Mars, the red planet, is the fourth planet from the Sun; it is named for the Roman god of war. Easily distinguished in the night sky by its reddish appearance, it is 1.5 times farther from the Sun than the Earth and only about half as large as the Earth. More than any other planet in the solar system, Mars has characteristics that make it an Earth-like world. Its period of rotation and the inclination of its axis are similar to Earth's. Its density indicates that it is made of rocky materials, although with proportionately less iron and more lightweight elements and volatiles than the Earth. Its atmosphere is thin enough to allow observation of the surface. Only the Earth and Mars among the terrestrial planets have satellites; the two very tiny bodies orbiting Mars were discovered in 1877 by Asaph Hall and named \TPhobos\t and \TDeimos\t. CHARACTERISTICS The rotation period of Mars is only 37 minutes longer than the Earth's, a fact already determined in the late 1600s by watching the motion of surface features. The rotation axis is tilted with respect to the orbital plane by almost 24 deg, so that both planets experience significant seasonal differences in the amount of sunlight falling on a given hemisphere during the year. The difference between winter and summer is more extreme on Mars, due to the greater eccentricity of the martian orbit. The distance of Mars from the Sun ranges from 206.7 million km (128.4 million mi) at perihelion to 249.1 million km (154.7 million mi) at aphelion, a difference of 42.4 million km (26.3 million mi). By contrast, the Earth's distance from the Sun changes by only about 2.5 million km (1.6 million mi). The red planet therefore receives about 40 percent more sunlight during southern summer, when nearest the Sun, than during southern winter, when the Sun is most distant. These conditions produce relatively hot southern summers and mild northern winters but cool northern summers and cold southern winters. Surface Appearance Mars is a small planet and even at most favorable times is never closer to the Earth than 56 million km (35 million mi). This makes it difficult to observe details on the surface. Resolving surface features is made even more of a problem by the blurring effects of two atmospheres. One of the best examples of the resulting confusion is the case of the martian canals, first reportedly observed by the Italian priest Pietro Secchi in 1876. Giovanni Schiaparelli published a map of Mars in 1877 on which he assigned names to the recognized bright and dark features, but which also included a large number of straight linear features that he and Secchi called canali. The mistranslation of this term into "canal" instead of "channel" by English-speaking countries carried a totally misleading connotation of artificial construction that was never intended by the two observers. The view that canals were the work of a heroic, intelligent race trying to tap the melting polar ice for water to irrigate their equatorial crops was persuasively championed by Percival Lowell in his book of 1895. Not all observers agreed with the canal theory, but the idea that Mars could and probably did support some kind of life became established, with little scientific evidence, and persisted until the middle of the 20th century. Changes take place in the surface features of Mars as the seasons change. The most obvious is the rapid spring shrinking of the polar cap from its large winter size, when it may reach 45 deg latitude. The polar cap grows back as summer turns to fall, reaching full size shortly before winter begins. The darker areas of Mars, call maria, sometimes respond to seasonal change, becoming darker as the polar cap retreats during spring. This occurrence was originally thought to represent vegetation growing, as water from the melting polar cap becomes available. More recent studies indicate instead that the spring darkening is due to winds blowing fine bright dust off darker rocks. Telescopic observers have long known that dust blows about the martian surface; dust storms have been observed to grow out of the orange deserts and spread over the landscape, occasionally covering the entire planet in a featureless red haze. Atmosphere Telescopic measurements of the spectrum of Mars showed a thin atmosphere composed mostly of carbon dioxide. As early as 1940 it was known that oxygen and water vapor were extremely rare, but the true composition of the atmosphere, as with the true nature of the martian surface, became well known only when spacecraft began to explore the planet at close range. Nitrogen, argon, and small traces of oxygen and water vapor were found, and an atmospheric pressure of 7.5 millibars, compared to 1,000 millibars on Earth at sea level. Flyby Results \TMariner\t 4 obtained the first close look at Mars in 1965. The carbon dioxide atmosphere was found to be exceptionally thin, only 1/100 that at sea level on the Earth. No magnetic field was detected. Twenty-two fuzzy pictures showed plains covered with large, flat-bottomed craters. In the 1% of the surface examined, no evidence of life, canals, or recent geologic activity was detected. Mars seemed more like the Moon than like the Earth. Two more flyby spacecraft carrying high-resolution television cameras refined this picture in 1969. Cratered plains were again observed, the craters themselves being highly eroded; the worn-down rims and shallow, filled-in floors were evidence of the abrasive action of wind-blown dust. Because the atmosphere of Mars is so thin, wind velocities up to several hundred kilometers per hour are required to raise the dust particles during a dust storm, and these fast-moving particles erode structures with a sand-blasting effect. Featureless terrain was also observed as regions devoid of any surface detail, again suggesting ongoing erosion. Areas of jumbled blocks, perhaps resulting from subsurface collapse, were named chaotic terrain. The south polar cap was photographed during early spring, revealing frost-covered crater rims poking through the thicker deposits of frozen carbon dioxide. Temperatures over the ice fields were 160 to 170 K (about -170 F). Orbiter Results A new era of exploration of the planet Mars began when orbiter spacecraft were sent there, first in 1971 (Mariner 9) and again in 1976 (Viking 1 and 2). The advantage of an orbiter is the ability to map nearly all of the planet's surface, and to be able to do so repeatedly in order to look for seasonal and long-term changes in surface features. When 100% of the surface of Mars came under examination, it became clear that in many respects Mars was similar to both the Earth and the Moon. Confronted by a global dust storm upon its arrival at Mars in 1971, Mariner 9 had to wait three months to send back to Earth pictures of one of the important discoveries. Poking through the top of the slowly settling dust were the tops of four immense volcanic mountains. The largest of these shield volcanoes, now called Olympus Mons (Mount Olympus), dwarfs the largest such feature on the Earth, the largest of the Hawaiian Islands. Shield volcanoes do not exist on the Moon; on the Earth and Mars they are produced by the repeated outpourings of fluid molten rock that, given enough time, slowly build into a huge mountain. Olympus Mons is over 500 km (310 mi) across at the base and stands 24 km (15 mi) above a smooth plain. By contrast, the largest of the Hawaiian Islands is 120 km (75 mi) across and 9 km (5.6 mi) above the ocean floor. The reason for this difference is that the plate motion of the Earth's surface produces a string of volcanoes along the ocean floor, while the lack of plate motion on Mars piles up the volcanic material in one place, producing a single immense volcano. Four huge shield volcanoes have been observed on Mars, plus a great number of smaller ones like those found on the Earth. Most of the major volcanic structures are located in two particular regions of Mars, where the crust has swelled upward. These crustal uplifts are also found on the Earth, but not on the Moon. Near the largest of the crustal swells, where the four extremely large shield volcanoes are located, is an immense crack in the crust of Mars. This rift valley is a series of canyons, each several hundred km long and up to 100 km (62 mi) wide, that spans a distance of 5,000 km (3,100 mi). Placed in the United States, it would stretch from San Diego, Calif., to Boston. Such rift valleys are well known on the Earth. In Africa they represent the breakup of that continent into two separate plates. Rifting is the beginning of plate formation, the first step of plate tectonics. It is clear that this process began on Mars, but that Mars cooled too much to have actively moving plates such as cause earthquakes and major volcanic eruptions on the Earth. Minor "Marsquakes" and some volcanic eruptions may still occur occasionally, but geologically Mars is a dying world. Despite these Earth-like geologic structures, most of Mars looks like the Moon. Elevated cratered plains dot the southern hemisphere, like the highlands of the Moon. The lower-lying northern hemisphere has smooth plains that seem to be similar to the dark volcanic maria of the Moon. About 80% of the martian surface indicates a history similar to that which ended 3 billion years ago on the Moon. In a few areas Mars began to experience more Earth-like geologic processes that modified the lunarlike surface. Other processes have modified the martian surface. Several hundred long, winding channels exist on Mars with characteristics similar to those of dried-up river channels. The structure of these channels makes them seem like those stream beds cut by sudden runoff in desert regions on the Earth. Many observers regard the presence of these channels as strong evidence for liquid water erosion, since lava and glacial erosion produce very different channels. If true, these channels are also evidence that the martian climate has changed drastically. No liquid water exists on Mars today and, in fact, cannot. The low atmospheric pressure would cause any subsurface water to boil into a vapor upon reaching the surface. Once in the atmosphere, it would tend to circulate to the poles and freeze out. The channels suggest a previous epoch of much thicker atmosphere, which, in turn, would mean generally warmer, and perhaps wetter, climatic conditions. Crater counts on the river channels suggest that they are several hundred million years old. There is good evidence that the water once responsible for carving channels on the surface may now be locked in residual polar caps buried under the two large carbon dioxide frost caps. The amount of trapped water, added to that which is probably frozen into the martian soil, might form an ocean in the low-lying northern hemisphere several kilometers deep if melted. The polar regions also provide further evidence for climate change on Mars. Under the polar ice is the polar layered terrain, revealed where erosion has cut steep valleys or exposed a sloping ridge. Individual layers of alternating bright and dark material appear with thicknesses as little as 50 m (165 ft). The alternating pattern suggests variations in the amounts of dust and ice deposited over long time spans, and implies that the variation was periodic, perhaps in response to slow changes in the polar climate. Viking Lander Results The first detailed measurements of the surface and atmospheric conditions of the red planet began on July 20, 1976, when the Viking 1 lander touched down on the rocky desert called Chryse Planitia ("plains of gold"), seven years to the day following the first Apollo landing on the Moon. During and after descent, the atmospheric composition and structure were measured. The atmosphere of Mars was found to consist of 95% carbon dioxide, 2 to 3% nitrogen, 1 to 2% argon, and tiny traces of oxygen and water vapor. Ozone to screen out ultraviolet light is almost nonexistent. Water vapor is almost totally absent near the winter polar cap, but in the summer hemisphere, where both Viking 1 and 2 landed, it exists in amounts about 5/100 of that found in the driest parts of the Earth. The atmospheric pressure at Chryse (which is a basin some 2 km/1.2 mi below an imaginary reference "sea level" on Mars) was about 7.5 millibars (sea-level pressure on Earth is about 1,000 millibars). Detailed analysis of the atmosphere provided evidence that in the past the atmospheric pressure was probably higher, supporting the conclusions derived from the observations of dried-up river channels. Temperatures at the Viking landing sites depend on the time of day. After dark the Viking 1 lander chilled to 187 K (-123 deg F), but warmed during the day to 244 K (-20 deg F). Soil temperatures are higher than those in the thin martian atmosphere near ground and rise above freezing during the summer months. By contrast, polar temperatures are, during winter, 146 K (-197 deg F). The more northerly Viking 2 recorded low temperatures in this range during the winter months. Winds are surprisingly gentle, 6 to 8 mph with occasional gusts of 30 to 40 mph. Surrounding the Viking 1 lander is a dusty reddish plain littered with both dark and reddish rocks, including a 1X3-m (3X10-ft) boulder. Drifting soil piles up near the rocks and forms a dune field not far from the lander. Much to the surprise of scientists, the martian sky is not deep, dark blue but a light pinkish-grey, presumably due to fine dust particles suspended in the atmosphere. Viking 1 continued to function until Nov. 13, 1982, when contact was lost. Viking 2, which had touched down on Mars on Sept. 3, 1976, ceased operating much earlier, on Apr. 2, 1980. Dust was less common at the Viking 2 landing site, also a rock-strewn plain, located 7,500 km (4,660 mi) from the Chryse site. Named Utopia, the region is much farther north (latitude 48 deg compared to the 22.5 degN latitude of Viking 1), and had at the time of landing nearly three times as much water vapor in the air as was observed over Chryse. Rocks are abundant at Utopia; most show the same reddish stain seen at the Viking 1 site, due to the presence of iron oxide. The soil of Mars where the Vikings landed is similar to basaltic lava but heavily enriched in iron and depleted in aluminum. The iron is in a highly oxidized state; results of analysis by the chemical and biological experiments suggest the presence of superoxides, peroxides, and ozonides. These unusual compounds, which contribute to the strong coloration of the martian deserts, are probably formed in the presence of a small amount of water vapor under the action of the ultraviolet light that reaches the surface of Mars. Remarkably little seismic noise was recorded on seismometers, indicating that Mars is less active than the Earth. Search for Life A major portion of the Viking effort was directed at searching for life processes based on a carbon biochemistry. Although the soil chemistry of Mars was found to be unusual and highly active, no conclusive evidence of biological activity has been found. The initial results of an experiment based on photosynthesis, for example, indicated that the martian soil is capable of breaking down carbon dioxide and making carbon compounds, as do green plants on Earth. A control sample, sterilized at high temperatures, showed almost no response. When the original experiment was repeated in the presence of water, which should have enhanced a biological process on a dry planet, the response again went to zero. Other experiments showed strong chemical reactions to nutrients used, and a test for organic compounds in the soil proved negative. The question of life on Mars, so important at the beginning of the century, remains open. Only a truly positive unambiguous result is meaningful. The inability of Viking to detect life may mean there is no life on Mars, or that the experiments were designed incorrectly. Mars remains the best candidate for life in the solar system outside of the Earth. Further searching will be required to resolve the conflicting results of the Vikings. A number of international programs are planned for placing probes on Mars during the 1990s, possibly followed by a manned mission to the planet in the early 21st century. Herbert Frey Bibliography: Arridson, R. E., et al., "Three Mars Years: Viking Lander 1 Imaging Observations," Science, Nov. 4, 1983; Baker, V. R., The Channels of Mars (1982); Carr, Michael, The Surface of Mars (1984); Chandler, David, Life on Mars (1979); Ezell, E. C. and L. N., On Mars (1984); Hartmann, W. K., "What's New on Mars?" Sky and Telescope, May 1989; Hoyt, W. G., Lowell and Mars (1977); Miles, Frank, and Booth, Nicholas, Race to Mars (1988); Schultz, Peter H., "Polar Wandering on Mars," Scientific American, December 1985; Wilford, John Noble, Mars Beckons: The Mysteries, the Challenges, the Expectations of Our Next Great Adventure in Space (1990). See also: \Tsolar system\t.