MARS

MARS

MARS, in astronomy, fourth planet from the sun in the solar system, named for the Roman god of war. It is the third in order of increasing mass. Mars has two small, heavily cratered moons, Phobos and Deimos, which some astronomers consider asteroidlike objects captured by the planet very early in its history. Phobos is about 21 km (about 13 mi) across; Deimos, only about 12 km (about 7.5 mi).

Appearance from Earth.

When viewed without a telescope, Mars is a reddish object of considerably varying brightness. At its closest approach to earth (55 million km/34 million mi), Mars is, after Venus, the brightest object in the night sky. Mars is best observed when it is at opposition (directly opposite the sun in earth’s sky) and also at its closest distance from earth. Such favorable circumstances repeat about every 15 years when the planet comes to perihelion (its closest approach to the sun) almost exactly at opposition.

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. (Because of the tilt of its axis and the eccentricity of its orbit, Mars has short, relatively warm southern summers and long, relatively cold southern winters.) The reddish color of the planet results from its heavily oxidized, or rusted, surface. The dark areas are thought to consist of rocks similar to terrestrial basalts, the surfaces of which have been weathered and oxidized. The brighter areas seem to consist of similar but even more weathered and oxidized material that apparently contains more fine, dust-size particles than do the dark regions. The mineral scapolite, relatively rare on earth, seems widespread; it may serve as a store for carbon dioxide (CO²) in the atmosphere.

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. Below this so-called polar hood, a thin cap of carbon dioxide frost is deposited during autumn and winter. By late winter, the cap may extend down to latitudes of 45°. By spring, and the end of the long polar night, 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. These remnant polar caps are believed to consist mostly of frozen water. The caps are 300 km (185 mi) wide at the south pole and 1000 km (620 mi) wide in the north. Although their true thickness is not known, they must contain frozen gases and water vapor to a thickness of possibly 2 km (1.3 mi).

In addition to the polar hoods—presumed to consist of clouds of frozen carbon dioxide—other clouds are common on the planet. High-altitude hazes and localized water ice clouds are observed. The latter result from the cooling associated with lifting air masses over elevated obstacles. Extensive yellow clouds, consisting of dust lifted by Martian winds, are especially prominent during southern summers.

Observation by Spacecraft.

The most detailed knowledge of Mars has come from missions carried out by automated U.S. spacecraft since 1964. The first views of Mars were obtained by Mariner 4 in 1964, and 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 of the planet 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 1992, the U.S. launched Mars Observer; the first U.S. mission since the Viking program, it failed to reach its Mars orbit. Four years later, the U.S. inaugurated a 10-year program of Mars exploration with two automated spacecraft missions.

Mars Global Surveyor, launched in November 1996, reached Mars orbit in September, the following year; in addition to carrying out many of the studies of the failed Mars Observer, it will begin mapping the planet in 1999.

Mars Pathfinder, launched in December 1996, landed on Mars in July 1997 and deployed Sojourner, a 22-pound roving vehicle designed to photograph and analyze Martian rocks. The first spacecraft to land on Mars in 20 years (and the first ever without orbiting prior to landing), Mars Pathfinder, together with Sojourner, provided unprecedented details on the climate, atmosphere, and geology of the planet, including panoramic three-dimensional photographs of the planet’s terrain. Radio contact was lost in October; the site of the spacecraft on Mars was named Carl Sagan Memorial Station in honor of the American astronomer who had died in December 1996.

In 1988 the Soviet Union sent two probes to land on the moon Phobos; both missions failed, although one relayed back some data and photographs before being lost to radio contact. Developed in collaboration with several European nations, the Russian craft Mars 96, launched in November 1996, spun out of control and fell back to earth the next day.

Atmosphere.

The Martian atmosphere consists of carbon dioxide (95 percent), nitrogen (2.7 percent), argon (1.6 percent), oxygen (0.2 percent), and trace amounts of water vapor, carbon monoxide, and noble gases other than argon. The average atmospheric pressure at the surface is close to 4.6 torrs, which is 0.6 percent that on earth and equal to the pressure at a height of 35 km (22 mi) in earth’s atmosphere. Surface temperatures vary greatly with time of day, season, and latitude. Maximum summer temperatures may reach 290 K (63° F), but average daily temperatures at the surface do not exceed 240 K (–27° F). Due to the thinness of the atmosphere, daily temperature variations of 100° C (180° F) are common. Poleward of about 50° lat, temperatures remain cold enough (less than 150 K/–189° F) throughout winter for the atmosphere’s major constituent, carbon dioxide, to freeze into the white deposits that make up the polar caps. The total atmospheric pressure on the surface fluctuates by about 30 percent due to the seasonal cycle of the polar caps.

The amount of water vapor present in the atmosphere is extremely slight and variable. The concentration of atmospheric water vapor is highest near the edges of the receding polar caps in spring. Mars is like a very cold, high-altitude desert. Surface temperatures are too cold and surface pressures too low for water to exist in the liquid state in most places on the planet. It has been suggested, however, that liquid water may exist just below the surface in a few localities.

At certain seasons, some areas on Mars are subject to winds strong enough to move sand on the surface and to suspend dust in the atmosphere. A major weather event occurs in the southern hemisphere between late spring and early summer when Mars is near perihelion and the heating of southern equatorial latitudes is most intense. Dust storms begin to form, and some reach global proportions, obscuring the planet’s surface for weeks or even months. The dust entrained in these clouds is very fine and takes a long time to settle.

Surface and Interior.

The Martian surface can be divided into two approximately hemispherical provinces by a great circle inclined at about 30° to the equator. The southern half consists of ancient cratered terrain dating from the planet’s earliest history, when Mars and the other planets were subjected to a much more intense meteoroidal bombardment that is the case today. Considerable erosion and filling of even the largest craters have occurred since then.

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. Some of the solar system’s largest volcanoes occur in Tharsis. Olympic Mons, a structure showing all the characteristics of a basaltic volcano, reaches an elevation of more than 25 km (15.5 mi) and measures more than 600 km (370 mi) across its base. No definite evidence exists of current volcanic activity anywhere on the planet. In 1997, the first-ever chemical analysis of a rock carried out on the surface of Mars was conducted by Sojourner; the data suggested that the rock contained large amounts of the minerals quartz, feldspar, and orthopyroxene, all common on earth; the rock was similar in composition to andesite, a terrestrial volcanic rock. This was the first evidence of quartz in extraterrestrial material, and its presence suggested that Mars may have undergone a cyclical process of heating and cooling comparable to that which led to the formation of quartz on earth. Data from soil samples taken by Sojourner concurred with the analysis of soil samples taken from two other locations during the 1976 Viking mission.

Faults and other features suggestive of crustal fracture due to local bulging and expansion are widespread on Mars. On the other hand, no features due to large-scale compression have been found. Specifically, folded mountain belts, so common on earth, are lacking, indicating an absence of plate tectonics. This suggests, in turn, that Mars may have a thicker crust and a cooler thermal history than earth. An escarpment near the Martian equator that was studied in 1988, however, may prove to be a strike-slip fault, which would indicate some plate-tectonic activity, after all.

Evidence of subsurface ice prevails, especially in the form of petal-shaped ejecta blankets around some craters, vast areas of collapsed chaotic terrain, and so-called patterned ground at high northern latitudes. By far the most spectacular geologic discovery has been the channels that superficially resemble the valleys of dried-up rivers. Two major types are known. Large outflow channels may have been formed by the sudden catastrophic release of vast amounts of liquid water from areas of collapsed chaotic terrain. Most of these channels drain from the higher southern hemisphere to the generally lower northern hemisphere. The cause of the localized melting of the ground ice in the source areas remains uncertain, but these features probably date from the first third of the planet’s 4.6-billion-year history. In addition to the large outflow channels, there are numerous small channellike features for which evidence of erosion by liquid water is less compelling, but possible. Because liquid water cannot exist on the surface of the planet today, the channels have been singled out as proof that Mars had higher pressures and warmer temperatures in the past. Pictures sent back to earth by Mars Pathfinder from the region known as Ares Vallis appeared to support the theory that the terrain in the area, some of the oldest on Mars, had been overrun by a massive flood one to three billion years ago.

Today, however, Mars is a windblown desert. Vast expanses of sand dunes and other wind-formed erosional features abound, all attesting to the efficacy of both depositional and erosional wind processes present in the current Mars environment.

Little is known about the interior of Mars. The planet’s relatively low mean density indicates that Mars cannot have an extensive metallic core. Furthermore, any core that may be present is probably not fluid, although instruments on Mars Global Surveyor reported that Mars has a weak, but stronger than expected, magnetic field. Judging from its ability to support such massive topological features as Tharsis, the crust of Mars may be as thick as 200 km (125 mi)—five or six times as thick as earth’s crust. A seismometer on board Viking 2 lander failed to detect any definite “Marsquakes.”

The Search for Life.

The idea that life can or even does exist on Mars has a long history. In 1877 the Italian astronomer Giovanni Schiaparelli claimed to have seen a planetwide system of channels. The American astronomer Percival Lowell then popularized these faint lines as canals and held them out as proof of a vast attempt by intelligent beings to irrigate an arid planet. Subsequent spacecraft observations have shown that there are no canals on the planet, and various other alleged proofs of life on Mars have turned out to be equally illusory. Not only are there no canals, but dark areas once thought to be oases are not green, and their spectra contain no evidence of organic materials. The seasonal changes in the appearance of these areas are not due to any vegetative cycle, but to seasonal Martian winds blowing sterile sand and dust. Water probably occurs only as ice on or below the surface or as trace amounts of vapor or ice crystals in the atmosphere. The strongest evidence against the presence of life on Mars, however, is the thinness of the atmosphere and the fact that the surface of the planet is exposed not only to lethal doses of ultraviolet radiation but also to the effects of highly oxidizing substances (such as hydrogen peroxide) produced by photochemistry.

Perhaps the most fundamental and far-reaching information obtained by the Viking landers and Sojourner is that the soil contains no organic material. Although small amounts of organic molecules are continually being supplied to the surface of Mars by carbonaceous meteoroids, apparently this material is destroyed before it has a chance to accumulate. The results of the soil analysis for organic molecules carried out by the Viking landers and Sojourner provide no evidence for the existence of life.

A more difficult question is whether life ever existed on Mars, given the strong evidence of climatic change and the indications of a previously warmer, thicker atmosphere. In August 1996 chemical evidence suggesting bacteria-size organisms was reported in a rock fragment of a meteorite from Mars that had been recovered from the Allan Hills region in Antarctica in 1984. The studies suggested that the 4.5 billion-year-old, 4-pound meteorite harbors evidence that bacteria existed on Mars 3.6 billion years ago, a conclusion that remains controversial.