Once-wet Mars has long been the primary focus of the search for life on other planets. But Saturn's moon Enceladus could be an even more promising place to start the search for extraterrestrials.

Startling new images from NASA's Cassini spacecraft indicate that Enceladus may contain pockets of liquid water below its icy crust.

These pockets, described in an article published today in the journal Science, may be ideal habitats for life-forms similar to those found in hydrothermal vents beneath the Earth's oceans.

"This is extraordinary," said Carolyn Porco, a Cassini team leader at the Space Science Institute in Boulder, Colorado and primary author of the new study.

"I think our results are significant enough to redirect the planetary exploration program, placing Enceladus as the primary target of astrobiological interest in our solar system."

(See our interactive solar system map.)

Thar She Blows

Launched in 1997, the Cassini orbiter has conducted numerous flybys of Saturn's frigid moons.

When Cassini imaged Enceladus's south pole early last year, researchers noticed plumes of what appeared to be a steamlike substance spewing from the 300-mile-wide (480-kilometer-wide) moon's crust.

At first Porco's team thought the billows might be water vapor rising from subsurface ice deposits. Then the scientists realized they were seeing something unprecedented: outer-space liquid-water geysers not unlike Yellowstone's Old Faithful.

What causes these geysers to form? According to Porco and her colleagues, unknown heat sources inside Enceladus melt ice into deposits of subsurface water. Under pressure, these water pockets burst through the icy crust in fountainlike jets.

"Once the water comes out it freezes, and that produces copious amounts of ice particles," Porco said.

Most of these just frozen particles fall back to the ground. Their highly reflective surfaces help give Enceladus its unofficial status as the shiniest object in our solar system.

The team's geyser investigations have yielded many new insights about Enceladus's interaction with Saturn, its rings, and its other moons.

Enceladus, Phone Home?

Porco and her colleagues are most enthusiastic about their work's implications for the search for alien life.

"What these findings tell us is that we may have a warm, water-based environment reasonably close to the surface—one that could be conducive to living organisms," Porco said.

"Enceladus will be a very exciting place to look for extraterrestial life."

Other moons in the solar system, such as Jupiter's Europa, have bodies of liquid water that might be able to support life. Europa's water, however, is several miles below the surface. Water deposits near Enceladus's south pole could be less than a hundred feet (30 meters) underground.

"Even though there's an ocean on Europa, it would be much more difficult to access," Porco said. "Also, there's a more intense radiation field on Europa, limiting the amount of time you could spend there with a lander."

Robert Pappalardo, an astrophysicist at the University of Colorado at Boulder, calls Porco's findings "a spectacular result."

"The first step in any search for life is to find liquid water, and Enceladus has that," he said.

"Next we need to add more detail. Have these geysers been active long enough to create a stable environment? Does the moon have a source of chemical energy that would be capable of supporting life?"

Warm Gooey Center?

The source of Enceladus's potentially life-sustaining heat is another unsolved mystery.

Porco speculates that the moon may have a hot inner region, partially made of molten rock, that sends waves of warmth to the surface.

United States Geological Survey researcher Jeffrey Kargel proposes an alternative theory in a companion article to in Science. He speculates that friction from shifting tectonic plates could be generating heat that is getting trapped inside the icy crust.

To help fill in some of these unknowns, Porco, the study author, is lobbying to extend the Cassini mission past its end date specifically for more Enceladus study.

"A closer flyby would let us improve our measurements of the type of material coming out of the vents and map the location of any thermal activity," she said.

"To really establish that there's life, you need to find a fossil or stick your hand in the ice and pull up a microbe. We can't do that with Cassini, but the instruments we have now [on the spacecraft] can at least provide us with additional clues."