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H. B. Hammel (SSI), I. de Pater (UCB), S. G. Gibbard (LLNL), G. W. Lockwood (Lowell), K. Rages (SETI)
On 4 July 2004 UT, we detected a southern hemispheric cloud feature on Uranus at K' (2.2 microns) using adaptive optics and the NIRC2 camera on the Keck 10-m. At H (1.6 microns), the feature had two components: a condensed core (2004-A) co-located with the K� feature at latitude �36.0 � 0.4 deg, and an extended feature (2004-B) at roughly �33.7 � 0.4 deg. When we observed again on 8 July UT, the core had faded at K�. By 9 July we were unable to detect a K� feature; the extended feature was still seen at H. The initial high K�-brightness of 2004-A indicates that the core�s scattering particles reached altitudes above the 1-bar level, with the larger feature at H, 2004-B, residing below the 1.1-bar level (assuming the model of de Pater et al. 2002, Icarus 160, 359). The core�s rapid disappearance at K� indicates dynamical processes in the local vertical aerosol structure, for example, subsidence of the feature�s cloud tops.
Features 2004-A and 2004-B had zonal velocities of 107 and 115 m/s, respectively (+/- 3 m/s), different from past velocities near these latitudes. These features are either new, or are long-lived but drifting in latitude (as has been seen on Neptune); see abstract by Rages et al. this volume. Regardless of their latitude, no southern features on Uranus have been detected at wavelengths of 2 microns or longward, indicating some change of activity. Continued observations of Uranus are strongly encouraged as its 2007 equinox approaches.
HBH acknowledges partial support from NASA grants NAG5-11961 and NAG5-10451. IdP acknowledges partial support from NSF and the Technology Center for Adaptive Optics, managed by UCSC under cooperative agreement No. AST-9876783. SGG�s work was performed under the auspices of the U.S. DoE, National Nuclear Security Administration by the UC, LLNL under contract No. W-7405-Eng-48.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.