Washington - It has methane rain and white crystalline sand dunes, and now scientists say Saturn's moon Titan also has many lakes.
The lakes are probably made up of methane, with a little ethane mixed in, and they are probably the source of the obscuring smog in the frigid moon's atmosphere, researchers reported on Friday.
"This is a big deal," said Steve Wall, deputy radar team leader at Nasa's Jet Propulsion Laboratory in Pasadena, California. "We've now seen a place other than Earth where lakes are present."
The numerous, well-defined dark patches were seen in radar images of Titan's high latitudes taken during a fly-by on Saturday by the Cassini-Huygens spacecraft, a co-operative project among Nasa, the European Space Agency and the Italian Space Agency.
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Titan's flat surface is very cold, with a temperature of minus 180°C, and scientists believe its thick atmosphere may occasionally rain methane.
Another team reported on Wednesday that the rain includes a persistent drizzle that keeps the surface of Saturn's largest moon damp. Fierce storms could produce huge droplets of methane.
Saturn has at least 47 moons. Titan, the largest, has geological features similar to those on Earth.
During the flyby, Cassini's radar spotted several dozen lakes, including one about 100km long.
"It was almost as though someone laid a bull's-eye around the whole north pole of Titan, and Cassini sees these regions of lakes just like those we see on Earth," said Larry Soderblom of the US Geological Survey in Flagstaff, Arizona.
Titan's dense, smoggy atmosphere makes it very difficult to get good visible images, so Cassini uses radar.
Dark regions in radar images generally mean smooth terrain, while bright regions suggest a rougher surface. Some of the new radar images show channels leading in or out of a variety of dark patches. The shape of the channels also strongly implies they were carved by liquid.
"We've always believed Titan's methane had to be maintained by liquid lakes or extensive underground 'methanofers,' the methane equivalent of aquifers. We can't see methanofers, but we can now say we've seen lakes," said Jonathan Lunine of the University of Arizona, Tucson.
The Cassini spacecraft was launched in 1997 and reached Saturn in 2004 after passing Venus and Jupiter. In May, Cassini's instruments spotted regions covered with dunes, possibly made out of methane ice crystals, and last year it saw what appeared to be a big lake on Titan's south pole.
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