Europa is an ice-encrusted moon of Jupiter with a global ocean flowing underneath its surface. NASA is planning a mission soon that will look for signs of possible life there.
Now, a new finding from old data makes that mission even more tantalizing.
In recent years, the Hubble Space Telescope has spotted what looks like plumes, likely of water vapor, reaching more than 100 miles above the surface.
The plumes, if they exist, could contain molecules that hint at whether Europa possesses the building blocks of life.
In a study published Monday in the journal Nature Astronomy, scientists are reporting a belated discovery that Galileo, an earlier NASA spacecraft that studied Jupiter, appears to have flown through one of the Europa plumes more than 20 years ago. And that occurred close to one of four regions where Hubble has observed plumes.
“That’s too many coincidences just to dismiss as ‘there’s nothing there’ or ‘we don’t understand the data,’” said Robert T. Pappalardo, the project scientist for NASA’s Europa Clipper mission, which may launch as soon as 2022. “It sure seems like there’s some phenomenon, and plumes seem consistent.”
Galileo, which launched in 1989, arrived at Jupiter in 1995 and spent almost eight years examining the planet and its moons until its mission ended with a swan dive into Jupiter in 2003.
During a flyby of Europa on Dec. 16, 1997, instruments on Galileo measured a swing in the magnetic field and a jump in the density of electrons. At the time, scientists noted the unusual readings, but they did not have an explanation.
Then, in 2005, another spacecraft passing by another moon around another planet made a startling observation.
NASA’s Cassini spacecraft — which completed its mission in September — found geysers of ice crystals erupting out of Enceladus, a small moon of Saturn. Enceladus, it turns out, also has an ocean of liquid water under its ice.
That spurred renewed curiosity about Europa and whether it too might burp bits of its ocean into space. The Hubble first recorded signs of possible plumes in 2012, then again in 2014 and 2016. But at other times, Hubble has looked and seen nothing. That suggests the plumes are sporadic.
Last year, Melissa McGrath, a senior scientist at the SETI Institute in Mountain View, California, who was not involved in the new study, took a look at some radio experiments conducted by Galileo that examined how signals bent as Europa passed between Earth and the spacecraft. The experiments showed Europa possesses an atmosphere.
Some of the flybys indicated a higher density of particles near the surface — possible plumes. Before heading to a meeting of scientists working on the Clipper mission, a thought occurred to McGrath: “Gee, I really should check to see if any of them line up with any of the claimed plume detections” from Hubble.
One of them indeed did.
Margaret G. Kivelson, an emeritus professor of space physics at UCLA who was the principal investigator for Galileo’s magnetometer, was at McGrath’s talk. She remembered the odd magnetic readings from 1997.
For years, she had been thinking of taking another look at the data for signs of plumes, but “there are always other things to do,” she said.
Then she did.
“With the Hubble data in hand,” Kivelson said, “we had an idea of how big a plume might be reasonable. That we could translate into how long it would take Galileo to move across a plume that had been proposed.”
The3-minute-long magnetic anomaly seemed to fit with the apparent size of the Hubble plume.
Next, they turned to William S. Kurth, an astronomer at the University of Iowa who contributed to Galileo’s plasma wave experiment, which listened to the radio waves generated as charged particles bobbed back and forth along magnetic fields around Jupiter and its moons. That instrument also noticed a burst of radio waves during the flyby — and it occurred right in the middle of the magnetic anomaly.
The final piece was a computer model of a plume by Xianzhe Jia, a professor of climate and space sciences and engineering at the University of Michigan, that created the same effects on the magnetic field and the plasma waves.
“It all seemed to hang together,” Kivelson said.
The location was close, though not exactly the same, as the site McGrath reported. But McGrath said the new paper was convincing. “They did a really good job of the modeling and made a strong case,” she said.
Also convinced is Rep. John Culberson, R-Texas, chairman of the House subcommittee that sets NASA’s budget. Culberson has been an enthusiastic supporter of the Clipper mission, repeatedly adding more money to the project than NASA officials requested. He has also been pushing for a follow-up mission that would land on Europa and drill into the ice.
At a subcommittee meeting this past week, Culberson handed out the article, which was not yet available for public discussion, to his colleagues. “It’s worth noting that the scientific journal Nature Astronomy just reported that the Galileo mission, back in 1997, flew through a water plume on Europa,” he said. “So, the ocean of Europa is venting directly into outer space.”
The scientists were amused as they were not allowed to talk publicly about their findings yet. “That was really funny, because we’ve been so careful,” Kivelson said. “And all of sudden, Rep. Culberson is throwing around the paper on the table. Very funny.”
Astronomers will certainly be taking more looks at Europa with the Hubble, trying to better understand how often the plumes erupt.
Pappalardo said it might be possible to adjust the trajectory of Europa Clipper so that at least one of the more than 40 planned flybys pass over a potential plume site. But that would have to be weighed against other science goals and how much fuel would be needed to nudge the spacecraft’s trajectory.
“Obviously this is a place we would want to suss out with the Europa Clipper mission in the future,” he said. “I think this is going to make for a lively debate at our next science mission meeting.”
A European Space Agency spacecraft, called Juice or Jupiter Icy Moons Explorer, will also fly by Europa, as well as two other Jovian satellites, Ganymede and Callisto. It could also launch as soon as 2022.