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Europa vents water, Hubble data suggest



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However, the failure of Galileo’s main communications antenna prevented astronomers from getting detailed photos and measurements. Since then, scientists have debated whether Europa supports a habitable ocean. For life to exist on Europa, water and heat beneath a kilometers-thick layer of ice would have to mix with organic chemicals on the surface.

Knowing that Enceladus, an icy moon of Saturn, shoots water into space (SN Online: 7/31/13), a team led by space scientist Lorenz Roth of the Southwest Research Institute in San Antonio decided to see if Europa emits similar plumes. The researchers analyzed hours-long observations of Europa taken by the Hubble Space Telescope in October 1999, November 2012 and December 2012. Hubble can detect ultraviolet radiation emitted by hydrogen and other atoms. The December 2012 data revealed unusually high concentrations of both hydrogen and oxygen hovering over Europa’s southern hemisphere, suggesting the presence of water molecules. No analogous signal showed up in the other two months.

The researchers conclude that last December, liquid water encased under Europa’s ice shell was escaping through cracks to form plumes of vapor reaching heights of about 200 kilometers, the approximate distance between Seattle and Vancouver. The water output rate is about 10 times as high as that on Enceladus, says study coauthor and University of California, Santa Cruz planetary scientist Francis Nimmo: about a thousand liters, or a hot tub’s worth, every second.

The plumes suggest that at least some of Europa’s buried water supply reaches the surface. Meanwhile, research by Schmidt and others has hinted that chemicals can work their way down through the ice (SN: 12/17/11, p. 5). Researchers at the American Geophysical Union meeting in San Francisco this week are presenting evidence that Europa has a form of plate tectonics, with sheets of ice that slide beneath one another and transport surface chemicals to the ocean.

However, says planetary scientist Carolyn Porco of the Space Science Institute in Boulder, Colo., “before people start jumping up and down over this, there needs to be some confirmation.” Her imaging team for the Cassini probe, which orbits Saturn, has released brilliant photos of Enceladus’ plumes of water vapor and ice crystals. Its geysers are most active when the moon is farthest from Saturn, and the Hubble detection similarly took place when Europa was farthest from Jupiter. Porco suggests taking follow-up measurements when Europa is again at this point in its 3.5-day-long orbit, as well as at other times when Jupiter’s gravity would presumably weaken the plumes. Nimmo agrees that additional plume detections would solidify the study’s conclusion.

Data in the Galileo archive may provide an additional line of evidence. During one of Galileo’s close passes to Europa, the probe detected a strange magnetic signal that astronomers could not explain; the new findings imply that it could be the signature of floating water molecules. “If this work is correct, then it’s an obvious answer to that puzzle,” says UCLA geophysicist Krishan Khurana, who analyzed the perplexing result for the Galileo team in 1997.

If Europa is indeed shooting its insides into space, it would increase the clamor by planetary scientists to send a dedicated robotic explorer there. Rather than having to drill through the ice to explore the moon’s ocean, a probe could fly through the plumes and scan the molecules that fall back to the surface. “It’s a free sample,” Nimmo says. Porco suggests including an instrument to collect and identify molecules in the plume such as fragments of proteins, which could indicate the presence of microorganisms.

NASA has proposed a $2 billion mission called the Europa Clipper, but a $230 million cut in planetary science funding this year and more proposed cuts for next year mean there’s no guarantee it will launch. In the meantime, astronomers will scrutinize data from NASA’s Juno probe, which is en route to Jupiter but won’t study Europa in detail, and JUICE, the European Space Agency’s Jupiter Icy Moons Explorer, which will cruise past Europa twice when it arrives around 2030. 

Editor's Note: This story was corrected January 6, 2014, to say that Europa has few impact craters.
