Saturn's visual appearance is controlled by the distribution and optical properties of clouds and haze. From thermochemical equilibrium theory we believe that we are seeing the top of an ammonia ice cloud which lies above deeper ammonium hydrosulfide and water/ammonia clouds. In addition there is a mixture of colored `chromophore' particles whose composition remains uncertain. The subtle contrasts and colors change with time and do not correlate well with the more stable zonal wind profile. The highest clouds are in the equatorial band. Cloud altitudes in middle latitudes respond to seasonal forcing. Cloud tops deepen during the summer as expected if absorbed solar radiation enhances the sublimation rate of ammonia ice and the static stability of the upper troposphere. Recent Cassini polarization images show that the polar regions are darker than other latitudes and are more highly polarizing. These attributes are found also in Jupiter's polar stratospheric haze and suggest a population of small (sub-micron) stratospheric haze particles or aggregates of very small particles produced from auroral chemistry. Unlike Jupiter, the polar haze reflectivity is very weak in the near-infrared methane-band images below 1-micron wavelength, so the Saturnian polar haze particles must be very small (less than ~0.1 micron radius). There is a sharp boundary to the polar haze indicative of polar vortex confinement. We anticipate many advances in our understanding of the haze and clouds with a wealth of new data to be acquired by the UVIS, ISS, VIMS and CIRS instruments on the Cassini orbiter beginning in February of 2004.
35th COSPAR Scientific Assembly
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