Radar scattering from Titan and Saturn's icy satellites using the Cassini spacecraft [electronic resource]
Keywords: Electrical Engineering

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. 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, penetrates the hazy atmosphere to detect the surface. RADAR operates in several modes. It calculates surface height profiles, measures the emissivity, and also maps the surface at resolutions as fine as 300 m. The high-resolution 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, or the backscatter function. We develop a real aperture processor to reduce the scatterometer data, and also extend this reduction to the other active modes of the RADAR instrument. We calibrate the different modes in order to combine the data sets globally. We correct the measured backscatter for incidence angle effects to produce a global backscatter map with real-aperture resolutions between 10 and 250 km. This is the first time Titan has been mapped globally at cm wavelengths. With all RADAR data processed and calibrated to the same scale, we obtain detailed scattering behavior at different locations on Titan over a range of incidence angles. We model the backscatter functions to obtain estimates of the surface composition and physical structure for specific Titan features. We infer dielectric constant values consistent with solid hydrocarbons over much of Titan's surface, but the brighter regions often appear more consistent with water ice bedrock. Almost all features are dominated by diffuse volume scatter, which comprises more than 80\% of the radar echo. Comparison of the feature model results demonstrates the heterogeneity of the surface scattering parameters across Titan, and contributes to the understanding of the geological processes responsible for each feature's formation and evolution. We extend the real aperture processor and modeling technique to other moons of Saturn. We compare the icy satellite backscatter models to those obtained for Titan features and detect similar diffuse scattering behavior. We further analyze the backscatter from Titan's largest southern lake, called Ontario Lacus. Altimetry observations over the lake reveal the first specular glints from a liquid surface on Titan. We model this data with specular reflection theory to constrain the height variation of the surface waves (if any). We find that Ontario Lacus must be mirror-smooth to reproduce the observed scattering levels, results that have direct implications for wind speed and wave generation on Titan's liquid surfaces. We further investigate off-nadir imaging and real aperture data over Ontario Lacus, employing a two-layer scattering model to produce bathymetry maps and depth profiles across the lake. We find that the lake is shallow, with mean depths around 3-5 meters, and likely has a volume of 50-80 cubic-km.