"The idea that Europa might possess plumes seems to be becoming more and more real, and that's very good news for future exploration," said Xianzhe Jia, a space physicist at the University of Michigan and the lead author of the new paper on the phenomenon.
The results of the Clipper and Juice missions, he continued, "could have huge implications" - nudging us Earthlings closer to understanding whether we are alone.
Scientists have suspected since 2012 that Europa might harbour plumes, after the Hubble Space Telescope observed water vapour spouting above the moon's frigid south pole. Another set of observations, taken in 2014 and 2016, found a recurring jet shooting from an unusually warm "hot spot" near the moon's equator.
The tallest of the plumes was so powerful that it extended 120 miles above the moon's surface; Old Faithful, the famous geyser at Yellowstone, reaches only 184 feet.
The interpretation of those images has been debated; the images pushed the limits of Hubble's sensitivity, and sometimes the space telescope was unable to spot the plumes altogether.
The ongoing debate called for some on-site observations, Jia said. But no spacecraft has gotten close to Europa since Galileo, which swooped just 250 miles above the moon's "hot spot" in December 1997.
That mission had a severe shortcoming: The spacecraft's more powerful antenna failed to deploy after launch, limiting the amount of data the spacecraft could send back to Earth.
Nevertheless, Jia - who was just a college student during the flyby - had a hunch that, if a plume existed, Galileo might have sensed its signatures with its magnetometer and plasma wave instruments. So he and three colleagues began poring over findings from the two decades-old mission.
First they found Europa's magnetic field intensified and shifted orientation just as Galileo made its closest approach to the moon. Then, data from the plasma wave instrument showed unusual emissions that could be associated with a high density of charged particles - just what you would expect to find near a speeding jet of salty water.
But the environment around the moon is complex - warped by Jupiter's strong magnetic fields and by Europa's own atmosphere. So they ran the data through a sophisticated modelling program, which compared the observations with what scientists might expect to see from a plume of the dimensions reported by Hubble.
"The result came out with very satisfying agreement," Jia said - scientist speak for "we were right."
The source of the plume is still unclear. The prevailing theory is the water comes directly from Europa's subsurface ocean and is being driven upward by hydrothermal activity much like that which powers geysers on Earth.
But the water could originate elsewhere, Jia cautioned. Some have suggested there might be a subsurface lake hiding between layers of Europa's thick ice sheets.
The behavior of the plumes is also unpredictable. Because they were initially seen when Europa was farthest from Jupiter, researchers thought they might be driven by tidal stress - the friction generated by Jupiter's gravitational pull that keeps Europa's interior liquid. But follow-up observations from Hubble were unable to confirm that idea.
Jia hopes this paper will inspire fellow researchers to keep looking at Europa's plumes. Perhaps someone else will find further clues by mining years-old data. Or, maybe when the powerful James Webb Space Telescope finally launches (it has been pushed back several times), it will get a clearer picture of what is happening on the alien moon.
The Clipper mission - now in the preliminary design phase - is projected to arrive at Jupiter sometime around 2030. There it will perform 45 flybys past Europa, getting as close as just 16 miles above the moon's surface.
"If Clipper does end up flying through the plumes and the instruments are able to measure the composition, by analysing those data we'll be able to assess whether or not Europa has conditions for life," Jia said.
