Uranus' apparent seasonal variability in 25 HST filters

Hubble Space Telescope (HST) images of Uranus taken in 1997 reveal that Voyager's view of the southern hemisphere was not representative of the rest of the planet. The banded structure with hemispherical asymmetry, presented in 25 filters between 0.27 and 2.03 mum wavelength, caused Uranus' full disk to darken by as much as 35% between 1985 and 2001, and probably much more over a full uranian season. The amplitude is strongly wavelength dependent. Hemispherical albedo asymmetries are as large as a factor of two over low latitudes and could be larger when they include higher latitudes that will be observable soon. Previous models of Uranus' atmosphere do not apply to northern latitudes. Some observed brightness variations of Uranus, explained as physical variations in previous studies, can now be fully explained by the geometry of changing subsolar latitudes. HST images taken between 1994 and 2000 show the latitudinal albedo structure and the number of discrete clouds in both hemispheres remaining roughly constant, although a slight darkening of high southern latitudes at some filters probing the methane haze indicates a slow decrease in its optical depth. A similar conclusion applies when including Voyager images of 1986. Physical changes in Uranus' atmosphere were many times faster before 1982 than since, and a similar rapid change can be expected in the near future. (C) 2001 Academic Press.