Saturn's Polar Atmosphere: Seasonal Change and Composition from Cassini
Abstract
Cassini's prime mission permitted the first detailed infrared studies of the polar atmosphere of Saturn, under both southern summertime and northern wintertime conditions. Both poles exhibited small, discrete warm vortices (`hotspots') in the troposphere and stratosphere, which were depleted in disequilibrium species and hazes, surrounded by strong prograde jets and characterised by distinct `eyewalls.' In addition, the south polar stratosphere had an extensive warm polar `hood' between 70-90° S, which was expected to be seasonal in nature and related to the presence of high-altitude stratospheric hazes, localised at the south pole. Finally, infrared observations revealed the continued presence of the northern hexagon discovered during the Voyager encounter in 1981, but observed no comparable features in the south. In August 2009 Saturn reached its equinox, so that the cold northern pole emerged into sunlight for the first time in almost 15 years. We present a time series of Composite Infrared Spectrom-eter (CIRS) observations from 2004 to 2009 which demonstrate (a) the continued presence of the polar vortices under spring and autumnal conditions; (b) the cooling of the southern stratospheric hood; and (c) the persistence of the northern hexagon. These are compared with ground-based observations of Saturn from the Very Large Telescope VISIR instrument at the time of ring-plane crossing to study compositional asymmetries between the two poles. Fur-thermore, we derive the spatial distribution of aerosols, phosphine, ammonia and arsine in the north polar hotspot and hexagon from Cassini VIMS 4.5-5.1 µm observations to reveal the mor-phology and composition of these dynamical features. These results will be discussed in terms of our present understanding of the polar circulation systems on Saturn, and their response to seasonal variations of insolation.
- Publication:
-
38th COSPAR Scientific Assembly
- Pub Date:
- 2010
- Bibcode:
- 2010cosp...38.1371F