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.

fourth planet in the solar system in order of distance from the Sun and seventh in size and mass. It is a conspicuous, sometimes quite bright, reddish object in the night sky. Mars is designated by the symbol ♂.

Planetary data for Mars Planetary data for MarsBlood-red in colour and sometimes called the Red Planet, Mars has long been associated with warfare and slaughter. It is named for the Roman god of war. As long as 3,000 years ago, Babylonian astronomer-astrologers called the planet Nergal for their god of death and pestilence. The planet's two moons, Phobos (Greek: “Fear”) and Deimos (“Terror”), were named for two of the sons of Ares and Aphrodite (the counterparts of Mars and Venus, respectively, in Greek mythology).

In recent times, Mars has intrigued people for more substantial reasons than its baleful appearance. The planet is the second closest to Earth, after Venus, and it is usually easy to observe in the night sky because its orbit lies outside Earth's. It is also the only planet whose solid surface and atmospheric phenomena can be seen in telescopes from Earth. Centuries of assiduous studies by earthbound observers, which have been extended by spacecraft observations since the 1960s, have revealed that Mars is similar to Earth in many ways. Like Earth, Mars has clouds, winds, a roughly 24-hour day, seasonal weather patterns, polar ice caps, volcanoes, canyons, and other familiar features. There are intriguing clues that billions of years ago Mars was even more Earth-like than today, with a denser, warmer atmosphere and much more water—rivers, lakes, flood channels, and perhaps oceans. By all indications, Mars is now a sterile frozen desert, but close-up images from the Mars Global Surveyor spacecraft of seemingly water-eroded gullies suggest that at least small amounts of water may have flowed on or near the planet's surface in geologically recent times and may still exist as a liquid in protected areas below the surface. The presence of water on Mars is considered a critical issue because, without water, life as it is presently understood cannot exist. If microscopic life-forms ever did originate on Mars, there remains a chance, albeit a remote one, that they may yet survive in these hidden watery niches. In 1996 a team of scientists reported what they concluded to be evidence for ancient microbial life in a piece of meteorite that had come from Mars, but most scientists have disputed their interpretation.

Mars spins on its axis once every 24 hours 37 minutes, making a day on Mars only a little longer than an Earth day. Its axis of rotation is inclined to its orbital plane by about 25°, and, as for Earth, the tilt gives rise to seasons (season) on Mars. (See the diagram-->.) The Martian year consists of 668.6 Martian solar days, called sols. As a result, southern summers (summer) are shorter (154 Martian days) and warmer than those in the north (178 Martian days). The situation, however, is slowly changing such that 25,000 years from now the northern summers will be the shorter and warmer ones. In addition, the tilt of the axis is slowly changing on a roughly one-million-year timescale. During some epochs the tilt is close to zero, and so Mars has no seasons; in other epochs the tilt may be as high as 35°, resulting in extreme seasonal differences.

Planetary data for MarsMars is a small planet, larger than only Mercury and slightly more than half the size of Earth. It has an equatorial radius of 3,396 km (2,110 miles) and a mean polar radius of 3,379 km (2,100 miles), both values accurately determined by the Mars Global Surveyor spacecraft, which began its primary mission in orbit around the planet in 1999. The mass of Mars is only one-tenth the terrestrial value, and its gravitational acceleration of 3.72 metres (12.2 feet) per second per second at the surface means that objects on Mars weigh a little more than a third of their weight on Earth's surface. Mars has only 28 percent of the surface area of Earth, but, because about three-fourths of Earth is covered by water, the land areas of the two planets are comparable. For additional orbital and physical data, see the table (Planetary data for Mars).

To the Earth-based telescopic observer, the Martian surface outside the polar caps is characterized by red-ochre-coloured bright areas on which dark markings appear superimposed. In the past, the bright areas were referred to as deserts, and the majority of large dark areas were originally called maria (Latin: “oceans” or “seas”; singular mare) in the belief that they were covered by expanses of water.

Images from spacecraft in Mars's orbit have documented a variety of low-lying clouds and fogs that are often confined within topographic depressions—i.e., valleys or craters. They have also revealed high, thin clouds, particularly at the morning terminator (the dividing line between the lit and unlit portions of the planet's disk). Orographic clouds, produced when moist air is lifted over elevated terrain and cooled, form around prominent topographic features such as craters and volcanoes, and winter at midlatitudes is characterized by westward-moving, spiral-shaped storm systems, similar to those on Earth. Most of these clouds are composed of water ice—the white clouds seen by the early observers.

Although too small to be observed from Earth, dust devils (dust devil) (see whirlwind) have been seen from Mars orbit and at the landing site of the Mars Pathfinder spacecraft. Narrow tracks, thought to be caused by dust devils, are also visible in high-resolution images taken from orbit by Mars Global Surveyor.

mars, composition of atmosphereDirect chemical analysis at the surface by the Viking landers and spectral observations from orbiting spacecraft have allowed scientists to determine the composition of the Martian atmosphere to high precision. Where the atmosphere is well mixed by turbulence—below an altitude of 125 km (about 80 miles)—95.3 percent of the atmosphere by weight is carbon dioxide (see the table (mars, composition of atmosphere)). This is a comparatively large amount—nine times the quantity now in Earth's much more massive atmosphere. Much of Earth's carbon dioxide, however, is chemically locked in sedimentary rocks; the amount in the Martian atmosphere is less than a thousandth of the terrestrial total. The balance of the Martian atmosphere consists of molecular nitrogen, water vapour, and noble gases (argon, neon, krypton, and xenon). There are also trace amounts of gases that have been produced from the primary constituents by photochemical reactions (photochemical reaction), generally high in the atmosphere; these include molecular oxygen, carbon monoxide, nitric oxide, and small amounts of ozone.

Although water is only a minor constituent of the Martian atmosphere (a few molecules per 10,000 at most), primarily because of low atmospheric and surface temperatures, it plays an important role in atmospheric chemistry and meteorology. The Martian atmosphere is effectively saturated with water vapour, yet there is no liquid water present on the surface. The temperature and pressure of the planet are so low that water molecules can exist only as ice or as vapour. Little water is exchanged daily with the surface despite the very cold nighttime surface temperatures.

Most of the information about atmospheric water on Mars has come from the Viking orbiters, which observed seasonal patterns of water content in the atmosphere over a full Martian year. Water vapour is mixed uniformly up to altitudes of 10–15 km (6–9 miles) and shows strong latitudinal gradients that depend on the season. The largest changes occur in the northern hemisphere. During summer in the north, the complete disappearance of the carbon dioxide cap leaves behind a water-ice cap. Sublimation of water from the residual cap results in a strong north-to-south concentration gradient of water vapour in the atmosphere. In the south, where a small carbon dioxide cap remains in summer and only a small amount of water ice has been detected, a strong water vapour gradient does not normally develop in the atmosphere.

The only direct measurements of wind speeds were made by the Viking and Pathfinder landers. Near-surface winds at these sites were usually regular in behaviour and generally light. Average speeds were typically less than 2 metres per second (4.5 miles per hour), although gusts up to 40 metres per second (90 miles per hour) were recorded. Other observations, including streaks of windblown dust and patterns in dune fields and in the many varieties of clouds, have provided additional clues about surface winds.

The seasonal behaviour of the Martian polar caps is well-documented. Early each southern spring, the southern carbon dioxide cap starts to recede from its maximum extent of 50° S. As spring progresses, the cap shrinks by as much as 1° of latitude every five days. The edge of the cap becomes ragged, controlled by local topography (e.g., craters), and eventually it breaks into well-defined fragments. From year to year, the location of the fragments remains about the same, although there are small variations in detail. Over the course of one-third of the Martian year, the cap recedes to its smallest extent; it is then not usually visible from Earth, but spacecraft observations have confirmed the presence of a small remnant throughout the summer.

The composition of the summer remnant caps, particularly the southern one, remains somewhat less certain despite considerable data on their thermal and radiative properties. Measurements by the Viking orbiters showed that the ice of the northern remnant is unquestionably frozen water. Added to this evidence is the large increase in the amount of water vapour detected in the atmosphere over the summer cap. The northern remnant cap, in fact, represents the largest known reservoir of available water on the planet. At the south pole, the carbon dioxide cap does not completely disappear in summer. A small remnant, consisting mostly of carbon dioxide but containing as much as 10 percent water, remains. Measurements by the Mars Express orbiter in 2004 revealed that water ice also is present just below the surface over wide areas around the remnant cap. Almost no water vapour is normally observed in the atmosphere above the southern remnant cap.

The character of the Martian terrain has been well established from spacecraft photography and altimetry. The entire planet was photographed by the Viking orbiters at a resolution of roughly 250 metres (820 feet) and selected areas at resolutions down to 10 metres (33 feet). Subsequently the camera on the Mars Global Surveyor spacecraft photographed selected areas with resolutions of 1.4 metres (4.6 feet), but it covered only a small fraction of the planet. The topography of the Martian surface was determined very accurately with the laser altimeter aboard Mars Global Surveyor, which mapped elevations with a vertical resolution of a few metres. (See the map-->.)

The southern terrain possesses several distinctive types of craters—huge impact basins; large, partially filled craters with shallow, flat floors and eroded rims; smaller, fresh-looking bowl-shaped craters like those on the Moon; and rampart and pedestal craters. Hellas is the largest impact basin on Mars. According to Mars Global Surveyor altimetry data, the feature is about 7,000 km (4,400 miles) across, including the broad elevated ring surrounding the depression,and 8 km (5 miles) deep—much larger than previously thought. Most of the craters measuring tens to hundreds of kilometres across are highly eroded. Because larger craters tend to be older than smaller ones, erosion rates on early Mars appear to have been much higher than subsequently. It is one piece of evidence that the climate on early Mars was very different from what it was for most of the planet's subsequent history.

Rampart and pedestal craters may be unique to Mars. A rampart crater is so named because the lobes of ejecta—the material thrown out from the crater and extending around it—are bordered with a low ridge, or rampart. The ejecta thus apparently flowed across the ground, which may indicate that it had a mudlike consistency. Some scientists have conjectured that the mud formed from a mixture of impact debris and water that was present under the surface. Around a pedestal crater, the ejected material forms a steep-sided platform, or pedestal, with the crater situated inside its border. The pedestal appears to have developed when wind carved away the surface layer of the surrounding region while leaving intact that portion protected by the overlying ejecta.

High-resolution Viking images revealed an additional characteristic of the ancient southern terrain—the pervasive presence of networks of small valleys that resemble terrestrial drainage systems created by flowing water. Examples include Nirgal Vallis, located in the southern hemisphere north of the Argyre impact basin, and Nanedi Vallis, located just north of the equator near the east end of Valles Marineris. Scientists have proposed two alternative mechanisms for their formation, either the runoff of rainfall on the surface or erosion by the outflow of groundwater that seeped onto the surface. In either case, warm climatic conditions may have been required for their formation. A major surprise of the Mars Global Surveyor mission was observation of small, fresh-appearing gullies on steep slopes at high latitudes. These features strongly resemble water-worn gullies in Earth's desert regions, but their origin remains controversial. Although the discoverers initially proposed water erosion as the cause, this was challenged by other researchers.

Large flood channels, termed outflow channels, are observed incised into the Martian surface in several areas. The channels are generally tens of kilometres across and hundreds of kilometres long. Most emerge full-size from rubble-filled depressions and continue downslope into the northern plains or the Hellas basin (Hellas) in the south. Many of the largest drain from the south and west into Chryse Planitia. These are true channels in that they were once completely filled with flowing water, as opposed to most river valleys, which have never been close to full but contain a much smaller river channel. The peak discharges of the floods that cut the channels are estimated to have been a hundred to a thousand times the peak discharge of the Mississippi River—truly enormous events. Some of the floods appear to have formed by catastrophic release of water from lakes. Others formed by explosive eruption of groundwater.

Close to the equator, centred on 70° W longitude, are several interconnected canyons collectively called Valles Marineris. Individual canyons are roughly 200 km (125 miles) across. At the centre of the system, several canyons merge to form a depression 600 km (375 miles) across and as much as 9 km (5.6 miles) deep—about five times the depth of the Grand Canyon. The entire system is more than 4,000 km (2,500 miles) in length, or about 20 percent of Mars's circumference. At several places within the canyons are thick sedimentary sequences, which suggest that lakes may have formerly occupied the canyons. Some of the lakes may have drained catastrophically to the east to form large outflow channels that start at the canyons' eastern end. How the canyons formed is not known, but faulting probably played a major role.

The canyons of Valles Marineris terminate to the west near the crest of the Tharsis rise, a vast bulge on the Martian surface more than 8,000 km (5,000 miles) across and 8 km (5 miles) high at its centre. Near the top of the rise are three of the planet's largest volcanoes (volcano)—Ascraeus Mons, Arsia Mons, and Pavonis Mons—which tower 18, 17, and 14 km (11.2, 10.5, and 8.7 miles), respectively, above the mean radius. Just off the rise to the northwest is the planet's tallest volcano, Olympus Mons, and at the north end is yet another large volcano, Alba Patera, which approaches 7 km (4.3 miles) in height. Between these giant landforms are several smaller volcanoes and lava plains. What formed Tharsis is not known; it may have resulted from a combination of uplift and the accumulation of huge volumes of volcanic deposits.

The presence of the Tharsis rise has caused stresses within, and deformation of, the crust. A vast system of fractures radiating from Tharsis and compressional ridges arrayed around the rise are evidence of this process. The radial faulting around Tharsis appears to have contributed to the formation of the Valles Marineris system.

Little was learned about the two moons of Mars, Phobos and Deimos, after their discovery in 1877 until orbiting spacecraft observed them a century later. Viking 1 flew to within 100 km (60 miles) of Phobos and Viking 2 to within 30 km (20 miles) of Deimos.

Moons of MarsBoth moons are irregular chunks of rock, roughly ellipsoidal in shape. Phobos is the larger of the two (see the table (Moons of Mars)). Phobos's rugged surface is totally covered with impact craters. The largest, the crater Stickney, is about half as wide as the satellite itself. Its surface also exhibits a widespread system of linear fractures, or grooves, many of which are geometrically related to Stickney. In contrast, the surface of Deimos appears smooth, as its many craters are almost completely buried by fine debris, and it shows no fracture system. The difference in appearance between the two moons is thought to be related to the final disposition of the debris produced by impacts. In the case of the inner, more massive Phobos, the ejected material either fell back to the surface or, if it left the satellite with enough velocity to go into space, subsequently fell on Mars. For the more distant, smaller Deimos, debris thrown off the satellite remained in orbit until it was recaptured, sifting down to blanket its surface.

Since the beginning of the space age, Mars has been a focus of planetary exploration for three main reasons: (1) it is the most Earth-like of the planets; (2) other than Earth, it is the planet most likely to have developed indigenous life; and (3) it will probably be the first extraterrestrial planet to be visited by humans. Between 1960 and 1980 the exploration of Mars was a major objective of both the U.S. (United States) and Soviet (Union of Soviet Socialist Republics) space programs. U.S. spacecraft successfully flew by Mars ( Mariners 4, 6, and 7), orbited the planet (Mariner 9 and Vikings 1 and 2), and placed lander modules on its surface ( Vikings 1 and 2). Three Soviet probes (Mars 2, 3, and 5) also investigated Mars, two of them reaching its surface. Mars 3 was the first spacecraft to soft-land an instrumented capsule on the planet, on December 2, 1971; landing during a planetwide dust storm, the device returned data for about 20 seconds.

Amid failures of several U.S. spacecraft missions to Mars in the 1990s, Mars Pathfinder successfully set down in Chryse Planitia (19° N, 33° W) on July 4, 1997, and deployed a robotic wheeled rover called Sojourner on the surface. This was followed by Mars Global Surveyor, which reached Mars in September 1997 and systematically mapped various properties of the planet from orbit for several years beginning in March 1999. These included Mars's gravity and magnetic fields, surface topography, and surface mineralogy. The spacecraft also carried cameras for making both wide-angle and detailed images of the surface at resolutions down to 1.5 metres (5 feet). Mars Odyssey safely entered Mars orbit in October 2001 and started mapping other properties, including the chemical composition of the surface, the distribution of near-surface ice, and the physical properties of near-surface materials.

Also launched in mid-2003 was the U.S. Mars Exploration Rover Mission (Mars Exploration Rover), which comprised twin robotic landers, Spirit and Opportunity. Spirit touched down in Gusev Crater (15° S, 175° E) on January 3, 2004. Three weeks later, on January 24, Opportunity landed in Meridiani Planum (2° S, 6° W), on the opposite side of planet. The six-wheeled rovers, each equipped with cameras and a suite of instruments that included a microscopic imager and a rock-grinding tool, analyzed the rocks, soil, and dust around their landing sites, which had been chosen because they appeared to have been affected by water in Mars's past. Both rovers found evidence of past water; perhaps the most dramatic was the discovery by Opportunity of rocks that appeared to have been laid down at the shoreline of an ancient body of salty water (seawater).

Observations made by Mariner 9 and subsequent Mars-orbiting spacecraft have led to many maps of topography, geology, temperature, mineral distributions, and a variety of other data. After Mariner 9, the prime meridian on Mars, the equivalent of the Greenwich meridian on Earth, was defined as passing through a small crater named Airy-0 within the larger crater Airy. Longitude was measured in degrees that increase to the west of this meridian completely around the planet. Later some scientists expressed a preference for a coordinate system having longitude that increases to the east of the prime meridian. Consequently, maps of Mars were published with either or both of these systems.

The biological (astrobiology) experiments aboard the two Viking landers addressed three issues: (1) the nature of organic material, if any, on the Martian surface, (2) the possible presence of objects on the surface whose appearance or motion would suggest living or fossilized organisms, and (3) the possible presence in Martian soil of agents that, under prescribed conditions, could indicate metabolic processes. The results related to the first issue were definite and unambiguous—a direct, extremely sensitive chemical analysis of samples at both lander sites showed no trace of complex organic materials. Addressing the second issue, the cameras on the landers found no evidence of biological agents or activity.

This strategy was underscored by the findings from the Martian meteorite ALH84001, which was collected from an Antarctic ice field in 1984. It is the oldest known rock from Mars, having formed about 4.5 billion years ago when the planet itself was accreting. It contains organic materials, evidence of mineral deposition from liquid water, and a number of structures similar to primitive terrestrial fossils. Although most scientists conclude that these findings do not provide good evidence for past life, they have stimulated closer examination of the Martian meteorites and have emphasized the need for obtaining samples from Mars, particularly of ancient rocks that were being formed or were already present when conditions on Mars were more Earth-like. (See the article extraterrestrial life.)

Donald Goldsmith, The Hunt for Life on Mars (1997), describes the events that led to the controversial claims for evidence of life in the Martian meteorite ALH84001. John Noble Wilford, Mars Beckons: The Mysteries, the Challenges, the Expectations of Our Next Great Adventure in Space (1990), provides a general introduction. All aspects of Mars science are addressed in Hugh H. Kieffer et al. (eds.), Mars (1992), a huge, comprehensive technical summary of existing knowledge. Victor R. Baker, The Channels of Mars (1982), is 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), is a profusely illustrated survey of scientists' perception of Mars after the Viking missions, and Water on Mars (1996) discusses the role that water has played in the evolution of the Martian surface. Volumes of papers detailing the results of the various Mars spacecraft missions appear in the Journal of Geophysical Research: for Viking, 82, no. 28 (1977); for Mars Pathfinder, 104, no. E4 (1999); and for Mars Global Surveyor, 106, no. E4 (2001). These results are summarized in several excellent papers in Nature 412:207–253 (July 12, 2001).Michael C. Malin Michael H. Carr