Phoebe: Albedo map and photometric properties

Using clear-filter images from Voyager 2 (effective wavelength 0.48 mu m), we have constructed the first-ever digital albedo map of Saturn's moon Phoe be. Most normal reflectances in this new map are between 0.07 and 0.11; the albedo histogram is largely bimodal, suggesting that the satellite is cove red predominantly by two different types of surface materials. The highest albedos are confined to isolated, quasi-circular spots 40 to 100 km across, including three spots of varying albedo in a band immediately south of the equator and one especially bright spot at latitude 60 degrees N (normal re flectance as high as 0.13, approximate to 50% brighter than the average sur face). The bright northern spot and the brightest of the southern spots occ ur at approximately the same longitude, an alignment that gives Phoebe its significant rotational lightcurve. The low resolution of the Voyager images does not permit interpretation of the bright spots' origin. Phoebe's global-average photometric function was determined by combining th e satellite's telescopic near-opposition phase curve (S. Kruse et al. 1986, Icarus 68, 167-175) with absolute disk-resolved reflectances measured from the Voyager images (triaxial-ellipsoid shape assumed with radii 115, 110, and 105 km). Modeling of the telescopic observations supports the presence of a significant opposition surge, although scatter in the data does not al low unambiguous determination of the surge's exact strength. The derived ph otometric function is consistent with the idea that Phoebe is a C-type obje ct-probably a primitive, captured body related to Chiron, Pholus, and the i nhabitants of the Kuiper Belt. The albedo map and photometric information w ill aid in the planning of high-resolution Cassini images of Phoebe, which will represent our first close look at this class of primitive outer-Solar- System object. (C) 1999 Academic Press.