Valles Marineris, a network of canyons that dwarfs Earth's Grand Canyon, forms a deep slash across the center of Mars in this mosaic of images from the Viking 1 orbiter. The canyon stretches almost 3,000 miles, which is the length of the United States. It is miles deep and up to dozens of miles wide. It is named for the Mariner 9 spacecraft, which first photographed the feature in the 1970s. [NASA/JPL]