Europa: Come for the Geysers, Stay for the Submarine Rides
Yellowstone National Park's Old Faithful is a superheated squirt gun compared to the geysers found on Jupiter's fourth-largest moon, Europa. Roughly the size of Earth's moon, its jets spew water more than 125 miles (201 kilometers) high when squeezed by the gravitational tidal forces of the gas giant and its major satellites [sources: Daniels; Lemonick].
Europa's icy surface makes it cue-ball smooth compared to Jupiter's other Galilean satellites (the four largest of Jupiter's 50-67 moons). Still, its icy surface possesses an exotic beauty traced out in veiny fractures across its crisscrossed shell. These cracks hint at the moon's possible structure, which scientists believe consists of a mile-thick icy covering overlaying a subsurface ocean. The same gravitational forces that drive Europa's geyser activity could provide enough energy to keep water from freezing solid, even on a moon half a billion miles from the sun. The plumes could also erupt from smaller lakes or ponds trapped in the ice [sources: Cook et al.; Daniels; Lemonick; NASA].
Europa's liquid saltwater ocean, which could reach 100 miles deep, explains why we recommend this moon's fountains over the ice-and-dust geysers of Saturn's Enceladus; outside of Earth, it alone offers a chance to dive a submarine into waters potentially populated by extraterrestrial life [sources: Cook et al.; Daniels; Lemonick].