An eruption of evidence may unlock Titan's mystery

Below the thick smog that made cameras all but useless and gave Titan the appearance of a giant orange cue ball, something curious had to be going on.

Amid the soup of gases that choked Saturn's planet-size moon, there were clear signs of methane. It was an odd discovery in this distant corner of the solar system, where neighboring moons were little more than barren blocks of ice. Methane is notoriously fickle, easily destroyed by the sun's ultraviolet light. If it was on Titan, there had to be a source beneath the shroud.

So before the Cassini spacecraft arrived at the Saturn system last year, scientists envisioned vast seas of methane covering Titan. Yet in eight months of searching, they have found none. Now, however, an infrared image snapped by Cassini appears to have uncovered a cause no less fantastic than the one that scientists originally imagined.

For the first time, researchers say they have found convincing evidence of a volcano that spews not searing lava but a frigid mix of water-ice sludge and other chemicals - including methane.

Astronomers have long guessed at the existence of cryovolcanism on moons in the outer reaches of the solar system. But this week's discovery is of particular import, as a world long veiled in mystery begins to reveal its secrets - hinting at an intoxicating complexity of eruptions and floods beneath Titan's oppressive orange haze.

"It's mind-boggling, the range of geological processes going on there," says Laurence Soderblom of the United States Geological Survey in Flagstaff, Ariz. "When we look at Titan, we see exotic and exciting things."

When Cassini dispatched its smaller Huygens probe to Titan months ago, the surface was perhaps even more exotic than scientists had predicted. In the half-light of the hazy Titan day, Huygens saw land forms that had been etched and eroded by liquids over eons - dark basins ringed by sharp shorelines, as well as what looked like springs and river channels. At minus 290 degrees F, Titan is too cold for liquid water. As expected, everything pointed to the presence of liquid methane. Yet scientists couldn't find any. There were basins and river beds, but nothing in them.

The most recent discovery, announced in the current edition of the journal Nature, begins to at least offer clues - and it presents a world nearly as dynamic as Earth. Specifically, it is a volcanic dome 19 miles wide, pushing up like a boomerang from the dark basin that surrounds it. Almost immediately, it caught researchers' eyes.

"It looks just like a volcano should look," says Bonnie Buratti, an astronomer at the Jet Propulsion Laboratory in Pasadena, Calif. "It really does remind me of the Hawaiian islands."

The comparison is apt. Scientists suggest that Titan's volcano should act very much like a volcano on Earth - a sort of Kilauea for the solar system's outer rim. The fundamental difference is that most of the moons of the outer solar system are made up primarily of water ice, while the planets of the inner solar system are made of rock. So while terrestrial volcanoes purge molten rock from beneath Earth's crust, volcanoes on an ice world - like Titan is thought to be - spew a slurry of water ice mixed with chemicals like ammonia that lower its freezing point and help it well up from below the surface.

Another chemical almost certainly involved in an eruption would be methane gas, not only replenishing the methane in the atmosphere but also potentially driving weather cycles that some researchers think must be at work on Titan. The methane might eventually fall as rain, cutting the river channels and spilling into the dark lowland playa, which are soggy, according to Huygens's splat landing. Moreover, eruptions would disgorge flows of watery sludge that resurfaced the moon as they froze, explaining why so much of Titan's surface lacks craters, which typically build up over time on dead moons.

It is only one piece in the grander puzzle of how Titan works. But it could be a significant one. Indeed, the presence of a volcano would confirm that Titan is still active - its icy interior warmed to a molten goo, most likely by the constant squeeze and release of Saturn's gravity as Titan makes its oblong circuit around the giant planet.

With at least three years left on the mission, researchers are confident that they can unravel Titan's mysteries - and maybe even catch a volcano in the act. Says Bob Brown, a coauthor of the report: "With 40 more flybys of Titan, we'll get it figured out."