Ever since 2012, when astronomers using the Hubble Space Telescope first spied inconclusive hints of watery plumes emanating from the subsurface ocean of Jupiter’s large, icy moon Europa, space scientists have fiercely debated the claim. Previous estimates had suggested the moon’s crust might be tens if not hundreds of kilometers thick—too thick, that is, to allow direct exploration of its potentially life-friendly ocean anytime soon. A plume venting some of Europa’s ocean water into space where it could be sampled by an orbiting spacecraft would change the whole equation—it seemed, in short, too good to be true.
Now, however, a new analysis of 21-year-old data from NASA’s Galileo spacecraft, which orbited Jupiter from 1995 to 2003, has found strong independent evidence in favor of the plume.
This discovery all but confirms Europa should be considered a high-priority peer of another “ocean world” in the outer solar system, namely Saturn’s moon Enceladus, which also sports an even more dramatic plume. The finding also bolsters hypotheses that posit parts of Europa’s crust are far thinner and more fractured than previously believed—conditions that may allow life-sustaining energy as well as exploratory robots easier entry into the moon’s lightless abyss. Xianzhe Jia, a University of Michigan space scientist, and his colleagues published their findings on May 14 in Nature Astronomy.
“Up until this paper, I was very skeptical that the plume existed, because all those Hubble measurements made before were at the sensitivity limit of the instruments. But this has made me a believer,” says Morgan Cable, an astrochemist at the NASA Jet Propulsion Laboratory who was not involved in the research.
Inspired by the Hubble images and discussions of planned missions to revisit Europa, Jia and his team dusted off the data from two instruments onboard Galileo, gathered during the spacecraft’s 11 close passes around the moon. One flyby in December 1997 came within 400 kilometers of the surface—much closer than Hubble, which in low Earth orbit is at least 600 million kilometers away. As Galileo flew near the moon, its instruments detected significant, sudden changes in the magnetic field and charged particles around Europa—as if the spacecraft was passing through a tenuous cloud of material rising from below. Together with careful modeling of the moon’s surrounding environment, those changes point to the presence of the plume and its location, not far from the putative source inferred from Hubble images. “It’s very interesting that one can look at old data like these with new analysis tools like models and simulations. It’s possible to find things in those data you didn’t initially expect,” says William Kurth, an astrophysicist at The University of Iowa and co-author of the study.
Jia used measurements from Galileo’s magnetometer to seek out small perturbations in the magnetic field during the closest flyby. Unlike Galileo researchers in the 1990s, he and his colleagues performed a more focused investigation, looking for little changes here and there and finally finding one. They also examined data from Galileo’s spectrometer, which measured the presence and behavior of charged particles. If a plume vented water vapor from the moon, some of the water’s constituent hydrogen and oxygen would become charged by being stripped of electrons by the radiation belts that surround Jupiter. Finally, those charged particles would interact with the surrounding magnetic field, generating fluctuations detectable by Galileo.
To make sense of those measurements and ensure they were not merely due to background fluctuations produced by Jupiter itself, Jia and his team compared them with computer simulations of how a plume would affect the plasma and magnetic field around Europa along the path Galileo took. By and large, those model predictions matched their observations, ruling out Jupiter as the source and strengthening the case for watery plumes erupting from Europa’s depths.
The presence of the plume means the moon is geologically active, which is promising for the possibility of alien life in the sunless sea. Because geologic processes move material from underground to the surface, the reverse may also be happening, transporting surface materials highly oxidized by Jupiter’s harsh radiation down through the ice. Wherever materials from above and below meet and mix, chemical gradients form that organisms could in theory exploit for energy. “If you have an exchange of those materials, that might provide the reactants needed for life. They’re like a battery, combining both positive and negative to make an energy source,” says Alison Murray, a biologist at the Desert Research Institute who studies organisms in extreme environments but did not take part in the research.
To determine whether Europa is indeed life-friendly and to look for possible evidence of those organisms themselves, scientists want spacecraft much more powerful than Galileo to return to the moon. “We can stare with really good telescopes in Earth orbit as much as we want, but to really answer the question, you have to go there and do the measurements. You have to grab the ice grains that are there and see what’s inside Europa,” Cable says.
These findings will inform and draw more interest in NASA’s Europa Clipper and the European Space Agency’s Jupiter Icy Moons Explorer (JUICE) missions, planned to launch in the mid-2020s and in 2022, respectively. Each spacecraft would reach the mysterious world less than three years after launch. Both are planned to fly close to the moon, using more sensitive instruments such as high-resolution cameras, spectrometers and ice-penetrating radar that could study organic materials on the surface, the thickness of the ice shell and the salinity of the vast ocean beneath. Potential passages through a plume would be a bonus, allowing both spacecraft to sniff out any signs of curious oceanic chemistry or even of life carried aloft in the tenuous vapor.
NASA scientists have also proposed a mission for a spacecraft that would actually land on the Europan surface, tentatively slated for a 2024 launch. Material jetted from a plume and snowing back down onto the moon’s surface would make landing sites in close proximity to the plume the most prized spots. Assuming, that is, NASA and Congress officially approve such a mission.
According to at least one high-powered congressman, the lander’s approval is already inevitable. “The lander will be built and flown. It’s going to happen. I made certain that the Europa orbiter and lander are the only missions it’s illegal for NASA not to fly,” says Rep. John Culberson (R–Texas), member of the House Appropriations Committee and chair of the subcommittee with jurisdiction over NASA. For years Culberson has advocated for both Europa missions, ensuring they are present and accounted for in multiple budget bills. “This discovery confirms the paramount importance of these missions, because it looks like Europa will be our best opportunity to discover life on another world,” he adds.