Mars

Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance.^[13] 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 caps of 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 highest known mountain within the Solar System, and of Valles Marineris, the largest canyon. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature.^[14][15]

Until the first successful flyby of Mars occurred 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, dark striations 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 suggest that Mars once had large-scale water coverage on its surface.^[16] In 2005, radar data revealed the presence of large quantities of water ice at the poles,^[17] and at mid-latitudes.^[18][19] 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.^[20]

Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Martian trojan asteroid. Mars is currently host to three functional orbiting spacecraft: Mars Odyssey, Mars Express, and the Mars Reconnaissance Orbiter. On the surface are the Mars Exploration Rover Opportunity and its recently decommissioned twin, Spirit, along with several other inert landers and rovers, both successful and unsuccessful. The Phoenix lander completed its mission on the surface in 2008. Observations by NASA's now-defunct Mars Global Surveyor show evidence that parts of the southern polar ice cap have been receding.^[21] Observations by NASA's Mars Reconnaissance Orbiter have revealed possible flowing water during the warmest months on Mars.^[22]

Mars can easily be seen from Earth with the naked eye. Its apparent magnitude reaches −3.0^[7] a brightness surpassed only by Jupiter, Venus, 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.^[23]

Physical characteristics

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.^[6] While Mars is larger and more massive than Mercury, 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.^[24]

Geology

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,^[25] 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 show 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.^[26] Much of the surface is deeply covered by finely grained iron(III) oxide dust.^[27][28]

Like Earth, this planet has undergone differentiation, resulting in a dense, metallic core region overlaid by less dense materials.^[29] Current models of the planet's interior imply a core region about 1794 ± 65 km in radius, consisting primarily of iron and nickel with about 16–17% sulfur.^[30] This iron sulfide core is partially fluid, and has twice the concentration of the lighter elements than 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 now 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, with a maximum thickness of 125 km.^[31] Earth's crust, averaging 40 km, is only one third as thick as Mars’ crust, relative to the sizes of the two planets.

Although Mars has no evidence of a current structured global magnetic field,^[32] 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. This paleomagnetism of magnetically susceptible minerals has properties that are very 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.^[33]

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.^[34]

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,^[35][36][37] while 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.^[14][15] 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.^[38][39]

The geological history of Mars can be split into many periods, but the following are the three primary periods:^[40][41]

• 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 Gyr ago 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.^[42] On February 19, 2008, images from the Mars Reconnaissance Orbiter showed evidence of an avalanche from a 700 m high cliff.^[43]

Soil

The Phoenix lander returned data showing Martian soil to be slightly alkaline and containing elements such as magnesium, sodium, potassium and chloride. These nutrients are found in gardens on Earth, and are necessary for growth of plants.^[44] Experiments performed by the Lander showed that the Martian soil has a basic pH of 8.3, and may contain traces of the salt perchlorate.^[45][46]

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 spreads 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.^[47] Several explanations have been put forward, some of which involve water or even the growth of organisms.^[48][49]

Hydrology

Liquid water cannot exist on the surface of Mars due to low atmospheric pressure, except at the lowest elevations for short periods.^[50][51] The two polar ice caps appear to be made largely of water.^[52][53] 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.^[54] A permafrost mantle stretches from the pole to latitudes of about 60°.^[52]

Large quantities of water ice are thought to be trapped underneath 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)^[17][55] and at mid-latitudes (November 2008).^[18] The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.^[20]

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 as outflow 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 hypothesised to result from the action of glaciers or lava.^[56][57] The youngest of these channels are thought to have formed as recently as only a few million years ago.^[58] 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 very strongly imply that they were carved by runoff resulting 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.^[59]

There are also thousands of features along crater and canyon walls 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,^[60][61] although others have argued for formation mechanisms involving carbon dioxide frost or the movement of dry dust.^[62][63] No partially degraded gullies have formed by weathering and no superimposed impact craters have been observed, indicating that these are very young features, possibly even active today.^[61]

Other geological features, such as deltas and alluvial fans preserved in craters, also argue very strongly for warmer, wetter conditions at some interval or intervals in earlier Mars history.^[64] 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.^[65] 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.^[66]

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.^[67] Some of the evidence believed to indicate ancient water basins and flows has been negated by higher resolution studies by the Mars Reconnaissance Orbiter.^[68] In 2004, Opportunity detected the mineral jarosite. This forms only in the presence of acidic water, which demonstrates that water once existed on Mars.^[69]

Polar caps

Mars has two permanent polar ice caps. During a pole's winter, it lies in continuous darkness, chilling the surface and causing the deposition of 25–30% of the atmosphere into slabs of CO₂ ice (dry ice).^[70] 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.^[71]

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, while the south cap has a permanent dry ice cover about eight metres thick.^[72] The northern polar cap has a diameter of about 1,000 kilometres during the northern Mars summer,^[73] and contains about 1.6 million cubic km of ice, which if spread evenly on the cap would be 2 km thick.^[74] (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.^[75] 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.^[76] 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.^[77][78]

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.^{[79][80][81][82]}

Geography

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’ surface features were permanent, and 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."^[83]

Today, features on Mars are named from a variety of sources. Albedo features are named for classical mythology. Craters larger than 60 kilometres (37 mi) are named for deceased scientists and writers and others who have contributed to the study of Mars. Craters smaller that 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.^[84]

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).^[85] 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.^[86] The permanent northern polar ice cap is named Planum Boreum, while 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 (at Greenwich), 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.^[87]

Since 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^[88] of Mars, analogous to the terrestrial geoid. Zero altitude is defined by the height at which there is 610.5 Pa (6.105 mbar) of atmospheric pressure.^[89] This pressure corresponds to the triple point of water, and is about 0.6% of the sea level surface pressure on Earth (0.006 atm).^[90] In practice, today this surface is defined directly from satellite gravity measurements.

Impact topography

The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. Research in 2008 has presented evidence regarding a theory proposed in 1980 postulating that, four billion years ago, the northern hemisphere of Mars was struck by an object one-tenth to two-thirds the size of the 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.^[14][15]

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.^[91] The largest confirmed of these is the Hellas impact basin, a light albedo feature clearly visible from Earth.^[92] Due to the smaller mass of Mars, the probability of an object colliding with the planet is about half that of the 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 comets, i.e., those that lie within the orbit of Jupiter.^[93] 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.^[94]

Tectonic sites

The shield volcano, Olympus Mons (Mount Olympus), at 27 km is the highest known mountain in the Solar System.^[95] It is an extinct volcano in the vast upland region Tharsis, which contains several other large volcanoes. Olympus Mons is over three times the height of Mount Everest, which in comparison stands at just over 8.8 km.^[96]

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 long and nearly 2 km 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. Another large canyon is Ma'adim Vallis (Ma'adim is Hebrew for Mars). It is 700 km long and again much bigger than the Grand Canyon with a width of 20 km and a depth of 2 km in some places. It is possible that Ma'adim Vallis was flooded with liquid water in the past.^[97]

Caves

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 Arsia Mons volcano.^[98] The caves, named after loved ones of their discoverers, are collectively known as the "seven sisters."^[99] 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, it is likely 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 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.^[100]

Atmosphere

Mars lost its magnetosphere 4 billion years ago,^[101] 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 these ionised atmospheric particles trailing off into space behind Mars.^[101][102] Compared to Earth, the atmosphere of Mars is quite rarefied. Atmospheric pressure on the surface ranges from a low of 30 Pa (0.030 kPa) on Olympus Mons to over 1,155 Pa (1.155 kPa) in the Hellas Planitia, with a mean pressure at the surface level of 600 Pa (0.60 kPa).^[103] The surface pressure of Mars at its thickest is equal to the pressure found 35 km^[104] above the Earth's surface. This is less than 1% of the Earth's surface pressure (101.3 kPa). The scale height of the atmosphere is about 10.8 km,^[105] which is higher than Earth's (6 km) 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 on Mars consists of 95% carbon dioxide, 3% nitrogen, 1.6% argon and contains traces of oxygen and water.^[6] 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.^[106]

Methane has been detected in the Martian atmosphere with a mole fraction of about 30 ppb;^[12][107] 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.^[108][109] The profiles suggest that there may be two local source regions, the first centered near 30° N, 260° W and the second near 0°, 310° W.^[108] It is estimated that Mars must produce 270 ton/year of methane.^[108][110]

The implied methane destruction lifetime may be as long as about 4 Earth years and as short as about 0.6 Earth years.^[108][111] 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.^[112]

Climate

Of all the planets in the Solar System, the seasons of Mars are the most Earth-like, due to the similar tilts of the two planets' rotational axes. The lengths of the Martian seasons are about twice those of Earth's, as 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 −87 °C (−125 °F) during the polar winters to highs of up to −5 °C (23 °F) in summers.^[50] 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.^[113] The planet is also 1.52 times as far from the sun as Earth, resulting in just 43% of the amount of sunlight.^[114]

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 °C (86 °F) warmer than the equivalent summer temperatures in the north.^[115]

Mars also has the largest dust storms in our 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.^[116]

Orbit and rotation

Mars’ average distance from the Sun is roughly 230 million km (1.5 AU) 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.^[6]

The axial tilt of Mars is 25.19 degrees, which is similar to the axial tilt of the Earth.^[6] As a result, Mars has seasons like the 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.^[9] Mars passed its perihelion in March 2010^[117] and its aphelion in March 2010.^[117] The next perihelion comes in January 2013 and the next aphelion in February 2012.

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.^[118] The Mars cycle of eccentricity is 96,000 Earth years compared to the Earth's cycle of 100,000 years.^[119] 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 the Earth and Mars will continue to mildly decrease for the next 25,000 years.^[120]

Moons

Mars has two relatively small natural moons, Phobos and Deimos, which orbit close to the planet. Asteroid capture is a long-favored theory but their origin remains uncertain.^[121] Both satellites were discovered in 1877 by Asaph Hall, and 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. Ares was known as Mars to the Romans.^[122][123]

From the surface of Mars, the motions of Phobos and Deimos appear very different from that of our own 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 very slowly. Despite the 30 hour orbit of Deimos, it takes 2.7 days to set in the west as it slowly falls behind the rotation of Mars, then just as long again to rise.^[124]

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 will either crash into Mars’ surface or break up into a ring structure around the planet.^[124]

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, very near the equator, which is very unusual for captured objects and the required capture dynamics are complex. Accretion early in the history of Mars is also 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 some kind of impact disruption.^[125] More recent lines of evidence for Phobos having a highly porous interior^[126] and suggesting a composition containing mainly phyllosilicates and other minerals known from Mars,^[127] point toward an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit,^[128] similar to the prevailing theory for the origin of Earth's moon. While 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.^[127]

Search for life

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 currently extends from just beyond Venus to about the semi-major axis of Mars.^[129] 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.^[130]

The lack of a magnetosphere and 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 sublimates to a gaseous state). Mars is also 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.^[131]

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 CO₂ production on exposure to water and nutrients. This sign of life was later disputed by some 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.^[132] Tests conducted by the Phoenix Mars lander have shown that the soil has a very alkaline pH and it contains magnesium, sodium, potassium and chloride.^[133] The soil nutrients may be able to support life but life would still have to be shielded from the intense ultraviolet light.^[134]

At the Johnson Space Center lab, some fascinating shapes have been found in the meteorite ALH84001, which is thought to have originated from Mars. Some scientists propose that these geometric shapes could be fossilized microbes extant on Mars before the meteorite was blasted into space by a meteor strike and sent on a 15 million-year voyage to Earth. An exclusively inorganic origin for the shapes has also been proposed.^[135]

Small quantities of methane and formaldehyde recently detected by Mars orbiters are both claimed to be hints for life, as these chemical compounds would quickly break down in the Martian atmosphere.^[136][137] It is remotely possible that these compounds may instead be replenished by volcanic or geological means such as serpentinization.^[112]

Exploration

Dozens of spacecraft, including orbiters, landers, and rovers, have been sent to Mars by the Soviet Union, the United States, Europe, and Japan to study the planet's surface, climate, and geology. As of 2008, the price of transporting material from the surface of Earth to the surface of Mars is approximately US$309,000 per kilogram.^[138]

Roughly two-thirds of all spacecraft destined for Mars have failed in one manner or another before completing or even beginning their missions, including the difficult late 20th century period of early pioneers and first-timers. Mission failures are typically ascribed to technical problems, and missions planners have to balance technology and mission goals.^[139] Failures since 1995 include Mars 96 (1996), Mars Climate Orbiter (1999), Mars Polar Lander (1999), Deep Space 2 (1999), Nozomi (2003), Beagle 2 (2003), and Fobos-Grunt (2011).

Past missions

The first successful fly-by of Mars was on July 14–15, 1965, by NASA's Mariner 4. On November 14, 1971 Mariner 9 became the first space probe to orbit another planet when it entered into orbit around Mars.^[140] The first objects to successfully land on the surface were two Soviet probes: Mars 2 on November 27 and Mars 3 on December 2, 1971, but both ceased communicating within seconds of landing. The 1975 NASA launches of the Viking program consisted of two orbiters, each having a lander; both landers successfully touched down in 1976. Viking 1 remained operational for six years, Viking 2 for three. The Viking landers relayed color panoramas of Mars^[141] and the orbiters mapped the surface so well that the images remain in use.

The Soviet probes Phobos 1 and 2 were sent to Mars in 1988 to study Mars and its two moons. Phobos 1 lost contact on the way to Mars. Phobos 2, while successfully photographing Mars and Phobos, failed just before it was set to release two landers to the surface of Phobos.^[142]

Following the 1992 failure of the Mars Observer orbiter, the NASA Mars Global Surveyor achieved Mars orbit in 1997. This mission was a complete success, having finished its primary mapping mission in early 2001. Contact was lost with the probe in November 2006 during its third extended program, spending exactly 10 operational years in space. The NASA Mars Pathfinder, carrying a robotic exploration vehicle Sojourner, landed in the Ares Vallis on Mars in the summer of 1997, returning many images.^[143]

The NASA Phoenix Mars lander arrived on the north polar region of Mars on May 25, 2008.^[144] Its robotic arm was used to dig into the Martian soil and the presence of water ice was confirmed on June 20.^{[145][146][146]} The mission concluded on November 10, 2008 after contact was lost.^[147]

The Dawn spacecraft flew by Mars in February 2009 for a gravity assist on its way to investigate Vesta and then Ceres.^[148]

Current missions

The NASA Mars Odyssey orbiter entered Mars orbit in 2001.^[149] Odyssey's Gamma Ray Spectrometer detected significant amounts of hydrogen in the upper metre or so of regolith on Mars. This hydrogen is thought to be contained in large deposits of water ice.^[150]

The Mars Express mission of the European Space Agency (ESA) reached Mars in 2003. It carried the Beagle 2 lander, which failed during descent and was declared lost in February, 2004.^[151] In early 2004 the Planetary Fourier Spectrometer team announced the orbiter had detected methane in the Martian atmosphere. ESA announced in June 2006 the discovery of aurorae on Mars.^[152]

In January 2004, the NASA twin Mars Exploration Rovers named Spirit (MER-A) and Opportunity (MER-B) landed on the surface of Mars. Both have met or exceeded all their targets. Among the most significant scientific returns has been conclusive evidence that liquid water existed at some time in the past at both landing sites. Martian dust devils and windstorms have occasionally cleaned both rovers' solar panels, and thus increased their lifespan.^[153] Spirit Rover (MER-A) was active until 2010, when it stopped sending data.

On March 10, 2006, the NASA Mars Reconnaissance Orbiter (MRO) probe arrived in orbit to conduct a two-year science survey. The orbiter began mapping the Martian terrain and weather to find suitable landing sites for upcoming lander missions. The MRO snapped the first image of a series of active avalanches near the planet's north pole, scientists said March 3, 2008.^[154]

The Mars Science Laboratory, named Curiosity, launched on November 26, 2011, and is expected to reach Mars in August 2012. It is larger and more advanced than the Mars Exploration Rovers, with a movement rate of 90 m/h. Experiments include a laser chemical sampler that can deduce the make-up of rocks at a distance of 13 m.^[155]

Future missions

In 2008, NASA announced MAVEN, a robotic mission in 2013 to provide information about the atmosphere of Mars.^[157] In 2018 the ESA plans to launch its first Rover to Mars; the ExoMars rover will be capable of drilling 2 m into the soil in search of organic molecules.^[158]

The Finnish-Russian MetNet, is mission concept where multiple small vehicles on Mars to establish a widespread observation network to investigate the planet's atmospheric structure, physics and meteorology.^[159] MetNet was considered for a piggyback launch on the Russian Fobos-Grunt mission, but not selected.^[160] It may be included on a future Mars-Net mission.^[161] Another Russian mission concept is Mars-Grunt, a Mars sample return mission.^[161]

InSight (formerly GEMS) is a Mars lander mission proposed under NASA's Discovery program, to place a geophysical lander on Mars to study its deep interior, and understand the processes that shaped the rocky planets of the inner solar system.^[162]

Manned mission goals

The ESA hopes to land humans on Mars between 2030 and 2035.^[163] This will be preceded by successively larger probes, starting with the launch of the ExoMars probe^[164] and a joint NASA-ESA Mars sample return mission.^[165]

Manned exploration by the United States was identified as a long-term goal in the Vision for Space Exploration announced in 2004 by then US President George W. Bush.^[166] The planned Orion spacecraft would be used to send a human expedition to Earth's moon by 2020 as a stepping stone to a Mars expedition. On September 28, 2007, NASA administrator Michael D. Griffin stated that NASA aims to put a man on Mars by 2037.^[167]

Mars Direct, a low-cost human mission proposed by Robert Zubrin, founder of the Mars Society, would use heavy-lift Saturn V class rockets, such as the Space X Falcon X, or, the Ares V, to skip orbital construction, LEO rendezvous, and lunar fuel depots. A modified proposal, called "Mars to Stay", involves not returning the first immigrant explorers immediately, if ever (see Colonization of Mars).^[168]

Astronomy on Mars

With the existence of various orbiters, landers, and rovers, it is now possible to study astronomy from the Martian skies. While Mars’ moon Phobos appears about one third the angular diameter of the full Moon as it appears from Earth, Deimos appears more or less star-like, and appears only slightly brighter than Venus does from Earth.^[169]

There are various phenomena, well-known on Earth, that have been observed on Mars, such as meteors and auroras.^[152] A transit of the Earth as seen from Mars will occur on November 10, 2084.^[170] There are also transits of Mercury and transits of Venus, and the moons Phobos and Deimos are of sufficiently small angular diameter that their partial "eclipses" of the Sun are best considered transits (see Transit of Deimos from Mars).^[171][172]

Viewing

Because the orbit of Mars is eccentric its apparent magnitude at opposition from the Sun can range from −3.0 to −1.4. The minimum brightness is magnitude +1.6 when the planet is in conjunction with the Sun.^[7] Mars usually appears a distinct yellow, orange, or reddish color; the actual color of Mars is closer to butterscotch, and the redness seen is just dust in the planet's atmosphere; considering this NASA's Spirit rover has taken pictures of a greenish-brown, mud-colored landscape with blue-grey rocks and patches of light red colored sand.^[173] When farthest away from the Earth, it is more than seven times as far from the latter as 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—Mars shows a wealth of surface detail to a telescope. Especially noticeable, even at low magnification, are the polar ice caps.^[174]

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.^[175]

Closest approaches

Relative

The point Mars’ geocentric longitude is 180° different from the Sun's is known as opposition, which is near the time of closest approach to the Earth. The time of opposition can occur as much as 8½ days away from the closest approach. The distance at close approach varies between about 54^[176] and about 103 million km due to the planets' elliptical orbits, which causes comparable variation in angular size.^[177] The last Mars opposition occurred on January 29, 2010. The next one will occur on March 3, 2012 at a distance of about 100 million km.^[178] 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.^[179]

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 (0.372719 AU), magnitude −2.88, on 27 August 2003 at 9:51:13 UT. This occurred when Mars was one day from opposition and about three days from its perihelion, making Mars 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.^[180] This record approach was only very 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.^[119]

An email sent during the close approach in 2003 has, in succeeding years, repeatedly spawned hoax emails saying that Mars will look as big as the Moon.^[181]

Historical observations

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

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.^[182] 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 also invented arithmetic methods for making minor corrections to the predicted positions of the planets.^[183][184]

In the fourth century BCE, Aristotle noted that Mars disappeared behind the Moon during an occultation, indicating the planet was farther away.^[185] Ptolemy, a Greek living in Alexandria,^[186] 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.^[187] Literature from ancient China confirms that Mars was known by Chinese astronomers by no later than the fourth century BCE.^[188] In the fifth century CE, the Indian astronomical text Surya Siddhanta estimated the diameter of Mars.^[189]

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.^[190] 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.^[191] The only occultation of Mars by Venus observed was that of October 13, 1590, seen by Michael Maestlin at Heidelberg.^[192] In 1610, Mars was viewed by Galileo Galilei, who was first to see it via telescope.^[193] The first person to draw a map of Mars that displayed any terrain features was the Dutch astronomer Christiaan Huygens.^[194]

Martian "canals"

By the 19th century, the resolution of telescopes reached a level sufficient for surface features to be identified. In September 1877, a perihelic opposition of Mars occurred on September 5. In that year, Italian astronomer Giovanni Schiaparelli used a 22 cm 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".^[195][196]

Influenced by the observations, the orientalist Percival Lowell founded an observatory which had a 300 and 450 mm telescope. 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.^[197] The canali were also found by other astronomers, like Henri Joseph Perrotin and Louis Thollon in Nice, using one of the largest telescopes of that time.^[198][199]

The seasonal changes (consisting of the diminishing of the polar caps and the dark areas formed during Martian summer) in combination with the canals lead 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 a 840 mm telescope, irregular patterns were observed, but no canali were seen.^[200]

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.^[201]

It was not until spacecraft visited the planet during NASA's Mariner missions in the 1960s that these myths were dispelled. The results of the Viking life-detection experiments started an intermission in which the hypothesis of a hostile, dead planet was generally accepted.^[202]

Some maps of Mars were made using the data from these missions, but it was not until the Mars Global Surveyor mission, launched in 1996 and operated until late 2006, that complete, extremely detailed maps of the martian topography, magnetic field and surface minerals were obtained.^[203] These maps are now available online, for example, at Google Mars.

In culture

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

Mars has been featured in science fiction media, and one theme is intelligent "Martians", responsible for the speculated "canals" and "faces" on the planet. Another theme is Mars being a future colony of the Earth, or human expedition.

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.^[139]

Intelligent "Martians"

The popular 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.^[204]

Many other observations and proclamations by notable personalities added to what has been termed "Mars Fever".^[205] 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.^[206]

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.^[207] Kelvin "emphatically" denied this report shortly before departing America: "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."^[208]

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 the Earth.^[209]

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.^[209]

Pickering later proposed creating a set of mirrors in Texas, intended to signal Martians.^[210]

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, some 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.^[196]

The depiction of Mars in fiction has been stimulated by its dramatic red color and by nineteenth century scientific speculations that its surface conditions not only might support life, but intelligent life.^[211] 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. A subsequent US radio adaptation of The War of the Worlds on October 30, 1938 by Orson Welles was presented as a live news broadcast, and became notorious for causing a public panic when many listeners mistook it for the truth.^[212]

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),^[213] and a number of Robert A. Heinlein stories before the mid-sixties.^[214]

Author 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.^[215]

A comic figure of an intelligent Martian, Marvin the Martian, appeared on television in 1948 as a character in the Looney Tunes animated cartoons of Warner Brothers, and has continued as part of popular culture to the present.^[216]

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.^[217]

The theme of a Martian colony that fights for independence from Earth is a major plot element in the novels of Greg Bear as well as the movie Total Recall (based on a short story by Philip K. Dick) and the television series Babylon 5. Some video games also use this element, including Red Faction and the Zone of the Enders series. Mars (and its moons) were also the setting for the popular Doom video game franchise and the later Martian Gothic.

Timeline

Timeline based on one by NASA Goddard Space Flight Center, with additions, as of 2011^[156]

See also

• Colonization of Mars
• Darian calendar—system of time-keeping
• Extraterrestrial life
• List of artificial objects on Mars
• List of chasmata on Mars
• List of craters on Mars
• List of valles on Mars
• Terraforming of Mars
• 2007 WD5—asteroid that had a possible impact with Mars on January 30, 2008
• Seasonal flows on warm Martian slopes

Notes

References

External links

• Mars at the Open Directory Project
• Mars Exploration Program
• On Mars: Exploration of the Red Planet 1958–1978 from the NASA History Office.
• Mars Unearthed—Comparisons of terrains between Earth and Mars
• Be on Mars—Anaglyphs from the Mars Rovers (3D)
• Mars articles in Planetary Science Research Discoveries
• Geody Mars—World's search engine that supports NASA World Wind, Celestia, and other applications
• Mars Society—The Mars Society, an international organization dedicated to the study, exploration, and settlement of Mars.
• NASA/JPL OnMars WMS Server for Mars Data—Work as Google Earth client overlays
• New Papers about Martian Geomorphology

Media

• Computer Simulation of a flyby through Mariner Valley
• Movie of Mars at National Oceanic and Atmospheric Administration
• Flight Into Mariner Valley—NASA/JPL/Arizona State University 3D flythrough of Valles Marineris
• Mars Astronomy Cast episode #52, includes full transcript
• 15 Amazing Pictures of the Red Planet – slideshow at The Huffington Post

Cartographic resources

• Gazeteer of Planetary Nomenclature—Mars (USGS)
• PDS Map-a-planet
• Viking Photomap
• MOLA (topographic) map
• 3D maps of Mars in NASA World Wind
• Google Mars—Interactive image of Mars
• Ralph Aeschliman's Online Atlas of Mars