Valles Marineris (Latin for Mariner Valleys, named after the Mariner 9 Mars orbiter of 1971–72 which discovered it) is a system of canyons that runs along the Martian surface east of the Tharsis region. At more than 4,000 km long, 200 km wide and up to 7 km deep, the Valles Marineris rift system is one of the larger canyons of the Solar System, surpassed only by the rift valleys of Earth and (in length only) by Baltis Vallis on Venus.

Valles Marineris is located along the equator of Mars, on the east side of the Tharsis Bulge, and stretches for nearly a quarter of the planet’s circumference. The Valles Marineris system starts in the west with the Noctis Labyrinthus; proceeding to the east are Tithonium and Ius chasmata, then Melas and Ophir chasmata, then Coprates Chasma, then Ganges, Capri and Eos chasmata; finally it empties into an outflow channel region containing chaotic terrain that ends in the basin of Chryse Planitia. Most researchers agree that Valles Marineris is a large tectonic "crack" in the Martian crust that formed as the crust thickened in the Tharsis region to the west, and was subsequently widened by erosional forces. However, near the eastern flanks of the rift there appear to be some channels that may have been formed by water or carbon dioxide.

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