Olivier Witasse, a scientist working on the ESA’s Jupiter Icy Moon Explorer project, thinks that Ganymede’s ocean has more water than Europa’s (JUICE). “Ganymede’s ocean has six times as much water as Earth’s ocean and three times as much as Europa’s.” In March 2020, NASA’s Hubble Space Telescope found the best evidence yet that Ganymede, Jupiter’s largest moon (it’s bigger than Mercury and not much smaller than Mars), has a saltwater ocean under its surface.

The search for habitable worlds beyond Earth and for life as we know it depends on finding liquid water. John Grunsfeld, who is now retired as assistant administrator of NASA’s Science Mission Directorate at NASA Headquarters, said, “This discovery is a big deal because it shows what only Hubble can do.” “In the 25 years that Hubble has been in space, it has found out a lot about our own solar system. Ganymede’s icy surface hides a deep ocean. This means that there could be life on other planets.
Ganymede is the biggest moon in our solar system and the only one with its own magnetic field. Around the north and south poles of the moon, aurorae, which are ribbons of glowing, hot, electrically charged gas, are caused by the moon’s magnetic field. Ganymede is part of Jupiter’s magnetic field because it is close to Jupiter. When Jupiter’s magnetic field changes, the aurorae on Ganymede also change and “rock” back and forth.

Ganymede isn’t as well-known as its sister ocean world Europa, which will be visited by NASA’s Europa Clipper mission in the 2020s. This is similar to how Saturn’s moon Dione is always overshadowed by Enceladus and Titan.

The Hubble Space Telescope has seen cycles of auroral activity on Ganymede’s surface. These cycles show oscillations in the moon’s magnetic field that are best explained by the tidal movement of a huge ocean hundreds of kilometers below the surface, which generates heat. JUICE will fly by the moons at distances of between 1,000 and 200 kilometers. It will orbit Ganymede for nine months, spending the last four months at an altitude of about 500 kilometers. Even though Jupiter’s moons probably have oceans buried deep below their icy crusts, radar will be able to help us figure out how they changed over time.

For example, it will look at the potentially active parts of Europa and be able to tell where the composition changes, like if there are shallow pools of water between layers of ice. It will be able to find subsurface layers that have been “deflected,” which will help scientists learn more about Ganymede’s tectonic history.