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Published December 1992 | public
Journal Article

Hubble Space Telescope Observations of the 1990 Equatorial Disturbance on Saturn: Images, Albedos, and Limb Darkening


In September 1990 a major equatorial eruption on Saturn produced a disturbance that spread in longitude until it completely girdled the planet. We report here on 150 images recorded in six passbands with the Wide Field/Planetary Camera (WF/PC) aboard the Hubble Space Telescope (HST) on 17 and 18 November 1990. For comparison, we used HST-WF/PC observations of Saturn obtained in three colors on 26 August 1990 before the onset of the disturbance, and in six colors on 5 and 6 June 1991 when almost no evidence of the disturbance remained. At both of those times, the equatorial belt was "normal" in appearance. Four of the passbands (with mean wavelengths of 336, 435, 546, and 716 nm) were selected for photometric analysis, and a patch of the B ring near the central meridian was used for photometric calibration. Using deconvolved images from all three epochs of observation, we measured reflectivities (I/F) of the disk along parallels of latitude as a function of longitudinal distance from the central meridian and also along the central meridian as a function of latitude from 0° to 90°. The longitudinal measurements cover essentially the whole visible disk; they were made at 1° intervals of planetographic latitude from 0° to 80°, and the results are expressed in terms of Minnaert coefficients k and Minnaert albedos (I/F)_0. We find that the cloud particles associated with the disturbance must differ in character from those that normally make up the visible cloud deck on Saturn. They were brighter and bluer, they had greater limb darkening, and their limb darkening was spectrally more neutral. The mutual relationship of those properties is such that features which stand out strongly near the meridian fade to invisibility when near the limb.

Additional Information

© 1992 by Academic Press. Received June 29, 1992; revised September 14, 1992. Our 150-exposure sequence required unusually complex HST programming. Marc Buie, Peggy Stanley, and many others at the Space Telescope Science Institute provided the dedication and high level of skill needed to make it a success. Don Parker of Coral Gables, FL, kindly provided groundbased telescopic videos to help in planning our HST-WF/PC observations. We are also grateful to Bob Light (UCSC) for help with image processing and to Ed Groth (Princeton) for the image rectification software used in producing the mosaics. We thank the HST-WF/PC Investigation Definition Team for making the June 1991 observations available. This work was supported in part by NASA Grants NAG5-1611, NAG5-1491, and NAGW-1956.

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