A large system of canyons on Mars, centred at about –11 ° lat., 60 ° W long. It is the largest canyon in the Solar System, 3800 km long, up to 500 km wide, and over 4 km deep in places. It is really a complex of several parallel troughs, the central ones being the Ius, Melas, Coprates, and Eos Chasmata, with Tithonium, Ophir, Candor, and Gangis Chasmata to the north. Hebes and Juventae Chasmata are separate from the main complex, lying further north. They are all thought to have been caused by tensional faulting within Mars, and all have been modified and enlarged by the action of ancient water channels running down their walls. Valles Marineris, named after the Mariner 9 spacecraft that discovered it in 1971, is visible from Earth as the dark streak Coprates. Its darkness may be due to dust covering part of the canyon floor.