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) and is probably differentiated into a rocky core and an icy mantle. A layer of liquid water may be present at the boundary between the mantle and the core. The surface of Oberon, which is dark and slightly red in color, appears to have been primarily shaped by asteroid and comet impacts. It is covered by numerous impact craters reaching 210 km in diameter. Oberon possesses a system of "chasmata" (graben or scarps) formed during crustal extension as a result of the expansion of its interior during its early evolution. The Uranian system has been studied up close only once: