Despite its modest size, Mars has some stupendously large landscape features. Its largest impact basins, volcanoes, and canyons are far bigger than any found on Earth.
The maps to the right illustrate the topography of Mars. The difference in elevation between the southern highlands and the northern lowlands exceeds 6 kilometers (3.7 miles) in some areas — comparable to the elevation difference between Earth's continents and ocean floors. Why such a vast northern lowland formed within a ring of ancient cratered highlands remains a mystery.
Mars has some of the largest impact basins in the solar system. They formed when asteroids collided with Mars, ejecting material over much of the planet. The Hellas Basin, the largest exposed impact feature on Mars, is about 9 kilometers (5.6 miles) deep and 2,300 kilometers (1,430 miles) across. Utopia Planitia and Isidis Planitia are the smooth-floored remnants of other large ancient basins.
The Tharsis Rise is the largest volcanic and tectonic feature on Mars. Here crustal materials have been pulled and broken apart to form troughs, and pushed together and buckled to form ridges. Some Tharsis landforms, such as the Olympus Mons volcano, 26 kilometers (15.5 miles) high, and the Valles Marineris canyon system, are the largest in the solar system. Valles Marineris, named after the Mariner 9 probe that first photographed it, is up to 8 kilometers (5 miles) deep and 4,500 kilometers (2,800 miles) long. The Grand Canyon could easily fit into one of its side canyons.
Mars Global Surveyor, Mars Orbiter Laser Altimeter