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. Mars has two small moons, Phobos and Deimos, both small and oddly shaped, possibly captured asteroids. The prefix areo- refers to Mars.
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 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, confirmed in March 2004 by the Mars Express Orbiter. The presence of methane is 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.
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.
In 2000, researchers studying meteorites which 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.
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 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. 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 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.
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" was chosen to conform to a line where the atmospheric pressure is 610 Pa (6.1 mbar), approximately 0.6% the atmospheric pressure at Earth's surface.
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 geographic control point network.
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.
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
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.
Mars Exploration Rover(s) (MER)
In 2003, NASA launched the twin Mars Exploration Rovers named Spirit (MER-A) and Opportunity (MER-B). The spacecraft landed successfully three weeks apart, activated their rover vehicles, sent back detailed panoramic photographs, and began geologic work. On March 2, 2004 NASA announced that the rovers had uncovered convincing evidence that some parts of the surface of Mars were once covered in liquid water. This increases the probability that some form of life once existed on Mars.
Although little problems have occurred with the MER Project, a week into Spirit's deployment, the rover experienced memory problems and was not able to communicate with the NASA Jet Propulsion Lab (JPL) in Pasadena, California. After what looked like the end for the doomed rover, NASA was able to reboot the onboard system and get the rover working again. After that, all has been successful. Also worth noting, the MER rovers were expected to last for 90 days before becoming inoperable. However, as of September 22nd, 2004, over 6 months since the rovers first landed on Mars, both Spirit and Oppurtuniy are still completely operable with no end in sight.
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).
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 14 April 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
There is today good evidence that the planet once was significantly more inhabitable than is the case 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.
Mars in religion
Water on Mars
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