The Cassini spacecraft has found its best evidence yet for Saturn's icy moon Enceladus habouring a wide-spread salty ocean.

Plumes of ice and water vapour were first seen jetting from the so-called tiger stripes at the moon's south pole in 2005. During three flybys in 2008 and 2009, Cassini's Cosmic Dust Analyzer soaked up the icy dust grains as it passed through the plumes. The grains vaporized instantly upon striking the detector; electrical fields inside the instrument then separated the various constituents of the resulting impact cloud for analysis.

The study revealed that far from the moon's surface the grains are small and low in salt content, but are larger and saltier closer to the surface.

“There currently is no plausible way to produce a steady outflow of salt-rich grains from solid ice across all the tiger stripes other than salt water under Enceladus’s icy surface,” states Frank Postberg, a Cassini team scientist at the University of Heidelberg, Germany, and the lead author on the paper that is published in Nature this week.

When water freezes, the salt is squeezed out, leaving pure water ice behind. So, if the plumes emanated from the surface ice they should have very little salt in them. Instead, the finding points to a layer of water between the moon's rocky core and icy mantle, which could be as deep as 80 kilometres. In this model, interaction of water with rocks dissolves the salt compounds and rises through fractures to pool in reserves nearer the surface. A plume breaks out if the outermost layer cracks open. Scientists estimate that some 200 kilograms of water vapor is being lost every second in the plumes, and that the water reserves must have large evaporating surfaces, or they would freeze easily and shut off the plumes.

Another Cassini instrument, the ultraviolet imaging spectrograph, has also found evidence for a subsurface ocean. Distinct jets show a difference in composition of ice grains close to the moon’s surface and those that make it out to contribute material to Saturn's faint E ring.

“The E Ring is made up predominately of such salt-poor grains, although we discovered that 99 percent of the mass of the particles ejected by the plumes was made up of salt-rich grains, which was an unexpected finding,” says CU-Boulder faculty member and study co-author Sascha Kempf of the Laboratory for Atmospheric and Space Physics. “Since the salt-rich particles were ejected at a lower speed than the salt-poor particles, they fell back onto the moon’s icy surface rather than making it to the E-Ring.”

Nicolas Altobelli, the European Space Agency’s project scientist for Cassini, adds: “Enceladus is a tiny icy moon located in a region of the outer Solar System where no liquid water was expected to exist, because of its large distance from the Sun. This finding is a crucial new piece of evidence showing that environmental conditions favorable to the emergence of life can be sustained on icy bodies orbiting gas giant planets.”