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Mars as seen by the Hubble Space Telescope
|Aphelion distance:||249,228,730 km (154.863,553 mi)
1.665 991 16 AU
|Perihelion distance:||206,644,545 km (128,402,967 mi)
1.381 333 46 AU
|Semi-major axis:||227,936,637 km (141,632,976 mi)
1.523 662 31 AU
|Orbital circumference:||~1,430,000,000 km (888,000,000 mi)
|Eccentricity:||0.093 412 33|
|Sidereal period:||686.9600 day
|Synodic period:||779.96 day
|Avg. orbital speed:||24.077 km/s
|Max. orbital speed:||26.499 km/s
|Min. orbital speed:||21.972 km/s
(5.65° to Sun's equator)
|Longitude of ascending node:||49.578 54°|
|Argument of perihelion:||286.462 30°|
|Equatorial radius:||3402.5 km (2114.2 mi)
|Polar radius:||3377.4 km (2098.6 mi)
|Surface area:||1.448×108 km²)
144 798 465 square kilometers (55,907,000 square miles)
|Mean density:||3.934 g/cm³|
|Equatorial surface gravity:||3.69 m/s2
|Escape velocity:||5.027 km/s (11,245 mi/h)|
|Sidereal rotation period:||1.025 957 day
(24.622 962 h)
|Rotation velocity at equator:||868.22 km/h (539.49 mi/h)|
|Right ascension of North pole:||317.681 43°
(21 h 10 min 44 s)
|Declination of North pole:||52.886 50°|
|Surface pressure:||0.7–0.9 kPa|
|Composition:||95.72% Carbon dioxide
0.07% Carbon monoxide
0.03% Water vapor
0.01% Nitric oxide
2.5 ppm Neon
300 ppb Krypton
130 ppb Formaldehyde
80 ppb Xenon
30 ppb Ozone
10 ppb Methane
- Note: This article contains special characters.
Mars (IPA: /ˈmɑɹz/ (GenAm); /ˈmɑːz/ (RP)) is the fourth planet from the Sun and is known as the Red Planet due to its reddish appearance as seen from Earth. The planet is named after Mars, the Roman god of war. A terrestrial planet, Mars has a thin atmosphere and surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts and polar ice caps of Earth. It has the highest mountain in the solar system, Olympus Mons, and the largest canyon, Valles Marineris. Mars' rotational period and seasonal cycles are also similar to those of the Earth.
Until the first flyby of Mars by Mariner 4 in 1965, it was speculated that there might be liquid water on Mars. This was based on observations of periodic variations in light and dark patches, particularly in the polar latitudes, which looked like seas and continents. Also, long dark striations were interpreted by some observers as irrigation channels of liquid water. These straight line features were later proven not to exist and were instead explained as optical illusions. Still, of all the planets in our solar system other than Earth, Mars is the most likely to harbor liquid water, and perhaps life.
Mars is currently host to four orbiting spacecraft: Mars Global Surveyor (non-functional), Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter. This is more than any planet except Earth. The surface is also home to the two Mars Exploration Rovers (Spirit and Opportunity). Geological evidence gathered by these and preceding missions suggests that Mars previously had large-scale water coverage, while observations also indicate that small geyser-like water flows have occurred in recent years. Observations by NASA's Mars Global Surveyor shows evidence that parts of the southern polar ice cap have been receding.
Mars has two moons, Phobos and Deimos, which are small and oddly shaped. These may be captured asteroids similar to 5261 Eureka, a Mars Trojan asteroid. Mars can be seen from Earth with the naked eye. Its apparent magnitude reaches −2.9, a brightness surpassed only by Venus, the Moon, and the Sun, though for much of the year Jupiter may appear brighter to the naked eye than Mars.
 Physical characteristics
Mars has half the radius of the Earth and only one-tenth the mass, being less dense, but its surface area is only slightly less than the total area of Earth's dry land. While Mars is larger and more massive than Mercury, Mercury has a higher density. This results in a slightly stronger gravitational force at Mercury's surface. The red-orange appearance of the Martian surface is caused by iron(III) oxide, more commonly known as rust.
Based on orbital observations and the examination of the Martian meteorite collection, the surface of Mars appears to be composed primarily of basalt. Some evidence suggests that a portion of the Martian surface is more silica-rich than typical basalt, and may be similar to andesitic stones on Earth. However, these observations may also be explained by silica glass. Much of the surface is deeply covered by a fine iron(III) oxide dust that has the consistency of talcum powder.
Although Mars has no intrinsic magnetic field, observations show that parts of the planet's crust have been magnetized. This paleomagnetism of magnetically-susceptible minerals has properties 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 4 billion years ago, before the planetary dynamo ceased to function.
Current models of the planet's interior infer a core region approximately 1,480 km in radius, consisting primarily of iron with about 15-17% sulfur. This iron sulfide core is partially fluid, and has twice the concentration of lighter elements than exist at the 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 inactive. The average thickness of the planet's crust is about 50 km, with a maximum thickness of 125 km. Earth's crust, averaging 40km, is only a third as thick as Mars's crust relative to the sizes of the two planets.
The geological history of Mars is split into three broad epochs:
- Noachian epoch (named after Noachis Terra): Formation of the oldest extant surfaces of Mars, 3.8 billion years ago to 3.5 billion years ago. Noachian age surfaces are scarred by many large impact craters. The Tharsis bulge volcanic upland is thought to have formed during this period, with extensive flooding by liquid water late in the epoch.
- Hesperian epoch (named after Hesperia Planum): 3.5 billion years ago to 1.8 billion years ago. The Hesperian epoch is marked by the formation of extensive lava plains.
- Amazonian epoch (named after Amazonis Planitia): 1.8 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.
Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure, but water ice is in no short supply, with two polar ice caps made largely of water ice. In March, 2007, NASA announced that 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 metres (36 feet). Additionally, a water ice permafrost mantle stretches down from the pole to lattitudes of about 60°. Much larger quantities of water are thought to be trapped underneath Mars' thick cryosphere, only to be released when the crust is cracked through volcanic action. The largest such release of liquid water is thought to have occurred when the Valles Marineris formed early in Mars' history, enough water being released to form river valleys across the planet. A smaller but more recent event of the same kind occurred when the Cerberus Fossae chasm opened about 5 million years ago, leaving a sea of frozen ice still visible today on the Elysium Planitia.
More recently the high resolution Mars Orbiter Camera on the Mars Global Surveyor has taken pictures which give much more detail about the history of liquid water on the surface Mars. Despite the many giant flood channels and associated tree-like network of tributaries found on Mars there are no smaller scale structures that would indicate the origin of the flood waters. It has been suggested that weathering processes have denuded these indicating the river valleys are old features. Higher resolution observations from spacecraft like Mars Global Surveyor also revealed at least a few hundred features along crater and canyon walls that appear similar to terrestrial seepage gullies. The gullies tended to be Equator facing and in the highlands of the southern hemisphere, and all poleward of 30° latitude. The researchers found no partially degraded (i.e. weathered) gullies and no superimposed impact craters, indicating that these are very young features.
In a particularly stiking example (see image) two photographs, taken six years apart, show a gully on Mars with what appears to be new deposits of sediment. Michael Meyer, the lead scientist for NASA's Mars Exploration Program, argues that only the flow of material with a high liquid water content could produce such a debris pattern and colouring. Whether the water results from precipitation, underground or another source remains an open question. However, alternative scenarios have been suggested, including the possibility that the deposits were caused by carbon dioxide frost or by the movement of dust on the Martian surface.
Further proof 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.
- See also: Category:Surface features of Mars
Although better remembered for mapping the Moon starting in 1830, Johann Heinrich Mädler and Wilhelm Beer were the first "areographers". They started off by establishing once and for all that most of Mars' surface features were permanent, and pinned down Mars' rotation period. In 1840, Mädler combined ten years of observations and drew the first ever map of Mars. Rather than giving names to the various markings they mapped, Beer and Mädler simply designated them with letters; Meridian Bay (Sinus Meridiani) was thus feature "a".
Today, features on Mars are named from a number of sources. 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).
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.
Since Mars has no oceans and hence no 'sea level', a zero-elevation surface or mean gravity surface must be selected. The zero altitude is defined by the height at which there is 610.5 Pa (6.105 mbar) of atmospheric pressure. This pressure corresponds to the triple point of water, and is approximately 0.6% of the sea level surface pressure on Earth.
The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. The surface of Mars as seen from Earth is thus 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.
The shield volcano, Olympus Mons (Mount Olympus), at 26 km is the highest known mountain in the solar system. It is an extinct volcano in the vast upland region Tharsis, which contains several other large volcanoes. It is over three times the height of Mt. Everest which in comparison only stands at 8848 m.
Mars is also scarred by a number of impact craters, of which a total of 43,000 craters with a diameter of 5 km or greater have been found. The largest of these is the Hellas impact basin, a light albedo feature clearly visible from Earth. Due to the smaller mass of Mars, the probability of an object colliding with the planet is about half that of the Earth. However, 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; those that lie within the orbit of Jupiter. In spite of this, there are far fewer craters on Mars compared with the Moon because Mars' atmosphere provides protection against small meteors. Some craters have a morphology that suggests that the ground was wet when the meteor impacted.
The large canyon, Valles Marineris (Latin for Mariner Valleys, also known as Agathadaemon in the old canal maps), has a length of 4000 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 Tharis 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.
Mars lost its magnetosphere 4 billion years ago, so the solar wind interacts directly with the Martian ionosphere, keeping the atmosphere thinner than it would otherwise be 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. As such the atmosphere of Mars is relatively thin; the atmospheric pressure on the surface varies from around 30 Pa (0.03 kPa) on Olympus Mons to over 1155 Pa (1.155 kPa) in the depths of Hellas Planitia, with a mean surface level pressure of 600 Pa (0.6 kPa). This is less than 1% of the surface pressure on Earth (101.3 kPa). The equivalent pressure of Mars' atmosphere can be found at a height of 35 km above the Earth's surface. The scale height of the atmosphere is about 11 km; higher than Earth's 6 km due to the lower gravity.
The atmosphere on Mars consists of 95% carbon dioxide, 3% nitrogen, 1.6% argon, and contains traces of oxygen and water. The atmosphere is quite dusty, containing dust particulates of about 1.5 µm across which give the Martian sky a tawny color when seen from the surface.
Several researchers claim to have detected methane in the Martian atmosphere with a concentration of about 10 ppb by volume. Since methane is an unstable gas that is broken down by ultraviolet radiation, typically lasting in the atmosphere for about 340 years, its presence on Mars could indicate that there is (or has been within the last few hundred years) a source of the gas on the planet. Volcanic activity, comet impacts, and the existence of life in the form of microorganisms such as methanogens are among possible sources. It was recently shown that methane could also be produced by a non-biological process involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.
In the winter months when the poles are in continuous darkness, the surface gets so cold that as much as 25–30% of the atmosphere condenses out into thick slabs of CO2 ice (dry ice). When the poles are again exposed to sunlight, the CO2 ice sublimes, creating enormous winds that sweep off the poles as fast as 400 km/h (250 mph). 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.
Of all the planets, Mars' seasons are the most Earth-like due to the similar tilts of the two planets' rotational axes. However, 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 approximately two Earth years in length. Martian surface temperatures vary from lows of approximately –140 °C (−220 °F) during the polar winters to highs of up to 20 °C (70 °F) in summers. 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.
If Mars had an Earthlike orbit, its seasons would be similar to Earth's because its axial tilt is similar to Earth's. However, the comparatively large eccentricity of the Martian orbit has a significant effect. Mars is near perihelion when it is summer in the southern hemisphere and winter in the north, and near aphelion when it is winter in the southern hemisphere and summer in the north. As a result, the seasons in the southern hemisphere are more extreme and the seasons in the northern are milder than would otherwise be the case. The summer temperatures in the south can be up to 30 K warmer than the equivalent summer temperatures in the north.
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.
Mars possesses polar caps at both poles, which mainly consist of water ice. However there is dry ice present on their surfaces. Frozen carbon dioxide (dry ice) accumulates as a 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. The northern polar cap has a diameter of approximately 1,000 kilometres during the northern Mars summer, and contains about 1.6 million cubic kilometres of ice, which if spread evenly on the cap would be 2 kilometres thick. (This compares to a volume of 2.85 million cubic kilometres for the Greenland ice sheet.) The southern polar cap has a diameter of 350 km and a thickness of 3 km. The total volume of ice in the south polar cap plus the adjacent layered deposits has also been estimated at 1.6 million cubic kilometres. Both polar caps show spiral troughs, which are believed to form as a result of differential solar heating, coupled with the sublimation of ice and condensation of water vapor. Both polar caps shrink and regrow following the temperature fluctuation of the Martian seasons.
 Orbit and rotation
Mars has a relatively pronounced orbital eccentricity of about 9%; of the other planets in the solar system, only Mercury shows greater eccentricity. 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.
Mars' axial tilt is 25.19 degrees, which is similar to the axial tilt of the Earth. As a result, Mars has seasons like the Earth, though Mars' are about twice as long given its longer year. Mars passed its aphelion in June 2006 and is now heading for its perihelion in June 2007.
The image to the left shows a comparison between Mars and Ceres, a dwarf planet in the Asteroid Belt, as seen from the ecliptic pole, while the image to the right is as seen from the ascending node. The segments of orbits below the ecliptic are plotted in darker colors. The perihelia (q) and aphelia (Q) are labelled with the date of the nearest passage.
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.
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.
Because Phobos' orbit 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 Mars.
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 requires that the orbit of a planet lie within a habitable zone, which for the Sun is currently occupied by Earth. Mars orbits half an astronomical unit beyond this zone and so water is frozen on its surface. The past flow of liquid water, however, demonstrates the planet's potential for habitability.
The lack of a magnetosphere and extremely thin atmosphere of Mars are a greater challenge: the planet has little heat transfer across its surface, poor insulation against bombardment and 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 stopped the recycling of chemicals and minerals between the surface and interior of the planet.
Evidence suggests that the planet was once significantly more habitable than it is today, but whether living organisms ever existed there is still unclear. The Viking probes of the mid-1970s carried experiments designed to detect microorganisms in Martian soil at their respective landing sites, and had some apparently positive results, including a temporary increase of CO2 production on exposure to water and nutrients. However this sign of life was later disputed by many scientists, resulting in a continuing debate, with NASA scientist Gilbert Levin asserting that Viking may have found life. A re-analysis of the now 30-year-old Viking data, in the light of modern knowledge of extremophile forms of life, has suggested that the Viking tests were also not sophisticated enough to detect these forms of life. The tests may even have killed a (hypothetical) life form.
At the Johnson space center lab organic compounds have been found in the meteorite ALH84001, which is supposed to have come from Mars. They concluded that these were deposited by primitive life forms extant on Mars before the meteorite was blasted into space by a meteor strike and sent on a 15 million-year voyage to Earth. Meanwhile small quantities of methane, and formaldehyde recently detected by Mars orbiters are both claimed to be hints for life, as these particles would quickly break down in the Martian atmosphere. It is possible that these compounds may be replenished by volcanic or geological means such as serpentinization.
Roughly two-thirds of all spacecraft destined for Mars have failed in one manner or another before completing or even beginning their missions. While this high failure rate can be ascribed to technical problems, enough have either failed or lost communications for causes unknown for some to search for other explanations. Examples include an Earth-Mars "Bermuda Triangle", a Mars Curse, or even a "Great Galactic Ghoul" that feeds on Martian spacecraft.
 Past missions
The first successful fly-by mission to Mars was NASA's Mariner 4 launched in 1964. The first successful objects to land on the surface were two Soviet probes, Mars 2 and Mars 3 from the Mars probe program, launched in 1971, but both lost contact within seconds of landing. Then came the 1975 NASA launches of the Viking program, which consisted of two orbiters, each having a lander; both landers successfully touched down in 1976 and remained operational for 6 and 3 years, for Viking 1 and Viking 2 respectively. The Viking landers relayed the first colour pictures of Mars and also mapped the surface of Mars so well that the images are still sometimes used to this day. The Soviet probes Phobos 1 and 2 were sent to Mars in 1988 to study Mars and its two moons, unfortunately Phobos 1 lost contact on the way to Mars, and Phobos 2, while successfully photographing Mars and Phobos, failed just before it was set to release two landers on Phobos' surface.
Following the 1992 failure of Mars Observer orbiter, NASA launched the Mars Global Surveyor in 1996. 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. Only a month after the launch of the Surveyor, NASA launched the Mars Pathfinder, carrying a robotic exploration vehicle, which landed in the Ares Vallis on Mars. This mission was another big success, and received much publicity, partially due to the many spectacular images that were sent back to Earth.
 Current missions
In 2001 NASA launched the successful Mars Odyssey orbiter, which is still in orbit as of August 2006. Odyssey's Gamma Ray Spectrometer detected significant amounts of hydrogen in the upper metre or so of Mars' regolith. This hydrogen is thought to be contained in large deposits of water ice.
In 2003, the ESA launched the Mars Express craft consisting of the Mars Express Orbiter and the lander Beagle 2. Beagle 2 failed during descent and was declared lost in early February 2004. In early 2004 the Planetary Fourier Spectrometer team announced it had detected methane in the Martian atmosphere. ESA announced in June 2006 the discovery of aurorae on Mars.
Also in 2003, NASA launched the twin Mars Exploration Rovers named Spirit (MER-A) and Opportunity (MER-B). Both missions landed successfully in January 2004 and have met or exceeded all their targets. Among the most significant science returns has been the 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.
On August 12, 2005 the NASA Mars Reconnaissance Orbiter probe was launched toward the planet, arriving in orbit on March 10, 2006 to conduct a two-year science survey. The orbiter will map the Martian terrain and weather to find suitable landing sites for the upcoming lander missions. It also contains an improved telecommunications link to Earth, with more bandwidth than all previous missions combined.
 Future missions
- Further information: Colonization of Mars
The next scheduled mission to Mars, not counting the brief flyby by the Dawn spacecraft to Ceres and Vesta, is the NASA Phoenix Mars lander, expected to launch in 2007 and set to arrive in the north polar region of Mars in 2009. The lander will have a robotic arm with a reach 2.5m and capable of digging a meter into the Martian soil. The lander will be in an area with an 80% chance of ice being less than 30cm below the surface, and has a microscopic camera capable of resolving to one thousanth the width of a human hair.
Phoenix will be followed by the Mars Science Laboratory in 2009, a bigger, faster (90m an hour) and smarter version of the Mars Exploration Rovers. Experiments include a laser chemical sample, that can deduce the make-up of rocks at a distance of 13m.
The joint Russian and Chinese Phobos-Grunt sample-return mission, to return samples of Mars' moon Phobos, is sheduled for a 2009 launch. In 2012 the ESA plans to launch its first Rover to Mars, the ExoMars rover will be capable of drilling 2m into the soil in search of organic molecules .
Manned Mars exploration by the United States has been explicitly identified as a long-term goal in the Vision for Space Exploration announced in 2004 by US President George W. Bush. NASA and Lockheed Martin have begun work on the Orion Crew Exploration Vehicle (CEV), which is currently scheduled to send a human expedition to Mars by 2025.
The European Space Agency hopes to land humans on Mars between 2030 and 2035 . This will be preceded by successively larger probes, starting with the launch of the ExoMars probe and a Mars Sample Return Mission.
 Astronomy on Mars
It is now possible, with the existence of various orbiters, landers, and rovers to study astronomy from the Martian skies. The Earth and the Moon are easily visible while Mars' moon Phobos appears about one third the angular diameter that the full Moon appears from Earth. On the other hand Deimos appears more or less starlike, and appears only slightly brighter than Venus does from Earth.
There are also various phenomena well-known on Earth that have now been observed on Mars, such as meteors and auroras. A transit of the Earth as seen from Mars will occur on November 10, 2084. There are also transits of Mercury and transits of Venus, and the moon Deimos is of sufficiently small angular diameter that its partial "eclipses" of the Sun are best considered transits (see Transit of Deimos from Mars).
 Viewing Mars
- See also: Aspects of Mars
To a naked-eye observer, Mars usually appears a distinct yellow, orange, or reddish color, and varies in brightness more than any other planet, as seen from Earth, over the course of its orbit. When farthest away from the Earth, it is more than seven times as far from the latter as when it is closest (when least favourably positioned, it can be lost in the Sun's glare for months at a time). At its most favourable times — which occur twice every 32 years, alternately at 15 and 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.
The point of Mars' closest approach to the Earth is known as opposition. The length of time between successive oppositions, or the Synodic period, is 780 days. Because of the eccentricities of the orbits, the times of opposition and minimum distance can differ by up to 8.5 days. The minimum distance varies between about 55 and 100 million km due to the planets' elliptical orbits. The next Mars opposition will occur on December 24, 2007.
On August 27, 2003, at 9:51:13 UT, Mars made its closest approach to Earth in nearly 60,000 years: 55,758,006 km (approximately 35 million miles). 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. However, this record approach was only very slightly closer than other recent close approaches. For instance, the minimum distance on August 22, 1924 was 0.37284 AU, compared to 0.37271 AU on August 27, 2003, and the minimum distance on August 24, 2208 will be 0.37278 AU. The orbital changes of Earth and Mars are making the approaches nearer: the 2003 record will be bettered 22 times by the year 4000.
As Mars approaches opposition it begins a period of retrograde motion, which means it will appear to moved backwards in a looping motion with respect to the background stars. The next retrograde period will begin on November 16, 2007 and last through to January 31, 2008 with Mars appearing to move backwards through the sky from the constellation Gemini into Taurus.
 Historical observations of Mars
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 approximately every 15-17 years, and are distiguished because Mars is close to perihelion making it even closer to Earth. Aristotle was first to view Mars, noting that, as it passed behind the moon, it was farther away than was originally believed. In 1609, Mars was viewed by Galileo, who was first to see it via telescope.
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, while in Milan, used a 22 cm telescope 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 was popularly mistranslated as canals.
Influenced by the observations the orientalist Percival Lowell founded an observatory which had a 12 and 18 inch 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 Mars which had a great influence on the public. The canali were also found by other astronomers, like Perrotin and Thollon in Nice, using one of the largest telescopes of that time.
The seasonal changes (consisting of the diminishing of the polar caps and the dark areas formed during Martian summer) in combination with the canals 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. However, as bigger telescopes were used, fewer long, straight canali were observed. During an observation in 1909 by Flammarion with a 33 inch telescope, irregular patterns were observed, but no canali were seen.
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.
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 hostile dead Mars was generally accepted.
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 were obtained. These maps are now available online.
 Mars in human culture
 Historic connections
Mars is named after the Roman god of war. In Babylonian astronomy, the planet was named after Nergal, their deity of fire, war, and destruction, most likely due to the planet's reddish appearance. When the Greeks equated Nergal with their god of war, Ares, they named the planet Ἄρεως ἀστἡρ (Areos aster), or "star of Ares". Then, following the identification of Ares and Mars, it was translated into Latin as stella Martis, or "star of Mars", or simply Mars. The Greeks also called the planet Πυρόεις Pyroeis meaning "fiery". In Hindu mythology, Mars is known as Mangala (मंगल). The planet is also called Angaraka in Sanskrit. He is the god of war and is celibate. He is the owner of the Aries and Scorpio signs, and a teacher of the occult sciences. The planet was known by the Egyptians as "Ḥr Dšr";;;; or "Horus the Red". The Hebrews named it Ma'adim (מאדים) - "the one who blushes"; this is where one of the largest canyons on Mars, the Ma'adim Vallis, gets its name. It is known as al-Mirrikh in both Arabic and Persian, and Merih in Turkish. The etymology of al-Mirrikh is unknown. Ancient Persians named it Bahram, the Zoroastrian god of faith. Ancient Turks called it Sakit. The Chinese, Japanese, Korean and Vietnamese cultures refer to the planet as 火星, or the fire star, a naming based on the ancient Chinese mythological cycle of Five Elements.
Its symbol, derived from the astrological symbol of Mars, a circle with a small arrow pointing out from behind it is a stylized representation of a shield and spear used by the Roman God Mars. Mars in Roman mythology was the God of War and patron of warriors. This symbol is also used in biology to describe the male sex. ♂ occupies Unicode position U+2642.
 In fiction
The depiction of Mars in fiction has been stimulated by its dramatic red color and by early scientific speculations that its surface conditions might be capable of supporting life.
Until the arrival of planetary probes, the traditional view of Mars derived from the astronomers Percival Lowell and Giovanni Schiaparelli, whose observation of supposedly linear features on the planet created the myth of canals on Mars. For many years, the standard notion of the planet was a drying, cooling, dying world with ancient civilizations constructing irrigation works.
Thus originated a large number of science fiction scenarios, the best known of which is H. G. Wells' The War of the Worlds, in which Martians seek to escape their dying planet by invading Earth. A subsequent radio version of War of the Worlds on October 30, 1938 was presented as a live news broadcast, and many listeners mistook it for the truth. Also influential was Ray Bradbury's The Martian Chronicles, in which human explorers accidentally destroy a highly advanced Martian civilization, as well as Burroughs' Barsoom series and a number of Robert A. Heinlein stories prior to the mid-sixties.
Famous literary author Jonathan Swift made reference to the moons of Mars, approximately 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.
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. However, 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.
Another popular theme, particularly among American writers, is the Martian colony that fights for independence from Earth. This is a major plot element in the novels of Greg Bear and Kim Stanley Robinson, as well as the movie Total Recall (based on a short story by Philip K. Dick) and the television series Babylon 5. Many video games also use this element, such as 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.
 See also
- Darian calendar
- Extraterrestrial life
- Mars Direct
- Mars in astrology
- List of artificial objects on Mars
- List of chasmata on Mars
- List of craters on Mars
- List of valles on Mars
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 External links
|Find more information on Mars by searching Wikipedia's sister projects|
|Dictionary definitions from Wiktionary|
|Textbooks from Wikibooks|
|Quotations from Wikiquote|
|Source texts from Wikisource|
|Images and media from Commons|
|News stories from Wikinews|
|Learning resources from Wikiversity|
- 3D maps of Mars in NASA World Wind
- Google Mars Interactive image of Mars
- Flight Into Mariner Valley NASA/JPL/Arizona State University 3D flythrough of Valles Marineris
- Marsgeo.com Mars Rover photos, videos & surface geology
- Nine Planets Mars page
- On Mars: Exploration of the Red Planet 1958-1978 from the NASA History Office.
- Martian Law - a CATO white paper
- Computer Simulation of a flyby through Mariner Valley
- Mars Unearthed - Comparisons of terrains between Earth and Mars
- Ralph Aeschliman's Online Atlas of Mars
- Geody Mars World's search engine that supports NASA World Wind, Celestia, and other applications.
- BBC News update on Mars Express' findings of polar water ice and water-eroded features on the surface
- BBC News Mars pictures reveal frozen sea
- Be on Mars Anaglyphs from the Mars Rovers (3D)
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