Mars

Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System. Its cold, dry surface is covered in iron-oxide dust, which gives the planet its reddish colour, and its thin atmosphere is made mostly of carbon dioxide. Like Earth, it has seasons, polar ice caps, a rotation period close to a day, and a rocky surface shaped by volcanism, impacts, wind and water. It is therefore one of the best places to test ideas about how terrestrial planets form and change over time, and how the atmosphere of Earth and climate might evolve under different conditions.

Mars matters for three main reasons. Its rock record preserves evidence that liquid water once flowed across its surface, which makes it a leading target in the search for past life. It is the most intensively explored planet beyond Earth, with orbiters, landers and rovers operating there for decades. And it is the most frequently proposed destination for crewed spaceflight beyond the Moon, which raises engineering, medical, legal and ethical questions that go well beyond planetary science.

This entry first describes the planet's orbit, geology, interior, atmosphere, water history and moons. It then covers the history of observation and naming, robotic exploration, the sample-return program and its current status, and upcoming missions. It ends with human exploration, resource use, planetary protection, the terraforming debate, and Mars in culture and fiction.

Orbit and basic physical properties

Mars orbits the Sun at an average distance of about 1.52 astronomical units, or roughly 228 million kilometres. The average distance from Mars to the Sun (called the semi-major axis) is 228 million km. Its orbit is much more stretched than Earth's. The eccentricity is about 0.093, compared with about 0.017 for Earth, and the distance swings from about 206.7 million km at perihelion to about 249.2 million km at aphelion. The orbit of Mars is the second most eccentric in the Solar System. Only Mercury's orbit is more eccentric. One circuit takes approximately 687 Earth days to complete, so a Martian year is about 1.88 Earth years long.

Mars is a small world. Its mean radius is about 3,390 km, and the planet is roughly half the width of Earth. Its mass is about 0.107 times Earth's, which gives it a mean surface gravity of about 3.7 m/s². Surface gravity (mean) is 3.73 m/s² on Mars, compared with 9.82 on Earth, a ratio of 0.380. A Martian day, called a sol, is only a little longer than an Earth day. The solar day (or sol) on Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds. Because of its small size and modest mass, the planet's gravity is weak enough that a person would weigh much less than on Earth.

Mars has an axial tilt of about 25.2 degrees relative to its orbital plane, which is slightly larger than Earth's tilt of about 23.5 degrees. As a result, Mars has seasons like Earth, though on Mars, they are nearly twice as long because its orbital period is that much longer. The eccentric orbit adds a strong asymmetry between the hemispheres. Mars is near perihelion when it is summer in the southern hemisphere and winter in the north, and near aphelion when it is winter in the southern hemisphere and summer in the north. Southern summers are therefore shorter but receive more solar energy, while northern summers are longer and cooler. Long-term changes in the tilt are discussed in the atmosphere and climate sections.

Mars has no global magnetic field today, and its modest mass also limits its gravitational grip on gases. The planet's escape velocity is about 5.0 km/s, escape velocity 5.027 km/s, which is less than half of Earth's value of about 11.2 km/s. Gas molecules, especially lighter ones, therefore reach escape speed more easily on Mars, so over billions of years a thicker early atmosphere could be progressively lost to space. The lack of a global magnetic field may also have allowed the solar wind to interact more directly with the upper atmosphere, but how much this contributed relative to gravity is still an active research question. The history of the planet's magnetic field is discussed in the interior section.

Surface geology and landforms

Mars's surface is shaped by a small number of enormous features, produced by volcanism, faulting, impact and wind. Because the planet has little or no present-day plate tectonics, volcanic hotspots could keep erupting from the same places for very long periods, and some of the resulting structures grew to sizes unmatched on Earth.

The largest of these is the Tharsis region, a broad volcanic rise near the equator that spans several thousand kilometres and stands roughly 10 km above the planet's mean elevation. Three large shield volcanoes, Arsia Mons, Pavonis Mons and Ascraeus Mons, sit along its crest, each around 350 to 450 km across. At the north-western edge of the rise lies Olympus Mons, about 600 km in diameter. Published figures for its height range from roughly 22 to 27 km, depending on how the base is defined, which makes it the tallest known volcano in the Solar System. Its flows are much longer than typical terrestrial lava flows, which is generally attributed to high eruption rates and Mars's weaker gravity.

East of Tharsis runs Valles Marineris, a system of canyons lying almost along the equator. It extends more than 4,000 km, a substantial fraction of the planet's circumference, and reaches depths of about 7 km. Reported widths vary between roughly 200 and 600 km depending on the measurement. The system was named after the Mariner 9 orbiter, which discovered it in 1971–72. Its origin is mainly tectonic: as the Tharsis crust swelled, it stretched and fractured, and blocks of ground dropped down along faults. Later modification involved collapse, landslides, groundwater loss and erosion by wind and possibly by liquid flows, which widened the canyons. The walls expose thick sequences of layered rock that help geologists reconstruct the planet's history, and the extent of past water activity there remains debated.

The most fundamental surface contrast is the martian crustal dichotomy, which divides the planet into the southern highlands and the northern lowlands. The highlands are heavily cratered and older, whereas the lowlands are smoother, sparsely cratered and younger. The boundary between them is abrupt in places, and the difference in elevation amounts to several kilometres, with the crust beneath the lowlands markedly thinner. Two families of explanation dominate. One attributes the dichotomy to mantle convection early in the planet's history. The other proposes that one or more giant impacts excavated the north, with the Borealis basin, a vast and roughly elliptical depression, as the candidate structure. Impact models can reproduce the basin's size and shape under suitable impact conditions, but the absence of strong magnetic signatures and of widespread volcanism in the north complicates that case. A 2025 study of marsquakes recorded in both hemispheres is cited by some researchers as supporting the mantle-convection view. The question remains unresolved.

Impact basins provide the clearest record of the planet's early bombardment. Hellas Planitia, in the southern hemisphere, is roughly 2,300 km across and is considered the largest well-preserved impact structure on Mars. Its floor lies several kilometres below the surrounding terrain, making it one of the lowest places on the planet and a region of unusually high atmospheric pressure. Its formation is generally placed in the early part of Mars's history, when large impactors were still common. Other large basins and impact craters show a range of degradation states, which lets researchers compare how surfaces age under different conditions.

Wind and ice have reshaped much of the younger surface. Polar layered deposits in the north and south consist of alternating layers of dust and water ice, and their stratigraphy records changes in climate over long timescales. Dark dune fields, many of them encircling the polar regions, are being moved and reshaped by wind at present. Dust storms can rearrange these deposits within days, so the surface seen from orbit is a snapshot of an active system. Taken together, these features show that the surface reflects the combined effects of volcanism, tectonics, impacts and climate, rather than any single process. Orbital imaging and radar have been central to mapping them.

Interior structure and seismology

Mars is usually modelled as a differentiated body with three main layers: a crust, a silicate mantle and a metallic core. For decades these layers were inferred indirectly, from the planet's mass, its moment of inertia (which describes how that mass is distributed toward the centre) and the motion of orbiters and landers. Seismology added the first direct measurements. Because Mars has no plate tectonics, its interior is often compared with that of the Moon and Earth, and seismic data are the main test of such comparisons.

NASA's InSight lander, which reached Mars in late 2018, carried the SEIS seismometer. InSight reached Mars in late 2018 and soon afterward detected the first known marsquake, and over the mission the lander's instruments have picked up more than a thousand temblors, most of them minor rumbles. The mission ended at the close of 2022. Many events came from a seismically active region far from the lander: many of those quakes originated at a seismically active region more than 1,000 kilometers away from the lander. Distance mattered for the core studies, because the seismic core shadow as seen from InSight's location covers half the surface of Mars, including the majority of potentially active regions, which limited how many usable events could be recorded.

Waves reflected and converted at underground boundaries gave the first direct crustal thickness. Beneath the lander, if the second interface is the boundary of the crust, the thickness is 20 ± 5 kilometers, whereas if the third interface is the boundary, the thickness is 39 ± 8 kilometers. Combining these results with global gravity and topography maps, researchers estimated that the average thickness of the martian crust lies between 24 and 72 kilometers. The uppermost layer is thin and weak: the first layer with a thickness of 6 to 11 km and an S-wave velocity between 1.2 and 2.1 km s–1 is consistent with fractured or altered material. Below the crust, the rigid lithosphere is unusually deep, and the crust itself appears to be highly enriched in radioactive elements that help to heat this layer at the expense of the interior.

The core is the least settled part of the picture, and its estimates have changed several times. The first seismic analysis placed the liquid core at a radius of 1830 ± 40 kilometers, and a mean core density of 5.7 to 6.3 grams per cubic centimeter indicated a substantial share of light elements dissolved in the iron-nickel alloy. A later reanalysis suggested a smaller core. It gave a decreased core radius of 1,675 ± 30 km associated with an increased density of 6.65 ± 0.1 g cm−3, and placed a molten silicate layer about 150 km thick above the metal. A further study used PKKP and PKiKP seismic phases, which travel through or reflect off the deep core, to infer the radius of the Martian inner core to about 613 ± 67 km. If confirmed, this would mean Mars has a solid inner core inside a liquid outer core, but the result is not yet universally accepted, and some researchers have argued that Mars may have no inner core at all.

Mars also preserves evidence of a vanished magnetic field. Mars Global Surveyor showed that Mars carries a strong crustal magnetic field in the absence of an active core dynamo. The magnetised rock is concentrated in the southern highlands, and the crustal topographic dichotomy is well aligned with the pattern of crustal magnetisation. The field's origin is therefore attributed to an earlier core dynamo, which the crust recorded as it cooled. The date of its shutdown is disputed. Analysis of craters suggests that the dynamo shut off at an absolute model age of 4.12–4.14 Ga. Other volcanic rocks that formed between 3.9 and 3.5 billion years ago appear strongly magnetized, suggesting the dynamo was active during their emplacement, which would push the end of the dynamo later.

The dynamo was most likely driven by heat leaving the core, so its history is closely linked to how the interior cooled. One study notes that the dynamo was probably powered by planetary cooling and so its termination is intimately tied to the thermochemical evolution and present-day physical state of the Martian core. Researchers have also proposed that the shutdown may have contributed to the loss of the atmosphere and surface water: the early dynamo may have played a key role in the planet's climatic evolution and habitability, with its termination potentially triggering the shift to Mars' present cold and dry conditions. This link, and the precise timing and cause of the shutdown, remain active research questions rather than settled conclusions.

Atmosphere, dust and seasonal cycles

Mars has a very thin atmosphere, and it is dominated by carbon dioxide. Measurements summarised by space agencies put the gas at roughly 95 percent by volume, with about 2.7 percent nitrogen, about 1.6 percent argon, and only around 0.13 percent oxygen. Water vapour is a trace component that varies with season and location, and the air also contains small amounts of carbon monoxide. Because the atmosphere is so thin, the average surface pressure is only about 6 millibars, which is less than one percent of the sea-level value on Earth. The low pressure and the average temperature of about −60 °C mean that liquid water cannot remain stable on the surface under present conditions.

The atmosphere also changes over the year, mainly because of carbon dioxide. In the winter of each hemisphere, the air becomes cold enough for CO₂ to condense out of the atmosphere, forming frost or snow on the polar caps. In spring and summer, the frost sublimates and returns to the air. This exchange removes roughly a quarter of the atmosphere's total mass during the winter, and it produces a seasonal surface-pressure cycle with a peak-to-peak amplitude of about 25 to 30 percent. Because the two poles behave in opposite ways, CO₂ vapour moves from the cap that is subliming to the cap that is freezing, and the global pressure rises and falls in step with the seasons. The pressure difference between hemispheres is greatest when the southern hemisphere is in summer and the northern hemisphere is in winter, which drives large-scale winds that blow toward the winter pole.

Dust is a permanent component of the Martian air and is the main driver of atmospheric heating and circulation. Winds lift fine, iron-rich particles from the surface, producing dust devils that are common in some regions and larger regional storms. Once in a while, a storm grows to cover the whole planet. Such global storms can raise dust to heights of around 20 kilometres and can persist for months. They tend to begin in the southern summer, when Mars is closest to the Sun and the southern CO₂ cap sublimates rapidly, and they are estimated to occur roughly once every three Mars years on average, although the timing is irregular. The dust thickens the atmosphere optically, reducing the sunlight that reaches the surface. This affects the output of solar arrays and the thermal balance of spacecraft, and it has forced missions into low-power modes or ended them, so battery capacity and power planning are central concerns for surface operations. Orbital remote sensing is used to track storm development so that operators can plan around them.

Mars's axial tilt, or obliquity, is close to Earth's at present, at about 25 degrees, but it varies much more over time. Earth's tilt is comparatively stable because the Moon stabilises it, whereas Mars's tilt is perturbed chaotically by the gravity of the other planets. Modelling studies disagree on the full range, with some suggesting values from near zero to about 60 degrees and others going higher, and the uncertainty grows the further back or forward the calculation is extended. When the tilt is high, sunlight is redistributed toward the poles and water ice migrates away from the caps, depositing ice and dust in mid-latitudes. When it is low, ice retreats back toward the poles. Orbiter data suggest that Mars has passed through several such cycles in the last few hundred million years, and that each glacial episode left less ice than the one before. Changes in the planet's orbit (eccentricity) add to this effect. These climate-model results are used to explain the layered polar deposits and the mid-latitude ice reservoirs, and they link atmospheric cycles to the longer paleoclimate record of the planet.

Water, ice and the ancient climate record

Liquid water once flowed across parts of the Martian surface, and the evidence for this is now extensive. Some valley networks end in features that look like eroded deltas, and their sinuous channels have proportions similar to those of rivers on Earth. Channel features on what are interpreted as eroded deltas at the foot of some networks are also uniquely associated with formation by flowing water. In Gale Crater, the Curiosity rover's investigations point to streams and lakes that were active between about 3.8 to 3.3 billion years ago, delivering sediment that slowly built up the lower layers of Mount Sharp. In Jezero crater, an inflowing stream dropped its sediment where it met a standing lake, building a delta that remains preserved Where water entered the lake, it deposited a delta, a fan of sediment that had been carried by the water.

Flooding also shaped the landscape. A study led by Timothy Goudge estimated that at least 25 percent of the valleys were created by lake breach flooding, where the water in the lakes overtopped their rims. Researchers have counted many more such basins: over 200 of these lake basins filled with sufficient water to breach the confining topography, causing catastrophic flooding and incision of outlet canyons. What supplied this water remains contested. A 2025 study from the University of Colorado Boulder compared erosion from rain or snow with erosion from melting ice, and the rain or snow model best explained the erosion features still seen on Mars today.

Today the planet holds large volumes of water, but almost all of it is frozen. Mars has long had abundant ice, both at its poles and underground. The surface itself is unfriendly to liquid water. The early flyby data showed that the atmospheric pressure was only around half a dozen millibars, and the average temperature was 50 degrees Celsius below freezing. Under those conditions, if I put a bucket of water out there, it would take a while, but it would freeze or evaporate. Ice exposed at the surface therefore tends to pass directly into vapour, a process known as sublimation, which is why the stability of any near-surface brine is a central problem in subsurface water studies.

The most debated present-day evidence concerns recurring slope lineae (RSL). These are dark streaks that are lengthening streaks on Mars that reappear each year during the warm seasons and fade during the cold seasons. In 2015, NASA reported that planetary scientists detected hydrated salts on these slopes at Hale crater, corroborating their original hypothesis that the streaks are indeed formed by liquid water. Other analyses have placed strict limits on the amount of water involved. Temperature data from Mars Odyssey imply an upper limit on how much water is present at these darkened streaks: about as much as in the driest desert sands on Earth. A 2025 study in Nature Communications argued that RSL and other streaks are not water-related, and its machine-learning analysis of over 500,000 streaks suggest only wind and dust are responsible. The question therefore remains open.

Minerals provide a longer-term record of the climate. Orbital spectroscopy shows that these minerals appear to have formed in two distinct climatic episodes: The Noachian period (4.2–3.7 Gy), presumably warmer and wetter, resulted in the formation of clays, whereas the intermediate Hesperian (3.7–3.0 Gy) appears to mark a transition to a dryer and more acidic climate, resulting in the formation of sulfates. The clay-to-sulfate change has been read as a planetary shift toward drier conditions, and a clear mineralogical transition from Fe/Mg smectite in older strata to hydrated Mg sulfate salts in younger strata was one reason Gale Crater was chosen as a landing site. Yet the rover record complicates this simple picture. A 2026 study of the Gale transition concluded that the preserved sequence record repeated fluctuations in water availability and sediment supply, reflecting a dynamic climate history rather than a single drying trend.

These observations sit uneasily with climate theory. Climate modelling indicates that long-term warm conditions were not sustainable, even though valley networks and deltas point to running water on ancient terrains. Reconciling the geological evidence with the models, including the question of how a surface stayed warm enough for lakes and rivers to persist, remains a central task of paleoclimate research on Mars.

Moons: Phobos and Deimos

Mars has two small moons, Phobos and Deimos, both discovered by Asaph Hall in 1877. Phobos has a mean diameter of about 22 kilometres and Deimos about 12 kilometres, so both are far smaller than Earth's Moon and too small to pull themselves into spheres. Their irregular shapes are typical of small bodies with weak self-gravity. Phobos orbits roughly 9,400 kilometres above the surface and completes a circuit in under eight hours, faster than Mars rotates. Deimos orbits much farther out, at roughly 23,000 kilometres, and takes about 30 hours per orbit.

The origin of the two moons is still unresolved. The first hypothesis is capture: the moons formed as asteroids elsewhere and were later gravitationally trapped by Mars. Their dark, low-reflectance surfaces and their spectra resemble those of carbonaceous, D-type asteroids, which supported this idea for decades. The difficulty is their orbits. Captured bodies would normally have elongated orbits tilted at random angles, whereas Phobos and Deimos circle Mars in nearly circular paths close to the planet's equator. The second hypothesis is a giant impact. A large collision could have thrown debris into orbit around Mars, and that material could have re-accreted into the moons. This scenario explains the near-equatorial, near-circular orbits naturally, and it is often described as the most widely favoured option, though it has its own problems. Simulations suggest the moons would contain a mixture of Martian and impactor material, and the heating of the impact would deplete volatile elements such as water. Other proposals, such as a single progenitor moon that broke apart or a disk made of captured asteroidal material, have also been argued, but they remain contested.

Both moons are slowly changing their orbits. Phobos lies inside Mars's synchronous orbit, the distance at which a satellite's orbital period matches the planet's rotation. Tidal forces from Mars therefore drag Phobos inward, and it is spiralling down toward the planet at a rate of about 1.8 centimetres per year. On timescales of tens of millions of years, it is expected to either break apart or crash onto the surface. Deimos lies outside the synchronous orbit, so tidal effects push it slowly outward instead. Phobos also carries grooves and the large Stickney crater, which record past impacts and stresses on the small body.

The Japanese Martian Moons eXploration (MMX) mission is the first attempt to settle the question directly. Developed by the Japan Aerospace Exploration Agency (JAXA) and approved for development in 2020, it is scheduled to launch on an H3 rocket from Tanegashima on 20 October 2026 Japan time, with a backup launch window running into early November. After roughly a year of cruise, the spacecraft is due to reach Mars in 2027. It will study both moons from orbit, and it will land on Phobos to collect at least 10 grams of material using dual samplers, with help from a small rover supplied by France and Germany. The samples are planned to return to Earth in 2031. The mission is an international project, with participation from NASA, CNES, DLR and ESA. Its instruments include a gamma-ray and neutron spectrometer, which can measure the elemental and hydrogen content of the surface.

If successful, MMX would be the first sample return from either moon. Laboratory analysis of the material on Earth could reveal isotopic ratios, volatile content and mineralogy with far greater precision than remote sensing allows. Those measurements could test whether Phobos is made of Martian and impactor material, which would support a giant impact origin, or whether it preserves the primitive composition of an asteroid, which would support capture. The answer would also constrain how Mars itself formed and how water and organic material were delivered to the inner Solar System.

Geological history and Martian meteorites

Planetary scientists split Mars's past into three epochs: the Noachian, Hesperian and Amazonian. The boundaries rest mainly on how densely impact craters pack each surface, not on rock type. The Noachian takes its name from Noachis Terra, a region of old, rugged southern highlands, which makes it the oldest period. Most estimates place the end of the Noachian near 3.7 billion years ago, and the Hesperian ended roughly 2.9 to 3.3 billion years ago. The Hesperian was first named after Hesperia Planum, but its starting point is now defined through crater statistics, because the original rocks no longer serve as a single marker. Crater counts are the main tool. Middle-Noachian terrain can carry more than 400 craters larger than 5 km per million square kilometres, while the start of the Amazonian carries only about 25 to 67. Turning these densities into years requires assumptions about how the impact rate changed over time, and the rate did not fall in a simple linear way. Absolute ages are therefore model-dependent, and the relative order of the periods is more secure than the exact dates.

The early planet was shaped by intense bombardment. Noachian terrain preserves the record of this heavy impact era. Volcanism and erosion were also active, and some researchers think the young planet may have had a much denser atmosphere, though this remains a hypothesis. The later Hesperian is marked by extensive lava plains. Over the Amazonian, impact craters became comparatively rare, and the surface was reshaped mostly by lava flows, wind and slow atmospheric loss.

Martian meteorites are the only physical samples of Mars that scientists have on Earth, apart from data from landers and rovers. Their origin is recognised because they were ejected by impacts that were strong enough to launch rock off the surface. Some have been determined to have come from Mars. Researchers identify them through their mineralogy and by the gases trapped in their glassy or fine-grained material, which match the atmospheric composition measured by Viking landers.

The most famous of these rocks is ALH84001, found in the Allan Hills ice field in Antarctica in 1984. It is a roughly 1.9-kilogram igneous rock that is thought to have crystallized from magma on Mars 4.5 billion years ago. Carbonate minerals were later introduced into shock-induced fractures, and a later impact blasted the rock into space. In 1996, a NASA team led by David McKay argued that the carbonates in the meteorite had been produced by Martian microorganisms. The evidence included carbonate globules, polycyclic aromatic hydrocarbons, iron-rich grains resembling those made by bacteria, and elongated features that looked like microfossils.

Other scientists quickly challenged each line of evidence. Most specialists concluded that each could be adequately explained by nonbiological processes. Later work added further alternatives: a 2011 study suggested the carbonates formed during the evaporation of surface water, a geological rather than biological origin. A 2002 study argued that magnetite chains in the rock were a robust biosignature, and some investigators still suggest that microbial life existed on Mars approximately 4 billion years ago. Sources differ on the status of the debate. Some popular accounts describe the claim as having been disproved, while other sources note that no agreement has appeared on whether meteorite ALH84001 was ever graced by Martian life. The consensus is that the rock records real water–rock interaction on early Mars, but the question of fossil life in it remains unresolved.

Habitability and the search for life

Whether Mars ever supported life depends on two linked questions: whether its environment was once suitable for microbial life, and whether any trace of such life survives. The present surface is hostile to life as far as is known, so most of the evidence comes from ancient rocks and from instruments that look for organic molecules and characteristic chemical patterns. Every line of evidence has been contested, and no finding has yet been accepted as proof.

The first direct test came from the two Viking landers, which reached the surface in 1976 and carried three biology experiments. The Labeled Release experiment produced a release of gas that looked like metabolism, while the Gas Exchange and Pyrolytic Release experiments gave responses that were harder to interpret. At the same time, the landers' gas chromatograph–mass spectrometers found no organic compounds in the soil. Most Viking scientists concluded that the missions had not detected life, but the results were never fully explained. The later discovery of perchlorate salts changed the picture. The Phoenix lander found about half a percent perchlorate in the soil in 2008, and perchlorate breaks down organic matter when heated, which could account for the missing organics. In 2013, Richard Quinn and colleagues showed that gamma-irradiated perchlorate releases trapped oxygen and can reproduce the Labeled Release and Gas Exchange responses without invoking biology. Some researchers still argue that the Labeled Release result has not been fully resolved.

Organic molecules have since been found repeatedly. Curiosity in Gale Crater and Perseverance in Jezero Crater have both detected carbon-based compounds in sedimentary rocks. Such molecules are not diagnostic of life, because they can be delivered by meteorites, formed by abiotic chemical reactions, or preserved from ancient environments without any living source. Their origin therefore remains unclear, and their presence shows only that the ingredients for organic chemistry existed on Mars, not that anything lived there.

The surface soil also complicates habitability. Perchlorate is a strong oxidant that is toxic to many organisms on Earth, and it can damage cell functions and interfere with the uptake of iodine. Some Earth microbes can nevertheless use perchlorate as an energy source, so its presence does not automatically rule out life. What it does is make the near-surface environment more difficult for common terrestrial organisms, and it changes how scientists think about where to look for preserved biology, which tends to favour sheltered or buried material.

The strongest recent candidate is the rock nicknamed Cheyava Falls, which Perseverance drilled in July 2024 in the Bright Angel formation, along the edge of an ancient river valley in Jezero Crater. The rock has pale and dark spots, nicknamed leopard spots, that are rich in iron and phosphate. On Earth, similar textures are often associated with microbial metabolism, although some purely chemical processes can produce them. The sample, named Sapphire Canyon, was analysed by Perseverance's SHERLOC instrument, which detected organic compounds. In September 2025, a paper in Nature described the features as potential biosignatures and reported that the abiotic explanations considered so far were not strongly supported, while stopping short of claiming life. Scientists use the term "potential biosignature" for a feature that could be biological but still needs more data and laboratory study. NASA's Confidence of Life Detection scale is intended to structure such judgements, and it also makes clear how much independent evidence is required before a claim is accepted.

Reports of seasonal methane on Mars follow a similar pattern. Variations measured at the surface have been difficult to confirm, because the signals are small and methane can have geological as well as biological sources. These reports are therefore treated as tentative. Confirming any of these signals will require more precise measurements and, for the most important samples, analysis in terrestrial laboratories, which is why the fate of returned samples is closely tied to the search for life.

Early observation and naming

Mars has been tracked since antiquity, and its reddish colour made it stand out among the wandering points of light in the sky. Babylonian astronomers recorded the planet's positions and identified its apparent retrograde motion, the temporary reversal of its path against the background stars that occurs when Earth overtakes it in its orbit. In Mesopotamian tradition the planet was associated with Nergal, a deity of war and destruction. Greek astronomers inherited and extended these records, and in Greek religion the planet was linked to Ares. When Roman writers adopted and adapted the Greek pantheon, the planet took the name of the Roman god Mars, and that name became the standard term in Western languages.

The modern scientific understanding of Mars' motion began with Greek geometrical models and reached its turning point in the early 17th century. Johannes Kepler worked with the unusually precise observations of Mars made by Tycho Brahe at his observatory on Hven. Kepler first tried to fit the planet's positions to circular orbits, but a mismatch of about eight arcminutes between prediction and observation persuaded him that the path could not be a circle. His analysis, published in 1609 in Astronomia Nova, established that Mars moves along an ellipse with the Sun at one focus, and that the planet sweeps out equal areas in equal times. These two findings, together with his third law, became the foundation of planetary theory, and gravitation later provided Isaac Newton with the physical explanation for them.

In the 19th century, improved telescopes revealed more surface detail, and the planet became a focus of popular speculation. During the close approach of 1877, the Italian astronomer Giovanni Schiaparelli, working at the Brera Observatory in Milan, mapped what he described as a network of linear features. He called them canali, an Italian word meaning channels or grooves. When the term was translated into English as "canals", it suggested artificial waterways. The American businessman and astronomer Percival Lowell took up the idea, founded the Lowell Observatory in Flagstaff, Arizona, in 1894, and published books arguing that the canals were irrigation works built by an ageing civilisation. Other observers challenged these claims. In 1909, Eugène Antoniadi, using a large refractor at Meudon, reported irregular patches rather than straight canals. The canal debate nevertheless shaped public imagination well beyond astronomy.

Other cultures gave the planet names that reflected their own systems of thought. In Chinese astronomy it is known as the "fire star" (火星, huoxing), a name linked to the element fire in the five-phase system. In Sanskrit it is called Mangala, a name associated with a god of war, and Indian astronomy also used the name Angaraka. These traditions show that Mars was widely seen as a red, restless, and ominous object long before it was studied as a world.

Modern surface nomenclature follows conventions set by the International Astronomical Union, whose working group on planetary system nomenclature approves names for features across the Solar System. Early albedo features on Mars were often given classical place names, a practice that goes back to 19th-century mapmakers. Craters are generally named after deceased scientists, writers and artists, while large valleys and other landforms follow different rules. Valles Marineris, for example, was named after the Mariner 9 spacecraft that mapped it in the early 1970s, which illustrates how the naming of Martian features moved from telescopic observation to spacecraft exploration.

Robotic landers and rovers

Robotic spacecraft began exploring Mars in the 1960s, and each generation of machines has widened what scientists could measure on the surface. Mariner 4 — no, keep plain: NASA's Mariner 4 flew past the planet in 1965 and sent back images of a heavily cratered landscape. Those pictures overturned the expectation that Mars resembled the Moon only in its craters and forced planetary scientists to rethink its geological history.

The first landings were Soviet and mostly unsuccessful. Mars 2 crashed in late 1971. Its sister lander, Mars 3, touched down on 2 December 1971 and became the first spacecraft to make a soft landing on the planet, but it fell silent after about 15 to 20 seconds. Sources differ on the exact figure: most give roughly 14.5 seconds, while the Planetary Society puts it at 20. Only a partial, featureless grey image reached Earth. Whether the fault lay in the lander or in the relay on the orbiter above is still unknown, and a dust storm that was raging at the time is a leading suspect. Both Soviet landers also carried small rovers that were never deployed. The United States followed with Viking 1 and Viking 2 in 1976. These were the first long-lived American landers. They sent back panoramas and weather data for years, and their onboard biology experiments, discussed elsewhere in this entry, remain controversial.

Mobile exploration began with Sojourner, which landed in July 1997 as part of NASA's Pathfinder mission. The six-wheeled rover weighed about 10.6 kilograms, was roughly the size of a microwave oven, and operated for 83 sols, far beyond its planned week. It depended on its lander for communications, and contact was lost when the lander's battery failed. Its twin successors, Spirit and Opportunity, were designed for 90-sol missions and landed in January 2004 on opposite sides of the planet. Spirit became stuck in soft soil in 2009 and stopped operating in 2010. Opportunity travelled roughly 45 kilometres and was ended by a global dust storm in 2018. Curiosity, a car-sized rover with a long robotic arm and a radioisotope-based nuclear power source, landed in Gale Crater in August 2012. Its instruments were designed to test whether the site had ever been habitable. Perseverance, which reached Jezero Crater in February 2021, carries a drill and sealed sample tubes for collecting rock cores, and it is designed to store them for a possible future return to Earth.

The progression from Sojourner to Perseverance can be read along three lines: mobility, instrumentation and the range of sites examined. Early rovers drove only short distances and relied on simple cameras and spectrometers. Later vehicles travelled tens of kilometres, carried more spectroscopic and chemical analysis tools, and could reach terrain that earlier machines could not cross. Solar arrays powered the first generations, but the long-lived Curiosity and Perseverance rely on nuclear sources that are not affected by dust-darkened skies. This change in power source is one reason these vehicles have operated through seasons and dust storms that would have ended earlier missions.

Flight was the next step. NASA's Ingenuity helicopter, a technology demonstration that carried no science instruments, made the first powered and controlled flight on another planet on 19 April 2021. It climbed about three metres, hovered, and landed after roughly 39 seconds in the air. Because the Martian atmosphere has less than one percent of Earth's surface pressure, its rotors had to spin very quickly to generate lift. Ingenuity rode to Mars attached to Perseverance and was then deployed onto the surface. Over nearly three years it completed 72 flights, serving as a scout for the rover, before its final flight on 18 January 2024.

Despite these successes, the record of Mars robotics is marked by setbacks. The Planetary Society estimates that roughly half of all Mars missions have failed, and the causes range from landing problems and dust to communication breakdowns and mechanical faults. Each landing requires autonomous guidance through the thin atmosphere, and each surface mission depends on radio links that are often routed through orbiters. These difficulties explain why the history of Mars landers and rovers is as much a story of engineering lessons learned as of scientific discoveries.

Orbiters and relay networks

Orbiters have been the workhorses of Mars exploration since the early 1970s. They map the surface, sample the atmosphere, and act as the communication backbone for everything on the ground. The fleet has changed over time, and several of its members have now been retired or are near the end of their lives.

Mars Global Surveyor, which reached Mars in September 1997, pioneered aerobraking, in which a spacecraft repeatedly skims the thin upper atmosphere to lower its orbit without burning much fuel. Its cameras returned more than 240,000 images, and its laser altimeter built a detailed topographic picture of the planet. Its data also helped select landing sites for three rovers and the Phoenix lander. Mars Odyssey, launched in April 2001, carried a gamma-ray spectrometer that produced the first global map of the chemical elements in the near-surface, along with a thermal imager. It continues to operate in 2026 and holds the record for the longest continuously active spacecraft in orbit around a planet other than Earth. The Mars Reconnaissance Orbiter, launched in 2005, combines high-resolution cameras, a visible and infrared spectrometer and a radar sounder that probes subsurface layers. MAVEN, which entered orbit in September 2014, studied how the upper atmosphere is lost to space. After it fell silent in December 2025, NASA formally ended the mission in June 2026.

The European, Indian, Emirati and Chinese orbiters have extended this work with different instruments and priorities. ESA's Trace Gas Orbiter, launched in 2016, searches for minor atmospheric constituents and, on average, now relays more data per day from the surface than any other spacecraft in the network. The UAE's Hope probe, launched in July 2020, reached orbit in February 2021 and is the country's first interplanetary mission; it studies the atmosphere and weather. China's Tianwen-1 orbiter entered orbit in the same month, carrying the Zhurong rover to the surface and later observing the planet from a long, polar elliptical orbit. India's Mars Orbiter Mission, launched in 2013, was the country's first mission to another planet. Each of these spacecraft adds instruments that complement those of the American orbiters, so that a combined picture of the atmosphere, surface and subsurface can be assembled over many years.

Surface assets depend on orbiters for communication. Landers and rovers have modest antennas and limited power, so they can exchange only small amounts of data directly with Earth. Curiosity, for example, can communicate directly at rates of up to about 32 kbit/s, but most of its science data travels through the Mars Reconnaissance Orbiter and Odyssey. Together, the orbiters that handle this traffic form the Mars Relay Network, which currently comprises five spacecraft. Relay links are scheduled around the times when each orbiter passes overhead, and data are stored and forwarded in bursts. Several orbiters carry Electra ultra-high-frequency transceivers for this purpose. The network proved its value during the Perseverance landing in 2021, when the Mars Reconnaissance Orbiter received the entry, descent and landing data and passed it to Earth almost immediately. The ageing state of the relay fleet has raised concern, and proposals such as NASA's Next Mars Orbiter, a dedicated telecommunications spacecraft, have been put on hold.

Orbital imaging and radar are also essential for planning surface operations. High-resolution images allow engineers to assess the slope, rock density and hazards of candidate landing sites before a lander commits to a descent. Repeated passes over the same terrain reveal changes such as the advance and retreat of seasonal frost, the movement of dust, and the evolution of slopes. Orbiter data also monitor global dust storms, which can alter atmospheric heating and reduce the power available to solar-powered rovers, so mission controllers can plan around them. In this sense, the orbiter fleet functions as both a communications network and an observatory that supports every surface mission.

Sample return and the status of the NASA-ESA program

The NASA–ESA Mars Sample Return (MSR) effort was built as a chain of three missions. The plan, approved in September 2022, relied on Perseverance as the sample collector, a retrieval lander carrying an ascent vehicle and helicopters, and ESA's Earth Return Orbiter. Each stage depended on the previous one, so a delay or overrun anywhere in the chain pushed back the whole campaign. Costs proved the central problem. Fears about the program's ballooning cost, which reached $11 billion in 2024, led to repeated congressional threats of cancellation, and the project survived those threats only in reduced form while NASA reworked its plans. NASA spent 2024 and 2025 evaluating alternative architectures, including concepts that relied on commercial landers or simpler mission profiles. Even so, NASA had not produced a workable plan, despite asking industry for feedback and seeing several companies publicly pitch their own ideas.

The White House's fiscal 2026 budget proposal, released in May 2025, called for ending the American side of the program. The Trump administration's proposal planned to cancel MSR on the U.S. side. Congress rejected most of the proposed cuts to NASA's overall budget, approving roughly $24.4 billion for the agency, but it accepted the decision to end MSR in its existing form. The decisive vote came in the House, which passed the Commerce, Justice, Science appropriations package, a "minibus" bill, on January 8, 2026. The fiscal year 2026 appropriations bill was signed on 23 January 2026.

Congress did not zero out the technology work entirely. Instead of MSR funding, the 2026 bill provided $110 million for a Mars Future Missions account. The amount was well below the $300 million proposed in the House spending bill. The account is meant to keep alive technologies such as entry, descent and landing systems for the thin Martian atmosphere. Those capabilities, however, are the ones that matter most for any future retrieval, and critics argue the funding is inadequate. In a letter to appropriators, four senators asked that the account receive at least $400 million in the fiscal year 2027 bill. A June 2026 memo from the Mars Exploration Program Analysis Group also warned that the science directorate had been directed to absorb the costs of a proposed nuclear-powered Mars mission within the existing Mars Future Missions line, which holds about $110 million.

ESA's position has been shaped by the same funding decision. The Earth Return Orbiter was ESA's main contribution, and ESA awarded Airbus Defence and Space a €491 million contract in October 2020 to develop and deliver the orbiter. In March 2026, following its 345th Council meeting, the European Space Agency announced that member states had formally proposed cancelling the Earth Return Orbiter program. ESA's Director of Human and Robotic Exploration later said that member states had asked for the mission to be cancelled, and the agency had begun talks with the prime contractor, Airbus Defence and Space, about how to manage the mission's remaining work and reuse some of its elements. Before this decision, ESA had explored a cheaper alternative. It proposed repurposing the orbiter as a dedicated European exploration mission, known as ZefERO, which would launch in 2032 to study Martian winds and carry out geological investigations while also serving as a communications relay.

The samples themselves remain on Mars. The tubes Perseverance drilled are still on the surface, with ten lying at Three Forks and the rest carried on the rover, and nothing has been funded to retrieve them. No authorised lander, Mars ascent rocket or Earth-return spacecraft is currently set to collect them. The rover continues to operate in Jezero Crater, and lawmakers did not order the samples abandoned, so a later Congress could fund a different architecture. The stakes are considerable: sample return has ranked as the top priority in the last two planetary science decadal surveys. China's Tianwen-3 mission, planned for the late 2020s, is designed to bring samples back and could become the first to do so.

Upcoming robotic missions

The nearest of the upcoming missions is Japan's Martian Moons eXploration (MMX). JAXA has set its launch for 20 October 2026 at 04:41 JST, on an H3 rocket from Tanegashima, with backup dates running to 7 November. The probe is designed to reach Phobos after a cruise of about a year, enter a quasi-satellite orbit around it, and collect samples before heading home. The target is at least 10 grams of material, and the spacecraft will also observe Deimos and monitor the Martian climate. The return to Earth is planned for around 2031. No spacecraft has yet brought material back from the Mars system, so MMX would set that first. The mission is led by JAXA with participation from NASA, CNES, DLR and ESA. Its scientific aim is to test whether the moons are captured asteroids or debris from a giant impact. The project has already slipped: it was first aimed at 2024, and problems with the H3 rocket pushed it back.

China's Tianwen-3

China's Tianwen-3 is the most ambitious of the planned sample-return efforts. The Chinese National Space Administration presented the plan in April 2026, targeting a launch around 2028 and a return of samples around 2031. The mission would require two Long March 5 launches: one would carry the lander and ascent vehicle, and the other would carry the orbiter and Earth-return module. The planned launch window is December 2028 to January 2029. Samples would be gathered in three ways: at several points on the surface, through drilling at fixed sites, and by a vehicle sampling in flight. The drill is intended to collect at least 500 grams of surface material. If successful, Tianwen-3 would achieve the first Mars sample return. The schedule has moved between 2028 and 2030 over the years, and Mars launch windows open only about every 26 months, so a slip of one window would delay the mission by more than two years.

ExoMars Rosalind Franklin

The European rover Rosalind Franklin, the second ExoMars mission, has been delayed by geopolitics. ESA suspended the earlier ExoMars 2022 launch in March 2022, and NASA rejoined the project in March 2023. The current plan calls for launch in October 2028 and landing on Mars at the end of November 2030 in Oxia Planum. The arrival date was chosen to avoid the planet's global dust storm season. The rover is designed to drill to about two metres and examine soil and subsoil. Its mass spectrometer, the Mars Organic Molecule Analyser, is meant to search the soil for organic compounds. NASA will procure a commercial US rocket for the launch and supply part of the propulsion system for the landing, along with heater units.

Other proposals

Several other projects remain at an earlier stage. Roscosmos plans to launch Mars-Grunt in the 2030s. Concepts such as the International Mars Ice Mapper, Indian lander and rover designs, and NASA's proposed orbiters, helicopters and nuclear-powered spacecraft are still proposals rather than funded flight programmes, so their timing is uncertain.

Human exploration: challenges and proposals

The biggest obstacle to crewed Mars missions is travel time. Using conventional chemical propulsion and low-energy trajectories, a spacecraft typically needs six to nine months to reach Mars, so a round trip that includes a surface stay could last two to three years. A 2025 study by a UCSB physicist argued that a Starship-class vehicle could reach Mars in about 90 to 104 days, but that is a proposed trajectory rather than a demonstrated capability. Shorter transits are attractive because the physiological and radiation risks of a crew in deep space grow with time.

Radiation is the most serious of these risks. Mars has no global magnetic field and only a very thin atmosphere, so neither deflects charged particles or absorbs much of the cosmic radiation that reaches the surface. Galactic cosmic rays arrive throughout the cruise, and solar particle events can deliver sudden, dangerous doses. The Curiosity rover's radiation detector was used to estimate the dose for NASA's template mission, which assumed 360 days of travel and 500 days on the surface; the result was about 1 sievert, roughly ten times the dose of a six-month stay on the International Space Station. NASA's career limit for radiation exposure is about 600 millisieverts, and some researchers argue that only nuclear propulsion can keep a mission within that limit, a view that newer trajectory studies contest.

The planet's environment also complicates landing and surface operations. The thin atmosphere provides too little drag for parachutes alone to slow heavy payloads, so entry, descent and landing systems must be scaled up well beyond what has flown to date, and every crewed landing needs a habitat that can be shielded and kept pressurised. Surface gravity of about 0.38 g is another unknown, because its effect on the human body over years is not well documented.

Human health is the second major area of concern. In weight-bearing bones, microgravity causes mineral density to fall by roughly 1 to 1.5 percent per month, and bone recovery after return may be incomplete. A 2020 model of Mars trajectories predicted that most astronauts on the longer missions would develop osteopenia after return, and that every crew on the longest conjunction-class trajectory would do so. Muscle atrophy, cardiovascular deconditioning, isolation and confinement add further risks. Communication is also delayed by up to about 20 minutes one way, which means that the crew must diagnose and treat medical emergencies, and repair technical faults, largely on its own. Mission planners therefore treat onboard medical capability and autonomous decision support as essential, not optional.

Timelines for crewed missions have been revised many times. Government programmes have shifted priorities, and private ambitions have moved even more often. SpaceX's leader, Elon Musk, has a record of revising Mars dates: a 2016 prediction of arrival by 2025 was later replaced by a target of an uncrewed mission in 2024, and the company's plans later shifted to 2026 for uncrewed flights with crewed missions to follow. In February 2026, SpaceX reportedly told investors that it would prioritise a lunar city and postpone Mars activity by roughly five to seven years, although reports differ on how far the first uncrewed attempt has slipped. Because launch opportunities recur only about every 26 months, a single missed window can add years to the schedule, which makes the dates for crewed flights highly uncertain.

Resource use and settlement engineering

Any long-term human presence on Mars would depend on in-situ resource utilisation (ISRU), the practice of making consumables from local materials rather than launching them from Earth. The two most discussed inputs are the atmosphere and subsurface water. Water could be split by electrolysis into hydrogen and oxygen, while carbon dioxide from the air could supply carbon. One proposed route combines carbon dioxide with water to yield methane and oxygen, a pairing that would allow a return vehicle to be fuelled on the surface.In addition to liquid water, through the reaction CO2 + 2 H2O → CH4 + 2 O2, water would also be useful for producing methane propellant. Because the atmosphere is so thin, the availability of ice near the surface is central to these plans, and polar ice is less useful for early missions than equatorial or mid-latitude deposits.

The clearest demonstration of this approach is the MOXIE instrument on the Perseverance rover. It uses a high-temperature solid-oxide cell that operates at roughly 800 °C, so the device must be built from heat-tolerant parts. MOXIE works by separating oxygen atoms from carbon dioxide molecules, and a waste product, carbon monoxide, is emitted into the Martian atmosphere. The first run took place in April 2021, and since Perseverance landed on Mars in 2021, MOXIE has generated a total of 122 grams of oxygen. At its most efficient, MOXIE was able to produce 12 grams of oxygen an hour, twice as much as NASA's original goals for the instrument, at 98% purity or better. The instrument ended operations after its sixteenth run in August 2023. Its output is tiny by crewed standards, but it showed that the chemistry works in Martian conditions. A scaled-up version is the practical goal: one analysis argued that a MOXIE-type system several hundred times larger, producing 2–3 kg/h oxygen production vs. MOXIE's 6–10 g/h, could supply the oxidiser for a Mars ascent vehicle for a crew.

Habitat design has to address several environmental constraints at once. Surface pressure is very low, dust is abrasive, temperatures swing widely, and perchlorate salts in the soil are a chemical hazard that must be kept out of living spaces. Radiation is the most demanding problem. Because the atmosphere and the lack of a global magnetic field offer little protection, exposure of astronauts to ionising radiation is widely considered to be the biggest threat to sustained Martian habitation. Materials rich in hydrogen perform best against galactic cosmic rays. One study found that hydrogen-rich materials (i.e., water ice) have a predictable response to GCRs and are therefore the best defense against cosmic rays. Regolith performs less well per unit mass, and a NASA analysis concluded that an in-situ shielding strategy will also be of little help unless several hundred g/cm^2 of regolith is utilized. Such a strategy would probably require large scale excavation making it an unlikely candidate. For this reason, many designs favour burial. One simulation estimated that a depth of ten meters would already be able to entirely dissolve the effect of radiation, and other concepts propose a mixture of regolith and water as a building material for surface structures.

Power is the final constraint. Solar arrays are the simplest option, but dust settling on panels reduces their output, and the output falls further at higher latitudes. The Martian night also requires energy storage, usually batteries or regenerative fuel cells, and the cycle of day and night means that the power system must be sized for the worst case rather than the average. Radioisotope generators, which convert the heat from the decay of an isotope into electricity, provide steady output regardless of weather or season, but they supply only modest power. A fission reactor could supply much larger amounts of power, and it would be independent of dust and latitude. The choice among these options depends on the scale of the settlement and on the mission's tolerance for dust storms and long nights, and it is still an open question in engineering studies.

Planetary protection and governance

Planetary protection covers the practices meant to stop biological contamination in both directions: Earth organisms reaching other worlds (forward contamination) and possible extraterrestrial organisms reaching Earth (backward contamination). Its legal foundation is the Outer Space Treaty. The treaty's membership stood at 118 countries in early 2026, and it has underpinned space law since its adoption in 1967.

The treaty's Article IX carries the core duties. Article IX imposes obligations to avoid forward and backward contamination and provides that states parties "shall adopt appropriate measures" to do so. Legal commentators generally read the article as combining three duties: acting with due regard for the interests of other states, avoiding harmful contamination, and consulting before activities that could cause potentially harmful interference. The article only provides for an active obligation to adopt appropriate measures in cases of backward contamination and not for forward contamination, as it may have been perceived to be the greater risk at the time. The treaty also makes governments answerable for private activity. States parties to the Outer Space Treaty (under Article VI) are responsible for any space activities in their countries, governmental and non-governmental.

Because the treaty is general, the technical rules come from the Committee on Space Research (COSPAR), whose Panel on Planetary Protection provides guidance for compliance with the Outer Space Treaty of 1967. COSPAR's recommendations have no binding force in international law, but they have become the common benchmark for meeting Article IX. The current structure dates from 1984, when COSPAR adopted a new policy centered on target-mission categorizations, an approach that has served since as the framework for planetary protection policy.

Under this framework, most Mars spacecraft fall into one of two groups. Orbiters and flybys are placed in Category III, where operators must show that contamination risk is limited, typically through a calculated probability of impact. Landers and rovers fall into Category IV. Within Category IV, the strictest limits apply to bioburden levels on the spacecraft's surface, which are capped both in total spore count and in average spores per square metre. A later subdivision was created after a 2002 COSPAR and IAU workshop in Williamsburg, Virginia, which established a new mission category—Category IVc—for spacecraft accessing a Special Region on Mars. COSPAR defines such regions as zones where Earth organisms could plausibly multiply, or where Martian life might plausibly exist. Missions that bring material back to Earth form a separate class: missions returning extraterrestrial samples to Earth are assigned to Category V, a planetary protection classification reserved for inbound missions.

Sample return and crewed missions raise the most demanding containment questions. If Martian biology exists, returned material and人员 exposure would need strict containment protocols, and any crewed landing would bring human-associated microorganisms into the same environments that scientists want to study. These requirements are costly, which is one reason they feature in debates about the feasibility of future missions.

Several governance questions remain open. The first concerns private actors. Commercial activity has grown, and there does not appear to be agreement that private actors should be held to the same standard as space agencies, since companies may not themselves be engaged in science that could be harmed by contamination. Some scholars argue that the growth of private missions means specific guidelines are needed to protect scientific work from forward contamination, and that such guidelines need not match COSPAR's policy exactly.

The second concerns the use of Mars itself. The treaty bars national appropriation of celestial bodies, but how that principle applies to resource extraction, fixed surface sites, or the boundaries of protected areas is not settled. Policy discussions continue over whether these activities should be regulated internationally, by national law, or through voluntary agreements among operators, and over how far the existing COSPAR categories are suited to them.

Terraforming and long-term settlement debate

Terraforming refers to the hypothetical engineering of a planet's environment so that it could support Earth-like conditions, and for Mars the process would begin with warming. Most proposals start by raising the temperature enough that frozen carbon dioxide and water ice in the polar caps and the regolith would sublimate into the air. Each release would add pressure and warmth, which would in turn free more gas, creating a feedback loop. Suggested ways to start this include placing giant orbital mirrors above the poles, dispersing industrial greenhouse gases into the atmosphere, and darkening the ice so that it absorbs more sunlight. Authors who have assessed these ideas generally describe them as physically conceivable but requiring enormous energy and infrastructure.

The most serious scientific objection concerns the amount of material available. Mars holds much less accessible carbon dioxide than early optimistic scenarios assumed, and recent work suggests that the surface and polar reservoirs may not be enough to build a dense, Earth-like atmosphere. A 2018 study concluded that, even if the gas existed, releasing it would require processing a large fraction of the planet's surface, which was beyond present-day technology. Even at the best case, the planet's gravity, which is about 38 percent of Earth's, would hold only a thin atmosphere of roughly 0.38 bar, so a terraformed Mars would remain far colder and thinner than Earth. Some proposals suggest importing volatiles such as water, carbon dioxide and nitrogen from icy bodies elsewhere in the Solar System, but the cost and engineering demands of moving such large masses are considered extreme.

A second obstacle is the loss of gas to space. Mars has no global magnetic field of the kind that shields Earth, so the Sun's solar wind interacts directly with the upper atmosphere and strips away its particles. Spacecraft have detected this ejection of atoms, and the present loss rate of carbon dioxide has been estimated at only about one millibar per billion years, which is slow on human timescales but would still undermine any attempt to maintain a thick atmosphere over the millennia needed for stable warming. Without a magnetic field, the surface is also exposed to intense solar and cosmic radiation, and the lack of a dense atmosphere offers little protection. One proposal addresses this by placing a large artificial magnet at the Sun–Mars L1 point, which would deflect the solar wind before it reaches the planet. Simulations by researchers including Green and colleagues suggest that a field of roughly 10,000 to 20,000 gauss could be sufficient, whereas current technology can produce only about 2,000 gauss at that location, so the idea is far beyond present capability.

The debate over whether terraforming should be pursued reflects broader disagreements about the purpose of space exploration. Advocates argue that a warmer, wetter, and eventually breathable Mars could become a long-term destination for human expansion, and that the planet's large reserves of ice and volatiles make it a plausible candidate for gradual transformation over centuries or longer. Critics question both the technical feasibility and the assumptions behind the estimates, and they argue that the process would be essentially irreversible. Ethical objections focus on the possibility that Mars harbours microbial life, since large-scale alteration could destroy any indigenous biosphere before it is understood. Such concerns are also linked to microbial life and the broader issue of whether a planet's natural state has value in itself.

These concerns have shaped discussions about the preservation of Mars in its present condition, a position that is closely tied to current planetary-protection policy. Under this view, scientific study of Martian environments, including the search for signs of past or present life, should take priority over large-scale engineering, at least until the biological and geological record is better understood. Opponents of this position reply that the protection of potential life should not rule out future settlement entirely, and that research on closed habitats, local resource use and atmospheric chemistry can proceed in parallel with scientific preservation. Because no terraforming method has yet been tested beyond small laboratory or modelling studies, the question remains largely theoretical, and it will likely depend on advances in climate modelling, engineering and international governance.

Mars in culture, literature and film

Long before telescopes revealed its surface, Mars was read through myth. Greek writers linked the reddish point of light to Ares, the god of war, and the Romans named the planet after their own war deity. Babylonian astronomers associated it with Nergal, a deity of war and pestilence, and Egyptian sources called it "Her Desher," meaning "the red one." In China it became the "fire star" (huoxing), and in Sanskrit Mangala. Medieval astrology gave Mars rule over the zodiac signs Aries and Scorpio, so the planet's colour and brightness came to stand for blood, fire and violence in art and calendars alike. These older associations shaped the way Mars was named, depicted and imagined, and they set the emotional tone that later writers inherited.

The modern image of Mars as a home for other civilisations owes much to the telescope era. H. G. Wells's novel The War of the Worlds, serialised in 1897 and published as a book in 1898, pictured Martians as an intelligent people on a cooling, drying world who invade Earth. Wells drew on the speculation about canals that Giovanni Schiaparelli and Percival Lowell had popularised, and his invaders are a dying species seeking resources. The story later became famous through Orson Welles's 1938 radio adaptation, although accounts of the panic it caused are contested. Edgar Rice Burroughs took a different route in A Princess of Mars (1912), the first of his Barsoom stories. In these tales Mars is an ancient, desiccated planet with canals, walled cities and rival peoples, and the adventure format helped establish the planet as a setting for pulp fiction and space opera.

Mid-20th-century writers used Mars to reflect on human affairs on Earth. Ray Bradbury's The Martian Chronicles (1950) is a collection of linked stories about Earth colonists arriving on a planet with a vanished native civilisation, and critics have read it as an allegory of colonial expansion, displacement and nuclear anxiety. Kim Stanley Robinson's trilogy Red Mars (1992), Green Mars (1993) and Blue Mars (1996) takes a more systematic approach. It follows the first settlers through decades of political conflict over the planet's future, and it treats terraforming, economics and ethics as matters of negotiation rather than engineering alone. The trilogy remains one of the most detailed literary treatments of the terraforming debate discussed elsewhere in this entry.

Film and television have followed a similar range, from spectacle to near-documentary realism. Early genre films such as Total Recall (1990) and Mars Attacks! (1996) used the planet as a backdrop for action and satire. Andy Weir's novel The Martian (2011) and its 2015 film adaptation, by contrast, built a survival story around plausible engineering and orbital mechanics, and it was widely praised for its technical detail. Television has been equally varied. The National Geographic series Mars (2016) combined dramatised scenes with expert interviews about crewed missions, while the alternate-history series For All Mankind (Apple TV+) imagined a space race in which Mars missions were pursued. Such productions often draw on the imagery and findings of real missions, so that the boundary between documentary and fiction is deliberately blurred.

Popular culture has also had a practical effect. Fiction created expectations that Mars was habitable and inhabited, expectations that the first flyby by Mariner 4 in 1965 undermined by revealing a heavily cratered, apparently lifeless surface. Public enthusiasm nonetheless continues to support government funding, science outreach and the naming of spacecraft and rovers. Mission names such as Sojourner, Spirit, Opportunity, Curiosity and Perseverance carry cultural meanings that are deliberately chosen to invite public identification. Fiction also raises questions that mission planners must answer, including who has the right to settle Mars, how indigenous life should be protected, and what a human presence would mean for the planet's future. These questions connect popular imagination with the ethical and legal frameworks that govern real exploration.

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