PARIS -- The pioneering US spacecraft Cassini has turned up fresh evidence that water exists on Enceladus, the beguiling moon of Saturn where, some experts believe, the potential ingredients for life exist.

Plumes of vapor, disgorged high above the satellite's surface, are probably being blasted out by jets of liquid water and gas expelled at high speed from nozzle-like vents, scientists report on Wednesday.

A worldlet measuring only 504 kilometers (315 miles) across, Enceladus is unique among Saturn's four dozen moons.

It is strangely unmarked by asteroid impacts, for its surface is encased in a dazzling white shell of ice that is otherwise pristine except for deep ridges near its south pole.

These fractures, dubbed "tiger stripes," have unleashed huge excitement because of geyser-like plumes of vapor and dust emitting from them.

Analysis of the vapor's light signature indicates water, which is extraordinary, for Enceladus inhabits a region of space where the temperature is close to absolute zero -- minus 273 degrees Celsius (minus 460 degrees Fahrenheit).

Theorists suggest the tiny moon is geologically active, thanks to a phenomenon called tidal heating.

It suffers a ripping gravitational pull from its giant mother, and from the nearby satellites of Dione and Janus.

As a result, according to their hypothesis, its guts are stretched and squeezed, causing friction that warms a sub-surface ocean, whose existence is visible in the plumes expelled into space.

If so, two of the three key elements for life as we know it -- liquid water and energy -- exist on Enceladus.

Not everyone is convinced. Dissidents hold that the vapour is not water per se, but carbon dioxide, nitrogen and methane gases released from an icy state called clathrates, with ice particles making up the rest of the mix.

In the latest bout in this intellectual joust, a team led by Candice Hansen of the Jet Propulsion Laboratory at the California Institute of Technology (Caltech), took a close look at four plumes when Cassini swung around Enceladus on October 24 2007, using a bright star as background illumination.

The plumes -- whose positions coincide with dust jets seen in an earlier flyby -- fan out to a width of 10 kilometres (6.2 miles) to a height of 15 kilometers (9.5 miles) above the surface.

The density of the column of water inside the plumes is twice that previously reported, the team says.

In addition, the speeds of the plumes support the hypothesis that the source is liquid water, expelled with gas "accelerated to supersonic velocity" from nozzle-like channels.

Their study appears in the journal Nature, published by the London-based Nature Publishing Group.

In March, US scientists said Cassini had detected temperatures at Enceladus' south pole that were as warm as -93 degrees Celsius (-135 degrees Fahrenheit), suggesting that sub-surface temperatures were high enough for the existence of water.

They also found simple and complex organic molecules -- the third ingredient for potential life -- in the March 12 flyby, which skimmed over the plumes to within 50 kms (30 miles) of the surface.