By Molly Bentley
Science reporter, San Francisco
An ocean is not the source of the jets emanating from Saturn's moon Enceladus, a new study concludes.
Next year, Cassini will attempt to fly through the jets
The research questions the moon's promise as a target in the search for life beyond Earth and has stirred controversy among scientists who dispute its conclusions.
A chemical analysis of Enceladus, led by University of Colorado planetary scientist Nick Schneider, failed to detect sodium, an element scientists say should be in a body of water that has had billions of years of contact with rock.
"If you have a long-lived ocean, it's going to have salt in it," said Dr Schneider, at the American Geophysical Union (AGU) meeting in San Francisco this week, "but that ocean, if it exists on Enceladus, isn't leaking out into space."
Spectral analysis with the Keck Telescope found no sodium in the plumes or in the vapour that's in orbit around the moon.
The source of the plumes is "very, very pure water," Dr Schneider concluded, and proposed clean ice, melt water or clathrates - a crystal of water, carbon dioxide and ammonia - as alternative sources.
The fountains on Enceladus tantalise scientists by suggesting an ocean beneath the moon's icy crust. An ancient sea is the best bet for where life might evolve off Earth, scientists say.
At stake is whether Saturn's moon could support alien life and is a worthy target for a US space agency (Nasa) exploratory mission to detect it. Such a mission to Enceladus is one of four currently under review for further development.
Dr Schneider didn't rule out the possibility of an ocean on Enceladus, only that it is the source of the spraying water.
Critics of the study accept his observations, but disagree with his conclusion; and it has led to some robust exchanges here at the AGU meeting this past week.
"There is tremendous dispute about his interpretation of the results," said Carolyn Porco, the Imaging Team leader on the Cassini spacecraft.
He may not have detected sodium, she said, but it did not follow that the plumes were not connected to an ocean.
The absence of detectable sodium might mean only that; it's not detectible, said Dr Porco. It could be in a solid form that eludes detection by this method, she said.
Dr Schneider used the Keck Telescope to look for a glow from sodium atoms, the same colour found in sodium streetlights. He failed to detect it in the plume or in the ring of particles that encircle Saturn at Enceladus' orbit.
Yet sodium is quite abundant in the Solar System, said Torrence Johnson, Cassini Imaging Team member at Nasa's Jet Propulsion Laboratory.
ENCELADUS ORBITER CONCEPT
To look at biological potential, composition and structure
Instruments to image and sample surface and jets
Lander to provide unique close-up view of surface processes
Probable cost about $3bn; 2018 launch on biggest Delta rocket
"It's a very surprising result not to find sodium at all," he explained. "So the question is: can you hide the sodium?"
One way to hide sodium is to put it in a salt crystal that becomes the nucleus of a water particle. If a sodium atom were tied up in a solid form, the Keck Telescope would not detect it. It only detects liberated sodium atoms, he said.
"If you took salt from a salt-shaker and threw it into the air, the telescope wouldn't see any sodium, even though half the salt is sodium," added Dr Johnson.
Dr Schneider said that the molecules would release sodium as they made their way into the particles that encircle Saturn.
Scientists have detected sodium around Jupiter's moons Io and Europa. Volcanoes produce Io's sodium and their heat liberates it. Scientists believe Jupiter's energetic radiation belt kicks the sodium out of minerals that entrap it on Europa.
But Saturn is cooler in temperature and its radiation levels may also be too weak to free sodium.
At any rate, said Dr Johnson, the sodium-free test results do not rule out the existence of an ocean on the moon or an ocean as the source of the plumes.
The Cassini spacecraft is scheduled to fly close to the jets in March of next year. It will analyse the water further, but is not able to test for sodium.
Nonetheless, the discovery of water spewing from cracks - dubbed "tiger stripes" -on Enceladus in 2006, has promoted the moon into an elite club of outer Solar System bodies that are considered high priority for future Nasa missions.
The possibility that the plumes tap directly into a lunar ocean is the impetus behind a flagship mission that would explore Enceladus further.
A spacecraft that flew through the spray might be designed to sample the water directly and run tests to detect the presence of alien microbes.
The mission is much more difficult - and expensive - if the data is not obtainable at altitude and a lander has to be put on the icy surface to reach it.
The Enceladus flagship mission is one of four - along with those to Europa, Titan and Jupiter - competing for funding and currently under review by Nasa.
Dr Johnson says Dr Schneider's study might influence how inclined people are to send a spacecraft to Enceladus and fly through the plumes.
"If Nick is right," he said, "all they'd see is pure water."
Nasa is scheduled to select which flagship missions will advance at the end of December.
The concept that eventually emerges in the process will launch no earlier than 2015 and is likely to include sizeable input from the Europeans.