An ocean within Jupiter's icy moon Europa may be intermittently venting plumes of water vapor into outer space, according to a new study in the Astrophysical Journal. The finding suggests the ocean, thought to lie underneath perhaps 100 kilometers of ice, may be more amenable to life—and more accessible to curious astrobiologists—than previously thought. “If there are plumes emerging from Europa, it is significant because it means we may be able to explore that ocean for organic chemistry or even signs of life without having to drill through unknown miles of ice,” says study lead William Sparks, an astronomer at the Space Telescope Science Institute. The plumes would also suggest a potent source of heat lurking within Europa that could sustain living things.
With the help of the Hubble Space Telescope's imaging spectrograph, Sparks and his team observed Europa 10 times between late 2013 and early 2015 as it crossed the face of Jupiter. Watching in ultraviolet light, in which Europa's icy surface appears dark, they looked for silhouettes of any plumes contrasted against Jupiter's bright, smooth cloudscapes. Intensive image processing unveiled what looked like three instances of ultraviolet shadows soaring over the southern edge of Europa's dark bulk. If the shadows were produced by plumes and not glitches in Hubble's instruments, they would collectively contain an estimated few million kilograms of water and reach about 200 kilometers above Europa's surface.
Sparks acknowledges that his team's results remain frustratingly hazy. “These observations are at the limit of what Hubble can do,” he says. “We do not claim to have proven the existence of plumes but rather to have contributed evidence that such activity may be present.” Previous evidence of similar plumes was reported in 2014 in Science, but after follow-up observations, those water vapor spouts seemed to have stopped—or were not there in the first place. In that regard, “this [new observation] is exactly as likely as the last detections” to be real, says Britney Schmidt, who is a planetary scientist at the Georgia Institute of Technology and was not involved with the research.
Such caution is justified—the presence (or absence) of Europa's plumes could profoundly alter the future of interplanetary exploration, redirecting billions of dollars in funding toward new exploratory missions. NASA and the European Space Agency already aim to lead missions to the tantalizing Jovian moon in the 2020s.