"A signature located in the MAG data acquired at Galileo's closest encounter with Europe is totally consistent with the expected disturbances if the spacecraft were to pass through a plume that rises above the nearby surface", states the science team's members in the study's report. Xianzhe Jia, the lead author of the study, told Mashable, "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".
The relatively small moon of Jupiter is thought to have a liquid water ocean beneath its icy shell, and it's considered one of the most likely places to host alien life in our solar system.
It is thought to have an iron core, a rocky mantle and a surface ocean of salty water, like Earth.
Near Enceladus' south pole, more than 100 individual geysers continuously blast water ice, organic molecules and other material far out into space - so far that this plume stuff forms Saturn's E ring. They believed these fluctuations might be due to perturbations from a water plume in the plasma surrounding the Moon. The particles that are spread by the plumes will be found in the atmosphere of Europa. In late 2012, Hubble spotted signs of such a feature near the moon's south pole.
"These results provide strong independent evidence of the presence of plumes at Europa", they wrote in Nature.
When they examined the information gathered during that flyby 21 years ago, sure enough, high-resolution magnetometer data showed something unusual.
Imaginary illustration of four best known Jupiter moons
During its time at Jupiter, Galileo performed 11 flybys of Europa.
The team used numerical models to try to explain exactly what Galileo saw on that fateful day in 1997, revealing that the spacecraft likely got sprayed with water shooting from Europa when it made a flyby of the moon.
The study shows that the spacecraft's instrument detected two critical phenomena in this area: a significant change in Europa's magnetic field and a surge of ionized gas hinting at a large increase in density of the moon's plasma. For example, in the 2014 and 2016 candidate detections, the possible plumes blocked some ultraviolet light emitted by Jupiter. Jia's team pulled that data as well, and it also appeared to back the theory of a plume.
Why did it take more than two decades to tease this result out of the Galileo data set?
"The data were there, but we needed sophisticated modeling to make sense of the observation", Jia said. "And I don't think those were available back 20 years ago". Researchers published their findings in Nature Astronomy. The agency is developing a $2 billion Jupiter-orbiting mission called Europa Clipper, which is scheduled to launch in the early to mid-2020s.
Europa's frozen surface has always been thought to cover a salty ocean about twice the size of our planet's, according to AFP.
"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", Pappalardo said.
Since Galileo's end, the Hubble Space Telescope has periodically observed the Jovian system.