Cassini ISS Update on Seasonally Evolving Northern Hemisphere, and a new Hexagon Model

Abstract:

We present updates on the continued Cassini ISS observation of Saturn’s northern hemisphere. Our continued imaging campaign reveals temporal evolution of cloud and haze morphologies in the northern hemisphere, which is approaching northern summer solstice in May 2017. Our analysis shows that the disturbance left in the aftermath of the Great Storm of 2010-2011 continues to have an impact on the morphology of the storm-affected latitudes between 35-45 degree N latitudes. The region surrounded by the northern hexagon and the north-polar vortex exhibits changes in the upper tropospheric haze opacity and reflectivity. 

We also present a new dynamical model of Saturn’s northern Hexagon. Observations show that the Hexagon is a near-stationary meandering atmospheric jetstream that acts as a transport barrier of materials; however, past experimental and numerical models of the Hexagon reproduced the six-sided morphology as a vortex street (VS). The VS model does not reproduce the near-stationary drift rate measured in System III reference frame, and a VS is also a poor transport barrier. Our new model demonstrates that a shallow atmospheric jet that is confined above the water condensation level exhibits many of these key characteristics better than the VS model.

Our study is supported by the Cassini Project, NASA Outer Planet Research Program grant NNX12AR38G, and NSF Astronomy and Astrophysics grant 1212216.