In 2005, NASA's Cassini probe, orbiting Saturn, made a tantalizing discovery: Enceladus, one of Saturn's moons, was venting something -- possibly liquid water -- into the airless space around it.
In the frozen wastes around Saturn, this was (please forgive the pun) really cool. How could there be water nearly 900 million miles from the sun? That's almost 10 times as far out as Earth is.
Enceladus. Even the name, coming from Greek mythology, rolls off the tongue mysteriously. En-CELL-a-dus.
It is a small world, 300 miles in diameter, covered in a thick layer of ice. It whips around Saturn in 1.4 days, meaning the gravitational pull of the great ringed planet on the little moon's innards must be fierce.
Saturn has more than 60 known moons; Enceladus is the sixth largest. The venting appears to come from fractures or fissures in the ice near its south pole.
But liquid water? You can find water in the form of vapor or ice all over the universe (think of comets, with their frozen cores and filmy tails), but liquid water is the stuff of life as we know it, and when NASA made its original announcement four years ago, it set minds racing. There were many doubters.
Now, a team of scientists has analyzed the chemical makeup of one of the plumes, and found some intriguing clues that support the liquid-water theory. Their report is in Thursday's edition of the journal Nature.
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They found ammonia, argon, various organic compounds and salts, some of which could act as a kind of celestial antifreeze, keeping the waters of Enceladus from freezing.
An Ocean in a Moon of Saturn?
"Given the temperatures measured by Cassini of the fractures where the plumes originate, having ammonia mixed with water makes it much more likely that liquid water is at the source of the plumes, and not just warm ice," Jonathan Lunine of the University of Arizona, a member of the Cassini team, said in an e-mail to ABC News.
What does all this tell us? The scientists paint a picture of a briny sea beneath Enceladus' icy crust. They think it is heated by the moon's rocky interior, which may be partly radioactive, and may also be cracked and pulled by Saturn's gravity.
Both those things would generate warmth, enough warmth to compensate for the feeble light of the sun, so far away. Temperatures elsewhere in Saturn's neighborhood are hundreds of degrees below zero.
Similar things may be happening to at least one of Jupiter's moons. Europa, examined by NASA's Gailileo probe in the 1990s, may also have an ocean covered with ice.
Where there's water, could there be more? Could there be life? Could life, at least microbes, be common in the solar system, implying that life on Earth is less unusual than we thought?
"We do not know," Lunine said. Cassini's instruments, he said, "measured water, ammonia, argon-40, and carbon-hydrogen molecules (organic molecules). These, together with the source of heat that makes Enceladus warm, would seem to provide a promising environment for life.
"But Cassini cannot test this," he said. "We must look to the next mission."