Uranus' Moons 1
Ariel is the twelfth of Uranus's known satellites with a diameter of 1158 kilometer, a mass of 1.27e21 kilogram, and an orbit that takes it 190,930 km from Uranus. Discovered by Lassell in 1851, it was named after the mischievous airy spirit in Shakespeare's The Tempest. Ariel is Umbriel's twin, but in size only. Ariel does, however, appear quite similar to Titania (Titania is 35% larger though).
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. Ariel is the lightest of the moons in color. It's surface is one of the least cratered, which also means one of the youngest. It resembles Titania more than Umbriel. Its surface displays a network of faults and rift valleys. On Ariel, volcanoes have resurfaced most of the terrain. The lava that is emitted is not molten rock like that of earth. Instead, it is a glacier-like mixture of ice and rock, slightly warmer than the surface of Ariel. This material may have flowed like a glacier. Water froze and expanded, shattering the crust and resulted in valleys.
Ariel'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. Some of the craters appear to be half-submerged. Ariel's surface is clearly relatively young (though older than some such as Enceladus); obviously some sort of resurfacing processes have been at work. Some ridges in the middle of the valleys are interpreted as upwellings of ice. Ariel may have been hot inside long ago, but it's cold now. Perhaps the valleys are cracks that formed when Ariel froze.