Astronomers Spy Shadowy Plumes Around Europa

Astronomers using the Hubble Space Telescope have found new evidence that a subsurface ocean within Jupiter’s icy moon Europa may be intermittently venting plumes of water vapor into outer space, the scientists announced at a NASA press conference Monday. The finding suggests Europa’s ocean, thought to be buried beneath perhaps 100 kilometers of ice, may be more amenable to life—and accessible to curious astrobiologists—than previously believed. The findings will be presented in an upcoming edition of The Astrophysical Journal.

“If there are plumes emerging from Europa, it is significant,” says study lead William Sparks, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland. “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.”

Using Hubble’s Space Telescope Imaging Spectrograph (STIS), 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 very dark, they looked for shadows of the plumes backlit against Jupiter’s bright, smooth cloudscapes. Three times, painstaking analysis and image processing unveiled what looked like ultraviolet shadows soaring over the southern edge of Europa’s silhouette. If they were plumes, they would contain an estimated few million kilograms of material and reach about 200 kilometers above Europa’s surface.

This is not the first time scientists have spied plumes on Europa. Lorenz Roth, an astronomer now at the Royal Institute of Technology in Stockholm, led a team of researchers who glimpsed what could be a single similarly sized and located plume in 2012. Those findings, reported in Science in 2013, also used Hubble’s STIS instrument. But instead of glimpsing shadows, the findings recorded the ultraviolet emission near Europa’s south pole of what could have been hydrogen and oxygen—exactly what would be produced by a plume of water vapor dissociating into its constituent atomic elements as it is bombarded by particles trapped in Jupiter’s powerful magnetic field.

Afterward, however, the putative plumes observed by Roth’s team vanished, failing to manifest in archival data or in every new search by other telescopes—until now. Perhaps, some thought, the plumes only appeared when Europa reached the farthest edge of its orbit, where the collective gravitational tugs of Jupiter and its other moons could flex and “tidally heat” Europa’s interior, opening fissures and melting ice to vent water into space. Or maybe it was a one-time event produced by an unseen asteroid or comet hitting Europa’s surface. Less-charitable skeptics speculated instead that perhaps plume-hungry scientists were just succumbing to pareidolia, the human mind’s tendency to find patterns in chaos and project significance onto meaningless noise.

With the new detections reported by Sparks’s team, the “tidal heating” hypothesis seems weaker than before—the possible plumes they spotted do not seem to occur when Europa’s tidal heating should be strongest. This means that, if the plumes do exist, they now lack an obvious source of heating that could also explain their observed dimensions and mysterious intermittency. Similarly, because Sparks’s team has witnessed the plumes apparently recurring, the “one-time impact” idea loses its luster, too. While these hypotheses fall to the wayside, the broader idea that the plumes are somehow simply illusory remains firmly in contention. Both detections lie at the edge of statistical significance and come from the same instrument upon the same telescope, albeit one that is arguably more used and deeply understood than any other observatory in history.

“This is exactly as likely as the last detections,” says Britney Schmidt, a planetary scientist at the Georgia Institute of Technology who was not involved with the research. “Both results showed statistically significant signals, at about the same level. So I’m fairly neutral.  I think we should expect plume-type behavior. What I don’t know is whether these are sensitive enough detections to really knock that out for good.”

Sparks fully acknowledges that his team’s results remain frustratingly hazy. “These observations are at the limit of what Hubble can do,” he says. “We’re not aware of any instrumental artifacts that could cause these features, and they are statistically significant, but we remain cautious … We do not claim to have proven the existence of plumes, but rather to have contributed evidence that such activity may be present.”

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. Europa, which is roughly the size of Earth’s moon, unquestionably possesses an ocean based on a wealth of evidence collected across decades by multiple spacecraft and telescopes. Some models estimate it may be 10 times deeper and three times more voluminous than Earth’s ocean, and that it likely has persisted in its liquid state for billions of years.

These characteristics make it markedly different from the other plume-venting poster-child of outer-world oceans, Saturn’s moon Enceladus, first seen spewing water vapor from its south pole in 2004 by the Cassini orbiter. Cassini’s studies of those plumes have produced astounding results, including indirect detections of hydrothermal vents on the moon’s hidden seafloor. But the fact remains that Enceladus is six times smaller than Europa and twice as far from Earth, bearing a much more diminutive subsurface reservoir that could conceivably just be a short-lived pocket of meltwater produced by tidal heating from Saturn. On balance, most astrobiologists would probably prefer to study Europa—provided they did not first have to break through a forbidding ceiling of thick ice.

Even so, there is no guarantee that Europa’s plumes, if confirmed, would even be connected to the subsurface ocean at all. They could instead be linked to molten pockets in the crust far above, perhaps formed at regions of “chaotic terrain” upon Europa’s ridged, fissured surface. Ice there has cracked into jumbled, tumbling blocks with softer, warmer material welling up from underneath. According to Schmidt, Europa’s variegated and active surface could ultimately be a mark against studying its plumes in comparison to Enceladus, which has a relatively inert crust save for the plume-venting fractures at its southern pole. “On Enceladus, the geology makes it obvious a little bit where activity has been,” Schmidt says. “On Europa, there’s activity everywhere, which could both lead to plumes or muddy the interpretation.”

Nevertheless, the earlier plume reported by Roth’s team has already helped transform NASA’s long-simmering plans for more robust exploration of Europa. In the past few years, a mission to the moon has now rapidly progressed from pipe dream to Congressionally-approved imperative, and is now slated for launch in the 2020s. NASA’s current design for the spacecraft includes a suite of nine instruments, all keenly designed for hunting and studying any plumes in addition to performing other run-of-the-mill science objectives. The European Space Agency is also building a probe called the Jupiter Icy Moons Explorer to investigate Europa, set to launch in 2022.

To minimize exposure to spacecraft-threatening radiation near Europa, NASA’s planned mission would not orbit the moon but would rather repeatedly fly by it in a series of orbital encounters, designed to take it swooping over the surface from almost every conceivable angle. If plumes exist the spacecraft could pinpoint their sources on the surface and even directly sample them to determine their composition, potentially sniffing out signs of life thriving or barely getting by deep beneath the surface.

There is already a NASA spacecraft at Jupiter called Juno, bearing a powerful ultraviolet spectrometer that could be of use in confirming Europa’s plumes. But Juno is in a polar orbit around Jupiter that is ill-suited for studying the planet’s moons, and even if it weren’t, is not sterilized to the degree NASA deems suitable for potential close encounters with a life-bearing world. “We took great pains to make sure that that spacecraft does not get anywhere near Europa, because we want to protect Europa from contamination,” says Curt Niebur, NASA’s program scientist for outer planets missions.

In the absence of up-close confirmation and study from interplanetary probes, further evidence that the plumes are genuine could come from continued Hubble monitoring, or from follow-ups using the James Webb Space Telescope scheduled to launch in 2018. Sparks’s team has observed two more transits of Europa, although their analysis of the gathered data remains incomplete. Alternatively, he says, “The other thing that could potentially nail it would be if somebody came in with a completely independent observing technique and the results were consistent.”

Since February of this year, Lucas Paganini, a NASA-affiliated astronomer at the Catholic University of America, has been doing just that—looking for Europa’s shadowy plumes in near-infrared light using one of the twin 10-meter telescopes at the Keck observatory on Mauna Kea in Hawaii.

“There are a lot of deniers of plumes out there, without real reason,” he says. “There is nothing crazy about saying plumes exist and are even common from icy moons. It’s not like anyone is trying to get famous from this, just by claiming detections.” In a preview of results to be presented at a meeting of the American Astronomical Society in October, Paganini says he has seen many “interesting things” in the Keck data, but nothing that would definitively rule out the latest reports of plumes—and nothing that would confirm them.



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