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A. Verbiscer (U. of Virginia), R. G. French, C. A. McGhee (Wellesley Coll.)
Due to their proximity to the bright A ring, the innermost of Saturn's classical satellites have historically proven challenging targets for the Earth-based observer. Since the ring plane crossings in 1995 and 1996 and in preparation for the upcoming Cassini mission in 2004, the Hubble Space Telescope has been monitoring the Saturn system with the WFPC2 camera at opposition and quadrature each year at solar phase angles between 0.26 and 6.4 degrees. From these observations, we have produced mutually consisitent solar phase and rotation curves for each broadband UVBRI filter as well as for filters at 785 and 1042nm. The absolute photometric accuracy and high spatial resolution of the WFPC2 images have enabled the first observation of the steep, narrow opposition effects of Mimas and Enceladus at visible wavelengths, thus necessitating an increase in their previously reported geometric albedos. Fits to the Hapke (Icarus, 157, 523 2002) photometric equation enable the distinction between the shadow-hiding and coherent backscatter opposition effects as well as other regolith properties including the single scattering albedo, macroscopic roughness, and the single particle phase function. Comparisons between derived photometric parameters for each satellite suggest the degree to which location -- either interior (Mimas and Enceladus) or exterior (Tethys, Dione, and Rhea) to the densest point in the E ring -- affects the physical properties of regolith particles. Mimas and Enceladus have brighter trailing hemispheres, while Tethys, Dione, and Rhea display the opposite hemispherical albedo dichotomy. Multiple observations during a single HST orbit provide a series of measurements of the brightness of a satellite at constant phase angle, longitude, and latitude in each UVBRI filter, occasionally spanning a broader range from 255 - 1042nm. These spectral data indicate the satellite's color and demonstrate the wavelength dependence of albedo.
This work was supported by the Levinson Enhancement Grant to the University of Virginia, NASA PG&G grant NAG5-10197, and STScI grant GO-08660.01A.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.