Sayanagi, K.M., Ewald, S.P., Ingersoll, A.P. (2009). "New Cassini ISS Observations of Saturn's Northern Hemisphere Ribbon and Hexagon" American Astronomical Society, DPS meeting #41, #10.08.


Abstract
We report new Cassini observations of the ribbon and the hexagon in Saturn's northern hemisphere. Their latitudes coincide with the zonal jets at 47 and 77 degrees planetographic latitude, and have been hypothesized to arise from shear instabilities in the jets (Allison et al., 1990; Godfrey and Moore, 1986). New modeling studies by Sayanagi et al. and Morales-Juberias et al. (both manuscripts under preparation) of the ribbon and the hexagon, respectively, show that both zonal jets are indeed susceptible to shear instabilities, and develop morphologies resembling the ribbon and the hexagon. In particular, our model predicts that the ribbon's meandering path continuously changes its course.

Using new Cassini ISS images, we first analyze the temporal characteristics of the ribbon to test the model prediction. We then re-examine the zonal mean wind profile of the ribbon jet. The wind measurement using Voyager data by Sanchez-Lavega et al. (2000) places the peak speed of the jet at 150 m/s, which matches the ribbon's propagation phase speed measured by Sromovsky et al. (1983). This raises the possibility that the cloud-tracking wind measurement was influenced by the Ribbon's propagation; thus we systematically isolate the wind speed from the wave propagation speed. In addition, we study the cloud morphology in the Hexagon region, which was in the winter polar night until recently since Cassini arrived the Saturnian system in 2004. Although infrared observations using Cassini's VIMS and CIRS have previously detected the hexagon in the deep troposphere and the stratosphere, it is the first time the whole hexagonal shape can be mapped out in visible light with Cassini. Using these new images, we examine the fine-scale cloud motions in the region to investigate the interaction between the hexagon and the numerous eddies that exist in the region.

Our study is funded by the Cassini Project.