Titania (moon) may have a subsurface ocean at the core–mantle boundary. The thickness of this ocean, if it exists, is up to and its temperature is around 190 K. However the present internal structure of Titania depends heavily on its thermal history, which is poorly known. Among Uranus's moons, Titania is intermediate in brightness between the dark Oberon and Umbriel and the bright Ariel and Miranda. Its surface shows a strong opposition surge: its reflectivity decreases from 35% at a phase angle of 0° (geometrical albedo) to 25% at an angle of about 1°. Titania has a relatively low Bond albedo of
Solar System icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets. Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune. Most of the planets in the Solar System have secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which, Titan and Ganymede, are larger than the planet Mercury), and, in