If there is life elsewhere in our solar system, Jupiter's large icy moons are a pretty good bet on where to find it.

Scientists believe vast oceans lurk within them, kept liquid by the jostling from Jupiter's immense gravitational field and protected from the planet's harsh radiation belts by thick ice sheets. “What we've learned on Earth is where you find water, you quite often find life,” says Mark Fox-Powell of the Open University in England. “When we look out in the solar system, places that have [liquid] water in the present day are really restricted to Earth and the moons of Jupiter and Saturn.” That last planet and its satellites, studied in detail by NASA and the European Space Agency's Cassini-Huygens mission from 2004 to 2017, still hold secrets that scientists will one day probe. For now all eyes are on Jupiter.

A new mission to visit our solar system's largest planet and investigate the habitability of its moons is now set to begin. ESA's JUICE—the Jupiter Icy Moons Explorer—was shipped to French Guiana in South America for its April launch on a European Ariane 5 rocket. The six-ton JUICE spacecraft will take eight years to reach Jupiter, saving fuel along the way by using gravitational assists from Earth, Venus and Mars. On its arrival in July 2031 the solar-powered machine will focus its 10 science instruments on three of the four largest Jovian moons—Europa, Ganymede and Callisto—all thought to harbor subsurface oceans. Ganymede, the solar system's largest moon, will receive most of JUICE's attention. After its initial reconnaissance, the spacecraft will enter orbit there in 2034. “We're trying to characterize what the habitability of Ganymede might be,” says Emma Bunce of the University of Leicester in England, part of the JUICE team.

ESA isn't the only space agency with Jupiter in its sights. The concept that would ultimately become JUICE emerged in 2008 as part of the Europa Jupiter System Mission (EJSM), a joint venture with NASA. This collaborative effort called for Europe to build a Ganymede-focused spacecraft, while NASA would construct a probe for Europa. Funding issues in the U.S., however, led NASA to pull the plug on EJSM in the early 2010s, leaving Europe flying solo. “We didn't have the money,” says Louise Prockter of the Johns Hopkins University Applied Physics Laboratory, part of the U.S. proposal team. “That killed the Europa part.” The situation was disappointing but not wholly unexpected. “These things happen,” says Michele Dougherty of Imperial College London, who worked on the European side of EJSM.

Redemption came in 2013, when NASA's efforts to explore Europa received renewed support and funding from Congress. Initially named the Europa Multiple Flyby Mission, the U.S. project eventually became Europa Clipper, after the “clipper” merchant ships of the 19th century. The international collaboration was reborn, mostly. “It's much reduced,” Prockter says, although she estimates about 70 percent of the originally planned joint science will still be possible. With these two missions, our knowledge of Jupiter and its moons is set to increase substantially. The spacecraft will tell us whether life could exist in some of these worlds' bewildering subsurface oceans, laying the groundwork for later missions to look directly for evidence of such life, possibly even by diving into the oceans themselves. We can't yet travel to alien worlds around other stars, but Jupiter might offer the next best thing.

The First Moons

The jovian arena is often regarded as a miniature solar system because of the complexity and variety of the planet's moons—particularly its four largest, the Galilean moons, named for Italian astronomer Galileo Galilei, who discovered them in 1610. Their identification shook people's understanding of the universe, revealing the first known objects orbiting a body that was not the sun or Earth and thereby validating the Copernican model of the cosmos, which did not have us at its center. Jupiter is now known to have 92 natural satellites. Yet even Galileo might not have appreciated how fascinating his moons would turn out to be 400 years later or how pivotal they might prove in the hunt for life elsewhere in the universe.

A view of the shadow of Jupiter’s moon Ganymede's, shown on the surface of the planet Jupiter; shown against a black background.
The shadow of the moon Ganymede hangs over Jupiter in this image from Juno, with color enhancement by a citizen scientist. Credit: NASA/JPL-Caltech/SwRI/MSSS (image data); Thomas Thomopoulos © CC BY 3.0 Unported (image processing)

The first spacecraft to venture into Jupiter's realm, moons and all, was NASA's Pioneer 10 spacecraft. It flew past the planet in December 1973, providing our first close-up images of the magnificent gas giant. The flyby of NASA's Voyager 1 spacecraft in March 1979 proved even more remarkable. The spacecraft's images of the moon Europa revealed that it had a bright, icy surface devoid of craters, hinting that some kind of resurfacing process was keeping its crust fresh and unblemished. The best bet was an unseen reservoir of liquid water below the surface, scientists surmised—an enticing option given that on Earth, life follows water.

In December 1995 NASA's Galileo mission became the first to orbit Jupiter, making numerous discoveries—for example, that the planet's third-largest moon, Io, is the most volcanically active world in the solar system. Data that Galileo took at Europa in 1996 found that something was disrupting Jupiter's magnetic field, offering stronger hints of a liquid sloshing under Europa's surface. The best evidence for a liquid ocean on Europa came two decades later, when the Hubble Space Telescope spotted plumes of water escaping from the moon's surface. The Galileo spacecraft orbited Jupiter for eight years, ending in 2003, and was “a fantastic mission,” says Olivier Witasse of ESA, the project scientist for JUICE. “We are really going on the shoulders of Galileo.”

No other probe would orbit Jupiter until the arrival of NASA's Juno spacecraft in 2016. Juno is still operational today, but it is focused on Jupiter itself, swinging past it in a looping orbit to probe the planet's interior, image its violent storms and monitor its immense magnetic field. The spacecraft has taken some images of Jupiter's moons, but it'll take dedicated missions to really expose their secrets. And that's where JUICE and Clipper come in.

Moon Hopping and Plume Spotting

Clipper will launch in fall 2024 on a SpaceX Falcon Heavy rocket. Despite its later launch date, its more powerful launch vehicle will allow the spacecraft to reach Jupiter more than a year before JUICE, in April 2030. It will not orbit Europa like JUICE will Ganymede, because Europa's proximity to Jupiter places it perilously deep within the planet's radiation belts. Instead Clipper will perform about 50 Europa flybys as it zips around the Jovian system, allowing it to map the moon's interior and work out the extent of its subsurface ocean while also studying other targets. “Putting an orbiter around Europa, because of the radiation environment, means you're only going to survive one to three months before the radiation kills you,” says Curt Niebur, Europa Clipper program scientist at NASA Headquarters in Washington, D.C. “We realized instead we could fly by, collect our data and get the heck out of town where the radiation is lower. That way we can last years, not months.”

During their overlapping missions, JUICE and Clipper will perform an intricate tango as they hop between Jupiter's attractions, with copious opportunities for collaboration. “To have two spacecraft in the same system will be really fantastic,” Witasse says. About 20 scientists from both missions are meeting virtually every week as part of the JUICE-Clipper Steering Committee, with the group formulating ideas for how the two spacecraft might sync up at Jupiter. “We're busy talking through the science opportunities and coming up with a plan” to present to NASA and ESA, says Bunce, who co-chairs the committee with Prockter. Whereas “some of the details are a little bit different” from the initial EJSM collaboration, Bunce says, the overall dream remains alive. “The original plan was one mission focused on Ganymede and another mission focused on Europa,” she says. “And that's what we've got.”

View of Jupiter’s moon Sol, shown against a black backdrop.
In 2021 Juno made a close flyby of Ganymede, the solar system's largest moon. Credit: NASA/JPL-Caltech/SwRI/MSSS (image data); Kalleheikki Kannisto © CC BY 3.0 Unported (image processing)

One possibility is that each spacecraft could act as a spotter for the other. JUICE, for example, could keep an eye on Europa from afar as Clipper prepares to swoop past—a valuable partnership, especially if there are indeed plumes of liquid water spouting from cracks in the overlying ice. Peering into these plumes could lead to studying oceanic ejecta that are just “minutes old,” Fox-Powell says. “It really gives us an opportunity to study something that's pristine.” As Clipper approaches Europa, JUICE could look for plumes erupting from the surface, allowing Clipper to train its eye in that direction. “If JUICE spotted one, that could tell us where to look,” Prockter says. Clipper may even fortuitously pass through some plumes, allowing it to directly sample them and look for signs of complex molecules that might hint at signs of life in the Europan ocean.

JUICE will perform two Europa flybys of its own prior to orbiting Ganymede. The one in July 2032 will be just four hours apart from a Clipper flyby. “We can make similar measurements at the same time,” Witasse says. That could allow some exciting science to be done, although the exact details have yet to be determined. “We won't fly over the same location, but it will for sure be very interesting,” he adds. “We could image similar surface features, or if there is a plume, we can observe it from different geometries.”

The joint emphasis on Europa is partially based on scientists' suspicions that the moon's liquid-water ocean is in direct contact with a rocky core. There hydrothermal vents—openings in the seafloor where heat from deeper within can escape—could supply sufficient energy and nutrients to sustain life. “On Earth we have hydrothermal vents where there are whole communities of organisms,” Fox-Powell says. “We have good reason to believe that similar kinds of chemical reactions are going on at Europa.” Ganymede's much larger bulk, however, means that higher-density ice may have sunk to the bottom of its ocean, forming a vent-blocking barrier. “It could seal the rocky core away,” Fox-Powell says. “Europa is not big enough to have that amount of gravity and pressure, so that high-pressure ice doesn't form.”

Two Missions, One Vision

None of this rules out Ganymede's chances of habitability, nor does it diminish that moon's scientific interest. After entering orbit around Ganymede in December 2034, JUICE will survey the entire surface, study the moon's magnetic field and attempt to map its aquatic inner layers. For an environment to be interesting for potential habitability, it needs “a heat source, liquid water, organic material and stability,” Dougherty says. “At [Saturn's moon] Enceladus we know we've got three. At Europa we've got three. And at Ganymede we're trying to find out.” Although it will start in a high orbit 5,000 kilometers above Ganymede, during a nine-month period JUICE will lower its altitude to just 200 kilometers over the moon's surface. Eventually, at the mission's end in 2035, the spacecraft will be deliberately crashed into the surface to minimize the chance of any debris contaminating Europa. Ganymede is not thought to have plume activity, but if it does or if its ice crust is found to be particularly thin, this finale may have to be rethought so as not to contaminate Ganymede's liquid ocean, too. “If there is something that indicates a connection with the inner ocean and the outer surface, we may need to change our orbit,” says Giuseppe Sarri of ESA, project manager for JUICE.

View of Jupiter’s moon Europa, shown against a black backdrop.
Cracks and ridges crisscross the surface of Europa in an image assembled from data taken by the Galileo spacecraft. Credit: NASA/JPL-Caltech/SETI Institute

Clipper will provide a similar level of knowledge about Europa and its ocean. It is not designed to find definitive evidence of life, however; at best, it will perhaps see the ingredients of life within the moon's plumes. Life detection may come on a later mission, such as NASA's much sought-after Europa Lander. A concept for the mission was drawn up years ago by scientists and engineers at NASA's Jet Propulsion Laboratory in California, but it awaits further funding. “Europa Lander has not been in the president's budget or the budget passed by Congress for a while,” Niebur says. A road map for U.S. interplanetary exploration produced by the U.S. National Academies in late 2021, meanwhile, placed a Europa Lander mission as a lower priority for NASA than other projects.

For now the work is archived, ready and waiting to be reborn. “I'm confident that what Europa Clipper will learn will make us want to go back, and a lander of some kind is the logical next step,” Niebur says. “But maybe Clipper will throw us a curveball, and a lander is not the right way to go. Maybe we'll want to hover in the plumes instead of landing.”

If scientists do want to take a dip in this alien ocean, breaking through the kilometers-thick ice poses its own challenges. One possibility is that a lander could include a heat probe to melt its way through the frozen crust. Last year Paula do Vale Pereira, now at the Florida Institute of Technology, led an experiment to see how long that might take, using a two-meter-high column of cryogenic ice called the Europa Tower to simulate the Europan surface. Presenting her work at the 241st meeting of the American Astronomical Society in Seattle in early January 2023, she found the task might take anywhere between three and 13 years—long times to wait, even for multidecadal missions to the outer solar system.

Besides the ticking of the clock, other obstacles abound. “Figuring out a way to have cables transfer power and information between the lander and the probe is a big, big problem that needs to be solved in the coming years,” do Vale Pereira says. The lander would have to carry perhaps several kilometers' worth of cable with it, and the cable would have to be resilient enough to endure water refreezing as ice around it during the probe's descent. The scientific value in solving such problems, however, is tremendous, not least the prospect of placing some kind of machine directly inside an alien ocean.

Such dreams are many years away. Any hope of making them a reality hinges on voyaging to Jupiter and confirming its icy moons are the attractive targets we believe them to be. Beginning with JUICE in April and Clipper next year, we are set to solve some of the most intriguing questions about Jupiter's moons that have long gone unanswered. The Galileo spacecraft “revealed to us that it's worth going back,” Niebur says. Now we're doing so with not one but two spacecraft—a transatlantic partnership to significantly advance the search for habitability around our sun. There is no world in our solar system quite like Earth, but perhaps places like Europa and even Ganymede are a close second. If life can survive here, who knows where else it might thrive?