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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.