Mars

Mars in astronomy, 4th planet from the sun, with an orbit next in order beyond that of the earth.

Physical Characteristics

Mars has a striking red appearance, and in its most favorable position for viewing, when it is opposite the sun, it is twice as bright as Sirius, the brightest star. Mars has a diameter of 4,200 mi (6,800 km), just over half the diameter of the earth, and its mass is only 11% of the earth's mass. The planet has a very thin atmosphere consisting mainly of carbon dioxide, with some nitrogen and argon. Mars has an extreme day-to-night temperature range, resulting from its thin atmosphere, from about 80°F (27°C) at noon to about -100°F (-73°C) at midnight; however, the high daytime temperatures are confined to less than 3 ft (1 m) above the surface.

Surface Features

A network of linelike markings first studied in detail (1877) by G. V. Schiaparelli was referred to by him as canali, the Italian word meaning "channels" or "grooves." Percival Lowell, then a leading authority on Mars, created a long-lasting controversy by accepting these "canals" to be the work of intelligent beings. Under the best viewing conditions, however, these features are seen to be smaller, unconnected features. The greater part of the surface area of Mars appears to be a vast desert, dull red or orange in color. This color may be due to various oxides in the surface composition, particularly those of iron. About one fourth to one third of the surface is composed of darker areas whose nature is still uncertain. Shortly after its perihelion Mars has planetwide dust storms that can obscure all its surface details.

Photographs sent back by the Mariner 4 space probe show the surface of Mars to be pitted with a number of large craters, much like the surface of our moon. In 1971 the Mariner 9 space probe discovered a huge canyon, Valles Marineris. Completely dwarfing the Grand Canyon in Arizona, this canyon stretches for 2,500 mi (4,000 km) and at some places is 125 mi (200 km) across and 2 mi (3 km) deep. Mars also has numerous enormous volcanoes—including Olympus Mons (c.370 mi/600 km in diameter and 16 mi/26 km tall), the largest in the solar system—and lava plains. In 1976 the Viking spacecraft landed on Mars and studied sites at Chryse and Utopia. They recorded a desert environment with a reddish surface and a reddish atmosphere. These experiments analyzed soil samples for evidence of microorganisms or other forms of life; none was found. In 1997, Mars Pathfinder landed on Mars and sent a small rover, Sojourner, to take soil samples and pictures. Among the data returned were more than 16,000 images from the lander and 550 images from the rover, as well as more than 15 chemical analyses of rocks and extensive data on winds and other weather factors. Mars Global Surveyor, which also reached Mars in 1997 and remained operational until 2006, returned images produced by its systematic mapping of the surface. The European Space Agency's Mars Express space probe went into orbit around Mars in late 2003 and sent the Beagle 2 lander to the surface, but contact was not established with the lander. The American rovers Spirit and Opportunity landed successfully in early 2004 and continue to explore the Martian landscape. In 2008 NASA's Phoenix lander touched down in the planet's north polar region; it conducted studies for five months.

Analysis of the satellite data indicates that Mars appears to lack active plate tectonics at present; there is no evidence of recent lateral motion of the surface. With no plate motion, hot spots under the crust stay in a fixed position relative to the surface; this, along with the lower surface gravity, may be the explanation for the giant volcanoes. However, there is no evidence of current volcanic activity.

There is evidence of erosion caused by floods and small river systems. The possible identification of rounded pebbles and cobbles on the ground, and sockets and pebbles in some rocks, suggests conglomerates that formed in running water during a warmer past some 2-4 billion years ago, when liquid water was stable and there was water on the surface, possibly even large lakes or oceans. Rovers have identified minerals that only form in the presence of liquid water. There is also evidence of flooding that occurred less that several million years ago, most likely as the result of the release of water from aquifers deep underground. However, other evidence suggests that the water would have been extremely salty and acidic. Data received beginning in 2002 from the Mars Odyssey space probe suggests that there is water in sand dunes found in the northern hemisphere, and the Mars Reconnaissance Orbiter, which went into orbit around the planet in 2006, collected radar data that indicates the presence of large subsurface ice deposits in the mid-northern latitudes of Mars. Most of the known water on Mars, however, lies in a frozen layer under the planet's large polar ice caps, which themselves consist of water ice and dry ice (frozen carbon dioxide); the lander Phoenix found and observed frozen water beneath the soil surface in the north polar region in 2008.

Seasonal Changes

Because the axis of rotation is tilted about 25° to the plane of revolution, Mars experiences seasons somewhat similar to those of the earth. One of the most apparent seasonal changes is the growing or shrinking of white areas near the poles known as polar caps. These polar caps, which are are composed of water ice and dry ice (frozen carbon dioxide). During the Martian summer the polar cap in that hemisphere shrinks and the dark regions grow darker; in winter the polar cap grows again and the dark regions become paler. The seasonal portion of the ice cap is dry ice. When the ice cap is seasonally warmed, geyserlike jets of carbon dioxide gas mixed with dust and sand erupt from the ice.

Astronomical Characteristics

The mean distance of Mars from the sun is about 141 million mi (228 million km); its period of revolution is about 687 days, almost twice that of the earth. At those times when the sun, earth, and Mars are aligned (i.e., in opposition) and Mars is at its closest point to the sun (perihelion), its distance from the earth is about 35 million mi (56 million km); this occurs every 15 to 17 years. At oppositions when Mars is at its greatest distance from the sun (aphelion) it is about 63 million mi (101 million km) from the earth. It rotates on its axis with a period of about 24 hr 37 min, a little more than one earth day.

Satellites of Mars

Mars has two natural satellites, discovered by Asaph Hall in 1877. The innermost of these, Phobos, is about 7 mi (11 km) in diameter and orbits the planet with a period far less than Mars's period of rotation (7 hr 39 min), causing it to rise in the west and set in the east. The outer satellite, Deimos, is about 4 mi (6 km) in diameter.

Bibliography

See J. K. Beatty and A. Chaikin, ed., The New Solar System (3d ed. 1991).

Mars Fourth major planet from the Sun. Mars appears red to the naked eye because of the high iron content of its surface crust, and is known as the Red Planet. The atmosphere consists mainly of 95% carbon dioxide, 2.5% nitrogen, and 1.5% argon, with smaller quantities of oxygen, carbon monoxide and water vapour. Its axial tilt is similar to the Earth's, so it passes through a similar cycle of seasons. The surface temperature on Mars varies between extremes of 130K and 290K. Its surface reveals a long and complex history of geological activity. The major difference in terrain is between the largely smooth, lowland volcanic plains of the northern hemisphere, and the heavily cratered uplands of the south. The biggest volcanic structure on Mars is Olympus Mons, which is hundreds of kilometres across and 27km (17mi) high. Other geographical features, such as giant canyons, are channels in which rivers once flowed. The variable polar ice caps appear to be composed of solid carbon dioxide with underlying caps of water ice. Mars has two tiny satellites in very close orbits, Phobos and Deimos. In 1996, scientists investigating a meteorite, thought to have originated on Mars, found fossilized microorganisms that some believe indicate the presence of primitive life on the planet. In 2003, two separate missions to Mars were launched. The British-built probe Beagle 2 failed to make contact with Earth. In 2004, the NASA funded Opportunity probe found evidence that water suitable for supporting life, once lay on the surface of Mars.

http://lpl.arizona.edu/nineplanets/nineplanets/mars.html; http://wr.usgs.gov

Mars (adj. martian) The fourth planet in the solar system, 1.524 AU from the Sun. Its radius is 3390 km, its density 3940 kg/m³, and the inclination of equator to orbit is 25.1°. It has a small atmosphere (7 mb) of CO₂. The polar caps are of water ice with seasonal solid CO₂. The northern-hemisphere crust is mainly basaltic plains and volcanoes; the southern an ancient cratered terrain (see CRATER). The Tharsis Bulge is an uplifted or volcanic plateau. Large canyons exist and there is evidence of former water erosion. Some basaltic meteorites are derived from Mars. It has two small satellites, Phobos and Deimos, probably captured asteroids.