NASA's Mariner 9 mission rewrote the book on Mars by discovering channels, polar layered materials, shield volcanoes, sand dunes, and an enormous canyon system. In this Mariner 9 image, a series of massive landslides are visible on the canyon floor. The older (bottom-most) landslide has longitudinal ridges which formed parallel to flow direction, while the younger slides have concentric ridges. Dunes are also visible near the bottom of the image.
Image by NASA/JPL/Arizona State University