MARS

ancient Roman deity, in importance second only to Jupiter. Little is known of his original character, and that character (chiefly from the cult at Rome) is variously interpreted. It is clear that by historical times he had developed into a god of war; in Roman literature he was protector of Rome, a nation proud in war. Mars's festivals at Rome occurred in the spring and the fallthe beginning and the end of both the agricultural and the military seasons. The month of March was especially filled with festivals wholly or partially in his honour; the members of the ancient priesthood of the Salii, who were particularly associated with Jupiter, Mars, and Quirinus, came out several times during the month to dance their ceremonial war dance in old-fashioned armour and chant a hymn to the gods. October was also an important month for Mars. At the festival of the October Horse on October 15, a two-horse chariot race was held in the Campus Martius, and on October 19 the Armilustrium marked the purification of the arms of war and their storage for the winter. The god was invoked in the ancient hymn of the Arval Brothers, whose religious duties had as their object to keep off enemies of all kinds from crops and herds. Until the time of Augustus, Mars had only two temples at Rome: one was in the Campus Martius, the exercising ground of the army; the other was outside the Porta Capena. Within the city there was a sacrarium (shrine, or sanctuary) of Mars in the regia, originally the king's house, in which the sacred spears of Mars were kept; upon the outbreak of war the consul had to shake the spears saying, Mars vigila (Mars, wake up!). Under Augustus the worship of Mars at Rome gained a new impetus; not only was he traditional guardian of the military affairs of the Roman state but, as Mars Ultor (Mars the Avenger), he became the personal guardian of the emperor in his role as avenger of Caesar. His worship at times rivaled that of Capitoline Jupiter, and about AD 250 Mars became the most prominent of the di militares (military gods) worshiped by the Roman legions. In literature and art he is hardly distinguished from the Greek Ares. in astronomy, fourth major planet from the Sun, named after the Roman god of war because of its reddish colour. Mars has an elliptical orbit, and so its distance from the Earth varies considerably. Its mean distance from the Sun is 228 million km (141 million miles), about half as far again as is the Earth. A Martian day, or sol, is 24.6 Earth hours, and the Martian year is approximately 687 Earth days. The planet has two small satellites, Phobos and Deimos (qq.v.). Like the Earth, Mars has seasons because of an oblique axis of rotation and the presence of an atmosphere. It is, however, much colder: the mean surface atmospheric temperature is only -23 C (-9.4 F). Mars is a small planet, having a mean diameter of 6,790 km (4,219 miles), approximately half that of the Earth. Also, its density, 3.933 grams per cubic centimetre, is lower than that of Earth. No magnetic field has been detected on Mars, and this, together with the low density, suggests the absence of a substantial metallic core. Mars's thin atmosphere is composed predominantly of carbon dioxide, with some nitrogen and argon. Traces of water vapour have also been detected. The perennial part of the ice caps consists of water ice and the seasonal parts of frozen carbon dioxide. Utopian plain, Mars, from Viking II. Studies conducted with Earth-based telescopes established the existence but not the nature of Mars's atmosphere, polar caps, albedo markings, and satellites. The first spacecraft images of the planet were obtained by the flyby missions of the U.S. planetary probes Mariner 4 (1964) and 6 and 7 (1969); they showed a cratered surface, similar to that of the Moon, in the southern hemisphere. The first orbiter spacecraft was Mariner 9 (1971), which obtained photographs of much of the Martian surface. These images revealed a dramatic difference between the southern and northern hemispheres. The southern hemisphere is old and cratered, whereas the northern hemisphere is less cratered because most of the terrain has been buried by younger materials, probably of volcanic and wind-blown origin. During the late 1970s two U.S. Viking spacecraft photographed the Martian surface in greater detail. The Viking landing craft performed various experiments on the surface, including tests designed to detect the presence of life forms such as microbes, but these yielded no positive results. This computer-generated image is a simulated flight through a Martian canyon. The Martian landscape Portions of six Viking orbiter photographs forming a mosaic image of the Martian volcano Central Valles Marineris, one of many valleys on Mars. This false-colour picture is a composite of Among the surface features of the planet that were revealed by the Mariner 9 and Viking probes were volcanoes, extensive lava plains, various types of channels and canyons, and remnants of landslides. Many of these features are large by terrestrial standards. Olympus Mons, for example, is the largest known volcano in the solar system, having a diameter of 550 km (342 miles) and a height of roughly 27 km (17 miles). Other large volcanoes occur in the same region as Olympus Mons, on the 2,000-km- (1,200-mile-) long Tharsis rise, which is a region up to 10 km (6.2 miles) higher than the surrounding northern plains. The Valles Marineris, a gigantic equatorial rift more than 4,000 km (2,500 miles) long, comprises the equatorial canyon lands. This rift has been interpreted as the beginning of crustal plate separation that was not sustained. Although faulting is common on the Martian surface, there is no evidence that plate tectonics (crustal movement) has operated further than the early stages. This may be attributed to the great thickness of the Martian lithosphere (crust and upper mantle). Some of the craters on Mars resemble those found on Mercury and the Moon, but a new type of impact crater was discovered on the Martian surface. Structures of this kind are called rampart craters because the ejecta blanket (i.e., the material thrown out by a meteorite colliding with the surface) has raised edges. This characteristic indicates that after the ejecta was emplaced, it flowed outward and thus extended over a greater area than expected from ballistic considerations. Such action may be attributed to the presence of ground ice. The major types of channels observed on Mars are classified as runoff, outflow, and fretted channels. Runoff channels are dendritic networks similar to some terrestrial river valleys. Their morphology is consistent with erosion by some sort of liquid. The outflow channels are much larger, originate in closed depressions called box canyons, and decrease in size downstream. Their formation is not fully understood; current theories propose erosion by melting of ground ice, by solid ice, or by wind. Fretted channels are a network of wide valleys that have been modified by landslides along their walls. Wind is an important element on Mars. Wind-formed deposits such as dunes and crater streaks are common. The largest dune field so far detected forms an annulus around the north polar cap. Crater streaks, flame-shaped accumulations of material, build up in the protected areas to the lee of obstacles to the wind. These landforms may be craters or small isolated hills. The satellites of Mars, Phobos and Deimos, were photographed by the Mariner 9 and Viking spacecraft. Closeup photographic images show that they are irregular, cratered lumps of rock. Both moons may have originated from residual material yielded during the formation of Mars, or they may be asteroids that were captured by the planet's gravity early in its history. Sunlit half of Mars, as seen by the Mars Global Surveyor spacecraft. The dark areas indicate fourth planet in order of distance from the Sun and seventh in order of diminishing size and mass. It orbits the Sun once in 687 Earth days and spins on its axis once every 24 hours and 37 minutes. For additional related data, see the Table. Owing to its blood-red colour, Mars, designated in astronomy, has often been associated with gods of war. It is named for the Roman god of war; as far back as 3,000 years ago, Babylonian astronomer-astrologers called the planet Nergal for their god of death and pestilence. The Greeks called it Ares for their god of battle; the planet's two satellites, Phobos (Fear) and Deimos (Terror), were named for two of the sons of Ares and Aphrodite. Additional reading John Noble Wilford, Mars Beckons (1990), provides a general introduction. An illustrated summary of the results of the Mariner 9 orbiter mission is found in Mars as Viewed by Mariner 9, rev. ed. (1976); while a synthesis of views on the nature of the planet between Mariner 9 and Viking is developed in Thomas A. Mutch et al., The Geology of Mars (1976). Papers detailing the results of these two missions are found in Journal of Geophysical Research: on Mariner 9, vol. 78, no. 20, pp. 40074440 (July 10, 1973); and on Viking, vol. 82, no. 28, pp. 39594680 (Sept. 30, 1977). Henry S.F. Cooper, Jr., The Search for Life on Mars: Evolution of an Idea (1980), recounts the Viking expedition and the people involved in it. The geomorphology of the planet is addressed in Victor R. Baker, The Channels of Mars (1982), a well-illustrated discussion of how these features came about and their implications for the history of Mars; Michael H. Carr, The Surface of Mars (1981), a profusely illustrated survey of modern knowledge; and R.M. Batson and J.L. Inge (eds.), Atlas of Mars: The Viking Global Survey (1992). Duke B. Reiber (ed.), The NASA Mars Conference (1988), summarizes existing knowledge about the planet and addresses planned and future unmanned exploration as well as options for manned exploration. Eric Burgess, Return to the Red Planet (1990), reviews the past, present, and future Mars exploration program. Michael C. Malin