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E. Karkoschka, M. G. Tomasko (University of Arizona, Lunar & Planetary Lab.)
We investigated the seasonal change of Uranus based on various data sets. First, we took our 1997 Hubble Space Telescope (HST) images of Uranus and determined photometric center-to-limb profiles for all visible latitudes. This let us to construct images from other viewing angles assuming no physical change. The resulting seasonal light curves of Uranus agree very well with the b- and y-filter data by Lockwood and Thompson (1999, Icarus 137, 2) since 1982, but deviate significantly during the period 1972-1982. Thus, the observed brightness variations since 1982 are mostly due to the changing illumination. Before 1982, the brightness variation of Uranus is partially due to physical change in Uranus' atmosphere.
We compared HST images taken between 1994 and 1998 in identical filters. When accounting for the changing illumination, there is remarkable agreement in the latitudinal structure throughout this period. We compared recent HST images with images taken in 1986 by the Voyager 2 spacecraft. We find that the latitudinal structure has remained almost constant with slight changes in the southern polar region. However, since the filters don't match each other completely, and since the phase angle of both data sets are different, it is difficult to evaluate the changes quantitatively. It is believed that the observed changes are probably real. Nevertheless, these variations are much too small to account for large variations determined by Walter and Marley (1998, Icarus 132, 285), such as a temporal variation of the haze optical depth by a factor of 20.
We compared the frequency of discrete cloud features observed by Voyager 2 in 1986 and by HST between 1994 and 1998. When accounting for the spatial resolution of each image and its wavelength together with spectral characteristics of these clouds, we find that the frequency of clouds has remained constant within the statistical fluctuations expected for small numbers. However, since Uranus' northern hemisphere is far more active than its southern hemisphere, during the current season more and more discrete clouds became visible from year to year. If Uranus' physical activity will remain constant, this trend is expected to continue during the following decades.
This work is supported by STScI grant GO-07429.01-96A.