By Irene Klotz
SAN FRANCISCO (Reuters) - New observations from the Hubble Space Telescope show jets of water vapor blasting off the southern pole of Europa, an ice-covered moon of Jupiter that is believed to hold an underground ocean, scientists said on Thursday.
If confirmed, the discovery could affect scientists' assessments of whether the moon has the right conditions for life, planetary scientist Kurt Retherford, with the Southwest Research Institute in San Antonio, Texas, told reporters at the American Geophysical Union conference in San Francisco.
"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," Retherford said.
Researchers using the Hubble Space Telescope found 125-mile-high (200-km-high) plumes of water vapor shooting off from Europa's south polar region in December 2012.
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.
Scientists believe the water vapor 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. Then, as it moves further away from Jupiter, it becomes un-squished, the pole moves outward and that's when the cracks open," said planetary scientist Francis Nimmo, with the University of California in Santa Cruz.
The plumes also could be the result of frictional heating from rubbing ice blocks or a fortuitously timed comet impact, scientists said.
Similar jets have been detected on Saturn's moon Enceladus, which because it has 12 times less gravity than Europa, can shoot its plumes much farther into space.
Scientists find it interesting that both Europa and Enceladus, which is being studied by the Saturn-orbiting Cassini spacecraft, are pumping out about the same amount of water vapor, roughly seven tons per second.
"We were really kind of surprised by the number ... and we're grasping what that means," 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," said NASA's planetary sciences chief, Jim Green.
"I think we'll have some other great results, or another controversy," he said.
(Reporting by Irene Klotz, editing by Jane Sutton and Cynthia Osterman)