Saturn's Thermal Emission at 2-cm Wavelength and Implications for Atmospheric Composition and Dynamics

The thermal emission from Saturn's atmosphere has been mapped using the passive radiometer of the Cassini RADAR instrument. The radiometer operates at a frequency of 13.78 GHz, or 2.18-cm wavelength, and uses the spacecraft's main communication antenna to form a beam of 0.36º width at half power. Images of absolute brightness at normal incidence were constructed using data obtained from raster scans during close periapsis passes in September, 2005, and February, 2008. In 2005 most of the equatorial region was mapped within latitudes +/- 60 degrees, while the polar regions were observed in 2008. The sensitivity was about 0.1 K and the best spatial resolution in the images is about 500 km, providing more than an order of magnitude improvement in both parameters over all previous microwave observations of Saturn. At 2-cm wavelength the radiometric weighting function lies at altitudes almost entirely within the ammonia saturation region, and the brightness temperature is therefore primarily sensitive to the cloud-level ammonia concentration. A variety of heretofore-unseen structure attributable to cloud-level ammonia variations was seen and implications for atmospheric circulations will be presented.