Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. This feature also earned it the name of "Red Planet". Mars has two small moons, Phobos and Deimos, both small and oddly shaped, possibly captured asteroids. The prefix areo- refers to Mars.
Its symbol is a stylized representation of the god Mars' shield and spear: a circle with a small arrow pointing out from behind it (Unicode: ♂).
Mars has always fascinated people. Its red, fiery appearance is mysterious and intriguing. Mars has only a quarter the surface area of the Earth and only 1/10th the mass (though because it lacks oceans the area of Mars' accessible dry land is approximately equal to that of the Earth's dry land). The solar day on Mars is almost the same length as it is on earth, at 24h 39m 35.244s.
Mars with polar ice caps visible
Mars' atmosphere is very thin: the surface air pressure is only 750 Pa (about 0.75% of the average on Earth). The atmosphere on Mars is 95% carbon dioxide, 3% nitrogen, 1.6% argon, with only a trace of oxygen and water. In 2003 methane was also discovered in the atmosphere by Earth-based telescopes, and possibly confirmed in March 2004 by the Mars Express Orbiter.
The presence of methane on Mars is 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 impact and the existence of life in the form of microorganisms such as methanogens are among possible but as yet unproven sources. The methane appears in patches, which suggests that it is being rapidly broken down (and so is presumably also continually being released to the atmosphere) before it has time to become uniformally distributed in the atmosphere. Plans are now being made to look for other 'companion' gases that may suggest which sources are most likely; in the Earth's oceans biological methane production tends to be accompanied by ethane, while volcanic methane is accompanied by sulfur dioxide.
Apart from methane release and breakdown, other aspects of the atmosphere are also dynamic, with water vapor moving from one pole to the other with the change between summer and winter, giving rise to Earth-like frost and large cirrus clouds  (http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20041213a/merb_sol290_clouds-B313R1_br.jpg) of water-ice that were photographed by the Opportunity rover in 2004.
Observations of the magnetic fields on Mars by the Mars Global Surveyor spacecraft have revealed that parts of the planet's crust has been magnetized in alternating bands, typically measuring 100 miles wide by 600 miles long, in a similar pattern to those found on the ocean floors of Earth. One interesting theory, published in 1999 is that these bands could be evidence of the past operation of plate tectonics on Mars, although this has yet to be proven  (http://photojournal.jpl.nasa.gov/catalog/PIA02008). If true, the processes involved may have helped to sustain an Earth-like atmosphere by transporting carbon rich rocks to the surface, while the presence of a magnetic field would have helped to protect the planet from cosmic radiation. Other explanations have also been proposed.
Microscopic rock forms indicating past signs of water taken by MER-B
Amongst the findings from the Opportunity rover is the presence of hematite on Mars in the form of small spheres on the Meridiani Planum. The spheres are only a few millimeters in diameter and are believed to have formed as rock deposits under watery conditions billions of years ago. Other minerals have also been found containing forms of sulfur, iron or bromine such as jarosite. This and other evidence lead a group of 50 scientists to conclude in the December 9, 2004 edition of the journal Science that "Liquid water was once intermittently present at the martian surface at Meridiani, and at times it saturated the subsurface. Because liquid water is a key prerequisite for life, we infer conditions at Meridiani may have been habitable for some period of time in martian history". On the opposite side of the planet the mineral goethite, which (unlike hematite) only forms in the presence of water, along with other evidence of water, has also been found by the Spirit rover in the "Columbia Hills".
In 1996, researchers studying a meteorite (ALH84001) believed to have originated from Mars reported features which they attributed to microfossils left by life on Mars. As of 2004, this interpretation remains controversial with no consensus having emerged.
North Polar region with icecap. (Courtesy NASA/JPL-Caltech.)
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.
Mars has polar ice caps that contain frozen water and carbon dioxide that change with the Martian seasons-the carbon dioxide ice melts in summer, uncovering a surface of layered rocks, and forms again in winter. An extinct shield volcano, Olympus Mons (Mount Olympus), is at 27 km the highest mountain in the solar system. It is in a vast upland region called Tharsis, containing several large volcanos. See list of mountains on Mars. 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. See list of craters on Mars.
The International Astronomical Union's Working Group for Planetary System Nomenclature is responsible for naming Martian surface features.
Topographic map of Mars, courtesy NASA/JPL-Caltech. Notable features include the Tharsis volcanoes in the west (including Olympus Mons
), Valles Marineris to the east of Tharsis, and Hellas Basin in the southern hemisphere.
Zero elevation: Since Mars has no oceans and hence no 'sea level', a zero-elevation surface or mean gravity surface must be selected. The datum for Mars is defined by the fourth-degree and fourth-order spherical harmonic gravity field, with the zero altitude defined by the 610.5 Pa (6.105 mbar) atmospheric pressure surface (approximately 0.6% of Earth's) at a temperature of 273.01 K. This pressure and temperature correspond to the triple point of water.
Zero meridian: 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 which was accepted by later observers. The German astronomers Wilhelm Beer and Johann Heinrich Mädler selected a small circular feature as a reference point when they produced the first systematic chart of Mars features in 1830-32. In 1877, their choice was adopted as the prime meridian by the Italian astronomer Giovanni Schiaparelli when he began work on his notable 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') along the line of Beer and Mädler, was chosen by Merton Davies of the RAND Corporation to provide a more precise definition of 0.0° longitude when he established a planetographic control point network.
Mars has an important place in human imagination due to the belief by some that life existed on Mars, due mainly to observations by Percival Lowell of Martian canals, apparently artificial linear features on the surface that he asserted were canals, and due to seasonal changes in the brightness of some areas that were thought to be caused by 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.
Orbits of Phobos and Deimos (to scale)
Both Phobos and Deimos are tidally locked with Mars, always pointing the same face towards it. Since Phobos orbits around Mars faster than the planet itself rotates, tidal forces are slowly but steadily decreasing its orbital radius. At some point in the future Phobos will be broken up by gravitational forces (see Roche limit). Deimos, on the other hand, is far enough away that its orbit is being slowly boosted instead.
Both satellites were discovered in 1877 by Asaph Hall, and are named after the characters Phobos and Deimos in Greek mythology, sons of the Greek god Ares.
Mars' natural satellites
|Name ||Diameter (km) ||Mass (kg) ||Mean orbital |
|Orbital period |
|Phobos ||22.2 (27 × 21.6 × 18.8) ||1.08×1016 ||9378 ||7.66 hours |
|Deimos ||12.6 (10 × 12 × 16) ||2×1015 ||23,400 ||30.35 hours |
As seen from Mars, Phobos has an angular diameter of about 12', while Deimos has an angular diameter of about 2'. The Sun's angular diameter, by contrast, is about 21'.
The exploration of Mars
Main article: Exploration of Mars
Viking Lander 1 site (click for detailed description).
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 incompetence, 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 of a Great Galactic Ghoul which subsists on a diet of Mars probes.
Among the most successful missions are the Mariner and Viking programs, Mars Global Surveyor, Mars Pathfinder, and Mars Odyssey. Global Surveyor has taken pictures of gullies and debris flow features that suggest there may be current sources of liquid water, similar to an aquifer, at or near the surface of the planet. Mars Odyssey determined that there are vast deposits of water ice in the upper three meters of Mars' soil within 60° latitude of the south pole.
In 2003, the ESA launched the Mars Express craft consisting of the Mars Express Orbiter and the lander Beagle 2. Mars Express Orbiter confirmed the presence of water ice and carbon dioxide ice at the planet's south pole. NASA had previously confirmed their presence at the north pole of Mars. 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; while a 90-day nominal mission was planned, as of February 2005, their missions have been extended twice and they continue to return science, although some mechanical faults have occurred. Among the most significant science return has been evidence of liquid water at both landing sites.
The "Ares Vallis" area as photographed by Mars Pathfinder (click for detailed description).
Earth passes Mars every 26 months at a distance of about 80,000,000 km. However, this varies because the orbits are elliptic.
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 close approach came about because 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 tiny fraction less than other recent close approaches that occur four times every 284 years. 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.
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).
Main article: 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 ALH 84001 meteorite thought to have come from Mars, shows some features that may be fossils of single-celled organisms, although this idea is controversial.
In Solar System Research (March 2004, vol 38, page 97) it was suggested that the unique Kaidun meteorite, recovered from 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.
Life on Mars
Main article: Life on Mars
Evidence exists that the planet once was significantly more habitable than today, but the question whether living organisms ever actually existed there is an open one. Some researchers think that a certain rock which is believed to have originated on Mars - specifically, meteorite ALH84001 - does contain evidence of past biologic activity, but no consensus about these claims has been achieved so far. The Viking probes carried experiments designed to detect microorganisms in Martian soil at their respective landing sites, but are thought to have come back empty in this respect. 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.
Mars in various cultures
In Ancient Egypt, this planet was known as "Horus the Red." Because of Mars' retrograde motion, Egyptians said that it "traveled backwards." Also, Cairo's name comes from "Al Qahira," which means the planet Mars in ancient Arabic.
In Jyotish, Mars is known in Sanskrit as Mangal (auspicious), Angaraka (burning coal), and Kuja (the fair one). It represents energetic action, confidence and ego.
Mars in fiction
Because of early scientific speculations that its surface conditions might be capable of supporting life, Mars has become one of the most often mentioned real planets in science fiction.
Mars in astrology
According to Marcus Manilius (1st century AD) in his epic (8000 verses) poem Astronomica, Mars is ardent, and presides over the genitals. Called "The Bringer Of War" in Alan Leo's What is a Horoscope?, since the planet is associated with the Roman god of war, Mars in Western astrology is associated with confident, aggressive personalities. Mars is the ruling planet of Aries. Before the discovery of Pluto, Mars was also the ruling planet of Scorpio (and still is for the more traditional astrologers).
In Chinese astrology, Mars is ruled by the element Fire.
Mars in religion
Microscopic rock forms indicating past signs of water taken by MER-B
Water on Mars
- Dr. Tony Phillips: "Making a Splash on Mars" (http://science.nasa.gov/headlines/y2000/ast29jun_1m.htm), Science@NASA article, June 29, 2000. Phillips describes the Martian "gullies" and explains the conditions under which liquid water can exist on the surface of Mars.
- BBC News story on subsurface ice deposits on Mars (http://news.bbc.co.uk/hi/english/sci/tech/newsid_2009000/2009318.stm)
- BBC News update on Mars Express' findings of polar water ice and water-eroded features on the surface (http://news.bbc.co.uk/1/hi/sci/tech/3426539.stm)
- Mars Rover Scientists Wring Water Story from Rocks (http://www.nasa.gov/vision/universe/solarsystem/opportunity_water.html) This image taken by Mars Rover Opportunity shows microscopic rock forms indicating past signs of water. Courtesy: NASA