Data gathered by NASA's Galileo spacecraft in 1997 may prove that it flew directly through a water plume from Jupiter's moon Europa.
The revelation has again emphasised the scientific consensus that Europa, which has a salty ocean twice the size of Earth's, could be home to extraterrestrial life.
Galileo Galilei discovered Europa in 1610, and the moon has been keenly observed by humanity ever since – including by the NASA space probe named after him.
When Galileo orbited Jupiter between 1995 and 2003 it collected reams of data, including from 11 flybys of the moon Europa.
"On one particular pass by Europa, the spacecraft came very, very close to the surface – as I remember less than 150 kilometers (93 miles) above the surface," said Professor Margaret Kivelson
"On that pass we saw signatures that we never really understood," Professor Kivelson, professor emerita of space physics at the University of California, Los Angeles, told NASA TV.
Researchers have now re-examined those signatures according to a report in the journal Nature Astronomy after one presented their findings from Hubble Space Telescope data about water plumes erupting from the moon.
The eruptions from Europa are called cryogeysers (cryo meaning icy cold), although it is not clear how they are generated.
Unlike geysers on Earth, it is unlikely that the eruptions are powered by heat from the moon's core.
Instead, scientists suspect that tidal heating caused by the gravitational pull of Jupiter could be pressurising the watery ocean beneath Europa's frozen surface and causing it to blow out through cracks in the surface.
NASA has reported evidence from the Hubble Space Telescope before about the existence of water plumes, although the scientific community has debated this interpretation of the data.
Dr Elizabeth Turtle, a research scientist at Johns Hopkins Applied Physics Laboratory, said the subsurface ocean "is likely the most habitable part of Europa because it is warmer and it is protected from the radiation environment by the ice shell".
Because the plumes erupt like geysers "there may be ways for that material from the ocean to come out above that ice shell and that means we would be able to sample it," she added.
NASA described the report as "good news" ahead of a new mission to the moon, named the Europa Clipper mission.
Costing $8bn, Europa Clipper will launch as early as June 2022 and will conduct a series of low-altitude flybys, collecting samples of frozen water and dust.
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"There now seem to be too many lines of evidence to dismiss plumes at Europa," said Dr Robert Pappalardo, Europa Clipper project scientist at NASA's Jet Propulsion Laboratory.
"If plumes exist, and we can directly sample what's coming from the interior of Europa, then we can more easily get at whether Europa has the ingredients for life," he added.
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