Titan River System: NASA's Cassini Spacecraft Finds Flowing Liquid On Saturn's Largest Moon (PHOTO)

Saturn's biggest moon, Titan, has served up a big surprise: a vast river system more than 200 miles long.

The river -- visible in a spectacular photo taken by NASA's Cassini space probe -- has drawn comparisons to the Nile River. It has its own "headwaters" and tributaries, eventually emptying into a vast sea known as Kraken Mare (which covers about 154,000 square miles, or five times the surface area of Lake Superior, according to Space.com.

Unlike the Nile, however, Titan's river carries no water. Scientists believe it's full of hydrocarbons like ethane and methane.

"Titan is the only place we've found besides Earth that has a liquid in continuous movement on its surface," Steve Wall, the radar deputy team lead at NASA's Jet Propulsion Laboratory, said in a written statement. "This picture gives us a snapshot of a world in motion. Rain falls, and rivers move that rain to lakes and seas, where evaporation starts the cycle all over again. On Earth, the liquid is water; on Titan, it's methane; but on both it affects most everything that happens."

In 2010, Cassini observed patches of Titan's surface that darkened after a rainfall, according to a report from the European Space Agency.

PHOTO of Titan's vast river system:

CORRECTION: An earlier version of this story mistakenly attributed the Cassini spacecraft to the European Space Agency (ESA). While there's a partnership between NASA, the ESA, and ASI for the Cassini mission, the spacecraft itself belongs to NASA.