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The deepest known gorge in the solar system is Valles Marineris on Mars. It is known to be the deepest gorge on any rocky planet in the solar system, as we have already mapped all four of them in detail (Mercury, Venus, Earth, and Mars) and have not found anything deeper. Valles Marineris is 4,500 km (2,800 mi) long, 200 km (125 mi) wide and as deep as 7.7 km (4.8 mi). By comparison, the Grand Canyon is 446 km (277 mi) long, ranges in width from 6.4 to 29 km) (4 to 18 mi), and attains a depth of around 1.6 km (1 mi). The deepest crevice on our planet, Earth's wet and geologically active surface is not well-suited to the formation of extremely deep gorges.
The Marianas Trench, located in the Pacific Ocean near the Philippines, is 10.9 km, 6.77 mi below sea level, but less than half as deep measured relative to the sea floor, putting it short of the depth of Valles Marineris. Though still unknown, it may be that deeper gorges exist in the solar system in currently veiled or distant locations, such as the rocky cores of the gas giants, or outer solar system bodies such as Pluto or Eris. The New Horizons spacecraft is en route to Pluto and will arrive there in 2015, letting us map Pluto's surface in detail and determine the presence of any deep gorges.
The cause of formation of Valles Merineris is believed by most scientists to be a tectonic crack formed during planetary cooling, and subsequently widened due to erosion. If so, the gorge may be extremely old. Valles Merineris means valley of the mariner in Latin, named after Mariner 9, a NASA Mars orbiter which discovered the chasm in 1971. Valles Merineris shares the planet with the solar system's tallest mountain, Olympus Mons, several times taller than Mount Everest.
Valles Merineris may be a favorable colonization target for humans in the future. Due to its depth, the valley is shielded from cosmic rays and the intense heat of the surface.