Neptune's zonal winds from near-IR Keck adaptive optics imaging in August 2001

We present H-band (1.4-1.8 mu m) images of Neptune with a spatial resolution of similar to 0.06aEuro(3), taken with the W.M. Keck II telescope using the slit-viewing camera (SCAM) of the NIRSPEC instrument backed with Adaptive Optics. Images with 60-second integration times span 4 hours each on UT 20 and 21 August, 2001 and similar to 1 hour on UT 1 September, 2001. These images were used to characterize the overall brightness distribution on Neptune, and to determine rotations periods (which translate into wind speeds) of individual cloud features. The images show that the spatial brightness distribution of cloud features, in particular the bright bands at mid-southern latitudes and near 30A degrees N, changed considerably between 1989 (Voyager era) and 2001. The brightest features extend latitudinally over several degrees, and despite the different velocities in different latitude bands, these bright features remain coherent. We show that these features are bright in part because of the foreshortening effect near the limb, which suggests that the features may be composed of small bright clouds that happen to line up near the limb. At certain latitudes (mid-southern and northern latitudes), there is considerable dispersion in relative rotation periods (and hence zonal velocities) of faint and moderately bright features, while there is essentially no velocity dispersion of features at 50A degrees S. While the zonal speeds of the brightest features are consistent with the Voyager-derived zonal-mean wind profile, there are many cloud features that do not appear to move with the flow. The data are further suggestive of oscillations in longitude, with periods > 4 hrs. We suggest that tidal forcing by Triton could play a role in exciting the waves responsible for the velocity variations of the observed period.