PHOTOMETRY OF SATURNS 1990 EQUATORIAL DISTURBANCE

Multicolor CCD images at selected wavelengths from 370 to 900 nm obtai ned at Pic-du-Midi Observatory during four different phases of the dev elopment of Saturn's equatorial disturbance are used to analyze its ph otometric properties. Three epochs and four morphologically different regions were studied: October 5, 1990, the source of the event, the Gr eat White Spot properly; October 6 and 8, 1990, an undisturbed area in the same latitude far away from the GWS; November 17, 1990, the matur e phase of the planetary disturbance; and July 7, 1991, the declined p hase, but still active, disturbance. The absolute spectral reflectivit y of the GWS single feature showed a marked increase at all wavelength s with respect to the undisturbed neighbourings. A simple interpretati on of these data indicates that optically thick cloud tops in the GWS nucleus were located at average pressures approximately 200 mbar, appr oximately two scale heights above the expected ammonia cloud deck at a pproximately 1.8 bar. The particles forming the GWS were very bright i n the red and less contaminated by the blue absorbing impurities. Limb to limb scans at latitudes 5-degrees, 13-degrees, and 20-degrees N ar e used to characterize the temporal spectral dependence of the Minnaer t's limb darkening coefficient K and the normalized albedo (1/F)0. Pro nounced increases in K and (1/F)0 were noted at continuum wavelengths in November 1990, but only moderate changes were apparent in July 1991 (both with respect to the undisturbed area observed in early October 1990). The particles constituting the upper cloud of the disturbance d uring its mature phase in November 1990 showed changes in their vertic al distribution, and had a higher single-scattering albedo at blue and yellow wavelengths, and were more forward scattering than those which normally form the equatorial haze and cloud of Saturn. (C) 1994 Acade mic Press Inc.