Neptune's Atmospheric Circulation and Cloud Morphology: Changes Revealed by 1998 HST Imaging

On 11–12 August 1998, we imaged Neptune using the Hubble Space Telescope WFPC2 CCD camera and NICMOS InSb camera to obtain high-spatial-resolution, absolutely calibrated imagery from the visible through the near infrared. We used nine HST orbits to intensively sample one rotation of Neptune, and four additional orbits to sample cloud evolution and motions one and two rotations later. In 1998 Neptune displayed more bright cloud features at southern mid-latitudes (mainly at 30°S and 45°S) than in 1996, producing small, but significant, increases in Neptune's disk-integrated albedo of 1.4±1, 1.9±1, and 9.5±1% for F467M, F673N, and F850LP filters respectively. Averages over longitude regions relatively free of discrete bright clouds did not show substantial brightness changes for most filters. Although we were unable to directly confirm the continued presence in 1998 of the 1996 Northern Great Dark Spot NGDS-32, we did find what we believe to be its bright northern companion cloud at 39°N. The increased extent of both northern and southern companions in 1998 may have obscured the normally dark core of this circulation feature. The 1996 Great Dark Spot NGDS-15 was not visible in 1998. The 1996 dark circumpolar band near 60°–70°S was present in 1998, with somewhat enhanced contrast. Significant evolution of cloud features was found over short time intervals of one and two rotations of Neptune, especially at 31°S, 45°S, and 70°S. Cloud motions in 1998 agree relatively well with the 1989 Voyager smoothed profile of L. A. Sromovsky et al. (1983, Icarus105, 110–141), with some exceptions associated with bright companion clouds, and with small but significant deviations that are consistent with those observed in 1995 and 1996 (Sromovsky et al. 2001, Icarus149, 459–488). These wind profile deviations contribute important modifications to the potential vorticity gradients that control latitudinal drift rates of GDS-scale vortices on Neptune (according to numerical simulations of R. P. LeBeau and T. E. Dowling (1998, Icarus132, 239–265)). However, these modifications alone do not explain the zero latitudinal drift of 1996 NGDS-32.