This HiRISE image shows a part of Nanedi Vallis, one of the Martian valley networks. The valley networks are thought to have formed by flowing water in the distant past when the climate on Mars was warmer and wetter than it is today.
Some scientists have suggested that the valley networks could have been produced in a climate like the dry, cold one of Mars today if the liquid water was protected by an overlying ice layer. Others think that glacial activity may be responsible for them, but the majority believe that the valley networks are evidence of ancient flowing water.
Valley networks are characterized by their blunt, theater-shaped heads and their approximately constant width along their reaches. They often have tributaries, as seen in this image, that connect with the main trunk of the valley.
Photo credit: NASA/JPL/University of Arizona