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L. A. Sromovsky, P. M. Fry (University of Wisconsin - Madison)
We observed Uranus with the Keck 2 telescope using NIRC2 and Adaptive Optics on 11-12 July and 11-12 August 2004. Imaging with J, H, K' filters provided crude cloud height discrimination. Only a few features were high enough to be visible in K' images; all of these were between 20N and 40N. AO imaging resolution reached 0.06 arcseconds in K' during some periods, allowing us to locate 48 trackable cloud features, with 19 providing wind speeds with better than 20 m/s accuracy. (For comparison, Voyager imaging only revealed 8 discrete cloud features.) Wind results are generally consistent with the Voyager and HST observations, but provide new values between 45N and 50N (centric). These results indicate a north south asymmetry in the zonal wind profile (as noted by Karkoschka (Science \bf 280, 570-72, 1998). Most of Uranus' cloud features do not evolve substantially within a transit, and many can be seen on subsequent rotations. A few seem to have long lifetimes, including a bright feature near 35 S, seen as early as 1998. Since then its latitude varied by 3.5 degrees and its planetographic longitudinal drift rate varied between -25.6 deg/day and -32.3 deg/day. Dramatic changes are also possible on Uranus. On 12 July 2004 we observed a very bright cloud feature near 28 N with a large pattern of associated streak features extending slightly to the east and 60 deg of longitude to the west. We saw the bright feature again in August, but this time with no pattern of associated streak features. At its brightest, the bright feature was twice as bright as its surroundings in H and about 25 times brighter than its surroundings in K'.
We thank the W. M. Keck Observatory, Keck Foundation, and NASA, for making these observations possible. This research was also supported by Planetary Astronomy Grant NAG5-12206.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.