SAN FRANCISCO — Jupiter’s icy moon Europa may be showing us its insides. Data from the Hubble space telescope suggests that enormous jets of water more than 200 kilometers tall (roughly twice as high as Earth's atmosphere) may be spurting intermittently from the moon’s surface.
The frozen body Europa is known to have a vast liquid water ocean beneath its cold crust, a potential home for life. Should these newly observed water plumes be tapping into some Europan sea, they could be bringing material to the surface that would otherwise stay hidden. Follow-up observations from Earth or with probes around Europa could sample the fountains, hunting for organic material and perhaps finding evidence of living organisms beyond Earth.
The findings, presented today at the annual meeting of the American Geophysical Union, await independent confirmation. But if the jets are real, the frozen world would join the tiny number of others known to have active jets, including Saturn’s moon Enceladus and Neptune’s moon Triton.
Scientists spotted the plumes in ultraviolet images from Hubble taken in December 2012. “We found that there’s one blob of emission at Europa’s south pole,” astronomer Lorenz Roth of the Southwest Research Institute in Texas, co-author of a paper about the research appearing today in Science. “It was always there over the 7 hours we observed and always at the same location.”
Previous observations from NASA’s Galileo mission, which visited the Jupiter system in the 1990s and early 2000s, suggest that Europa’s south pole is full of ridges and cracks quite similar to features called tiger stripes on Enceladus that spew water.
Lorenz and his team looked back through previous Hubble data to see if the plumes could have been spotted earlier but saw nothing, suggesting that they are likely transient. Europa happened to be at the point in its orbit where it was farthest away from Jupiter in December 2012, which could explain why the jets appeared only then. Researchers recently determined that Enceladus’ plumes are weakest when the moon is closest to Saturn, likely because the ringed planet’s gravity squeezes the tiger stripes shut. When the moon is far away, its crust relaxes, tripling the plumes power.
“We actually saw this press release on Enceladus,” said astronomer Kurt Retherford, also of SwRI and another co-author. “And we thought, ‘Oh my god! This is the explanation’” for why Europa’s plumes might only appear when it's far from Jupiter.
Because of its oceanic subsurface, scientists have looked in the past for Europa jets. When the Voyager probes flew by in the 70s, one image showed a fuzzy spot that some thought to be a plume, though most considered it an artifact of imaging. Galileo also saw a row of dark spots on a ridge of Europa, perhaps similar to spots that appear on Earth in active places like Hawaii or Iceland before an eruption begins. But nothing conclusive was ever seen.
Because of previous false positives, scientists should be cautious when interpreting these newest results, said planetary scientist Robert Pappalardo of JPL, who was not involved in the recent work. The findings are exciting and impressive, he says, but also “on the hairy edge” of Hubble’s signal.
“This is really pushing the limits of what can be observed from Earth,” he added. “I’ll sleep better when it’s confirmed."
Even with those reservations, Pappalardo, who leads the planning team for a proposed mission to Europa, said that he’s already discussing with other scientists how these new results should affect their study priorities. Some future orbiter headed to Europa could for instance carry detectors specifically to search for heavy organic molecules that could be indicative of life in the subsurface. When it passed over the geyser’s spray, it would be bathed in material from the moon’s interior, giving scientists a window into Europa’s ocean.
Pappalardo hopes that the finding will help push Europa to a place of high priority in NASA’s exploration agenda. Administrators have recently made comments regarding the agency’s lack of funding for a big costly mission. But a probe to Europa was singled out as the one mission that could justify the expenditure.