LOS ANGELES, March 14: Scientists for the first time have discovered what appear to be sea-size bodies of liquid on the surface of Saturn's largest moon, including one about as big as the Caspian Sea on Earth.

The discovery by the international Cassini spacecraft was welcomed by researchers, who have long theorised that Titan possessed hydrocarbon seas because of methane and other organic compounds in its thick, largely nitrogen atmosphere. Until now, Cassini had only spotted clusters of small lakes on the planet-size moon.

''They're very obvious. There's nothing subtle about them,'' said Cassini scientist Jonathan Lunine of the University of Arizona, Tucson.

Researchers using visual and radar imaging uncovered evidence of at least two seas on Titan's hazy north pole.

Cassini's camera last month imaged a large, irregular feature stretching 1,094 kilometres long with a surface area similar to the landlocked Caspian Sea. Its radar instrument swept over the feature's northern tip and determined it likely contains liquid methane or ethane because of its smooth appearance. However, scientists do not know whether the entire area is filled with liquid.

The spacecraft also discerned another body one-fifth that size. With a surface area of about 119,140 sq. kilometres, it is larger than Lakes Superior and Ontario combined, scientists said.

While there is no scientific definition of what constitutes a lake or sea on Titan, the newly found features are significantly larger than previously discovered bodies of liquid on the frigid moon and should be considered seas, Lunine said.

Results were presented on Tuesday at the annual Lunar and Planetary Science Conference in Texas.Titan is one of the few objects in the outer solar system that possesses a significant atmosphere, and scientists have long puzzled over its source. Methane is a flammable gas on Earth but liquid on Titan because of the moon's intense atmospheric pressure and cold.

Judging by their sizes and depths, Lunine said the newly discovered seas likely are not responsible for replenishing the long-term methane found in the moon's atmosphere. Instead, Lunine said that source likely comes from underground methane reservoirs that vent to the surface.

The discovery raises questions about why hydrocarbon pools are concentrated at Titan's poles, said Carolyn Porco, a Cassini imaging scientist from the Space Science Institute in Boulder, Colorado. It also gives researchers confidence that a feature the size of Lake Ontario spied on the moon's south pole two years ago may also be liquid-filled, she said.

In 2005, Cassini launched a probe that parachuted to the surface of Titan where it found evidence of an active world with liquid methane rain and a landscape of ridges, peaks and features formed by erosion.—AP