This could be the best evidence yet that Europa has a subsurface ocean. With liquid water and energy from the moon's internal tidal forces, Europa could fit the scientific definition of a habitable world, a place where life could exist, dark and chilly though that existence might be.
The hidden ocean has long been suspected, but scientists have never seen anything as dramatic and overt as plumes of water vapor more than 100 miles high. If this finding holds up, it will boost Europa even further as a target for robotic exploration.
"If there's a geyser 200 kilometers tall, and you could fly a spacecraft through it and sample the water coming out from Europa, that would be phenomenal. What if there are organics in it? That's getting to the question of 'Are we alone in the universe?'" said John Grunsfeld, NASA's top official for space science.
The discovery, detailed in a paper led by Lorenz Roth at the Southwest Research Institute, San Antonio, and published Thursday online by the journal Science, is the subject of a news conference in San Francisco at the fall meeting of the American Geophysical Union.
Since the late 1970s, when a NASA probe took the first close look at Europa, scientists have thought the moon could have an ocean beneath its cracked, icy crust. Late last year, the Hubble Space Telescope took a closer look. It didn't see the plumes but rather saw an occasional surplus of hydrogen and oxygen appearing in a spatially confined area over a period of roughly seven hours.
The implication is that tidal forces within the moon -- created by Jupiter's immense gravity -- cause Europa to contract and expand, a bit like a tennis ball being squeezed and released. The Hubble spotted the signs of plumes when Europa was farthest from Jupiter in its slightly elliptical orbit of the planet. The likely scenario is that, when the crust decompresses slightly, liquid water squeezes through a crack and squirts into the cold vacuum of space.
The water would quickly change form, freezing and then sublimating into water vapor. Those water molecules would be split into atomic hydrogen and oxygen in the harsh radiation environment of the Jupiter system.
This is not the first moon to show signs of geysers. Another candidate for exploration is Enceladus, a moon of Saturn, which has similar south pole plumes and might have a subsurface sea, though perhaps not a global ocean as Europa appears to have.
Carolyn Porco, leader of the imaging team for NASA's spacecraft Cassini, which is exploring the Saturn system, said of the Europa announcement, "If it really is a plume of material coming from the ocean beneath the ice shell, that is truly extraordinary. It would put it in the same league as Enceladus as an accessible target."
Alan Stern, a former NASA associate administrator for science who is principal investigator for New Horizons, a probe on its way to Pluto, said, "I think it's game-changing."
He said any probe looking to sample geyser water would be able to get to Europa much more quickly than Enceladus, which is almost twice as far away.
"If Europa is truly venting water, then that is a slam dunk on the liquid ocean," Stern said.
NASA has plans for a robotic mission known as the Europa Clipper, though it is in the formulation phase and has not yet been fully approved. In recent weeks, budget pressures have made new, expensive NASA robotic missions look increasingly less likely to be funded.
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