The Ganymede Ocean is believed to contain more water than Europa’s,” says Olivier Witasse, a project scientist working on ESA’s 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 2020, 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 our solar system’s largest moon and the only one with its own magnetic field. The magnetic field creates aurorae, which are ribbons of bright, hot electrified gas that circle the moon’s north and south poles. Ganymede is surrounded by Jupiter’s magnetic field due to its proximity. The aurorae on Ganymede vary as Jupiter’s magnetic field changes, “rocking” back and forth.
Ganymede’s fame is eclipsed by its sister ocean planet, Europa, which is scheduled for flybys by NASA’s Europa Clipper mission in the 2020s, just as Saturn’s moon, Dione, is perpetually overshadowed by Enceladus and Titan.
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.