So they ran the data through a sophisticated modeling program, which compared the observations with what scientists might expect to see from a plume of the dimensions reported by Hubble.
That mission, now dubbed Europa Clipper, could be informed by research published online Monday in the journal Nature Astronomy.
First they found that Europa's magnetic field intensified and shifted orientation just as Galileo made its closest approach to the moon.
If some of the water from Europa's interior can be collected without actually landing on the moon, it could be a boon for exploration in that it's much easier to send a spacecraft on a flyby mission than to actually land something on the surface of the world.
By reanalyzing old data from Galileo instruments, researchers discovered evidence that the robot had flown through a giant jet or plume of water sprayed from Europa's ocean. Now, researchers report in a new study that NASA's Galileo Jupiter probe, which orbited the planet from 1995 to 2003, also detected a likely Europa plume, during a close flyby of the icy moon in 1997. The European Space Agency will be sending JUICE (JUpiter ICy moons Explorer), while NASA will be sending the Europa Clipper, which will be focusing on Europa to see if it has the right conditions to be able to host life.
Jia hopes that this paper will inspire fellow researchers to keep looking at Europa's plumes.
"This is potentially great news for future exploration of Europa, because spacecraft may have a chance to directly sample materials that are linked to the subsurface ocean", Xianzhe Jia, lead author of the study, said via email. But the evidence has been hard to come by.
The best so far came from the Hubble Space Telescope, which watched Europa repeatedly pass in front of Jupiter. But scientists think the Jovian satellite harbors a huge amount of liquid water - perhaps twice as much water as Earth does, in fact - in a deep global ocean sloshing beneath the object's ice shell.
That said, the new study doesn't necessarily confirm the existence of a plume, either.
The evidence gathered to date suggests that the processes producing Europa's plumes - if they do indeed exist - may not be continuous like the geysers of Enceladus but instead intermittent.
The space agency is priming two probes, including one that will land on its surface, to explore the distant moon in detail within the next decade, the agency says.
The Europa Clipper mission will study Europa for two years, spending time orbiting Jupiter and making at least 40 close flybys of Europa.
The other mission is NASA's roughly $2 billion Europa Clipper, which may launch sometime between 2022 and 2025 and arrive about half a decade later. Gravity maps confirmed the plumes originated from the moon's surface. That could give it access to visuals of plumes and the ability to taste of their water for salts, organic compounds, and other chemicals.
Bob Pappalardo, a planetary scientist at NASA's Jet Propulsion Laboratory who wasn't involved in the new work, previously told Business Insider that oceans on moons like Europa "may be the most common habitats for life that exist in the universe". Then, data from the plasma wave instrument showed unusual emissions that could be associated with a high density of charged particles - just what you'd expect to find near a speeding jet of salty water.
He added: "If there's life at Europa, it'd nearly certainly be an independently evolved form of life".
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