Umbriel (moon) that obliterated its older surface. Covered by numerous impact craters reaching in diameter, Umbriel is the second most heavily cratered satellite of Uranus after Oberon. The most prominent surface feature is a ring of bright material on the floor of Wunda crater. This moon, like all moons of Uranus, probably formed from an accretion disk that surrounded the planet just after its formation. The Uranian system has been studied up close only once, by the spacecraft "Voyager 2" in January 1986. It took several images of Umbriel, which allowed mapping of about 40% of the moon’s surface. Umbriel, along with
Oberon (moon) by the expansion of Oberon by about 0.5%, which occurred in two phases corresponding to the old and young canyons. The nature of the dark patches, which mainly occur on the leading hemisphere and inside craters, is not known. Some scientists hypothesized that they are of cryovolcanic origin (analogs of lunar maria), while others think that the impacts excavated dark material buried beneath the pure ice (crust). In the latter case Oberon should be at least partially differentiated, with the ice crust lying atop the non-differentiated interior. Oberon is thought to have formed from an accretion disc or subnebula: a