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H. B. Hammel (SSI), G. W. Lockwood (Lowell), K. Rages (SPRI/Ames), M. Marley (Ames)
On 5-8 June 2000, disk-resolved images and spectroscopy of Uranus from 0.8 to 2.5 microns were obtained at the NASA IRTF. The spectra reveal hemispheric asymmetry, and will allow discrimination of vertical aerosol models at different latitudes on the planet. At wavelengths where Uranus is dark, spectra of the ring ansae are spatially resolved from the planet's disk.
On 9-11 August 2000, images were also obtained in the near infrared (roughly J, H, and K') with the Keck 10-m adaptive optics system. (In addition, HST obtained visible wavelength images on 16-18 June and 7 August 2000; see Rages et al., this meeting, for details.) The Keck images show at least four discrete features roughly at latitudes -31, -25, +25, and +42 deg (+/- 5 deg in the preliminary reconnaissance); further processing may reveal additional features. The two southern features were seen on two nights, and have wind speeds commensurate with earlier determinations for those latitudes (Karkoschka, E., 1998, Science 280, 57-575; henceforth K98). The two northern features were each detected on only one night, but comparison with HST data may allow accurate wind speeds to be determined.
In both June and August 2000, the highest-contrast features were seen near +42 deg on the far northern limb. In 1998, the highest-contrast features and several other features were located near +30 deg, at that time on the northern limb (Karkoschka 1998). Those features, which now should be well away from the limb, are presently not detectable at visible and broad-band near-infrared wavelengths. One possible explanation is that the northernmost features are relatively short-lived, appearing briefly on the newly sunlit limb and then fading within a few years. On the other hand, features at southern midlatitudes have been consistently detected since 1994, perhaps suggesting that different mechanisms for discrete cloud formation are active on Uranus.
Support for the Keck observations was provided by NASA grant NAG5-7853; additional Uranus work is supported in part through AR-08351.04-97A, provided by NASA through a grant from STScI (operated by AURA, Inc, under NASA contract NAS5-26555).
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