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E. Karkoschka, M. G. Tomasko (U. Arizona)
Images taken with the Hubble Space Telescope in 1997 show a number of discrete clouds and zonal latitude bands. Consistent with the views of Uranus by Voyager 2, these features are hardly visible at wavelengths of Voyager's filters. On the other hand, at red and near infrared wavelengths, they display very large contrasts with intrinsic albedo variations of up to a factor of 10.
This uncommon wavelength dependence can be explained by a very clear upper troposphere and hazes or clouds of moderate optical depth. Zonal features seems to be caused by variations of optical depth at lower altitudes than those of discrete clouds. The aerosols causing both kinds of features are larger than the thin underlying population expected in the upper troposphere.
The large zonal contrasts cause the geometric albedo to vary by possibly up to a factor of 2 between equinoxes and solstices. Thus, observed seasonal variations of the albedo of Uranus may be largely caused by the changing geometry rather than by physical changes on Uranus.
The albedo contrasts on Uranus may be unique in the solar system. All other investigated solar system objects have smaller albedo variations deriving from the changing seasonal geometry.
This work is supported by STScI grant GO 07429-0196A and by NSF grant AST 9417368.