March 9 (Bloomberg) -- The surface of Saturn's moon Enceladus is composed mostly of water ice and there may be a cold ocean beneath that holds some form of life, according to studies of images taken by NASA's Cassini spacecraft last year.

Images show a plume of gases and water spouting from the moon's southern pole, similar to the Old Faithful geyser in Yellowstone National Park in the western U.S., said Andrew Ingersoll, a member of the Cassini imaging team and an atmospheric scientist at the California Institute of Technology.

Other moons, such as Jupiter's Europa, have oceans covered by ice more than a kilometer (0.6 miles) thick. Enceladus's plume shows that liquid water may be as close as 10 meters underground and may hold an extreme form of life, said Ingersoll, a co-author of one of the studies being published in tomorrow's edition of the journal Science.

``Any life that existed could not be luxuriant and would have to deal with low temperatures, feeble metabolic energy and perhaps a severe chemical environment,'' said Jeffrey Kargel of the University of Arizona in Tucson in an accompanying article. ``Nevertheless we cannot discount the possibility that Enceladus might be life's distant outpost.''

Enceladus, the sixth-largest of Saturn's 47 moons, was discovered by astronomer William Herschel in 1789. It has a diameter of about 500 kilometers, smaller than France, and orbits Saturn about every 1.4 days. The Cassini spacecraft, which has been circling the planet since July 2004, flew by the moon three times between February and July of 2005.

Active Volcano

The observations make Enceladus the fourth known body in the solar system with active volcanoes, in addition to Earth, Jupiter's moon Io and Neptune's moon Triton, said John Spencer, a scientist with the Southwest Research Institute in Boulder, Colorado, in a statement. The moon is also the third known body, besides Earth and Io, with internal heat that can be measured remotely.

The plume likely spews liquid water, which breaks down into oxygen and hydrogen that travels thousands of kilometers into Saturn's atmosphere, where it helps to replenish the planet's so- called E ring. Some of the material may fall back to the surface of the moon as snow.

Scientists concluded last year that Enceladus had a ``significant'' atmosphere full of electrically charged water vapor particles after analyzing data from Cassini's magnetometer instrument taken during the first two fly-bys, suggesting an underground source of gases close to the surface.

Extreme Cold

It's not clear why there is liquid water on a body that could fit into New Mexico and has a surface temperature of about -201 degrees Celsius (-330 degrees Fahrenheit), said Bruce Betts, a planetary scientist and director of projects for the Pasadena, California-based Planetary Society.

Astrobiologists have been searching the solar system for decades for bodies that have liquid water, one of three things required for life to exist on Earth, Betts said. So far, scientists have theorized that water flowed on four bodies: Earth, Mars, Europa and Enceladeus.

NASA's Mars Exploration Rovers have found signs of minerals that indicate that water once flowed on the surface of the planet, and the agency's Mars Reconnaissance Orbiter, scheduled to arrive tomorrow, will search for further evidence.

``If there's liquid water, there's the possibility of life,'' Betts said in a telephone interview. ``But we only have Earth as a laboratory.''

More Flights

Cassini will fly by Enceladus again in 2008. Yet the spacecraft won't be able to verify that life exists, Kargel wrote, and Ingersoll said the findings may lead to a future NASA mission to Enceladus.

``It's a bit of a stretch,'' Ingersoll said of the possibility that life exists on Enceladus. ``But at least the ingredients are there.''

Cassini was launched from Cape Canaveral, Florida, in 1997 and traveled more than 2.2 billion miles before entering orbit around Saturn on July 1. The craft has sent back detailed pictures of the ringed planet and its moons, and at least 76 orbits are planned before the mission is scheduled to end in 2008.

To contact the reporter on this story: Chris Dolmetsch in New York at cdolmetsch@bloomberg.net.

Last Updated: March 9, 2006 16:08 EST