The discovery, based on photos from the international Cassini spacecraft, is being welcomed by researchers, who have long theorized that Titan possessed hydrocarbon seas because of methane and other organic compounds in its thick, largely nitrogen atmosphere. Until now, Cassini had spotted only 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 spied a large, irregular feature stretching 680 miles long with a surface area similar to Asia's landlocked Caspian Sea. Its radar instrument swept over the feature's northern tip and determined that 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 the size of Titan's "Caspian Sea." With a surface area of about 46,000 square miles, it is larger than Lakes Superior and Ontario combined, scientists said.
While there's 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 Tuesday at the annual Lunar and Planetary Science Conference in Texas.
Titan is one of the few objects in the outer solar system with a significant atmosphere, and scientists have long puzzled over its source. Methane is a flammable gas on Earth but is liquid on Titan because of the moon's intense atmospheric pressure and cold.
Judging by their sizes and depths, the newly discovered seas likely are not responsible for replenishing the long-term methane found in the moon's atmosphere, Lunine said. Instead, the source likely is underground methane reservoirs that vent to the surface.
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.
Cassini, on a mission to study the ringed planet and its many moons, is a project of NASA, the European Space Agency and the Italian Space Agency. The spacecraft is managed by NASA's Jet Propulsion Laboratory in Pasadena.
By Alicia Chang