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

行星名称

• 古中国：取其「荧荧如火、亮度与位置变化甚大使人迷惑」之意，命名「荧惑」。《尚书·舜典》记载：「在璇玑玉衡以齐七政。」孔颖达疏：「七政，其政有七，于玑衡察之，必在天者，知七政谓日月与五星也。木曰岁星，火曰荧惑星，土曰镇星，金曰太白星，水曰辰星。」今日则取名「火星」。

• 古希腊：因火红之色而取名「Ares」（音：阿瑞斯），源自希腊神话的战神，天神宙斯的儿子阿瑞斯（希腊语：）。

• 古罗马：因火红之色而取名「Mars」（音：马尔斯），源自罗马神话的战神玛尔斯（拉丁语：）。

物理特徵

以直径、质量、表面重力来说，火星约介于地球和月球中间：火星直径约为地球的一半、月球的两倍，质量约为地球的九分之一、月球的九倍，表面重力约为地球的38%、月球的2.4倍。火星体积约为地球的15%，质量约为11%，表面积略小于地球陆地面积，密度则比其他三颗类地行星还要小很多。2012年8月，加利福尼亚大学洛杉矶分校的教授尹安在分析了100张来自火星勘测轨道飞行器的卫星图片后发现，火星有类似地球主要板块划分的构造特点。

长期观测火星发现，南半球地势比北半球高，北极盆地显示有过大撞击，推论约45亿年前遭冥王星大小天体撞击之后，不但形成火卫一和火卫二，亦逼使内核热能散溢出上地幔、内部搅拌逐渐停止，无法以发电机原理持续对流生成磁场。由于火星比地球小，相对表面积与体积成反比而较大，因此火星核心也冷却得比地球的快，地质活动趋缓，磁场和板块运动消逝，太阳风带走大气变薄导致气压偏低，而造成液态水在低温就会沸腾、无法稳定存在。

内部结构

地质

火星基本上是沙漠行星，地表沙丘、砾石遍布，没有稳定的液态水体。二氧化碳为主的大气既稀薄又寒冷，沙尘悬浮其中，每年常有尘暴发生。与地球相比，地质活动不活跃。

火星地表地貌大部份于远古较活跃的时期形成，充满撞击坑，有密布的陨石坑、火山与峡谷，包括太阳系最高的山：奥林帕斯山和最大的峡谷：水手号峡谷。另一个独特的特徵是南北半球的明显差别：南方是古老、充满陨石坑的高地，北方则是较年轻的平原，两极皆有主要以水冰组成的极冠，而上覆的乾冰会随季节消长。

基于撞击坑密度的撞击坑计数法可判别出地表年龄：撞击坑大而密集处较老，反之则年轻，进而将地质年代分为四个阶段：前诺亚纪、诺亚纪、赫斯珀利亚纪和亚马逊纪。前诺亚纪没有留下实质地表，此时地形南北差异形成，有全球性磁层；诺亚纪有大量陨石撞击，火山活动旺盛，可能有温暖潮湿的大气、河川和海洋，侵蚀旺盛，但到末期这些活动已减弱很多；赫斯珀利亚纪，火山活动仍然继续；亚马逊纪则是大气稀薄乾燥，以冰为主要活动，如极冠、冰冻层、冰河，并有周期性变迁，沟壑也是这时期形成，火山活动趋缓并集中在塔尔西斯与埃律西昂。

现今火星风成地形遍布，如吹蚀、磨蚀等风蚀作用，和沙尘遇地形阻碍而填积、侵积等风积作用。（名词解释：）前者形成如广泛分布于梅杜莎槽沟层的风蚀脊，后者则如大瑟提斯高原上撞击坑下风处的沙尘堆积，和撞击坑中常见的沙丘。

地理与命名

火星和地球一样拥有多样的地形，有高山、平原和峡谷。南北半球的地形有著强烈的对比：北方是被熔岩填平的低原，南方则是充满撞击坑的古老高地，而两者之间以明显的斜坡分隔；火山地形穿插其中，众多峡谷分布各地，南北极有以水冰与乾冰组成的极冠，而风成沙丘广布整个星球。随著卫星拍摄的越来越多，更发现很多耐人寻味的地形景观。

20世纪早期地面以无线电波测量火星地形。1976年海盗号进行的地形测量，发现了峡谷和南北半球的巨大差异，而衍生出北方平原本是海洋的假说。火星全球勘测者自1999年起以雷射进行更精确的地形测量，得出目前使用的全球地形图，以火星大地水准面（Areoid）为基准，最高点在奥林帕斯山，高21,229公尺；最低点在希腊平原，低于基准8,200公尺。现在很多探测器如火星勘察卫星、火星快车号和火星探测漫游者运用航照图的地形判别方法，以视差法来测量区域地形，并制成高解析度立体照片。

火星的经度坐标采用东经0至360度，不是地球的东西经各180度。

来自火星奥德赛号上热辐射成像系统（THEMIS）的影像显示阿尔西亚山北坡有七个可能的深洞，照片中光线无法抵达底部，推测底部可能更深、更宽，可能免受微陨星、紫外线、太阳闪焰和其他高能粒子的侵害，可能是未来寻找液态水或生命痕迹的可行地点。但后来火星勘察卫星的更高解析度HiRISE影像部分推翻了之前猜测，认为只是光线角度造成深不见底的样子。

大气

火星大气层相对较薄，平均地表气压只有6百帕，约为地球表面气压的0.6%，相当于地球表面算起35公里高的气压，如此低的气压使声音传播的距离只有在地球上的1.5%。随著季节的变化，火星气压变化可达20%。火星大气层按高度可分为低层大气、中层大气、上层大气和外气层。其中低层大气由于气悬微尘与地表的热，这部份相对温暖；中层大气存在有高速气流；上层大气（或热气层）温度很高，大气分子也不再像下层那样分布均匀；外气层高度在200公里以上，大气渐渐过度到太空，无明显外层边界。

火星大气成分为95%的二氧化碳，3%的氮气，1.6%氩气，很少的氧气、水气等，亦充满著很多悬浮尘埃，吸收蓝光使天空成黄褐色。2003年火星大冲时地面望远镜在大气中发现了甲烷；2004年3月，火星奥德赛号确认了这一发现。由于甲烷易被紫外线分解，存在甲烷表示现在或者最近几百年内在火星上存在制造甲烷的来源，火山作用、地质作用、彗星或小行星撞击甚至生物来源如甲烷古菌等都有可能。

2013年9月19日，根据从好奇号得到的进一步测量数据，NASA科学家报告，并没有侦测到大气甲烷（atmospheric methan）存在迹象，测量值为 ppbv，对应于1.3 ppbv上限（95%置信限），因此总结甲烷微生物活性概率很低，可能火星不存在生命。但是，很多微生物不会排出任何甲烷，仍旧可能在火星发现这些不会排出任何甲烷的微生物。

由于火星比地球离太阳远，日射量较少，表面温度应较低，计算值约210K，但实际观测地表平均约240K，则是因为大量的二氧化碳所造成的温室效应。由于大气层很薄，无法保留很多热，使地表日夜温差很大，某些地区地表温度白天可达28℃，夜晚可低至-132℃，平均-52℃。火星大气环流主要为单胞环流，由赤道相对热空气上升，漂至极区下沉，再沿地面回到赤道。另外，在火星的北半球，极冠的二氧化碳升华进入大气，使气压升高；而南半球由于二氧化碳凝华，气压下降，由于进出大气的二氧化碳量高达25%，造成南北压力差，空气便倾向由高压的夏半球流向低压的冬半球，形成另一依季节而变向的环流。因此火星的天气系统趋向成为全球性的，例如尘暴。http://www.adlerplanetarium.org/cyberspace/planets/mars/circulation.html

火星天气重覆次数较高，比地球容易预测。如果一个气象事件在一年的特定时间中发生，可提供的资料（相当稀疏）指出，很可能在下一年几乎同一个位置再发生一次，误差最多一个星期。2008年9月29日，凤凰号拍下了降雪事件，是在接近凤凰号登陆地点附近海姆达尔撞击坑之上，高 4.5 公里的云降雪。这次降水在到达火星表面时就已蒸发，这现象称为幡状云。火星上的风速要超过地球100倍。

水文

火星地表遍布著流水的遗迹，有些是洪水刻画而成，有些则是降雨或地下水流动而形成，但多半年代久远。冲蚀沟（gullies）则是另一类规模较小的地形，但形成年代十分年轻，常分布于撞击坑壁，型态多样。关于成因有两派说法，一派认为是由流动的水造成，另一方则认为是凹处累积的乾冰促使了松软物质滑动。

火星南北极有明显的极冠，曾被认为是由乾冰组成，但实际上绝大部分为水冰，只有表面一层为乾冰。这层乾冰在北极约1公尺厚，在南极则约8公尺厚，是冬季时凝华而成，到夏季则再度升华进入大气，不过南极的乾冰并不会完全升华。夏季仍存在的部分称为永久极冠，而整体构造称做极地层状沉积（Polar Layered Deposits），和地球南极洲与格陵兰冰层一样为一层层的沉积构造。北极冠宽达1,100公里，厚达2公里，体积82.1万立方公里；南极冠宽达1,400公里，最厚达3.7公里，体积约1.6百万立方公里。两极冰冠皆有独特的螺旋状凹谷，推论主要是由光照与夏季接近升华点的温度使沟槽两侧水冰发生差异融解和凝结而逐渐形成的。

2011年由火星勘察卫星的浅地层雷达发现南极冠有部分原本认为是水冰的地层其实是乾冰，所含二氧化碳量相当于大气含量的80%，这比以往认为的要多很多。根据此的模拟结果，十万年一周期的气候变迁中藉由乾冰升华、凝结，大气总质量的变化幅度会达数倍。由这些乾冰沉积上方地表的下陷与裂隙判断，乾冰正在慢慢升华。

自海盗号即发现，火星北半球中纬度有几处峡谷底含有条纹流动状的地表特徵，但不确定是富含冰的山崩、含冰土的流动或是尘砾覆盖的冰河。但根据更新任务的资料与比对地球的相关地形，支持这些是冰河，且推测是自转轴倾角较大时的气候状态下所累积的。

由火星奥德赛号X射线光谱仪的中子侦测器得知，自极区延伸至纬度约60°的地方表层一公尺的土壤含冰量超过60%，推论有更大量的水冻在厚厚的地下冰层（cryosphere）。

另外一个关于火星上曾存在液态水的证据，就是发现特定矿物，如赤铁矿和针铁矿，而这两者都需在有水环境才能形成。

对于于火星上有冰存在的直接证据在2008年6月20日被凤凰号发现，凤凰号在火星上挖掘发现了八粒白色的物体，当时研究人员揣测这些物体不是盐（在火星有发现盐矿）就是冰，而四天后这些白粒就凭空消失，因此这些白粒一定升华了，盐不会有这种现象。2008年7月31日，美国航空航天局科学家宣布，凤凰号火星探测器在火星上加热土壤样本时鉴别出有水蒸气产生，从而最终确认火星上有水存在。

2013年9月26日，美国航空航天局科学家报告，火星探测车好奇号发现火星土壤含有丰富水分，大约为1.5至 3重量百分比，显示火星有足够的水资源供给未来移民使用。

2015年9月28日，美国航空航天局宣布，在火星上发现液态的盐水。根据火星勘测轨道飞行器配备的光谱仪获得的数据，研究人员在火星的神秘斜坡上发现了水合矿物。这些暗色条纹表明火星地表随时间变化有流水存在。在较温暖的季节，这些线条的颜色变得更深，表明水流在斜坡上出现，在较冷的季节，这些地表特徵变浅。在火星的部分地区，最高温度可以达到摄氏零下23度，此时深色线条最明显。

2018年7月25日，据意大利媒体报道，该国科学家在火星上首度发现一个地下液态水湖。该研究称，「火星地下及电离层高级探测雷达」在火星南极冰层下1.5千米处发现一个大型液态水湖，里面含有盐。湖的直径约为20千米，温度至少为零下10度。

运动规律

火星与太阳平均距离为1.52AU，公转周期为1.88地球年，687地球日，或668.6火星日。火星公转轨道和地球的一样，受太阳系其他天体影响而不断变动。轨道离心率有两个变化周期，分别是9.6万年和210万年，于0.002至0.12间变化；而地球的是10万年和41.3万年，于0.005至0.058间变化（见米兰科维奇循环）。

火星日平均为24小时39分35.244秒，或1.027地球日。火星目前自转轴倾角为25.19度，和地球的相近，但可在13度至40度间变化，周期为一千多万年，不像地球的稳定处于22.1和24.5度间，是因为火星没有如月球般的巨大卫星来维持自转轴。由于没有大卫星的潮汐作用，火星自转周期变化小，不像地球的会被慢慢拉长。

火星自转轴有明显倾斜，日照的年变化形成明显的四季变化，而一季的长度约为地球的两倍。由于火星轨道离心率大，为0.093（地球只有0.017），使各季节长度不一致，又因远日点接近北半球夏至，北半球春夏比秋冬各长约40天。2009年10月26日为北半球春分，2010年5月13日为夏至，目前北半球处春季。虽然火星没有地球般受海洋影响的复杂气候，但仍有以下特殊之处：火星轨道离心率比地球大，造成日射量在一年当中变化更大，位于近日点时，南半球处夏季，比北半球远日点夏季所造成的升温更强；随季节交替，二氧化碳和水气会升华和凝结而在两极冠间迁移，驱动大气环流；地表反照率特徵，因颜色深浅和沙、岩性质差异而造成的容积热容不同，可影响大气环流；易发生的尘暴会将沙尘粒子卷入高空，沙尘粒子吸收日光与再辐射会使高层大气增温，但遮蔽天空的沙尘会使地表降温；自转轴倾角和轨道离心率的长期变化则造成了气候的长期变迁。火星表面的平均温度比地球低30度以上。

目前火星与地球最短距离正慢慢减小。当地球与火星之间的距离最小时，称为火星冲日。火星相邻两次冲日的时间间隔约为779天，最近一次出现在2018年7月27日，下一次火星冲日将出现在2020年10月13日。当地球与太阳和火星连成一线时，在火星上便可看到地球凌日，在太阳的位置可看到地球的黑点通过，同理还有水星凌日 (火星)，在地球上则不会看到火星凌日。

卫星

火星有两个天然卫星——火卫一（Phobos）与火卫二（Deimos），最长直径各为27公里和16公里，形状不规则并充满撞击坑，以近圆形的轨道于接近火星赤道面处公转。它们虽然很小，但由于接近火星，使火卫一从火星上看约有满月直径的二分之一至三分之一大，而视星等火卫一可达-7，火卫二可达-5，白天可能可见。和月球一样，这两颗卫星都被火星潮汐锁定，因此他们总是以一面对著火星。火卫一的公转周期比火星自转更快，所以在火星上来看是西升东落的，且只花了约4个小时；而火卫二的公转周期只比火星自转慢一些，东升西落要花约2.4个火星日。因为火卫一离火星很近，火星的潮汐力会慢慢但稳定地减小它的轨道半径，预计再过约760万年，火卫一将因轨道低于3620公里，也就是火星的洛希极限而被瓦解。另一方面火卫二因为离火星足够远，所以它的轨道反而正在慢慢地被推进。

两卫星可能是捕获的小行星，但新研究认为可能是撞击事件、或原本的卫星被火星潮汐力拉碎后，由散布轨道上的岩屑再度吸积而形成。

两颗卫星是在1877年被阿萨夫·霍尔发现的，以希腊神话中的福波斯和得摩斯命名，两者皆为战神阿瑞斯的儿子。

观测探测

古代

火星的火红色，自古就吸引著人们，希腊人称为战神。此时火星观测和其他天体般，大部分是为了占星，而后渐渐涉及科学方面，如克卜勒探索行星运动定律时是依据第谷积累的大量而精密的火星运行观测资料。

望远镜出现后，人们对火星可以进行更进一步的观测。使用望远镜观测星空的伽利略所见的火星只是一个橘红小点，然而随著望远镜的发展，观测者开始辨别到一些明暗特徵。惠更斯依此测出火星自转周期约为24.6小时，而他亦为首次纪录火星南极冠的人。一开始由于各人各自观测，意见不一致，地名也未统一（例如用绘制者名字命名）。后来义大利的乔范尼·斯基亚帕雷利统合了各家说法而绘制了地图，地名取自地中海、中东等的地名和圣经等作为来源，而其馀则依照旧有的观念：暗区被认为是湖（lacus）海（mare）等水体，如太阳湖、塞壬海、明显的暗大三角——大瑟提斯；而亮区则是陆地，如亚马逊。这个命名系统一直延续下来。

当时，斯基亚帕雷利和同期观测者一样，观察到了火星表面似乎有一些从暗区延伸出的细线，因为对于暗区是水体的传统，这些细线命名为水道（canali）。而后来观察到暗区会在冬季时缩小、夏季时扩张，有人提出暗区是植物覆盖、而暗区的扩大缩小则是消长所引起的，改变以往认为暗区是水的说法。帕西瓦尔·罗威尔观察到并宣称那些「水道」其实是人工挖掘的「运河」，用来灌溉植物，因为水道应太细不可见，而看到的细线应是灌溉出的大片植物。风靡大众的火星科幻和火星人即源于此。不过这些细线大多已证明是不存在的，部分则是峡谷或陨石坑后延伸出的深色沙子。而火星表面颜色的改变则是因为沙被风吹移，或发生火星尘暴。

到了太空时代，水手4号传回的充满陨石坑的火星照片粉碎了人们对火星文明的幻想，认为火星只是一处如月球般布满陨石坑的死寂星球。但随著往后水手9号等的巨大峡谷、火山和疑似流水遗迹的发现，火星的独特性、液态水和生命的可能又重新引起人们的兴趣。（见#近代探测）

近代

苏联、美国、欧洲、日本、印度、中国 和阿拉伯联合大公国共已发射数十艘太空船研究火星表面、地质和气候。这些太空船包括轨道卫星、登陆器和漫游车，但大约有三分之二的任务在完成前或刚要开始时就因种种原因而失败。目前将一公斤物体由地球表面送往火星平均要花费约30,900美元。

1965年水手4号飞掠火星。1971年水手9号进入火星轨道，成为第一个环绕火星的探测船。1971年苏联火星计画火星2号的登陆器坠毁后数日，相同的火星3号的登陆器成功登陆火星，是第一个成功登陆火星的探测器，但登陆十几秒后随即失去联系。1975年NASA发射海盗号，包括两组轨道卫星和登陆器。海盗1号和2号轨道卫星各运作了六年和三年。两个登陆器皆于1976年成功登陆，并传送了第一张火星地景的彩色照片，而轨道卫星也绘制了很好的火星地图，甚至到今天都还在使用。

1988年苏联发射弗伯斯1号、2号以探测火星和两个卫星。弗伯斯1号于抵达前失去联系，而弗伯斯2号虽然成功拍摄了火星和火卫一，但在放出两艘登陆器到火卫一前也失去联系。

在1992年火星观察者失败后，NASA于1996年11月发射了火星全球勘测者。火星全球勘测者出色地完成任务，它在2001年完成了地图绘制的任务，并三次延长任务，直到2006年11月2日失去联系而结束，总计共花了10年在太空中工作。在火星全球勘测者发射一个月后，NASA发射了火星探路者，包括了一个登陆器和漫游车——旅居者号（Sojourner），于1997年7月登陆在阿瑞斯峡谷。这任务也很成功，而且也广为人知，其中的原因是因为传回了大量照片。

NASA的火星勘测98计画于1998、99年发射了火星气候卫星与火星极地登陆者，前者预计研究气候、水与二氧化碳等，后者则预计于南极登陆，船上的搭载深空2号则计划于火星极地登陆者进入大气时与它分离，直接降落并穿入地表进行研究。但整个计划在2000年到达火星时失败。

NASA于2007年8月发射凤凰号，于2008年5月登陆在火星北纬68度的极区。凤凰号登陆器有一支可伸及2.5公尺的机械手臂，并可挖掘土壤1公尺深。它还搭载一座显微镜，解析度达人类头发宽度的千分之一。2008年6月20日确认在2008年6月15日发现的地表白色物质为水冰。2008年11月10日进入冬季而无法继续联系凤凰号，任务结束。

2001年NASA发射了2001火星奥德赛号，任务成功进行并延续到2010年9月。船上的伽玛射线光谱仪于地表下一公尺内侦测到大量的氢，也就是大量的水分子。

2003年欧洲太空总署发射了火星快车号，包括轨道卫星和登陆器——小猎犬2号，而小猎犬2号于2004年2月降落时失败。2004年船上的行星傅立叶光谱仪于大气中侦测到甲烷。2006年6月ESA宣布火星快车号发现极光。

2003年NASA发射了两台相同的火星探测漫游者——精神号（MER-A）和机会号（MER-B）。两台皆于2004年1月成功登陆并工作超过预定时间。传回的资料中最有价值的大概是两地过去有水的确实证据。尘卷风和风暴偶尔清除了太阳能板上的沙尘，使它们能以超过预定任务时间继续工作。

2005年8月NASA发射了火星勘察卫星，于2006年3月进入火星轨道展开为期2年的工作。它搭载更进步的通讯系统，频宽比之前任务总和还宽，且传回的资料远多于过去任务的总和。拥有解析度高达0.3公尺的相机——HiRISE，拍摄地表和天气以寻找未来任务的适合登陆地点。2008年2月19日拍摄到北极冠边缘的一系列雪崩影像。

2007年2月25日，探测彗星的罗塞塔号近距离飞掠火星并拍照，有拍到很高的云。

2009年2月17日，黎明号飞掠火星以重力助推前往目的地灶神星和谷神星，并在接近火星时拍了照。

中俄合作的福布斯-土壤号于2011年升空，将会送回火卫一土壤样本。而该探测器还将搭载一颗重110公斤的火星探测器，也就中国第一艘无人驾驶火星探测船萤火一号（YH-1），预计乘坐俄罗斯的联盟号运载火箭升空，航程大约10个月。萤火一号主要研究火星的电离层及周围空间环境，火星磁场等。该探测器发射到近地轨道后，因为与地面失去联系变轨失败，探测器的碎片于莫斯科时间2012年1月15日坠落在太平洋海域。

继凤凰号之后，NASA于2011年的发射的火星科学实验室（好奇号），在2012年8月6日05:31UTC成功登陆火星的盖尔撞击坑。它和火星探测漫游者一样是火星车，但比火星探测漫游者更大、速度更快，而且设备更完善。它搭载雷射化学检测仪，可在13公尺外分析岩石组成。比起之前其它火星任务，它携带了更多先进科学仪器。本次任务的总成本达到了25亿美元，是历来最贵的火星探测任务。

2008年9月15日NASA发表了MAVEN任务，预计2013年以各种机器研究火星大气。

芬兰、俄罗斯的合作计划MetNet包括数十个登陆器组成观测网，以研究火星的大气结构、物理和天气。这任务的前导任务将会于2011年先发射一至数个登陆器，有可能是和火卫一-土壤号并在一起发射。往后的发射会持续到2019年。

2016年ESA计划发射第一台火星车——ExoMars，它可挖掘两公尺深以寻找有机物甚至火星生命。

2004年美国总统布什宣布载人火星任务为太空探索展望中的长期目标。NASA和洛克希德·马丁已开始研究猎户座太空船，计划于2020年以前送人类到月球，作为人类登陆火星的准备。2007年9月28日，NASA执行长麦可·D·格里芬声明NASA预计于2037年以前送人类到火星。

2021年5月15日，中国国家航天局的天问一号著陆器和祝融号火星车在火星乌托邦平原南部预选著陆区成功著陆。

ESA希望于2030至2035年间送人类上火星。但在这之前还有其他探测任务，包括ExoMars和火星样本取回任务。

直达火星是罗伯·祖宾——火星学会的创始人和主席——提出的极低成本载人火星任务，使用重载的农神五号级火箭，如战神五号或太空探索技术公司（SpaceX）的猎鹰九号，省略轨道组装、低地轨道会合和月球燃料补给站而直接用小的太空船前往火星。修改后的计划，叫做Mars to Stay ，改成先不送回第一批登陆者，狄恩·尤尼克说明送回一开始的四到六人所花费用比送他们到火星还高，反而可再送二十人。

2007 WD5：2007年11月20日NASA JPL近地天体观测计划发现，一颗直径约50公尺的小行星2007 WD5可能会在2008年1月30日撞击火星，但随著观测资料越多，终把撞击机率降至0.01%，小行星则于1月30日掠过火星。

火星生命

2000年，美国科学家在南极洲发现了一块火星陨石。这是一块碳酸盐陨石，后被编号为ALH84001。美国国家航空航天局声称在这块陨石上发现了一些类似微体化石的结构，有人认为这可能是火星生命存在的证据，但也有人认为这只是自然生成的矿物晶体。直到2004年，争论的双方仍然没有任何一方占据上风。

有证据显示火星曾比现在更适合生命存在，但生命在火星上到底是否真正存在过还没有确切的结论。某些研究者认为源自火星的ALH84001陨石有过去生命活动的证据，但这个看法至今尚未得到公认。另有反对的观点认为，自几十亿年前产生以来，该陨石从未长期处于液态水存在的温度下，因而不会曾有生命活动。

海盗号曾做实验检测火星土壤中可能存在的微生物。实验只分析了海盗号著陆点处的土壤并给出了阳性的结果，但随后即被许多科学家所否定，而这一结果也仍就处在争议之中。现存生物活动也是火星大气中存在微量甲烷的解释之一，但亦有其它与生命无关的解释。

人类若对外星殖民，由于火星的适宜条件（同其他行星相比，火星最像地球，而且距离相对较近），它将是人类的首选地点。

2018年6月6日，美国太空总署宣布，好奇号探测车在火星的古老湖床的岩石里，发现有机物质。这可能对寻找生命给出重要线索。

相关文化及网络用语

中国古人认为火星在位置及亮度上都常变不定，故称为「荧惑」，在星占学上象徵残、疾、丧、饥、兵等恶象。「荧惑守心」是火星留守在心宿（天蝎座）的天文现象，心宿主要有三颗星，中间这颗最亮，代表皇帝，旁边的两颗代表太子、庶子。荧惑守心是很罕见的天象，被认为最不祥，可能出现两种结果一是皇帝驾崩，或是宰相下台。西汉成帝绥和二年（前7年），天文台观测到了荧惑守心，宰相翟方进被汉成帝赐了毒酒自杀。翟方进死没几天，汉成帝突然暴毙，王莽后来称帝，翟方进之子翟义起兵反王莽。

台湾国立清华大学黄一农教授在他的专书《名家专题精讲系列—社会天文学史十讲》内的其中一篇文章《中国星占学上最凶的天象──「荧惑守心」》提到，现在以电脑推算发现当年并未发生此天象，中国史籍中记载荧惑守心共二十三次，但有十七次是伪造的。中国历史上实际发生过的荧惑守心则共有三十八次，且在中国史籍多无记录。

关于火星的神话传说有：

• 阿瑞斯，希腊战神

• 玛尔斯，罗马战神

• 内尔伽勒，巴比伦神祇

• 提尔，北欧神话中的战神

• 火星 (妖怪)，中国神话中的妖怪，记载于《搜神记》

火星文是中文互联网曾经流行的一种特殊的文本，大量使用同音字、音近字、特殊符号来表音，难以阅读。称为火星文，取「地球人看不懂的文字」的讽刺意味。