Iapetus as seen by the Cassini microwave radiometer

On September 10, 2007, the Cassini spacecraft encountered Saturn's third largest moon, Iapetus, during its closest targeted flyby to-date (IA49). During this flyby, the RADAR instrument scanned the antenna beam in a north-south raster pattern over an area confined to latitudes between 60°S and 75°N and longitudes between 0°W and 150°W, collecting a unique passive (radiometry) and active (scatterometry) data set at the wavelength of 2.2 cm. Measurements were thus acquired mostly over the dark terrains of the leading hemisphere of Iapetus, named Cassini Regio (CR), but also over the bright terrains of the mid/high latitudes, namely Roncevaux Terra (RT) in the north and Saragossa Terra (ST) in the south. Radiometry and scatterometry measurements are real-aperture data with a footprint size of about 120 km, while Iapetus is 718 km in radius. Radiometry data hold important clues for Iapetus' surface. At a wavelength of 2.2 cm, the radiometer probes several tens of cm into the subsurface depending on absorbing impurities, or up to ~5 m in clean water ice. The brightness temperatures inferred from the calibrated antenna temperatures reflect both variations in emissivity and physical temperature of the near-surface across the targeted body. Combined with the concurrent active data set, they thus can shed light on the composition, structure and thermal properties of the Iapetus' dark and bright terrains. Preliminary results inferred from the comparison to a thermal model indicate that the thermal inertias sensed by the Cassini microwave radiometer over both CR and the bright high latitude (RT and ST) terrains exceed that measured by the CIRS (Cassini's Composite Infrared Spectrometer) instrument (Spencer et al., 2007). This suggests a gradient in density with depth. The seasonal contrast between ST (summer at the time of IA49) and RT was captured by the radiometer while the different local solar times of the equatorial observations seem to be responsible for a variation of less than 10 K in the brightness temperature recorded over CR.