Voyager 2 Photopolarimeter observations of the Uranian satellites

Abstract

We report the ultraviolet (0.25 µm) and infrared (0.75 µm) geometric albedos, phase curves, and phase coefficients for the large Uranian satellites using full disk photometric observations obtained by the photopolarimeter subsystem during the Voyager 2 encounter with the Uranian system. The phase coefficients and phase curves of the Uranian satellites are consistent with surfaces that have a loosely packed regolith with a heavily cratered terrain. Ariel, Titania, and Oberon exhibit a reddening with increasing phase angle of observation. We do not observe this effect for Umbriel. We have determined preliminary phase integrals for Ariel, Umbriel, Titania, and Oberon, and we have used this information and the geometric albedos reported herein and previously reported by the Voyager imaging subsystem to derive Bond albedos for the satellites. These are 0.22 ± 0.1, 0.07 ± 0.05, 0.16 ± 0.12, and 0.19 ± 0.22 for Ariel, Umbriel, Titania, and Oberon, respectively. We have compared the geometric albedos and color ratios of the Uranian satellites with comparable sets of data from the literature on other large satellites in the solar system, and we find that the Uranian satellites as a class are separate and distinct from the large Jovian and Saturnian satellites. This indicates a compositional difference between the three classes of large icy satellites. This may be due to different conditions at the time of formation and/or differences in the subsequent evolution of the surfaces of these objects. This finding is consistent with the hypothesis that a common surficial modification process exists for all the satellites in the Uranian system which is different from the processes which modify the surfaces of the Jovian and Saturnian satellites.