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   =Mars= From Wikipedia, the free encyclopedia This article is about the planet. For other uses, see Mars (disambiguation).[1]

Mars [2]
[3]

Mars image taken by the Hubble Space Telescope (2003).

Designations
Pronunciation [4]i/ˈmɑrz/
Adjectives Martian
Orbital characteristics[2]
EpochJ2000
Aphelion 1.6660 AU

249.2 million km

Perihelion 1.3814 AU

206.7 million km

Semi-major axis 1.523679 AU

227,939,100 km

Eccentricity 0.0935±0.0001
Orbital period 1.8808 Julian years

686.971 d 668.5991 sols

Synodic period 779.96 days

2.135 Julian years

Average orbital speed 24.077 km/s
Mean anomaly 19.3564°
Inclination 1.850° to ecliptic

5.65° to Sun's equator 1.67° to invariable plane[1]

Longitude of ascending node 49.562°
Argument of perihelion 286.537°
Satellites 2
Physical characteristics
Mean radius 3389.5±0.2 km[a][3]
Equatorial radius

3396.2±0.1 km[a][3]

0.533 Earths

Polar radius

3,376.2±0.1 km[a][3]

0.531 Earths

Flattening 0.00589±0.00015
Surface area

144,798,500 km2

0.284 Earths

Volume

1.6318×1011 km3[4]

0.151 Earths

Mass

6.4185×1023 kg[4]

0.107 Earths

Mean density 3.9335±0.0004[4]g/cm³
Surface gravity

3.711 m/s²[4]

0.376 g

Moment of inertia factor 0.3662±0.0017[5]
Escape velocity 5.027 km/s
Sidereal rotation period

1.025957 d

24h 37m 22s[4]

Equatorial rotation velocity 868.22 km/h (241.17 m/s)
Axial tilt 25.19°
North poleright ascension

21h 10m 44s

317.68143°

North poledeclination 52.88650°
Albedo

0.170 (geometric)[6]

0.25 (Bond)[7]

Surface temp. min mean max
Kelvin 130 K 210 K[7] 308 K
Celsius −143 °C[9] −63 °C 35 °C[10]
Apparent magnitude +1.6 to −3.0[8]
Angular diameter 3.5–25.1″[7]
Atmosphere[7][14]
Surface pressure 0.636 (0.4–0.87) kPa
Composition by volume *95.97% carbon dioxide

Mars is the fourth planet from the Sun and the second smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is often described as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance.[15] Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts, and polar ice capsof Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of Olympus Mons, the second highest known mountain within the Solar System (the tallest on a planet), and of Valles Marineris, one of the largest canyons. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature.[16][17] Mars has two moonsPhobos and Deimos, which are small and irregularly shaped. These may be captured asteroids,[18][19] similar to 5261 Eureka, a Mars trojan.

Until the first successful Mars flyby in 1965 by Mariner 4, many speculated about the presence of liquid water on the planet's surface. This was based on observed periodic variations in light and dark patches, particularly in the polar latitudes, which appeared to be seas and continents; long, darkstriations were interpreted by some as irrigation channels for liquid water. These straight line features were later explained as optical illusions, though geological evidence gathered by unmanned missions suggests that Mars once had large-scale water coverage on its surface at some earlier stage of its life.[20] In 2005, radar data revealed the presence of large quantities of water ice at the poles[21] and at mid-latitudes.[22][23] The Mars rover Spirit sampled chemical compounds containing water molecules in March 2007. The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.[24]

Mars is host to seven functioning spacecraft: five in orbit – the Mars OdysseyMars ExpressMars Reconnaissance OrbiterMAVEN and Mars Orbiter Mission – and two on the surface – Mars Exploration RoverOpportunity and the Mars Science Laboratory Curiosity. Defunct spacecraft on the surface include MER-A Spirit and several other inert landers and rovers such as the Phoenix lander, which completed its mission in 2008. Observations by the Mars Reconnaissance Orbiter have revealed possible flowing water during the warmest months on Mars.[25] In 2013, NASA's Curiosity rover discovered that Mars' soil contains between 1.5% and 3% water by mass (about two pints of water per cubic foot or 33 liters per cubic meter, albeit attached to other compounds and thus not freely accessible).[26]

Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches −3.0,[8] which is surpassed only byJupiterVenus, the Moon, and the Sun. Optical ground-based telescopes are typically limited to resolving features about 300 km (186 miles) across when Earth and Mars are closest because of Earth's atmosphere.[27]

ContentsEdit

  [hide*1 Physical characteristics

§Physical characteristicsEdit

[5]Earth compared with Mars.Mars - animation (00:40) showing major features. 

Mars has approximately half the diameter of Earth. It is less dense than Earth, having about 15% of Earth's volume and 11% of the mass. Its surface area is only slightly less than the total area of Earth's dry land.[7]Although Mars is larger and more massive thanMercury, Mercury has a higher density. This results in the two planets having a nearly identical gravitational pull at the surface—that of Mars is stronger by less than 1%. The red-orange appearance of the Martian surface is caused by iron(III) oxide, more commonly known as hematite, or rust.[28] It can also look like butterscotch,[29] and other common surface colors include golden, brown, tan, and greenish, depending on minerals.[29]

§Internal structureEdit

Like Earth, this planet has undergone differentiation, resulting in a dense, metallic core region overlaid by less dense materials.[30] Current models of the planet's interior imply a core region about 1,794 ± 65 kilometres (1,115 ± 40 mi) in radius, consisting primarily of iron and nickel with about 16–17% sulfur.[31] This iron sulfidecore is partially fluid, and it has twice the concentration of the lighter elements that exist at Earth's core. The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but it now appears to be dormant. Besides silicon and oxygen, the most abundant elements in the Martian crust are iron,magnesiumaluminumcalcium, 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).[32] Earth's crust, averaging 40 km (25 mi), is only one third as thick as Mars' crust, relative to the sizes of the two planets. The InSight lander planned for 2016 will use aseismometer to better constrain the models of the interior.

§Surface geologyEdit

Main article: Geology of Mars

Mars is a terrestrial planet that consists of minerals containing silicon and oxygenmetals, and other elements that typically make up rock. The surface of Mars is primarily composed of tholeiitic basalt,[33] although parts are moresilica-rich than typical basalt and may be similar to andesitic rocks on Earth or silica glass. Regions of low albedoshow 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 also been found.[34] Much of the surface is deeply covered by finely grained iron(III) oxide dust.[35][36]

[6]Geologic Map of Mars (USGS; July 14, 2014)

(full map / video)[37][38][39]

Although Mars has no evidence of a current structured global magnetic field,[40]observations show that parts of the planet's crust have been magnetized, and that alternating polarity reversals of its dipole field have occurred in the past. Thispaleomagnetism of magnetically susceptible minerals has properties that are similar to the alternating bands found on the ocean floors of Earth. One theory, published in 1999 and re-examined in October 2005 (with the help of the Mars Global Surveyor), is that these bands demonstrate plate tectonics on Mars four billion years ago, before the planetary dynamo ceased to function and the planet's magnetic field faded away.[41]

During the Solar System's formation, Mars was created as the result of a stochastic process of run-away accretion out of 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 removed from areas closer to the Sun by the young star's energetic solar wind.[42]

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,[43][44][45] 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 km by 8,500 km, or roughly four times larger than the Moon's South Pole – Aitken basin, the largest impact basin yet discovered.[16][17] 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.[46][47]

The geological history of Mars can be split into many periods, but the following are the three primary periods:[48][49]

  • Noachian period (named after Noachis Terra): Formation of the oldest extant surfaces of Mars, 4.5 billion years ago 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 billion years ago to 2.9–3.3 billion years ago. The Hesperian period is marked by the formation of extensive lava plains.
  • Amazonian period (named after Amazonis Planitia): 2.9–3.3 billion years ago to present. Amazonian regions have few meteorite impact craters, but are otherwise quite varied. Olympus Mons formed during this period, along with lava flows elsewhere on Mars.

Some geological activity is still taking place on Mars. The Athabasca Valles is home to sheet-like lava flows up to about 200 Mya. Water flows in the grabens called the Cerberus Fossae occurred less than 20 Mya, indicating equally recent volcanic intrusions.[50] On February 19, 2008, images from the Mars Reconnaissance Orbiter showed evidence of an avalanche from a 700 m high cliff.[51]

Notable rocks on Mars
[7] [8] [9] [10] [11] [12] [13] [14]
Adirondack

(Spirit)

Barnacle Bill

(Sojourner)

Bathurst Inlet

(Curiosity)

Big Joe*

(Viking)

Block Island

(Opportunity)M

Bounce

(Opportunity)

Coronation

(Curiosity)

El Capitan

(Opportunity)

[15] [16] [17] [18] [19] [20] [21] [22]
Esperance*

(Opportunity)

Goulburn

(Curiosity)

Heat Shield

(Opportunity)M

Home Plate

(Spirit)

Hottah

(Curiosity)

Jake Matijevic

(Curiosity)

Last Chance

(Opportunity)

Link

(Curiosity)

[23] [24] [25] [26] [27] [28] [29] [30]
Mackinac Island

(Opportunity)M

Mimi*

(Spirit)

Oileán Ruaidh

(Opportunity)M

Pot of Gold

(Spirit)

Rocknest 3

(Curiosity)

Shelter Island

(Opportunity)M

Tintina

(Curiosity)

Yogi

(Sojourner)

This box: 

 

(Notes: * = links to relevant article; M = Meteorite)

 ===§Soil===

Main article: Martian soil[31]Exposure of silica-rich dust uncovered by the Spiritrover

The Phoenix lander returned data showing Martian soil to be slightly alkaline and containing elements such as magnesiumsodiumpotassium and chlorine. These nutrients are found in gardens on Earth, and they are necessary for growth of plants.[52]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.[53][54][55][56]

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. Sometimes, the streaks start in a tiny area which then spread out for hundreds of metres. They have also 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.[57] Several explanations have been put forward, some of which involve water or even the growth of organisms.[58][59]

§HydrologyEdit

Main article: Water on Mars[32]Photomicrograph taken by Opportunity showing a gray hematite concretion, indicative of the past presence of liquid water

Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is about 100 times thinner than Earth's,[60] except at the lowest elevations for short periods.[61][62] The two polar ice caps appear to be made largely of water.[63][64] 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.[65] A permafrost mantle stretches from the pole to latitudes of about 60°.[63]

Large quantities of water ice are 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 both at the poles (July 2005)[21][66] and at mid-latitudes (November 2008).[22] The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.[24]

Landforms visible on Mars strongly suggest that liquid water has at least at times existed on the planet's surface. Huge linear swathes of scoured ground, known asoutflow channels, cut across the surface in around 25 places. These are thought to record erosion which occurred during the catastrophic release of water from subsurface aquifers, though some of these structures have also been hypothesized to result from the action of glaciers or lava.[67][68] One of the larger examples, Ma'adim Vallis is 700 km long and much bigger than the Grand Canyon with a width of 20 km and a depth of 2 km in some places. It is thought to have been carved by flowing water early in Mars' history.[69] The youngest of these channels are thought to have formed as recently as only a few million years ago.[70] 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 runoffresulting from rain or snow fall 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.[71]

Along crater and canyon walls, there are also 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 demands the involvement of liquid water, probably from melting ice,[72][73] although others have argued for formation mechanisms involving carbon dioxide frost or the movement of dry dust.[74][75] No partially degraded gullies have formed by weathering and no superimposed impact craters have been observed, indicating that these are young features, possibly even active today.[73]

Other geological features, such as deltas and alluvial fans preserved in craters, also argue strongly for warmer, wetter conditions at some interval or intervals in earlier Mars history.[76] Such conditions necessarily require the widespread presence of crater lakes across a large proportion of the surface, for which there is also independent mineralogical, sedimentological and geomorphological evidence.[77] Some authors have even gone so far as to argue that at times in the Martian past, much of the low northern plains of the planet were covered with a true ocean hundreds of meters deep, though this remains controversial.[78]

[33]Composition of"Yellowknife Bay" rocks –rock veins are higher incalcium and sulfur than "Portage" soil – APXSresults – Curiosity rover(March 2013).

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.[79] Some of the evidence believed to indicate ancient water basins and flows has been negated by higher resolution studies by the Mars Reconnaissance Orbiter.[80] In 2004, Opportunity detected the mineral jarosite. This forms only in the presence of acidic water, which demonstrates that water once existed on Mars.[81] More recent evidence for liquid water comes from the finding of the mineralgypsum on the surface by NASA's Mars rover Opportunity in December 2011.[82][83]Additionally, the study leader Francis McCubbin, a planetary scientist at the University of New Mexico in Albuquerque looking at hydroxals in crystalline minerals from Mars, states that the amount of water in the upper mantle of Mars 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).[84]

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.[85][86][87] 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, in the rover's traverse from the Bradbury Landing site to the Yellowknife Bayarea in the Glenelg terrain.[85]

In March 2015, scientists stated that evidence exists for an ancient Martian ocean, likely in the planet's northern hemisphere and about the size of Earth's Arctic Ocean. This finding was derived from the ratio of water anddeuterium in the modern Martian atmosphere compared to the ratio found on Earth. Eight times as much deuterium was found at Mars than 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 presence of an ocean. Other scientists caution that this new study has 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.[88]

§Polar capsEdit

Main article: Martian polar ice caps[34]North polar early summer ice cap (1999)[35]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 CO2ice (dry ice).[89] When the poles are again exposed to sunlight, the frozen CO2 sublimes, creating enormous winds that sweep off the poles as fast as 400 km/h. 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.[90]

The polar caps at both poles consist primarily 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.[91] 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.[92] The northern polar cap has a diameter of about 1,000 kilometres during the northern Mars summer,[93] and contains about 1.6 million cubic km of ice, which, if spread evenly on the cap, would be 2 km thick.[94] (This compares to a volume of 2.85 million cubic km (km3) for the Greenland ice sheet.) The southern polar cap has a diameter of 350 km and a thickness of 3 km.[95] The total volume of ice in the south polar cap plus the adjacent layered deposits has also been estimated at 1.6 million cubic km.[96] 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.[97][98]

The seasonal frosting of some 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 spider-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.[99][100][101][102]

§Geography and naming of surface featuresEdit

Main article: Geography of MarsSee also: Category:Surface features of Mars[36]MOLA-based topographic map showing highlands (red and orange) dominating the southern hemisphere of Mars, lowlands (blue) the northern. Volcanic plateaus delimit the northern plains in some regions, whereas the highlands are punctuated by several large impact basins.

Although better remembered for mapping the Moon, Johann Heinrich Mädler andWilhelm Beer were the first "areographers". They began by establishing that most of Mars' 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".[103]

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.[104]

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).[105] 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.[106] The permanent northern polar ice cap is named Planum Boreum, whereas the southern cap is called Planum Australe.

Mars' equator is defined by its rotation, but the location of its Prime Meridian was specified, as was Earth's (atGreenwich), by choice of an arbitrary point; Mädler and Beer selected a line in 1830 for their first maps of Mars. 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.[107]

Because Mars has no oceans and hence no "sea level", a zero-elevation surface also had to be selected as a reference level; this is also called the areoid[108] 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.[109] This pressure corresponds to thetriple point of water, and it is about 0.6% of the sea level surface pressure on Earth (0.006 atm).[110] In practice, today this surface is defined directly from satellite gravity measurements.

§Map of quadranglesEdit

The following imagemap of the planet Mars is divided into the 30 quadrangles defined by the United States Geological Survey[111][112] The quadrangles are numbered with the prefix "MC" for "Mars Chart."[113] Click on the quadrangle and you will be taken to the corresponding article pages. 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.

 [37]0°N 180°W0°N 0°W90°N 0°WMC-01 Mare BoreumMC-02 DiacriaMC-03 ArcadiaMC-04 Mare AcidaliumMC-05 Ismenius LacusMC-06 CasiusMC-07 CebreniaMC-08 AmazonisMC-09 TharsisMC-10 Lunae PalusMC-11 Oxia PalusMC-12 ArabiaMC-13 Syrtis MajorMC-14 AmenthesMC-15 ElysiumMC-16 MemnoniaMC-17 PhoenicisMC-18 CopratesMC-19 MargaritiferMC-20 SabaeusMC-21 IapygiaMC-22 TyrrhenumMC-23 AeolisMC-24 PhaethontisMC-25 ThaumasiaMC-26 ArgyreMC-27 NoachisMC-28 HellasMC-29 EridaniaMC-30

Mare Australe

 ====§Impact topography====

[38]Bonneville crater andSpirit 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 km long by 8,500 km wide, 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.[16][17]

[39]Fresh asteroid impact on Mars 3.34°N 219.38°E -before/March 27 &after/March 28, 2012 (MRO).[114]

Mars is scarred by a number of impact craters: a total of 43,000 craters with a diameter of 5 km or greater have been found.[115] The largest confirmed of these is the Hellas impact basin, a light albedo feature clearly visible from Earth.[116] 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 also more likely to be struck by short-period cometsi.e., those that lie within the orbit of Jupiter.[117] 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. Some craters have a morphology that suggests the ground became wet after the meteor impacted.[118]

§VolcanoesEdit

[40]Viking orbiter view ofOlympus Mons[41]MOLA colorized shaded-relief map of western hemisphere of Mars showing Tharsis bulge (shades of red and brown). Tall volcanoes appear white.Main article: Volcanism on 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.[119] 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 high.[120][121]

§Tectonic sitesEdit

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 and a depth of up to 7 km. 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 also a plate boundary where 150 km of transverse motion has occurred, making Mars a planet with possibly a two-plate tectonicarrangement.[122][123]

§HolesEdit

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.[124] The caves, named after loved ones of their discoverers, are collectively known as the "seven sisters."[125] Cave entrances measure from 100 m to 252 m wide and they are believed to be at least 73 m to 96 m deep. Because light does not reach the floor of most of the caves, perhaps 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 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.[126]

§AtmosphereEdit

Main article: Atmosphere of Mars[42]Escaping atmosphere on Mars (carbonoxygen, and hydrogen) by MAVEN inUV.[127]

Mars lost its magnetosphere 4 billion years ago,[128] possibly because of numerous asteroid strikes,[129] 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,[128][130] and this atmospheric loss will be studied by the upcoming 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).[131] The highest atmospheric density on Mars is equal to that found 35 km (22 mi)[132] 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),[133] 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.

[43]The tenuousatmosphere of Marsvisible on the horizon.

The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogenalong with traces of oxygen and water.[7][134] 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.[135]

Methane has been detected in the Martian atmosphere with a mole fraction of about 30 ppb;[13][136] it occurs in extended plumes, and the profiles imply that the methane was released from discrete regions. In northern midsummer, the principal plume contained 19,000 metric tons of methane, with an estimated source strength of 0.6 kilogram per second.[137][138] 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.[137] It is estimated that Mars must produce 270 tonnes per year of methane.[137][139]

The implied methane destruction lifetime may be as long as about 4 Earth years and as short as about 0.6 Earth years.[137][140] This rapid turnover would indicate an active source of the gas on the planet. Volcanic activity,cometary impacts, and the presence of methanogenic microbial life forms are among possible sources. Methane could also be produced by a non-biological process called serpentinization[b] involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.[141]

[44]Potential sources and sinks of methane (CH4) on Mars.

The Curiosity rover, which landed on Mars in August 2012, is able to make measurements that distinguish between different isotopologues of methane,[142] 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.[143] 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.[144][145][146][147]On September 19, 2013, NASA scientists, from further measurements by Curiosity, reported no detection of atmospheric methane with 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.[148][149][150] The Mars Trace Gas Mission orbiter planned to launch in 2016 would further study the methane,[151][152] as well as its decomposition products such as formaldehyde and methanol.

On 16 December 2014, NASA reported the Curiosity rover detected a "tenfold spike", likely localized, in the amount ofmethane 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.[153][154]

Ammonia was also tentatively detected on Mars by the Mars Express satellite, but with its relatively short lifetime, it is not clear what produced it.[155] Ammonia is not stable in the Martian atmosphere and breaks down after a few hours. One possible source is volcanic activity.[155]

§ClimateEdit

Main article: Climate of MarsDust storm on Mars.[45]November 18, 2012[46]November 25, 2012 Opportunity andCuriosity rovers are noted.

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' 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 (at the winter polar caps)[9] to highs of up to 35 °C (in equatorial summer).[10] 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.[156] The planet is also 1.52 times as far from the Sun as Earth, resulting in just 43% of the amount of sunlight.[157]

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 reach up to 30 kelvins warmer than the equivalent summer temperatures in the north.[158]

Mars also has the largest dust storms in the Solar System. 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.[159]

§Orbit and rotationEdit

Main article: Orbit of Mars[47]Mars is about 143 million miles from the Sun; its orbital period is 687 (Earth) days - depicted in red - Earth's orbit in blue.

Mars' average distance from the Sun is roughly 230 million km (1.5 AU, or 143 million miles), 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.[7]

The axial tilt of Mars is 25.19 degrees, which is similar to the axial tilt of Earth.[7] As a result, Mars has seasons like Earth, though on Mars, they are nearly twice as long given its longer year. Currently, the orientation of the north pole of Mars is close to the star Deneb.[14] Mars passed an aphelion in March 2010[160] and its perihelion in March 2011.[161] The next aphelion came in February 2012[161] and the next perihelion came in January 2013.[161]

Mars has a relatively pronounced orbital eccentricity of about 0.09; of the seven other planets in the Solar System, only Mercury shows greater eccentricity. It is known that in the past, Mars has had a much more circular orbit than it does currently. At one point, 1.35 million Earth years ago, Mars had an eccentricity of roughly 0.002, much less than that of Earth today.[162] Mars' cycle of eccentricity is 96,000 Earth years compared to Earth's cycle of 100,000 years.[163] Mars also 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.[164]

§Search for lifeEdit

Main articles: Life on Mars and Viking spacecraft biological experiments[48]Viking 1 Lander - sampling arm created deep trenches, scooping up material for tests (Chryse Planitia).

The current understanding of planetary habitability—the ability of a world to develop and sustain life—favors planets that have liquid water on their surface. This most often requires that the orbit of a planet lie within the habitable zone, which for the Sun extends from just beyond Venus to about the semi-major axis of Mars.[165] During perihelion, Mars dips inside this region, but the planet'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. Some recent evidence has suggested that any water on the Martian surface may have been too salty and acidic to support regular terrestrial life.[166]

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