One of the most promising places in the solar system to search for life is Jupiter's moon Europa, an icy world with a slushy, salty, subsurface ocean. Peering through the crust to the gooey center below is a challenge, but luckily it seems that the water is meeting us halfway. Old data has turned up new evidence of water plumes spraying through the icy shell into space.
The first hints of plumes erupting from Europa were spotted by Hubble in 2012, in the form of water vapor over the south pole. In 2016, astronomers observed the moon as it passed in front of Jupiter, and saw what looked like jets of material in several images.
Now, a new study has lent more weight to the idea, using data that's much older but much closer to the source. Things kicked off after a presentation by Melissa McGarth, a scientist on the Europa Clipper mission, highlighting places of interest for the upcoming probe. Scientists in attendance realized that some of that data may have already been gathered.
"One of the locations she mentioned rang a bell," says Xianzhe Jia, lead author of the new study. "Galileo actually did a flyby of that location, and it was the closest one we ever had. We realized we had to go back. We needed to see whether there was anything in the data that could tell us whether or not there was a plume."
The Galileo probe, which orbited Jupiter between 1995 and 2003, performed a flyby of Europa in 1997, coming within 124 mi (200 km) of the surface. And as it turns out, Galileo passed right over a plume – but scientists at the time didn't know to look for it.
This time around though, the team knew how to spot the telltale signs of a plume, having studied them on Saturn's similarly-icy moon, Enceladus. That work revealed that material in the plumes becomes ionized, creating a brief, small-scale bend in the moon's magnetic field. Sure enough, data from Galileo's high-resolution magnetometer showed exactly that signature, which scientists hadn't been able to interpret before now.
To back up the find, the team also pulled data from Galileo's Plasma Wave Spectrometer, which detected plasma wave signatures consistent with a plume.
Altogether, the new evidence for plumes is exciting news for the upcoming Europa Clipper mission, which will buzz the intriguing moon at low altitude in search of evidence of microbial life.
"There now seem to be too many lines of evidence to dismiss plumes at Europa," says Robert Pappalardo, project scientist on Europa Clipper. "This result makes the plumes seem to be much more real and, for me, is a tipping point. These are no longer uncertain blips on a faraway image. 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. That's what the mission is after. That's the big picture."
The research was published in the journal Nature Astronomy.
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