NASA's Mars Reconnaissance Orbiter took this picture with its High Resolution Imaging Experiment camera (HIRISE) from about 166 miles above the planet. It shows an area called Aeolis Planum where rivers once flowed billions of years ago, when Mars had an atmosphere. The planet dried up when that atmosphere was stripped away, although water still exists in ice form at the poles. But wait- this picture doesn't look like river channels at all. It looks like those lines are jutting out of the ground!
This happened because the ancient rivers had rocks of all sizes flowing with the water. When the banks overflowed, the rivers would dump smaller sand and silt over the sides, but the heavier gravel and larger rocks remained in the channel. Once the rivers dried up, the silt and sand blew away from the banks, leaving a ridge of larger rocks in place where the river had been. So they are ridges now, but are evidence of ancient rivers. Scientists call these "inverted channels."
Read more about the image and the orbiter that took it at Mashable.
(Image credit: NASA/JPL-Caltech/University of Arizona)