The big picture: Cassini spies Titan's tall mountains.

A spacecraft has discovered the largest mountains known on Titan, Saturn's smog-shrouded moon. A combination of infrared detectors and penetrating radar on the Cassini spacecraft recorded images of the 1.5-kilometer-high structures, planetary scientists announced Dee. 12 at a meeting of the American Geophysical Union in San Francisco. The infrared images reveal the shadows cast by the mountains, while the radar reveals their shape.

Coated with multiple layers of organic material and blanketed by clouds, the icy mountains are topped by bright, white material that could be methane snow, says Cassini scientist Larry Soderblom of the U.S. Geological Survey in Flagstaff, Ariz.

He speculates that some of the layers might be composed of material that has fallen out of Titan's atmosphere as rain, dust, or smog. The mountains might have formed when tectonic forces pulled apart Titan's crust, permitting material to rise from below, as the midocean ridges arose on Earth.

But it's also possible that the Titan mountains formed as the result of crust squeezing together, notes Cassini researcher Jonathan Lunine of the University of Arizona in Tucson. "I don't think we yet have a sufficiently global picture of Titan's crust to make an assessment," he says.

Cassini has been touring Saturn and its moons since 2004. The craft observed the tall mountains on Oct. 25, when it flew within 12,000 km of Titan. The passage was the mission's 22nd flyby of Titan, and Cassini is scheduled to make 23 more passes during the next 2 years.