NASA's Cassini spacecraft has found evidence of the long-sought seas of Titan, planetary scientists announced Tuesday.
Several instruments aboard the craft, which has been orbiting Saturn and its largest moon for the last two years, have identified large, dark features at the moon's north polar region. The new areas are flat and undifferentiated, several hundred miles across and have sharp, shoreline-type features.
Cassini scientists said the latest images were still short of "smoking gun" proof but represented the best evidence so far that there were large bodies of liquid, probably methane and ethane, on the surface of the solar system's second largest moon.
Large seas "are the simplest explanation that fits the data we've seen so far," said Jonathan Lunine, an astronomer and Cassini scientist at the University of Arizona.
Scientists have been looking for seas or oceans of methane on Titan since NASA's Voyager mission 25 years ago found evidence of a smog-choked atmosphere around the moon. There must be some source for the hydrocarbon muck surrounding the planet, they theorized, or else the atmosphere would have disappeared billions of years ago.
Scientists originally speculated that there must be a global ocean of methane. But Cassini and its Huygens probe, which landed on the surface of the moon, found nothing resembling large bodies of liquid. Huygens took bizarre pictures of ice balls strewn across an orange landscape -- but no liquid.
Some radar images indicated the possible presence of lakes, but nothing large enough to fill Titan's skies with hydrocarbon aerosols. As the hunt progressed, some scientists began wondering whether the methane was coming from volcanic eruptions.
Perhaps, they reasoned, Titan was a refrigerated sand dune with a gooey center like a candy bar.
In recent weeks, however, Cassini focused its radar and infrared mapping instruments on Titan's north pole. Vast dark regions started unfolding. One of the spacecraft's cameras, which can take pictures of the entire moon, found a large, irregular dark feature corresponding to the radar image. It stretches over an area 600 miles across.
The largest possible body of liquid is being compared to the Caspian Sea, Earth's largest lake at about 143,000 square miles.
Aside from the large sea, Cassini's instruments have turned up a number of small lakes in the extreme northern hemisphere, scientists said.
As a result of the new findings, scientists leading the Cassini mission, based at the Jet Propulsion Laboratory in La Canada Flintridge, plan to reposition the spacecraft's radar and other instruments during a May flyby of the moon.