By studying the subtle shifts of aurorae on Ganymede, scientists working with the Hubble Space Telescope have concluded that Jupiter's largest moon hosts a massive subterranean ocean. Quite suddenly, the outer reaches of our solar system appear to be a very wet place, indeed.
Ganymede, in addition to being the largest moon in the solar system (its radius is 0.413 Earths), is also the only moon to feature its own magnetic field. This magnetic field gives rise to aurorae, bands of hot electrified gas which circle the moon's north and south poles. Interestingly, Ganymede is embedded in Jupiter's magnetic field, so when the gas giant's magnetic field changes, the aurorae on Ganymede "rock" back and forth.
Ganymede's auroral belts are shown in blue. (NASA/ESA)
By measuring this rocking motion, the NASA scientists have determined that a large amount of saltwater must exist beneath the crust. This ocean is creating a secondary magnetic field that's countering Jupiter's field. This "magnetic friction" is suppressing the rocking of the aurorae. And in fact, the ocean is fighting Jupiter's magnetic field to such an extent that it's reducing the rocking of the aurorae by a factor of 2 degrees instead of the expected 6 degrees if an ocean was not present. The scientists were able to make these measurements by studying the ultraviolet images captured by the Hubble Space Telescope.
And wow, is there a lot of water on Ganymede. Calculations suggest that Ganymede's ocean is 60 miles (100 km) thick, which is 10 times deeper than Earth's oceans. All this water is buried under a 95-mile (150-km) crust of mostly ice.
In addition to Ganymede, we're now fairly certain that Europa and Enceladus have subterranean oceans as well. It's time for NASA to start considering these objects as we continue the search for alien life in our own solar system.
[ NASA ]