Scientists released what they called the best pictures yet of the frozen surface of Saturn's enormous moon Titan but said they were puzzled that the Cassini spacecraft hadn't glimpsed any evidence of liquids.
The latest images of Titan revealed a single set of clouds about the size of Poland and dark and light shapes across the moon that the imaging team continued to analyse.
The shots of the moon's surface features were taken during Cassini's first pass Friday at a distance of about 200,000 miles.
"It's different from anything we've ever seen before," imaging scientist Elizabeth Turtle said Saturday. "We're still trying to understand the surface of Titan."
One indistinct circular shape could be a huge crater, and a linear shape could be a hydrocarbon river or a fault line, scientists said.
Scientists believe the moon could have chemical compounds much like those that existed on Earth billions of years ago before life appeared.
Big enough to be a planet in its own right, Titan has an atmosphere 1 1/2 times as dense as Earth's, containing organic _ meaning carbon-based _ compounds. Scientists believe there could be hydrocarbon seas or lakes.
{{/usCountry}}Big enough to be a planet in its own right, Titan has an atmosphere 1 1/2 times as dense as Earth's, containing organic _ meaning carbon-based _ compounds. Scientists believe there could be hydrocarbon seas or lakes.
{{/usCountry}}Turtle said initial data analysis suggested the moon is the site of some type of geologic activity that could include wind and erosion and development of the lakes or rivers.
Kevin Baines, a member of the visual and infrared spectrometer team, said scientists were disappointed that they hadn't seen evidence of liquids through reflections of sunlight on smooth surfaces of the moon.
"We thought we'd see some flashes, and we haven't seen any. So we're a little perplexed," he said after a news conference at NASA's Jet Propulsion Laboratory in Pasadena.
Among the new pictures released were four images of a cluster of clouds near Titan's south pole that are believed to be composed of methane. They were the only brightly distinct spots on otherwise fuzzy images of Titan.
"Someone likened it to a melting ice cream sundae," Turtle said.
Near-infrared images of Titan prompted scientists to question previous theories about the moon's surface. They had thought brighter areas of the moon were thick layers of ice and darker areas were a mixture of substances including hydrocarbons, but they said it appears the opposite is true.
There will be many more chances to uncover the face of Titan during Cassini's planned four-year tour. The spacecraft will make 45 more flybys of the moon — coming within 600 miles of Titan at times — and then send a probe into its atmosphere in January.
The probe, named Huygens, will send pictures back to Cassini as it makes a 2 1/2-hour descent by parachute through the atmosphere. Titan was Cassini's first encounter since the spacecraft began orbiting Saturn earlier this week.
The $3.3 billion mission, funded by NASA, the European Space Agency and the Italian Space Agency, was launched in 1997. The spacecraft flew 2.2 billion miles on a roundabout route to Saturn.