Saturn moon spewing water vapour
One of Saturn's moons, Enceladus, is spewing out a giant plume of water vapour that is probably feeding one of the planet's rings, scientists say.
The findings, published in the journal Science, suggest that tiny Enceladus could have a liquid ocean under its icy surface which in theory could sustain primitive life, similar to Jupiter's moon Europa.
The plume was spotted by Cassini, a joint US-European spacecraft that is visiting Saturn.
"We realise that this is a radical conclusion, that we may have evidence for liquid water within a body so small and so cold," says Dr Carolyn Porco, Cassini imaging team leader at the Space Science Institute in Boulder, Colorado.
"However, if we are right, we have significantly broadened the diversity of solar system environments where we might possibly have conditions suitable for living organisms."
Scientists have long known that many of Saturn's moons have water. They took an especially close look at Enceladus because it seemed to have a smooth surface, suggesting recent geological activity that, in turn, could mean liquid water.
Liquid water is a key requirement for life. Several moons have been found to have evidence of liquid water and the chemical elements needed to make life, including Europa.
But scientists are far more intrigued by the plume itself, a gigantic geyser of water vapour and tiny ice particles.
"It's basically this giant plume of gas coming out of the south pole of Enceladus," says Dr Candy Hansen of NASA's Jet Propulsion Laboratory in California.
"The plume is half the size of the moon. It's huge," says Hansen, a planetary scientist.
"Water is being spewed out of this moon. It solves some real mysteries that we have been struggling with over the years."
Indirect observations have shown the moon, discovered in 1789 by William Herschel, is rich in oxygen and hydrogen. But whether this is because of water is not clear.
Both water vapour and water particles were observed, as well as a smattering of other compounds such as methane and carbon dioxide, the international team of scientists reports in a series of papers in Science.
It is possible the plume comes directly from ice, but more likely there is a liquid source, the researchers say. It would have to be under the moon's surface, which is covered with ice.
"If a wet domain exists at the bottom of Enceladus' icy crust, like a miniature Europan ocean, Cassini may help to confirm it," Dr Jeffrey Kargel of the University of Arizona at Tucson writes in a commentary.
"Might it be a habitat? Cassini cannot answer this question," Kargel adds.
"Any life that existed could not be luxuriant and would have to deal with low temperatures, feeble metabolic energy, and perhaps a severe chemical environment. Nevertheless, we cannot discount the possibility that Enceladus might be life's distant outpost."
Hansen is cautious.
"I think the best you can say is there is biological potential. We have liquid water probably, and it is in contact with rocks so there are minerals," she says.
"And there is energy. But we haven't detected life."
The findings help to confirm theories that Enceladus was the source of Saturn's outer E-ring, the researchers say.
Saturn has at least 47 known moons. Enceladus is named after a Titan in Greek mythology who was defeated in battle and buried under Mount Etna by the goddess Athena.