Mars

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This article is about the planet. For the deity, see Mars (mythology). For other uses, see Mars (disambiguation). Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System after Mercury. In English, Mars carries a name of the Roman god of war, and is often referred to as the "Red Planet"[15] [16]  because the iron oxide prevalent on its surface gives it a reddish appearance that is distinctive among the astronomical bodies visible to the naked eye.[17]  Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.

The days and seasons are likewise comparable to those of Earth, because the rotational period as well as the tilt of the rotational axisrelative to the ecliptic plane are very similar. Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature.[18] [19]  Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids,[20] [21]  similar to 5261 Eureka, a Mars trojan.

There are ongoing investigations assessing the past habitability potential of Mars, as well as the possibility of extant life. Future astrobiology missions are planned, including the Mars 2020 and ExoMars rovers.[22] [23] [24] [25]  Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is less than 1% of the Earth's,[26]  except at the lowest elevations for short periods.[27] [28]  The two polar ice caps appear to be made largely of water.[29] [30]  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 11 meters (36 ft).<sup id="cite_ref-nasa070315_33-0">[31]  In November 2016, NASA reported finding a large amount of underground ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior.<sup id="cite_ref-34">[32] <sup id="cite_ref-35">[33] <sup id="cite_ref-NASA-20161122_36-0">[34]

Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches −2.94,<sup id="cite_ref-Mallama_and_Hilton_13-1">[11]  which is surpassed only by Jupiter, Venus, the Moon, and the Sun. Optical ground-based telescopes are typically limited to resolving features about 300 kilometers (190 mi) across when Earth and Mars are closest because of Earth's atmosphere.<sup id="cite_ref-usra_37-0">[35]

Contents

 * 1Physical characteristics
 * 1.1Internal structure
 * 1.2Surface geology
 * 1.3Soil
 * 1.4Hydrology
 * 1.5Geography and naming of surface features
 * 1.6Atmosphere
 * 1.7Climate
 * 2Orbit and rotation
 * 3Habitability and search for life
 * 4Moons
 * 5Exploration
 * 5.1Future
 * 6Astronomy on Mars
 * 7Viewing
 * 7.1Closest approaches
 * 8Historical observations
 * 8.1Ancient and medieval observations
 * 8.2Martian "canals"
 * 8.3Spacecraft visitation
 * 9In culture
 * 9.1Intelligent "Martians"
 * 10See also
 * 11Notes
 * 12References
 * 13External links
 * 13.1Images
 * 13.2Videos
 * 13.3Cartographic resources

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.<sup id="cite_ref-nssdc_11-5">[9]  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.<sup id="cite_ref-rust_38-0">[36]  It can look like butterscotch;<sup id="cite_ref-ismars_39-0">[37]  other common surface colors include golden, brown, tan, and greenish, depending on the minerals present.<sup id="cite_ref-ismars_39-1">[37]



Comparison: Earth and Mars

showing major features of Mars

showing how three NASA orbiters mapped the gravity field of Mars

Internal structure
Like Earth, Mars has differentiated into a dense metallic core overlaid by less dense materials.<sup id="cite_ref-Nimmo_2005_40-0">[38]  Current models of its interior imply a core with a radius of about 1,794 ± 65 kilometers (1,115 ± 40 mi), consisting primarily of iron and nickel with about 16–17% sulfur.<sup id="cite_ref-icarus213_2_451_41-0">[39] This iron(II) sulfide core is thought to be twice as rich in lighter elements as Earth's.<sup id="cite_ref-jacque03_42-0">[40]  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, aluminum, calcium, and potassium. The average thickness of the planet's crust is about 50 km (31 mi), with a maximum thickness of 125 km (78 mi).<sup id="cite_ref-jacque03_42-1">[40]  Earth's crust averages 40 km (25 mi).

Surface geology
Main article: Geology of Mars

Mars is a terrestrial planet that consists of minerals containing silicon and oxygen, metals, and other elements that typically make up rock. The surface of Mars is primarily composed of tholeiitic basalt,<sup id="cite_ref-science324_5928_736_43-0">[41]  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.<sup id="cite_ref-jgr107_E6_44-0">[42] Much of the surface is deeply covered by finely grained iron(III) oxide dust.<sup id="cite_ref-sci300a_45-0">[43] <sup id="cite_ref-sci300b_46-0">[44]



Geologic map of Mars (USGS, 2014)<sup id="cite_ref-USGS-20140714_47-0">[45]

Although Mars has no evidence of a structured global magnetic field,<sup id="cite_ref-magnetosphere_48-0">[46]  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.<sup id="cite_ref-plates_49-0">[47]

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.<sup id="cite_ref-ssr96_1_4_197_50-0">[48]

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,<sup id="cite_ref-zharkov93_51-0">[49] <sup id="cite_ref-icarus165_1_52-0">[50] <sup id="cite_ref-barlow88_53-0">[51]  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 10,600 by 8,500 km (6,600 by 5,300 mi), or roughly four times the size of the Moon's South Pole – Aitken basin, the largest impact basin yet discovered.<sup id="cite_ref-northcratersn_20-1">[18] <sup id="cite_ref-northcraterguard_21-1">[19]  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.<sup id="cite_ref-sciam080627_54-0">[52] <sup id="cite_ref-nyt080626_55-0">[53]



Artist's impression of how Mars may have looked four billion years ago<sup id="cite_ref-56">[54]

The geological history of Mars can be split into many periods, but the following are the three primary periods:<sup id="cite_ref-jog91_57-0">[55] <sup id="cite_ref-ssr_96_1_4_58-0">[56] 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.<sup id="cite_ref-ag44_4_59-0">[57]  On February 19, 2008, images from the Mars Reconnaissance Orbiter showed evidence of an avalanche from a 700-metre-high (2,300 ft) cliff.<sup id="cite_ref-dc080304_60-0">[58]
 * 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.

Soil
Main article: Martian soil



Exposure of silica-rich dust uncovered by the Spirit rover

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.<sup id="cite_ref-bbc080627_61-0">[59]  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.<sup id="cite_ref-marssalt_62-0">[60] <sup id="cite_ref-jpl_soil_63-0">[61] <sup id="cite_ref-64">[62] <sup id="cite_ref-65">[63]  This is a very high concentration and makes the Martian soil toxic (see also Martian soil toxicity).<sup id="cite_ref-toxicmars_66-0">[64] <sup id="cite_ref-toxicsoil1_67-0">[65]

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.<sup id="cite_ref-jpl_dust_devil_68-0">[66]  Several other explanations have been put forward, including those that involve water or even the growth of organisms.<sup id="cite_ref-gpl29_23_41_69-0">[67] <sup id="cite_ref-oleb33_4_515_70-0">[68]

Hydrology
Main article: Water on Mars

Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is less than 1% that of Earth's,<sup id="cite_ref-nasa.gov_28-1">[26]  except at the lowest elevations for short periods.<sup id="cite_ref-h_29-1">[27] <sup id="cite_ref-jgr110_30-1">[28]  The two polar ice caps appear to be made largely of water.<sup id="cite_ref-kostama_31-1">[29] <sup id="cite_ref-sci299_32-1">[30]  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 11 meters (36 ft).<sup id="cite_ref-nasa070315_33-1">[31]  A permafrost mantle stretches from the pole to latitudes of about 60°.<sup id="cite_ref-kostama_31-2">[29]  Large quantities of water iceare thought to be trapped within the thick cryosphere of Mars. Radar data from Mars Express and the Mars Reconnaissance Orbiter show large quantities of water ice at both poles (July 2005)<sup id="cite_ref-specials1_71-0">[69] <sup id="cite_ref-bbc040124_72-0">[70]  and at middle latitudes (November 2008).<sup id="cite_ref-jsg.utexas.edu_73-0">[71]  The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.<sup id="cite_ref-spacecraft1_74-0">[72]



Photomicrograph by Opportunityshowing a gray hematite concretion, nicknamed "blueberries", indicative of the past existence of liquid water

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.<sup id="cite_ref-Kerr2005_75-0">[73] <sup id="cite_ref-Jaeger2007_76-0">[74]  One of the larger examples, Ma'adim Vallis is 700 km (430 mi) long, much greater than the Grand Canyon, with a width of 20 km (12 mi) and a depth of 2 km (1.2 mi) in places. It is thought to have been carved by flowing water early in Mars's history.<sup id="cite_ref-lucchita_rosanova_77-0">[75] The youngest of these channels are thought to have formed as recently as only a few million years ago.<sup id="cite_ref-nature434_78-0">[76]  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.<sup id="cite_ref-CraddockHoward2002_79-0">[77]

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,<sup id="cite_ref-sci288_80-0">[78] <sup id="cite_ref-nasa061206_81-0">[79]  although others have argued for formation mechanisms involving carbon dioxide frost or the movement of dry dust.<sup id="cite_ref-bbc061206_82-0">[80] <sup id="cite_ref-nasa061206b_83-0">[81]  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.<sup id="cite_ref-nasa061206_81-1">[79]  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.<sup id="cite_ref-Lewis2006_84-0">[82]  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.<sup id="cite_ref-Matsubara2011_85-0">[83]



A cross-section of underground water ice is exposed at the steep slope that appears bright blue in this enhanced-color view from the MRO.<sup id="cite_ref-86">[84] The scene is about 500 meters wide. The scarp drops about 128 meters from the level ground. The ice sheets extend from just below the surface to a depth of 100 meters or more.<sup id="cite_ref-87">[85]

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.<sup id="cite_ref-nasa040303_88-0">[86]  In 2004, Opportunity detected the mineral jarosite. This forms only in the presence of acidic water, which demonstrates that water once existed on Mars.<sup id="cite_ref-nasa101001_89-0">[87]  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.<sup id="cite_ref-nasa_90-0">[88] <sup id="cite_ref-nationalgeographic_91-0">[89]  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 200–1,000 m (660–3,280 ft).<sup id="cite_ref-nationalgeographic1_92-0">[90]

In 2005, radar data revealed the presence of large quantities of water ice at the poles<sup id="cite_ref-specials1_71-1">[69]  and at mid-latitudes.<sup id="cite_ref-jsg.utexas.edu_73-1">[71] <sup id="cite_ref-esa050221_93-0">[91]  The Mars rover Spiritsampled chemical compounds containing water molecules in March 2007. The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.<sup id="cite_ref-spacecraft1_74-1">[72]

On March 18, 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.<sup id="cite_ref-NASA-20130318_94-0">[92] <sup id="cite_ref-BBC-20130319_95-0">[93] <sup id="cite_ref-MSN-20130120_96-0">[94]  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 60 cm (24 in), during the rover's traverse from the Bradbury Landing site to the Yellowknife Bay area in the Glenelg terrain.<sup id="cite_ref-NASA-20130318_94-1">[92]  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.<sup id="cite_ref-97">[95] <sup id="cite_ref-98">[96] <sup id="cite_ref-Ojhaetal2015_99-0">[97]  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.<sup id="cite_ref-SeasonalFlows_100-0">[98]  The streaks contain hydrated salts, perchlorates, which have water molecules in their crystal structure.<sup id="cite_ref-101">[99]  The streaks flow downhill in Martian summer, when the temperature is above −23 degrees Celsius, and freeze at lower temperatures.<sup id="cite_ref-102">[100]  On September 28, 2015, NASA announced the presence of briny flowing salt water on the Martian surface.<sup id="cite_ref-103">[101]



Perspective view of Korolev crater shows 1.9 km (1.2 mi) deep water ice. Image taken by ESA’s Mars Express.

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.<sup id="cite_ref-Head1999_104-0">[102]  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.<sup id="cite_ref-NYT-20150305_105-0">[103]

Near the northern polar cap is the 81.4 kilometres (50.6 mi) wide Korolev Crater, where the Mars Express orbiter found it to be filled with approximately 2,200 cubic kilometres (530 cu mi) of water ice.<sup id="cite_ref-DLR_106-0">[104]  The crater floor lies about 2 kilometres (1.2 mi) below the rim, and is covered by a 1.8 kilometres (1.1 mi) deep central mound of permanent water ice, up to 60 kilometres (37 mi) in diameter.<sup id="cite_ref-DLR_106-1">[104] <sup id="cite_ref-TG1218_107-0">[105]

Polar caps
Main article: Martian polar ice caps



North polar early summer ice cap (1999)



South polar midsummer ice cap (2000)

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).<sup id="cite_ref-icarus169_108-0">[106]  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.<sup id="cite_ref-clouds_109-0">[107]

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.<sup id="cite_ref-malin2001_110-0">[108]  The northern polar cap has a diameter of about 1,000 km (620 mi) during the northern Mars summer,<sup id="cite_ref-mira_111-0">[109]  and contains about 1.6 million cubic kilometres (380,000 cu mi) of ice, which, if spread evenly on the cap, would be 2 km (1.2 mi) thick.<sup id="cite_ref-brown_112-0">[110]  (This compares to a volume of 2.85 million cubic kilometres (680,000 cu mi) for the Greenland ice sheet.) The southern polar cap has a diameter of 350 km (220 mi) and a thickness of 3 km (1.9 mi).<sup id="cite_ref-phillips_113-0">[111]  The total volume of ice in the south polar cap plus the adjacent layered deposits has been estimated at 1.6 million cubic km.<sup id="cite_ref-sci315_114-0">[112]  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.<sup id="cite_ref-Onset_and_migration_of_spiral_troughs_on_Mars_revealed_by_orbital_radar_115-0">[113] <sup id="cite_ref-Mystery_Spirals_on_Mars_Finally_Explained_116-0">[114]

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.<sup id="cite_ref-2006-100_117-0">[115] <sup id="cite_ref-Kieffer2000_118-0">[116] <sup id="cite_ref-Portyankina_119-0">[117] <sup id="cite_ref-Hugh2006_120-0">[118]

Geography and naming of surface features


A MOLA-based topographic map showing highlands (red and orange) dominating the southern hemisphere of Mars, lowlands (blue) the northern. Volcanic plateaus delimit regions of the northern plains, whereas the highlands are punctuated by several large impact basins.



These new impact craters on Mars occurred sometime between 2008 and 2014, as detected from orbit

Main article: Geography of Mars

For more on how geographic references are determined, see Geodetic datum.

See also: Category:Surface features of Mars

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".<sup id="cite_ref-sheehan_ch04_121-0">[119]

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.<sup id="cite_ref-usgs_122-0">[120]

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).<sup id="cite_ref-viking_1950_2000_123-0">[121]  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.<sup id="cite_ref-seds_huygens_124-0">[122]  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 for the definition of 0.0° longitude to coincide with the original selection.<sup id="cite_ref-archinal_caplinger_125-0">[123]

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<sup id="cite_ref-NASAMola2007_126-0">[124]  of Mars, analogous to the terrestrial geoid. Zero altitude was defined by the height at which there is 610.5 Pa (6.105 mbar) of atmospheric pressure.<sup id="cite_ref-pers66_127-0">[125]  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).<sup id="cite_ref-lunine99_128-0">[126]  In practice, today this surface is defined directly from satellite gravity measurements.

Map of quadrangles
Main article: List of quadrangles on Mars

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.



0°N 180°W

0°N 0°W

90°N 0°W

MC-01

Mare Boreum

MC-02

Diacria

MC-03

Arcadia

MC-04

Mare Acidalium

MC-05

Ismenius Lacus

MC-06

Casius

MC-07

Cebrenia

MC-08

Amazonis

MC-09

Tharsis

MC-10

Lunae Palus

MC-11

Oxia Palus

MC-12

Arabia

MC-13

Syrtis Major

MC-14

Amenthes

MC-15

Elysium

MC-16

Memnonia

MC-17

Phoenicis

MC-18

Coprates

MC-19

Margaritifer

MC-20

Sabaeus

MC-21

Iapygia

MC-22

Tyrrhenum

MC-23

Aeolis

MC-24

Phaethontis

MC-25

Thaumasia

MC-26

Argyre

MC-27

Noachis

MC-28

Hellas

MC-29

Eridania

MC-30

Mare Australe



Clickable image of the 30 cartographic quadrangles of Mars, defined by the USGS.<sup id="cite_ref-mapping_mars_129-0">[127] <sup id="cite_ref-130">[128]  Quadrangle numbers (beginning with MC for "Mars Chart")<sup id="cite_ref-131">[129]  and names link to the corresponding articles. North is at the top; 0°N 180°W is at the far left on the equator. The map images were taken by the Mars Global Surveyor.
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Impact topography


Bonneville crater and Spirit rover's lander

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 10,600 by 8,500 km (6,600 by 5,300 mi) 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.<sup id="cite_ref-northcratersn_20-2">[18] <sup id="cite_ref-northcraterguard_21-2">[19]



Fresh asteroid impact on Mars at 3.34°N 219.38°E. These before and after images of the same site were taken on the Martian afternoons of March 27 and 28, 2012 respectively (MRO)<sup id="cite_ref-NASA-20140522_132-0">[130]

Mars is scarred by a number of impact craters: a total of 43,000 craters with a diameter of 5 km (3.1 mi) or greater have been found.<sup id="cite_ref-wright03_133-0">[131]  The largest confirmed of these is the Hellas impact basin, a light albedo feature clearly visible from Earth.<sup id="cite_ref-ucar_geography_134-0">[132]  Due to the smaller mass 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.<sup id="cite_ref-emp9_135-0">[133]  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.<sup id="cite_ref-emp45_136-0">[134]

Volcanoes


Viking 1 image of Olympus Mons. The volcano and related terrain are approximately 550 km (340 mi) across.

Main article: Volcanology of Mars

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 8.8 km (5.5 mi).<sup id="cite_ref-scsdes49_137-0">[135]  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 21 to 27 km (13 to 17 mi) high.<sup id="cite_ref-138">[136] <sup id="cite_ref-glenday09_139-0">[137]

Tectonic sites


Valles Marineris (2001 Mars Odyssey)

The large canyon, Valles Marineris (Latin for "Mariner Valleys", also known as Agathadaemon in the old canal maps), has a length of 4,000 km (2,500 mi) and a depth of up to 7 km (4.3 mi). 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 446 km (277 mi) long and nearly 2 km (1.2 mi) 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 150 km (93 mi) of transverse motion has occurred, making Mars a planet with possibly a two-tectonic plate arrangement.<sup id="cite_ref-tectonic_140-0">[138] <sup id="cite_ref-Lin,_An_141-0">[139]

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.<sup id="cite_ref-cushing_titus_wynn07_142-0">[140]  The caves, named after loved ones of their discoverers, are collectively known as the "seven sisters".<sup id="cite_ref-nau070328_143-0">[141]  Cave entrances measure from 100 to 252 m (328 to 827 ft) wide and they are estimated to be at least 73 to 96 m (240 to 315 ft) 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 130 m (430 ft) 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.<sup id="cite_ref-bbc070317_144-0">[142]

Atmosphere
Main article: Atmosphere of Mars



The tenuous atmosphere of Mars visible on the horizon

Mars lost its magnetosphere 4 billion years ago,<sup id="cite_ref-swind_145-0">[143]  possibly because of numerous asteroid strikes,<sup id="cite_ref-146">[144]  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 Expresshave detected ionised atmospheric particles trailing off into space behind Mars,<sup id="cite_ref-swind_145-1">[143] <sup id="cite_ref-swind2_147-0">[145]  and this atmospheric loss is being studied by the MAVENorbiter. Compared to Earth, the atmosphere of Mars is quite rarefied. Atmospheric pressure on the surface today ranges from a low of 30 Pa(0.030 kPa) on Olympus Mons to over 1,155 Pa (1.155 kPa) in Hellas Planitia, with a mean pressure at the surface level of 600 Pa (0.60 kPa).<sup id="cite_ref-bolonkin09_148-0">[146] The highest atmospheric density on Mars is equal to that found 35 km (22 mi)<sup id="cite_ref-atkinson07_149-0">[147]  above Earth's surface. The resulting mean surface pressure is only 0.6% of that of Earth (101.3 kPa). The scale height of the atmosphere is about 10.8 km (6.7 mi),<sup id="cite_ref-carr06_150-0">[148]  which is higher than Earth's, 6 km (3.7 mi), 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.<sup id="cite_ref-nssdc_11-6">[9] <sup id="cite_ref-Abundance_151-0">[149]  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.<sup id="cite_ref-dusty_152-0">[150]  It may take on a pink hue due to iron oxide particles suspended in it.<sup id="cite_ref-Rees2012_18-1">[16]



Potential sources and sinks of methane (CH 4) on Mars

Methane has been detected in the Martian atmosphere;<sup id="cite_ref-methane-me_153-0">[151] <sup id="cite_ref-methane_154-0">[152]  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 ppbduring the summer.<sup id="cite_ref-Sample2018_155-0">[153]  In northern midsummer 2003, the principal plume contained 19,000 metric tons of methane, with an estimated source strength of 0.6 kilograms per second.<sup id="cite_ref-plumes_156-0">[154] <sup id="cite_ref-hand08_157-0">[155]  The profiles suggest that there may be two local source regions, the first centered near 30°N 260°W and the second near 0°N 310°W.<sup id="cite_ref-plumes_156-1">[154]  It is estimated that Mars must produce 270 tonnes per year of methane.<sup id="cite_ref-plumes_156-2">[154] <sup id="cite_ref-results_158-0">[156]

Methane can exist in the Martian atmosphere for only a limited period before it is destroyed—estimates of its lifetime range from 0.6–4 years.<sup id="cite_ref-plumes_156-3">[154] <sup id="cite_ref-nature460_159-0">[157]  Its presence despite this short lifetime indicates that an active source of the gas must be present. Volcanic activity, cometaryimpacts, and the presence of methanogenic microbial life forms are among possible sources. Methane could be produced by a non-biological process called serpentinization<sup id="cite_ref-serpentinization_160-0">[c]  involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.<sup id="cite_ref-olivine_161-0">[158]



Escaping atmosphere on Mars (carbon, oxygen, and hydrogen) by MAVEN in UV<sup id="cite_ref-NASA-20141014-NJ_162-0">[159]

The Curiosity rover, which landed on Mars in August 2012, is able to make measurements that distinguish between different isotopologues of methane,<sup id="cite_ref-163">[160]  but even if the mission is to 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.<sup id="cite_ref-164">[161]  The first measurements with the Tunable Laser Spectrometer (TLS)indicated that there is less than 5 ppb of methane at the landing site at the point of the measurement.<sup id="cite_ref-165">[162] <sup id="cite_ref-Science-20121102_166-0">[163] <sup id="cite_ref-Space-20121102_167-0">[164] <sup id="cite_ref-NYT-20121102_168-0">[165]  On September 19, 2013, NASA scientists, from further measurements by Curiosity, reported no detection of atmospheric methanewith a measured value of 0.18±0.67 ppbv corresponding to an upper limit of only 1.3 ppbv (95% confidence limit) and, as a result, conclude that the probability of current methanogenic microbial activity on Mars is reduced.<sup id="cite_ref-SJ-20130919_169-0">[166] <sup id="cite_ref-SCI-20130919_170-0">[167] <sup id="cite_ref-NYT-20130919_171-0">[168]

The Mars Orbiter Mission by India is searching for methane in the atmosphere,<sup id="cite_ref-payload_172-0">[169]  while the ExoMars Trace Gas Orbiter, launched in 2016, would further study the methane as well as its decomposition products, such as formaldehyde and methanol.<sup id="cite_ref-Mustard_173-0">[170]  During measurements lasting from April to August 2018, the satellite failed to detect any atmospheric methane, hinting at a new and hitherto unknown process by which CH4 is rapidly removed from the atmosphere in a short space of time.<sup id="cite_ref-174">[171]

On December 16, 2014, NASA reported the Curiosity rover detected a "tenfold spike", likely localized, in the amount of methane in the Martian atmosphere. Sample measurements taken "a dozen times over 20 months" showed increases in late 2013 and early 2014, averaging "7 parts of methane per billion in the atmosphere." Before and after that, readings averaged around one-tenth that level.<sup id="cite_ref-NASA-20141216-GW_175-0">[172] <sup id="cite_ref-NYT-20141216-KC_176-0">[173]

Ammonia was tentatively detected on Mars by the Mars Express satellite, but with its relatively short lifetime, it is not clear what produced it.<sup id="cite_ref-davidw_177-0">[174]  Ammonia is not stable in the Martian atmosphere and breaks down after a few hours. One possible source is volcanic activity.<sup id="cite_ref-davidw_177-1">[174]

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.<sup id="cite_ref-nasa20170929_178-0">[175]

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 degrees 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.<sup id="cite_ref-179">[176]

On March 18, 2015, NASA reported the detection of an aurora that is not fully understood and an unexplained dust cloud in the atmosphere of Mars.<sup id="cite_ref-NASA-20150318_180-0">[177]

Climate
Main article: Climate of Mars

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 −143 °C (−225 °F) at the winter polar caps<sup id="cite_ref-cold_14-2">[12]  to highs of up to 35 °C (95 °F) in equatorial summer.<sup id="cite_ref-hot_15-2">[13]  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.<sup id="cite_ref-nasa_surface_181-0">[178]  The planet is 1.52 times as far from the Sun as Earth, resulting in just 43% of the amount of sunlight.<sup id="cite_ref-disc920901_182-0">[179]

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 30 °C (54 °F).<sup id="cite_ref-goodman97_183-0">[180]

Mars has the largest dust storms in the Solar System, reaching speeds of over 160 km/h (100 mph). 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.<sup id="cite_ref-philips01_184-0">[181]



Mars (before/after) dust storm (July 2018)

Dust storms on Mars



November 18, 2012



November 25, 2012



June 6, 2018<sup id="cite_ref-SPC-20180612_185-0">[182]

Locations of the Opportunity and Curiosity rovers are noted

Orbit and rotation
Main article: Orbit of Mars



Mars is about 230 million km (143 million mi) from the Sun; its orbital period is 687 (Earth) days, depicted in red. Earth's orbit is in blue.

Mars's average distance from the Sun is roughly 230 million km (143 million mi), 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.<sup id="cite_ref-186">[183]  A Martian year is equal to 1.8809 Earth years, or 1 year, 320 days, and 18.2 hours.<sup id="cite_ref-nssdc_11-7">[9]

The axial tilt of Mars is 25.19 degrees relative to its orbital plane, which is similar to the axial tilt of Earth.<sup id="cite_ref-nssdc_11-8">[9]  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.<sup id="cite_ref-barlow08_16-1">[14]

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.<sup id="cite_ref-mars_eccentricity_187-0">[184]  Mars's cycle of eccentricity is 96,000 Earth years compared to Earth's cycle of 100,000 years.<sup id="cite_ref-Meeus2003_188-0">[185]  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.<sup id="cite_ref-Baalke2003_189-0">[186]

Habitability and search for life
Main articles: Life on Mars, Viking lander biological experiments, and Colonization of Mars



Viking 1 lander's sampling arm scooped up soil samples for tests (Chryse Planitia)

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.<sup id="cite_ref-Nowack_190-0">[187]  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.<sup id="cite_ref-saltlife_191-0">[188]



Detection of impact glass deposits (green spots) at Alga crater, a possible site for preserved ancient life<sup id="cite_ref-NASA-20150608_192-0">[189]

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.<sup id="cite_ref-hannsson97_193-0">[190]

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 CO 2 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.<sup id="cite_ref-WSU2006_194-0">[191]  Tests conducted by the Phoenix Mars lander have shown that the soil has a alkaline pH and it contains magnesium, sodium, potassium and chloride.<sup id="cite_ref-nutrient_195-0">[192]  The soil nutrients may be able to support life, but life would still have to be shielded from the intense ultraviolet light.<sup id="cite_ref-UV_196-0">[193]  A recent analysis of martian meteorite EETA79001 found 0.6 ppm ClO− 4, 1.4 ppm ClO− 3, and 16 ppm NO− 3, most likely of Martian origin. The ClO− 3 suggests the presence of other highly oxidizing oxychlorines, such as ClO− 2 or ClO, produced both by UV oxidation of Cl and X-ray radiolysis of ClO− 4. Thus, only highly refractory and/or well-protected (sub-surface) organics or life forms are likely to survive.<sup id="cite_ref-197">[194]



This image from Gale crater in 2018 prompted speculation that some shapes were worm-like fossils, but they were geological formations probably formed under water.<sup id="cite_ref-nasa2018-02_198-0">[195]

A 2014 analysis of the Phoenix WCL showed that the Ca(ClO 4) 2 in the Phoenix soil has not interacted with liquid water of any form, perhaps for as long as 600 Myr. If it had, the highly soluble Ca(ClO 4) 2 in contact with liquid water would have formed only CaSO 4. This suggests a severely arid environment, with minimal or no liquid water interaction.<sup id="cite_ref-199">[196]

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.<sup id="cite_ref-am89_200-0">[197]

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.<sup id="cite_ref-icarus172_201-0">[198] <sup id="cite_ref-form_202-0">[199]  Alternatively, these compounds may instead be replenished by volcanic or other geological means, such as serpentinization.<sup id="cite_ref-olivine_161-1">[158]



Location of subsurface water in Planum Australe

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.<sup id="cite_ref-brown20140418_203-0">[200] <sup id="cite_ref-Schultz2014_204-0">[201]  Likewise, the glass in impact craters on Mars could have preserved signs of life if life existed at the site.<sup id="cite_ref-nasapr20150608_205-0">[202] <sup id="cite_ref-brown20150608_206-0">[203] <sup id="cite_ref-sciam20150612_207-0">[204]

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.<sup id="cite_ref-PO-20170509_208-0">[205] <sup id="cite_ref-NC-20170509_209-0">[206]

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.<sup id="cite_ref-nasa2018-02_198-1">[195]

On June 7, 2018, NASA announced that the Curiosity rover had discovered organic compounds in sedimentary rocks dating to three billion years old,<sup id="cite_ref-210">[207]  indicating that some of the building blocks for life were present.<sup id="cite_ref-SPC-20180607_211-0">[208] <sup id="cite_ref-NYT-20180607_212-0">[209]

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 1.5 km (0.9 mi) below the surface at the base of the southern polar ice cap and is about 20 km (12 mi) wide.<sup id="cite_ref-SCI-20180725_213-0">[210] <sup id="cite_ref-NYT-20180725_214-0">[211]  The lake was discovered using the MARSISradar on board the Mars Express orbiter, and the profiles were collected between May 2012 and December 2015.<sup id="cite_ref-Suppl_material_215-0">[212]  The lake is centered at 193°E, 81°S, 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.<sup id="cite_ref-SCI-20180725_213-1">[210]

Moons
Main articles: Moons of Mars, Phobos (moon), and Deimos (moon)



Enhanced-color HiRISE image of Phobos, showing a series of mostly parallel grooves and crater chains, with Stickney crater at right



Enhanced-color HiRISE image of Deimos (not to scale), showing its smooth blanket of regolith

Mars has two relatively small (compared to Earth's) natural moons, Phobos (about 22 km (14 mi) in diameter) and Deimos(about 12 km (7.5 mi) in diameter), which orbit close to the planet. Asteroid capture is a long-favored theory, but their origin remains uncertain.<sup id="cite_ref-esa31031_216-0">[213]  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.<sup id="cite_ref-theoi_217-0">[214] <sup id="cite_ref-qjras19_218-0">[215]  In modern Greek, the planet retains its ancient name Ares (Aris: Άρης).<sup id="cite_ref-Greek_Names_of_the_Planets_219-0">[216]

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.<sup id="cite_ref-phobos.html_220-0">[217]



Orbits of Phobos and Deimos (to scale)

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.<sup id="cite_ref-phobos.html_220-1">[217]

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.<sup id="cite_ref-ellis07_221-0">[218]  More-recent lines of evidence for Phobos having a highly porous interior,<sup id="cite_ref-Andert_222-0">[219]  and suggesting a composition containing mainly phyllosilicates and other minerals known from Mars,<sup id="cite_ref-Giuranna_223-0">[220]  point toward an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit,<sup id="cite_ref-Blast_224-0">[221]  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.<sup id="cite_ref-Giuranna_223-1">[220]

Mars may have moons smaller than 50 to 100 metres (160 to 330 ft) in diameter, and a dust ring is predicted to exist between Phobos and Deimos.<sup id="cite_ref-adler_23-1">[21]

Exploration
Main article: Exploration of Mars



Mars Science Laboratory under parachute during its atmospheric entry at Mars

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

As of 2018, Mars is host to eight functioning spacecraft: six in orbit—2001 Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, MAVEN, Mars Orbiter Mission and ExoMars Trace Gas Orbiter—and two on the surface—Mars Science Laboratory Curiosity (rover) and InSight (lander). Another rover, Opportunity, is inactive now, but NASA still hopes to reestablish contact with it. The public can request images of Mars via the Mars Reconnaissance Orbiter's HiWish program.

The Mars Science Laboratory, named Curiosity, launched on November 26, 2011, and reached Mars on August 6, 2012 UTC. It is larger and more advanced than the Mars Exploration Rovers, with a movement rate up to 90 m (300 ft) per hour.<sup id="cite_ref-home_225-0">[222]  Experiments include a laser chemical sampler that can deduce the make-up of rocks at a distance of 7 m (23 ft).<sup id="cite_ref-laser_226-0">[223]  On February 10, 2013, the Curiosity rover obtained the first deep rock samples ever taken from another planetary body, using its on-board drill.<sup id="cite_ref-227">[224]  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).<sup id="cite_ref-Guardian_228-0">[225]  Observations by the Mars Reconnaissance Orbiter had previously revealed the possibility of flowing water during the warmest months on Mars.<sup id="cite_ref-nasa20110804_229-0">[226]

On September 24, 2014, Mars Orbiter Mission (MOM), launched by the Indian Space Research Organisation, reached Mars orbit. ISRO launched MOM on November 5, 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.<sup id="cite_ref-230">[227]

The European Space Agency, in collaboration with Roscosmos, launched the ExoMars Trace Gas Orbiter and Schiaparelli lander on March 14, 2016.<sup id="cite_ref-bbcnews20160314_231-0">[228]  While the Trace Gas Orbiter successfully entered Mars orbit on October 19, 2016, Schiaparelli crashed during its landing attempt.<sup id="cite_ref-232">[229]

Future
Main article: Exploration of Mars § Timeline of Mars exploration



Concept for a Bimodal Nuclear Thermal Transfer Vehicle in low Earth orbit

In May 2018 NASA's InSight lander was launched, along with the twin MarCO CubeSats that will fly by Mars and provide a telemetry relay for the landing. The mission arrived at Mars in November 2018.<sup id="cite_ref-233">[230] <sup id="cite_ref-sfnow20160309_234-0">[231]  NASA plans to launch its Mars 2020 astrobiology rover in July or August 2020.<sup id="cite_ref-Mars2020launch_235-0">[232]

The European Space Agency will launch the ExoMars rover and surface platform in July 2020.<sup id="cite_ref-LaunchMoved2020_236-0">[233]

NASA will launch their Mars Rover between July 17 and August 5, 2020.<sup id="cite_ref-237">[234]

The United Arab Emirates' Mars Hope orbiter is planned for launch in 2020, reaching Mars orbit in 2021. The probe will make a global study of the Martian atmosphere.<sup id="cite_ref-238">[235]

Several plans for a human mission to Mars have been proposed throughout the 20th century and into the 21st century, but no active plan has an arrival date sooner than the 2020s. SpaceX founder Elon Musk presented a plan in September 2016 to, optimistically, launch space tourists to Mars in 2024 at an estimated development cost of US$10 billion.<sup id="cite_ref-nyt20160927_239-0">[236]  In October 2016, President Barack Obama renewed U.S. 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.<sup id="cite_ref-CNN-20161011_240-0">[237] <sup id="cite_ref-NYT-20161011_241-0">[238]  The NASA Authorization Act of 2017 directed NASA to get humans near or on the surface of Mars by the early 2030s.<sup id="cite_ref-242">[239]

Astronomy on Mars
Main article: Astronomy on Mars

See also: Solar eclipses 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.<sup id="cite_ref-pl_org_deimos_243-0">[240]

Various phenomena seen from Earth have also been observed from Mars, such as meteors and auroras.<sup id="cite_ref-aurora_244-0">[241]  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).<sup id="cite_ref-nature436_245-0">[242] <sup id="cite_ref-sd040317_246-0">[243]  Transits of Mercury and Venus have been observed from Mars. A transit of Earth will be seen from Mars on November 10, 2084.<sup id="cite_ref-jbaa93_247-0">[244]

On October 19, 2014, Comet Siding Spring passed extremely close to Mars, so close that the coma may have enveloped Mars.<sup id="cite_ref-NASA-20141019_248-0">[245] <sup id="cite_ref-NYT-20141019_249-0">[246] <sup id="cite_ref-ESA-20141020_250-0">[247] <sup id="cite_ref-ISRO_MOM_safe_after_Mars_comet_flyby_251-0">[248] <sup id="cite_ref-SD-20131201_252-0">[249] <sup id="cite_ref-NS-20131206_253-0">[250]



Earth and the Moon (MRO HiRISE, November 2016)<sup id="cite_ref-NYT-20170109_254-0">[251]



Phobos transits the Sun (Opportunity, March 10, 2004)



Tracking sunspots from Mars

Viewing


Animation of the apparent retrograde motion of Mars in 2003 as seen from Earth

The mean apparent magnitude of Mars is +0.71 with a standard deviation of 1.05.<sup id="cite_ref-Mallama_and_Hilton_13-2">[11]  Because the orbit of Mars is eccentric, the magnitude at opposition from the Sun can range from about −3.0 to −1.4.<sup id="cite_ref-MallamaSky_255-0">[252]  The minimum brightness is magnitude +1.86 when the planet is in conjunction with the Sun.<sup id="cite_ref-Mallama_and_Hilton_13-3">[11]  At its brightest, Mars (along with Jupiter) are second only to Venus in luminosity.<sup id="cite_ref-Mallama_and_Hilton_13-4">[11]  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.<sup id="cite_ref-lloyd06_256-0">[253]  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- 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.<sup id="cite_ref-shallowsky_257-0">[254]

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.<sup id="cite_ref-zeilik02_258-0">[255]

Relative


Geocentric animation of Mars's orbit relative to Earth from January 2003 to January 2019 Mars ·   Earth



Mars distance from Earth

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 54 and 103 million km (34 and 64 million mi) due to the planets' elliptical orbits, which causes comparable variation in angular size.<sup id="cite_ref-Laskar2003_259-0">[256] <sup id="cite_ref-nasa051103_260-0">[257]  The last Mars opposition occurred on July 27, 2018,<sup id="cite_ref-NYT-20180801_261-0">[258]  at a distance of about 58 million km (36 million mi).<sup id="cite_ref-sheehan970202_262-0">[259]  The next Mars opposition occurs on October 13, 2020, at a distance of about 63 million km (39 million mi).<sup id="cite_ref-sheehan970202_262-1">[259]  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.<sup id="cite_ref-astropro_263-0">[260]

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, 55,758,006 km (0.37271925 AU; 34,646,419 mi), magnitude −2.88, on August 27, 2003, at 9:51:13 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 September 12, 57,617 BC, the next time being in 2287.<sup id="cite_ref-rao030822_264-0">[261]  This record approach was only slightly closer than other recent close approaches. For instance, the minimum distance on August 22, 1924, was 0.37285 AU, and the minimum distance on August 24, 2208, will be 0.37279 AU.<sup id="cite_ref-Meeus2003_188-1">[185]

Historical observations
Main article: History of Mars observation

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


Galileo Galilei, first person to see Mars via telescope in 1610.<sup id="cite_ref-jha15_265-0">[262]

The ancient Sumerians believed that Mars was Nergal, the god of war and plague.<sup id="cite_ref-Rabkin2005_266-0">[263]  During Sumerian times, Nergal was a minor deity of little significance,<sup id="cite_ref-Rabkin2005_266-1">[263]  but, during later times, his main cult center was the city of Nineveh.<sup id="cite_ref-Rabkin2005_266-2">[263]  In Mesopotamian texts, Mars is referred to as the "star of judgement of the fate of the dead".<sup id="cite_ref-267">[264]  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.<sup id="cite_ref-paob85_268-0">[265]  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.<sup id="cite_ref-north08_269-0">[266] <sup id="cite_ref-swerdlow98_270-0">[267]  In Ancient Greek, the planet was known as Πυρόεις.<sup id="cite_ref-271">[268]

In the fourth century BCE, Aristotle noted that Mars disappeared behind the Moon during an occultation, indicating that the planet was farther away.<sup id="cite_ref-poor08_272-0">[269]  Ptolemy, a Greek living in Alexandria,<sup id="cite_ref-google_273-0">[270]  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.<sup id="cite_ref-google7_274-0">[271]  Literature from ancient China confirms that Mars was known by Chinese astronomers by no later than the fourth century BCE.<sup id="cite_ref-needham_ronan85_275-0">[272]  In the fifth century CE, the Indian astronomical text Surya Siddhanta estimated the diameter of Mars.<sup id="cite_ref-jse97_276-0">[273]  In the East Asian cultures, Mars is traditionally referred to as the "fire star" (Chinese: 火星), based on the Five elements.<sup id="cite_ref-277">[274] <sup id="cite_ref-278">[275] <sup id="cite_ref-279">[276]

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.<sup id="cite_ref-taton03_280-0">[277]  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.<sup id="cite_ref-hirschfeld01_281-0">[278]  The only occultation of Mars by Venus observed was that of October 13, 1590, seen by Michael Maestlin at Heidelberg.<sup id="cite_ref-sat57_282-0">[279]  In 1610, Mars was viewed by Italian astronomer Galileo Galilei, who was first to see it via telescope.<sup id="cite_ref-jha15_265-1">[262]  The first person to draw a map of Mars that displayed any terrain features was the Dutch astronomer Christiaan Huygens.<sup id="cite_ref-arizona_283-0">[280]

Martian "canals"


Map of Mars by Giovanni Schiaparelli



Mars sketched as observed by Lowell before 1914 (south on top)



Map of Mars from the Hubble Space Telescope as seen near the 1999 opposition (north on top)

Main article: Martian canal

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 September 5, 1877. In that year, the Italian astronomer Giovanni Schiaparelli used a 22 cm (8.7 in) 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".<sup id="cite_ref-snyder01_284-0">[281] <sup id="cite_ref-sagan80_285-0">[282]

Influenced by the observations, the orientalist Percival Lowell founded an observatory which had 30 and 45 cm (12 and 18 in) 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.<sup id="cite_ref-basalla06_286-0">[283] <sup id="cite_ref-NYT-20151001_287-0">[284]  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.<sup id="cite_ref-maria_lane05_288-0">[285] <sup id="cite_ref-ba3_289-0">[286]

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 Flammarion with an 84 cm (33 in) telescope, irregular patterns were observed, but no canali were seen.<sup id="cite_ref-zahnle01_290-0">[287]

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.<sup id="cite_ref-science136_3510_291-0">[288]

Spacecraft visitation
Main article: Exploration of Mars

Once spacecraft visited the planet during NASA's Mariner missions in the 1960s and 70s, 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.<sup id="cite_ref-ward_brownlee00_292-0">[289]

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.<sup id="cite_ref-Distant_worlds:_milestones_in_planetary_exploration_293-0">[290]  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.<sup id="cite_ref-294">[291]

In culture
Main articles: Mars in culture and Mars in fiction



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.<sup id="cite_ref-dinerman04_295-0">[292]

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.<sup id="cite_ref-prion_296-0">[293]



An 1893 soap ad playing on the popular idea that Mars was populated

Many other observations and proclamations by notable personalities added to what has been termed "Mars Fever".<sup id="cite_ref-fergus04_297-0">[294]  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.<sup id="cite_ref-tesla01_298-0">[295] 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.<sup id="cite_ref-cheney81_299-0">[296]  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."<sup id="cite_ref-nyt020511_300-0">[297]

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.<sup id="cite_ref-nyt2_301-0">[298] 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.<sup id="cite_ref-nyt2_301-1">[298] Pickering later proposed creating a set of mirrors in Texas, intended to signal Martians.<sup id="cite_ref-fradin99_302-0">[299]



Martian tripod illustration from the 1906 French edition of The War of the Worlds by H. G. Wells

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.<sup id="cite_ref-sagan80_285-1">[282] 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.<sup id="cite_ref-lightman97_303-0">[300]  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),<sup id="cite_ref-sanford09_304-0">[301]  and a number of Robert A. Heinlein stories before the mid-sixties.<sup id="cite_ref-buker02_305-0">[302]

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.<sup id="cite_ref-jonathan_swift_306-0">[303]

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.<sup id="cite_ref-rabkin05_307-0">[304]

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.<sup id="cite_ref-miles_peters_308-0">[305]