... A riel and Titania appear quite similar though Titania is 35% larger. All of Uranus' large moons are a mixture of about 40-50% water ice with the rest rock, a somewhat larger fraction of rock than Saturn 's large moons such as Rhea . A riel's surface is a mixture of cratered terrain and systems of interconnected valleys hundreds of kilometers long (left, above) and more than 10 km deep. This is similar to, but much larger and more extensive than the situation on Titania. ... Uranus .. ...