Jupiter’s moon Europa, thought to harbor a vast saltwater ocean under a crust of ice, might end up teasing astrobiologists with a taste of its water. In a press call today, astronomers announced new hints that water from the moon is intermittently leaking out into space—plumes of water vapor that could be picked up and tested for life-friendly chemistry by a NASA spacecraft scheduled to arrive in the late 2020s.
On three separate occasions in 2014, the Hubble Space Telescope saw what may be plumes of material extending about 200 kilometers into space from Europa’s southern hemisphere. It’s a tentative but hopeful sign, soon to be published in a paper in The Astrophysical Journal. “We do not claim to have proven the existence of plumes, but rather to have contributed evidence that such activity may be present,” said William Sparks, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, who led the team.
If plumes exist to spray ocean water onto the moon’s surface and into space, as they do on Saturn’s moon Enceladus, it would make studying the prospects of life in the ice-capped ocean enormously easier. Instruments flying through the plume could search for evidence of hydrothermal vents at the base of the ocean by assessing trace chemicals and particles mixed into the water. Skewed molecular ratios could indicate the possible presence of metabolic activity.
But the search for plumes has been exasperating. In 2012, Hubble picked up emissions from hydrogen and oxygen atoms above Europa’s southern hemisphere, as if water molecules were being split apart by electrons whirring through Jupiter’s extended magnetic fields. In Science, a team led by planetary scientist Lorenz Roth, now at Sweden’s Royal Institute of Technology in Stockholm, argued that the emission came from two plumes of water vapor rising 200 kilometers from the moon’s surface.
Yet those plumes hadn’t appeared before in identical observations, and haven’t shown up since, even with dedicated monitoring by Hubble. “Our only insight is it’s even harder to explain than we initially thought,” said Kurt Retherford, a planetary scientist at the Southwest Research Institute (SwRI) in San Antonio, Texas, and a member of that team. “It’s a frustrating place to be in.”
Rather than look for emission, Sparks’s team took a different approach. When Europa transits in front of the bright giant planet, any plumes around the moon should be backlit. Of the 10 transits Hubble observed, “off-limb features, possibly plumes, were found on three occasions,” Sparks said in the press call.
Two of these match the same spatial location as the plume found by Roth’s team, and they all seem consistent in terms of size. “It’s definitely supporting evidence,” Retherford says. “There’s more confirmation needed. I think it’s fair to say that both techniques have pushed Hubble to its limits.”
Even a little support comes as a relief for instrument scientists on NASA’s Europa mission, which could launch as early as 2022. Of the nine instruments chosen to fly on the spacecraft last year, several were picked with the idea that they might help explore Roth’s tentative plumes. “You would be ill advised to go there without some capability to look at plumes if you found them,” says Hunter Waite, also at SwRI.
An ultraviolet spectrograph, headed up by Retherford, will characterize any plumes up close with similar techniques to those that Hubble has used at a distance. A dust analyzer will sample any ice and dust grains kicked up, and a thermal mapper will look for heat flowing from the spot on the surface where they are launched. And the instrument Waite heads, a sensitive mass spectrometer, could identify organic molecules in the spray up to 500 or 1000 mass units in size—including amino acids and fatty acids that might serve as ingredients for, or products of, life. Waite’s instrument could also look at the ratio of molecules like methane and molecular hydrogen, which might hint at the metabolic activity of bacteria.
It’s still an open question, though, whether the Europa mission will also include a lander to descend and perform tests on the surface. U.S. Representative John Culberson (R–TX) has talked about adding money to NASA’s budget to pay for such a lander, although the agency has so far kept its budget requests limited to a more streamlined “Clipper” mission that would fly past Europa 45 times. But more evidence for plumes can’t hurt that case, Waite thinks. “The enthusiasm for that has to be increased,” he says. “Not only is there a potential that you’re accessing the interior, but it’s also like a road sign that’s going, ‘Land here!’”