Mars

For the Roman god, see Mars (mythology). For other uses of this term, Mars (disambiguation).

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Template:Planet Infobox/Mars

Mars is the fourth planet from the Sun in our solar system. It is named after Mars, the Roman god of war (Ares in Greek mythology). Mars is also known as "The Red Planet" due to the reddish appearance it has when seen from Earth at night. Mars has two moons, Phobos and Deimos, which are small and oddly-shaped, and possibly captured asteroids. Mars can be seen from Earth by the naked eye with a brightness of up to -2.9 magnitude, only surpassed by Venus, the Moon and the Sun.

The prefix areo- refers to Mars in the same way geo- refers to Earth—for example, areology versus geology. Areology is also used to refer to the study of Mars as a whole rather than just the geological processes of the planet. The astronomical symbol for Mars is ♂, a circle with an arrow pointing northeast. This symbol is a stylized representation of the shield and spear of the god Mars, and in biology it is used as a sign for the male sex.

The Chinese, Japanese, Korean and Vietnamese cultures refer to the planet as 火星, or fire star, a naming based on the ancient Chinese mythological cycle of Five Elements.

Name

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". 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.

Physical characteristics

The red, fiery appearance of Mars is caused by iron(III) oxide (rust) on its surface.^[1] 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.

Atmosphere

The atmosphere of Mars is thin; the atmospheric pressure on the surface is only 0.7-0.9 kPa, compared to Earth's 101.3 kPa. However, the scale height of the atmosphere is about 11 km, somewhat higher than Earth's 6 km. 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, giving the Martian sky a tawny color when seen from the surface; data from the Mars Exploration Rovers indicates the suspended dust particles are roughly 1.5 micrometres across^[2].

Mars from Hubble Space Telescope October 28, 2005 with sandstorm visible.

In March 2004 the Mars Express Orbiter reported they had found methane in the Martian atmosphere, with a concentration of about 10 ppb by volume ^[3]. The presence of methane on Mars would be very intriguing, since as an unstable gas it indicates that there must be (or have 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 but as yet unproven sources. It was also recently shown that methane could be produced by a non-biological process involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.^[4]

The thin atmosphere cannot hold heat and is the cause of the lower temperatures on Mars. The maximum temperature is roughly 20 degrees Celsius (68 degrees Fahrenheit). Recently, evidence has been discovered suggesting that Mars may be warming in the short term.^[5]

In the winter months when the poles are in continual darkness, the surface gets so cold that as much as 25% of the entire atmosphere condenses out into thick slabs of CO₂ ice (dry ice). When the poles are again exposed to sunlight the CO₂ ice sublimates, creating enormous winds that sweep off the poles as fast as 250 mph (400 km/h). These seasonal actions transport large amounts of dust and water vapor giving rise to Earth-like frost and large cirrus clouds. Clouds of water-ice were photographed by the Opportunity rover in 2004.^[6]

Geology

The surface of Mars is thought to be primarily composed of basalt, based upon the Martian meteorite collection and orbital observations. There is some evidence that some portion of the Martian surface might be more silica-rich than typical basalt, perhaps similar to andesitic rocks on Earth, though these observations may also be explained by silica glass. Much of the surface is deeply covered by dust as fine as talcum powder.

Observations of the magnetic fields on Mars by the Mars Global Surveyor spacecraft have revealed that parts of the planet's crust have been magnetized. This magnetization has been compared to alternating bands found on the ocean floors of Earth. One theory, published in 1999 and reexamined in October 2005 with the help of the Mars Global Surveyor, is that these bands are evidence of the past operation of plate tectonics on Mars.^[7]

Shortly after the landing of the Mars Exploration Rovers in 2004, it was announced by a large group of scientists that they had conclusive evidence that liquid water existed at one time on the surface of Mars. Key discoveries leading to this conclusion include the detection of various minerals such as hematite and goethite which usually only form in the presence of water.

Geography

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 the surface features were permanent, and pinned down Mars' rotation period. In 1840, Mädler combined ten years of observations and drew the first map of Mars ever made. 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 derive 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, 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 originally selected line.

Topography

The dichotomy of Martian topography is striking: northern plains flattened by lava flows contrast with the southern highlands, pitted and cratered by ancient impacts. The surface of Mars as seen from Earth is consequently 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.

Mountains and Craters

The shield volcano, Olympus Mons (Mount Olympus), is at 26 km the highest known mountain in the solar system. It is in a vast upland region called Tharsis, which contains several large volcanos. The Tharsis region of Mars also has the solar system's largest canyon system, Valles Marineris or the Mariner Valley, which is 4000 km long and 7 km deep. Mars is also scarred by a number of impact craters. The largest of these is the Hellas impact basin, covered with light red sand.

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 (approximately 0.6% of Earth's). This pressure corresponds to the triple point of water.

Moons

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

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. Speedy Phobos rises in the west, sets in the east, and rises again in just 11 hours, while Deimos, being only just outside synchronous orbit, rises as expected in the east but very slowly. Despite its 30 hour orbit, it takes 2.7 days to set in the west as it slowly falls behind the rotation of Mars, and as long again to rise.

The exploration of Mars

File:Vikinglander-view.jpg

Viking Lander 1 site

Dozens of spacecraft, including orbiters, landers, and rovers, have been sent to Mars by the Soviet Union, the United States, Europe, and Japan to study the planet's surface, climate, and geography. Roughly two-thirds of all spacecraft destined for Mars have failed in one manner or another before completing or even beginning their missions. Part of this high failure rate can be ascribed to technical problems, but enough have either failed or lost communications for no apparent reason that some researchers half-jokingly speak of an Earth-Mars "Bermuda Triangle", or a Mars Curse.

Mars 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 from the Mars probe program, launched in 1971, but both lost contact within seconds of landing. Then was 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 many years.

Following the 1992 failure of NASA's Mars Observer orbiter, they launched the Mars Global Surveyor in 1996. This mission was a complete success, having finished its primary mapping mission in early 2001. 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.

Following various failures in the late 90s, in 2001 NASA launched the successful Mars Odyssey orbiter, which is still in orbit as of March 2006. It notably determined that there are significant deposits of water ice in the upper meter or so of Mars' regolith within 30° of the north and south pole.

In 2003, the ESA launched the Mars Express craft consisting of the Mars Express Orbiter and the lander Beagle 2. It was announced in early 2004 that the orbiter detected methane in the atmosphere, which is a critical factor in determining if there is life on Mars. Unfortunately attempts to contact the Beagle 2 failed and it was declared lost in early February 2004.

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 are known to be passing over the Rovers, cleaning their solar panels, and thus extending their lifespan.

On August 12 2005 the NASA Mars Reconnaissance Orbiter probe was launched toward the planet, to conduct a two-year science survey. The purpose of the mission is to do more studies and prepare the upcoming lander missions. It arrived in orbit on March 10, 2006. The next scheduled mission to Mars is the NASA Phoenix Mars lander, expected to launch in 2007.

Astronomy on Mars

It is now possible, with the existence of various orbiters, landers, and rovers to study astronomy from the martian skies. In particular, the Earth and the Moon would easily be visible to the naked eye. Also, one could observe the two moons of Mars. The moon Phobos appears about one third the angular diameter that the full Moon appears from Earth, and when it is full it is bright enough to cast shadows. On the other hand Deimos appears more or less starlike, and appears only slightly brighter than Venus does from Earth.

There are also various phenomenon well-known on Earth that have now been observed on Mars, such as meteors and auroras. The first meteor photographed on Mars was on March 7 2004 by the Spirit rover. Auroras occur on Mars, but they do not occur at the poles as on Earth, because Mars has no planetwide magnetic field. Rather, they occur near magnetic anomalies in Mars's crust, which are remnants from earlier days when Mars did have a magnetic field. They would probably be invisible to the naked eye, being largely ultraviolet phenomena.

Colonization of Mars

Plans for manned interplanetary exploration beyond the Moon have mostly assumed Mars as the first target; serious proposals arguably date from the late 1950's with Project Orion. 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.

Orbital Characteristics

Mars has a fairly high eccentricity compared to other planets in the solar system, and has an average distance from the Sun of roughly 230 million km (1.5 AU). Its orbital period is 687 (Earth) days, but the solar day (or sol) on Mars is only slightly more than an Earth day: 24 hours, 39 minutes, and 35.244 seconds. See Timekeeping on Mars.

Close Approaches to Earth

Approximately every 780 days opposition occurs, which is when Mars is nearest to Earth. This minimum distance varies between about 55 and 100 million km due to the planets' elliptical orbits.

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) without Light-time correction. 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. Detailed analysis of the solar system's gravitational landscape forecasts an even closer approach in 2287. However, to keep this in perspective, this record approach was only an imperceptibly small distance 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.

Observing Mars

To a naked-eye observer, Mars usually shows a distinct yellow, orange or reddish colour, and varies in brightness more than any other planet as seen from Earth over the course of its orbit, due to the fact that when furthest away from the Earth it is more than seven times as far from the latter as when it is closest (and can be lost in the Sun's glare for months at a time when least favourably positioned). 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.

Transits and occultations

Photograph of a Martian sunset taken by Spirit at Gusev crater, May 19th, 2005.

A transit of the Earth as seen from Mars will occur on November 10, 2084. At that time the Sun, the Earth and Mars will be exactly in a line. 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).

The only occultation of Mars by Venus to be observed was that of October 3, 1590, seen by M. Möstlin at Heidelberg.

Life on Mars

Some evidence suggests that the planet once was significantly more habitable than today, but the question on whether living organisms ever actually existed there is an open one.

The Viking probes of the mid-1970s carried experiments designed to detect microorganisms in Martian soil at their respective landing sites, and had some positive results, later denied by many scientists, resulting in ongoing controversy. Also, present biologic activity is one of the explanations that have been suggested for the presence of traces of methane within the Martian atmosphere, but other explanations not involving life are generally considered more likely.

Martian meteorites

A handful of objects are known that are surely meteorites and may be of Martian origin. Two of them may show signs of ancient bacterial activity. On August 6, 1996 NASA announced that analysis of the ALH84001 meteorite, thought to have come from Mars, shows some features that may be fossils of single-celled organisms, although this idea is controversial. In fact recent research indicates that the rock, since its creation several billion years ago, has never been exposed to temperatures for extended periods of time that would allow for liquid water.

In March 2004, it was suggested that the unique Kaidun meteorite landed on March 12, 1980 in Yemen, may have originated on the Martian moon of Phobos.

On April 14, 2004, NASA revealed that a rock known as "Bounce", studied by the Mars Exploration Rover Opportunity, was similar in composition to the meteorite EETA79001-B, discovered in Antarctica in 1979. The rock may have been ejected from the same crater as the meteorite, or from another crater in the same area of the Martian surface.

Canals

The belief that there may be life on Mars was popularized in the 19th century, mainly due to the announcement of Martian canals from the observations by Percival Lowell and Giovanni Schiaparelli. Schiaparelli called these observed features canali, meaning channels in Italian. This was popularly mistranslated as 'canals', and the myth of the Martian canals began. They were apparently artificial linear features on the surface that were asserted to be canals, and together with the seasonal changes in the brightness of some areas, they were thought to be linked with vegetation growth. This gave rise to many stories concerning Martians. The linear features are now known to be mostly non-existent or, in some cases, dry ancient watercourses. The color changes have been ascribed to dust storms.

The Mars flag

In early 2000, a proposed Mars flag flew aboard the space shuttle Discovery on STS-103. Designed by NASA engineer and Flashline Mars Arctic Research Station task force leader Pascal Lee and carried aboard by astronaut John Mace Grunsfeld, the flag consists of three vertical bars (red, green, and blue), symbolizing the transformation of Mars from a barren planet (red) to one bearing sustainable life (green), and finally to a fully terraformed planet with open bodies of water (blue). This design was suggested by the Kim Stanley Robinson sci-fi trilogy Red Mars, Green Mars, and Blue Mars. While other designs have been proposed, the republican tricolor has been adopted by the Mars Society as its own official banner. In a statement released after the launch of the mission, the Society said that the flag "has now been honored by a vessel of the leading spacefaring nation on Earth," and added that "it is fitting that this action occurred when it did: at the dawning of a new millennium."^[8]

Mars in Human Culture

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 Greek God Mars. Mars in Greek mythology was the God of War and patron of warriors. This symbol is also used in biology to describe the male sex.

Mars 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.

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. Mars (and its moons) were also the setting for the popular Doom video game franchise and the later Martian Gothic .