Flowing rivers have carved deep canyons on Titan, the largest moon of Saturn, such as Vid Flumina at the top left of this image. The rivers are made of liquid methane and ethane, and they flow into lakes and seas (the dark features on the right of this image, made from radar observations by the Cassini spacecraft). Some of the canyons are quite steep, setting up the possibility that there could be ultra-cold rapids on the big moon. [NASA/JPL/ASI]