The U.S. Cassini spacecraft's first look at Saturn's largest moon, Titan, is thrilling scientists and changing their ideas about what its surface is made of. Researchers are hoping to find water there, but the first glimpse does not show it.

Cassini passed 320,000 kilometers from Titan and saw what mission photographic interpreter Elizabeth Turtle calls spectacular sights.

"It's our first good look at Titan, and Titan hasn't disappointed us," she said. "It's different from anything we've ever seen before."

Titan is a planet-sized moon, bigger than Mercury and Pluto, with a hazy methane atmosphere that requires special filters on Cassini's camera to see the surface. Scientists are interested in Titan because U.S. Voyager satellites that flew by in the 1980s revealed organic material, and its atmosphere is thought to be similar to what Earth's was millions of years ago before life appeared and oxygen increased in it. It may even have a sea.

Mission scientists say they are disappointed that the first Cassini images do not show extremely bright spots that would signal the presence of water. Yet Ms. Turtle says the features they do see are intriguing because they hint of a body alive with geologic activity.

"We see some surface features that are circular. Others are linear," she said. "There are actually concentric features as well. The fact that we're not just seeing circular features on the surface suggests that it's not just a heavily cratered body, that there has been geologic activity on Titan. Now, we can't say what the geologic activity has been."

Titan's shading runs from light to dark, with some features sharply defined and others rather fuzzy looking. Ms. Turtle says the hazy atmosphere may be causing the fuzzy appearance of those ground features, or they may actually have softly defined boundaries.

Despite the moon's atmosphere, only a single cloud of methane vapor hovers near Titan's south pole.

"Right now, it's the southern summer and the south pole is actually illuminated 24 hours a day and that may be why there are clouds only in this part of the planet, because that's where the surface is being hearted the most," Ms. Turtle said.

Until Cassini sent back these photos from Titan, scientists had assumed its dark regions were areas where hydrocarbons had rained out of the methane atmosphere, with the lighter areas being regions of ice, which would reflect more light.

But the new images hint at something different. They show organic materials in the lighter areas mixed with ice, suggesting that the darker areas might where the pure ice is and not hydrocarbons after all.

"The data have turned the theories on their heads and we're still trying to find out what what actually the composition of the surface of Titan is," Ms. Turtle said.

Scientists expect to get many answers because the Cassini spacecraft will make 44 more of its orbits around Saturn near Titan, several within just 1,000 kilometers. In addition, the European Huygens craft attached to Cassini will separate in late December and plunge to a Titan landing.