Mars

Mars is the fourth planet outward from the Sun and one of the Solar System's rocky, or terrestrial, planets. Its radius is about half Earth's. A Martian rotation takes about 24.6 hours, while one orbit around the Sun takes about 687 Earth days. Its tilted axis gives it seasons, though they last longer than Earth's because the Martian year is longer. The planet has two small, irregularly shaped moons, Phobos and Deimos. Iron-bearing minerals in Martian rocks and dust oxidize, giving much of the surface its familiar reddish appearance.

Mars has a thin atmosphere composed mostly of carbon dioxide, with nitrogen and argon among its other gases. Its landscape includes impact craters, enormous volcanoes such as Olympus Mons and the canyon system Valles Marineris. Landforms such as ancient river valleys, deltas and lakebeds, together with minerals formed in water, are evidence that liquid water once flowed or collected on the surface. Water is present today chiefly as ice, including at the poles and beneath the surface; the low atmospheric pressure makes persistent liquid water on the open surface difficult. Dust and weather continue to modify the landscape.

Mars preserves evidence useful for studying how rocky planets evolve. Its surface records impacts, volcanic activity and ancient water-related processes, while its thin atmosphere and present cold, dry conditions contrast with evidence for a warmer, wetter past. Orbital observations, surface geology and atmospheric measurements address how that transition occurred and what conditions might once have been habitable. The question of life remains open. A 2025 peer-reviewed study of the Perseverance rover's “Sapphire Canyon” core, collected from the Cheyava Falls rock in Jezero Crater, described mineral and organic associations as potential biosignatures. NASA and the researchers stressed that a potential biosignature is not proof of life; further study, including analysis with laboratory instruments on Earth, is needed to determine the materials' origin. Earlier, NASA's Viking landers found no clear evidence of living microorganisms in soil near their landing sites.

Knowledge of Mars has grown from telescopic observation to robotic missions using flybys, orbiters, landers and rovers. NASA's Mariner 4 returned the first spacecraft images of Mars during a flyby in 1965. In 1976, Viking 1 became the first U.S. mission to land safely on the planet and send images from its surface. Later international orbiters and rovers have mapped terrain, analyzed rocks and soils, and measured the atmosphere. ESA's Mars Express, in orbit since 2003 and conducting science operations from 2004, has mapped atmospheric chemistry and investigated the planet's history of water. These missions help scientists assess Mars as a world in its own right and plan future exploration.