Deep valleys and grooves cross the surface of Ariel, a moon of Uranus, in this 1986 view from the Voyager 2 spacecraft. It is the sharpest image ever snapped of the moon. Ariel is roughly 720 miles (1,160 km) in diameter, or roughly one-third the size of Earth’s moon. Its rugged surface hints that Ariel once had a deep ocean of liquid water beneath its icy crust. [NASA/JPL]
Find out more in our radio program entitled Deep Ocean.