Close to the equator, centred on 70° W longitude, are several enormous interconnected canyons collectively called Valles Marineris. Individual canyons are roughly 200 km (125 miles) across. At the centre of the system, several canyons merge to form a depression 600 km (375 miles) across and as much as 9 km (5.6 miles) deep—about five times the depth of the Grand Canyon. The entire system is more than 4,000 km (2,500 miles) in length, or about 20 percent of Mars's circumference, almost the width of the United States. At several places within the canyons are thick, sulfate-rich sedimentary sequences, which suggest that lakes may have formerly occupied the canyons. Some of the lakes may have drained catastrophically to the east to form large outflow channels that start at the canyons' eastern end. In contrast to the Grand Canyon, which formed by erosion, the Valles Marineris formed mainly by faulting, although they have been enlarged by erosion.
Valles Marineris, the largest canyon system on Mars, shown in a composite of images taken by the Viking 1 and 2 orbiters. NASA/JPL/Caltech
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