NASA scientists have spotted thelongest extraterrestrial river system ever - on Saturn's moon Titan - and it appears to be a miniature version of Earth's Nile river.

The river valley on Titan stretches more than 400 kilometres from its "headwaters" to a large sea, a NASA Jet Propulsion Laboratory statement said.

In comparison, the Nile river on Earth stretches about 6,700 kilometres.

Images by NASA's Cassini mission have revealed for the first time a river system this vast and in such high resolution anywhere other than Earth.

Titan is known to have vast seas - the only other body in the solar system, apart from Earth, to possess a cycle of liquids on its surface.

However, the thick Titan atmosphere is a frigid one, meaning liquid water couldn't possibly flow. The liquids on Titan are therefore composed of hydrocarbons such as methane and ethane, Discovery News reported.