Ganymede is the only moon in the solar system known to have its own magnetic field, and that magnetic field creates ribbons of glowing electrified gas, like the aurora on Earth. Scientists observed that when Jupiter's magnetic field changes, Ganymede's glowing ribbons of gas also changed -- rocking back and forth.
The changes in the aurorae allowed scientists to determine that a large amount of water is beneath Ganymede's surface affecting the moon's magnetic field.
"If you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon’s interior," Joachim Saur, a co-author of the study, said in a statement.
The ocean is believed to be buried under 95 miles of ice. Scientists estimated the ocean is 60 miles thick, making Ganymede's ocean ten times deeper than any ocean found on Earth, even though Ganymede itself has a radius that is about four-tenths that of the Earth.
Perhaps most exciting of all -- the discovery hints at the possibility that there could be a home for life beyond Earth.