Satellites of Uranus: Disk‐integrated photometry from Voyager imaging observations

Abstract

Voyager 2 imaging observations over a wide range of phase angles are used to determine the fundamental photometric parameters for the five largest satellites of Uranus. Over the spectral range covered by Voyager cameras (approximately 0.35–0.60 µm) the disk‐averaged colors are moderately gray (no redder than the spectrum of Saturn's satellite Phoebe). Geometric albedos range from 0.19 for Umbriel to 0.40 for Ariel. Phase coefficients determined generally between phase angles of 10° and 60° vary from 0.021 magnitude per degree (mag/deg) for Ariel to 0.028 mag/deg for Miranda. Phase integrals lie in the range of 0.5–0.65. We estimate the following Bond albedos: about 0.1 for Umbriel and about 0.2 for the other satellites. The most complete phase angle coverage occurred in the case of Titania, for which data are available between 0.8° and 152.6°. The Titania phase curve shows a pronounced opposition surge, which can be modeled in detail by using Hapke's theory. A regolith texture more porous than that of the lunar surface is indicated. Observations at high phase angles (>139°) can be used to constrain the large‐scale roughness of the satellites. For Titania, Umbriel, and Oberon we find values of (the large‐scale roughness parameter in Hapke's model) comparable to those of the Moon. For Ariel a significantly rougher surface is suggested.