Ganymede is thought to hide a saltwater ocean beneath more than 100 kilometers of ice.NASA/JPL-CALTECH/SWRI/MSSS/KALLEHEIKKI KANNISTO

Ganymede, Jupiter’s largest moon, is practically a planet. Larger than Mercury, it is the only moon with its own magnetic field, produced by churning molten iron in its core. Its icy crust, more than 100 kilometers thick, is cut by ridges and grooves, evidence of past tectonic activity. Smooth plains suggest ice-spewing cryovolcanoes may have paved over parts of the moon. And beneath the crust, many researchers believe, is a salty ocean, kept warm by the moon’s inner heat and Jupiter’s gravitational kneading. “It’s now clear you can find liquid water much farther away from the Sun than we ever thought,” says Michele Dougherty, a space physicist at Imperial College London.

Ganymede is one of three jovian moons that may hold hidden oceans, all potential habitats for life. They are the targets of the $1.6 billion Jupiter Icy Moons Explorer (Juice), a European Space Agency (ESA) mission set for a 13 April launch on an Ariane 5 rocket from French Guiana. Boosted by gravitational assists from Earth and Venus, Juice will take 8 years to reach Jupiter. It will spend another 3 years promenading among the moons, eventually ending up in a tight orbit around Ganymede—the first time a spacecraft will orbit a moon other than Earth’s.

Ganymede’s sister moon, Europa, has long gotten more attention as a possible home for life and is the target of another spacecraft, NASA’s Europa Clipper, to launch in October 2024. Europa’s icy shell is much thinner than Ganymede’s, perhaps just 15 kilometers thick, and its ocean may sometimes breach the surface—perhaps even sending plumes of water erupting into space. But Europa orbits closer to Jupiter’s intense radiation field, which would disrupt the electronics of any spacecraft lingering nearby. The Clipper, which will arrive a year ahead of Juice thanks to a more powerful rocket, will inspect Europa by swooping past it 50 times.

Juice will also fly past the moon, twice. If the plumes exist, Juice’s mass spectrometer might be able to sample their outer reaches and hunt for organic molecules. The spacecraft’s radar, capable of penetrating ice 9 kilometers thick or more, might see the ocean if the crust is thin enough, or at least subsurface pockets of water embedded in the ice. Eruptions from these pockets could be responsible for the “chaos terrains,” a landscape of fissures that cover one-fourth of the moon’s surface. Juice will also use a laser altimeter—an instrument Clipper doesn’t possess—to search for depressions where subsurface water might be close to bursting through. “We’ll switch on all the instruments to gather as much as we possibly can,” says Olivier Witasse, Juice’s project scientist at ESA’s European Space Research and Technology Centre.

Callisto, the most distant of Jupiter’s moons, is another target for Juice. It does not derive much energy from Jupiter’s tidal tugs, and data from NASA’s Galileo spacecraft in the 1990s suggest it never got hot enough to separate into distinct layers, leaving its ice and rock jumbled together. “We think it’s not active at all, a dead moon,” Witasse says. Yet Galileo’s magnetometer found hints that Callisto, too, might hold an ocean: faint signs of a magnetic field, perhaps generated as Jupiter’s strong magnetic field sweeps through electrically conductive saltwater in the moon’s interior and induces currents and magnetism. By carefully looking for this induced field, Juice could divine whether this ocean exists.

In late 2034 Juice will finally settle into orbit 500 kilometers above Ganymede. One task will be delimiting the ocean, which some researchers think could exist in multiple layers, separated by high-pressure species of ice. Ganymede’s crust is too thick for Juice’s radar to penetrate, but the ocean’s structure could be revealed through careful study of the way its sloshing contributes to orbital wobbles, combined with measures of induced magnetic fields. The radar can also hunt for pockets of water that could be fueling eruptive episodes that create the smooth plains, says Lorenzo Bruzzone, a remote-sensing scientist at the University of Trento. By measuring how deep the radar penetrates, the team can also learn how salty the ice is, an important factor in its freezing temperature.

Juice’s mass spectrometer can provide another handle on the composition of the crust. It will sniff the moon’s thin atmosphere for molecules that have escaped the surface. “You get the ground truth by directly sampling the material,” says Audrey Vorburger, a planetary scientist at the University of Bern. Meanwhile, the magnetometer will help determine the size of the core and the ocean by measuring their separate contributions to the moon’s field.

NASA may end the Europa Clipper by crashing it into Jupiter, lest microbes on the spacecraft seed Europa’s ocean with earthly life. Ganymede’s thick shell means the Juice team doesn’t have to worry as much about contamination. It plans to end the mission sometime after 2035 by colliding with its target, leaving one more crater on its icy crust.

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