An artistic illustration of Europa's icy surface with a water jet in the foreground. K Retherford/Southwest Research Institute
Towering plumes of water vapour have been observed blasting off Jupiter's icy moon Europa, which is believed to hold an underground ocean.
If confirmed as water vapour, the discovery could affect scientists' ideas on whether the moon has the right conditions for life, Southwest Research Institute planetary scientist Kurt Retherford said.
"We've only seen this at one location right now, so to try to infer that there's a global effect as a result of this is a little difficult at this time," Dr Retherford said.
The 200-kilometre-high plumes of water vapour were observed by researchers using the Hubble Space Telescope in December last year.
The jets were not seen during Hubble observations of the same region in October 1999 and November 2012. The now-defunct Galileo spacecraft, which made nine passes by Europa in the late 1990s, likewise did not detect any plumes.
"We have probably observed only one of the largest plumes on Europa," Dr Retherford told the BBC.
"There could be a lot of plumes, more like 10 to 50 kilometres high, and we're just not seeing them with our current data-sets."
Scientists believe the water vapour may be escaping from cracks in Europa's southern polar ice that open due to gravitational stresses when the moon is farthest from Jupiter.
"When Europa is close to Jupiter, it gets stressed and the poles get squished and the cracks close up," University of California planetary scientist Francis Nimmo said.
"Then, as it moves further away from Jupiter, it becomes un-squished, the pole moves outward and that's when the cracks open."
Scientists say the the plumes also could be the result of frictional heating from rubbing ice blocks or a fortuitously timed comet impact.
Similar jets have been detected on Saturn's moon Enceladus. Because Enceldus has 12 times less gravity than Europa, its plumes shoot much farther into space.
"We were really kind of surprised by the number, and we're grasping what that means," Dr Retherford said.
Additional Hubble observations are planned, as well as a review of archived Galileo data taken when Europa was farthest away from Jupiter.
"Now that we know where (the plumes) are, that narrows the window that we have in comparison to the passes that we've made," NASA planetary sciences chief Jim Green said.
"I think we'll have some other great results, or another controversy."
Hypothetical advanced underwater life such as might exist in Europa's ocean.
Image: © Adolf Schaller
Advanced life on Europa?
If microscopic organisms developed on Europa, is there the possibility that some of them have evolved into large, multicellular forms? Much depends on how freely oxygen, generated at the surface by the interaction between surface ice and intense incident radiation (from Jupiter's powerful magnetosphere), has been able to make its way down into the ocean. If the Europan sub-surface ocean is rich in ocean it would have the potential to support a rich diversity of complex life, including creatures as large or larger than the great whales on Earth.