Mars is the fourth planet from the Sun and a terrestrial planet with a rocky surface, a thin atmosphere, polar ice deposits, extensive volcanic terrain and a long geological history. It is often called the Red Planet because iron bearing minerals on its surface have oxidised, producing abundant reddish dust and rocks. Mars is considerably smaller than Earth and has lower surface gravity, approximately 38 percent of Earths gravitational acceleration.
Its atmosphere is very thin compared with Earths and consists primarily of carbon dioxide, with nitrogen and argon as important secondary constituents. Mars has two small irregular moons, Phobos and Deimos. It has polar caps containing water ice and seasonal carbon dioxide frost, while underground water ice is widespread in many regions. Evidence from geology, mineralogy and spacecraft observations indicates that ancient Mars once had environments where liquid water flowed or stood at the surface, although its present surface is generally cold and dry.
Mars has a differentiated interior consisting of a core, mantle and crust. The crust is dominated by basaltic volcanic rocks, while the mantle contains silicate minerals. Mars possesses enormous volcanic structures, including Olympus Mons, the largest known volcano in the Solar System by elevation relative to its surrounding plains, and extensive volcanic provinces such as Tharsis. The planet also contains Valles Marineris, a gigantic canyon system extending thousands of kilometres and vastly exceeding the dimensions of Earths Grand Canyon.
Martian geology records impacts, volcanism, tectonic deformation, erosion, sediment deposition and interactions with water and ice. Unlike Earth, Mars does not currently possess active global plate tectonics. Its interior has cooled more substantially because of its smaller size, although seismic activity still occurs. The InSight lander detected marsquakes and used seismic observations to investigate the planets internal structure.
Mars has a thin atmosphere with a surface pressure generally less than one percent of Earths average sea level pressure. Carbon dioxide dominates the atmosphere, and dust particles can remain suspended for long periods because the thin atmosphere produces unusual aerodynamic behaviour. Mars experiences dust devils and regional or global dust storms. Its atmosphere is too thin to provide Earth like insulation, so surface temperatures vary strongly with time and location.
The planet has seasons because its axial tilt is approximately 25 degrees, similar to Earths, although its orbital eccentricity is greater, producing a larger seasonal asymmetry between hemispheres. Carbon dioxide freezes at the poles during winter and sublimates during warmer seasons, producing seasonal changes in polar carbon dioxide ice. Water also exists as atmospheric vapour, surface or subsurface ice and minerals formed through ancient aqueous processes.
Evidence for ancient water is one of the most important aspects of Martian science. Spacecraft have identified dried river channels, valley networks, lake deposits, deltas, sedimentary rocks and minerals that form or alter in the presence of water. Some ancient terrains show extensive evidence of weathering by liquid water, while other regions appear to have been shaped by volcanic or dry processes.
Mars may once have possessed a thicker atmosphere capable of sustaining more stable surface liquid water, although the precise climate history remains uncertain. As the atmosphere thinned and the planet cooled, stable surface liquid water became increasingly rare. Water nevertheless persists today as polar ice, subsurface ice and possibly transient brines under particular conditions. Robotic missions have investigated rocks and sediments to determine the chemistry and environmental conditions of ancient habitats. The question of whether Mars ever hosted life is unresolved, and current research focuses on identifying environments that could have supported microbial life and preserving potential biosignatures.
Mars has a complex interaction with the Sun because it lacks a strong global magnetic field. Ancient Mars appears to have possessed a dynamo that generated a planetary magnetic field, but that dynamo ceased relatively early in Martian history. Remanent magnetisation preserved in ancient crust provides evidence for the former field.
Without a strong global magnetosphere, the upper atmosphere has been more directly exposed to the solar wind, contributing to atmospheric escape along with other processes driven by ultraviolet radiation and particle interactions. Atmospheric loss is one reason researchers investigate how Mars changed from a potentially wetter ancient environment to the cold and dry planet observed today. However, atmospheric evolution also depends on volcanic outgassing, surface mineral reactions, polar reservoirs, dust, impacts and other processes. The loss of atmosphere and water is therefore a long term planetary evolution problem rather than the result of a single event.
Mars has been explored by numerous spacecraft, including orbiters, landers and rovers. The Viking landers performed the first successful soft landings and biological experiments on Mars. Later missions such as Pathfinder, Spirit, Opportunity, Curiosity and Perseverance greatly expanded knowledge of Martian geology and surface chemistry. Curiosity has investigated Gale Crater and found evidence of ancient environments that could have supported microbial life. Perseverance has explored Jezero Crater, including an ancient river delta, and is collecting scientifically selected rock samples for potential future return to Earth. Orbiters provide global observations of mineralogy, topography, atmosphere and seasonal change. Mars exploration therefore combines remote sensing, geochemistry, geology, atmospheric science and astrobiology.
Mars is important to planetary science because it provides a relatively accessible example of a terrestrial planet that evolved differently from Earth. Both worlds have rocky surfaces, seasons, polar ice, atmospheres and evidence of ancient water, yet Mars became much colder and less atmospherically dense. Its lower gravity, smaller size, lack of present global plate tectonics and loss of its ancient dynamo contributed to a different evolutionary pathway. Mars also provides an important natural laboratory for studying planetary habitability.
Habitability does not mean that life is known to exist there. It refers to conditions potentially suitable for life, including sources of energy, appropriate chemistry and environments where liquid water could exist. The search for life therefore focuses on ancient environments, preserved organic molecules, mineral evidence and possible biosignatures rather than assuming that life has been found.
Mars has two tiny moons, Phobos and Deimos, whose origins remain under investigation. Phobos is particularly interesting because it is slowly spiralling inward and is expected eventually to break apart or collide with Mars on astronomical timescales. Mars also has an enormous elevation range and strong seasonal dust activity. Its atmosphere produces clouds made largely of water ice, and frost can form on the surface during cold periods.
The planet appears red from Earth because iron minerals have undergone oxidation and become incorporated into dust that is distributed widely across the surface. A common misconception is that Mars is permanently covered in red sand. Much of its surface is rocky, and the reddish appearance is caused primarily by fine iron rich dust and oxidised material. Mars remains one of the most intensively studied planets because it combines geological evidence of ancient water with a realistic possibility that its early environments could have been habitable for microbial life.
