NASA's Cassini spacecraft has found evidence of huge seas -- one of them bigger than any of North America's Great Lakes -- on Saturn's largest moon, scientists said on Tuesday.

Scientists studying the images taken by the probe, which blasted off a decade ago, said the seas on Titan were likely filled with liquid methane or ethane and that the discovery reinforced previous theories.

"We've long hypothesized about oceans on Titan, and now with multiple instruments we have a first indication of seas that dwarf the lakes seen previously," said Jonathan Lunine, a University of Arizona scientist who works on Cassini data.

Scientists at the U.S. space agency said Cassini's radar instruments captured several very dark features near Titan's north pole. The largest measures at least 100,000 square km (39,000 square miles).

Titan is the second-largest moon in the solar system, after Jupiter's Ganymede, and is about 50 percent larger than the Earth's moon.

NASA said although there was no proof the seas contain liquid, their shape and dark appearance on radar indicates smoothness.

The liquid is probably methane or ethane because those compounds are abundant in clouds in Titan's atmosphere.

The scientists said the presence of the seas reinforced current thinking that Titan's surface must be resupplying methane to its atmosphere.

Because of the new images, the Cassini mission team is repositioning the spacecraft's radar instruments during a May fly-by so it can pass directly over the dark areas seen by the cameras.

Cassini was launched in October 1997 and entered into orbit around Saturn in July 2004. The mission is a project between NASA, the European Space Agency and the Italian Space Agency.