The Cassini probe has captured the clearest picture ever of a river beyond our Earth - a huge, cold river of liquid methane or ethane on Saturn's icy moon Titan.
Scientists describe the 250-mile river valley as like an 'extraterrestrial version of the Nile'.
It is the first time images have revealed a river system this vast and in such high resolution anywhere beyond Earth.
Scientists deduce the river is filled with liquid because it appears dark in the high-resolution radar image, indicating a smooth surface.
[Related: Gene find hints why homosexuality runs in families]
Titan is the only other world we know of that has stable liquid on its surface - icy seas of hydrocarbons such as ethane and methane.
“Though there are some short, local meanders, the relative straightness of the river valley suggests it follows the trace of at least one fault line, 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, USA.
“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.”
Images from Cassini’s visible-light cameras in late 2010 revealed regions that darkened after recent rainfall.
Cassini’s visual and infrared mapping spectrometer confirmed liquid ethane at a lake in Titan’s southern hemisphere known as Ontario Lacus in 2008.
“This radar-imaged river by Cassini provides another fantastic snapshot of a world in motion, which was first hinted at from the images of channels and gullies seen by ESA’s Huygens probe as it descended to the moon’s surface in 2005,” says Nicolas Altobelli, ESA’s Cassini Project Scientist.