the backscatter response for individual features (e.g. crater ejecta and dune fields) rather than the average response over regional areas, we can constrain the composition and structure of specific units on Titan's heterogeneous surface, and thus the processes responsible for the feature's formation and evolution. Unfortunately, scatterometer-data alone does not provide adequate angular sampling of a specific feature's backscatter response. Of the 17 scatterometry scans acquired to-date, there are only eight distinct areas of overlap giving multi-angle coverage. But by applying the real-aperture processing techniques of the scatterometer to the other active radar modes, we can combine similarly calibrated datasets and complete the angular sampling. Altimetry provides the very low angle response (less than 1°), while SAR gives the mid-range response (10° to 40°). We separate the backscatter response into two different regimes: surface scatter dominates at low angles and volume scattering dominates at large angles. We use traditional facet scattering models (such as Hagfors' or Gaussian laws) to describe the quasi-specular scatter of the surface term, which yields the tightest constraints on dielectric constant and surface slopes. We consider two different approaches to model the diffuse volume term: an empirical cosine-law and a model that incorporates the radiometer emission measurements. We present the results of both models applied to specific features' backscatter curves.