Huge geysers on Saturn's moon Enceladus may be fed by a salty sea below its surface, boosting the odds of extraterrestrial life in our own Solar System, according to a study released Wednesday.
Researchers in Europe detected salt particles in the volcanic vapour-and-ice jets that shoot hundreds of kilometres (miles) into space, the strongest evidence to date of a liquid ocean under the moon's icy crust.
Scientists already knew that tiny Enceladus, only 500 kilometers across, had two of the three essential ingredients for the emergence of life.
One is an energy source, produced in this case by "tidal warming" driven by the shifting gravitational tug of its parent planet during the moon's lopsided orbit, and perhaps by other forces too.
The Cassini spacecraft circling Saturn since 2004 has also found a potentially life-sustaining mix of organic chemicals in Enceladus' plumes, ejected from a quartet of 120-kilometer (75-mile) long fractures -- known as "tiger stripes" -- aligned on the moon's south pole.
That left the third critical ingredient: liquid water.
Since their discovery in 2005, the giant geysers have fueled intense speculation on the presence of a subterranean ocean, and the new discovery goes a long way toward resolving one of the most hotly debated topics in planetary science.
A team led by Frank Postberg of the University of Heidelberg studied data from Cassini's Cosmic Dust Analyzer, and tested their findings in laboratory experiments.
Their results, published in the British journal Nature, show that ice grains in the Enceladus plumes contain substantial quantities of sodium salts, and that the moon's hidden sea -- if there is one -- could be as salty as Earth's oceans.
"The abundance of various salt components in the particles ... exhibit a compelling similarity to the predicted composition of a subsurface Enceladus ocean in contact with its rock core," the researchers conclude.
"Individual plume sources stay active for years, implying outflow from a large reservoir."
It is highly soluble, "so any Enceladan water that has prolonged contact with the moon's silicate core should be rich is sodium salts, like Earth's oceans," he noted in a commentary for Nature.