Scientists with the Cassini-Huygens mission have just announced they identified a river on Saturn's moon Titan. The river appears in radar images taken in September of this year by the Cassini spacecraft, a joint project run by NASA and the European and Italian Space Agencies. The extraterrestrial river extends 250 miles through the moon's north polar regions before it drains into a large sea called the Ligeia Mare. Since the river's course is relatively straight, scientists say it likely occurs along a fault line. The river valley appears dark in the radar image---an indication of a smooth surface---so scientists suggest the river is actually flowing. The liquid between its banks isn't water, though. In contrast to Earth's water-based hydrologic cycle, Titan's cycle operates instead on hydrocarbons such as ethane and methane. Titan is the only other world scientists know to have stable liquid on its surface. Image courtesy of the European Space Agency.