Scientists have uncovered the biggest evidence yet that water may be spewing from the surface of Jupiter’s icy moon Europa — a revelation that was buried deep within the archives of a long-dead NASA spacecraft. In 1997 the Jupiter probe, Galileo, flew near a geyser when it passed close by Europa, collecting data that went overlooked at the time. But now, the rendezvous has been unearthed 20 years later, and it provides scientists with their first up-close measurement of one of Europa’s water plumes.

Up until now, scientists have strongly suspected that water is pouring out from Europa, but the matter hasn’t been fully settled. The only evidence we have for these geysers comes from NASA’s Hubble Space Telescope, which captured images of water escaping from the distant moon in 2012 and 2016. But Hubble’s pictures were taken from afar and are pretty fuzzy; they haven’t been considered definitive proof. This discovery from Galileo, detailed today in Nature Astronomy, “is the strongest evidence we have so far in terms of seeing signatures of a plume at Europa,” Xianzhe Jia, a planetary scientist at the University of Michigan and lead author of the study, tells The Verge.

Europa’s plumes are thought to stem from a global saltwater ocean lurking underneath the moon’s crust. It’s a feature that’s made this icy world a prime candidate in the search for life elsewhere in our Solar System. Water is vital for life here on Earth, so many have wondered if organisms might be able to survive in Europa’s waters as well. And plumes offer a great opportunity for studying what’s inside this ocean. Scientists don’t need to send a spacecraft to land on the moon and drill into its icy surface; they can simply send a vehicle to fly by the moon and sample its plumes — a much easier type of mission to pull off.

That’s exactly the kind of mission NASA plans to do in the early 2020s. The space agency has been working on a robotic spacecraft called Europa Clipper, which will fly by Europa’s surface upward of 40 times to get a taste of these plumes. Before today’s findings, it was still somewhat unclear if Europa Clipper would actually see any plumes while it was out there. Now, scientists are much more sure. “It really suggests that Clipper has a good opportunity to directly fly through a plume and tell us a lot more about its properties,” says Jia.

Galileo’s secret

Jia says that the Hubble images are what inspired his team to revisit Galileo. Launched in 1989, the robotic probe explored Jupiter and its moons from 1995 to 2003, before it plunged into the gas giant and broke apart. But while it was out there, it flew by Europa 11 times to gather data of the world with its various instruments. At the time, there wasn’t any evidence of water stemming from Europa, so the spacecraft wasn’t specifically looking for plumes on the moon. However, it did take measurements of Europa’s magnetic field and how particles behaved around the icy world. “We were thinking maybe we had a chance of seeing something in that data,” Jia says.

After going through all 11 flybys, the researchers narrowed in on one that occurred on December 16th, 1997. They noticed that the data from this trip looked strange: Galileo measured a sharp drop in the strength of the magnetic field. And the field seemed to bend and rotate, too. At the same time, the spacecraft also noticed that the space near Europa had a higher amount of charged particles than usual. “This one certainly stood out as very special,” says Jia. “We don’t see anything like that on all the other flybys.”

To make sense of all these measurements, the research team created a computer model of Europa’s plumes, based on the images taken from Hubble. They wanted to see how a simulated plume might affect the environment around the moon. And sure enough, the model had the same effects on the space environment that Galileo saw. Given how long it took Galileo to make these measurements, the researchers estimate the plume was about 620 miles (1,000 kilometers) thick.

The team thinks that all those weird changes in the magnetic field occur because some of the particles within Europa’s plumes are charged, and are clashing with other charged particles already circulating around the moon. The space around Europa is filled with particles streaming in from another moon of Jupiter — the volcanically active Io that’s constantly burping up gas and plasma. So when a plume erupts from Europa, the particles slam into the ones from Io, causing an electrically charged traffic jam that creates twists and turns in the moon’s magnetic field.

Backing up the data

There’s even more evidence that Galileo mingled with a plume during the December 1997 flyby. For one, the event occurred pretty close to where Hubble spotted water spewing from the moon in 2016. And this region of Europa is special: it’s a hotspot, where the temperature is higher than the rest of the moon. It’s possible this heating is linked with the geyser activity in some way. Perhaps heated water is flowing upward inside Europa, warming the surface and causing cracks in the ice that allow water to burst through. Or maybe the plumes themselves are to blame for the heating. The ejected water could be falling back onto Europa, changing the structure of the ice so that it can hold heat longer than the rest of the surface.

The December 1997 flyby was also the closest Galileo ever got to Europa’s surface, coming within 125 miles (200 kilometers) of the moon. So it makes sense that this was the only flyby to pick up any activity. Europa’s plumes don’t seem to extend very far into space. In order to really see the effects of the plume, the spacecraft had to be very close to the surface, says Jia.

That means that any follow-up probes will need to get as close as possible to the moon. The good news is that’s exactly what Europa Clipper is designed to do. During its 40-plus flybys, it will get much closer to Europa than Galileo ever got, at one point coming within 16 miles (25 kilometers) of the moon’s surface. Clipper will have much more sensitive instruments, too, aimed at piecing together what’s inside the water. “I think it bodes well for Clipper that the few flybys that Galileo had, with poorer conditions for observing this phenomenon, turned up a pretty solid plume detection,” Carol Raymond, a NASA scientist and principal investigator of the magnetic field instrument on the Europa Clipper mission, tells The Verge.

Looking ahead to Europa

The Galileo study is so supportive of the Europa Clipper design that one enthusiastic lawmaker is using this research to clinch further funding for the mission. During a House subcommittee meeting last week, a group of lawmakers approved a new House funding bill for NASA, one that would give $545 million to the Europa Clipper mission in fiscal year 2019, Space News reports. At the meeting Rep. John Culberson (R-TX), chairman of the appropriations subcommittee for NASA and a big Europa buff, cited today’s Nature Astronomy study (under embargo at the time) as a big reason why funding the Europa Clipper mission is so vital.

“So, the ocean of Europa is venting into outer space,” Culberson said, according to Space News. “The science community has wanted to go there for years... and this bill makes that happen.”

Culberson is also pushing NASA to make a lander that will go to Europa, and the House bill secures $195 million for that purpose, too. However, drilling through Europa’s ice is going to be tough: the icy crust may reach as deep as 12 miles (20 kilometers) before it turns into liquid. Plus, the lander isn’t supposed to launch until 2024, whereas Europa Clipper is slated to launch in 2022. Clipper can get a feel for Europa’s oceans much sooner and much more easily before NASA attempts the difficult task of digging into the moon’s ice. “It’s just a great demonstration of how powerful the Clipper data sets are going to be,” says Raymond. “It’s making it all seem a little bit more realistic that we’re going to capture some plume activity in process.”