Infrared images snapped by the orbiting Cassini reveal a 20-mile-wide dome that appears to be a cryovolcano, a volcanic-like vent that spews forth ice instead of lava. Scientists theorize the volcano at one time spat out icy plumes that released methane into Titan's atmosphere.
The findings appear in Thursday's issue of the journal Nature.
Titan is the only moon in the solar system that has a significant atmosphere made up of nitrogen and methane. Its atmosphere is similar to that of primordial Earth and scientists believe that studying it could provide clues to how life began.
Scientists have long speculated that the organic materials in Titan's atmosphere were formed by seas or lakes of methane or ethane, but the latest Cassini images did not show any evidence that Titan is awash in pools of methane. Methane is a highly flammable gas on Earth, but it is liquid on Titan because of the intense atmospheric pressure and cold.
"Interpreting this feature as a cryovolcano provides an alternative explanation for the presence of methane in Titan's atmosphere," said Christophe Sotin, a team member of the Cassini visual and infrared mapping spectrometer.
Although the ice volcano is inactive, scientists believe similar volcanoes may exist elsewhere on Titan that ooze a methane-water ice mixture to the surface.
In an accompanying editorial, Louise Prockter of the Applied Physics Laboratory at Johns Hopkins University noted that while the ice volcano hypothesis was intriguing, higher-resolution images could reveal that the structure is something other than a volcano.
The Cassini-Huygens mission is a joint NASA-European Space Agency project. The combined craft was launched in 1997 and arrived in orbit of the ringed planet last year. Huygens, a probe developed and controlled by the ESA, touched down earlier this year.
The latest finding is based on a Cassini flyby of Titan on Oct. 26, 2004. Forty-five flybys are planned during Cassini's four-year mission. The next one will be Aug. 22.