Phoebe: Albedo Map and Photometric Properties

Abstract

Using clear-filter images from Voyager 2 (effective wavelength 0.48 μm), we have constructed the first-ever digital albedo map of Saturn's moon Phoebe. 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 covered 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°N (normal reflectance as high as 0.13, ≈50% brighter than the average surface). The bright northern spot and the brightest of the southern spots occur 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 the satellite's telescopic near-opposition phase curve (S. Kruseet al.1986,Icarus68, 168–175) 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 allow unambiguous determination of the surge's exact strength. The derived photometric function is consistent with the idea that Phoebe is a C-type object—probably a primitive, captured body related to Chiron, Pholus, and the inhabitants of the Kuiper Belt. The albedo map and photometric information will 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.