By Helen Briggs
Science reporter, BBC News
Saturn's moon Titan may have a deep, hidden ocean, according to data published in the journal Science.
Future observations by Cassini will help test the prediction
Radar images from the Cassini-Huygens mission reinforce predictions that a reservoir of liquid water exists beneath the thick crust of ice.
If confirmed, it would mean that Titan has two of the key components for life - water and organic molecules.
Currently, three other Solar System objects are suspected of having deep oceans: Ganymede, Callisto and Europa.
The Cassini-Huygens mission is a cooperative project of the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (Asi).
When Cassini began to observe the largest of Saturn's moons in 2004, the surface was thought to be completely covered with an ocean of hydrocarbons.
But when the spacecraft turned its radar on the moon for the first time in 2004, and the Huygens probe parachuted to the surface a year later, a different picture emerged.
Much of the surface was found to be solid, with geological features such as dunes, channels and impact craters, punctuated by vast "lakes".
Cassini's latest fly-by of Titan is providing a new glimpse of these features, which to researchers' surprise, are not in the place they should be.
Scientists would like to send an instrumented balloon to Titan
Coupled with models of how the moon spins, the data suggests that the observed seasonal variation in spin rate could only exist if a liquid ocean lay beneath the solid crust.
The researchers, led by Dr Ralph Lorenz of Johns Hopkins University Applied Physics Laboratory, US, say their predictions can be checked in the proposed extended Cassini mission or in future missions.
John Zarnecki, Professor of Space Science, at the UK's Open University, who was not part of the study, said the motivation to go back to Titan with a more sophisticated space probe was "overwhelming".
Evidence suggests that Titan has two of the key constituents for the formation of life - water and organic molecules, and possibly a third - a source of energy, he said.
Prof Zarnecki told BBC News: "We know there are organic molecules, the place is swarming in organics.
Titan: The second largest moon in the Solar System
"Titan is 50% water-ice. If it is liquid, as this paper is implying some of it is, it looks as though we've got at least two of the things to initiate the chemistry that leads to life.
"It wouldn't be too far fetched to imagine certain spots on Titan where there would be a source of energy - maybe geothermal energy, as we have on Earth at the bottom of the oceans."
Titan is the second largest moon in the Solar System; only Jupiter's Ganymede satellite is bigger.
Past observations have shown that Titan in many ways resembles a very early Earth, particularly in the composition of its atmosphere. The major difference is the frigid temperatures out near Saturn.
Prof Zarnecki added: "We've got to go back again with balloons and rovers and really understand this place."