The Cassini spacecraft orbiting Saturn's moon Titan has snapped a picture of the longest liquid river yet found in the solar system outside of Earth. And it's not water.
The river, which is over 400 kilometers (240 miles) long, is likely composed of hydrocarbon, probably methane, an explosive gas here on the comparably balmy Earth.
The latest image is not the first picture of a river on Titan, but it is the highest resolution image taken yet, and it shows the longest river seen so far.
The image clearly shows a network of dark lines that look very much like tributaries feeding a river, which winds down a mountain and spreads out in a delta, just like the Nile River in Egypt, before emptying into a much larger body.
"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," said 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."
Temperatures on the surface of Titan are around -180 degress Celcius, so cold that all water is rock-hard ice, and chemicals usually found as gases on Earth are cooled into their liquid form.
Titan, which is larger than Mercury, has its own "water cycle," much like the one here on Earth, only based on methane and a similar compound called ethane.
They evaporate into the atmosphere, form clouds of gas, rain back onto the surface, collect into pools and rivers, and flow down mountains and crevasses to form large lakes.
"Titan is the only place we've found besides Earth that has a liquid in continuous movement on its surface," said Steve Wall, the radar deputy team lead 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."