On alien moon, hints of early Earth?

While imaging scientists have seen interesting features on the surface - what could be long fault scars, shorelines, or collections of what look like chains of hydrocarbon lakes - they haven't picked up clear evidence of changing terrain that would help them draw firmer conclusions.

So far, the sun has been behind Cassini when cameras have been trained on the moon. This orientation and the light-scattering haze pervading the thick atmosphere have eliminated surface shadows that might hint at topography. And so far, Cassini's radar-map coverage has not dovetailed with parts of the surface other instruments have observed. So researchers are having a tougher time than they anticipated figuring out what their data mean.

In short, the moon's surface has remained something of a Rorschach test for planetary scientists.

Cassini's first close flyby of Titan Oct. 26 nevertheless yielded several surprises, notes Bonnie Buratti, a planetary scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. Dr. Buratti, a member of the team using the orbiter's visual and infrared mapping spectrometer, notes that so far, Titan appears to be virtually free of impact craters, suggesting that the surface is geologically young.

In addition, recent radar images show "what looks to geologists like a huge volcano," whose flanks are likewise free of impact craters, she says.

Researchers also have spotted wispy short-lived clouds at mid-latitudes, far from the larger cloud formations that seem to hug the moon's southernmost region at this time in its solar year. Some think the clouds are signs of seasonal change, while others suggest they could be plumes from icy volcanic action on the surface. When Voyager 2 hurtled past Neptune and its moon Triton in 1989, the craft's cameras recorded geysers spewing nitrogen gas and dust high into the moon's atmosphere. The same process could be at work on Titan, she speculates.

Absent from the images so far is evidence of liquid oceans or lakes.

The prospect that portions of Titan's surface are liquid received a boost in 2003 when a team led by Cornell University astronomer Donald Campbell reported that it had detected radar reflections consistent with mirror-like glints from liquid hydrocarbons.

But so far, Cassini has failed to pick up equivalent glints in the wavelengths its instruments are using. That could mean the standing bodies of liquid hydrocarbons aren't there; that they exist but are covered with organic scum built up over millions of years; or that they're trapped in the ground.

"The lack of indications so far doesn't demonstrate a lack of existence," cautions Ralph Lorenz, a University of Arizona planetary scientist working with Cassini and Huygens.

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