Uranus at equinox: Cloud morphology and dynamics

As the 7 December 2007 equinox of Uranus approached, collaboration between ring and atmosphere observers in the summer and fall of 2007 produced a substantial collection of ground-based observations using the 10-m Keck telescope with adaptive optics and space-based observations with the Hubble Space Telescope. Both near-infrared and visible-wavelength imaging and spatially resolved near-infrared spectroscopic observations were obtained. We used observations spanning the period from 7 June 2007 through 9 September 2007 to identify and track cloud features, determine atmospheric motions, characterize cloud morphology and dynamics, and define changes in atmospheric band structure. Atmospheric motions were obtained over a wider range of latitudes than previously was possible, extending to 73°N, and for 28 cloud features we obtained extremely high wind-speed accuracy through extended tracking times. We confirmed the existence of the suspected northern hemisphere prograde jet, locating its peak near 58°N. The new results confirm a small N–S asymmetry in the zonal wind profile, and the lack of any change in the southern hemisphere between 1986 (near solstice) and 2007 (near equinox) suggests that the asymmetry may be permanent rather than seasonally reversing. In the 2007 images, we found two prominent groups of discrete cloud features with very long lifetimes. The one near 30°S has departed from its previous oscillatory motion and started a significant northward drift, accompanied by substantial morphological changes. The complex of features near 30°N remained at a nearly fixed latitude, while exhibiting some characteristics of a dark spot accompanied by bright companion features. Smaller and less stable features were used to track cloud motions at other latitudes, some of which lasted over many planet rotations, though many could not be tracked beyond a single transit. A bright band has developed near 45°N, while the bright band near 45°S has begun to decline, both events in agreement with the idea that the asymmetric band structure of Uranus is a delayed response to solar forcing, but with a surprisingly short delay of only a few years.