Clues hidden in data from NASA’s Galileo spacecraft—which explored the Jupiter system between 1995 and 2003—suggest very strongly that the icy moon Europa is venting water into space, scientists report today in Nature Astronomy.
Long considered to be one of the most promising places to search for alien life in the solar system, Europa is known to have a global ocean containing vastly more water than all of Earth’s combined. Finding plumes raises the possibility that the ocean tucked beneath its icy shell might erupt into outer space, which means that tasting that alien sea and searching it for signs of life could be as simple as sending a spacecraft zooming through a plume of ejected water vapour.
That’s still not exactly easy, but it is less complicated than asking a probe to fly all the way to Europa, safely land, burrow through a miles-thick crust of rock-hard ice, and then get to work being an extraterrestrial ocean explorer.
It’s also possible—and perhaps more likely—that any plumes come from a lake or some other reservoir trapped in the ice. But that still means an orbiting spacecraft, like the Europa Clipper mission that’s tentatively scheduled to launch in the early 2020s, could sample a plume and get a glimpse of what lies beneath the moon’s ruddy, crisscrossed rind.
“It’s unlikely that one of these plumes is going to throw a fish into space that’s going to whack into Europa Clipper,” says Cynthia Phillips of NASA’s Jet Propulsion Laboratory. “It’s more likely to come from pockets of liquid that are closer to the surface – so, not free ocean samples, but free subsurface samples.”
For years, planetary scientists have argued over whether Europa might be spitting water into space, as Saturn’s moon Enceladus does.
An image of the icy plumes spouting from Saturn's moon Enceladus, as seen by NASA's Cassini spacecraft.
PHOTOGRAPH BY NASA/JPL/SPACE SCIENCE INSTITUTE
In late 2013, tantalizing images from the Hubble Space Telescope revealed possible 125-mile-high plumes of water vapour erupting from a region in the moon’s southern hemisphere. At the time, though, scientists were skeptical, as the plumes were at the very limit of Hubble’s ability to see. Follow-up observations revealed nothing.
But in 2016, and again in 2017, scientists reported that more Hubble images pointed to the presence of a plume, though something less dramatically exuberant than the geysers of Enceladus, which fly so high that they create a ring around Saturn. Even so, Europa’s eruptions could be equally as dense as its cousin’s, and easily visible by an orbiting spacecraft.
In May 2017, the SETI Institute’s Melissa McGrath gave a presentation at one of the Europa Clipper science team meetings. In it, she reviewed all the evidence for potential plumes coming from the moon, including the intriguing possibility that the Galileo spacecraft had spotted a plume two decades earlier.
That’s when the University of Michigan’s Xianzhe Jia and colleagues decided to revisit those archival data and see what kinds of nuggets they could find.
“I was asking myself, Why didn’t we start looking at this earlier? Why wait so long? The data is there, publicly available for almost 20 years,” Jia says.
Something in the Way
Europa’s gravity is strong enough that any erupted water vapour would hug the moon quite closely, and Galileo only swung low enough to detect such an eruption twice, most notably in December 1997. During that pass, the spacecraft took about five minutes to traverse Europa’s face.
Jia and his colleagues pulled observations from the spacecraft’s magnetometer, which measures magnetic fields, and from a second instrument that measures the density of charged particles. In the sequence of numbers produced by those two instruments, they immediately spotted something unusual: Anomalous blips, lasting about three minutes, centred around Galileo’s closest approach to the moon.
If a plume were erupting, Jia says, the erupted water vapour and dust particles would be affected by magnetic fields, which is what the spacecraft detected. And the density of charged particles surrounding the spacecraft would change as the spacecraft entered, flew through, and then exited the plume.
We saw very peculiar changes in the magnetic signal, something I don’t think has been explained in the past,” Jia says. “We also pulled out the plasma wave data from Galileo, and surprisingly, around the same time, the plasma wave showed anomalous emissions. So when you put those two together, that indicated that something very special had happened during that interval.”
In other words, the spacecraft had flown through a localized plume maybe 620 miles wide somewhere near the moon’s equator. But Jia and his team wanted to make sure. So they simulated the observations such a spacecraft would make if it flew through a plume of the size and density spotted by Hubble. The Galileo observations matched the simulation almost perfectly.
Now, with two spacecraft and several independent instruments reporting similar findings, it’s harder to deny that Europa is venting water vapour into space.
“If you look at any one piece of evidence by itself, it’s not very convincing,” McGrath says. “But when you start bringing in completely independent sets of observations and they all seem to be telling us the same thing, it’s sort of like, yeah. That’s what is starting to convince people.“
Come As You Are
As exciting as the Galileo data are, they don’t necessarily prove the existence of continual plumes. Instead, Phillips says, the observation is another clue that can be added to a body of evidence suggesting that Europa does—at least occasionally—vent water into space.
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“I think it tells you there’s probably more plumes than we can see right now, because the odds that we’d happen to fly through the only one that exists are pretty low,” McGrath says.
What does this mean for the Clipper spacecraft? Turns out, the team designing the probe has already planned to include a suite of instruments capable of tasting a plume, should the spacecraft fly through. What it finds will be anyone’s guess.
“Even with our wildest imagination, we always see stuff that we totally did not expect,” McGrath says. “We’ll surely see something we totally don’t expect at Europa.”