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Smog on Saturn Moon May Be From Volcano

A spacecraft flyby has captured the image of a feature on the surface of Saturn's largest satellite that could be a source of underground methane.

June 09, 2005|John Johnson Jr. | Times Staff Writer

The possible discovery of an ice volcano on Saturn's moon Titan may solve one of the lingering mysteries about the strange satellite with the smog-choked atmosphere.

Scientists have wondered for decades where the methane in Titan's atmosphere came from. Titan, Saturn's largest moon, is the only moon in the solar system with a substantial atmosphere. Most of the atmosphere consists of nitrogen, but up to 3% is methane.

Investigators theorized that it could have come from a methane-rich hydrocarbon ocean that covered much of the moon. But instruments aboard the Cassini spacecraft, which has been investigating the moon since late last year, have failed to turn up evidence of such a sea.

A recent flyby of Titan has produced an image of a circular feature about 19 miles in diameter with two wings extending to the west, scientists reported today in the journal Nature.

Scientists believe the wings represent repeated flows from a volcano that spews ice and liquid methane, instead of molten rock. Researchers have noted the presence of similar physical features surrounding volcanoes on Earth and Venus.

This raises the possibility that the atmospheric methane comes from a subsurface source of liquid methane that is vented to the atmosphere by erupting volcanoes.

The eruptions are possibly caused by heat generated by tidal movements of the liquid methane inside Titan. The moon is thought to be subject to extreme variations of its internal tides because it has a strongly elliptical orbit that sometimes draws it close to Saturn and other times sends it spinning farther out in space.

"We all thought volcanoes had to exist on Titan, and now we've found the most convincing evidence to date," said Bonnie Buratti, a member of the team at the Jet Propulsion Laboratory analyzing results of Cassini's infrared mapping spectrometer. "This is exactly what we've been looking for."

In the center of the geologic feature is a dark area that appears to be a depression similar to volcanic calderas seen on Earth.

Releases from an ice volcano could explain the black channels spotted by the Huygens probe, which landed on the surface of Titan early this year. Those channels could have been formed by methane rainstorms after a volcanic eruption, according to Cassini scientists.

Planetary scientists have considered other explanations for the volcano-like structure. For a time, they thought it might be a stationary cloud. But its shape is so uniform and static that researchers decided it had to be a surface feature.

They also speculated it could be a buildup of particles similar to a sand dune on Earth. However, the circular shape of the feature raised doubts about that. Known wind patterns on Titan also do not match the shape of the mound and its wings.

Scientists are now awaiting the results of radar observations from Cassini to confirm their theory.

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