The mysterious plume of icy material spewing from the bottom of Saturn’s moon Enceladus appears to be made up of salty water vapor that scientists now think may come from a vast ocean under the moon’s surface.

New measurements taken closer to the moon, collected during 2008 and 2009 flybys of the Cassini­-Huygens spacecraft, found for the first time large grains of ice with substantial amounts of salt. The original discovery of the Enceladus plume was made when Cassini passed through its outer reaches, where the ice particles were smaller and salt less prevalent.

Scientists think the ice grains are coming from surface fractures at the moon’s southern pole, areas that have been deemed “tiger stripes.”

That the stripes are the apparent source of salty ice is considered a significant breakthrough with implications for understanding planets and moons, and for detecting possible sites hospitable to life.

“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 the saltwater under Enceladus’s icy surface,” said Frank Postberg of the University of Germany, lead author of a study being published in Nature on June 23.

The Cassini spacecraft is operated by NASA, in collaboration with the European Space Agency and the Italian Space Agency.

Nicolas Altobelli, ESA’s project scientist for the Cassini-Huygens mission, said the presence of a liquid and salty ocean beneath the moon’s surface is surprising.

“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,” he said. “This finding is therefore a crucial new piece of evidence showing that environmental conditions favorable to the emergence of life may be sustainable on icy bodies orbiting gas giant planets.”

The researchers think that a layer of water exists between the moon’s rocky core and the icy mangle, one perhaps 50 miles beneath the crust. The layer could be kept warm enough to be in a liquid state by tidal forces created by the gravitational pull of Saturn and several neighboring moons, or heat generated by radioactive decay.

Saturn has 62 confirmed moons, but only seven are large enough to have spherical shapes.

Jupiter’s moon Europa is also thought to have a vast ocean beneath a thick layer of ice.

Cassini uses a Cosmic Dust Analyzer to measure the composition of freshly ejected plume grains. The icy particles hit the detector target at speeds of up to 11 miles a second and instantly turn to vapor. Electrical fields inside the instrument then separate the various parts of the resulting impact cloud for analysis.

That analysis helped determine that the particle had high concentrations of sodium and potassium and that they most likely come from the evaporation of liquid saltwater.

The paper estimated that about 400 pounds of water vapor flows out in the plume every second, and some smaller amount as ice grains as well. The size of the release suggests that the amount of liquid water beneath the surface is substantial.