“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.”
"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."
"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.”