Icarus 160, 359–374 (2002) doi:10.1006/icar.2002.6966 Keck Adaptive Optics Images of Uranus and Its Rings Imke de Pater Astronomy Department, 601 Campbell Hall, University of California, Berkeley, Berkeley, California 94720 E-mail: imke@floris.berkeley.edu S. G. Gibbard and B. A. Macintosh Lawrence Livermore National Laboratory, Livermore, California 94550 H. G. Roe Astronomy Department, 601 Campbell Hall, University of California, Berkeley, Berkeley, California 94720 and D. T. Gavel and C. E. Max Lawrence Livermore National Laboratory, Livermore, California 94550 Received February 5, 2001; revised April 25, 2002 We present adaptive optic images of Uranus obtained with the 10-m W. M. Keck II telescope in June 2000, at wavelengths be- tween 1 and 2.4 µm. The angular resolution of the images is ∼0.06– 0.09 ′′ . We identified eight small cloud features on Uranus’s disk, four of which were in the northern hemisphere. The latter features are ∼1000–2000 km in extent and located in the upper troposphere, above the methane cloud, at pressures between 0.5 and 1 bar. Our data have been combined with HST data by Hammel et al. (2001, Icarus 153, 229–235); the combination of Keck and HST data al- lowed derivation of an accurate wind velocity profile. Our images further show Uranus’s entire ring system: the asymmetric ǫ ring, as well as the three groups of inner rings (outward from Uranus): the rings 6 + 5 + 4, α + β , and the η + γ + δ rings. We derived the equivalent I/F width and ring particle reflectivity for each group of rings. Typical particle albedos are ∼0.04–0.05, in good agreement with HST data at 0.9 µm. c  2002 Elsevier Science (USA) 1. INTRODUCTION Due to Uranus’s small angular extent and lack of large-scale brightness variations, images of the planet obtained several decades ago were of limited interest. Attempts to observe Uranus were renewed after star occultation measurements by Elliot et al. (1977a,b) indicated the existence of a ring system surrounding the planet. To image this ring system, however, one had to some- how avoid contamination from Uranus’s scattered light. This could be accomplished by imaging the planet at near-infrared wavelengths in methane absorption bands where Uranus itself is dark, and/or by using coronagraphy to effectively block out the light from the planet (Matthews et al. 1982, Terrile and Smith 1986). These techniques are still widely used to image faint material next to bright planets or stars. The best ground- based infrared images of Uranus published to date are those by Baines et al. (1998) and Sromovsky et al. (2000). In both sets of images the ǫ ring with its asymmetric brightness distribution was easily observed. Due to the relatively low spatial resolution (typically at best ∼0.4–0.5 ′′ from the ground), light from this ring is blended with that of several faint inner rings. Since the ǫ ring is so much brighter than any of the other rings, light from the ring system has usually been completely attributed to the ǫ ring. A wealth of information regarding the planet and its ring sys- tem was obtained by the Voyager spacecraft (e.g., Smith et al. 1986). The planet itself appeared to be rather uninteresting in the sense that practically no cloud features were detected. Yet the circumpolar bands on Uranus were similar to those seen on Saturn and Jupiter, although in a completely different sun–planet geometry. This provided much information regarding zonal wind systems on giant planets. Unfortunately, due to an overall lack of cloud features it was difficult to determine an accurate wind pro- file in Uranus’s atmosphere. Moreover, since only the southern (visible) hemisphere 1 was imaged by Voyager, no information was obtained on its northern counterpart. In addition to atmo- spheric observations, Voyager imaged the ring system, discov- ered a tenth ring, and imaged sheets of dust in between rings at 1 We use the IAU convention of the location of the north pole: at the time of the observations the south pole is visible, and the planet’s rotation is retrograde, i.e., in the westward direction. 359 0019-1035/02 $35.00 c  2002 Elsevier Science (USA) All rights reserved.