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H. B. Hammel (Space Science Institute), K. A. Rages (SETI/Ames), A. J. Friedson (JPL)
Analysis of Hubble Space Telescope (HST) images of Uranus taken between 1994 and 2002 shows evidence for seasonal change in the south polar region. We see the development of a bright ring around Uranus' south pole at ~70 degrees S between 1994 and 2001. Specifically, latitudes from 69-72 degrees S have darkened less than their surroundings in the F791W HST filter, with the result that in 2001 they are about 2% brighter than the surrounding latitudes. The pole itself changes from being the brightest region in the southern hemisphere to a relatively dark one.
The most likely cause of the overall darkening of Uranus' south polar regions over the past decade is a decrease in the opacity of the condensed methane cloud at ~1.3 bar. The narrow band developing at 70 degrees S cannot be located at this altitude, however, since there is no trace of it in the 619-nm methane band filter, which is quite sensitive to variations near the 1-bar pressure level. The narrow band may represent latitudinal structure in Uranus' deeper tropospheric cloud (possibly composed of hydrogen sulfide) with a top at 3-4 bars; e.g., the cloud tops may be slightly higher at 70 degrees S than at surrounding latitudes.
Also between 1997 and 2002, the "collar" at the edge of the polar cap near 45 degrees S became more prominent in all filters for which data are available from both years. Comparison of images through different filters suggests that the change is occurring at pressures of 2-4 bars in the atmosphere. The observed changes are not predicted by existing dynamical models of Uranus' atmosphere. Continued observations of Uranus as the planet approaches its 2007 equinox may well reveal further atmospheric variability, perhaps including the development of a bright cap over the northern pole.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.