After making the closest-ever flyby of mysterious Saturn moon Titan this week, the Cassini probe has sent back radar data that reveals some facts about the planet-like moon's surface.

NASA said the images show a very complex geological surface that may be relatively young. Until now, the surface was hidden behind thick, hazy atmosphere.

"Unveiling Titan is like reading a mystery novel," said Dr. Charles Elachi, director of NASA's Jet Propulsion Laboratory. "Each time you flip the page you learn something new, but you don't know the whole story until you've read the whole book."

Approximately 1 percent of Titan's surface was mapped during the flyby. The images show features down to 1,000 feet across on the northern hemisphere. A wide variety of geologic terrain types can be seen. There are bright areas that correspond to rougher terrains and darker areas that are thought to be smoother.

Optical imaging cameras on Cassini showed streaks on the surface. The streaking may be caused by movement of a material over the surface by wind, flowing hydrocarbon liquids, or a moving ice sheet like a glacier.

Titan's atmosphere is about four times more dense than Earth's.

On Dec. 24, Cassini is expected to drop the Huygens probe, which should enter the atmosphere Jan. 14, 2005.