36th DPS Meeting, 8-12 November 2004
Session 5 Uranus and Neptune
Oral, Monday, November 8, 2004, 1:30-3:00pm, Clark

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[5.05] Adaptive Optics imaging of small cloud features on Neptune: zonal wind variability and detections of oscillations in longitude

S.C. Martin, I. de Pater (UC Berkeley), S.G. Gibbard (LLNL/IGPP), P. Marcus (UC Berkeley), H.G. Roe (Caltech), B.A. Macintosh (LLNL/IGPP), C.E. Max (LLNL/IGPP, UC Santa Cruz)

We present the results of an imaging experiment designed to track the motions of clouds in the upper atmosphere of Neptune. Images were taken in H band (1.4-1.8 microns) with a resolution of .06 arcseconds using the NIRSPEC/AO system on the W. M. Keck II telescope August 20 2001 UT. This dataset is unique in that it is densely sampled in time: 56 images were taken during 4 hours, and the time interval between images ranged from 1 to 32 minutes. The positions as a function of time were determined for 51 cloud features at southern midlatitudes (15-50 degrees South.) In this region, cloud features are visually organized into bands of clouds that almost follow lines of constant latitude. The two major findings of this analysis are: 1. The drift rates of clouds (as determined from the linear fit of longitude versus time curves) are highly variable for a given latitude band. Rotation periods range over several hours per cloud band, yielding relative velocities that are in some cases supersonic. 2. Graphs of longitude versus time are not strictly linear but rather show an oscillation in longitude. We have subtracted the linear drift rate and made empirical fits to the residuals of 12 features which have the longest time baselines of observations. Amplitudes of oscillations are 2-4 degrees of longitude, and periods are comparable to the 4 hour observation time. This is the first detection of small oscillations in the atmosphere of Neptune. Sromovsky, Limaye and Frye (1993) measured oscillations of much greater amplitudes and periods in the Great Dark Spot (GDS) and in the Second Dark Spot (DS2) of Neptune using Voyager data.

This research was supported in part by the STC Program of the National Science Foundation under Agreement No. AST-9876783, and in part under the auspices of the US Department of Energy at Lawrence Livermore National Laboratory, Univ. of Calif. under contract No. W-7405-Eng-48. Data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.