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Europa and plumes
This composite image shows suspected plumes of water vapor erupting at the 7 o’clock position off the limb of Jupiter’s moon Europa. The Hubble data were taken on Jan. 26, 2014. The image of Europa, superimposed on the Hubble data, is assembled from data from the Galileo and Voyager missions. (Credit: W. Sparks / STScI / NASA / ESA / USGS Astrogeology Science Center)

It’s not aliens. And it’s not exactly surprising, despite NASA’s advance billing. But new evidence of water plumes emanating from Europa, an ice-covered moon of Jupiter, have added to the excitement over a proposed mission that could sample the water for signs of life.

The evidence comes in the form of splotchy ultraviolet images captured by the Hubble Space Telescope, operating at the limits of its sensitivity. Scientists say the images appear to show intermittent emissions of water vapor near Europa’s south pole.

“If plumes exist, this is an exciting finding,” William Sparks, an astronomer at the Space Telescope Science Institute in Baltimore, told reporters today during a teleconference.

Sparks is the principal author of a paper describing the observations that will appear in the Astrophysical Journal. The ultraviolet readings were taken in 2014 by Hubble’s Space Telescope Imaging Spectrograph, or STIS.


Three years ago, a different team of researchers said spectrographic readings from STIS pointed to towering water plumes at Europa, but they weren’t able to confirm those observations. A study based on data from the Cassini mission turned up no evidence of the plumes, raising doubts.

Sparks and his colleagues tried a different method. They had Hubble look at Europa while it was passing across the disk of Jupiter, which reflects an even glow of ultraviolet light from the sun. Then they painstakingly analyzed millions of readings to find spots along Europa’s edge where the light was apparently filtered through the plumes of water vapor.

The fact that two different methods pointed to water plumes strengthens scientists’ confidence that the phenomenon is real and not just a quirk in the data. However, Sparks emphasized that still more data would be needed to nail down the hypothesis. NASA’s James Webb Space Telescope, a successor to Hubble that’s due for launch in 2018, could deliver the goods.

Sparks also said the evidence suggests that the water isn’t always there. Only three of Hubble’s 10 readings showed evidence of plumes. “If they’re real, they have to be intermittent,” he said.

Europa images
Three ultraviolet images captured by the Hubble Space Telescope appear to show signs of plumes near Europa’s south pole. Seven others didn’t. In these pictures, a visible-light image of Europa has been added as a visual aid. (Credit: W. Sparks / STScI / NASA / ESA)

Sparks estimated that the plumes should contain a few million kilograms of water, and rise about 125 miles above Europa’s surface. That would put them in a league with the more obvious water plumes that have been seen emanating from Enceladus, an ice-covered moon of Saturn.

Like Enceladus, Europa is thought to have a deep water ocean beneath its ice, potentially heated by tidal stresses and strains in its rocky core. The presence of liquid water would make Europa and Enceladus prime candidates for missions to gauge their potential habitability.

The fact that the moons are covered with ice poses a challenge: How do you get to the water? Now scientists are more confident that a proposed Europa mission could analyze the plumes during repeated flybys, including close flybys that would let the probe “taste” the water as it passed through.

mission that’s already on the books for launch in the 2020s could make such observations, said Curt Niebur, Europa mission program scientist at NASA Headquarters in Washington.


Niebur emphasized that the Europa spacecraft would not be built specifically to detect the presence of life. “We know how to measure habitability. … When it comes to finding life, we don’t have as much experience,” he said.

And even if microbes (or freeze-dried fish) were to spew up from beneath the surface, the Jupiter system’s radiation environment and the cold vacuum of space would kill them off, Sparks said. “We’d have to look for the remains of something that was once protected in the ice, or by the ice,” he said.

Before today’s briefing, NASA said the scientists would be sharing “surprising evidence” of activity that may be related to Europa’s ocean – while making clear that they wouldn’t be reporting the discovery of alien life.

NASA spokesman Steve Cole jokingly referred to the alien angle at the top of the briefing. “Spoiler alert: Despite what some media have reported recently, this finding has nothing to do with detecting life on Europa,” he told reporters.

The scientists distanced themselves even more from the hype. Sparks told GeekWire he was “neutral” on the question of whether the findings were surprising. Britney Schmidt, a planetary scientist at Georgia Tech who has been studying Europa for years, struck a similarly circumspect tone.

“I usually say to people, I’m not surprised,” Schmidt said. “I’m excited, and skeptical.”

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