The plumes of gas and ice shooting from the south pole of the Saturnian moon Enceladus contain sodium salts, which is the best evidence so far that the satellite harbors a liquid water ocean.

NASA's Cassini probe observed the salts in Saturn's outermost ring, which is believed to be composed of material ejected from Enceladus. That news, published Wednesday in Nature, is sure to excite life-hunters hoping to find extraterrestrial microbes within our solar system.

"Those salty grains provide our current best smoking (or steaming) gun pointing to present-day liquid water near the surface of Enceladus," space scientist John Spencer of the Southwest Research Institute, who was not involved with the research, wrote in an essay accompanying the findings.

Since 2005, when Cassini spotted plumes jetting out from Enceladus, the moon has become one of the hottest topics in solar-system science. In 2008, water vapor was discovered in the plumes, and Enceladus joined Mars and Jupiter’s moon Europa as the likeliest places to find liquid water — and therefore life as we know it — outside Earth. Though the planet is covered with ice and too far from the sun to derive much warmth, the gravitational field in the Saturnian system is believed to warm the moon by a frictional process called tidal heating, possibly allowing it to maintain a deep liquid water reservoir.

But the presence of the hypothesized subsurface ocean isn't as simple to confirm as it sounds.

A second study in Nature Wednesday that looked at the same plumes from ground based telescopes found no evidence of sodium vapor. That rules out the possibility that the plumes are just near-surface ocean water blasted out into space, complicating our understanding of the moon's internal dynamics.

"I'm still a little skeptical," said astronomer Nick Schneider of the University of Colorado, Boulder, lead author of the second study. "There are other ways to explain the results."

Taking both papers together, we now know that an intermediate step is necessary to explain the plume, if liquid water is indeed present under the tiny planet's icy glaze. Scientists had hoped that water from a deep ocean was simply making its way up through cracks in the ice towards the surface, where it was erupted into space.

"That scenario is out according to both of our results," Schneider said.

Explaining both results requires some sort of distillation process that would give you pure vapor and then some salt pieces that are carried along during eruptions.

"Our picture of its subsurface must now be expanded to include the possibility of misty ice caverns floored with pools and channels of salty water," Spencer wrote. "What else may lurk in those salty pools, if they exist, remains to be seen."

There are other explanations of Enceladus' behavior. For example, Susan Kieffer of the University of Illinois said the simplest explanation of the planet's internal structure doesn't require water at all. The presence of clathrates, ice-like lattice structures that can trap gases, could just as easily explain what we see on the moon.

"We proposed that the crust of Enceladus was composed of two layers: One, a surface layer of ice with carbon dioxide, and two, starting at no more than 3 kilometers, a mixture of icy clathrates that overlaid the core," Kieffer said. "Gas is released from the clathrates by the earthquakes associated with the tectonic activity at the south pole. In that way, we were able to account quantitiatvely for the gases observed in the plume. "

Support among scientists for the ocean hypothesis has gathered a lot of steam from the range of Cassini observations.

Fortunately, we might be able to settle the disputes with further observations. Cassini will be flying by the moon at least four more times by mid-2010, and could make as many as 12 fly-bys before 2015, if NASA extends its mission.

Unfortunately, NASA has no current plans to send a probe crashing through the ice anytime soon. Enceladus lost out in the most recent round of mission planning to the Jovian moon, Europa, to be NASA's marquee outer planet mission.

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