Miranda is only 300 miles in diameter, but contains some of the most peculiar terrain observed on any Solar system object. Part of the surface is heavily cratered ancient terrain, but all sorts of weird fissures, canyons and cliffs (some more than 5 miles high) are scattered across one side of the Moon. Various explanations have been proposed to explain the features, ranging from collisions breaking the moon up over and over again, to upwelling of material from the moon's inside as it suffers internal melting caused by tidal forces from Uranus (Miranda is the closest of the 'larger' moons of Uranus). However, all of the various explanations are merely educated guesses, as we have too little information to truly understand what is going on. And since there are no plans for further exploration of Uranus or Neptune, the intriguing and puzzling photographs now on hand may be all that we have to look at or speculate about for the best part of a century (or more). All of the large satellites of Uranus appear to be about half ice and half rock.

     Ariel and Titania are 700 and 1000 miles in diameter. Each of them has a surface that has been extensively reworked by some kind of activity, creating large fissures and ice ridges. Some of the canyon floors appear to have been smoothed out by some kind of fluid flow, but the nature of the fluid is unknown, other than that it is unlikely to have been water, as the low temperatures so far from the Sun would make other volatile materials such as ammonia or methane, far more likely to be liquids.

     Umbriel and Oberon are, like Ariel and Titania, about 700 and 1000 miles diameter, but they are both very heavily cratered, and have presumably had much less internal activity than Ariel and Titania. Why these two moons look so different from the other two large moons is completely unknown, but one possibility is that all four have heavily cratered regions and geologically evolved regions, and the Voyager 2 flyby just happened to occur at a time when the cratered sides of Umbriel and Oberon were turned toward the spacecraft, and the cratered sides of Ariel and Titania weren't. In other words, in the absence of good data almost any theory can be proposed without any possibility of disproving it. (Note: The cause of the bright ring at the top of the photograph of Umbriel is unknown, but it might be sunlight reflecting off the walls of a large crater.)