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E. Karkoschka, M. G. Tomasko (Univ. of Arizona)
We used Hubble Space Telescope images of Saturn to infer changes in Saturn's vertical cloud structure. Our images cover the period 1991-2004, or early summer to early winter on Saturn's northern hemisphere. The spectral range of 231-2370 nm includes six methane and one hydrogen absorption band. We combined calibrated albedo measurements within each 0.5-degree interval of latitude into 18,000 center-to-limb curves. We found that 98 percent of the variations between these curves can be explained by the superposition of four principal structural features:
The first feature is an opacity variation in the tropospheric cloud and a variation in the cloud-top altitude. It is highly correlated with Saturn's wind speed profile. Its well-known zonal structure is best observable in methane bands, where the Equatorial Zone is the brightest among 10 zones. This feature varied globally with Saturn's seasons, but remained constant on zonal scales.
The second feature is an opacity variation in small, stratospheric aerosols, which are best observable at ultraviolet wavelengths. The highest opacities occur in the polar regions, indicating an origin from auroral bombardment. This feature remained constant.
The third feature is a variation in the tropospheric aerosol size. It is best observable near Saturn's limb at continuum wavelengths near 1000 nm. It varied globally with Saturn's seasons, but had no small-scale structure. Aerosols were smallest in the winter and largest in the summer.
The fourth feature is a variation in the phase function of the tropospheric aerosols, possibly due to variable aerosol shapes. Its irregular latitudinal structure is best observable at visible continuum wavelengths. It structure changed within months.
This work is supported by STScI grant HSTGO0935401A and NASA grant NAG5-12014.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.