No ocean, but maybe volcanoes on Saturn's moon

Images from the Cassini-Huygens space probe, a project headed up largely by the European Space Agency, seem to have sunk a theory that a dark region 19 miles in diameter on Titan might be a liquid or viscous sea. Instead, scientists interpret the feature as an "ice volcano" that spews plumes of frozen methane, which were also theorized to exist.

"The suite of instruments onboard Cassini and the observations at the Huygens landing site reveal that a global ocean is not present," Christophe Sotin, a member of the Cassini mapping team and professor at the Universite de Nantes, said in a statement this week. "Interpreting this feature as a cryovolcano provides an alternative explanation for the presence of methane in Titan's atmosphere."

Titan has long intrigued astronomers because it is the only moon in the solar system with a thick atmosphere. Primarily consisting of nitrogen, Titan's atmosphere also contains 2 percent to 3 percent methane. One of the goals of Cassini-Huygens is to try to figure out what replenishes that methane.

The foggy atmosphere has also, until recently, made it extremely difficult to get clear images of the surface. But infrared imaging on the space probe has improved the situation.

Scientists studying the recent images said the dark region resembles a caldera, a bowl-shaped structure seen on volcanoes on Earth and Venus. The material erupting from the volcano might be a methane-water ice mixture. Energy from an internal heat source may cause these materials to emerge from the volcano and vaporize as they reach the surface.

Channels near the region may be the result of erosion. Although the ocean theory appears to be incorrect, scientists also expected to find volcanoes on Titan before the images were obtained.

Determining the geographic features of a distant planet from images harvested from telescopes or space probes, particularly in the early stages, often involves posing, and subsequently discarding, analogies from Earth. For years, scientists thought bright streaks on Dione, another one of Saturn's moons, were ice because it looked like ice, said Carolyn Porco, imaging team leader for the Cassini-Huygens project, in December. Instead, Cassini-Huygens' images have shown that the streaks are cliffs.

Although the European Space Agency only revealed its conclusions about the dark region this week, the images came from the first flyby of the Cassini-Huygens probe last October. In all, 45 flybys of Titan are planned during Cassini's four-year prime mission. The next one is Aug. 22.