Lee Billings

 This passage is adapted from Lee Billings, “Astronomers Spy 

Shadowy Plumes around Europa.” ©2016 by Scientific 

American, a division of Nature America, Inc.

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 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.

“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 

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.”