High-quality images of Titan, Saturn's mysterious moon, tantalized and bewildered scientists yesterday, and they warned that NASA's Cassini spacecraft may have to make several passes over the frozen world before it gives up its secrets.
"We've been saying that Titan was the solar system's last great mystery, and [now] the solar system has become a smaller place," said the Cassini imaging team's leader, Carolyn Porco of the Space Science Institute in Boulder, Colo. "But we are still mystified and not quite sure what we're looking at."
Cassini, flying to within 745 miles of Titan's surface, beamed back more than 500 images Tuesday and yesterday after finishing a 32-hour flyby of the smog-shrouded moon, the first of 45 close approaches the $3.3 billion spacecraft will make during a four-year exploration of Saturn and its environs.
The flyby also served as a scouting trip for Cassini's Christmas Eve tour de force, when it will release the European Space Agency's Huygens probe for a parachute drop into Titan's atmosphere. Huygens will transmit a steady stream of data and images until it dies an icy death on the moon's surface in temperatures 330 degrees below zero Fahrenheit.
Images of Huygens's target area showed bright and dark patches, prompting Huygens mission manager Jean-Pierre Lebreton to remark, "It looks like a very very, interesting place."
There appeared to be little dissent among the scientists gathered at NASA's Jet Propulsion Laboratory in Pasadena, Calif. The images transmitted by Cassini on Tuesday night and early yesterday offered both insights and intriguing puzzles about a celestial body whose thick hydrocarbon-and-nitrogen atmosphere has shielded it from prying eyes until now.
Titan is the second-largest moon in the solar system, after Jupiter's Ganymede, and the only moon with a significant atmosphere. It is a prime target for study because its composition suggests a "prebiotic" environment that is perhaps similar to Earth's at the dawn of geologic time 4.5 billion years ago.
During the flyby, Cassini, a joint venture between NASA, the European Space Agency and the Italian Space Agency, trained a radar at Titan, and two imaging spectrometers to analyze the moon's composition. Analysis of the data will continue over the next few days.
"The biggest surprise is the fact we're still not seeing craters or evidence of structures," said JPL's Torrence Johnson. The images suggested a smooth, perhaps flat, surface with light and dark patches.
The instruments suggested the patches were made of similar materials, perhaps from "some sort of a coating effect," said the University of Arizona's Robert Brown, leader of the Visual Infrared Mapping Spectrometer team.
Johnson said the atmosphere may be depositing a steady drizzle of small particles on the surface -- like pollen from trees -- which over time has cloaked the planet in a snowdrift-like blanket.
He noted that exploration of the solar system until now has focused -- with the exception of Venus -- on "airless," or nearly airless, planets and satellites whose surfaces light up brilliantly in the sun to show crags and craters.
"This is the first place we've looked at with atmosphere and precipitation," Johnson said in a telephone interview. "It's more like Earth."
But not that much like Earth: "There's sharp boundaries between the bright and dark regions, but there is no topography in our images," Porco said. "Everyhing could be perfectly flat. We just don't know."
Other images showed lines or streaks across flat surfaces that could be caused by wind or geologic forces, Porco said. "They could be cracks in bedrock ice," she added. "There's a pattern and a process going on."
Porco also said "the jury is out" on whether there are liquids on Titan's surface. Scientists had speculated that Titan might have lakes or oceans of liquid ethane, but Cassini found no evidence of either.
And while the images found a cloud over Titan's south pole, the rest of the moon showed clear, albeit smoggy, skies. "There's something else going on in that region that is very different," Brown said.
The researchers cautioned that analysis of the images was unlikely to provide definitive answers. "We're really glad we have another 40-plus opportunities," Johnson said.