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Mars (planet) I INTRODUCTION

Mars (planet), fourth planet in distance from the Sun in the solar system. Mars is of special scientific interest because of its similarities to Earth. It has an atmosphere with seasons and changing weather, and its surface shows evidence of ancient water and volcanoes. The length of its day and the tilt of its axis are similar to those of Earth. Mars takes about two years to circle the Sun at an average distance of 228 million km (141.7 million mi). The possibility of life on Mars, now or in the distant past, is one of the major questions in astronomy. More space probes have been sent to Mars than to any other planet. Mars is named for the Roman god of war. It is sometimes called the red planet because it appears fiery red in Earth’s night sky, the result of rusty, iron-oxide mineral dust that covers its surface.

Mars is a relatively small planet, with a diameter of about 6,794 km (4,222 mi) or about half the diameter of Earth. Mars has about one-tenth Earth’s mass. The force of gravity on the surface of Mars is about three-eighths 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.

Mars has two moons, Phobos and Deimos, which are named after the sons 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.

II OBSERVATION FROM EARTH

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 passed 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 with a telescope 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 telescope 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.

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. Orbiting spacecraft around Mars furnish constant data about the planet. However, they orbit so close to the planet and are in a fixed orientation relative to the Sun that they cannot see features in the early morning or late afternoon parts of the Martian day. As a result, astronomers still need telescopes like the HST to study Mars, particularly its early morning and late afternoon cloud formations.

III ORBIT AND ROTATION

Mars orbits the Sun at an average distance of about 228 million km (141.7 million mi), or 1.524 astronomical units (AU). An AU is equal to the average distance between the Earth and the Sun, or about 150 million km (93 million mi). However, Mars’s orbit is more elliptical than Earth’s—its nearest point to the Sun (perihelion) is about 42 million km (26 million mi) closer than its farthest point (aphelion), compared with only a 5 million km (3 million mi) difference between perihelion and aphelion for Earth. Mars’s year, or the time it takes to revolve once around the Sun, is about two Earth years long (687 Earth days). Mars receives less than half the amount of sunlight Earth does and is much colder.

Mars is tilted on its axis by about 25° (Earth is tilted at 23.5°). 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 spring and summer period in the southern hemisphere, great dust storms have sometimes been observed through telescopes as bright yellow clouds. Sometimes white clouds of water vapor are visible, especially during the northern summer when Mars is near its farthest point from the Sun and its thin atmosphere is the coldest.

Like Earth, Mars turns counterclockwise on its axis (from west to east) when seen from its north pole and orbits the Sun in a counterclockwise direction. It takes Mars 24 hours and 37 minutes to rotate once on its axis (its sidereal day). Its solar day (the time between when the Sun next crosses the noon point in the sky) is about 24 hours and 39 minutes—its orbital motion around the Sun adds two minutes to its rotation period. (Earth’s solar day (24 hours) is four minutes longer than its rotation period.) The Martian solar day is sometimes called a sol.

IV THE INTERIOR OF 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. If other light elements, particularly sulfur, exist there as well, the core may be larger than presently thought. 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 indicate that Mars probably did have a strong magnetic field early in its history, suggesting that the core of Mars may have been at least partially liquid at one time.

Tectonics on the Earth is dominated by the relative motions and collisions of a few dozen large, moving lithospheric plates. Earthlike plate tectonics does not appear to be active on Mars today. However, there is considerable debate over whether Mars may have had plate tectonics in the distant past, when the core may have been molten. Ancient magnetic field patterns preserved in the crust show some similarities to magnetic field patterns that arise from plate tectonic processes on Earth. Because Mars is so much smaller than Earth, however, its more rapid cooling and crustal thickening after formation may have favored the creation of a one-plate planet rather than Earthlike plate tectonics.

Heat that melted at least some of the Martian interior has 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. Scientists do not know if the interior of Mars is still geologically active. No evidence for active volcanism or tectonic movement has been found on the planet. However, images from orbiting spacecraft suggest that some of the Tharsis volcanoes have been periodically active in the last 100 to 350 million years, and perhaps as recently as 2 million years ago. Smaller volcanic cones discovered around the north pole may have erupted as recently as 1 million years ago.

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.

V THE SURFACE OF MARS

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). The surface’s famous reddish color comes from iron oxide minerals in the dust, similar to rust on Earth. 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.

A Distinct Hemispheres

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. Radar and topographic studies of the northern hemisphere by Mars orbiters have revealed ancient impact craters beneath the plains, however, indicating that the underlying crust may be the same age as the southern highlands. Astronomers do not know why the northern and southern hemispheres of Mars are now so different; figuring out the reason is an important goal of Mars exploration.

B The Tharsis Bulge

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 giant volcanoes and valleys. The largest volcano in the solar system, Olympus Mons, is located in the Tharsis region. It is over 21 km (13 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 these volcanoes appears to be presently active, but there is some evidence of small eruptions in the last 100 to 350 million years, and perhaps as recently as 2 million years ago.

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.

C Water Channels

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. A small number of valley networks, however, observed at the highest resolution by the Mars Global Surveyor orbiter, look like they may have been formed from rainfall or surface runoff. Mars Global Surveyor images of Eberswalde Crater southeast of the Valles Marineris canyon system also show a fan-shaped deposit that closely resembles a river delta, further suggesting that water sometimes flowed for an extended period of time.

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.

Small-scale water events may still be occurring on Mars. Outflows of liquid water may have formed gullies seen on the walls of craters by the Mars Global Surveyor. Comparison of images taken between 1999 and 2005 showed fresh flows of bright material on the inner walls of several small craters. These flows could result from subsurface liquid water erupting onto the surface. Under current conditions of extreme cold and low air pressure, liquid water cannot exist for long on the surface. Water mixed with salts that lowered its freezing point or water erupting after being under pressure from overlying rock layers might flow for a short distance, however. New, higher-resolution images from the Mars Reconnaissance Orbiter suggest that these features might represent dusty avalanches rather than watery flows, however. Scientists are still actively debating the origin of these features.

D Ice Caps

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 have important differences and similarities. 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. While carbon dioxide may also make up some of the southern permanent cap, it is now thought to consist largely of water ice, like the northern permanent cap. Radar on the Mars Express orbiter found evidence for deep layers of frozen water under the south pole. If the amount of ice apparently indicated were melted, it could cover the entire planet in almost 11 m (36 ft) of water.

Both polar caps and their surrounding deposits show spectacular, fine-scale striped layering of dust, rock, and ice 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.

E Subsurface Water Ice

Instruments on the Mars Odyssey orbiter suggest that large deposits of water ice may lie below the Martian surface in the mid-latitudes. The data show that 50 percent or more of the uppermost meter of the soil may be made up of ice in some places. The depth and thus the total amount of this subsurface ice are not known, however. Ice may also exist in the subsurface near the equator. A region of Elysium Planitia near the equator has been interpreted from Mars Express images as a possible “frozen sea” (resembling pack ice seen on Earth) covered in dusty soil or volcanic ash only a few centimeters thick. However, the features in this region could also be platy or fractured lava flows without any associated subsurface ice.

VI THE ATMOSPHERE OF MARS A Composition

The atmosphere of Mars is 95 percent carbon dioxide (CO2), 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 78 percent nitrogen and 21 percent oxygen, with about 1 percent argon and only 0.04 percent carbon dioxide. The larger relative proportion of argon in the air on Mars may indicate that its atmosphere was much thicker in the past. Compared to other gases, argon is a relatively heavy gas that is not as easily lost into space over time. As a noble gas, argon does not combine chemically with other substances in the atmosphere or on the surface.

In 2004 scientists reported detecting small amounts of methane (CH4) in the Martian atmosphere, at about 10 parts per billion. The observations are controversial, however, because such small levels of abundance are involved. If the methane abundance is real, then some recent or ongoing process on Mars may be releasing fresh methane, because otherwise the gas would quickly break down from the ultraviolet radiation from the Sun. Possible sources for the methane on Mars could include volcanic or hydrothermal activity, chemical reactions between water and minerals in the crust, ancient deposits of methane ice, or even biological activity. However, there is currently no strong evidence for active volcanic or hydrothermal processes on Mars, which should also release sulfur dioxide (SO2), a gas not yet detected there. The apparent concentration of methane in places where subsurface water ice is thought to be present might argue more for a water-rock chemical reaction origin. At present there is no real consensus on the presence or origin of methane on Mars.

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 the temperature gets so low at the poles of Mars 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.

B Clouds

Even though the Martian atmosphere contains less than 1 percent 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, these features all reveal the Martian atmosphere to be quite dynamic.

Data collected by the Mars Global Surveyor spacecraft indicate that much denser water ice clouds can form on the night side of Mars not visible from Earth. These clouds are about five times as thick as the water ice clouds sometimes seen on the day side and form lower in the atmosphere, creating a foglike layer above parts of the surface. A clue to the presence of such clouds comes from the ground temperature on the night side—during the Martian summer some areas near the equator in the northern hemisphere can be up to 20°C (68°F) warmer than predicted. The clouds overhead prevent some of the heat built up during the day from radiating away into space at night. Most of these water ice clouds quickly disappear after dawn.

Clouds consisting of carbon dioxide ice crystals also form in the Martian atmosphere, mainly over the polar regions in winter when the temperature is lowest. The CO2 ice crystals strongly scatter thermal radiation, reducing the loss of heat into space over the poles.

C Sky Colors

Rovers and landers on the surface of Mars have photographed the planet’s striking sky colors, including sunrises and sunsets. On Earth, the mid-day sky is blue because of the way air molecules efficiently scatter blue light in our dense, relatively clear atmosphere. When the Sun is low in Earth’s sky, sunlight travels through much more of the air and so scattering of red light by dust and haze particles becomes more important than scattering by air molecules and our skies turn reddish and pink. On Mars, the sky color is almost totally determined by dust (only a feeble amount of molecular scattering is possible in the thin Martian atmosphere). Based on their sizes and shapes, dust particles scatter different colors of light more efficiently in certain directions, however. The mid-day Mars sky is reddish to pink because scattering of red light by fine dust particles is most efficient in that geometry. When the Sun is low, scattering of blue light is more efficient. Thus, the pinkish mid-day skies give way to bluish sunsets. Martian sky colors are to some extent opposite to those of Earth.

D Winds and Dust Storms

During most of the year wind speeds are fairly low—about 7 km/h (about 4 mph)—but during dust storms they can exceed 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. The dust is not sandy, as in a sandstorm on Earth, but has the consistency of flour. 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. A similar large dust storm occurred in 2007. Smaller local or regional dust storms can occur any time during the Martian year. Some scientists think dust storms may generate a strong static electric charge that affects the chemistry of the soil, creating hydrogen peroxide that would break down organic substances on the surface or methane in the air.

Dust devils are also an important feature of Martian weather. These swirling columns of dust and sand occur during the Martian summer when surface temperatures can warm to 20°C (68°F). Starting as heated air at ground level, Martian dust devils can reach heights of 10 km (6 mi) and rotate at 30 meters per second (70 mph). Although the force of such winds is weak because of the planet’s thin air, dust devils on Mars can grow vastly larger than dust devils on Earth—ten times as large as Earth tornados. Because the rotating winds transport dust into the atmosphere, dust devils may have a major effect on the planet’s overall weather and climate. Dust devils also may carry a strong static electric charge. Such electrified dust devils could pose a hazard to future robotic or human explorers, coating equipment with dust and sand, interfering with radio communications, and even discharging lightning.

VII THE MARTIAN PAST

Space probes have provided scientists with enough information to decipher some of the history of Mars as a planet. Surface features indicate that the environment on the surface of Mars has changed dramatically over time, from geologically active early periods when a relatively thick atmosphere and liquid water may have been present to today’s frozen world with a thin atmosphere.

A Formation of Mars

The chemical composition of Mars is similar overall to that of the Earth, although there are important differences in the abundances of iron and of volatile elements like water and sulfur. These differences probably exist because Mars likely formed further away from the Sun than the Earth, in a different region of the disk of rock, ice, and gas from which all of the planets formed.

Like Earth, Mars went through a period of massive bombardment from asteroids and comets from its formation about 4.6 billion years ago until about 4.2 billion years ago. Conditions would have been hostile to the rise of life, but the bombardment also provided Mars with some of the same chemical building blocks that made life possible on Earth. These substances include organic carbon compounds.

B Climate History

While there is still intense debate and scientific study of Martian climate change, a number of models of the planet’s evolution have been proposed to try to match the steady stream of new observations.

B1 Earthlike Mars Model

In one model, Mars appears to have had more Earthlike surface conditions between about 4.2 and 3.5 billion years ago. A thick CO2 atmosphere may have trapped more solar heat through the greenhouse effect, allowing the surface to warm up. Along with higher atmospheric pressure, the warm temperatures allowed water to remain liquid for long periods of time and to possibly cover extensive areas of the surface. Water reacted with the surface and subsurface rocks, creating clays and other hydrated minerals and possibly also carbonate rocks.

Volcanic eruptions over time released large amounts of sulfur dioxide (SO2) into the atmosphere, slowly changing the chemical environment. The added sulfur turned the surface water acidic, dissolving most of the clays and any carbonate rocks that may have formed during earlier more Earthlike periods. When the sulfur-rich surface water evaporated it left behind deposits of salty sulfate minerals.

B2 Dry Mars Models

Other models of the Martian past paint a “drier” picture of early Mars. In these scenarios, liquid water may have existed on the surface or in the shallow subsurface for perhaps only intermittent periods or only in small regions of the planet. These models lead to less optimistic implications for the possibility of life on Mars but are still consistent with the information that space probes have obtained. Astronomers focus significant efforts on trying to distinguish between these very different models for the Martian past, including designing future missions to try to resolve the controversy.

C Thinning Atmosphere

Regardless of the specific details of the climate of early Mars, it seems clear to most astronomers that beginning around 2.5 billion years ago Mars began to evolve into the frozen, dry world it is today. It became a planet with little volcanic activity and a carbon dioxide atmosphere too thin to allow liquid water on the surface or to sustain a significant greenhouse effect. Exactly what could have happened to most of the thicker early atmosphere that might have existed is still a mystery. One theory is that loss of the planet’s magnetic field after Mars’s liquid core began to turn solid allowed charged particles from the solar wind to collide with the atmosphere, knocking atoms and molecules off into space. Other theories propose that part of the atmosphere may have been blown away in a catastrophic impact event, or that the gases reacted with water and are chemically combined in rocks and minerals on the surface and in the subsurface.

D Loss of Liquid Water

Scientists also wonder where the liquid water that appears to have formerly existed at the Martian surface went. Some astronomers believe that it seeped into the ground and is still there as ice or possibly liquid water in the subsurface today. Others think that much of 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 below the surface are among the most important goals of Mars exploration today.

E Shifting Axis Tilt

A major difference between Earth and Mars is the stability of the tilt of the planet’s axis. Due to gravitational effects of the Moon, Earth maintains a relatively stable 23.5 degree tilt on its axis, with only a slight wobble (obliquity) between about 22.1° to 24.6° over a timescale of millions of years. Without a large moon, and being closer to the strong gravitational pull of Jupiter, Mars has a much more dramatic wobble to its axis, resulting in a tilt that may shift from 0° to 60° and causing major climate shifts over time, between massive ice ages and relative warm periods. When the tilt of Mars is nearly vertical, the polar caps may expand almost to the equator. At its greatest tilt, one pole is warmed almost directly by the Sun, likely releasing the deposits of frozen water and gas into the atmosphere.

VIII SPACECRAFT MISSIONS TO MARS

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. Beginning in the 1960s and into the 21st century, more than 40 spacecraft have been launched to Mars. About half of those probes have failed, mainly from technical problems. The most successful missions 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 from the surface.

A United States Exploration of Mars

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 spacecraft to the planet during the 1990s and in the early part of the 21st century.

A1 Mariner and Viking

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 Earth-like organisms to survive.

A2 Later Missions

After a 17-year interval, NASA launched its next Mars mission, Mars Observer, in 1992, but contact was lost with the spacecraft just three 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. MGS operated until the end of 2006, when the probe lost contact with Earth. It carried instruments to measure the composition and topography of the surface and to monitor weather conditions in much more detail than scientists can from Earth. MGS also carried 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, which 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.

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 and discovered evidence of extensive subsurface ice deposits later that year. Major discoveries include evidence of extensive subsurface ice deposits and some surface areas covered with salt deposits, possibly left by bodies of water that evaporated.

NASA launched the Mars Reconnaissance Orbiter in August 2005, and it successfully went into orbit around Mars in 2006. The Mars Reconnaissance Orbiter has the most powerful camera ever sent to another body in space. Its mission is to perform high-resolution imaging of objects as small as 25 cm (1 ft) across, which could help the search for landing sites for future missions in the Mars Exploration Program. It also performs atmospheric, mineral mapping, and subsurface radar studies.

A3 The Mars Exploration Rover Mission

The Mars Exploration Rover mission consisted of two spacecraft, Spirit and Opportunity, which were highly capable rovers equipped with the scientific instruments needed to determine whether liquid water once existed on some parts of the Martian surface. The solar-powered, six-wheeled robotic vehicles were much more sophisticated and mobile than Pathfinder, and could communicate directly with Earth. Launched in 2003, the two rovers successfully landed on Mars a few weeks apart in January 2004. Spirit landed in Gusev Crater, which is 160 km (100 mi) wide and lies about 15 degrees of latitude south of the Martian equator; Opportunity landed in a shallow crater in an equatorial region known as Meridiani Planum. Cameras on the Opportunity rover sent back images of the first exposed bedrock on the surface of Mars.

The original mission for each rover was scheduled to last 90 days. However, both Spirit and Opportunity proved to be remarkably hardy and able to explore and photograph the Martian surface for years despite some software and hardware problems. In addition to cameras to provide both panoramic and microscopic images of the surface, the Mars Exploration Rovers carried a variety of scientific instruments for measuring the composition of soil and rocks. An adjustable arm on each rover included a rock-abrasion tool. A grinding wheel on the tool could remove dust and weathered crusts from rocks to expose fresh areas for chemical analysis.

The wealth of data returned by both rovers has helped scientists understand much more about the early history of Mars, especially the presence and chemistry of water on or near the Martian surface. The mobility and long life of the rovers allowed a wide variety of geological structures to be examined in detail, including layers of ancient bedrock exposed inside impact craters and ancient eroded volcanic terrains.

The rovers detected deposits of sulfate salts that must have formed in wet conditions. Scientists also found evidence of geologic curved patterns called cross-beds, which can occur when water currents cause rock layers to be deposited at an angle to other layers, and they found puzzling pebble-size spherical grains of rock similar to those that result when minerals form out of porous, water-soaked sediments. The combination of all these findings led the scientists to conclude that only the existence of liquid water in this area of Mars in the distant past could explain all of these features. Later studies indicated that upwelling groundwater could also have been the source of the salt and mineral deposits left by evaporation, rather than exclusively a shallow surface lake or sea. Additional finds have included silica minerals similar to those formed by volcanic fumaroles or hot springs on Earth.

The rovers also photographed the Martian sky, detecting ice clouds. A major dust storm in 2007 temporarily reduced solar power to both robots to dangerously low levels, but allowed scientists to learn about conditions during such atmospheric events.

B Exploration of Mars by Other Countries B1 The Soviet Missions

From 1960 to 1971 the Union of Soviet Socialist Republics (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 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. In 1996 the Russian Space Agency's ambitious Mars 96 orbiter mission suffered an unsuccessful launch and crashed back to Earth. The Russians are planning a 2009 or 2011 launch of a mission that would return a sample from the Martian moon Phobos. That mission will also involve significant cooperation with the China National Space Administration and the European Space Agency. In the meantime, Russian astronomers continue to participate as collaborators on Mars missions of other nations.

B2 ESA and Japanese Missions

Many other nations have participated in Mars exploration, either by contributing scientific knowledge and instrumentation to missions led by the United States and Russia or by launching their own spacecraft. The European Space Agency (ESA) successfully placed the Mars Express spacecraft in orbit around Mars in December 2003 after launching it from the Baikonur Cosmodrome in Kazakhstan in June 2003. However, ESA officials were unable to make contact with the spacecraft’s lander, the British-built Beagle 2, after it separated from the Mars Express orbiter and descended to the surface of Mars. The lander, named after the ship that carried British naturalist Charles Darwin to the Galápagos Islands, was to use its instruments to search for past or present signs of life on Mars. Nevertheless, the seven remote-sensing instruments on board the Mars Express orbiter are returning valuable data about both the Martian surface and atmosphere. For example, Mars Express instruments have found evidence for clays, sulfates, and other minerals on the surface, supporting and extending the results from the Mars rovers.

Japan also launched a spacecraft to Mars, but like so many other Martian missions, it failed. The spacecraft called Nozomi was launched in July 1998 and initially went into orbit around the Sun. In December 2003 Japanese space officials announced that malfunctions had caused the spacecraft to go off its intended trajectory to Mars and that they were abandoning efforts to correct its course.

C Future Exploration of Mars

The United States and other countries are in the midst of an ambitious, long-term program of Mars exploration. NASA’s Phoenix Mars Lander mission was launched in 2007 and scheduled to land in May 2008. The Phoenix Mars Lander mission is intended to land a spacecraft in an ice-rich area of the north polar region of Mars and to scoop up samples of soil for detailed analysis. It is part of the Mars Scout program, which is conceived as a series of low-cost missions to Mars. The Mars Science Laboratory in 2009 is intended to put a rover on Mars that can travel over a region at least 20 km (12.5 mi) wide. A possible sample-return mission from the surface of Mars is being studied for a potential 2018 or 2020 launch, although no specific plans have yet been announced for this mission.

ESA is considering an advanced Mars rover called ExoMars as part of its Aurora planetary exploration program. If approved for launch, ExoMars would search for signs of present or past life on Mars. That mission could launch in 2013 and land in 2014. A European-led sample-return mission from the Martian surface is also under study, possibly in collaboration with NASA and other countries. Russia and China have announced plans to study joint missions to return samples to Earth from the moon Phobos and possibly from the surface of Mars.

What would be the most ambitious Mars mission yet was announced in January 2004 by U.S. president George W. Bush, who called for the establishment of an astronaut base on the Moon that would provide a launching pad for a later human mission to Mars. NASA announced more details about this new round of human space exploration in 2006 in what is being called the Constellation Program. The primary vehicles would be the piloted Orion space capsule, which carries a crew of six, and the Ares booster rocket. Flights to Mars would require a much more powerful version of the booster called Ares V. Plans for this ambitious return of humans to the Moon and then on to Mars are still being intensely discussed and debated in the United States, and specific launch and mission schedules have not yet been established.

IX SEARCH FOR LIFE ON MARS

Mars is the most Earth-like 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.

A Early Speculations

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 were unable 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, although numerous channels carved naturally by flowing water were discovered.

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.

B Current Scientific Knowledge

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 think that it is highly unlikely 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. Static electricity generated by dust storms and dust devils may also create hydrogen peroxide, which can break down organic chemicals.

However, a growing number of scientists believe that some form of life could possibly exist on Mars today in more protected environments such as underground or inside pores and cracks in rocks—places sheltered 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. See also Hydrothermal Vent.

A more basic question is whether conditions on Mars were ever Earthlike enough, and for a long enough time, for some form of life to have evolved. Liquid water has been essential for life as we know it to evolve and survive on Earth. The Spirit and Opportunity Rovers both found evidence that liquid water once existed on and below the surface of Mars billions of years ago, but the results seem to indicate that water would have been salty and extremely acidic, resembling dilute sulfuric acid. Although preexisting Martian microorganisms could possibly have adapted to to saltiness and high acidity, it is less clear that life itself could start in such a harsh environment.

Ancient terrains examined from orbit hint that conditions more favorable for the evolution of life may have existed at an earlier period on Mars before volcanic eruptions changed the surface chemistry with large amounts of sulfur. Detailed images from the Mars Reconnaissance Orbiter also show evidence that water or other fluids may have flowed underground between layers of rock, providing a possible environment for microorganisms.

The Odyssey orbiter may also have found large areas of salt on the surface that could have been left by bodies of water that evaporated. Some impact craters also show evidence that they once contained lakes. The discovery by the Spirit rover of silica deposits from apparent hot springs suggests that isolated spots on the surface may once have been more favorable to life. If life once existed on Mars but died out, the preserved chemical signatures of life, and possibly even actual fossils, may exist in ancient sediments.

C Meteorites from Mars

Rare types of meteorites found on Earth have a chemistry very similar to rocks on Mars. Scientists think these meteorites are chunks of Mars that were blasted into space by large impacts on the planet’s surface. The rocks went into orbit around the Sun and eventually fell to Earth as meteorites. Some of these meteorites have been studied for possible signs of Martian life.

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 Allen Hills meteorite was found in Antarctica and had 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.

Later studies of volcanic rocks formed by eruptions in freezing conditions on Earth found tiny spheres of carbonate minerals very similar to those in the Martian meteorite. Nonetheless, the discovery that chemical reactions with rock could create organic material in association with minerals such as magnetite indicates that organic compounds could form under conditions that exist on Mars.

In 2006 a group of scientists suggested that microscopic etch marks found in another meteorite from Mars could be possible evidence that Martian bacteria had once fed on minerals in the rock. Underground bacteria found in Earth leave similar etch marks when they extract chemical energy from iron minerals in rock. The meteorite was a volcanic rock that is thought to have been exposed to water about 600,000 years ago on Mars and later blasted into space by a large impact. No evidence of DNA was found in the sample.

Even if these kinds of Martian meteorites do not preserve actual evidence of life on Mars, their most important message may be that Mars is one of the few places in the solar system where we know that the conditions were habitable and life was at least possible. The goal now is to figure out how and where to look for more convincing evidence such as traces of organic material. Robotic Mars missions such as the Mars Science Laboratory, ExoMars, and sample returns are the next logical steps.