West, R., Ingersoll, A., del Genio, A., Vasavada, A., Barbara, J., Porco, C., et al. (2004). "The Saturn Atmosphere: An Early Cassini ISS Perspective" Eos Trans. AGU 85(47), Fall Meet. Suppl., Abstract P44A-05.


Abstract

During the approach and early part of the Cassini tour of the Saturn system the Imaging Science Subsystem (ISS) obtained a large number of images of Saturn. At this early phase of the mission we report quantitative results for some of the ISS atmospheric science objectives. Positional measurements of small spots show the same zonal wind profile reported from Voyager investigations (data from 1980-81) at latitudes outside of +- 18 degrees. In the latitude range within +- 18 degrees most of the Cassini measurements cluster around 270 m/s (Eastward), whereas the Voyager results are higher (300-470 m/s). The lower ISS values agree with values reported by Sanches-LaVega et al. (Nature 2003, 423, 623-625) measured from Hubble Space Telescope images between 1994 and 2002. A few of the Cassini measurements within this latitude band give speeds in the range 350-390 m/s which overlap (in latitude) the other points with lower values. Latitudinal temperature gradients point to a decrease in wind with height so some or all of the differences might be interpreted as differences in pressure levels of the cloud features used to track winds, with the Voyager measurements sensing the deepest. Two large storms in 1990 and 1995 put clouds at higher altitudes in the HST and Cassini epochs. Cloud height models from images in methane bands will be used to assess this hypothesis. We also observed spot interactions. The westward-moving jet at approximately 36 degrees south (the one closest to the equator in the southern hemisphere) was particularly active. It had many counter-clockwise rotating spots, which merged and divided during a few months of observation. A combination of polarization images from the violet to the near-infrared, methane-band images and ultraviolet images show a complex latitudinal behavior for cloud and haze structure and haze properties at high southern latitudes.