NASA's Cassini spacecraft has spotted a river valley on Titan that looks a lot like the Nile - the largest river system ever seen anywhere but Earth.
Stretching more than 200 miles across the north polar region, the river ends up in a large sea. The team believes it's filled with liquid hydrocarbons as it appears dark along its entire length, indicating a smooth surface.
"Though there are some short, local meanders, the relative straightness of the river valley suggests it follows the trace of at least one fault, similar to other large rivers running into the southern margin of this same Titan sea," says Jani Radebaugh, a Cassini radar team associate at Brigham Young University.
"Such faults - fractures in Titan's bedrock - may not imply plate tectonics, like on Earth, but still lead to the opening of basins and perhaps to the formation of the giant seas themselves."
Titan is the only other body where stable liquid has been observed on the surface, but has a 'methane cycle' based on hydrocarbons rather than water. The effects of rainfall were spotted in 2010, and Cassini has also identified tropical lakes.
"Titan is the only place we've found besides Earth that has a liquid in continuous movement on its surface," says Steve Wall, the radar deputy team lead, based at NASA's Jet Propulsion Laboratory.
"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."