Nomenclature

In English, the planet is named after the Roman god of war, an association made because of its red color, which suggests blood. The adjectival form of Latin is , which provides the English words Martian, used as an adjective or for a putative inhabitant of Mars, and Martial, used as an adjective corresponding to Terrestrial for Earth. In Greek, the planet is known as , with the inflectional stem . From this come technical terms such as areology, as well as the adjective Arean and the star name Antares.

Mars is also the basis of the name of the month of March (from Latin 'month of Mars'), as well as of Tuesday (Latin 'day of Mars'), where the old Anglo-Saxon god Tíw was identified with Roman god Mars by Interpretatio germanica.

Due to the global influence of European languages in astronomy, a word like Mars or Marte for the planet is common around the world, though it may be used alongside older, native words. A number of other languages have provided words with international usage. For example:

• Arabic – which connotes fire – is used as the (or a) name for the planet in Persian, Urdu, Malay and Swahili, among others

• Chinese 火星 'fire star' (in Chinese the five classical planets are identified with the five elements) is used in Korean, Japanese and Vietnamese.

• India uses the Sanskrit term Mangal derived from the Hindu goddess Mangala.

• A long-standing nickname for Mars is the "Red Planet". That is also the planet's name in Hebrew, , which is derived from , meaning 'red'.

• The archaic Latin form is seen, but only very rarely, in English, though the adjectives Mavortial and Mavortian mean 'martial' in the military rather than planetary sense.

Physical characteristics

Mars is approximately half the diameter of Earth, with a surface area only slightly less than the total area of Earth's dry land. Mars is less dense than Earth, having about 15% of Earth's volume and 11% of Earth's mass, resulting in about 38% of Earth's surface gravity. The red-orange appearance of the Martian surface is caused by iron(III) oxide, or rust. It can look like butterscotch; other common surface colors include golden, brown, tan, and greenish, depending on the minerals present.

Internal structure

Like Earth, Mars has differentiated into a dense metallic core overlaid by less dense materials. Scientists initially determined that the core is at least partially liquid. Current models of its interior imply a core with a radius of about , consisting primarily of iron and nickel with about 16–17% sulfur. This iron(II) sulfide core is thought to be twice as rich in lighter elements as Earth's. The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but it appears to be dormant. Besides silicon and oxygen, the most abundant elements in the Martian crust are iron, magnesium, aluminium, calcium, and potassium. The average thickness of the planet's crust is about , with a maximum thickness of . Earth's crust averages .

Mars is seismically active, with InSight recording over 450 marsquakes and related events in 2019. In March 2021, NASA reported, based on measurements of over 500 Marsquakes by the InSight lander on the planet Mars, that the core of Mars is between , about half the size of the core of Earth, and significantly smaller — suggesting a core of lighter elements — than thought earlier.

Surface geology

Mars is a terrestrial planet whose surface consists of minerals containing silicon and oxygen, metals, and other elements that typically make up rock. The Martian surface is primarily composed of tholeiitic basalt, although parts are more silica-rich than typical basalt and may be similar to andesitic rocks on Earth, or silica glass. Regions of low albedo suggest concentrations of plagioclase feldspar, with northern low albedo regions displaying higher than normal concentrations of sheet silicates and high-silicon glass. Parts of the southern highlands include detectable amounts of high-calcium pyroxenes. Localized concentrations of hematite and olivine have been found. Much of the surface is deeply covered by finely grained iron(III) oxide dust.

Although Mars has no evidence of a structured global magnetic field, observations show that parts of the planet's crust have been magnetized, suggesting that alternating polarity reversals of its dipole field have occurred in the past. This paleomagnetism of magnetically susceptible minerals is similar to the alternating bands found on Earth's ocean floors. One theory, published in 1999 and re-examined in October 2005 (with the help of the Mars Global Surveyor), is that these bands suggest plate tectonic activity on Mars four billion years ago, before the planetary dynamo ceased to function and the planet's magnetic field faded.

It is thought that, during the Solar System's formation, Mars was created as the result of a stochastic process of run-away accretion of material from the protoplanetary disk that orbited the Sun. Mars has many distinctive chemical features caused by its position in the Solar System. Elements with comparatively low boiling points, such as chlorine, phosphorus, and sulphur, are much more common on Mars than Earth; these elements were probably pushed outward by the young Sun's energetic solar wind.

After the formation of the planets, all were subjected to the so-called "Late Heavy Bombardment". About 60% of the surface of Mars shows a record of impacts from that era, whereas much of the remaining surface is probably underlain by immense impact basins caused by those events. There is evidence of an enormous impact basin in the Northern Hemisphere of Mars, spanning , or roughly four times the size of the Moon's South Pole – Aitken basin, the largest impact basin yet discovered. This theory suggests that Mars was struck by a Pluto-sized body about four billion years ago. The event, thought to be the cause of the Martian hemispheric dichotomy, created the smooth Borealis basin that covers 40% of the planet.

The geological history of Mars can be split into many periods, but the following are the three primary periods:

• Noachian period (named after Noachis Terra): Formation of the oldest extant surfaces of Mars, 4.5 to 3.5 billion years ago. Noachian age surfaces are scarred by many large impact craters. The Tharsis bulge, a volcanic upland, is thought to have formed during this period, with extensive flooding by liquid water late in the period.

• Hesperian period (named after Hesperia Planum): 3.5 to between 3.3 and 2.9 billion years ago. The Hesperian period is marked by the formation of extensive lava plains.

• Amazonian period (named after Amazonis Planitia): between 3.3 and 2.9 billion years ago to the present. Amazonian regions have few meteorite impact craters but are otherwise quite varied. Olympus Mons formed during this period, with lava flows elsewhere on Mars.

Geological activity is still taking place on Mars. The Athabasca Valles is home to sheet-like lava flows created about 200 Mya. Water flows in the grabens called the Cerberus Fossae occurred less than 20 Mya, indicating equally recent volcanic intrusions. On 19 February 2008, images from the Mars Reconnaissance Orbiter showed evidence of an avalanche from a cliff.

Soil

The Phoenix lander returned data showing Martian soil to be slightly alkaline and containing elements such as magnesium, sodium, potassium and chlorine. These nutrients are found in soils on Earth, and they are necessary for growth of plants. Experiments performed by the lander showed that the Martian soil has a basic pH of 7.7, and contains 0.6% of the salt perchlorate. This is a very high concentration and makes the Martian soil toxic (see also Martian soil toxicity).

Streaks are common across Mars and new ones appear frequently on steep slopes of craters, troughs, and valleys. The streaks are dark at first and get lighter with age. The streaks can start in a tiny area, then spread out for hundreds of metres. They have been seen to follow the edges of boulders and other obstacles in their path. The commonly accepted theories include that they are dark underlying layers of soil revealed after avalanches of bright dust or dust devils. Several other explanations have been put forward, including those that involve water or even the growth of organisms.

Hydrology

Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is less than 1% that of Earth's, except at the lowest elevations for short periods. The two polar ice caps appear to be made largely of water. The volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of . A permafrost mantle stretches from the pole to latitudes of about 60°. Large quantities of ice are thought to be trapped within the thick cryosphere of Mars. Radar data from Mars Express and the Mars Reconnaissance Orbiter (MRO) show large quantities of ice at both poles (July 2005) and at middle latitudes (November 2008). The Phoenix lander directly sampled water ice in shallow Martian soil on 31 July 2008.

Landforms visible on Mars strongly suggest that liquid water has existed on the planet's surface. Huge linear swathes of scoured ground, known as outflow channels, cut across the surface in about 25 places. These are thought to be a record of erosion caused by the catastrophic release of water from subsurface aquifers, though some of these structures have been hypothesized to result from the action of glaciers or lava. One of the larger examples, Ma'adim Vallis, is long, much greater than the Grand Canyon, with a width of and a depth of in places. It is thought to have been carved by flowing water early in Mars's history. The youngest of these channels are thought to have formed as recently as only a few million years ago. Elsewhere, particularly on the oldest areas of the Martian surface, finer-scale, dendritic networks of valleys are spread across significant proportions of the landscape. Features of these valleys and their distribution strongly imply that they were carved by runoff resulting from precipitation in early Mars history. Subsurface water flow and groundwater sapping may play important subsidiary roles in some networks, but precipitation was probably the root cause of the incision in almost all cases.

Along crater and canyon walls, there are thousands of features that appear similar to terrestrial gullies. The gullies tend to be in the highlands of the Southern Hemisphere and to face the Equator; all are poleward of 30° latitude. A number of authors have suggested that their formation process involves liquid water, probably from melting ice, although others have argued for formation mechanisms involving carbon dioxide frost or the movement of dry dust. No partially degraded gullies have formed by weathering and no superimposed impact craters have been observed, indicating that these are young features, possibly still active. Other geological features, such as deltas and alluvial fans preserved in craters, are further evidence for warmer, wetter conditions at an interval or intervals in earlier Mars history. Such conditions necessarily require the widespread presence of crater lakes across a large proportion of the surface, for which there is independent mineralogical, sedimentological and geomorphological evidence.

Further evidence that liquid water once existed on the surface of Mars comes from the detection of specific minerals such as hematite and goethite, both of which sometimes form in the presence of water. In 2004, Opportunity detected the mineral jarosite. This forms only in the presence of acidic water, which demonstrates that water once existed on Mars. More recent evidence for liquid water comes from the finding of the mineral gypsum on the surface by NASA's Mars rover Opportunity in December 2011. It is estimated that the amount of water in the upper mantle of Mars, represented by hydroxyl ions contained within the minerals of Mars's geology, is equal to or greater than that of Earth at 50–300 parts per million of water, which is enough to cover the entire planet to a depth of .

In 2005, radar data revealed the presence of large quantities of water ice at the poles and at mid-latitudes. The Mars rover Spirit sampled chemical compounds containing water molecules in March 2007.

On 18 March 2013, NASA reported evidence from instruments on the Curiosity rover of mineral hydration, likely hydrated calcium sulfate, in several rock samples including the broken fragments of "Tintina" rock and "Sutton Inlier" rock as well as in veins and nodules in other rocks like "Knorr" rock and "Wernicke" rock. Analysis using the rover's DAN instrument provided evidence of subsurface water, amounting to as much as 4% water content, down to a depth of , during the rover's traverse from the Bradbury Landing site to the Yellowknife Bay area in the Glenelg terrain. In September 2015, NASA announced that they had found conclusive evidence of hydrated brine flows on recurring slope lineae, based on spectrometer readings of the darkened areas of slopes. These observations provided confirmation of earlier hypotheses based on timing of formation and their rate of growth, that these dark streaks resulted from water flowing in the very shallow subsurface. The streaks contain hydrated salts, perchlorates, which have water molecules in their crystal structure. The streaks flow downhill in Martian summer, when the temperature is above −23° Celsius, and freeze at lower temperatures.

Researchers suspect that much of the low northern plains of the planet were covered with an ocean hundreds of meters deep, though this remains controversial. In March 2015, scientists stated that such an ocean might have been the size of Earth's Arctic Ocean. This finding was derived from the ratio of water to deuterium in the modern Martian atmosphere compared to that ratio on Earth. The amount of Martian deuterium is eight times the amount that exists on Earth, suggesting that ancient Mars had significantly higher levels of water. Results from the Curiosity rover had previously found a high ratio of deuterium in Gale Crater, though not significantly high enough to suggest the former presence of an ocean. Other scientists caution that these results have not been confirmed, and point out that Martian climate models have not yet shown that the planet was warm enough in the past to support bodies of liquid water.

Near the northern polar cap is the wide Korolev Crater, where the Mars Express orbiter found it to be filled with approximately of water ice. The crater floor lies about below the rim, and is covered by a deep central mound of permanent water ice, up to in diameter.

In February 2020, it was found that dark streaks called recurring slope lineae (RSL), which appear seasonably, are caused by briny water flowing for a few days annually.

Polar caps

Mars has two permanent polar ice caps. During a pole's winter, it lies in continuous darkness, chilling the surface and causing the deposition of 25–30% of the atmosphere into slabs of CO2 ice (dry ice). When the poles are again exposed to sunlight, the frozen CO2 sublimes. These seasonal actions transport large amounts of dust and water vapor, giving rise to Earth-like frost and large cirrus clouds. Clouds of water-ice were photographed by the Opportunity rover in 2004.

The caps at both poles consist primarily (70%) of water ice. Frozen carbon dioxide accumulates as a comparatively thin layer about one metre thick on the north cap in the northern winter only, whereas the south cap has a permanent dry ice cover about eight metres thick. This permanent dry ice cover at the south pole is peppered by flat floored, shallow, roughly circular pits, which repeat imaging shows are expanding by meters per year; this suggests that the permanent CO2 cover over the south pole water ice is degrading over time. The northern polar cap has a diameter of about during the northern Mars summer, and contains about of ice, which, if spread evenly on the cap, would be thick. (This compares to a volume of for the Greenland ice sheet.) The southern polar cap has a diameter of and a thickness of . The total volume of ice in the south polar cap plus the adjacent layered deposits has been estimated at 1.6 million cubic km. Both polar caps show spiral troughs, which recent analysis of SHARAD ice penetrating radar has shown are a result of katabatic winds that spiral due to the Coriolis Effect.

The seasonal frosting of areas near the southern ice cap results in the formation of transparent 1-metre-thick slabs of dry ice above the ground. With the arrival of spring, sunlight warms the subsurface and pressure from subliming CO2 builds up under a slab, elevating and ultimately rupturing it. This leads to geyser-like eruptions of CO2 gas mixed with dark basaltic sand or dust. This process is rapid, observed happening in the space of a few days, weeks or months, a rate of change rather unusual in geology – especially for Mars. The gas rushing underneath a slab to the site of a geyser carves a spiderweb-like pattern of radial channels under the ice, the process being the inverted equivalent of an erosion network formed by water draining through a single plughole.

Geography and naming of surface features

Although better remembered for mapping the Moon, Johann Heinrich Mädler and Wilhelm Beer were the first areographers. They began by establishing that most of Mars's surface features were permanent and by more precisely determining the planet's rotation period. In 1840, Mädler combined ten years of observations and drew the first map of Mars. Rather than giving names to the various markings, Beer and Mädler simply designated them with letters; Meridian Bay (Sinus Meridiani) was thus feature "a".

Today, features on Mars are named from a variety of sources. Albedo features are named for classical mythology. Craters larger than 60 km are named for deceased scientists and writers and others who have contributed to the study of Mars. Craters smaller than 60 km are named for towns and villages of the world with populations of less than 100,000. Large valleys are named for the word "Mars" or "star" in various languages; small valleys are named for rivers.

Large albedo features retain many of the older names but are often updated to reflect new knowledge of the nature of the features. For example, Nix Olympica (the snows of Olympus) has become Olympus Mons (Mount Olympus). The surface of Mars as seen from Earth is divided into two kinds of areas, with differing albedo. The paler plains covered with dust and sand rich in reddish iron oxides were once thought of as Martian "continents" and given names like Arabia Terra (land of Arabia) or Amazonis Planitia (Amazonian plain). The dark features were thought to be seas, hence their names Mare Erythraeum, Mare Sirenum and Aurorae Sinus. The largest dark feature seen from Earth is Syrtis Major Planum. The permanent northern polar ice cap is named Planum Boreum, whereas the southern cap is called Planum Australe.

Mars's equator is defined by its rotation, but the location of its Prime Meridian was specified, as was Earth's (at Greenwich), by choice of an arbitrary point; Mädler and Beer selected a line for their first maps of Mars in 1830. After the spacecraft Mariner 9 provided extensive imagery of Mars in 1972, a small crater (later called Airy-0), located in the Sinus Meridiani ("Middle Bay" or "Meridian Bay"), was chosen by Merton Davies of the Rand Corporation for the definition of 0.0° longitude to coincide with the original selection.

Because Mars has no oceans and hence no "sea level", a zero-elevation surface had to be selected as a reference level; this is called the areoid of Mars, analogous to the terrestrial geoid. Zero altitude was defined by the height at which there is of atmospheric pressure. This pressure corresponds to the triple point of water, and it is about 0.6% of the sea level surface pressure on Earth (0.006 atm).

Map of quadrangles

For mapping purposes, the United States Geological Survey divides the surface of Mars into thirty cartographic quadrangles, each named for a classical albedo feature it contains. The quadrangles can be seen and explored via the interactive image map below.

Impact topography

The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. Research in 2008 has presented evidence regarding a theory proposed in 1980 postulating that, four billion years ago, the Northern Hemisphere of Mars was struck by an object one-tenth to two-thirds the size of Earth's Moon. If validated, this would make the Northern Hemisphere of Mars the site of an impact crater in size, or roughly the area of Europe, Asia, and Australia combined, surpassing the South Pole–Aitken basin as the largest impact crater in the Solar System.

Mars is scarred by a number of impact craters: a total of 43,000 craters with a diameter of or greater have been found. The largest confirmed of these is the Hellas impact basin, a light albedo feature clearly visible from Earth. Due to the smaller mass and size of Mars, the probability of an object colliding with the planet is about half that of Earth. Mars is located closer to the asteroid belt, so it has an increased chance of being struck by materials from that source. Mars is more likely to be struck by short-period comets, i.e., those that lie within the orbit of Jupiter. In spite of this, there are far fewer craters on Mars compared with the Moon, because the atmosphere of Mars provides protection against small meteors and surface modifying processes have erased some craters.

Martian craters can have a morphology that suggests the ground became wet after the meteor impacted.

Volcanoes

The shield volcano Olympus Mons (Mount Olympus) is an extinct volcano in the vast upland region Tharsis, which contains several other large volcanoes. Olympus Mons is roughly three times the height of Mount Everest, which in comparison stands at just over . It is either the tallest or second-tallest mountain in the Solar System, depending on how it is measured, with various sources giving figures ranging from about high.

Tectonic sites

The large canyon, Valles Marineris (Latin for "Mariner Valleys", also known as Agathadaemon in the old canal maps), has a length of and a depth of up to . The length of Valles Marineris is equivalent to the length of Europe and extends across one-fifth the circumference of Mars. By comparison, the Grand Canyon on Earth is only long and nearly deep. Valles Marineris was formed due to the swelling of the Tharsis area, which caused the crust in the area of Valles Marineris to collapse. In 2012, it was proposed that Valles Marineris is not just a graben, but a plate boundary where of transverse motion has occurred, making Mars a planet with possibly a two-tectonic plate arrangement.

Holes

Images from the Thermal Emission Imaging System (THEMIS) aboard NASA's Mars Odyssey orbiter have revealed seven possible cave entrances on the flanks of the volcano Arsia Mons. The caves, named after loved ones of their discoverers, are collectively known as the "seven sisters". Cave entrances measure from wide and they are estimated to be at least deep. Because light does not reach the floor of most of the caves, it is possible that they extend much deeper than these lower estimates and widen below the surface. "Dena" is the only exception; its floor is visible and was measured to be deep. The interiors of these caverns may be protected from micrometeoroids, UV radiation, solar flares and high energy particles that bombard the planet's surface.

Atmosphere

Mars lost its magnetosphere 4 billion years ago, possibly because of numerous asteroid strikes, so the solar wind interacts directly with the Martian ionosphere, lowering the atmospheric density by stripping away atoms from the outer layer. Both Mars Global Surveyor and Mars Express have detected ionised atmospheric particles trailing off into space behind Mars, and this atmospheric loss is being studied by the MAVEN orbiter. Compared to Earth, the atmosphere of Mars is quite rarefied. Atmospheric pressure on the surface today ranges from a low of on Olympus Mons to over in Hellas Planitia, with a mean pressure at the surface level of . The highest atmospheric density on Mars is equal to that found above Earth's surface. The resulting mean surface pressure is only 0.6% of that of Earth . The scale height of the atmosphere is about , which is higher than Earth's, , because the surface gravity of Mars is only about 38% of Earth's, an effect offset by both the lower temperature and 50% higher average molecular weight of the atmosphere of Mars.

The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water. The atmosphere is quite dusty, containing particulates about 1.5 µm in diameter which give the Martian sky a tawny color when seen from the surface. It may take on a pink hue due to iron oxide particles suspended in it.

Methane

Methane has been detected in the Martian atmosphere; it occurs in extended plumes, and the profiles imply that the methane is released from discrete regions. The concentration of methane fluctuates from about 0.24 ppb during the northern winter to about 0.65 ppb during the summer.

Estimates of its lifetime range from 0.6 to 4 years, so its presence indicates that an active source of the gas must be present. Methane could be produced by non-biological process such as serpentinization involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars. Methanogenic microbial life forms in the subsurface are among possible sources. But even if rover missions determine that microscopic Martian life is the source of the methane, the life forms likely reside far below the surface, outside of the rover's reach.

Aurora

In 1994, the European Space Agency's Mars Express found an ultraviolet glow coming from "magnetic umbrellas" in the Southern Hemisphere. Mars does not have a global magnetic field which guides charged particles entering the atmosphere. Mars has multiple umbrella-shaped magnetic fields mainly in the Southern Hemisphere, which are remnants of a global field that decayed billions of years ago.

In late December 2014, NASA's MAVEN spacecraft detected evidence of widespread auroras in Mars's Northern Hemisphere and descended to approximately 20–30° North latitude of Mars's equator. The particles causing the aurora penetrated into the Martian atmosphere, creating auroras below 100 km above the surface, Earth's auroras range from 100 km to 500 km above the surface. Magnetic fields in the solar wind drape over Mars, into the atmosphere, and the charged particles follow the solar wind magnetic field lines into the atmosphere, causing auroras to occur outside the magnetic umbrellas.

On 18 March 2015, NASA reported the detection of an aurora that is not fully understood and an unexplained dust cloud in the atmosphere of Mars.

In September 2017, NASA reported radiation levels on the surface of the planet Mars were temporarily doubled, and were associated with an aurora 25 times brighter than any observed earlier, due to a massive, and unexpected, solar storm in the middle of the month.

Climate

Of all the planets in the Solar System, the seasons of Mars are the most Earth-like, due to the similar tilts of the two planets' rotational axes. The lengths of the Martian seasons are about twice those of Earth's because Mars's greater distance from the Sun leads to the Martian year being about two Earth years long. Martian surface temperatures vary from lows of about at the winter polar caps to highs of up to in equatorial summer. The wide range in temperatures is due to the thin atmosphere which cannot store much solar heat, the low atmospheric pressure, and the low thermal inertia of Martian soil. The planet is 1.52 times as far from the Sun as Earth, resulting in just 43% of the amount of sunlight.

If Mars had an Earth-like orbit, its seasons would be similar to Earth's because its axial tilt is similar to Earth's. The comparatively large eccentricity of the Martian orbit has a significant effect. 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. As a result, the seasons in the Southern Hemisphere are more extreme and the seasons in the northern are milder than would otherwise be the case. The summer temperatures in the south can be warmer than the equivalent summer temperatures in the north by up to .

Mars has the largest dust storms in the Solar System, reaching speeds of over . These can vary from a storm over a small area, to gigantic storms that cover the entire planet. They tend to occur when Mars is closest to the Sun, and have been shown to increase the global temperature.

Orbit and rotation

Mars's average distance from the Sun is roughly , and its orbital period is 687 (Earth) days. The solar day (or sol) on Mars is only slightly longer than an Earth day: 24 hours, 39 minutes, and 35.244 seconds. A Martian year is equal to 1.8809 Earth years, or 1 year, 320 days, and 18.2 hours.

The axial tilt of Mars is 25.19° relative to its orbital plane, which is similar to the axial tilt of Earth. 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. In the present day epoch, the orientation of the north pole of Mars is close to the star Deneb.

Mars has a relatively pronounced orbital eccentricity of about 0.09; of the seven other planets in the Solar System, only Mercury has a larger orbital eccentricity. It is known that in the past, Mars has had a much more circular orbit. At one point, 1.35 million Earth years ago, Mars had an eccentricity of roughly 0.002, much less than that of Earth today. Mars's cycle of eccentricity is 96,000 Earth years compared to Earth's cycle of 100,000 years. Mars has a much longer cycle of eccentricity, with a period of 2.2 million Earth years, and this overshadows the 96,000-year cycle in the eccentricity graphs. For the last 35,000 years, the orbit of Mars has been getting slightly more eccentric because of the gravitational effects of the other planets. The closest distance between Earth and Mars will continue to mildly decrease for the next 25,000 years.

Habitability and search for life

The current understanding of planetary habitability – the ability of a world to develop environmental conditions favorable to the emergence of life – favors planets that have liquid water on their surface. Most often this requires the orbit of a planet to lie within the habitable zone, which for the Sun extends from just beyond Venus to about the semi-major axis of Mars. During perihelion, Mars dips inside this region, but Mars's thin (low-pressure) atmosphere prevents liquid water from existing over large regions for extended periods. The past flow of liquid water demonstrates the planet's potential for habitability. Recent evidence has suggested that any water on the Martian surface may have been too salty and acidic to support regular terrestrial life.

The lack of a magnetosphere and the extremely thin atmosphere of Mars are a challenge: the planet has little heat transfer across its surface, poor insulation against bombardment of the solar wind and insufficient atmospheric pressure to retain water in a liquid form (water instead sublimes to a gaseous state). Mars is nearly, or perhaps totally, geologically dead; the end of volcanic activity has apparently stopped the recycling of chemicals and minerals between the surface and interior of the planet.

In situ investigations have been performed on Mars by the Viking landers, Spirit and Opportunity rovers, Phoenix lander, and Curiosity rover. Evidence suggests that the planet was once significantly more habitable than it is today, but whether living organisms ever existed there remains unknown. The Viking probes of the mid-1970s carried experiments designed to detect microorganisms in Martian soil at their respective landing sites and had positive results, including a temporary increase of production on exposure to water and nutrients. This sign of life was later disputed by scientists, resulting in a continuing debate, with NASA scientist Gilbert Levin asserting that Viking may have found life. A re-analysis of the Viking data, in light of modern knowledge of extremophile forms of life, has suggested that the Viking tests were not sophisticated enough to detect these forms of life. The tests could even have killed a (hypothetical) life form. Tests conducted by the Phoenix Mars lander have shown that the soil has an alkaline pH and it contains magnesium, sodium, potassium and chloride. The soil nutrients may be able to support life, but life would still have to be shielded from the intense ultraviolet light. A recent analysis of martian meteorite EETA79001 found 0.6 ppm , 1.4 ppm , and 16 ppm , most likely of Martian origin. The suggests the presence of other highly oxidizing oxychlorines, such as or , produced both by UV oxidation of Cl and X-ray radiolysis of . Thus, only highly refractory and/or well-protected (sub-surface) organics or life forms are likely to survive.

A 2014 analysis of the Phoenix WCL showed that the in the Phoenix soil has not interacted with liquid water of any form, perhaps for as long as 600 million years. If it had, the highly soluble in contact with liquid water would have formed only . This suggests a severely arid environment, with minimal or no liquid water interaction.

Scientists have proposed that carbonate globules found in meteorite ALH84001, which is thought to have originated from Mars, could be fossilized microbes extant on Mars when the meteorite was blasted from the Martian surface by a meteor strike some 15 million years ago. This proposal has been met with skepticism, and an exclusively inorganic origin for the shapes has been proposed.

Small quantities of methane and formaldehyde detected by Mars orbiters are both claimed to be possible evidence for life, as these chemical compounds would quickly break down in the Martian atmosphere. Alternatively, these compounds may instead be replenished by volcanic or other geological means, such as serpentinite.

Impact glass, formed by the impact of meteors, which on Earth can preserve signs of life, has been found on the surface of the impact craters on Mars. Likewise, the glass in impact craters on Mars could have preserved signs of life if life existed at the site.

In May 2017, evidence of the earliest known life on land on Earth may have been found in 3.48-billion-year-old geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Pilbara Craton of Western Australia. These findings may be helpful in deciding where best to search for early signs of life on the planet Mars.

In early 2018, media reports speculated that certain rock features at a site called Jura looked like a type of fossil, but project scientists say the formations likely resulted from a geological process at the bottom of an ancient drying lakebed, and are related to mineral veins in the area similar to gypsum crystals.

On 7 June 2018, NASA announced that the Curiosity rover had discovered organic compounds in sedimentary rocks dating to three billion years old, indicating that some of the building blocks for life were present.

In July 2018, scientists reported the discovery of a subglacial lake on Mars, the first known stable body of water on the planet. It sits below the surface at the base of the southern polar ice cap and is about wide. The lake was discovered using the MARSIS radar on board the Mars Express orbiter, and the profiles were collected between May 2012 and December 2015. The lake is centered at 193° East, 81° South, a flat area that does not exhibit any peculiar topographic characteristics. It is mostly surrounded by higher ground except on its eastern side, where there is a depression.

Moons

Mars has two relatively small (compared to Earth's) natural moons, Phobos (about in diameter) and Deimos (about in diameter), which orbit close to the planet. Asteroid capture is a long-favored theory, but their origin remains uncertain. Both satellites were discovered in 1877 by Asaph Hall; they are named after the characters Phobos (panic/fear) and Deimos (terror/dread), who, in Greek mythology, accompanied their father Ares, god of war, into battle. Mars was the Roman counterpart of Ares. In modern Greek, the planet retains its ancient name Ares (Aris: Άρης).

From the surface of Mars, the motions of Phobos and Deimos appear different from that of the Moon. Phobos rises in the west, sets in the east, and rises again in just 11 hours. Deimos, being only just outside synchronous orbit – where the orbital period would match the planet's period of rotation – rises as expected in the east but slowly. Despite the 30-hour orbit of Deimos, 2.7 days elapse between its rise and set for an equatorial observer, as it slowly falls behind the rotation of Mars.

Because the orbit of Phobos is below synchronous altitude, the tidal forces from the planet Mars are gradually lowering its orbit. In about 50 million years, it could either crash into Mars's surface or break up into a ring structure around the planet.

The origin of the two moons is not well understood. Their low albedo and carbonaceous chondrite composition have been regarded as similar to asteroids, supporting the capture theory. The unstable orbit of Phobos would seem to point towards a relatively recent capture. But both have circular orbits, near the equator, which is unusual for captured objects and the required capture dynamics are complex. Accretion early in the history of Mars is plausible, but would not account for a composition resembling asteroids rather than Mars itself, if that is confirmed.

A third possibility is the involvement of a third body or a type of impact disruption. More-recent lines of evidence for Phobos having a highly porous interior, and suggesting a composition containing mainly phyllosilicates and other minerals known from Mars, point toward an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit, similar to the prevailing theory for the origin of Earth's moon. Although the VNIR spectra of the moons of Mars resemble those of outer-belt asteroids, the thermal infrared spectra of Phobos are reported to be inconsistent with chondrites of any class.

Mars may have moons smaller than in diameter, and a dust ring is predicted to exist between Phobos and Deimos.

Exploration

Dozens of crewless spacecraft, including orbiters, landers, and rovers, have been sent to Mars by the Soviet Union, the United States, Europe, India, the United Arab Emirates, and China to study the planet's surface, climate, and geology.

, Mars is host to fourteen functioning spacecraft: eight in orbit – 2001 Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, MAVEN, Mars Orbiter Mission, ExoMars Trace Gas Orbiter, the Hope orbiter, and the Tianwen-1 orbiter – and six on the surface – the Mars Science Laboratory Curiosity rover, the InSight lander, the Perseverance rover, the Ingenuity helicopter, the Tianwen-1 lander, and the Zhurong rover. The public can request images of Mars via the Mars Reconnaissance Orbiter HiWish program.

The Mars Science Laboratory, named Curiosity, launched on 26 November 2011, and reached Mars on 6 August 2012 UTC. It is larger and more advanced than the Mars Exploration Rovers, with a movement rate up to per hour. Experiments include a laser chemical sampler that can deduce the make-up of rocks at a distance of . On 10 February 2013, the Curiosity rover obtained the first deep rock samples ever taken from another planetary body, using its on-board drill. The same year, it discovered that Mars's soil contains between 1.5% and 3% water by mass (albeit attached to other compounds and thus not freely accessible). Observations by the Mars Reconnaissance Orbiter had previously revealed the possibility of flowing water during the warmest months on Mars.

On 24 September 2014, Mars Orbiter Mission (MOM), launched by the Indian Space Research Organisation (ISRO), reached Mars orbit. ISRO launched MOM on 5 November 2013, with the aim of analyzing the Martian atmosphere and topography. The Mars Orbiter Mission used a Hohmann transfer orbit to escape Earth's gravitational influence and catapult into a nine-month-long voyage to Mars. The mission is the first successful Asian interplanetary mission.

The European Space Agency, in collaboration with Roscosmos, launched the ExoMars Trace Gas Orbiter and Schiaparelli lander on 14 March 2016. While the Trace Gas Orbiter successfully entered Mars orbit on 19 October 2016, Schiaparelli crashed during its landing attempt.

In May 2018, NASA's InSight lander was launched, along with the twin MarCO CubeSats that flew by Mars and acted as telemetry relays during the landing. The mission arrived at Mars in November 2018. InSight detected potential seismic activity (a "marsquake") in April 2019.

In 2019, MAVEN spacecraft mapped high-altitude global wind patterns at Mars for the first time. It was discovered that the winds which are miles above the surface retained information about the land forms below.

The United Arab Emirates' Mars Hope orbiter was launched on 19 July 2020, and successfully entered orbit around Mars on 9 February 2021. The probe will conduct a global study of the Martian atmosphere. With this accomplishment, UAE became the second country, after India, to reach Mars on its first attempt.

NASA launched the Mars 2020 mission on 30 July 2020. The Perseverance rover and the Ingenuity helicopter successfully landed on the surface of Mars on 18 February 2021. The mission will cache samples for future retrieval and return of them to Earth.

China's Tianwen-1 lander-rover vehicle successfully landed on Mars on 14 May 2021 (15 May Beijing Time).

Future

The current concept for the Mars sample-return mission would launch in 2026 and feature hardware built by NASA and ESA.

The European Space Agency will launch the ExoMars rover and surface platform sometime between August and October 2022.

Several plans for a human mission to Mars have been proposed throughout the 20th and 21st centuries, but no human mission has yet launched. SpaceX founder Elon Musk presented a plan in September 2016 to, optimistically, launch a crewed mission to Mars in 2024 at an estimated development cost of US$10 billion, but this mission is not expected to take place before 2027. In October 2016, President Barack Obama renewed United States policy to pursue the goal of sending humans to Mars in the 2030s, and to continue using the International Space Station as a technology incubator in that pursuit. The NASA Authorization Act of 2017 directed NASA to get humans near or on the surface of Mars by the early 2030s.

Astronomy on Mars

With the presence of various orbiters, landers, and rovers, it is possible to practice astronomy from Mars. Although Mars's moon Phobos appears about one-third the angular diameter of the full moon on Earth, Deimos appears more or less star-like, looking only slightly brighter than Venus does from Earth.

Various phenomena seen from Earth have also been observed from Mars, such as meteors and auroras. The apparent sizes of the moons Phobos and Deimos are sufficiently smaller than that of the Sun; thus, their partial "eclipses" of the Sun are best considered transits (see transit of Deimos and Phobos from Mars). Transits of Mercury and Venus have been observed from Mars. A transit of Earth will be seen from Mars on 10 November 2084.

On 19 October 2014, comet Siding Spring passed extremely close to Mars, so close that the coma may have enveloped Mars.

Viewing

The mean apparent magnitude of Mars is +0.71 with a standard deviation of 1.05. Because the orbit of Mars is eccentric, the magnitude at opposition from the Sun can range from about −3.0 to −1.4. The minimum brightness is magnitude +1.86 when the planet is in conjunction with the Sun. At its brightest, Mars (along with Jupiter) is second only to Venus in luminosity. Mars usually appears distinctly yellow, orange, or red. NASA's Spirit rover has taken pictures of a greenish-brown, mud-colored landscape with blue-grey rocks and patches of light red sand. When farthest away from Earth, it is more than seven times farther away than when it is closest. When least favorably positioned, it can be lost in the Sun's glare for months at a time. At its most favorable times — at 15-year or 17-year intervals, and always between late July and late September — a lot of surface detail can be seen with a telescope. Especially noticeable, even at low magnification, are the polar ice caps.

As Mars approaches opposition, it begins a period of retrograde motion, which means it will appear to move backwards in a looping motion with respect to the background stars. The duration of this retrograde motion lasts for about 72 days, and Mars reaches its peak luminosity in the middle of this motion.

Closest approaches

Relative

The point at which Mars's geocentric longitude is 180° different from the Sun's is known as opposition, which is near the time of closest approach to Earth. The time of opposition can occur as much as 8.5 days away from the closest approach. The distance at close approach varies between about due to the planets' elliptical orbits, which causes comparable variation in angular size. The second-to-last Mars opposition occurred on 27 July 2018, at a distance of about . The last Mars opposition occurred on 13 October 2020, at a distance of about . The average time between the successive oppositions of Mars, its synodic period, is 780 days; but the number of days between the dates of successive oppositions can range from 764 to 812.

As Mars approaches opposition it begins a period of retrograde motion, which makes it appear to move backwards in a looping motion relative to the background stars. The duration of this retrograde motion is about 72 days.

Absolute, around the present time

Mars made its closest approach to Earth and maximum apparent brightness in nearly 60,000 years, , magnitude −2.88, on 27 August 2003, at 09:51:13 UTC. This occurred when Mars was one day from opposition and about three days from its perihelion, making it particularly easy to see from Earth. The last time it came so close is estimated to have been on 12 September 57,617 BC, the next time being in 2287. This record approach was only slightly closer than other recent close approaches. For instance, the minimum distance on 22 August 1924, was , and the minimum distance on 24 August 2208, will be .

Every 15 to 17 years, Mars comes into opposition near its perihelion. These perihelic oppositions make a closer approach to earth than other oppositions which occur every 2.1 years. Mars comes into perihelic opposition in 2003, 2018 and 2035, with 2020 and 2033 being close to perihelic opposition.

Historical observations

The history of observations of Mars is marked by the oppositions of Mars when the planet is closest to Earth and hence is most easily visible, which occur every couple of years. Even more notable are the perihelic oppositions of Mars, which occur every 15 or 17 years and are distinguished because Mars is close to perihelion, making it even closer to Earth.

Ancient and medieval observations

The ancient Sumerians believed that Mars was Nergal, the god of war and plague. During Sumerian times, Nergal was a minor deity of little significance, but, during later times, his main cult center was the city of Nineveh. In Mesopotamian texts, Mars is referred to as the "star of judgement of the fate of the dead." The existence of Mars as a wandering object in the night sky was recorded by the ancient Egyptian astronomers and, by 1534 BCE, they were familiar with the retrograde motion of the planet. By the period of the Neo-Babylonian Empire, the Babylonian astronomers were making regular records of the positions of the planets and systematic observations of their behavior. For Mars, they knew that the planet made 37 synodic periods, or 42 circuits of the zodiac, every 79 years. They invented arithmetic methods for making minor corrections to the predicted positions of the planets. In Ancient Greece, the planet was known as Πυρόεις|label=none.

In the fourth century BCE, Aristotle noted that Mars disappeared behind the Moon during an occultation, indicating that the planet was farther away. Ptolemy, a Greek living in Alexandria, attempted to address the problem of the orbital motion of Mars. Ptolemy's model and his collective work on astronomy was presented in the multi-volume collection Almagest, which became the authoritative treatise on Western astronomy for the next fourteen centuries. Literature from ancient China confirms that Mars was known by Chinese astronomers by no later than the fourth century BCE. In the East Asian cultures, Mars is traditionally referred to as the "fire star" (Chinese: 火星), based on the Five elements.

During the seventeenth century, Tycho Brahe measured the diurnal parallax of Mars that Johannes Kepler used to make a preliminary calculation of the relative distance to the planet. When the telescope became available, the diurnal parallax of Mars was again measured in an effort to determine the Sun-Earth distance. This was first performed by Giovanni Domenico Cassini in 1672. The early parallax measurements were hampered by the quality of the instruments. The only occultation of Mars by Venus observed was that of 13 October 1590, seen by Michael Maestlin at Heidelberg. In 1610, Mars was viewed by Italian astronomer Galileo Galilei, who was first to see it via telescope. The first person to draw a map of Mars that displayed any terrain features was the Dutch astronomer Christiaan Huygens.

Martian "canals"

By the 19th century, the resolution of telescopes reached a level sufficient for surface features to be identified. A perihelic opposition of Mars occurred on 5 September 1877. In that year, the Italian astronomer Giovanni Schiaparelli used a telescope in Milan to help produce the first detailed map of Mars. These maps notably contained features he called canali, which were later shown to be an optical illusion. These canali were supposedly long, straight lines on the surface of Mars, to which he gave names of famous rivers on Earth. His term, which means "channels" or "grooves", was popularly mistranslated in English as "canals".

Influenced by the observations, the orientalist Percival Lowell founded an observatory which had telescopes. The observatory was used for the exploration of Mars during the last good opportunity in 1894 and the following less favorable oppositions. He published several books on Mars and life on the planet, which had a great influence on the public. The canali were independently found by other astronomers, like Henri Joseph Perrotin and Louis Thollon in Nice, using one of the largest telescopes of that time.

The seasonal changes (consisting of the diminishing of the polar caps and the dark areas formed during Martian summer) in combination with the canals led to speculation about life on Mars, and it was a long-held belief that Mars contained vast seas and vegetation. The telescope never reached the resolution required to give proof to any speculations. As bigger telescopes were used, fewer long, straight canali were observed. During an observation in 1909 by Camille Flammarion with an telescope, irregular patterns were observed, but no canali were seen.

Near the end of the nineteenth century, it was widely accepted in the astronomical community that Mars had life-supporting qualities, including oxygen and water. However, in 1894 W. W. Campbell at Lick Observatory observed the planet and found that "if water vapor or oxygen occur in the atmosphere of Mars it is in quantities too small to be detected by spectroscopes then available". This contradicted many of the measurements of the time and was not widely accepted. Campbell and V. M. Slipher repeated the study in 1909 using better instruments, but with the same results. It wasn't until the findings were confirmed by W. S. Adams in 1925 that the myth of the Earth-like habitability of Mars was finally broken. However, even in the 1960s, articles were published on Martian biology, putting aside explanations other than life for the seasonal changes on Mars. Detailed scenarios for the metabolism and chemical cycles for a functional ecosystem have been published.

Spacecraft visitation

Once spacecraft visited the planet during NASA's Mariner missions in the 1960s and 1970s, these concepts were radically broken. The results of the Viking life-detection experiments aided an intermission in which the hypothesis of a hostile, dead planet was generally accepted.

Mariner 9 and Viking allowed better maps of Mars to be made using the data from these missions, and another major leap forward was the Mars Global Surveyor mission, launched in 1996 and operated until late 2006, that allowed complete, extremely detailed maps of the Martian topography, magnetic field and surface minerals to be obtained. These maps are available online; for example, at Google Mars. Mars Reconnaissance Orbiter and Mars Express continued exploring with new instruments and supporting lander missions. NASA provides two online tools: Mars Trek, which provides visualizations of the planet using data from 50 years of exploration, and Experience Curiosity, which simulates traveling on Mars in 3-D with Curiosity.

In culture

Mars is named after the Roman god of war. In different cultures, Mars represents masculinity and youth. Its symbol, a circle with an arrow pointing out to the upper right, is used as a symbol for the male gender.

The many failures in Mars exploration probes resulted in a satirical counter-culture blaming the failures on an Earth-Mars "Bermuda Triangle", a "Mars Curse", or a "Great Galactic Ghoul" that feeds on Martian spacecraft.

Intelligent "Martians"

The fashionable idea that Mars was populated by intelligent Martians exploded in the late 19th century. Schiaparelli's "canali" observations combined with Percival Lowell's books on the subject put forward the standard notion of a planet that was a drying, cooling, dying world with ancient civilizations constructing irrigation works.

Many other observations and proclamations by notable personalities added to what has been termed "Mars Fever". In 1899, while investigating atmospheric radio noise using his receivers in his Colorado Springs lab, inventor Nikola Tesla observed repetitive signals that he later surmised might have been radio communications coming from another planet, possibly Mars. In a 1901 interview, Tesla said:

It was some time afterward when the thought flashed upon my mind that the disturbances I had observed might be due to an intelligent control. Although I could not decipher their meaning, it was impossible for me to think of them as having been entirely accidental. The feeling is constantly growing on me that I had been the first to hear the greeting of one planet to another.

Tesla's theories gained support from Lord Kelvin who, while visiting the United States in 1902, was reported to have said that he thought Tesla had picked up Martian signals being sent to the United States. Kelvin "emphatically" denied this report shortly before leaving: "What I really said was that the inhabitants of Mars, if there are any, were doubtless able to see New York, particularly the glare of the electricity."

In a New York Times article in 1901, Edward Charles Pickering, director of the Harvard College Observatory, said that they had received a telegram from Lowell Observatory in Arizona that seemed to confirm that Mars was trying to communicate with Earth.

Early in December 1900, we received from Lowell Observatory in Arizona a telegram that a shaft of light had been seen to project from Mars (the Lowell observatory makes a specialty of Mars) lasting seventy minutes. I wired these facts to Europe and sent out neostyle copies through this country. The observer there is a careful, reliable man and there is no reason to doubt that the light existed. It was given as from a well-known geographical point on Mars. That was all. Now the story has gone the world over. In Europe, it is stated that I have been in communication with Mars, and all sorts of exaggerations have spring up. Whatever the light was, we have no means of knowing. Whether it had intelligence or not, no one can say. It is absolutely inexplicable.

Pickering later proposed creating a set of mirrors in Texas, intended to signal Martians.

In recent decades, the high-resolution mapping of the surface of Mars, culminating in Mars Global Surveyor, revealed no artifacts of habitation by "intelligent" life, but pseudoscientific speculation about intelligent life on Mars continues from commentators such as Richard C. Hoagland. Reminiscent of the canali controversy, these speculations are based on small scale features perceived in the spacecraft images, such as "pyramids" and the "Face on Mars". Planetary astronomer Carl Sagan wrote:

Mars has become a kind of mythic arena onto which we have projected our Earthly hopes and fears.

The depiction of Mars in fiction has been stimulated by its dramatic red color and by nineteenth-century scientific speculations that its surface conditions might support not just life but intelligent life. Thus originated a large number of science fiction scenarios, among which is H. G. Wells' The War of the Worlds, published in 1898, in which Martians seek to escape their dying planet by invading Earth.

Influential works included Ray Bradbury's The Martian Chronicles, in which human explorers accidentally destroy a Martian civilization, Edgar Rice Burroughs' Barsoom series, C. S. Lewis' novel Out of the Silent Planet (1938), and a number of Robert A. Heinlein stories before the mid-sixties.

Jonathan Swift made reference to the moons of Mars, about 150 years before their actual discovery by Asaph Hall, detailing reasonably accurate descriptions of their orbits, in the 19th chapter of his novel Gulliver's Travels.

A comic figure of an intelligent Martian, Marvin the Martian, appeared in Haredevil Hare (1948) as a character in the Looney Tunes animated cartoons of Warner Brothers, and has continued as part of popular culture to the present.

After the Mariner and Viking spacecraft had returned pictures of Mars as it really is, an apparently lifeless and canal-less world, these ideas about Mars had to be abandoned, and a vogue for accurate, realist depictions of human colonies on Mars developed, the best known of which may be Kim Stanley Robinson's Mars trilogy. Pseudo-scientific speculations about the Face on Mars and other enigmatic landmarks spotted by space probes have meant that ancient civilizations continue to be a popular theme in science fiction, especially in film.

Interactive Mars map

行星名稱

• 古中國：取其「熒熒如火、亮度與位置變化甚大使人迷惑」之意，命名「熒惑」。《尚書·舜典》記載：「在璿璣玉衡以齊七政。」孔穎達疏：「七政，其政有七，於璣衡察之，必在天者，知七政謂日月與五星也。木曰歲星，火曰熒惑星，土曰鎮星，金曰太白星，水曰辰星。」今日則取名「火星」。

• 古希臘：因火紅之色而取名「Ares」（音：阿瑞斯），源自希臘神話的戰神，天神宙斯的兒子阿瑞斯（希臘語：）。

• 古羅馬：因火紅之色而取名「Mars」（音：馬爾斯），源自羅馬神話的戰神瑪爾斯（拉丁語：）。

物理特徵

以直徑、質量、表面重力來說，火星約介於地球和月球中間：火星直徑約為地球的一半、月球的兩倍，質量約為地球的九分之一、月球的九倍，表面重力約為地球的38%、月球的2.4倍。火星體積約為地球的15%，質量約為11%，表面積略小于地球陸地面積，密度則比其他三顆類地行星還要小很多。2012年8月，加利福尼亞大學洛杉磯分校的教授尹安在分析了100張來自火星勘測軌道飛行器的衛星圖片後發現，火星有類似地球主要板塊劃分的構造特點。

長期觀測火星發現，南半球地勢比北半球高，北極盆地顯示有過大撞擊，推論約45億年前遭冥王星大小天體撞擊之後，不但形成火衛一和火衛二，亦逼使內核熱能散溢出上地幔、內部攪拌逐漸停止，無法以發電機原理持續對流生成磁場。由於火星比地球小，相對表面積與體積成反比而較大，因此火星核心也冷卻得比地球的快，地質活動趨緩，磁場和板塊運動消逝，太陽風帶走大氣變薄導致氣壓偏低，而造成液態水在低溫就會沸騰、無法穩定存在。

內部結構

地質

火星基本上是沙漠行星，地表沙丘、礫石遍佈，沒有穩定的液態水體。二氧化碳為主的大氣既稀薄又寒冷，沙塵懸浮其中，每年常有塵暴發生。與地球相比，地質活動不活躍。

火星地表地貌大部份於遠古較活躍的時期形成，充滿撞擊坑，有密佈的隕石坑、火山與峽谷，包括太陽系最高的山：奧林帕斯山和最大的峽谷：水手號峽谷。另一個獨特的特徵是南北半球的明顯差別：南方是古老、充滿隕石坑的高地，北方則是較年輕的平原，兩極皆有主要以水冰組成的極冠，而上覆的乾冰會隨季節消長。

基於撞擊坑密度的撞擊坑計數法可判別出地表年齡：撞擊坑大而密集處較老，反之則年輕，進而將地質年代分為四個階段：前諾亞紀、諾亞紀、赫斯珀利亞紀和亞馬遜紀。前諾亞紀沒有留下實質地表，此時地形南北差異形成，有全球性磁層；諾亞紀有大量隕石撞擊，火山活動旺盛，可能有溫暖潮濕的大氣、河川和海洋，侵蝕旺盛，但到末期這些活動已減弱很多；赫斯珀利亞紀，火山活動仍然繼續；亞馬遜紀則是大氣稀薄乾燥，以冰為主要活動，如極冠、冰凍層、冰河，並有週期性變遷，溝壑也是這時期形成，火山活動趨緩並集中在塔爾西斯與埃律西昂。

現今火星風成地形遍佈，如吹蝕、磨蝕等風蝕作用，和沙塵遇地形阻礙而填積、侵積等風積作用。（名詞解釋：）前者形成如廣泛分布於梅杜莎槽溝層的風蝕脊，後者則如大瑟提斯高原上撞擊坑下風處的沙塵堆積，和撞擊坑中常見的沙丘。

地理與命名

火星和地球一樣擁有多樣的地形，有高山、平原和峽谷。南北半球的地形有著強烈的對比：北方是被熔岩填平的低原，南方則是充滿撞擊坑的古老高地，而兩者之間以明顯的斜坡分隔；火山地形穿插其中，眾多峽谷分布各地，南北極有以水冰與乾冰組成的極冠，而風成沙丘廣布整個星球。隨著衛星拍攝的越來越多，更發現很多耐人尋味的地形景觀。

20世紀早期地面以無線電波測量火星地形。1976年海盜號進行的地形測量，發現了峽谷和南北半球的巨大差異，而衍生出北方平原本是海洋的假說。火星全球勘測者自1999年起以雷射進行更精確的地形測量，得出目前使用的全球地形圖，以火星大地水準面（Areoid）為基準，最高點在奧林帕斯山，高21,229公尺；最低點在希臘平原，低於基準8,200公尺。現在很多探測器如火星勘察衛星、火星快車號和火星探測漫遊者運用航照圖的地形判別方法，以視差法來測量區域地形，並製成高解析度立體照片。

火星的經度坐標採用東經0至360度，不是地球的東西經各180度。

來自火星奧德賽號上熱輻射成像系統（THEMIS）的影像顯示阿爾西亞山北坡有七個可能的深洞，照片中光線無法抵達底部，推測底部可能更深、更寬，可能免受微隕星、紫外線、太陽閃焰和其他高能粒子的侵害，可能是未來尋找液態水或生命痕跡的可行地點。但後來火星勘察衛星的更高解析度HiRISE影像部分推翻了之前猜測，認為只是光線角度造成深不見底的樣子。

大氣

火星大氣層相對較薄，平均地表氣壓只有6百帕，約為地球表面氣壓的0.6%，相當於地球表面算起35公里高的氣壓，如此低的氣壓使聲音傳播的距離只有在地球上的1.5%。隨著季節的變化，火星氣壓變化可達20%。火星大氣層按高度可分為低層大氣、中層大氣、上層大氣和外氣層。其中低層大氣由於氣懸微塵與地表的熱，這部份相對溫暖；中層大氣存在有高速氣流；上層大氣（或熱氣層）溫度很高，大氣分子也不再像下層那樣分布均勻；外氣層高度在200公里以上，大氣漸漸過度到太空，無明顯外層邊界。

火星大氣成分為95%的二氧化碳，3%的氮氣，1.6%氬氣，很少的氧氣、水氣等，亦充滿著很多懸浮塵埃，吸收藍光使天空成黃褐色。2003年火星大衝時地面望遠鏡在大氣中發現了甲烷；2004年3月，火星奧德賽號確認了這一發現。由於甲烷易被紫外線分解，存在甲烷表示現在或者最近幾百年內在火星上存在製造甲烷的來源，火山作用、地質作用、彗星或小行星撞擊甚至生物來源如甲烷古菌等都有可能。

2013年9月19日，根據從好奇號得到的進一步測量數據，NASA科學家報告，並沒有偵測到大氣甲烷（atmospheric methan）存在跡象，測量值為 ppbv，對應於1.3 ppbv上限（95%置信限），因此總結甲烷微生物活性概率很低，可能火星不存在生命。但是，很多微生物不會排出任何甲烷，仍舊可能在火星發現這些不會排出任何甲烷的微生物。

由於火星比地球離太陽遠，日射量較少，表面溫度應較低，計算值約210K，但實際觀測地表平均約240K，則是因為大量的二氧化碳所造成的溫室效應。由於大氣層很薄，無法保留很多熱，使地表日夜溫差很大，某些地區地表溫度白天可達28℃，夜晚可低至-132℃，平均-52℃。火星大氣環流主要為單胞環流，由赤道相對熱空氣上升，漂至極區下沉，再沿地面回到赤道。另外，在火星的北半球，極冠的二氧化碳昇華進入大氣，使氣壓升高；而南半球由於二氧化碳凝華，氣壓下降，由於進出大氣的二氧化碳量高達25%，造成南北壓力差，空氣便傾向由高壓的夏半球流向低壓的冬半球，形成另一依季節而變向的環流。因此火星的天氣系統趨向成為全球性的，例如塵暴。http://www.adlerplanetarium.org/cyberspace/planets/mars/circulation.html

火星天氣重覆次數較高，比地球容易預測。如果一個氣象事件在一年的特定時間中發生，可提供的資料（相當稀疏）指出，很可能在下一年幾乎同一個位置再發生一次，誤差最多一個星期。2008年9月29日，鳳凰號拍下了降雪事件，是在接近鳳凰號登陸地點附近海姆達爾撞擊坑之上，高 4.5 公里的雲降雪。這次降水在到達火星表面時就已蒸發，這現象稱為幡狀雲。火星上的風速要超過地球100倍。

水文

火星地表遍佈著流水的遺跡，有些是洪水刻畫而成，有些則是降雨或地下水流動而形成，但多半年代久遠。沖蝕溝（gullies）則是另一類規模較小的地形，但形成年代十分年輕，常分布於撞擊坑壁，型態多樣。關於成因有兩派說法，一派認為是由流動的水造成，另一方則認為是凹處累積的乾冰促使了鬆軟物質滑動。

火星南北極有明顯的極冠，曾被認為是由乾冰組成，但實際上絕大部分為水冰，只有表面一層為乾冰。這層乾冰在北極約1公尺厚，在南極則約8公尺厚，是冬季時凝華而成，到夏季則再度昇華進入大氣，不過南極的乾冰並不會完全昇華。夏季仍存在的部分稱為永久極冠，而整體構造稱做極地層狀沉積（Polar Layered Deposits），和地球南極洲與格陵蘭冰層一樣為一層層的沉積構造。北極冠寬達1,100公里，厚達2公里，體積82.1萬立方公里；南極冠寬達1,400公里，最厚達3.7公里，體積約1.6百萬立方公里。兩極冰冠皆有獨特的螺旋狀凹谷，推論主要是由光照與夏季接近昇華點的溫度使溝槽兩側水冰發生差異融解和凝結而逐漸形成的。

2011年由火星勘察衛星的淺地層雷達發現南極冠有部分原本認為是水冰的地層其實是乾冰，所含二氧化碳量相當於大氣含量的80%，這比以往認為的要多很多。根據此的模擬結果，十萬年一週期的氣候變遷中藉由乾冰昇華、凝結，大氣總質量的變化幅度會達數倍。由這些乾冰沉積上方地表的下陷與裂隙判斷，乾冰正在慢慢昇華。

自海盜號即發現，火星北半球中緯度有幾處峽谷底含有條紋流動狀的地表特徵，但不確定是富含冰的山崩、含冰土的流動或是塵礫覆蓋的冰河。但根據更新任務的資料與比對地球的相關地形，支持這些是冰河，且推測是自轉軸傾角較大時的氣候狀態下所累積的。

由火星奧德賽號X射線光譜儀的中子偵測器得知，自極區延伸至緯度約60°的地方表層一公尺的土壤含冰量超過60%，推論有更大量的水凍在厚厚的地下冰層（cryosphere）。

另外一個關於火星上曾存在液態水的證據，就是發現特定礦物，如赤鐵礦和針鐵礦，而這兩者都需在有水環境才能形成。

對於於火星上有冰存在的直接證據在2008年6月20日被鳳凰號發現，鳳凰號在火星上挖掘發現了八粒白色的物體，當時研究人員揣測這些物體不是鹽（在火星有發現鹽礦）就是冰，而四天後這些白粒就憑空消失，因此這些白粒一定昇華了，鹽不會有這種現象。2008年7月31日，美國航空航天局科學家宣布，鳳凰號火星探測器在火星上加熱土壤樣本時鑑別出有水蒸氣產生，從而最終確認火星上有水存在。

2013年9月26日，美國航空航天局科學家報告，火星探測車好奇號發現火星土壤含有豐富水分，大約為1.5至 3重量百分比，顯示火星有足夠的水資源供給未來移民使用。

2015年9月28日，美國航空航天局宣佈，在火星上發現液態的鹽水。根據火星勘測軌道飛行器配備的光譜儀獲得的數據，研究人員在火星的神秘斜坡上發現了水合礦物。這些暗色條紋表明火星地表隨時間變化有流水存在。在較溫暖的季節，這些線條的顏色變得更深，表明水流在斜坡上出現，在較冷的季節，這些地表特徵變淺。在火星的部分地區，最高溫度可以達到攝氏零下23度，此時深色線條最明顯。

2018年7月25日，據意大利媒體報道，該國科學家在火星上首度發現一個地下液態水湖。該研究稱，「火星地下及電離層高級探測雷達」在火星南極冰層下1.5千米處發現一個大型液態水湖，裡面含有鹽。湖的直徑約為20千米，溫度至少為零下10度。

運動規律

火星與太陽平均距離為1.52AU，公轉週期為1.88地球年，687地球日，或668.6火星日。火星公轉軌道和地球的一樣，受太陽系其他天體影響而不斷變動。軌道離心率有兩個變化週期，分別是9.6萬年和210萬年，於0.002至0.12間變化；而地球的是10萬年和41.3萬年，於0.005至0.058間變化（見米蘭科維奇循環）。

火星日平均為24小時39分35.244秒，或1.027地球日。火星目前自轉軸傾角為25.19度，和地球的相近，但可在13度至40度間變化，週期為一千多萬年，不像地球的穩定處於22.1和24.5度間，是因為火星沒有如月球般的巨大衛星來維持自轉軸。由于沒有大衛星的潮汐作用，火星自轉週期變化小，不像地球的會被慢慢拉長。

火星自轉軸有明顯傾斜，日照的年變化形成明顯的四季變化，而一季的長度約為地球的兩倍。由於火星軌道離心率大，為0.093（地球只有0.017），使各季節長度不一致，又因遠日點接近北半球夏至，北半球春夏比秋冬各長約40天。2009年10月26日為北半球春分，2010年5月13日為夏至，目前北半球處春季。雖然火星沒有地球般受海洋影響的複雜氣候，但仍有以下特殊之處：火星軌道離心率比地球大，造成日射量在一年當中變化更大，位於近日點時，南半球處夏季，比北半球遠日點夏季所造成的升溫更強；隨季節交替，二氧化碳和水氣會昇華和凝結而在兩極冠間遷移，驅動大氣環流；地表反照率特徵，因顏色深淺和沙、岩性質差異而造成的容積熱容不同，可影響大氣環流；易發生的塵暴會將沙塵粒子捲入高空，沙塵粒子吸收日光與再輻射會使高層大氣增溫，但遮蔽天空的沙塵會使地表降溫；自轉軸傾角和軌道離心率的長期變化則造成了氣候的長期變遷。火星表面的平均溫度比地球低30度以上。

目前火星與地球最短距離正慢慢減小。當地球與火星之間的距離最小時，稱為火星衝日。火星相鄰兩次衝日的時間間隔約為779天，最近一次出現在2018年7月27日，下一次火星衝日將出現在2020年10月13日。當地球與太陽和火星連成一線時，在火星上便可看到地球凌日，在太陽的位置可看到地球的黑點通過，同理還有水星凌日 (火星)，在地球上則不會看到火星凌日。

衛星

火星有兩個天然衛星——火衛一（Phobos）與火衛二（Deimos），最長直徑各為27公里和16公里，形狀不規則並充滿撞擊坑，以近圓形的軌道於接近火星赤道面處公轉。它們雖然很小，但由於接近火星，使火衛一從火星上看約有滿月直徑的二分之一至三分之一大，而視星等火衛一可達-7，火衛二可達-5，白天可能可見。和月球一樣，這兩顆衛星都被火星潮汐鎖定，因此他們總是以一面對著火星。火衛一的公轉週期比火星自轉更快，所以在火星上來看是西升東落的，且只花了約4個小時；而火衛二的公轉周期只比火星自轉慢一些，東昇西落要花約2.4個火星日。因為火衛一離火星很近，火星的潮汐力會慢慢但穩定地減小它的軌道半徑，預計再過約760萬年，火衛一將因軌道低於3620公里，也就是火星的洛希極限而被瓦解。另一方面火衛二因為離火星足夠遠，所以它的軌道反而正在慢慢地被推進。

兩衛星可能是捕獲的小行星，但新研究認為可能是撞擊事件、或原本的衛星被火星潮汐力拉碎後，由散佈軌道上的岩屑再度吸積而形成。

兩顆衛星是在1877年被阿薩夫·霍爾發現的，以希臘神話中的福波斯和得摩斯命名，兩者皆為戰神阿瑞斯的兒子。

觀測探測

古代

火星的火紅色，自古就吸引著人們，希臘人稱為戰神。此時火星觀測和其他天體般，大部分是為了占星，而後漸漸涉及科學方面，如克卜勒探索行星運動定律時是依據第谷積累的大量而精密的火星運行觀測資料。

望遠鏡出現後，人們對火星可以進行更進一步的觀測。使用望遠鏡觀測星空的伽利略所見的火星只是一個橘紅小點，然而隨著望遠鏡的發展，觀測者開始辨別到一些明暗特徵。惠更斯依此測出火星自轉週期約為24.6小時，而他亦為首次紀錄火星南極冠的人。一開始由于各人各自觀測，意見不一致，地名也未統一（例如用繪製者名字命名）。後來義大利的喬范尼·斯基亞帕雷利統合了各家說法而繪製了地圖，地名取自地中海、中東等的地名和聖經等作為來源，而其餘則依照舊有的觀念：暗區被認為是湖（lacus）海（mare）等水體，如太陽湖、塞壬海、明顯的暗大三角——大瑟提斯；而亮區則是陸地，如亞馬遜。這個命名系統一直延續下來。

當時，斯基亞帕雷利和同期觀測者一樣，觀察到了火星表面似乎有一些從暗區延伸出的細線，因為對於暗區是水體的傳統，這些細線命名為水道（canali）。而後來觀察到暗區會在冬季時縮小、夏季時擴張，有人提出暗區是植物覆蓋、而暗區的擴大縮小則是消長所引起的，改變以往認為暗區是水的說法。帕西瓦爾·羅威爾觀察到並宣稱那些「水道」其實是人工挖掘的「運河」，用來灌溉植物，因為水道應太細不可見，而看到的細線應是灌溉出的大片植物。風靡大眾的火星科幻和火星人即源于此。不過這些細線大多已證明是不存在的，部分則是峽谷或隕石坑後延伸出的深色沙子。而火星表面顏色的改變則是因為沙被風吹移，或發生火星塵暴。

到了太空時代，水手4號傳回的充滿隕石坑的火星照片粉碎了人們對火星文明的幻想，認為火星只是一處如月球般佈滿隕石坑的死寂星球。但隨著往後水手9號等的巨大峽谷、火山和疑似流水遺跡的發現，火星的獨特性、液態水和生命的可能又重新引起人們的興趣。（見#近代探測）

近代

蘇聯、美國、歐洲、日本、印度、中國 和阿拉伯聯合大公國共已發射數十艘太空船研究火星表面、地質和氣候。這些太空船包括軌道衛星、登陸器和漫遊車，但大約有三分之二的任務在完成前或剛要開始時就因種種原因而失敗。目前將一公斤物體由地球表面送往火星平均要花費約30,900美元。

1965年水手4號飛掠火星。1971年水手9號進入火星軌道，成為第一個環繞火星的探測船。1971年蘇聯火星計畫火星2號的登陸器墜毀後數日，相同的火星3號的登陸器成功登陸火星，是第一個成功登陸火星的探測器，但登陸十幾秒後隨即失去聯繫。1975年NASA發射海盜號，包括兩組軌道衛星和登陸器。海盜1號和2號軌道衛星各運作了六年和三年。兩個登陸器皆於1976年成功登陸，並傳送了第一張火星地景的彩色照片，而軌道衛星也繪製了很好的火星地圖，甚至到今天都還在使用。

1988年蘇聯發射弗伯斯1號、2號以探測火星和兩個衛星。弗伯斯1號於抵達前失去聯繫，而弗伯斯2號雖然成功拍攝了火星和火衛一，但在放出兩艘登陸器到火衛一前也失去聯繫。

在1992年火星觀察者失敗後，NASA於1996年11月發射了火星全球勘測者。火星全球勘測者出色地完成任務，它在2001年完成了地圖繪製的任務，並三次延長任務，直到2006年11月2日失去聯繫而結束，總計共花了10年在太空中工作。在火星全球勘測者發射一個月後，NASA發射了火星探路者，包括了一個登陸器和漫遊車——旅居者號（Sojourner），於1997年7月登陸在阿瑞斯峽谷。這任務也很成功，而且也廣為人知，其中的原因是因為傳回了大量照片。

NASA的火星勘測98計畫於1998、99年發射了火星氣候衛星與火星極地登陸者，前者預計研究氣候、水與二氧化碳等，後者則預計於南極登陸，船上的搭載深空2號則計劃於火星極地登陸者進入大氣時與它分離，直接降落並穿入地表進行研究。但整個計劃在2000年到達火星時失敗。

NASA於2007年8月發射鳳凰號，於2008年5月登陸在火星北緯68度的極區。鳳凰號登陸器有一支可伸及2.5公尺的機械手臂，並可挖掘土壤1公尺深。它還搭載一座顯微鏡，解析度達人類頭髮寬度的千分之一。2008年6月20日確認在2008年6月15日發現的地表白色物質為水冰。2008年11月10日進入冬季而無法繼續聯繫鳳凰號，任務結束。

2001年NASA發射了2001火星奧德賽號，任務成功進行並延續到2010年9月。船上的伽瑪射線光譜儀於地表下一公尺內偵測到大量的氫，也就是大量的水分子。

2003年歐洲太空總署發射了火星快車號，包括軌道衛星和登陸器——小獵犬2號，而小獵犬2號於2004年2月降落時失敗。2004年船上的行星傅立葉光譜儀於大氣中偵測到甲烷。2006年6月ESA宣布火星快車號發現極光。

2003年NASA發射了兩台相同的火星探測漫遊者——精神號（MER-A）和機會號（MER-B）。兩台皆於2004年1月成功登陸並工作超過預定時間。傳回的資料中最有價值的大概是兩地過去有水的確實證據。塵捲風和風暴偶爾清除了太陽能板上的沙塵，使它們能以超過預定任務時間繼續工作。

2005年8月NASA發射了火星勘察衛星，於2006年3月進入火星軌道展開為期2年的工作。它搭載更進步的通訊系統，頻寬比之前任務總和還寬，且傳回的資料遠多於過去任務的總和。擁有解析度高達0.3公尺的相機——HiRISE，拍攝地表和天氣以尋找未來任務的適合登陸地點。2008年2月19日拍攝到北極冠邊緣的一系列雪崩影像。

2007年2月25日，探測彗星的羅塞塔號近距離飛掠火星並拍照，有拍到很高的雲。

2009年2月17日，黎明號飛掠火星以重力助推前往目的地灶神星和穀神星，並在接近火星時拍了照。

中俄合作的福布斯-土壤號于2011年升空，將會送回火衛一土壤樣本。而該探測器還將搭載一顆重110公斤的火星探測器，也就中國第一艘無人駕駛火星探測船螢火一號（YH-1），預計乘坐俄羅斯的聯盟號運載火箭升空，航程大約10個月。螢火一號主要研究火星的電離層及周圍空間環境，火星磁場等。該探測器發射到近地軌道後，因為與地面失去聯繫變軌失敗，探測器的碎片于莫斯科時間2012年1月15日墜落在太平洋海域。

繼鳳凰號之後，NASA於2011年的發射的火星科學實驗室（好奇號），在2012年8月6日05:31UTC成功登陸火星的蓋爾撞擊坑。它和火星探測漫遊者一樣是火星車，但比火星探測漫遊者更大、速度更快，而且設備更完善。它搭載雷射化學檢測儀，可在13公尺外分析岩石組成。比起之前其它火星任務，它攜帶了更多先進科學儀器。本次任務的總成本達到了25億美元，是歷來最貴的火星探測任務。

2008年9月15日NASA發表了MAVEN任務，預計2013年以各種機器研究火星大氣。

芬蘭、俄羅斯的合作計劃MetNet包括數十個登陸器組成觀測網，以研究火星的大氣結構、物理和天氣。這任務的前導任務將會於2011年先發射一至數個登陸器，有可能是和火衛一-土壤號併在一起發射。往後的發射會持續到2019年。

2016年ESA計劃發射第一台火星車——ExoMars，它可挖掘兩公尺深以尋找有機物甚至火星生命。

2004年美國總統布什宣布載人火星任務為太空探索展望中的長期目標。NASA和洛克希德·馬丁已開始研究獵戶座太空船，計劃於2020年以前送人類到月球，作為人類登陸火星的準備。2007年9月28日，NASA執行長麥可·D·格里芬聲明NASA預計於2037年以前送人類到火星。

2021年5月15日，中國國家航天局的天問一號著陸器和祝融號火星車在火星烏托邦平原南部預選著陸區成功著陸。

ESA希望於2030至2035年間送人類上火星。但在這之前還有其他探測任務，包括ExoMars和火星樣本取回任務。

直達火星是羅伯·祖賓——火星學會的創始人和主席——提出的極低成本載人火星任務，使用重載的農神五號級火箭，如戰神五號或太空探索技術公司（SpaceX）的獵鷹九號，省略軌道組裝、低地軌道會合和月球燃料補給站而直接用小的太空船前往火星。修改後的計劃，叫做Mars to Stay ，改成先不送回第一批登陸者，狄恩·尤尼克說明送回一開始的四到六人所花費用比送他們到火星還高，反而可再送二十人。

2007 WD5：2007年11月20日NASA JPL近地天體觀測計劃發現，一顆直徑約50公尺的小行星2007 WD5可能會在2008年1月30日撞擊火星，但隨著觀測資料越多，終把撞擊機率降至0.01%，小行星則於1月30日掠過火星。

火星生命

2000年，美國科學家在南極洲發現了一塊火星隕石。這是一塊碳酸鹽隕石，後被編號為ALH84001。美國國家航空航天局聲稱在這塊隕石上發現了一些類似微體化石的結構，有人認為這可能是火星生命存在的証據，但也有人認為這只是自然生成的礦物晶體。直到2004年，爭論的雙方仍然沒有任何一方占據上風。

有証據顯示火星曾比現在更適合生命存在，但生命在火星上到底是否真正存在過還沒有確切的結論。某些研究者認為源自火星的ALH84001隕石有過去生命活動的証據，但這個看法至今尚未得到公認。另有反對的觀點認為，自幾十億年前產生以來，該隕石從未長期處于液態水存在的溫度下，因而不會曾有生命活動。

海盜號曾做實驗檢測火星土壤中可能存在的微生物。實驗只分析了海盜號著陸點處的土壤並給出了陽性的結果，但隨後即被許多科學家所否定，而這一結果也仍就處在爭議之中。現存生物活動也是火星大氣中存在微量甲烷的解釋之一，但亦有其它與生命無關的解釋。

人類若對外星殖民，由于火星的適宜條件（同其他行星相比，火星最像地球，而且距離相對較近），它將是人類的首選地點。

2018年6月6日，美國太空總署宣布，好奇號探測車在火星的古老湖床的岩石裏，發現有機物質。這可能對尋找生命給出重要線索。

相關文化及網絡用語

中國古人認為火星在位置及亮度上都常變不定，故稱為「熒惑」，在星占學上象徵殘、疾、喪、飢、兵等惡象。「熒惑守心」是火星留守在心宿（天蠍座）的天文現象，心宿主要有三顆星，中間這顆最亮，代表皇帝，旁邊的兩顆代表太子、庶子。熒惑守心是很罕見的天象，被認為最不祥，可能出現兩種結果一是皇帝駕崩，或是宰相下台。西漢成帝綏和二年（前7年），天文台觀測到了熒惑守心，宰相翟方進被漢成帝賜了毒酒自殺。翟方進死沒幾天，漢成帝突然暴斃，王莽後來稱帝，翟方進之子翟義起兵反王莽。

台灣國立清華大學黃一農教授在他的專書《名家專題精講系列—社會天文學史十講》內的其中一篇文章《中國星占學上最凶的天象──「熒惑守心」》提到，現在以電腦推算發現當年並未發生此天象，中國史籍中記載熒惑守心共二十三次，但有十七次是偽造的。中國歷史上實際發生過的熒惑守心則共有三十八次，且在中國史籍多無記錄。

關于火星的神話傳說有：

• 阿瑞斯，希臘戰神

• 瑪爾斯，羅馬戰神

• 內爾伽勒，巴比倫神祇

• 提爾，北歐神話中的戰神

• 火星 (妖怪)，中國神話中的妖怪，記載于《搜神記》

火星文是中文互聯網曾經流行的一種特殊的文本，大量使用同音字、音近字、特殊符號來表音，難以閱讀。稱為火星文，取「地球人看不懂的文字」的諷刺意味。