They used the Hubble telescope to observe (under ultraviolet light) how big each aurora's movements are. You see, a saltwater ocean would create its own magnetic field that would counter Jupiter's, restricting the lights' movement and causing them to perform an unenthusiastic dance -- and that's exactly what's happening on Ganymede. The team believes that beneath the moon's 95-mile-thick ice crust, there's a 60-mile-deep saltwater ocean: that's 10 times deeper than our planet's.

NASA already has its eyes set on a number of icy moons: Titan, Europa and Enceladus. But since the presence of water is essential to its study of life beyond our planet, we wouldn't be surprised if the agency conjures up a mission to Ganymede in the future.

[Image credit: NASA/ESA]