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Mars (planet)

Larger image with caption
Orbital characteristics
Mean radius227,936,640 km
Revolution period686.98 days
Synodic period779.95 days
Avg. Orbital Speed24.1309 km/s
Number of satellitess2
Physical characteristics
Equatorial diameter6,794.4 km
Surface area144 million km2
Mass6.4191 × 1023 kg
Mean density3.94 g/cm3
Surface gravity3.71 m/s2
Rotation period24.6229 hours
Axial tilt25.19°
Escape Speed5.02 km/s
Surface temp
Atmospheric characteristics
Atmospheric pressure0.7-0.9 kPa
Carbon dioxide95.32%
Carbon monoxide0.07%
Water vapor0.03%

Mars is the fourth planet from the Sun in the solar system, named for the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. Mars has two small moonss, Phobos and Deimos, both small and oddly shaped, possibly captured asteroids.

Table of contents
1 Physical characteristics
2 Mars' moons
3 The exploration of Mars
4 Miscellaneous
5 Related articles
6 External links

Physical characteristics

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). Mars' atmosphere is very thin: the surface air pressure is only 7.5 millibars compared to an average 1013 millibars on Earth. The atmosphere on Mars is 95% carbon dioxide, 3% nitrogen, 1.6% argon, with only a trace of oxygen and water.

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 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 know to be non-existent or, in some cases, dry ancient watercourses. The color changes have been ascribed to dust storms. There is as yet no conclusive evidence that there is or ever has been life on Mars.

See an image comparison:


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 basin). The dark features were thought to be seas, hence their names Mare Erytherium, Mare Sirenum and Aurorae Sinus. The largest dark feature seen from Earth is Syrtis Major.

Mars with polar ice caps visible.

Mars has polar ice caps that contain frozen water and carbon dioxide. An extinct shield volcano, Olympus Mons (Mount Olympus), is at 27 km the tallest 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 scar of Mars, 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.

Other notes:

Zero elevation: Since Mars has no oceans and hence no 'sea level', a zero-elevation surface or mean gravity surface must be selected.

Zero meridian: Mars' equator is defined by its rotation, but its equivalent of Earth's Prime Meridian must be chosen arbitrarily. A crater in the Sinus Meridiani ('Equatorial Gulf') has been arbitrarily selected to mark the zero meridian.

Mars' moons

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 impact on Mars's surface. 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's natural satellites
Name Diameter (km) Mass (kg) Mean orbital
radius (km)
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

The exploration of Mars

Main article: Exploration of Mars

This image was acquired at the Viking Lander 1 site with camera number 1. The large rock just left of center is about 2 meters wide. This rock was named "Big Joe" by the Viking scientists. The top of the rock is covered with red soil. Those portions of the rock not covered are similar in color to basaltic rocks on Earth. Therefore, this may be a fragment of a lava flow that was ejected by an impact crater. ()

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's 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. Initial attempts to contact the lander failed. Also that year, NASA launched the two Mars Exploration Rovers named Spirit (MER-A) and Opportunity (MER-B).

The "Ares Vallis" area, which is among the rockiest parts of Mars, as photographed by the Mars Pathfinder lander in its 1997 mission. The "twin peaks" are seen in the distance. ()


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 UTC, Mars made its closest approach to Earth in nearly 60,000 years: approximately 34,646,416 miles (55,758,000 kilometers). This close approach while in opposition made Mars particularly easy to see from Earth. The last time it came so close is estimated to be in 57,617 BC. Detailed analysis of the solar system's gravitational landscape forecasts an even closer approach in 2287.

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.

Related articles

External links

The Solar System

Sun - Mercury - Venus - Earth - Mars - Asteroids - Jupiter - Saturn - Uranus - Neptune - Pluto - Comets - Kuiper belt - Oort cloud