Mars

Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury. Commonly known as the "Red Planet" because of the iron oxide, or rust, that gives its surface a distinctive reddish hue, Mars is a cold, rocky terrestrial world with a thin carbon-dioxide atmosphere, two small natural satellites, and a geological record that points to a far wetter and warmer past. Owing to its proximity, accessibility, and intriguing history, Mars has become the most intensively explored planetary body beyond the Earth and the leading focus of astrobiological research and human spaceflight planning.

Etymology and Cultural Background

The planet is named after Mars, the ancient Roman god of war, corresponding to the Greek god Ares. This martial association arose early in antiquity, when the planet's reddish colour was linked to blood and conflict by observers in many civilizations. Egyptian astronomers recorded Mars as "Har Decher," the Red One, while Babylonian scribes associated it with Nergal, their deity of destruction and plague. In Chinese astronomy it was known as Yinghuo (火星), the "fire star," and in Vedic literature as Mangala. The astronomical symbol for Mars, ♂, depicts a shield and spear and has also entered general usage as a symbol of the male sex.

History of Observation

Mars has been observed since prehistoric times because it is easily visible to the unaided eye, reaching a brightness that occasionally rivals the brightest stars. Ancient and medieval astronomers, notably in Babylon, Greece, India, China, and the Islamic world, carefully tracked its motion, including its puzzling periods of apparent retrograde movement across the sky. The Ptolemaic geocentric model accounted for this behaviour with elaborate epicycles, but the puzzle of Mars's motion proved decisive for astronomy: in the early seventeenth century, Johannes Kepler used Tycho Brahe's precise observations of Mars to demonstrate that planetary orbits are ellipses rather than circles, publishing his laws of planetary motion in 1609.

Telescopic study began with Galileo Galilei in 1610. Christiaan Huygens drew the first identifiable surface feature, the dark region now called Syrtis Major, in 1659 and estimated the planet's rotation period. Giovanni Domenico Cassini reported the polar ice caps in 1666, and William Herschel in the 1780s measured the planet's axial tilt and argued for the presence of a thin atmosphere. In 1877, the American astronomer Asaph Hall discovered the two moons, Phobos and Deimos, from the United States Naval Observatory. That same year, the Italian astronomer Giovanni Schiaparelli drew a network of linear features he called "canali" (channels). mistranslated into English as "canals," these features inspired Percival Lowell and others to speculate about an intelligent Martian civilization, speculation that persisted in popular culture until spacecraft visits in the 1960s and 1970s showed the "canals" to be optical illusions.

Physical Characteristics

Mars has an equatorial radius of about 3,390 kilometres, roughly half that of the Earth, and a mass of about 6.42 × 10²³ kilograms, or approximately 10.7 percent of Earth's mass. Its surface gravity is about 3.72 metres per second squared, some 38 percent of terrestrial gravity. The planet orbits the Sun at an average distance of roughly 228 million kilometres (about 1.52 astronomical units) and completes one revolution in 687 Earth days. Its orbit is noticeably eccentric, which produces significant variation in the amount of sunlight received over the Martian year and the cycle of global dust storms.

Mars rotates on its axis once every 24 hours, 39 minutes, and 35 seconds—the length of time known as a "sol"—making its day remarkably similar to Earth's. Its axial tilt of about 25.2 degrees closely resembles Earth's 23.4-degree tilt, so Mars experiences Earth-like seasons, though each lasts roughly twice as long and is tempered by the planet's thin atmosphere. Surface temperatures average about −63 °C but range from approximately −143 °C at the winter polar caps to around 20–35 °C on summer afternoons at the equator.

Geology and Surface Features

The Martian surface displays a striking hemispheric dichotomy: the northern hemisphere consists largely of smooth, low-lying volcanic plains, while the southern hemisphere is an ancient, heavily cratered highland terrain billions of years old. The largest known impact structure on the planet, the Hellas basin, spans roughly 2,300 kilometres and reaches depths of about 7 kilometres.

Mars hosts the largest volcano in the Solar System, Olympus Mons, a shield volcano rising about 22 kilometres above the surrounding plains with a base nearly 600 kilometres across. It is part of the Tharsis region, a vast volcanic plateau also bearing the giant volcanoes Ascraeus, Pavonis, and Arsia Mons. Equally dramatic is Valles Marineris, a canyon system stretching over 4,000 kilometres—nearly a quarter of the planet's circumference—and plunging as deep as 7 kilometres, far exceeding any canyon on Earth.

Abundant evidence indicates that liquid water once flowed on the surface. Dendritic valley networks, massive outflow channels, layered sedimentary deposits, and hydrated clay and sulfate minerals all point to sustained aqueous activity, particularly during the Noachian period more than 3.5 billion years ago. The delta deposits within Jezero crater, now explored by NASA's Perseverance rover, record an ancient river-fed lake. Water ice survives today in the polar caps—which also contain frozen carbon dioxide—and in large deposits beneath the surface at mid-to-high latitudes. Radar observations have also suggested the possible presence of briny liquid water beneath the south polar cap, though this interpretation remains debated.

Atmosphere and Climate

The Martian atmosphere is very thin, exerting an average surface pressure of only about 6 millibars—less than 1 percent of Earth's sea-level pressure. It consists predominantly of carbon dioxide (roughly 95 percent), with nitrogen, argon, oxygen, and trace gases making up the remainder, along with variable amounts of water vapour and suspended dust. Because the atmosphere cannot retain heat effectively, temperature swings between day and night are extreme.

Dust plays a dominant role in Martian weather. Regional dust storms occur frequently, and at times dust encircles the entire planet, as happened in 2018, blanketing the surface and blocking sunlight for weeks. The planet also exhibits water-ice clouds, ground fog, and seasonal frost. Over long timescales, changes in Mars's orbital and rotational parameters drive ice ages, redistributing polar ice across the surface, while the loss of most of its original atmosphere—likely tied to the shutdown of its protective global magnetic field early in its history—has left the modern climate far colder and drier than the ancient one.

Moons

Mars possesses two small moons, Phobos and Deimos, both discovered in 1877. Phobos, the larger, measures about 22 kilometres across and orbits so close to Mars— completing a revolution in under eight hours—that it rises in the west and sets in the east twice each Martian day. Tidal forces are drawing Phobos slowly inward, and calculations suggest it will either break apart into a ring or crash into the planet within roughly 50 million years. Deimos, about 12 kilometres across, orbits more distant ly in roughly 30 hours. Both moons are dark and irregularly shaped, and their origin remains uncertain: they may be captured asteroids, or they may have formed from debris of a giant impact on Mars, a hypothesis supported in part by recent close orbital observations.

Exploration of Mars

Spacecraft exploration began in the 1960s. NASA's Mariner 4 executed the first successful flyby in July 1965, returning images of a cratered, apparently Moon-like surface that dampened expectations of a habitable world. Mariner 9, entering orbit in 1971, mapped the entire planet and revealed the giant volcanoes and canyons. The Viking landers of 1976 achieved the first successful soft landings and operated for years, conducting the first on-site biology experiments.

After a lull marked by several high-profile failures, exploration resumed vigorously with Mars Pathfinder and its small rover Sojourner in 1997, followed by the long-lived rovers Spirit and Opportunity, which landed in 2004 and found strong mineralogical evidence of ancient water. The Phoenix lander confirmed near-surface water ice in 2008, and the Curiosity rover, delivered by the Mars Science Laboratory mission in 2012, documented an ancient habitable lake environment in Gale crater. The InSight lander (2018–2022) studied the planet's interior and recorded hundreds of marsquakes, allowing scientists to estimate the size of the Martian core. In February 2021, a remarkable set of arrivals included NASA's Perseverance rover—which is caching rock samples for a future return to Earth—and the Ingenuity helicopter, which performed the first powered, controlled flights on another planet, along with the United Arab Emirates' Hope orbiter and China's Tianwen-1 mission, whose Zhurong rover operated successfully on the surface. Europe, India, and Japan have also sent or are planning missions, and NASA and the European Space Agency continue to develop a Mars Sample Return campaign, while several national agencies and private companies study technologies for eventual crewed expeditions.

Habitability and the Search for Life

The central scientific question motivating Mars exploration is whether life has ever arisen there. The Viking landers reported ambiguous biological results in 1976, and the 1996 announcement that the Martian meteorite ALH84001, found in Antarctica, might contain fossil traces of microbial life proved highly controversial but galvanized planetary science. Subsequent findings—a formerly habitable environment at Gale crater, seasonal variations in atmospheric methane of uncertain origin, and traces of organic molecules preserved in ancient rocks—sustain interest without providing proof. Most researchers consider subsurface environments, where liquid water may persist and conditions are shielded from radiation, the most promising places to search. Strict planetary-protection protocols govern missions to avoid contaminating potential biological niches.

Significance and Cultural Impact

Mars holds a special place in science and society. For geologists and climatologists, it serves as a natural laboratory of comparative planetology: a world that once resembled the early Earth but followed a different evolutionary path, offering insights into climate change, planetary magnetism, and habitability in general. For biology, it defines the outer limits within which life might plausibly exist in the Solar System. For human civilization, Mars has long exercised a powerful imaginative pull, from H. G. Wells's The War of the Worlds (1898) and Edgar Rice Burroughs's Martian adventures to Ray Bradbury's The Martian Chronicles and countless modern films and games. Its colonization has been discussed seriously since the mid-twentieth century, and it remains the stated long-term destination of governmental space agencies and private ventures alike, symbolizing humanity's broader aspiration to become a multi-planet species.

See also

Solar System
Planet
Astrobiology
Space exploration
Phobos (moon)