The largest ocean world in our solar system has just been confirmed by Hubble, and it is not Earth.

“The Ganymede Ocean (the largest and most massive moon of Jupiter and in the Solar System) is believed to contain more water than Europa’s,” says Olivier Witasse, a project scientist working on European Space Agency’s (ESA) future Jupiter Icy Moon Explorer (JUICE). “Six times more water in Ganymede’s ocean than in Earth’s ocean, and three times more than Europa.”

In March of 2016, NASA’s Hubble Space Telescope revealed the best evidence yet for an underground saltwater ocean on Ganymede, Jupiter’s largest moon –larger than Mercury and not much smaller than Mars. Identifying liquid water is crucial in the search for habitable worlds beyond Earth and for the search for life, as we know it.

“This discovery marks a significant milestone, highlighting what only Hubble can accomplish,” said John Grunsfeld, now retired assistant administrator of NASA’s Science Mission Directorate at NASA Headquarters. “In its 25 years in orbit, Hubble has made many scientific discoveries in our own solar system. A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth.“

Ganymede is the largest moon in our solar system and the only moon with its own magnetic field. The magnetic field causes aurorae, which are ribbons of glowing, hot electrified gas, in regions circling the north and south poles of the moon. Because Ganymede is close to Jupiter, it is also embedded in Jupiter’s magnetic field. When Jupiter’s magnetic field changes, the auroras on Ganymede also change, “rocking” back and forth.

Just as Saturn’s moon, Dione is perennially overshadowed by Enceladus and Titan, Ganymede’s fame is eclipsed by its sister ocean world, Europa, slated for flybys by NASA’s Europa Clipper mission in the 2020s.

Ganymede’s cycles of auroral activity on the surface, detected by the Hubble Space Telescope, reveal oscillations in the moon’s magnetic field best explained by the internal heat-generating tidal sloshing of a huge ocean hundreds of kilometers below the surface.

JUICE will fly by the moons at distances between 1000 and 200 kilometers, orbiting Ganymede for nine months, with the latter four months at an altitude of about 500 km. While the oceans of Jupiter’s moons are likely buried at significant depth below their icy crusts, radar will be able to help piece together clues as to their complex evolution.

For example, it will explore Europa’s potentially active regions and be able to distinguish where the composition changes, such as if there are local, shallow reservoirs of water sandwiched between icy layers. It will be able to find ‘deflected’ subsurface layers, which will help to determine the tectonic History of Ganymede in particular.

The distinction between ice and non-ice materials will also be possible, perhaps enabling the detection of buried cyrovolcanic reservoirs. On Callisto, radar profiling will help to understand the evolution of large impact crater structures that are apparent on the surface, which typically display multiple rims and a central dome. Their nature provides clues to the nature of the surface and subsurface at the time of the impact.

“Seeing into the subsurface of these moons with radar will be like looking back in time, helping us to determine the geological evolution of these enigmatic worlds,” says Witasse. On the way, reports NASA the space craft will make several flybys of another potentially ocean-bearing Jovian moon, Callisto. “We think that Callisto also harbors a subsurface ocean, but the available data is unclear,” Witasse says. “What we hope to do is to check whether there is an ocean or not—and if yes, at which depth.”

[This is an updated version of a previous article. New information was added today.]