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the fourth planet from the Sun
Mars as seen by the Hubble Space Telescope on
August 27, 2003
popular symbol for Mars
The seventh largest planet is named for the Roman god of War, primarily because of its red color.
View from Earth
When viewed without a telescope, Mars is a reddish object of considerably varying brightness. At its closest approach to Earth, Mars is the third brightest object in the night sky (after the Moon and Venus). It is best observed when directly opposite the sun in Earth's sky.
Through a telescope Mars can be seen to have bright orange regions and darker, less red areas, the outlines and tones of which change with Martian seasons. Conspicuous bright caps, apparently made of frost or ice, mark the planet's polar regions. Their seasonal cycle has been followed for almost two centuries. Each Martian "autumn," bright clouds form over the appropriate pole. By late "winter," the cap may extend down to latitudes of 45°. By "spring," the polar hood dissipates, revealing the winter frost cap; the cap's boundary then gradually recedes poleward as sunlight evaporates the accumulated frost. By "midsummer" the steady recession of the annual cap stops, and a bright deposit of frost and ice survives until the following "autumn."
The first spacecraft to visit Mars was Mariner 4 in 1965. Further information was gained by the flyby missions of Mariners 6 and 7 in 1969. The first Mars orbiter -- Mariner 9, launched in 1971 -- studied the planet for almost a year, giving planetary scientists their first comprehensive global view of the planet and the first detailed images of its two moons. In 1976 two Viking lander craft touched down successfully on the surface and carried out the first direct investigations of the atmosphere and surface. The second Viking lander ceased operating in April 1980; the first lander worked until November 1982. The Viking mission also included two orbiters that studied the planet for almost two full Martian years.
In 1988, the Soviet Union sent two probes to land on the moon Phobos. Both missions failed, but one did relay back some data and photographs before being lost to radio contact.
On July 4, 1997, the Mars Pathfinder landed successfully on the Martian surface. In 2004, the Mars Expedition Rovers "Spirit" and "Opportunity" landed on the surface and almost immediately began sending back geologic data and many pictures; they continued to send back data for more than a year. Curiosity landed on the Martian surface on August 6, 2012, and has been sending pictures and experiment results to Earth-bound scientists since.
Mars has a very thin atmosphere composed mostly of carbon dioxide (95.3%), nitrogen (2.7%), argon (1.6%), and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface is less than 1% of Earth's, but varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. The atmosphere is just thick enough to support very strong winds and vast dust storms that on occasion engulf the entire planet for months. The thin atmosphere produces a greenhouse effect, but it is only enough to raise the surface temperature by 5 degrees Kelvin.
The Martian surface can be divided into two approximately hemispherical provinces. The southern half consists of ancient cratered terrain dating from the planet's earliest history, when Mars was subjected to intense meteoroidal bombardment. Considerable erosions and filling of even the largest craters have occured since then. One crater, Hellas Planitia, is more than 3.5 mi (6 km) deep and some 1,240 mi (2,000 km) in diameter.
The northern half of Mars has a much less cratered, and hence younger, surface, believed to consist of volcanic flows. Two major centers of past volcanic activity have been identified: the Elysium Plateau and the Tharsis Bulge. The latter is about 2,480 mi (4,000 km) across and 6.2 mi (10 km) high. Some of the Solar System's largest volcanoes occur in Tharsis. Olympus Mons, the largest mountain in the Solar System, rises 15.5 mi (24 km) above the surrounding Martian plain and mesures more than 370 mi (600 km) across at its base. No evidence of current volcanic activity exists anywhere on the planet, however.
Mars appears to lack active plate tectonics at present, as there is no evidence of recent horizontal motion of the surface such as the folded mountains common on Earth. With no lateral plate motion, hot-spots under the crust stay in a fixed position relative to the surface. This likely accounts for the Tharis Bulge and the enormous volcanoes seen on the surface. Although there is no evidence of current volcanic activity, data from the Mars Global Surveyor indicates that Mars most likely did have tectonic activity sometime in its past.
There is also evidence of erosion in many places on Mars, including the remains of large floods and small river systems. It is quite likely that at some time in the past some sort of fluid flowed over the Martian surface. There may have been large lakes or even oceans, evidence for which was strengthened by images of layered terrain taken by the Mars Global Surveyor and mineralology results from MER Opportunity. Channels across the cratered terrain of Lunae Planum, near the equator, were carved by a massive flood, the source of which is not known. Sharp young ridges on some Martian channels raise the possibility of periodic flash flooding from melted subsurface permafrost.
channels across the cratered terrain of Lunae
Mars has permanent ice caps at both poles; they're visible from Earth even with a small telescope. We now know that they're composed of water ice and solid carbon dioxide ("dry ice"), and that they exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the northern "summer" the carbon dioxide completely sublimes, leaving a residual layer of water ice.
South Polar Ice Cap as seen by the Viking Orbiter
Little is known about the interior of Mars. The planet's relatively low mean density indicates that Mars cannot have an extensive metallic core. And, since Mars does not have a measurable magnetic field, it is unlikely that the core is fluid. Judging from its ability to support such massive topological features as Tharsis, the crust of Mars may be as thick as 125 mi (200 km) -- five or six times as thick as Earth's crust.
left: Phobos and Mars
Both Phobos and Deimos are heavily cratered. They are considered by most astronomers to be asteroidlike objects captured by the planet very early in Mars' history.
Phobos is closer to its primary than any other moon in the Solar System. Because it orbits Mars below the synchronous orbit radius, it rises in the west, moves very rapidly across the sky and sets in the east, usually twice a day. It is so close to the surface that it cannot be seen above the horizon from all points on the surface of Mars. Phobos is also doomed. Tidal forces are gradually lowering its orbit at a rate of about 1.8 meters per century. In about 50 million years it will either crash onto the surface of Mars or (more likely) break up into a ring.
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