by an icy mantle. A layer of liquid water (ocean) rich in dissolved ammonia may have formed at the core–mantle boundary. The eutectic temperature of this mixture is 176 K. The ocean, however, is likely to have frozen long ago. Among Uranian moons Umbriel was least subjected to endogenic resurfacing processes, although it may like other Uranian moons have experienced a very early resurfacing event. Exploration So far the only close-up images of Umbriel have been from the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986. Since the closest distance between Voyager 2 and Umbriel was , the best images of this moon have a spatial resolution of about 5.2 km. The images cover about 40% of the surface, but only 20% was photographed with the quality required for geological mapping. At the time of the flyby the southern hemisphere of Umbriel (like those of the other moons) was pointed towards the Sun, so the northern (dark) hemisphere could not be studied. No other spacecraft has ever visited Uranus or its moons. See also List of natural satellites Notes References External links Umbriel page (including a labelled map of Umbriel) at Views of the Solar System'' Umbriel Nomenclature from the USGS Planetary Nomenclature web site 18511024 Moons with a prograde orbit