Here on Earth we're impressed by the Pacific Ocean's Marianas Trench, which reaches a depth of 6.8 miles. But the ocean on the Jupiter moon of Europa put ours to shame. Although Europa is covered in a thick crust of scarred and cross-hatched ice, measurements made by NASA's Galileo spacecraft and other probes strongly suggest that a liquid ocean lies beneath that surface. Some measurements put the ocean's depth at 62 miles. The interior is warmed, researchers believe, by the tidal stresses exerted on Europa by Jupiter and several other large moons, as well as by radioactivity.

Europa's huge liquid ocean makes it one of the most promising places to look in the search for extraterrestrial life. NASA and the European Space Agency are therefore hard at work on a joint mission that may launch in 2020, and which will examine Jupiter, Europa, and another moon named Ganymede. A major objective is to determine the thickness of Europa's ice crust, which has implications for the moon's potential to sustain life.