Another hint of Europa’s watery plumes found in 20-year-old Galileo data

Jupiter’s icy moon Europa may have been spitting into space for at least 20 years. Analyzing old Galileo mission data suggests that the NASA spacecraft flew through a plume of water vapor from the moon during a 1997 flyby, researchers report May 14 in Nature Astronomy.

“We now have very compelling support for the idea that Europa does possess plumes,” says study coauthor Xianzhe Jia, a planetary scientist at the University of Michigan in Ann Arbor.

Whether the moon, Jupiter’s fourth largest, has such geysers has been a lingering mystery. One of the most tantalizing results from Galileo, which orbited Jupiter from 1995 to 2003, was evidence that Europa harbors a deep ocean of liquid water beneath an icy shell. Then in 2012, data from the Hubble Space Telescope revealed high concentrations of charged hydrogen and oxygen atoms, or ions, over Europa’s southern hemisphere, a potential sign of water vapor escaping into space (SN: 1/25/14, p. 6).

The putative plumes have played peekaboo ever since (SN Online: 1/11/18), continuing to intrigue astronomers hoping to search the moon’s water for signs of life. 

Jia and colleagues examined data from Galileo’s closest Europa flyby, which brought the probe within 206 kilometers of the moon’s surface. Over one spot near the equator, the spacecraft detected sudden changes in its magnetic field and plasma instruments. Using up-to-date computer simulations, the team showed that these changes are best explained by Galileo flying through an active plume. Radiation from Jupiter’s magnetic field would have split water molecules into oxygen and hydrogen ions, and those particles, in turn, would have changed the direction of the magnetic field and the density of plasma within the plume. That spot is also near where Hubble picked up a second sign of escaping water vapor in 2014 (SN Online: 9/26/16) .

Galileo found no other signs of plumes during its 10 other Europa flybys, which were farther away from the moon. That suggests that any plumes are relatively small, extending not much higher than 200 kilometers above the surface. It’s also unclear if Europa plumes would be spewing constantly or be turning on and off.

Jia is working on magnetic field and plasma instruments for two future missions to Jupiter and its moons: the 2022 European JUICE mission and NASA’s Europa Clipper spacecraft, also planned for the early 2020s. The Clipper spacecraft will make much lower flybys over Europa, skimming as low as 25 kilometers above the surface. The new Galileo results should help in planning those flight paths, he says.

NASA will host a live discussion of these findings at 1:00 p.m. EDT on May 14. You can watch on various platforms, including NASA Television, Facebook Live and YouTube.

Planetary scientist Cynthia Phillips of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who worked on Galileo as a graduate student but was not involved in the new analysis, says it’s exciting that scientists are using new tools on the old mission data.

“During Galileo, we’d always known there was something weird during this flyby,” she says. But she is still holding out for visual confirmation of the plumes. “Pictures, or it didn’t happen.”