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{{Infobox Planet| bgcolour = #E8AB79| name = Mars| symbol = | image = | caption = Mars as seen by the Hubble Space Telescope| aphelion = 249,228,730 km
1.66599116 [Astronomical unit| perihelion = 206,644,545 km
1.38133346 AU| semimajor = 227,936,637 km
1.52366231 AU| eccentricity = 0.09341233
| period = 686.9600 day
1.8808 julian year (astronomy)
668.5991 Timekeeping on Mars| synodic_period = 779.96 day
2.135 Julian years| avg_speed = 24.077 km/s| inclination = 1.85061°
5.65° to Sun's Equator| asc_node = 49.57854°| arg_peri = 286.46230°| satellites = 2| physical_characteristics = yes| equatorial_radius = 3,396.2 ± 0.1 kmBest fit ellipsoid
0.533 Earths| polar_radius = 3,376.2 ± 0.1 km
0.531 Earths| flattening = 0.00589 ± 0.00015 | surface_area = 144,798,465 km²
0.284 Earths| volume = 1.6318 km³
0.151 Earths| mass = 6.4185 kg
0.107 Earths| density = 3.934 g/cm³| surface_grav = 3.69 Acceleration
0.376 g-force| escape_velocity = 5.027 km/s| sidereal_day = 1.025957 day
24.622962 h | rot_velocity = 868.22 km/h| axial_tilt = 25.19°| right_asc_north_pole = 21 h 10 min 44 s
317.68143°| declination = 52.88650°| albedo = 0.15| magnitude = +1.8 to -2.91 | angular_size = 3.5" — 25.1" | temperatures = yes| temp_name1 = Kelvin| min_temp_1 = 186 K| mean_temp_1 = 227 K| max_temp_1 = 268 K{{cite web| title = Mars: Facts & Figures | publisher = NASA | url = http://solarsystem.jpl.nasa.gov/planets/profile.cfm?Object=Mars&Display=Facts&System=Metric | accessdate = 2007-03-06 --> | temp_name2 = Celsius| atmosphere = yes| surface_pressure = 0.7–0.9 [Pascal (unit)| atmosphere_density =| atmosphere_composition = 95.72% Carbon dioxide
2.7% Nitrogen
1.6% Argon
0.2% Oxygen
0.07% Carbon monoxide
0.03% Water vapor
0.01% Nitric oxide
2.5 Parts per million Neon
300 Parts per billion Krypton
130 Parts per billion Formaldehyde
80 Parts per billion Xenon
30 Parts per billion Ozone
10 Parts per billion Methane
-->Mars () is the fourth planet from the Sun in the Solar System. The planet is named after Mars (mythology), the Roman mythology List of war deities. It is also referred to as the "Red Planet" because of its Iron(III) oxide as seen from Earth.

A terrestrial planet with a thin atmosphere, Mars has surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts and polar ice caps of Earth. It is the site of Olympus Mons, the highest known mountain in the solar system, and of Valles Marineris, the largest canyon. In addition to its geographical features, Mars’ rotational period and seasonal cycles are likewise similar to those of Earth.

Until the first flyby of Mars by Mariner 4 in 1965, it was speculated that there might be liquid water on the planet's surface. This was based on observations of periodic variations in light and darkness patches, particularly in the polar latitudes, which looked like seas and continents, while long, dark striations were interpreted by some observers as irrigation channels for 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 three functional orbiting spacecraft: 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 rover and Opportunity rover). 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 show evidence that parts of the southern polar ice cap have been receding.

Mars has two natural satellite, Phobos (moon) and Deimos (moon), which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Martian 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 most of the time Jupiter will appear brighter to the naked eye than Mars.

Physical characteristics , Venus, Earth, and MarsMars has half the radius of 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 (planet), 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 hematite, or rust.{{cite web| last = Peplow | first = Mark | title = How Mars got its rust | url = http://www.bioedonline.org/news/news-print.cfm?art=953 | accessdate = 2007-03-10 -->

Geology 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 and that alternating polarity reversals of its dipole field have occurred. This paleomagnetism of magnetically-susceptible minerals has properties that are very similar to the Plate tectonics#Explanation of magnetic striping. 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 1000000000 (number) years ago, before the planetary Dynamo theory ceased to function and caused the planet's magnetic field to fade away.{{cite web| last = Goddard Space Flight Center | first = | title = New Map Provides More Evidence Mars Once Like Earth | url = http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html | accessdate = 2006-03-17 -->

Current models of the planet's interior imply a core region approximately 1,480 kilometres in radius, consisting primarily of iron with about 15–17% sulfur. 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 inactive. The average thickness of the planet's crust is about 50 km, with a maximum thickness of 125 km. Earth's crust, averaging 40 km, is only a third as thick as Mars’ crust relative to the sizes of the two planets.

The geological history of Mars can be split into many epochs, but the following are the three main ones:

• 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.

Hydrology

Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure, except at the lowest elevations for short periods{{Citation| journal = Journal of Geophysical Research| date=March 7, 2005| last= Heldmann et al.| first= Jennifer L.| title= Formation of Martian gullies by the action of liquid water flowing under current Martian environmental conditions| url= http://daleandersen.seti.org/Dale%20Andersen/Articles_files/Heldmann%20et%20al.2005.pdf| volume=110| pages=Eo5004| doi=10.1029/2004JE002261| date= 2005/05/07| accessdate=2007-08-12--> 'conditions such as now occur on Mars, outside of the temperature-pressure stability regime of liquid water' ... 'Liquid water is typically stable at the lowest elevations and at low latitudes on the planet because the atmospheric pressure is greater than the vapor pressure of water and surface temperatures in equatorial regions can reach 273 K for parts of the day et al., 2001' but water ice is in no short supply, with two polar ice caps made largely of ice.{{Citation| journal=Geophysical Research Letters| volume = 33| pages = L11201| date = June 3, 2006| date=June 3, 2006| last=Kostama| first=V.-P.| last2=Kreslavsky| first2=M. A.| last3=Head| first3=J. W.| title=Recent high-latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement| url=http://www.agu.org/pubs/crossref/2006/2006GL025946.shtml| doi=10.1029/2006GL025946| accessdate=2007-08-12--> 'Martian high-latitude zones are covered with a smooth, layered ice-rich mantle' 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.{{cite web| publisher = NASA | date=March 15, [ | title = Mars' South Pole Ice Deep and Wide | url= http://jpl.nasa.gov/news/news.cfm?release=2007-030 | accessdate = 2007-03-16 --> Additionally, an ice [permafrost mantle stretches down from the pole to latitudes of about 60°.

Much larger quantities of water are thought to be trapped underneath Mars's 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's 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.{{Citation| last = Murray et al. | first = John B. | author-link = | title = Evidence for a frozen sea close to Mars' equator | journal = Nature | volume = 434 | pages = 352–355 | date = March 17, 2005 | url = http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=36832 | accessdate=2007-03-11 -->

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 of 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 tend to be located in the highlands of the southern hemisphere and to face the Equator; all are 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 striking 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 of the deposits being caused by carbon dioxide frost or by the movement of dust on the Martian surface.{{cite news| publisher = NASA | date=December 6, [ | title = Water May Still Flow on Mars, NASA Photo Suggests | url= http://www.npr.org/templates/story/story.php?storyId=6587226 | accessdate = 2006-04-30 -->

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.

Nevertheless, some of the evidence believed to indicate ancient water basins and flows has been negated by higher resolution studies taken at resolution about 30 cm by the Mars Reconnaissance Orbiter. See A. S. McEwen et al, Science 317, 1706-1709, 21 Sept 2007.

Geography Although better remembered for mapping the Moon, Johann Heinrich Mädler and Wilhelm Beer were the first "areographers". They began by establishing once and for all that most of Mars’ surface features were permanent, and 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."{{cite web] 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 also had to be selected. Zero altitude is defined by the height at which there is 610.5 Pascal (unit) (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.{{cite web], 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 stands at only 8.848 km.

Mars is also scarred by a number of impact craters: a total of 43,000 craters with a diameter of 5 km or greater have been found. The largest of these is the Hellas Planitia, 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, i.e., those that lie within the orbit of Jupiter. In spite of this, there are far fewer craters on Mars compared with Moon because Mars's atmosphere provides protection against small meteors. Some craters have a morphology that suggests the ground was wet when the meteor impacted.

The large canyon, Valles Marineris (Latin for Mariner program 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.{{cite web ] (THEMIS) aboard NASA's 2001 Mars Odyssey have revealed seven possible cave entrances on the flanks of the Arsia Mons volcano. The caves, named Dena, Chloe, Wendy, Annie, Abbey, Nikki and Jeanne after loved ones of their discoverers, are collectively known as the "seven sisters." 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. Some researchers have suggested that this protection makes the caves good candidates for future efforts to find liquid water and signs of life.

Mars has two permanent polar ice caps: the northern one at Planum Boreum and the southern one at Planum Australe.

Atmosphere 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.{{cite web| last = Philips | first = Tony | title = The Solar Wind at Mars | publisher=Science@NASA | year = 2001 | url = http://science.nasa.gov/headlines/y2001/ast31jan_1.htm | accessdate = 2006-10-08--> The celestial body atmosphere of Mars is now relatively thin. Atmospheric pressure on the surface varies from around 30 pascal (unit) (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). Mars's mean surface pressure equals the pressure found 35 km above the Earth's surface. The scale height of the atmosphere, about 11 km, is 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 particulates about 1.5 µm in diameter 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 Parts per billion by volume. {{cite news| date = March 30, 2004 | title = Mars Express confirms methane in the Martian atmosphere | publisher = [ESA | url = http://www.esa.int/SPECIALS/Mars_Express/SEMZ0B57ESD_0.html | accessdate = 2006-03-17 --> Since methane is an unstable gas that is broken down by ultraviolet radiation, typically lasting about 340 years in the Martian atmosphere,

Climate October 28, 2005 with dust storm visible.Of all the planets, Mars's 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 Celsius during the polar winters to highs of up to 20 °C in summers. 106(E10), 23,317–23,326. ( abstract, full paper requires purchase or AGU subscription) The wide range in temperatures is due to the thin atmosphere which cannot store much solar heat, the low atmospheric pressure, and the low Volumetric heat capacity of Martian soil.{{cite web| title = Mars' desert surface... | work = MGCM Press release | publisher = NASA | url = http://www-mgcm.arc.nasa.gov/mgcm/HTML/WEATHER/surface.html | accessdate = 2007-02-25-->

If Mars had an Earth-like 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 Apsis when it is summer in the southern hemisphere and winter in the north, and near Apsis 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 warmerthan 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.

The polar caps at both poles consist primarily 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.{{cite web| last = Darling | first = David | title = Mars, polar caps, ENCYCLOPEDIA OF ASTROBIOLOGY, ASTRONOMY, AND SPACEFLIGHT | url = http://www.daviddarling.info/encyclopedia/M/Marspoles.html | accessdate = 2007-02-26 --> The northern polar cap has a diameter of approximately 1,000 kilometres during the northern Mars summer,{{cite web | title = MIRA's Field Trips to the Stars Internet Education Program | author = | publisher = Mira.org | url = http://www.mira.org/fts0/planets/097/text/txt002x.htm | accessdate = 2007-02-26--> 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.{{cite web| last = Phillips | first = Dr. Tony | title = Mars is Melting, Science at NASA | url = http://science.nasa.gov/headlines/y2003/07aug_southpole.htm | accessdate = 2007-02-26 --> 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 seven other planets in the solar system, only Mercury (planet) shows greater eccentricity. However, 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 only 0.2%, much less than that of Venus or Neptune today. Although Mars takes twice as long as the Earth to orbit the Sun, its main cycle of eccentricity variation is slightly shorter than Earth's, with cycles taking 95,000 Earth years. However, there is a much longer cycle of eccentricity with a period of several million Earth years, and this overshadows the 95,000 year cycle in the eccentricity graph of the past three million years. Presently, Mars is approaching an eccentricity maximum, which will be reached in a thousand years.

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 Timekeeping on Mars) 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.

Mars's 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 on Mars they are about twice as long given its longer year. Mars passed its aphelion in June 2006 and is now passing its perihelion since June 2007.

{| class="wikitable"| The image to the left shows a comparison between Mars and 1 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 perihelion (q) and aphelion (Q) are labelled with the date of the nearest passage.|}

Moons

Mars has two tiny natural moons, Phobos (moon) and Deimos (moon), which orbit very close to the planet and are thought to be captured asteroids.

Both satellites were discovered in 1877 by Asaph Hall, and are named after the characters Phobos (mythology) (panic/fear) and Deimos (mythology) (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 the planet.

It is not well understood how or when Mars came to capture its two moons. Both have circular orbits, very near the equator, which is very unusual in itself for captured objects. Phobos's unstable orbit would seem to point towards a relatively recent capture. There is no known mechanism for an airless Mars to capture a lone asteroid, so it is likely that a third body was involved—however, asteroids as large as Phobos and Deimos are rare, and binaries rarer still, outside of the asteroid belt.

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 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 this, along with the planet's thin atmosphere, causes water to freeze 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.{{cite book| last = Hannsson | first = Anders | authorlink = | title = Mars and the Development of Life. | publisher = [Wiley | date = 1997 | isbn = 0-471-96606-1 -->

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 program 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 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 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. Also, 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. {{cite news| date = February 25, 2005 | title = Formaldehyde claim inflames Martian debate | url = http://www.nature.com/news/2005/050221/full/050221-15.html | accessdate = 2006-03-19 | publisher=Nature | doi = 10.1038/news050221-15--> It is possible that these compounds may be replenished by volcanic or geological means such as [serpentinization.

Exploration site

Dozens of spacecraft, including orbiters, landers, and rover (space exploration), have been sent to Mars by the Soviet space program, the NASA, ESA, and JAXA to study the planet's surface, climate, and geology.

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 Exploration of mars#Mars Curse, or even the long-standing NASA in-joke, the "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 Union 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 color 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 program 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's surface.

Following the 1992 failure of the 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 Sojourner (rover), 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 lander on MarsIn 2001 NASA launched the successful Mars Odyssey orbiter, which is still in orbit as of June 2007. Odyssey's Gamma Ray Spectrometer detected significant amounts of hydrogen in the upper metre or so of Mars's regolith. This hydrogen is thought to be contained in large deposits of water ice.

In 2003, the European Space Agency 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 Aurora (astronomy) on Mars.

Also in 2003, NASA launched the twin Mars Exploration Rover Mission named Spirit rover (MER-A) and Opportunity rover (MER-B). Both missions landed successfully in January 2004 and 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. Dust devils#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 upcoming lander missions. It also contains an improved telecommunications link to Earth, with more bandwidth than all previous missions combined.

Future missions .

The next scheduled mission to Mars, not counting the brief flyby by the Dawn Mission spacecraft to Ceres (dwarf planet) and 4 Vesta, is the NASA Phoenix (spacecraft) Mars lander, which launched August 4 2007 and is scheduled to arrive on the north polar region of Mars on May 25 2008. The lander has a robotic arm with a 2.5 m reach 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 30 cm below the surface, and has a microscopic camera capable of resolving to one-thousandth the width of a human hair.

Phoenix will be followed by the Mars Science Laboratory in 2009, a bigger, faster (90 m/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 13 m.

The joint Russian and Chinese Phobos-Grunt sample-return mission, to return samples of Mars's moon Phobos, is scheduled for a 2009 launch. In 2012 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.{{cite web] announced in 2004 by US President George W. Bush. NASA and Lockheed Martin have begun work on the Orion (spacecraft), which is currently scheduled to send a human expedition to Earth's moon by 2020 as a stepping stone to an expedition to Mars thereafter.

The European Space Agency hopes to land humans on Mars between 2030 and 2035.{{cite web| date=October 11, 2002 ] probe and a Mars Sample Return Mission.

On September 28, 2007, NASA administrator Michael Griffin stated that NASA aims to put a man on Mars by 2037: in 2057, "we should be celebrating 20 years of man on Mars." Yahoo.com, NASA aims to put man on Mars by 2037

Astronomy on Mars 2005.With the existence of various orbiters, landers, and rovers, it is now possible 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 of the full Moon as it appears from Earth. On the other hand Deimos appears more or less star-like, 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 aurora (phenomenon).empty A transit of Earth from Mars will occur on November 10, 2084. There are also transit of Mercury from Mars and transit of Venus from Mars, 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 To the naked-eye, 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. The apparent magnitude of Mars varies from +1.8 at conjunction to as high as -2.9 at perihelic Opposition (astronomy). 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 (astronomy). 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 (astronomy) 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' ellipse orbits. The next Mars opposition will occur on December 24, 2007.

As Mars approaches opposition it begins a period of Retrograde motion#Apparent retrograde motion .28.29, which means it will appear to move backwards in a looping motion with respect to the background stars.

2003 closest approach 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. This occurred when Mars was one day from Astronomical 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, Middle Paleolithic., 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 astronomical unit, 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.

2007-2008 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.{{cite web| last = Beish | first = Jefrey | title = The 2007 Aphelic Apparition of Mars | url= http://www.tnni.net/~dustymars/2007_MARS.htm | accessdate = 2007-02-28-->

{| width=640 class="wikitable"|
This chart shows the path of Mars in the sky during the opposition of 2007. Each dot represents one day's motion. Mars shown on December 18 in Gemini, when it is closest to earth (0.59 AU distance, 15.9" visual diameter), 6 days before opposition on December 24.|}

Historical observations The history of observations of Mars is marked by the opposition (astronomy) of Mars, when the planet is closest to Earth {{Infobox Planet| bgcolour = #E8AB79| name = Mars| symbol = | image = | caption = Mars as seen by the Hubble Space Telescope| aphelion = 249,228,730 km
1.66599116 [Astronomical unit| perihelion = 206,644,545 km
1.38133346 AU| semimajor = 227,936,637 km
1.52366231 AU| eccentricity = 0.09341233
| period = 686.9600 day
1.8808 julian year (astronomy)
668.5991 Timekeeping on Mars| synodic_period = 779.96 day
2.135 Julian years| avg_speed = 24.077 km/s| inclination = 1.85061°
5.65° to Sun's Equator| asc_node = 49.57854°| arg_peri = 286.46230°| satellites = 2| physical_characteristics = yes| equatorial_radius = 3,396.2 ± 0.1 kmBest fit ellipsoid
0.533 Earths| polar_radius = 3,376.2 ± 0.1 km
0.531 Earths| flattening = 0.00589 ± 0.00015 | surface_area = 144,798,465 km²
0.284 Earths| volume = 1.6318 km³
0.151 Earths| mass = 6.4185 kg
0.107 Earths| density = 3.934 g/cm³| surface_grav = 3.69 Acceleration
0.376 g-force| escape_velocity = 5.027 km/s| sidereal_day = 1.025957 day
24.622962 h | rot_velocity = 868.22 km/h| axial_tilt = 25.19°| right_asc_north_pole = 21 h 10 min 44 s
317.68143°| declination = 52.88650°| albedo = 0.15| magnitude = +1.8 to -2.91 | angular_size = 3.5" — 25.1" | temperatures = yes| temp_name1 = Kelvin| min_temp_1 = 186 K| mean_temp_1 = 227 K| max_temp_1 = 268 K{{cite web| title = Mars: Facts & Figures | publisher = NASA | url = http://solarsystem.jpl.nasa.gov/planets/profile.cfm?Object=Mars&Display=Facts&System=Metric | accessdate = 2007-03-06 --> | temp_name2 = Celsius| atmosphere = yes| surface_pressure = 0.7–0.9 [Pascal (unit)| atmosphere_density =| atmosphere_composition = 95.72% Carbon dioxide
2.7% Nitrogen
1.6% Argon
0.2% Oxygen
0.07% Carbon monoxide
0.03% Water vapor
0.01% Nitric oxide
2.5 Parts per million Neon
300 Parts per billion Krypton
130 Parts per billion Formaldehyde
80 Parts per billion Xenon
30 Parts per billion Ozone
10 Parts per billion Methane
-->Mars () is the fourth planet from the Sun in the Solar System. The planet is named after Mars (mythology), the Roman mythology List of war deities. It is also referred to as the "Red Planet" because of its Iron(III) oxide as seen from Earth.

A terrestrial planet with a thin atmosphere, Mars has surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts and polar ice caps of Earth. It is the site of Olympus Mons, the highest known mountain in the solar system, and of Valles Marineris, the largest canyon. In addition to its geographical features, Mars’ rotational period and seasonal cycles are likewise similar to those of Earth.

Until the first flyby of Mars by Mariner 4 in 1965, it was speculated that there might be liquid water on the planet's surface. This was based on observations of periodic variations in light and darkness patches, particularly in the polar latitudes, which looked like seas and continents, while long, dark striations were interpreted by some observers as irrigation channels for 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 three functional orbiting spacecraft: 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 rover and Opportunity rover). 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 show evidence that parts of the southern polar ice cap have been receding.

Mars has two natural satellite, Phobos (moon) and Deimos (moon), which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Martian 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 most of the time Jupiter will appear brighter to the naked eye than Mars.

Physical characteristics , Venus, Earth, and MarsMars has half the radius of 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 (planet), 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 hematite, or rust.{{cite web| last = Peplow | first = Mark | title = How Mars got its rust | url = http://www.bioedonline.org/news/news-print.cfm?art=953 | accessdate = 2007-03-10 -->

Geology 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 and that alternating polarity reversals of its dipole field have occurred. This paleomagnetism of magnetically-susceptible minerals has properties that are very similar to the Plate tectonics#Explanation of magnetic striping. 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 1000000000 (number) years ago, before the planetary Dynamo theory ceased to function and caused the planet's magnetic field to fade away.{{cite web| last = Goddard Space Flight Center | first = | title = New Map Provides More Evidence Mars Once Like Earth | url = http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html | accessdate = 2006-03-17 -->

Current models of the planet's interior imply a core region approximately 1,480 kilometres in radius, consisting primarily of iron with about 15–17% sulfur. 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 inactive. The average thickness of the planet's crust is about 50 km, with a maximum thickness of 125 km. Earth's crust, averaging 40 km, is only a third as thick as Mars’ crust relative to the sizes of the two planets.

The geological history of Mars can be split into many epochs, but the following are the three main ones:

• 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.

Hydrology

Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure, except at the lowest elevations for short periods{{Citation| journal = Journal of Geophysical Research| date=March 7, 2005| last= Heldmann et al.| first= Jennifer L.| title= Formation of Martian gullies by the action of liquid water flowing under current Martian environmental conditions| url= http://daleandersen.seti.org/Dale%20Andersen/Articles_files/Heldmann%20et%20al.2005.pdf| volume=110| pages=Eo5004| doi=10.1029/2004JE002261| date= 2005/05/07| accessdate=2007-08-12--> 'conditions such as now occur on Mars, outside of the temperature-pressure stability regime of liquid water' ... 'Liquid water is typically stable at the lowest elevations and at low latitudes on the planet because the atmospheric pressure is greater than the vapor pressure of water and surface temperatures in equatorial regions can reach 273 K for parts of the day et al., 2001' but water ice is in no short supply, with two polar ice caps made largely of ice.{{Citation| journal=Geophysical Research Letters| volume = 33| pages = L11201| date = June 3, 2006| date=June 3, 2006| last=Kostama| first=V.-P.| last2=Kreslavsky| first2=M. A.| last3=Head| first3=J. W.| title=Recent high-latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement| url=http://www.agu.org/pubs/crossref/2006/2006GL025946.shtml| doi=10.1029/2006GL025946| accessdate=2007-08-12--> 'Martian high-latitude zones are covered with a smooth, layered ice-rich mantle' 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.{{cite web| publisher = NASA | date=March 15, [ | title = Mars' South Pole Ice Deep and Wide | url= http://jpl.nasa.gov/news/news.cfm?release=2007-030 | accessdate = 2007-03-16 --> Additionally, an ice [permafrost mantle stretches down from the pole to latitudes of about 60°.

Much larger quantities of water are thought to be trapped underneath Mars's 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's 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.{{Citation| last = Murray et al. | first = John B. | author-link = | title = Evidence for a frozen sea close to Mars' equator | journal = Nature | volume = 434 | pages = 352–355 | date = March 17, 2005 | url = http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=36832 | accessdate=2007-03-11 -->

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 of 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 tend to be located in the highlands of the southern hemisphere and to face the Equator; all are 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 striking 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 of the deposits being caused by carbon dioxide frost or by the movement of dust on the Martian surface.{{cite news| publisher = NASA | date=December 6, [ | title = Water May Still Flow on Mars, NASA Photo Suggests | url= http://www.npr.org/templates/story/story.php?storyId=6587226 | accessdate = 2006-04-30 -->

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.

Nevertheless, some of the evidence believed to indicate ancient water basins and flows has been negated by higher resolution studies taken at resolution about 30 cm by the Mars Reconnaissance Orbiter. See A. S. McEwen et al, Science 317, 1706-1709, 21 Sept 2007.

Geography Although better remembered for mapping the Moon, Johann Heinrich Mädler and Wilhelm Beer were the first "areographers". They began by establishing once and for all that most of Mars’ surface features were permanent, and 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."{{cite web] 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 also had to be selected. Zero altitude is defined by the height at which there is 610.5 Pascal (unit) (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.{{cite web], 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 stands at only 8.848 km.

Mars is also scarred by a number of impact craters: a total of 43,000 craters with a diameter of 5 km or greater have been found. The largest of these is the Hellas Planitia, 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, i.e., those that lie within the orbit of Jupiter. In spite of this, there are far fewer craters on Mars compared with Moon because Mars's atmosphere provides protection against small meteors. Some craters have a morphology that suggests the ground was wet when the meteor impacted.

The large canyon, Valles Marineris (Latin for Mariner program 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.{{cite web ] (THEMIS) aboard NASA's 2001 Mars Odyssey have revealed seven possible cave entrances on the flanks of the Arsia Mons volcano. The caves, named Dena, Chloe, Wendy, Annie, Abbey, Nikki and Jeanne after loved ones of their discoverers, are collectively known as the "seven sisters." 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. Some researchers have suggested that this protection makes the caves good candidates for future efforts to find liquid water and signs of life.

Mars has two permanent polar ice caps: the northern one at Planum Boreum and the southern one at Planum Australe.

Atmosphere 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.{{cite web| last = Philips | first = Tony | title = The Solar Wind at Mars | publisher=Science@NASA | year = 2001 | url = http://science.nasa.gov/headlines/y2001/ast31jan_1.htm | accessdate = 2006-10-08--> The celestial body atmosphere of Mars is now relatively thin. Atmospheric pressure on the surface varies from around 30 pascal (unit) (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). Mars's mean surface pressure equals the pressure found 35 km above the Earth's surface. The scale height of the atmosphere, about 11 km, is 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 particulates about 1.5 µm in diameter 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 Parts per billion by volume. {{cite news| date = March 30, 2004 | title = Mars Express confirms methane in the Martian atmosphere | publisher = [ESA | url = http://www.esa.int/SPECIALS/Mars_Express/SEMZ0B57ESD_0.html | accessdate = 2006-03-17 --> Since methane is an unstable gas that is broken down by ultraviolet radiation, typically lasting about 340 years in the Martian atmosphere,

Climate October 28, 2005 with dust storm visible.Of all the planets, Mars's 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 Celsius during the polar winters to highs of up to 20 °C in summers. 106(E10), 23,317–23,326. ( abstract, full paper requires purchase or AGU subscription) The wide range in temperatures is due to the thin atmosphere which cannot store much solar heat, the low atmospheric pressure, and the low Volumetric heat capacity of Martian soil.{{cite web| title = Mars' desert surface... | work = MGCM Press release | publisher = NASA | url = http://www-mgcm.arc.nasa.gov/mgcm/HTML/WEATHER/surface.html | accessdate = 2007-02-25-->

If Mars had an Earth-like 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 Apsis when it is summer in the southern hemisphere and winter in the north, and near Apsis 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 warmerthan 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.

The polar caps at both poles consist primarily 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.{{cite web| last = Darling | first = David | title = Mars, polar caps, ENCYCLOPEDIA OF ASTROBIOLOGY, ASTRONOMY, AND SPACEFLIGHT | url = http://www.daviddarling.info/encyclopedia/M/Marspoles.html | accessdate = 2007-02-26 --> The northern polar cap has a diameter of approximately 1,000 kilometres during the northern Mars summer,{{cite web | title = MIRA's Field Trips to the Stars Internet Education Program | author = | publisher = Mira.org | url = http://www.mira.org/fts0/planets/097/text/txt002x.htm | accessdate = 2007-02-26--> 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.{{cite web| last = Phillips | first = Dr. Tony | title = Mars is Melting, Science at NASA | url = http://science.nasa.gov/headlines/y2003/07aug_southpole.htm | accessdate = 2007-02-26 --> 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 seven other planets in the solar system, only Mercury (planet) shows greater eccentricity. However, 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 only 0.2%, much less than that of Venus or Neptune today. Although Mars takes twice as long as the Earth to orbit the Sun, its main cycle of eccentricity variation is slightly shorter than Earth's, with cycles taking 95,000 Earth years. However, there is a much longer cycle of eccentricity with a period of several million Earth years, and this overshadows the 95,000 year cycle in the eccentricity graph of the past three million years. Presently, Mars is approaching an eccentricity maximum, which will be reached in a thousand years.

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 Timekeeping on Mars) 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.

Mars's 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 on Mars they are about twice as long given its longer year. Mars passed its aphelion in June 2006 and is now passing its perihelion since June 2007.

{| class="wikitable"| The image to the left shows a comparison between Mars and 1 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 perihelion (q) and aphelion (Q) are labelled with the date of the nearest passage.|}

Moons

Mars has two tiny natural moons, Phobos (moon) and Deimos (moon), which orbit very close to the planet and are thought to be captured asteroids.

Both satellites were discovered in 1877 by Asaph Hall, and are named after the characters Phobos (mythology) (panic/fear) and Deimos (mythology) (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 the planet.

It is not well understood how or when Mars came to capture its two moons. Both have circular orbits, very near the equator, which is very unusual in itself for captured objects. Phobos's unstable orbit would seem to point towards a relatively recent capture. There is no known mechanism for an airless Mars to capture a lone asteroid, so it is likely that a third body was involved—however, asteroids as large as Phobos and Deimos are rare, and binaries rarer still, outside of the asteroid belt.

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 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 this, along with the planet's thin atmosphere, causes water to freeze 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.{{cite book| last = Hannsson | first = Anders | authorlink = | title = Mars and the Development of Life. | publisher = [Wiley | date = 1997 | isbn = 0-471-96606-1 -->

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 program 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 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 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. Also, 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. {{cite news| date = February 25, 2005 | title = Formaldehyde claim inflames Martian debate | url = http://www.nature.com/news/2005/050221/full/050221-15.html | accessdate = 2006-03-19 | publisher=Nature | doi = 10.1038/news050221-15--> It is possible that these compounds may be replenished by volcanic or geological means such as [serpentinization.

Exploration site

Dozens of spacecraft, including orbiters, landers, and rover (space exploration), have been sent to Mars by the Soviet space program, the NASA, ESA, and JAXA to study the planet's surface, climate, and geology.

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 Exploration of mars#Mars Curse, or even the long-standing NASA in-joke, the "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 Union 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 color 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 program 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's surface.

Following the 1992 failure of the 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 Sojourner (rover), 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 lander on MarsIn 2001 NASA launched the successful Mars Odyssey orbiter, which is still in orbit as of June 2007. Odyssey's Gamma Ray Spectrometer detected significant amounts of hydrogen in the upper metre or so of Mars's regolith. This hydrogen is thought to be contained in large deposits of water ice.

In 2003, the European Space Agency 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 Aurora (astronomy) on Mars.

Also in 2003, NASA launched the twin Mars Exploration Rover Mission named Spirit rover (MER-A) and Opportunity rover (MER-B). Both missions landed successfully in January 2004 and 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. Dust devils#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 upcoming lander missions. It also contains an improved telecommunications link to Earth, with more bandwidth than all previous missions combined.

Future missions .

The next scheduled mission to Mars, not counting the brief flyby by the Dawn Mission spacecraft to Ceres (dwarf planet) and 4 Vesta, is the NASA Phoenix (spacecraft) Mars lander, which launched August 4 2007 and is scheduled to arrive on the north polar region of Mars on May 25 2008. The lander has a robotic arm with a 2.5 m reach 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 30 cm below the surface, and has a microscopic camera capable of resolving to one-thousandth the width of a human hair.

Phoenix will be followed by the Mars Science Laboratory in 2009, a bigger, faster (90 m/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 13 m.

The joint Russian and Chinese Phobos-Grunt sample-return mission, to return samples of Mars's moon Phobos, is scheduled for a 2009 launch. In 2012 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.{{cite web] announced in 2004 by US President George W. Bush. NASA and Lockheed Martin have begun work on the Orion (spacecraft), which is currently scheduled to send a human expedition to Earth's moon by 2020 as a stepping stone to an expedition to Mars thereafter.

The European Space Agency hopes to land humans on Mars between 2030 and 2035.{{cite web| date=October 11, 2002 ] probe and a Mars Sample Return Mission.

On September 28, 2007, NASA administrator Michael Griffin stated that NASA aims to put a man on Mars by 2037: in 2057, "we should be celebrating 20 years of man on Mars." Yahoo.com, NASA aims to put man on Mars by 2037

Astronomy on Mars 2005.With the existence of various orbiters, landers, and rovers, it is now possible 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 of the full Moon as it appears from Earth. On the other hand Deimos appears more or less star-like, 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 aurora (phenomenon).empty A transit of Earth from Mars will occur on November 10, 2084. There are also transit of Mercury from Mars and transit of Venus from Mars, 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 To the naked-eye, 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. The apparent magnitude of Mars varies from +1.8 at conjunction to as high as -2.9 at perihelic Opposition (astronomy). 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 (astronomy). 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 (astronomy) 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' ellipse orbits. The next Mars opposition will occur on December 24, 2007.

As Mars approaches opposition it begins a period of Retrograde motion#Apparent retrograde motion .28.29, which means it will appear to move backwards in a looping motion with respect to the background stars.

2003 closest approach 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. This occurred when Mars was one day from Astronomical 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, Middle Paleolithic., 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 astronomical unit, 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.

2007-2008 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.{{cite web| last = Beish | first = Jefrey | title = The 2007 Aphelic Apparition of Mars | url= http://www.tnni.net/~dustymars/2007_MARS.htm | accessdate = 2007-02-28-->

{| width=640 class="wikitable"|
This chart shows the path of Mars in the sky during the opposition of 2007. Each dot represents one day's motion. Mars shown on December 18 in Gemini, when it is closest to earth (0.59 AU distance, 15.9" visual diameter), 6 days before opposition on December 24.|}

Historical observations The history of observations of Mars is marked by the opposition (astronomy) of Mars, when the planet is closest to Earth

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