Ancient Martian riverbed

This image of the Martian surface, captured by the High-Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter (MRO), shows an ancient riverbed, marked out by a dark ridge meandering across the surface. The river which once carved its way across this scene in Mawrth Vallis flowed billions of years ago. The old riverbed, marked by the dark ridge, appears raised due to the erosion of softer material around it. These ridges are known as inverted channels, and many have been spotted in this particular area of Mars.