Valles Marineris is also known as Mariner Valleys. It has been named after the Mariner 9 Mars Orbiter of 1971-72 which discovered it.
It is a system of canyons which runs along the Martian surface east of the Tharsis region. Around 4000 km (2500 mi) long, 200 km (120 mi) wide and up to 7 km (23,000 ft) deep, it is one of the largest canyons of the Solar System, surpassed in length only by the rift Valleys of Earth.
Valles Marineris is situated 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 canyon system starts in the west with Noctis Labyrinthus; proceeding to the east are Tithonium and lus chasmata, then Melas, Candor and Ophir chasmata, then coprates chasma, then Ganges, Capri and Eos chasmata; finally it empties into an outflow channel region containing chaotic terrain which ends in the basin of chryse planitia.
Recently, it has been suggested that Valles Marineris is a large tectonic crack in the Martian crust. Many researchers claim that this formed as the crust thickened in the Tharsis region to the west, and was subsequently widened by erosion. Near the eastern flanks of the rift, there appear to be channels which may have been formed by water or carbon dioxide. It has also been said that Valles Marineris is a large channel which has been formed by the erosion of lava flowing from the flank of Pavouis Mons.