Ariel is a relatively small satellite and is the brightest moon of Uranus. The surface is pock-marked with craters, but the most outstanding features are long rift valleys stretching across the entire surface.
Canyons much like the ones on Mars appear in the pictures. The canyon floors appear as though they have been smoothed by a fluid. The fluid could not have been water because water acts like steel at these temperatures. The flow marks might have been made by ammonia, methane or even carbon monoxide.
NASA (Voyager 2) NOAA