The Science from Cassini CIRS, Today and over the Next Three Years

Abstract:

The longevity of the Cassini Mission has provided an unusual opportunity to study the Saturn system, and we report major results on temperatures, atmospheric composition, and condensates from CIRS thermal-infrared spectra. Titan has a pronounced seasonal cycle, and its winter poles are characterized by strong circumpolar winds enclosing a region of cold stratospheric temperatures, enhanced trace organic gases, and condensates. Cassini arrived in early northern winter and now it is mid-spring. The northern vortex has dissolved gradually, but the buildup of Titan's south polar vortex has been quite rapid, evidenced by the marked increase of trace organic gases and condensates. Saturn has seasonal and more irregular behavior. Like Earth, its stratosphere has an equatorial oscillation, with a descending pattern detected in Cassini's extended tour. The great northern storm that erupted in late 2010 was unexpected, leading to perturbations in tropospheric composition, and stratospheric anticyclones. Although the visible storm died out in a few months, the highly disturbed stratosphere persisted until this year. In the remainder of Cassini's tour, there will also be new observations of the thermally anomalous "Pac-Man" features on Mimas, Tethys, and Dione, which will provide better constraints on their physical surface properties and spatial distribution. Regions of Hyperion, Epimetheus, Janus, Pandora and Atlas will be mapped for the first time or at significantly better spatial-resolution than before. Two close flybys of Enceladus in late 2015 will allow the sampling of the structure of tiger stripe thermal emission on sub-kilometer scales. As the southern winter deepens, its endogenic signature will be less polluted by passive emission, providing the best opportunity to determine its heat flow and constrain its source. The F-ring and Proximal orbits will provide close views to obtain limb sounding of Saturn's equatorial region and map the thermal structure and composition of its north and south poles. CIRS will resolve structures in the rings previously inaccessible in the thermal infrared, including the plateaux in the C ring and broad structures associated with resonances in the A ring dubbed 'haloes,' and it will obtain the best resolution and signal-to-noise on the F ring of the entire tour.