This image was taken by the space probe Cassini, and shows what appears to be a massive river system on Saturn's moon Titan. The European Space Agency reports that it flows 400 km across the cloudy moon's surface, where it meets a large sea. This is most likely a river made of liquid ethane or methane, not water. Though previous Cassini observations of Titan revealed what appeared to be seasonal lakes there, this is the first confirmation we've had that the moon also has long, meandering rivers that form tributaries just like water does on Earth.
Here is the full image. Click to enlarge.
According to the ESA:
Scientists deduce that the river is filled with liquid because it appears dark along its entire extent in the high-resolution radar image, 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, 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."
Titan is the only other world we know of that has stable liquid on its surface. While Earth's hydrologic cycle relies on water, Titan's equivalent cycle involves hydrocarbons such as ethane and methane.
Images from Cassini's visible-light cameras in late 2010 revealed regions that darkened after recent rainfall.
Scientists have speculated that the fluid in Titan's waterways might be viscous, like oil or even tar.