Mars has a canyon system about 4,000 kilometers long, roughly ten times longer than the Grand Canyon.

Valles Marineris is a vast system of canyons near Mars's equator. It is about ten times longer than Arizona's Grand Canyon and several times deeper. If it were placed on Earth, it would stretch roughly from New York to Los Angeles. On Mars, which is smaller than Earth, it wraps around about a fifth of the planet.

Unlike the Grand Canyon, which was carved mainly by the Colorado River, Valles Marineris formed chiefly as the crust cracked and pulled apart. Scientists link it to the Tharsis region next door, a huge volcanic bulge that includes Olympus Mons. As Tharsis grew, stresses in the crust opened enormous rifts, which later widened through landslides, collapse and possibly some water erosion.

Its sheer scale means that from the canyon floor, the far wall would often lie beyond the horizon.

The canyon was discovered by NASA's Mariner 9, the first spacecraft to orbit another planet, which reached Mars in 1971. Its name, Mariner Valleys in Latin, honours that mission. Later orbiters, including Viking in the 1970s and modern spacecraft such as Mars Reconnaissance Orbiter and Mars Express, have mapped it in remarkable detail.

Elevation data from the Mars Global Surveyor laser altimeter measured the canyon's depth and shape across its length. High resolution images reveal layered rock in its walls, giant landslide deposits and minerals that formed in water, giving clues to Mars's wetter past. Some of these layers record billions of years of the planet's history.

Earth has rift valleys too, such as the East African Rift, where the crust is slowly pulling apart. Valles Marineris resembles a rift, but on a much larger scale.

Studying it helps scientists understand how planetary crusts break under stress, and how different the outcome can be on a world without plate tectonics.

Mars holds several solar system records. Olympus Mons is the tallest known volcano, and Valles Marineris is among the largest canyons. Both are linked to the Tharsis region, a raised volcanic area so massive it may have shifted the planet's crust.

Mars probably lacks moving tectonic plates like Earth's. Without plates to carry crust away, volcanoes could keep growing over the same hot spots for very long periods, building features far larger than anything on Earth.

The canyon system is not one single trench. It is a network of connected chasms, with names such as Ius, Melas, Candor and Coprates Chasma, joined along much of its length. At its western end lies Noctis Labyrinthus, a maze of intersecting valleys. At its eastern end, the chasms open into chaotic terrain and broad outflow channels, where huge floods may once have poured across the Martian surface toward the northern lowlands.