For more than two decades, scientists have been convinced Europa has a liquid water ocean sloshing around beneath its icy outer crust.
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.
A rendering of how NASA researchers believe Galileo flew through plumes of water high above Europa.
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NASA originally wanted to use the Clipper mission to land a probe on Europa for direct sampling, and one idea was for a robot melt its way down through the ice and into the waters below, but budget cuts put paid to that. "But this has made me a believer", says Morgan Cable, an astrochemist at the NASA Jet Propulsion Laboratory who was not involved in the research. Jia and his team took an in-depth look at information the probe gathered during the closest of these encounters - a 1997 flyby that brought Galileo within 128 miles (206 km) of the moon's frigid, fractured surface.
It comes from measurements made from much closer up during a flyby of Nasa's now-expired Galileo spacecraft. The magnetic detectors recorded a kink in the magnetic field and the on-board plasma wave spectrometer picked up increased levels of ionized particles. These changes were all consistent with computer models that showed how the plumes Hubble saw should have been affecting the environment around the planet. "It's very interesting that one can look at old data like these with new analysis tools like models and simulations".
The result that emerged, with a simulated plume, was a match to the magnetic field and plasma signatures the team pulled from the Galileo data. But in 2008 NASA's Cassini spacecraft swing by the Saturnian moon of Enceladus and intentionally flew through one of the plumes of matter than the body periodically emits to examine what it contained.
"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. That's another requirement for life that might be checked off the list - in fact, some scientists theorize that life on Earth started in the deep sea vents that erupt in geysers. Because geologic processes move material from underground to the surface, the reverse may also be happening, transporting surface materials highly oxidized by Jupiter's harsh radiation down through the ice.
As suggestive as these results were, they didn't represent a definitive Europa plume discovery, researchers said at the time.
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If we're going to find extraterrestrial life inside of our own Solar System there's only a handful of places where scientists believe we might find it. Each spacecraft would reach the mysterious world less than three years after launch.
"It's unlikely that plumes, if they exist, come directly from a subsurface ocean layer, since the surface ice layer is thought to be kilometers thick". Potential passages through a plume would be a bonus, allowing both spacecraft to sniff out any signs of curious oceanic chemistry or even of life carried aloft in the tenuous vapor.
Clipper isn't slated to launch until the 2020s. There it will perform 45 flybys past Europa, getting as close as 25km above the moon's surface. And NASA is working on a mission that could do just that.
According to at least one high-powered congressman, the lander's approval is already inevitable.
On its closest flyby, Galileo swept over Europa at more than 2,230 miles per hour.
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