This image from NASA's Cassini spacecraft shows a vast river system on Saturn's moon Titan. The image was acquired on Sept. 26, 2012, on Cassini's 87th close flyby of Titan. It stretches more than 200 miles. 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. That liquid is presumably ethane mixed with methane.