Europa, an ice-swaddled jovian satellite that has long fascinated both scientists and science fiction writers, just got a bit more interesting. Data gathered by the Hubble Space Telescope suggest that plumes of water vapor hundreds of kilometers tall, possibly originating in a subsurface ocean, spew from the moon’s south pole. The phenomenon is similar to the sprays of ice particles found emanating from the saturnian moon Enceladus  almost a decade ago.
Oxygen and hydrogen atoms emit or absorb certain wavelengths of ultraviolet light. If these telltale signs appear together in light from a distant object, they hint that water vapor might be present there, explains Lorenz Roth, a planetary astronomer at the Southwest Research Institute in San Antonio, Texas. So, late in 2012, in hopes of detecting vapor plumes, he and his colleagues used the Hubble Space Telescope to observe Europa in ultraviolet light. Oxygen is often present in Europa’s tenuous atmosphere, and sometimes it’s more concentrated in the space above the moon’s southern hemisphere, Roth says. But for one lengthy interval during the observations, the team spotted emissions from hydrogen (at a wavelength of 121.6 nanometers) in the same region. Because the satellite’s surface is covered with ice, the clearest observations came from portions of the purported plumes that were silhouetted against space rather than against the moon itself.
That whiff of hydrogen, which apparently lasted no more than 7 hours, burst forth when Europa was farthest from Jupiter in its orbit , the researchers report today in San Francisco at the fall meeting of the American Geophysical Union and online in Science. Intermittent and short-lived plumes of water vapor, possibly as much as 200 kilometers tall, are the best explanation for the observations, the researchers contend.
The orbital timing of the plumes, probably not coincidentally, is the same as that for Enceladus, Roth says. That moon’s sprays of ice are most profuse when Enceladus is farthest from Saturn in its orbit. Most likely, scientists have proposed, the tidal flexing induced in a moon’s icy surface causes cracks in polar regions to open widest while the satellite is farthest from its parent planet but clamp shut at other times. It’s not clear whether Europa’s plumes, like those on Enceladus, are triggered by icy surfaces of a surface fracture rubbing together—somewhat akin to the sides of a tectonic fault scraping past one another—or whether they represent water vapor spewing from Europa’s subsurface ocean through narrow cracks in the moon’s polar ice at supersonic speeds, Roth says.
The new finding “is very exciting,” says Geoffrey Collins, a planetary scientist at Wheaton College in Norton, Massachusetts. “I’m not sure that this paper clinches the case for Europa plumes, but you can bet that it will inspire a lot of follow-on work.” Nevertheless, he notes: “If the plumes are real, this shows that Enceladus isn’t the only icy moon where this happens. … The resemblance between Europa and the south polar terrain on Enceladus has always been striking to me, so perhaps it shouldn’t be surprising if they share more in common than just looks.”
Christophe Sotin, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, calls the new results “quite compelling.” But it’s important to note, he says, that what’s been discovered is evidence of individual hydrogen and oxygen atoms. To show that those atoms are bonded together into water vapor or a similar substance, observations at other wavelengths would be needed.
If Europa’s vapor plumes are confirmed by more observations, “that would change the kinds of instruments you’d want to install on future probes to the moon,” Sotin continues. In particular, sensors that could analyze the chemical composition of the plumes, either remotely or by sampling the material as it whizzed through the vapor, could provide keen insights into the chemical processes taking place on or deep beneath Europa’s icy surface.