Earth-Based Detection of Uranus' Lambda Ring

Abstract

The 11 July 1992 occultation of Uranus by U103 (mK= 10.1) was observed using the 5-m Hale telescope on Palomar Mountain in California, the 4-m telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile, and the nearby 2.2-m telescope at European Southern Observatory (ESO). All observations were taken using InSb aperture photometers andK(λ ∼ 2.2 μm) filters and provided high-quality profiles of the uranian rings. The ESO and Palomar ring event times have been incorporated into a comprehensive orbit model for the elliptical rings using all available Earth-based and Voyager occultation data. The kinematical model was used to determine the radius scale and geometry of the U103 occultation, as well as to provide absolute timing calibration for the CTIO light curve, which was reconstructed from two-dimensional digital scans of a high-speed strip chart. All nine classical rings were clearly observed during both ingress and egress. In addition, a sharp feature appeared in the CTIO ingress light curve with an orbital radius of 50,026.89 ± 0.39 km, very close to the mean radius of the λ ring of 50,026.3 ± 0.6 km found by M. R. Showalter (Science267, 490–493, 1995) from an analysis of Voyager 2 images. The CTIO feature is well above the noise: its equivalent width ofE= 0.32 ± 0.14 km corresponds to a 5–σ detection. It is the only such signature in the entire CTIO lightcurve, other than the previously known rings. The characteristic ring-like shape of the profile, its high SNR, and its radial location all support the identification of this feature as an Earth-based detection of the λ ring. At the same time, if the ring were azimuthally homogeneous withE≳ 0.32 km, it would have been clearly visible in the CTIO egress and Palomar light curves, but no such features were found. The λ ring is thus azimuthally clumpy, resembling to some degree Neptune's arc-like Adams ring. At the nearby ESO station, a suggestive ingress feature appeared with a radius of 50,023.01 ± 0.28 km, but the data are too noisy to permit a positive identification. The orbital characteristics of the λ ring and its azimuthal brightness profile cannot be uniquely extrapolated from the Voyager encounter epoch to the time of the U103 occultation because the mean motion and orbital radius of the ring are too uncertain. However, additional detections in archival Earth-based occultation data or from future observations could allow a much better determination of the dynamics of this dusty and clumpy ring.