Europa has been a high priority for scientists because, as an ice-covered moon with a subsurface salty liquid ocean, it has been identified as one of the ideal spots for hosting life in our solar system.
The question of whether plumes of water vapor and icy materials erupt on Europa has circulated for decades.
During its closest flyby of Europa in 1997, less than 93 miles above the surface, the Galileo craft collected signatures of changes in Europa's magnetic field that the scientists didn't understand, said Margaret Kivelson
, study author and professor emerita of space physics at the University of California, Los Angeles.
Kivelson was the principal investigator for Galileo's magnetometer and helped make the discovery of the subsurface ocean on Europa.
Kivelson and Xianzhe Jia, associate professor in the department of climate and space sciences and engineering at the University of Michigan, Ann Arbor, decided to take a second look at those signatures.
Untangling a mystery
According to a study
released last year, the Hubble Space Telescope showed a water plume erupting on the warmest part of the surface of Europa, whose ocean contains twice as much water as Earth's seas. This was the second time a plume has been observed in this exact spot, which had researchers excited that it could prove to be a feature on the surface. Observations of plumes were made by Hubble in 2012 and 2016.
But the sensitivity of telescopic data is limited, and more was needed to be sure that these were really plumes, the researchers said.
Jia was inspired by the Hubble detections to look back at the Galileo flyby data. The 1997 flyby was close to the site of the repeat detection by Hubble.
The Galileo data were unique because the magnetometer was trying to detect changes in Europa's magnetic field as the probe approached the moon, like a boat going through water and creating waves. But the data didn't match that signature for this flyby. Something was wrong: There was a three-minute "bend" where there shouldn't be.
"The data were there, but we needed sophisticated modeling to make sense of the observation," Jia said. "One of the locations mentioned rang a bell. 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."
They also looked at the Hubble observations for dimensions of the plumes and Galileo's Plasma Wave Spectrometer data, which measured signatures of plasma waves that are caused by charged particles in the gases around Europa's atmosphere.
With 3-D modeling to tie all these factors together, the data signature lined up perfectly to suggest that during the 1997 flyby, Galileo flew through a plume, Kivelson said. This fortuitous happenstance is the best evidence of plumes to date, the study said.
"It's amazing how hard it is to anticipate something that just hasn't happened before," Kivelson said. "It had been discussed as a possibility, but it was certainly not something we thought was highly probable, especially 20 years after acquiring the data."
How could Europa host life?
The necessary ingredients for life as we know it include liquid water, energy sources and chemicals such as carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus.
But we've also learned that life finds a way in the harshest of Earth's environments, like vents in the deepest parts of the ocean floor. There, microbes don't receive energy from sunlight but use methanogenesis, a process that reduces carbon dioxide with hydrogen, to form methane.
Europa and Saturn's moon Enceladus are showcasing some of these key ingredients for life in their oceans, which is why researchers believe they are the best chance for finding life beyond Earth in our own solar system.
Research suggesting the possibility of an ocean on Europa was published as early as 1977, after the Voyager mission saw long lines and dark spots, as opposed to a cratered surface similar to other moons. Then the Galileo mission reached Europa in 1996, and Kivelson's detections revealed that there was an ocean on another planet.
NASA plans to further explore ocean worlds in our solar system through the Europa Clipper mission, the first to explore an alien ocean. The Europa Clipper, named for the innovative, streamlined ships of the 1800s, will launch in the 2020s and arrive at Europa after a few years.
'Sniffing' the moon
Elizabeth Turtle, a research scientist at Johns Hopkins Applied Physics Laboratory, said that with these new data, it may be possible to tweak the trajectory for Europa Clipper to include this new spot of interest where Hubble and Galileo have evidence of plumes.
This literal "hotspot" where the plumes happen shows the warmest temperatures on the planet that is otherwise minus 279 degrees Fahrenheit. That's because heat from the interior of the moon is being forced upward in the plumes.
"This is significant, because the rest of the planet isn't easy to predict or understand, and it's happening in the warmest spot," said Britney Schmidt, second author on the 2017 Europa study. Schmidt, an assistant professor at the Georgia Institute of Technology's School of Earth and Atmospheric Sciences, is also one of the architects of the project that became the Europa Clipper mission.
Schmidt will be an investigator for the ice-penetrating radar instrument that will be housed on the Europa Clipper. It will act like an X-ray, peering through the unknown thickness of Europa's icy crust the same way scientists use earthquakes to assess the interior of the Earth.
Whereas Galileo didn't know that it was flying through a plume and was incapable of collecting material from the plume, Europa Clipper will be able to gather material from plumes if it can fly through them. This would allow scientists a first look at the material inside Europa's ocean that's spewing through the icy crust and revealing whether Europa's ocean is habitable and supports life.
Europa Clipper's instruments will be capable of "sniffing" the atmosphere of Europa, with more than 40 planned flybys. The flybys will be less than 228 miles above the surface, which is in the observed range of the plumes, which can reach 124 to 228 miles above the surface.
"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," said Robert Pappalardo, Europa Clipper project scientist. "That's what the mission is after. That's the big picture."