Saturn holds a tiny secret

July 23, 2007— -- Icy chasms on one of Saturn's most humble moons, hidden amid its glorious rings, have overtaken the sands of Mars and the stratosphere of Venus as the most intriguing potential hiding place for alien life in our solar system.

Enceladus, a shining ball of ice hugging Saturn's rings, was first caught in the act of spewing a watery geyser from its south pole two years ago by the international Cassini mission. Water, life's most crucial ingredient, was blasting 270 miles into space, actually hitting the orbiting spacecraft, from cracks on the frozen moon dubbed "tiger stripes."

Astronomers and astrobiologists, who are always looking for signs of life far from Earth, were caught by surprise — and they remain so, unable to explain how such a small celestial body (only 318 miles wide at its equator ) can pump out so much water.

"Nobody has figured it out," says Andrew Dombard of Johns Hopkins University's Applied Physics Laboratory in Laurel, Md. "Enceladus has jumped to the top of astrobiologists' list for a mission."

And as for the big question — Does life exist there? — the answer is the same: Who knows?

The most recent Cassini flyby of Enceladus, a distant one at 55,000 miles, was on June 28. Each such visit has heightened the interest of planetary scientists, who have erupted with their own flurry of theories, including two reports published in the journal Nature in May.

"Probably with 20/20 hindsight, we can say we expected something from Enceladus," says Cassini scientist Jonathan Lunine of the University of Arizona in Tucson. He notes that since the Voyager missions flew past Saturn in the early 1980s, the moon has been known as the brightest object in the solar system because of its coating of fresh ice.

But the eruption of the geyser from the moon's south pole rather than from the equator, which is subject to the strongest gravitational pull from Saturn, and the curious tiger-stripe vents, cracks about 80 miles long, were "a big surprise," Lunine says. "These are very exotic kinds of features."

Every eight seconds, the geyser spotted in a flyby of Enceladus in December 2005 dumped about a ton of not just water but also a mixture of life's building blocks — organic compounds such as methane, propane, acetylene and carbon dioxide, as well as nitrogen — into Saturn's outer "E" ring. Cassini carries a spectrometer that performed a chemical analysis of the plume.

Acetylene and propane bubbling up in the geysers indicates that "a very hot environment" — perhaps 440 to 980 degrees — hides under the ice of Enceladus, says a study published in April in the journal Icarus. The study was led by Dennis Matson of NASA's Jet Propulsion Laboratory, which runs the Cassini spacecraft.

"I think we will someday find liquid water under the surface of Enceladus," Lunine says. "It's very exciting and raises a lot of questions."

The moon and the myth

Enceladus is named for a giant in Greek mythology who was buried under a volcano after battling the gods, his throbbing wounds rumbling the mountain. And that may be the only explanation for the moon's activities that's not now in scientific play.

"Yes, there were some pre-Cassini hints that Enceladus may have been recently active. But that still doesn't prepare you at the gut level for the spectacular images of geysers shooting out of the south pole," says planetary scientist Geoffrey Collins of Wheaton College in Norton, Mass.

"If we didn't have any close-up images, it would be easy to write Enceladus off as being too small to be very active or geologically interesting," Collins says. "Now that we've seen all of this intense current activity on such a tiny world, we need to explain why it's happening there."

Last year, Cassini detected that the surface and interior of the tiger-stripe crevices are warmer than their surroundings, which nails them as the source or conduit of the geysers, Lunine says. And the cracks are likely only 10 to 1,000 years old, a blink of an eye to astronomers. But knowing where the geysers erupt doesn't explain how a tiny moon can generate the heat needed to fire up such eruptions.

Earth, nearly 8,000 miles wide, has an internal dynamo, a core of molten iron and rock that powers its volcanism. Io, a volcanic moon of Jupiter about 2,255 miles across, is riven by gravitational tides from the massive planet it orbits, and the tides churn its hellish landscape.

For Enceladus, the various explanations generated by a wide range of scientists can get tricky:

•"Cold faithful." A lake of near-frozen liquid water, squeezed like a vise between the ice above and rock below, may spurt the occasional geyser.

•Hot start. Radioactive elements decaying within 7 million years of Enceladus' birth may have sparked the moon's core, creating an ocean under the ice.

•Tidal friction. Gravitational tides produced by the oblong orbit of Enceladus around Saturn may rub the edges of the tiger stripes together, melting ice that jets into the vacuum of space.

•Clathrate cages. Gases seen in the plumes may simply be trapped in ice "clathrates." When the cracks shift, the ice may be exposed to space's vacuum, giving the gases enough "oomph" to escape from the lattice-like cages.

All the theories match the data to some extent, Dombard says, but none fully explains the source of the heat needed to power the geysers. To make his point, he and others point to a similar-size moon, Mimas. Mimas orbits closer to Saturn than Enceladus, on a more elongated orbit, and thus experiences stronger tides. It formed near Enceladus, probably getting the same starting helping of radioactivity. "And Mimas is basically a cracked ice ball, dead," Dombard says. "So it's a real puzzle."

Cassini discovered small signs of cryovolcanism in a pair of small Saturnian moons, Tethys and Dione, in June flybys, but nothing to match the scale of the releases from Enceladus. Dust flying off the moons may refill the material in Saturn's sparkling rings. Without refills, the rings would fade, slowly eaten away by charged particles shot from Saturn's magnetic field.

The search for life

Next year, Cassini will take its final close pass by Enceladus at an altitude of about 220 miles. Any closer and mission planners worry about ice from a geyser damaging the spacecraft, Lunine says. Cassini's managers have proposed at least seven similar flybys if the mission is extended, a decision expected by the end of the year.

Collins, one of the scientists who has proposed a liquid-sea explanation for the geysers, hopes such flybys will reel in enough gravitational measures from Enceladus to reveal what lies beneath the ice. But he shares doubts with other experts about whether Cassini by itself will ever answer the big question: Could life exist there?

"Of the classic three ingredients for life, Enceladus has liquid water (at least in our model), it has organic molecules (they're shooting out of the geysers!), but does it have an energy source?" Collins says by e-mail. If liquid water on Enceladus is in contact with warm rock below, microbes might extract energy from chemical reactions there, much like some deep-sea bugs on Earth, Collins and others reason.

A National Research Council committee report July 6 titled "The Limits of Organic Life in Planetary Systems" cautioned that astrobiologists may be too fixated on water as a pointer to alien life in places like Enceladus. The panelists suggest instead "that we make a conscious effort to broaden our ideas of where life is possible and what forms it might take."

The report points to frozen methane lakes on Saturn's big moon, Titan, and the aerosol region high above Venus as places where some very unearthly life may take root.

Still, "everyone is interested in Enceladus, Europa and Mars. Those will always be our top three priorities," says the report's chairman, John Baross of the University of Washington in Seattle. The Mars rover mission has been searching for signs of water and life on the Red Planet for several years.

Researchers such as JPL's Matson have argued that the combination of heat, chemistry and water on Enceladus make it a place hospitable for life, and therefore ripe for more scientific investigation.

Trying to figure out what sort of probe would best unlock the little moon's secrets "really is something we are concerned about," Dombard says.

Similar missions have been proposed for Europa, one of Jupiter's large moons with a liquid ocean probably locked under its icy crust. "But unlike Europa, we actually get stuff on Enceladus shooting out of the ice," Dombard notes. That means a robotic sampler wouldn't have to drill for signs of life, as has been proposed for Europa.

"Lots of people are thinking about this sort of mission," he says.