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For years, the motto among astrobiologists - people who look for life on distant worlds, and try to understand what life is, exactly - has been "follow the water." You have to start the search somewhere, and scientists have started with liquid water because it's the essential agent for all biochemistry on Earth.
Now they've followed the water to a small, icy moon orbiting Saturn. Scientists reported Thursday that Enceladus, a shiny world about 300 miles in diameter, has a subsurface "regional sea" with a rocky bottom.
Volume of Lake Superior
This cryptic body of water is centered around the south pole and is upwards of 5 miles deep. It has a volume similar to that of Lake Superior, according to the new research, which was published in the journal Science.
The moon's liquid reservoir had already been inferred from the presence of plumes of water vapor emerging from the south pole. The plumes stunned scientists when they were detected by NASA's Cassini spacecraft in 2005. This latest report adds the detail of the rocky sea floor, which is significant because the contact between liquid water and rock creates the potential for the kind of interesting chemistry that gets astrobiologists excited.
This bulletin from the outer solar system could boost Enceladus as a possible target of a future robotic space mission. A spacecraft could fly through the plumes and study whatever's coming out of the moon - something Cassini has done, but with instruments from the previous century that were not designed to look for signs of life.
To become a target for a new mission, however, Enceladus would likely need to outshine Jupiter's moon Europa, which also appears to have subsurface ocean and also has plumes shooting water vapor into space.
NASA is putting together preliminary designs for a possible Europa mission, though budgetary pressures for now make any new major, costly venture in the outer solar system problematic.
"I love Mars, but I think the two of them" - Enceladus and Europa - "provide the highest probability of finding extant life," said Mary Voytek, senior scientist for NASA's astrobiology program. She said she is "torn" about which moon would be the better destination for a robotic probe.
The description of the subsurface sea on Enceladus is based entirely on indirect evidence. The body of water, if it actually exists, is covered with at least 20 miles of ice, according to the new report. But there are several lines of evidence that point to its presence.
The first is gravitational: The Cassini spacecraft, which has been exploring the Saturn system for nearly a decade, has made multiple fly-bys of Enceladus. Faint changes in the wavelengths of radio signals sent back to Earth have enabled scientists to calculate how the moon's gravity tugs on the spacecraft. These are not simple calculations, because the gravitational effects have to be disentangled from other effects, including the drag on the spacecraft as it flies through the plumes of water vapor.
But ultimately the scientists created a model for the moon's interior and what appears to be a striking gravitational asymmetry. Around the moon's south pole, there's something that's slightly off, and the calculations seem to be begging for the model of the interior to include some material denser than water ice. Liquid water - about 7 percent denser than ice in those conditions - seems to be the answer.
Another line of evidence is the moon's shape: It has a shallow dimple, a depression, at the south pole. There's missing mass. This fits with the hypothesis that there's denser water down below, deforming the planet's shape.
"We know the composition of the shell. We know that it's water ice. So it's pretty obvious to think that some of the ice is molten and, therefore, if you melt part of the ice, if you transform it, the volume of it reduces, and you create a depression," said Luciano Iess (pronounced "Yes"), a professor of space systems at the University of Sapienza in Rome and the lead author of the Science paper. (The Cassini mission, which included the Huygens probe dropped to the surface of Saturn's moon Titan, is a joint endeavor of NASA, the European Space Agency and the Italian Space Agency.)
Finally, there are those plumes, which spew water vapor into space. It's possible to generate such a phenomenon without geysers; instead, you could make plumes by rubbing blocks of ice together. But the plumes could be created by a deep ocean sending water up through cracks and into space.
Question: Why is there an ocean only at the south pole and not, apparently, the north pole?
"I don't think anybody knows why it is only at the south," said David Stevenson, a Caltech planetary scientist and co-author of the Science paper. He said once a liquid reservoir forms, it helps drive more melting. A pole - north or south - is where you'd expect to find such a sea given the kind of tidal stresses on the planet generated by the immense gravity of Saturn.
"If you are going to use tidal heating to melt the ice, it is natural to do it at either the north pole or the south pole, because that is the place where the flexing and heating of the ice is greatest," Stevenson said.
So could there be life there? This now enters a speculative realm.
"Liquid water's not enough-not enough for the origin of life certainly," said Carol Cleland, a University of Colorado professor of philosophy who has written about astrobiology. For example, "You need an energy source so that you can drive thermodynamically uphill processes."
Chris McKay, a NASA astrobiologist who has been a major advocate for a new Enceladus mission, says this moon has the essentials for life as we know it. There's the liquid water, obviously, and energy from tidal forces, plus such essential elements as carbon and nitrogen, which were detected by Cassini when it flew through the plumes.
But although he's an optimist about life beyond Earth, he knows that the people in his field have yet to find a single sample.
"It's the occupational hazard of astrobiology to jump to the conclusion that you want to be true. I have to constantly chide myself and my colleagues for doing that," McKay said.
He went on: "It's looking good. We're more optimistic, but we can't actually point to a single fact that says this is evidence of life somewhere else. It's all still circumstantial evidence."