Observations Of Miranda And Other Satellites
Since Voyager 2 passed fairly close to Miranda, it was possible to determine Miranda's mass from its perturbation
of Voyager 2's hyperbolic orbit past Uranus found from an analysis of the Voyager 2 radio data. Ariel's mass was then determined from its perturbations of the orbits of Miranda and Voyager 2. Voyager 2 did not approach Umbriel, Titania, and Oberon closely enough for reliable determination of their masses from perturbations of its flyby orbit. Instead their masses are determined from their perturbations of Ariel's orbit and each other's orbits using both Voyager 2 observations and Earth-based observations made over many years. Accurate radii were found for all five satellites from Voyager 2 images; this allowed the calculation of mean densities for these satellites. Their mean densities, which range from 1.20 grams/cm3 for Miranda to 1.69 grams/cm3 for Titania, are compatible with their interiors being largely composed of water ice, which was detected earlier on their surfaces. All five satellites were found to be tidally locked to Uranus, as predicted by theory, so their rotation periods are the same as their orbital periods of revolution. Their rotation axes have become aligned nearly parallel to Uranus' rotation axis, so that only their southern hemispheres could be imaged. Voyager 2 infrared measurements near the south poles of Miranda and Ariel gave temperatures of -304.6°F (-187°C) and -308.2°F (-189°C), respectively, considerably warmer than the cloud layer of Uranus' atmosphere, but understandable for a surface that has been in continuous sunlight for about 20 years. None of the satellites show an appreciable atmosphere.
Voyager 2 obtained detailed images of parts of the sunlit surface of all five previously known satellites and also of Puck. Bright and dark (albedo) regions, craters with or without bright ray systems around them, mountains, cliffs, scarps, valleys, canyons, graben, faults, and other geological features are clearly seen on these images. Maps of the parts of the satellite surfaces that have been imaged have been made, and names have been assigned to many surface features. Summaries of the surface features of the six satellites with imaged surface features are given below. The images with the best resolution obtained were Ariel and Miranda because Voyager 2 flew closest to them.