Mars (planet)

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.

Atmosphere

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.

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.

North Polar region with icecap

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.

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. The datum for Mars is defined by the fourth-degree and fourth-order spherical harmonic gravity field, with the zero altitude defined by the 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' 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 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.

The exploration of Mars

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 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 November 2004, 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.