Oberon (moon) at the core–mantle boundary. The eutectic temperature of this mixture is 176 K. If the temperature dropped below this value the ocean would have frozen by now. Freezing of the water would have led to expansion of the interior, which may have also contributed to the formation of canyon-like graben. Still, present knowledge of the evolution of Oberon is very limited. So far the only close-up images of Oberon have been from the "Voyager 2" probe, which photographed the moon during its flyby of Uranus in January 1986. Since the closest approach of "Voyager 2" to Oberon was 470,600 km,
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