Ten Years Orbiting Saturn: What Have We Learned about Saturn's Atmosphere?

Abstract:

In the 10 years that Cassini has been orbiting Saturn, a wealth of observations have been returned that have probed a wide range of altitudes in Saturn's atmosphere from <1 microbar to roughly 3 bars. Radar and 5-micron VIMS images reveal spectacular cloud structure at the 3-bar level. ISS, CIRS, and VIMS probe the upper troposphere where storms originate. CIRS probes the stratosphere and upper troposphere, while UVIS sounds the stratosphere and mesosphere.

Some of the highlights include: 1) the Great Northern Storm of 2010-2011. This planet-encircling storm is believed to have originated in the water cloud (P>10 bars) with dramatic effects on the cloud structure in the upper troposphere (0.5 bars), and, quite surprisingly, the generation of localized heated regions in Saturn's stratosphere near 2 mbars. Cassini's full instrument complement was used to study the storm, leading to the detection of fresh ammonia ice in the troposphere and enhanced temperatures and hydrocarbons, such as ethylene, in the stratosphere.

2) The detection of hurricane-like features at both the North and South Pole of Saturn. These exhibit interesting structure at the spatial resolution of ISS and VIMS, while CIRS has detected enhanced temperatures at both poles.

3) The persistence of the Northern Hexagon at 79N. This 6-sided feature was detected by Voyager and it has been studied by the full suite of Cassini remote sensing instruments. It is detectable as a warm region at 100 mbars, but there is no thermal signature in the stratosphere.

4) Seasonal reversal of photochemically produced aerosols. When Cassini arrived at Saturn, the winter northern hemisphere appeared blue, while the southern hemisphere exhibited an orange appearance. Shortly after equinox in 2009, the appearance reversed quite abruptly.

During the last stage of the mission in 2016-2017, Cassini will obtain measurements at very high spatial resolution during the F-ring and Proximal orbits. The Ion and Neutral Mass Spectrometer will directly sample the upper atmosphere, while on other orbits, precise measurements of Saturn's gravity will provide valuable constraints to models of Saturn's interior.