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Mars (planet), one of the planets in the solar system, it is the fourth planet from the Sun and orbits the Sun at an average distance of about 228 million km (about 141 million mi). Mars is named for the Roman god of war and is sometimes called the red planet because it appears fiery red in Earth’s night sky.

Mars is a relatively small planet, with about half the diameter of Earth and about one-tenth Earth’s mass. The force of gravity on the surface of Mars is about one-third of that on Earth. Mars has twice the diameter and twice the surface gravity of Earth’s Moon. The surface area of Mars is almost exactly the same as the surface area of the dry land on Earth. Mars is believed to be about the same age as Earth, having formed from the same spinning, condensing cloud of gas and dust that formed the Sun and the other planets about 4.6 billion years ago.

The Martian day—that is, the time it takes Mars to rotate once on its axis—is about a half an hour longer than an Earth day and is sometimes called a sol. Its year, or the time it takes to revolve once around the Sun, is about two Earth years long. Mars has two moons, Phobos and Deimos, which are named after the dogs of the Roman god Mars. These tiny bodies are heavily cratered dark chunks of rock and may be asteroids captured by the gravitational pull of Mars. Phobos orbits Mars once in less than one Martian day, so it appears to rise in the west and set in the east, usually twice each day. Deimos has the more ordinary habit of rising in the east and setting in the west.

Mars appears as a fairly bright, red, starlike object in Earth’s night sky. Because of the relative movements of Earth and Mars around the Sun, Mars appears to move backward in the sky for a short time around opposition, which is the time when the two planets are closest. As Mars and Earth orbit the Sun, the distance between them varies from about 56 million km (about 35 million mi) at their closest approaches to about 375 million km (about 233 million mi) when the planets are on opposite sides of the Sun. This change in distance causes the apparent size of Mars to vary by more than a factor of 5 and its brightness to vary by a factor of 25. Because the orbit of Mars is elliptical and not circular, Earth and Mars approach each other more closely during some orbits than others. For example, in late August 2003 Earth and Mars will pass closer to each other than at any time since 1924. The two planets will not get that close again until the year 2287.

When Mars is viewed through a telescope, it looks like a reddish-orange disk. When Mars is close to Earth, an observer can usually see white ice caps at the north and south poles of Mars. These polar caps grow and shrink throughout the Martian year, just as the polar caps of Earth do. The darker areas of Mars’s surface may look greenish to the observer, but this is an optical illusion caused by the contrast in color between the dark patches and the redder, brighter areas. Scientists believe that the dark areas are regions of relatively unweathered dark rocks and sand, while the bright areas are regions with deposits of dusty, fine-grained oxidized iron minerals. Scientists now believe that the “canals” people observed on Mars during the 19th century are actually another optical illusion, caused by the mind’s tendency to draw connections between irregular patches in a fuzzy image.

Mars is tilted on its axis by about 25°. This tilt gives Mars seasons similar to Earth’s seasons. The elliptical orbit of Mars, however, causes the planet to have seasons of unequal lengths. For example, the southern hemisphere’s summer on Mars is about 25 days shorter than the northern summer. The intensity of sunlight also changes substantially during the Martian year: solar heating during the southern summer, when Mars is closer to the Sun, is 40 percent more intense than in the northern summer. During the warmer southern Martian spring and summer, great dust storms have sometimes been observed through telescopes as bright yellow clouds. The largest of these storms can cover the entire planet and last for months. An unusually large dust storm covered the planet in 2001 and was the largest storm seen since 1971. Smaller local or regional dust storms can occur any time during the Martian year. Sometimes white clouds of water vapor are visible, especially in the northern summer when Mars is near its farthest from the Sun and its thin atmosphere is the coldest.

The Hubble Space Telescope (HST) provides the clearest Earth-based views of Mars, and astronomers use it to study the composition of the surface and to monitor the weather on the planet. HST has provided detailed images of local and global dust storms, enormous spiral-shaped water ice cloud systems, and changes in the bright and dark surface markings that have occurred since the first detailed images were taken during the 1970s. The telescope also has enabled spectroscopic measurements that provide comprehensive information on atmospheric chemistry and on the nature and variability of ices and minerals on the surface. Using HST images and other data, astronomers have determined that the atmosphere of Mars is generally cooler and clearer when the planet is farther from the Sun and warmer and dustier when it is closer. There also appear to be longer-term trends in the Martian climate, but as is the case for Earth’s climate, scientists are only now beginning to untangle the complexities required to understand and perhaps one day even predict climate changes on Mars.

The density of Mars is about 30 percent less than that of Earth (3.94 g/cm³ vs. 5.52 g/cm³). Based on spacecraft measurements of the Martian gravitational field, scientists believe that the planet’s interior consists of a crust, mantle, and core like Earth’s interior. While the relative sizes of these components are not known for certain, the planet’s lower density combined with spacecraft mapping of the structure of its gravity field suggest that the planet’s iron-rich core and mantle are a smaller fraction of its volume than in the case of Earth. Mars therefore probably has a relatively thick crust compared to Earth. Beneath the Tharsis bulge, an area of volcanic activity in the northern hemisphere, the crust may be as thick as 130 km (80 mi). But the crustal thickness appears to vary significantly. For example, beneath the landing site of the United States spacecraft Viking 2, it may be as thin as 15 km (9 mi). The Martian core is probably much like Earth’s, consisting mostly of iron, with a small amount of nickel. Other light elements, particularly sulfur, could exist in the core as well. If so, the core may be quite large. From studying Earth’s magnetic field and core, scientists theorize that the motions of the liquid rock in Earth’s core generate its magnetic field. Mars does not have a significant magnetic field, so scientists believe that Mars’s core is probably solid. However, spacecraft data indicates that Mars probably did have a strong magnetic field early in its history, suggesting that the core of Mars may have been liquid at one time.

Characteristics of Mars

Mars does not, and probably did not ever, have active plate tectonics—crust made up of separate sections that move about and sometimes crash into each other as on Earth. Because Mars is so much smaller than Earth, it cooled more quickly after formation and the crust thickened, forming one solid piece and eliminating any possibility of Earthlike plate tectonics. Heat that melted at least some of the Martian interior has, however, sculpted parts of the planet’s surface. In some places molten rock broke through the crust to form volcanoes. In other places, large-scale motions of the partially molten mantle cracked the crust to form large rifts and canyon systems. Although no evidence for active volcanism or tectonic movement has been found on Mars, scientists do not know if the interior of the planet is still geologically active.

Additional details about the Martian interior may have to await a time when more sophisticated spacecraft or even astronauts bring instruments such as seismometers to the planet, providing information similar to that which scientists routinely obtain for Earth’s interior today.

The surface of Mars would be a harsh place for humans, but it is more like the surface of Earth than that of any other planet. The temperature on Mars never gets much warmer than the temperature at Antarctica, and it is usually much colder. At the surface the average temperature is about –55°C (about –67°F) and at the extremes it ranges from about -140° to 15°C (about -225° to 60°F). During most of the year wind speeds are fairly low—about 7 km/h (about 4 mph)—but during dust storms they can approach 70 to 80 km/h (40 to 50 mph). These winds often originate in large basins in the southern hemisphere and carry great volumes of dust from the basins to other regions, sometimes covering the entire planet in the storm. The dust is not sandy, as in a sandstorm on Earth, but has the consistency of flour. The most interesting surface features of Mars include two very distinct hemispheres, an enormous bulge called Tharsis littered with volcanoes and cut by an enormous rift valley, channels apparently carved by water, and polar ice caps similar to Earth’s.

The northern and southern hemispheres of Mars have different characteristics. The southern hemisphere has many impact craters and has a generally much higher elevation than the northern hemisphere. The southern highlands are probably the oldest terrain on Mars, dating back to the early history of the solar system when large impact events were much more common than they are today. The southern highlands, with their pervasive craters, resemble the surface of the Moon. Hellas Planitia is a giant impact basin in the southern hemisphere. The impact of a large asteroid formed the basin long ago. At 6 km (3.8 mi) deep and with a diameter of about 2,000 km (about 1,250 mi), it is the largest and deepest basin on Mars. A few other large basins and thousands of large craters can be found on the surface, mostly concentrated in the lunar-like southern highlands. The northern hemisphere of Mars contains a much wider variety of geologic features, including large volcanoes, a great rift valley, and a variety of channels. The northern hemisphere also contains large expanses of relatively featureless plains. Astronomers do not know why the northern and southern hemispheres of Mars are so different; figuring out the reason is an important goal of Mars exploration.

Mars has an enormous bulge in its surface called Tharsis. Tharsis is 10 km (6 mi) high and 4,000 km (2,486 mi) wide, and contains stupendous volcanoes and valleys. The largest volcano in the solar system, Olympus Mons, is located in the Tharsis region. It is 26 km (16 mi) high (more than twice as high as Earth’s Mount Everest) and covers an area comparable to the state of Arizona. Near it, three other volcanoes almost as large—Arsia Mons, Pavonis Mons, and Ascraeus Mons—form a line running from southwest to northeast. These four volcanoes are the most noticeable features of Tharsis. Another volcano, Alba Patera, is also part of the Tharsis bulge but is quite different in appearance. It is probably less than 6 km (4 mi) high but has a diameter of more than 1,600 km (1,000 mi). None of Mars’s volcanoes appear to be presently active.

The Tharsis bulge has had a profound effect on the appearance of the surface of Mars. It includes many smaller volcanoes and stress fractures in addition to the large volcanoes. Its presence affects the weather on Mars and its formation may have changed the climate by changing the rotational axis of the planet. Valles Marineris (named for the U.S. Mariner spacecraft that discovered it) is the most notable stress feature associated with the Tharsis bulge. It is a great rift valley and interconnected canyon system extending from the Tharsis region to the east-southeast. Valles Marineris is about the same length as the distance from New York to California (about 4,000 km or 2,500 mi). This canyon system reaches widths of 700 km (440 mi) and depths of 7 km (4 mi) in some places. High-resolution spacecraft images have revealed a spectacular variety of layered landforms in and around the canyon system. These layers may represent different episodes of volcanic eruptions, or they may be sedimentary deposits laid down when the canyons were possibly water-filled. The origin of this enigmatic layering on Mars is presently unknown, but most astronomers agree that understanding it will be critical to understanding the history of the planet.

Two main types of channels, valley networks and outflow channels, can be found on Mars. Both were probably formed by the action of liquid water. These channels are unrelated to the “canals” thought to be seen in early telescopic views of Mars.

Valley networks are similar in general appearance to streambeds on Earth and occur in the southern highlands. These channels may date from a time early in Mars’s history when the atmosphere was thicker and liquid water could flow readily on or near the surface. High-resolution images reveal important differences between these Martian valley networks and terrestrial valley networks, however. Specifically, Martian valley networks do not appear to have formed from rainfall or surface runoff, but instead may have formed primarily from the action of underground liquid water.

Outflow channels, formed by giant floods, occur primarily on the boundary between the southern highlands and the northern plains regions. Ares Vallis, where the Mars Pathfinder spacecraft landed in 1997, is one of these outflow channels. An important difference between outflow channels and valley networks is that outflow channels appear to have been formed quickly by the sudden and catastrophic release of enormous volumes of liquid water, with no particular requirements on climatic conditions, while valley networks appear to have required geologically long periods of time and perhaps much warmer and wetter climate conditions to form.

Mars has small, permanent ice caps at its north and south poles that increase in size with the addition of seasonal ice caps during the winter of each hemisphere. The polar caps in the north and south are quite different from one another. The northern permanent ice cap is composed of water ice and is about 1,000 km (about 620 mi) across. A seasonal cap of frozen carbon dioxide adds to the northern ice cap in the northern winter. The southern permanent ice cap is one-third the diameter of the northern cap because summer in the southern hemisphere is warmer than in the north. The southern seasonal cap is larger than the northern cap—more carbon dioxide is frozen out in the south than the north because Mars is farthest from the Sun, and therefore coldest, in the southern winter. Carbon dioxide may also make up some of the southern permanent cap. Both polar caps and their surrounding deposits show spectacular fine-scale striped layering of dust, rock, and ice, right down to the limits of the resolution of the best available pictures. Like similar layering found in Earth’s polar regions, these Martian polar layers may provide evidence of both short-term and long-term changes in the planet’s climate. The true origin of the Mars polar layering is unknown at present, but it may have been caused by climate cycles similar to ice ages on Earth. Understanding the polar layering is yet another important motivator for continued exploration of the planet.

The atmosphere of Mars is 95 percent carbon dioxide, nearly 3 percent nitrogen, and nearly 2 percent argon with tiny amounts of oxygen, carbon monoxide, water vapor, ozone, and other trace gases. Earth’s atmosphere is mostly nitrogen and oxygen, with only 0.03 percent carbon dioxide. The pressure of the Martian atmosphere varies with the seasons, ranging from 6 to 10 millibars, or about 1 percent of the air pressure at Earth’s surface. The variation in pressure occurs because in the fall and winter at the poles of Mars, the temperature gets so low that carbon dioxide snows out of the atmosphere and forms meters-thick deposits of dry ice on the surface. In the springtime as the surface warms up, the dry ice evaporates back into the atmosphere. The pressure also varies with altitude just as it does on Earth and is about ten times lower on the top of Olympus Mons than on the floor of Hellas Planitia.

Even though the Martian atmosphere contains less than 1/100 as much water vapor as Earth’s atmosphere, clouds and frosts form on Mars and have been studied in detail by telescopes and spacecraft. Wave clouds, spiral clouds, clouds formed near topographic obstacles such as volcanoes, wispy cirrus-like clouds, and a wide variety of hazes and fogs have all been observed. Along with the dust storms and related clouds described above, these features all reveal the Martian atmosphere to be quite dynamic.

Some surface features provide evidence for a very different climate early in the planet’s history, which may indicate that the atmosphere of Mars was much thicker long ago than it is now. A thicker atmosphere would have been able to trap more solar heat through the greenhouse effect, possibly allowing the surface to warm up to the point where water could have remained liquid for long periods of time. Scientists do not know, however, what the composition of this thicker atmosphere was, and where it went. They theorize that it may have been driven off in a catastrophic impact event, or that the gases reacted with water and got trapped in rocks and minerals on the surface. Scientists also wonder where the liquid water that formerly existed at the surface went. Some astronomers believe that it seeped into the ground and is still there as ice in the subsurface today. Others think that it may have evaporated and slowly trickled off into space as sunlight broke apart the water vapor molecules over long periods of time. Determining the history of the Martian atmosphere and finding out whether sizable quantities of water still exist there are among the most important goals of Mars exploration today.

Space probes have provided the most detailed information about Mars. But getting a spacecraft to Mars and operating it there successfully is a difficult and risky process. As of 2001, 34 space missions have been launched to Mars, most of them by the former Union of Soviet Socialist Republics (USSR) and the United States. Of these, less than half (only 15) have survived launch and the interplanetary cruise to return at least some data from Mars, and only eight have been completely successful. The missions that have been successful returned vast amounts of data about the chemical and physical characteristics of Mars and a large number of digital photographs of its surface. Several missions are currently returning data from Mars as part of an international effort to intensively study the planet from orbit and the surface over the next decade and beyond.

The U.S. exploration of Mars by the National Aeronautics and Space Administration (NASA) falls into two separate phases. A series of Mariner and Viking missions were sent to Mars during the 1960s and 1970s. NASA then abandoned the exploration of Mars for a number of years but began sending some individual spacecraft to the planet during the 1990s.

NASA launched its first Mars spacecraft, called Mariner 3 and 4, in 1964. Mariner 4 was successful and performed the first flyby of Mars in July 1965, taking dozens of close-up pictures and other measurements. These pictures had a powerful impact because the only features seen in the images taken of the parts of the southern hemisphere that Mariner 4 happened to pass over were impact craters like those on the Moon. These first close-up images did not reveal any evidence of the advanced civilizations that people in the 19th and early 20th centuries imagined might exist on Mars, or even any interesting and potentially Earth-like geologic or atmospheric features that modern astronomers were hoping to see. The 1969 flybys of Mariners 6 and 7 took much more detailed pictures of the Martian surface as well as measurements of the planet’s gravitational field and atmospheric composition. Even these more extensive views of the red planet, however, were just glimpses that did not reveal the true character of Mars.

Mariner 9, launched in 1971, was the first spacecraft to orbit Mars, and the resulting detailed and systematic study from orbit revealed the enormous volcanoes, canyons, and enigmatic channels that have come to characterize the modern view of Mars. Much of Mariner 9’s mission was hampered by a global dust storm that shrouded most of the surface from view during much of 1971. However, once the dust settled, Mariner 9’s ultimate legacy was showing that the planet was much more like Earth than the Moon.

NASA launched an even more ambitious series of probes to Mars—Viking 1 and 2—in 1975. These spacecraft provided scientists with incredible high-resolution views of the planet’s surface and atmosphere. The Viking probes included orbiters, which mapped Mars and made global studies of its geology and meteorology, and the first successful landers, which measured the composition of the surface, studied the planet’s daily and seasonal weather patterns, and searched for signs of life. The Viking Landers revealed a landscape much like some of the cold, dry deserts of Earth, except that the soils were found to be completely sterile, and the environment overall much too harsh for Earthlike organisms to survive.

After a 17-year interval, NASA launched its next Mars mission, Mars Observer, in 1992. Observer was lost just 3 days before it reached Mars. Its replacement, Mars Global Surveyor (MGS), was launched in 1996 and successfully went into orbit around the planet in 1997. It carries instruments to measure the composition and topography of the surface and to monitor the weather conditions in much more detail than scientists can from Earth. MGS also carries cameras that can resolve details as small as 1.5 m (about 5 ft) on the surface. Some of the MGS images reveal erosion patterns on the planet’s surface that appear to have been formed by relatively recent near-surface liquid water. This discovery is both exciting and puzzling, because ice, not liquid water, is expected to exist at such low pressures and temperatures.

MGS provided images of an enormous number of dunes and other windblown landforms that appear to be the only active geology on the planet today. It also discovered the many enigmatic layered deposits at the poles and discovered and mapped remnants of a once-strong planetary magnetic field preserved in certain parts of the Martian surface. Other important results from MGS include global mapping of the planet’s topography to an accuracy better than is available for most of Earth’s topography, global mapping of volcanic and other minerals on the surface, and daily mapping of the planet’s clouds and polar caps. In addition, in 2001 the spacecraft captured detailed multi-instrument measurements of the largest dust storm observed on Mars since 1971. As of early 2002, MGS continued to acquire new images and other data from Mars every day.

In 1997 the Mars Pathfinder spacecraft became the third successful mission to land on Mars. Pathfinder consisted of a lander and a small rover named Sojourner. The lander took digital camera images of the geology of the landing site and studied the weather conditions on Mars. The rover, a separate autonomous spacecraft about the size and weight of a microwave oven, was able to travel a few meters per day around and away from the lander, taking close-up images and chemical measurements of surface materials that were inaccessible to the lander itself. Pathfinder operated for 83 Martian days and discovered evidence, preserved in the geology and chemistry of the rocks and soils at the landing site, for the action of liquid water long ago.

NASA launched two spacecraft to Mars in 1998 and 1999. The first spacecraft, Mars Climate Orbiter, reached the planet in September 1999 but crashed into Mars instead of orbiting the planet because of a navigational error. The second spacecraft, Mars Polar Lander, reached Mars in December 1999, but it too crashed into the planet’s surface. Engineers believe the craft fired its landing rockets too early. Mars Polar Lander also carried two independent surface penetrator probes called Deep Space 2, but these also failed to perform successfully.

Another U.S. mission to Mars, an orbiter called Mars Odyssey, went into orbit around the planet in late 2001. Odyssey carries instruments to make geochemical measurements of the surface and to map the planet’s rock and mineral deposits in greater detail than MGS. This orbiter began its primary mapping mission in early 2002.

From 1960 to 1971 the USSR sent 12 probes to Mars before their first partial successes with missions Mars 4, 5, and 6 in 1973. The Soviets did not explore Mars again until the Phobos missions in 1988. The Phobos probes were primarily designed to study the planet’s moon Phobos. Phobos 1 was lost on its way to Mars, but Phobos 2 went into Martian orbit and sent back information on the composition of both Phobos and Mars.

Russia has continued scientific study of Mars after the dissolution of the Soviet Union in 1991, although on a more modest scale than the Soviet space program. To date, the most ambitious Russian mission involved an orbiter called Mars 96, which suffered an unsuccessful launch and crashed back to Earth. While no firm plans for future Russian Mars missions have been announced, Russian astronomers continue to participate as collaborators on Mars missions of other nations.

Many other nations have participated in Mars exploration, contributing scientific knowledge and instrumentation to missions led by the U.S. and Russia. The first other nation to lead its own dedicated Mars mission was Japan. In July 1998 a Japanese spacecraft called Nozomi was launched to Mars, and it is scheduled to reach the planet in 2003. Nozomi carries a color camera, an atmospheric probe, and several other scientific instruments designed primarily to study the planet’s upper atmosphere and weak magnetic field.

Mars is the most Earthlike place in the solar system besides Earth itself, and so it is only natural to wonder if the similarities extend to the existence of life. People have speculated about the possibility of life on Mars for centuries, and one of the major justifications for sending spacecraft to Mars is the search for direct evidence of past or present life.

Astronomers have often fueled the speculation that life may exist on Mars. For example, the 19th century Italian astronomer Giovanni Schiaparelli reported that he saw long, straight markings on Mars that he called canali (Italian for “channels”). He and other astronomers of that era also reported seeing evidence for seasonal color changes on Mars that could be interpreted as evidence for vegetation. Some astronomers of the early 20th century, as well as American entrepreneur and amateur astronomer Percival Lowell, turned Schiaparelli’s canali into the now-famous “canals,” forever changing the public’s perception of the red planet. Lowell believed that the canals indicated the existence of an advanced civilization on Mars. He wrote several books and magazine and newspaper articles on the subject and lectured extensively about his theory around the country to sold-out audiences. He proposed that the canals were a planetary-scale irrigation project, carrying water from the wet polar regions to the dry equatorial deserts. As telescopes improved, however, and as it became possible to record photographs of Mars on film instead of relying on human vision alone, astronomers found it more and more difficult to see repeatable evidence for Lowell’s canals. Close-up images of Mars from the Mariner spacecraft finally proved that the canals did not exist.

Scientists now know that windblown dust causes the color changes and that the canals are no more than an optical illusion caused by the limitations of human eyesight at the telescope. But Lowell’s beliefs about civilization on Mars have had a powerful and lasting effect on human perception of the planet. British author H. G. Wells’s The War of the Worlds (1898) and American actor and director Orson Welles’s 1939 nationally-broadcast radio hoax based on that novel put a sinister face on our interplanetary neighbors. American author Edgar Rice Burroughs’s series of Mars books, starting with A Princess of Mars (1912), provided a more benevolent expansion of the influence of Lowell’s ideas and inspired a generation of would-be planetary explorers. For a while in the 1970s, some people even thought there were human faces and pyramid-like structures carved into landforms in places on Mars, until better images revealed these, too, to be optical illusions. Even today, science fiction stories, movies, and television shows about Mars and Martians continue to be popular around the world.

A major focus of the Viking missions was to search for actual scientific evidence of life. Several instruments on the Viking landers were designed specifically to detect organic molecules in the soil, and to test soil samples for evidence of metabolism, growth, or photosynthesis of possible Martian life forms. Even though all of these experiments were sensitive enough to have been able to detect life even in the most arid, cold, or otherwise hostile environments on Earth, none of them showed any convincing evidence for the presence of life on Mars.

Most scientists today do not believe that there is any life on the surface of Mars. Conditions at the surface are extremely hostile to life as we know it. Temperatures are usually well below the freezing point of water, and the atmosphere is extremely thin and dry. Without a protective ozone layer like Earth’s, ultraviolet radiation bathes the surface and would destroy any organic molecules exposed there. However, a growing number of scientists believe that some form of life could exist on Mars today, underground or inside pores and cracks in rocks, where there is protection from the extreme conditions of the surface and where liquid water could exist even at very low temperatures. This new appreciation for the possibility of life on Mars has been driven by the discovery, only in the last decade or so, of simple life forms on Earth tenaciously surviving and in some cases even thriving in what used to be considered inhospitable conditions. On Earth, life has been found at great depths on the ocean floor, deep underground in volcanic rocks, in highly acidic cave waters, in near-boiling hot springs, and in almost permanently frozen tundra sediments. If life can maintain a foothold in even these extreme environments on Earth, then it may also be able to exist on Mars.

In 1996 a group of NASA scientists announced that a meteorite thought to have come from Mars contained possible fossil evidence of bacteria-like life forms. The meteorite traveled through space to Earth millions of years ago after being blasted from the surface of Mars, probably by the impact of a large meteor. The scientists’ evidence was based on the presence of certain chemicals and minerals—as well as microscope pictures of bacteria-like features—within the meteorite. Intense scrutiny of this meteorite by other scientists has not provided support for this theory, however, and most scientists believe that the meteorite may somehow have been contaminated by Earth life. Even though it is still possible that the rock does contain fossilized bacteria from Mars, most scientists now focus instead on what can be agreed upon about this mysterious meteorite. Liquid water probably once flowed through the meteorite, sources of heat such as meteorite impacts or volcanism acted upon it, and it probably contains at least some simple organic molecules that may have originated on Mars. These conditions—liquid water, heat, and organic molecules—are the requirements for life to exist on a planet. So even if the Mars meteorite does not preserve actual evidence of life on Mars, its most important message may be that Mars is one of the few places in the solar system where we know that the conditions were at least possible. The goal now is to figure out how and where to look for more convincing evidence.

The United States and other countries have planned an ambitious, long-term program of Mars exploration. In 2003 NASA’s Mars Exploration Rover mission will send two rovers to separate landing sites on Mars to search for evidence of water and life that may be preserved in the geology, chemistry, and mineralogy of the surface. These rovers are based on the Mars Pathfinder Sojourner rover design, but with enhanced mobility to allow them to traverse up to 100 m (about 300 ft) per Martian day. The European Space Agency (ESA), a consortium of 15 European nations, plans to launch its own mission called Mars Express in 2003. The European mission will include an orbiter to study and map details of the surface and atmosphere and a very small lander called Beagle 2. NASA plans to launch the Mars Reconnaissance Orbiter in 2005 to perform even higher resolution imaging and atmospheric studies, partly to search for landing sites for a planned 2007 enhanced lander-and-rover mission. The lander-and-rover mission would be followed, probably around the turn of the decade, by the first missions to retrieve samples from the surface of Mars for examination in laboratories on Earth. Sample-return missions could in turn provide data required before NASA and other space agencies would move forward with serious plans for the first human missions to Mars, perhaps by the middle to end of the second decade of this century.