From infrared images that show variations in brightness and texture, a geological map of the circular feature, shown in this photograph, thought to be a volcano, has been obtained using Cassini's visual and infrared mapping spectrometer. (Reuters photo)

WASHINGTON, June 8 (Xinhuanet) -- The Cassini spacecraft has spotted a possible ice volcano on Saturn's largest moon Titan, a finding that could provide an explanation for the presence of methane in Titan's atmosphere.

    The US National Aeronautics and Space Administration (NASA) said infrared images taken by Cassini show a circular feature about 30 km across. Scientists believe it is an "ice volcano," a dome formed by upwelling icy plumes that release methane into Titan's atmosphere.

    The findings are published in Thursday's issue of the journal Nature.

    "Interpreting the feature as a cryovolcano provides an alternative explanation for the presence of methane in Titan's atmosphere. Such an interpretation is supported by models of Titan's evolution," said Dr. Christophe Sotin, a team member of the Cassini visual and infrared mapping spectrometer.

    Titan is the only known moon in the solar system that has a significant atmosphere made up of nitrogen and methane. One goal of the Cassini-Huygens mission is to find out the source of the moon's atmosphere. Scientists have long thought there might be seas or lakes of methane or ethane on Titan. However, no such evidence has been found since Cassini orbited Saturn from June 30 last year.

    The Cassini-Huygens mission is a joint project between NASA andthe European Space Agency (ESA). Huygens, a probe developed and controlled by the ESA, separated from Cassini and landed on Titan's surface in last January.

    NASA said the new finding was based on Cassini's first targetedflyby of Titan on Oct. 26, 2004, at a distance of 1,200 km from Titan's surface.

    Cassini is expected to make 45 flybys of Titan during its four-year mission. The next one is on Aug. 22. Enditem