If life exists 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, 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 from 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 latter planet and its satellites, studied in detail by NASA and the European Space Agency’s (ESA) Cassini-Huygens mission from 2004 to 2017, still holds secrets that scientists will one day probe. For now, all eyes are on Jupiter.

The mission to visit our solar system’s largest planet will be ESA’s JUICE spacecraft—the Jupiter Icy Moons Explorer. Now undergoing testing in France, the six-ton spacecraft will soon be shipped to French Guiana in South America for its launch this April on a European Ariane 5 rocket. JUICE 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 spacecraft 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, however. 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 at the University of Leicester in England, part of the JUICE team.

ESA isn’t the only space agency with Jupiter in its sights, of course—although recent history would almost suggest otherwise. The concept that would ultimately become JUICE emerged in 2008, as part of a joint venture with NASA dubbed the Europa Jupiter System Mission (EJSM). 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. (A NASA spacecraft, Juno, is presently operational at Jupiter, but is more focused on the gas giant planet than on any of its moons.) “We didn’t have the money,” says Louise Prockter at the Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Maryland, 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 at 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 called the Europa Multiple Flyby Mission, the U.S. project eventually became Europa Clipper, named for the “clipper” merchant ships of the 19th century. The international collaboration was reborn, albeit in watered-down fashion. “It’s much reduced,” Prockter says, although she estimates about 70 percent of the originally planned joint science will still be possible.

Clipper will launch in fall 2024 on a SpaceX Falcon Heavy rocket. Despite its later launch date, its more powerful launch vehicle will allow Clipper to reach Jupiter earlier, more than a year before JUICE, in April 2030. It will not orbit Europa like JUICE will Ganymede, as the former’s proximity to Jupiter places it perilously deep within Jupiter’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, the Europa Clipper program scientist at NASA Headquarters in Washington, D.C. “We realized instead we could flyby, collect our data, and get the heck out of town where the radiation is lower. That way we can last years, not months.”

Moon-Hopping and Plume-Spotting

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,” says Olivier Witasse at ESA, the project scientist for JUICE. About 20 scientists from both missions are currently 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. While “some of the details are a little bit different” from the initial EJSM collaboration, Bunce says, the initial 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.”

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. It’s thought that Europa’s subsurface ocean, like that within Saturn’s moon Enceladus, occasionally spurts out plumes of liquid water 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 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 also perform two Europa flybys of its own prior to orbiting Ganymede. One of those, 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 for some interesting science to be done, although the exact details have yet to be determined. “We will not fly over the same location, but it will for sure be very interesting,” he adds. “We could image similar surfaces features or, if there is a plume, we can observe it from different geometries.”

The joint emphasis on Europa is partially due to scientists’ suspicions that the moon’s liquid water ocean is in direct contact with a rocky core. There, hydrothermal vents—openings in the sea floor 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,” says Fox-Powell. “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 Moons, Two Missions, One Vision

None of this rules out Ganymede’s chances of habitability, nor diminishes that moon’s scientific interest. JUICE, after entering orbit around Ganymede in December 2034, 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, you need a heat source, liquid water, organic material, and stability,” Dougherty says. “At 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 at ESA, project manager for JUICE.

Clipper, meanwhile, will provide a similar level of knowledge about Europa and its ocean. It is not geared to find definitive evidence of life, however, instead—at best—only perhaps seeing 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 NASA’s Jet Propulsion Laboratory (JPL) in California, but awaits further funding. “Europa has not been in the president’s budget or the budget passed by Congress for a while,” Niebur says. An authoritative 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.”

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 into Europa’s hidden ocean. 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, 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 are big, big problems that need to be solved in the coming years,” Pereira says. The lander would have to carry perhaps several kilometers worth of cable with it, and any probe would have to be resilient enough to endure water refreezing as ice around it during a descent. The scientific value in solving such problems, however, are tremendous.

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 unlock more secrets of the Jupiter system, itself an analogue for many of the exoplanet systems we see around other stars, than ever before. “It’s a mini solar system,” Sarri says. “We’re looking for potential habitats that can sustain life.” 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?

Editor’s Note (3/14/23): This article was edited after posting to correct the spelling of Curt Niebur’s name.