Titania, Uranus' largest and most massive satellite, shows similar geological features to those found on Oberon. Heavily cratered plains are the most extensive surfaces found here. Oberon also shows a global rift valley network related to global tectonics. Image resolution is somewhat better for Titania than Oberon, since Voyager 2 was closer to Titania. A prominent system of canyons and scarps is the most noticeable class of features on Titania's surface; shadows indicate some of them may be as deep as 3.7 mi (6 km). Moderately cratered and smooth plains are also observed, which indicate resurfacing. These features indicate that more geologic activity occurred on Titania than on Oberon after the end of the heavy bombardment (crater formation) phase of their histories. Here the resurfacing material should be liquid or icy water, ammonia, or methane, perhaps mixed with rocky material, rather than terrestrial type volcanic lava. Impact and/or internal heating may have melted or softened these ices, which then erupted and flowed over the satellite surfaces, resurfacing them. This phenomenon is called cryovolcanism (sometimes "water volcanism"), and it may have been important in the geological histories of many of the satellites of the Jovian planets.