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Mars moves through our skies in its stately dance, distant and enigmatic, a world awaiting exploration.

Carl Sagan, Mars: A New World to Explore (December 1967)

Mars has been a mystery to humans for thousands of years. Even though much is known about it, there is still much more to learn. Mars is the fourth planet from the Sun, and the planet most like Earth in the solar system. It is named after the mythical god of war whom the Romans called Mars and the Greeks called Ares. Mars is also known as the Red Planet, because it looks reddish from Earth. Mars is a dusty, cold world. The average temperature is 64 degrees Fahrenheit. Rays of ultraviolet radiation beat down on the surface continuously. The atmosphere is nearly all carbon dioxide.

People on Earth have always been fascinated with the idea of life on Mars. Ancient people could see Mars as a pale reddish light in the nighttime sky. They believed that it was stained with the blood of fallen warriors. Once telescopes were invented, people had a better view of the planet, but many still thought it was inhabited. Patterns of straight lines could be seen on the surface. To some, these were evidence of water canals dug into the ground by hard-working Martians. The notion lingered for decades in the public imagination.

At the dawn of the space age, humans sent robotic probes to Mars to settle the question once and for all. These probes found a frozen wasteland of fine powdery dust. Neither canals nor Martians could be located. There was some water vapor in the atmosphere and some frozen water at the planets poles. Where there is water, there is the potential for life similar to that found on Earth. Scientists continue to send probes to search for water and life.

In January 2004 President George W. Bush (1946) proposed that astronauts travel to Mars and explore the planet. It will be expensive and difficult. It takes six months to fly to Mars. The United States will need new rockets and spacecraft and some clever ways to keep astronauts healthy and happy on such a long journey. These are great challenges, but the idea is tantalizinghumans standing on another planet. Finally, there would be some life on Mars.


The Italian astronomer Galileo Galilei (1564-1642) was probably the first to see Mars through a telescope. He noticed that sometimes it appeared larger than at other times. He believed that its distance from Earth was changing over time. During the seventeenth century Johannes Kepler (1571-1630) studied Marss movement for years. His observations helped him to develop the laws of planetary motion for which he would become famous.

As telescopes improved, astronomers reported seeing dark and light patches on Mars that also varied in size over time. Some people thought these were patches of vegetation changing in response to the changing seasons. Others believed that they represented contrasting areas of land and sea.

In 1659 the Dutch astronomer Christiaan Huygens (1629-1695) recorded his Mars observations and noticed an odd-shaped feature that came to be called the hourglass sea. Huygens kept an eye on the location of the sea over time and determined that the Martian day lasts about twenty-four hours. The same conclusion was reached independently by the French astronomer Giovanni Cas-sini (1625-1712).

During the 1700s astronomers performed more detailed observations of the light and dark patches on Mars, particularly the whitish spots at the north and south poles. They could see that the spots changed in size over time, but they did not guess that these were polar ice caps. It was commonly believed that Mars was inhabited by some kind of beings. In 1774 the English astronomer William Herschel (1738-1822) speculated that Martians lived on a world much like Earth, with oceans on the surface and clouds flying overhead.


During the late 1800s Mars became the topic of a debate that would go on for decades. The controversy was sparked by the observations of the Italian astronomer Giovanni Schiaparelli (1835-1910). He created some of the first maps of the planet and assigned names to prominent features. Schiaparellis naming system relied on place names taken from the Bible and ancient mythology.

Schiaparelli said he saw straight lines on the Martian surface and called them canali. In Italian canali can mean either channels or canals. Many people interpreted the word to mean that there were artificial canals on Mars. The Suez Canal had recently been constructed in Egypt to connect the Mediterranean Sea to the Red Sea. Obviously, if there were artificial canals on Mars, they had been built by Martians.

Some other Mars observers also reported seeing the canals and claimed they connected light and dark patches on the planet. This reinforced the mistaken idea that the patches were areas of land and water.


In August 1877 the American astronomer Asaph Hall (1829-1907) discovered that Mars has two moons. Centuries before him, Kepler guessed that Mars had two moons, but this had never been verified.

Hall reported that the moons were small and orbited close to the planets surface, which made them difficult to observe. He made the discovery using a powerful new telescope recently installed at the U.S. Naval Observatory in Washington, D.C. He named the moons Phobos (meaning fear) and Deimos (meaning flight or panic). These are two characters mentioned in ancient Greek mythology as being servants to the god Mars.


Percival Lowell (1855-1916) was a mathematician and amateur astronomer who greatly popularized the idea that Mars was inhabited by intelligent beings.

In 1894 he founded the Lowell Observatory in Flagstaff, Arizona. Perched at an altitude of seven thousand feet, the observatory provided one of the best views yet of the cosmos, including Mars. For fifteen years Lowell studied the Red Planet and wrote about his observations. He was convinced that there were artificial canals on Mars, because he could see hundreds of straight lines on the surface that intersected large patches of contrasting colors. Lowell argued that the canals must have been built to move water from the melting polar ice caps toward the desert regions near the planets equator.

Lowell publicized his theories in many articles and three popular books: Mars (1895), Mars and Its Canals (1906), and Mars as the Abode of Life (1908). In 1901 he constructed a globe of Mars showing large geological features crisscrossed by a network of lines that intersected at certain points around the planet. Lowell called these intersections oases, because he imagined they were fertile green areas amid the desert.

In a series of articles published during 1895 in the Atlantic Monthly, Lowell explained his theories in detail. He believed that Mars had a thin air-based atmosphere containing lots of water vapor and that fresh water flowed from the polar ice caps through irrigation canals built by the highly intelligent Martians.


Lowells ideas were not shared by most astronomers of the time. In 1908 the distinguished journal Scientific American noted that Lowell is practically alone in the astronomical world in believing that he has proven that Mars is inhabited.

Scientists pointed out that artificial canals would have to be hundreds of miles wide to be visible from Earth. Furthermore, Mars was believed to be extremely cold, because of its great distance from the Sun. This made it even more unlikely that open flowing water was present on the planets surface. Agnes Mary Clerke (1842-1907) was a science writer trained in astronomy who wrote well-respected books about astronomical discoveries. She called Lowells idea hopelessly unworkable.

Lowell did have his supporters. The French astronomer Camille Flammarion (1842-1925) also believed that the lines on Mars were artificial canals built by an advanced civilization. Flammarion insisted that the reddish appearance of Mars was due to the growth of red vegetation on the planet.

In 1907 the natural scientist Alfred Russel Wallace (1823-1913) wrote the book Is Mars Habitable ?, which examined Lowells claims one by one and attacked them with scientific data and reasoning. The book is considered a pioneering work in the field of exobiology (the investigation of possible life beyond Earth).

Wallace argued that Mars was a frozen desert and that the polar caps were probably frozen carbon dioxide, instead of water ice. Wallace ended the book with the following definitive statement: Mars, therefore, is not only uninhabited by intelligent beings such as Mr. Lowell postulates, but is absolutely UNINHABITABLE.

Many astronomers of the time admitted seeing fine lines on the Martian surface. Most believed that these lines were either natural geological features or an optical illusion. The astronomer William H. Pickering (1858 1938) believed the lines to be cracks in Marss volcanic crust. He speculated that hot gases and water escaped through the cracks and supported vegetative growth. This explained the appearance of different colored splotches on the planet. The general public and science-fiction writers much preferred Lowells explanation.


Around the turn of the nineteenth century Mars became a popular topic of science fiction. Before that time there is little mention of the Red Planet. One notable exception is a fanciful story published in 1726 by the Irish writer Jonathan Swift (1667-1745). Gullivers Travels mentions the discovery of two Martian moons by astronomers living on the fictional island of Laputa. Oddly enough, Mars does have two moons, but they were not discovered until 151 years after the book was published.

After Schiaparelli and Lowell popularized the idea of intelligent life on Mars, science-fiction writers gleefully embraced the notion. In 1898 Herbert George Wells (1866-1946) portrayed Martians as lethal invaders in War of the Worlds. The insectlike creatures come to Earth looking for water and leave destruction in their path. They are finally wiped out by a common Earth germ to which they do not have immunity. The story was famously adapted for radio in 1938 and for film more than once, most recently in 2005.

Beginning in 1910 Edgar Rice Burroughs (1875 1950) wrote a series of adventure books in which an Earth man battles and romances his way around Mars. In his books the planet is called Barsoom by its exotic inhabitants. They come in various shapes, sizes, and colors.

In 1924 the motion picture Aelita: Queen of Mars debuted in the Soviet Union. It featured an engineer who takes a spaceship to Mars and falls in love with the planets beautiful queen. At the end of the film, he wakes up and discovers the journey was just a dream.

Three decades later Martians became popular characters in American media. Ray Bradbury (1920-) published a series of stories called The Martian Chronicles in which well-intentioned humans travel to Mars and accidentally spread deadly Earth germs among the Martian population. It was an interesting twist on the theme introduced by Wells a half-century before.

Evil invaders from Mars were common villains in low-budget horror movies of the 1950s. Historians now believe these sinister creatures symbolized the threat that Americans felt from the Soviet Union during the cold war. During the early 1960s the television show My Favorite Martian featured a friendly and wise Martian who crash lands on Earth and befriends a newspaper reporter.


Mars is a small planet. Its diameter is about half that of Earth. Mars is twice as large as Earths Moon.

A Martian day lasts twenty-four hours and thirty-nine minutes and is called a sol. It takes Mars 687 days to travel around the Sun. The planet has different seasons throughout its orbit, because it is tilted, just like Earth. During a Martian winter, the temperature at the poles can drop to 200 degrees Fahrenheit. At the equator during the summer, the temperature can reach 80 degrees Fahrenheit.

The force of gravity is much weaker on Mars than it is on Earth. An astronaut standing on Mars would feel only 38% as much gravity as on Earth.

Martian Geology and Atmosphere

Mars is called a terrestrial planet, because it is composed of rocky material, like Mercury, Venus, and Earth. Mars has some of the same geological features as Earth, including volcanoes, valleys, ridges, plains, and canyons.

Most Martian features have two-word names. One of the words is a geological term, and is usually from Latin or Greek, for example, mons for mountain, planitia for plains, and vallis for valley. The other word comes from the classical naming system begun by Schiaparelli during the 1800s or from later astronomers. Beginning in 1919 the International Astronomical Union (IAU) became the official designator of names for celestial objects and the features on them. Only the IAU has this authority.

There are two particularly prominent features on Mars. The first is the volcano Olympus Mons that is about seventeen miles high. This is three times higher than Mount Everest on Earth. In English Olympus Mons means Mount Olympus. This was the home of the gods in ancient Greek mythology. The other notable geological feature on Mars is the canyon Valles Marineris (Mariner Valleys). This enormous canyon is twenty-five hundred miles long by sixty miles wide and up to six miles deep in places. It was named after the Mariner spacecraft that photographed it during the 1960s.

The surface of Mars is covered with a fine powdery dust with a pale reddish tint. This is due to the presence of oxidized iron minerals (like rust) on the planets surface. The Martian atmosphere is thin and contains more than 95% carbon dioxide. There is a tiny amount of oxygen, but not enough for humans to breathe. It is windy on Mars. Strong winds sometimes engulf the planet in dust storms that turn the atmosphere a hazy yellowish-brown color. The wind also blows clouds around the sky.

The Martian poles are covered by solid carbon dioxide (dry ice) layered with dust and water ice. These polar caps change in size as the seasons change. Sometimes during the summer the uppermost dry ice evaporates away, only to re-form again when the weather turns cold.

Martian Moons

The two Martian moons Phobos and Deimos are not round spheres like Earths Moon. They are shaped like lopsided potatoes. Phobos is seventeen miles long and twelve miles wide and is approximately fifty-eight hundred miles from Mars; Deimos is ten miles long and six miles wide and is nearly fifteen thousand miles away.

The Martian moons are small compared to other moons in the solar system. Many scientists believe that Phobos and Deimos are actually asteroids that wandered too close to Mars and were captured by its gravity. There is a large asteroid belt located between the orbits of Mars and Jupiter. This could be where Phobos and Deimos originated.

Mars in Orbit and Opposition

Because their orbital paths are different, Earth and Mars each take a different amount of time to complete an orbit around the Sun. This means that Mars and Earth are constantly changing position in relation to each other. At their most distant point the two planets are 233 million miles apart. At their closest point they are less than thirty-five million miles apart. This explains why in some years Mars looks closer to Earth than in others. During August 2003 Mars was only 34.7 million miles from Earth. It will not be this close again until 2287.

About every twenty-six months the Sun, Earth, and Mars line up in a row with Earth lying directly in the middle. This configuration is called Mars in opposition. It means that Mars is closer to Earth than usual and is easier to observe. Most of the historic discoveries about Mars occurred when the planet was in opposition. This was particularly true for the 1877 opposition associated with the findings of Schiaparelli and Hall. Scientists now know that Mars was only thirty-five million miles from Earth during that opposition.

The most recent Mars opposition occurred in December 2007. The next one will be in January 2010. Oppositions are the best times to send spacecraft to Mars.


After the Moon the planet Mars was the destination of choice during the early days of space travel. The Soviet Union was particularly eager to reach the Red Planet before the United States. A historical log of all Mars missions from 1960 to 2007 is presented in Table 7.1.

Many Failures

Historically, spacecraft have had a difficult time making it to Mars in working order and staying that way. More than half of the missions intended for Mars have failed for one reason or another. (See Table 7.1.) Some were plagued by launch problems, whereas others suffered malfunctions during flight, descent, or landing.

Mars missions undertaken during the 1960s by the former Soviet Union were particularly trouble-prone. All six of them failed. Even though the next decade showed some improvement, little usable data were obtained from the spacecraft that reached their destination. The one attempt to reach Mars by the Russian Space Agency, in 1996, failed when the spacecraft was unable to leave Earth orbit.

In contrast to the Soviet Unions Mars attempts, most of the National Aeronautics and Space Administrations (NASA) Mars missions conducted during the 1960s achieved their objectives. There was also notable success over the next decade with the Viking spacecraft. After the Viking mission, there was a long lull in NASAs Mars exploration program.

During the 1990s NASA launched five separate spacecraft to Mars: Mars Observer (1992), Mars Global Surveyor (1996), Mars Pathfinder (1996), Mars Climate Orbiter (1998), and Mars Polar Lander (1999). Only two of the spacecraft were successful (Mars Global Surveyor and Mars Pathfinder ). The other spacecraft were lost on arrival.

NASA lost contact with the Mars Observer just before it was to go into orbit around Mars. It is believed that some kind of fuel explosion destroyed the spacecraft as it began its maneuvering sequence. The Mars Observer carried a highly sophisticated gamma-ray spectrometer designed to map the Martian surface composition from orbit. Failure of the mission resulted in a loss estimated at $1 billion. This was by far the most expensive of NASAs failed Mars missions.

In September 1999 the Mars Climate Orbiter was more than sixty miles off course when it ran into the Martian atmosphere and was destroyed. The loss of the $85 million spacecraft was particularly embarrassing for NASA, because it was due to human error. An investigation revealed that flight controllers had made mistakes doing unit conversions between metric units and English units. This resulted in erroneous steering commands being sent to the spacecraft. Outside investigators complained that the problem was larger than some mathematical errors. They blamed overconfidence and poor oversight by NASA management during the mission.

NASAs embarrassment deepened a few months later when the Mars Polar Lander was lost. The loss was attributed to a software problem that caused the spacecraft

TABLE 7.1 Historical log of Mars expeditions, 1960-2007
Mission Sponsor Launch date Purpose Results
SOURCE: Adapted from Historical Log, in NASAs Mars Exploration Program, National Aeronautics and Space Administration, Jet Propulsion Laboratory, California Institute of Technology, November 6, 2007, (accessed January 1, 2008)
Korabl 4 USSR 10/10/1960 Mars flyby Did not reach Earth orbit
Korabl 5 USSR 10/14/1960 Mars flyby Did not reach Earth orbit
Korabl 11 USSR 10/24/1962 Mars flyby Achieved Earth orbit only
Mars 1 USSR 11/1/1962 Mars flyby Radio failed at 65.9 million miles (106 million km)
Korabl 13 USSR 11/4/1962 Mars flyby Achieved Earth orbit only
Mariner 3 U.S. 11/5/1964 Mars flyby Shroud failed to jettison
Mariner 4 U.S. 11/28/1964 First successful Mars flyby 7/14/65 Returned 21 photos
Zond 2 USSR 11/30/1964 Mars flyby Passed Mars but radio failed, returned no planetary data
Mariner 6 U.S. 2/24/1969 Mars flyby 7/31/69 Returned 75 photos
Mariner 7 U.S. 3/27/1969 Mars flyby 8/5/69 Returned 126 photos
Mariner 8 U.S. 5/8/1971 Mars orbiter Failed during launch
Kosmos 419 USSR 5/10/1971 Mars lander Achieved Earth orbit only
Mars 2 USSR 5/19/1971 Mars orbiter/lander arrived 11/27/71 No useful data, lander destroyed
Mars 3 USSR 5/28/1971 Mars orbiter/lander, arrived 12/3/71 Some data and few photos
Mariner 9 U.S. 5/30/1971 Mars orbiter, in orbit 11/13/71 to 10/27/72 Returned 7,329 photos
Mars 4 USSR 7/21/1973 Failed Mars orbiter Flew past Mars 2/10/74
Mars 5 USSR 7/25/1973 Mars orbiter, arrived 2/12/74 Lasted a few days
Mars 6 USSR 8/5/1973 Mars orbiter/lander, arrived 3/12/74 Little data return
Mars 7 USSR 8/9/1973 Mars orbiter/lander, arrived 3/9/74 Little data return
Viking 1 U.S. 8/20/1975 Mars orbiter/lander, orbit 6/19/76-1980, lander 7/20/76-1982 Combined, the Viking orbiters and landers returned 50,000+ photos
Viking 2 U.S. 9/9/1975 Mars orbiter/lander, orbit 8/7/76-1978, lander 9/3/76-1980 Combined, the Viking orbiters and landers returned 50,000+ photos
Phobos 1 USSR 7/7/1988 Mars/Phobos orbiter/lander Lost 8/88 en route to Mars
Phobos 2 USSR 7/12/1988 Mars/Phobos orbiter/lander Lost 3/89 near Phobos
Mars Observer U.S. 9/25/1992 Orbiter Lost just before Mars arrival 8/21/93
Mars Global Surveyor U.S. 11/7/1996 Orbiter, in orbit 9/12/97-2006 Conducted prime mission of science mapping
Mars 96 Russia 11/16/1996 Orbiter and landers Launch vehicle failed
Mars Pathfinder U.S. 12/4/1996 Mars lander and rover, landed 7/4/97 Last transmission 9/27/97
Nozomi (Planet-B) Japan 7/4/1998 Mars orbiter Could not achieve Martian orbit due to propulsion problem
Mars Climate Orbiter U.S. 12/11/1998 Orbiter Lost on arrival at Mars 9/23/99
Mars Polar Lander/Deep Space 2 U.S. 1/3/1999 Lander/descent probes to explore Martian south pole Lost on arrival 12/3/99
Mars Odyssey U.S. 4/7/2001 Orbiter, arrived 10/24/2001 Currently conducting prime mission of science mapping
Mars Express Europe 6/2/2003 Orbiter and lander, arrived 12/24/2003 Orbiter currently collecting planetary data. Beagle 2 lost during descent.
Mars Exploration U.S. 6/10/03 (Spirit) and 7/7/03 (Opportunity) Two rovers: Spirit, landed 1/4/2004, and Opportunity, landed 1/25/2004 Rovers landed in January 2004. Currently exploring planet surface.
Mars Reconnaissance Orbiter U.S. 8/12/2005 Orbiter, arrived 3/10/2006 Currently photographing the planet, identifying surface minerals, and studying how dust and water are transported in the Martian atmosphere
Phoenix Mars Lander U.S. 8/4/2007 Lander will use robotic arms to sample Marss icy northern pole Scheduled to land in May 2008

to think it had touched down on the surface even though it had not. The computer apparently shut down the engines during descent and let the spacecraft plummet at high speed into the ground, where it was destroyed. The cost of the failed spacecraft was estimated at $120 million.


The Mariner program included a series of spacecraft launched by NASA between 1962 and 1973 to explore the inner solar system (Mercury, Venus, and Mars). These were relatively low-cost missions conducted with small spacecraft launched atop Atlas-type rockets. Each spacecraft weighed between four hundred and twenty-two hundred pounds. They were designed to operate for several years and collect specific scientific data about Earths nearest planetary neighbors.

Six of the Mariner spacecraft were scheduled for Mars missions. Two of these spacecraft failed. In 1964 Mariner 3 malfunctioned after takeoff and never made it to Mars. In 1971 Mariner 8 failed during launch. This left four successful Mariner Mars spacecraft: Mariner 4, Mariner 6, Mariner 7, and 9.

Mariner 4

In July 1965 Mariner 4 achieved the first successful flyby of Mars. A planetary flyby mission is one in which a spacecraft is put on a trajectory that takes it near enough to a planet for detailed observation, but not close enough to be pulled in by the planets gravity.

During its flyby, Mariner 4 took twenty-one photos, the first close-ups ever obtained of Mars. They showed a world pockmarked with craters, probably from meteor strikes.

Mariner 6 and 7

In 1969 Mariner 6 and Mariner 7 conducted a dual mission to Mars. Both spacecraft flew by the planet, and together sent back 201 photos. These photos revealed that the features once thought to be canals were not canals after all. Instead, it appears that a number of small features or shadows on Mars only looked like they were aligned when viewed through Earth-based telescopes. The illusion was perpetuated by a human tendency to see order in a random collection of shapes. The mystery of the canali had finally been solved.

Mariner 9

Mariner 9 turned out to be the most fruitful of the Mariner missions. In November 1971 the spacecraft went into orbit around Mars after a five-and-a-half-month flight from Earth. It was the first artificial satellite ever to be placed in orbit around the planet.

When it first arrived, Mariner 9 found the entire planet engulfed in a massive dust storm. The spacecraft remained in orbit for nearly a year and returned 7,329 photos of the planets surface. For the first time scientists got a good look at Mars surface features, such as volcanoes and valleys. Mariner 9 showed geological features that looked like dry flood channels. It also captured the first close-up photos of Phobos and Deimos.

Scientists learned from Mariner data that Mars had virtually no magnetic field and was bombarded with ultraviolet radiation. Earths extensive magnetic field (or magnetosphere) helps protect the planet from dangerous electromagnetic radiation traveling through space. Scientists knew that lack of such protection on Mars would make it exceedingly difficult for life to exist on the planet.


Within only four years NASA went from orbiting Mars to landing on the planet. In 1976 the Viking mission was the first American spacecraft to land safely on Mars. For the mission NASA built two identical spacecraft, each containing an orbiter and lander. The two spacecraft entered orbit around Mars and released their landers to descend to the planets surface.

The spacecraft were launched only weeks apart during the summer of 1975. It took them nearly a year to reach Mars. On July 20, 1976, the Viking 1 lander set down on the western slope of Chryse Planitia (Plains of Gold). On September 3, 1976, the Viking 2 lander set down at Utopia Planitia (Plains of Utopia).

The landers provided NASA with constant weather reports. They detected nitrogen in the atmosphere. Scientists reported that a thin layer of water frost formed on the ground during the winter near the Viking 2 lander. Temperatures varied between 184 degrees Fahrenheit in the winter to 7 degrees Fahrenheit in the summer at the lander locations.

The orbiters mapped 97% of the Martian surface and observed more than a dozen dust storms. Scientists examined Viking images and decided that some geologic features on Mars could have been carved out millions of years ago by flowing water. The Viking 2 orbiter continued functioning until July 1978, and its lander ended communications in April 1980. The Viking 1 orbiter was powered down in August 1980, and its lander continued to make transmissions to Earth until November 1982.

The landers were unique because they were powered by generators that created electricity from heat released during the natural decay of plutonium, a radioactive element. This method of power generation was selected because NASA feared that sunlight on the planet would not be consistent enough to provide solar power.

The Viking orbiters carried high-resolution cameras and were able to map atmospheric water vapor and surface heat from orbit. The landers included cameras and a variety of scientific instruments designed to investigate seismology, magnetic properties, meteorology, atmospheric conditions, and soil properties. They also tested for the presence of living microorganisms in the soil, but found no clear evidence of them. They did learn that the surface of Mars contains iron-rich clay. The Viking images revealed that Mars has a light yellowish-brown atmosphere due to the presence of airborne dust. In other words, the Red Planet is actually more the color of butterscotch.

Many scientists associated with the Viking project concluded that Mars is self-sterilizing. This means that the natural planetary conditions are such that living organisms cannot form. The high radiation levels and the unique soil chemistry are actually destructive to life. The Martian soil was found to be extremely dry and oxidizing. Oxidizing agents destroy organic chemicals considered necessary for life to form. The self-sterilizing theory is not universally accepted, however, and remains controversial.


On December 27, 1984, a meteorite hunter found a four-pound rock on the Allan Hills ice field in Antarctica. The rock was grayish-green and covered with pits and gouges. It was given the designation ALH84001.

The National Science Foundation (NSF) conducts annual searches in Antarctica looking for meteorites (rocks that have traveled through space to Earth). Each possible candidate is collected and assigned a tracking code. The letters in the tracking code represent the location of the find (Allan Hills). The first two numbers indicate the year of the find (1984), and the last three digits indicate the order in which the rock was processed by the NSF that year. ALH84001 was recognized immediately as a significant find, so it was the first rock investigated during that sampling year. In fact, the article X. ALH84001 (Charles Meyer, comp., The Mars Meteorite Compendium, 2003) notes that the person who found it wrote Yowza-Yowza across the field notes.

Scientists also got excited when they examined the rock, because they found gas trapped within it that matched the known atmosphere of Mars. They concluded that the rock formed on Mars 4.5 billion years ago. About sixteen million years ago an asteroid probably slammed into the planet and sent the rock hurtling through space. Scientists believe the rock arrived on Earth thirteen thousand years ago.

The rock contains a small amount of carbonate (a carbon-containing compound). Some scientists believe that the carbonate formed inside the rock in the presence of liquid water. This would mean that liquid water existed on Mars billions of years ago. However, the exact origin of the carbonate is still under debate.

Rocks determined to be meteorites are kept in special laboratories at NASA or the Smithsonian Institution. NASA reports in Meteorites from Mars (March 26, 2007, that there have been thirty-one known Martian meteorites found on Earth since 1815. ALH84001 is the oldest meteorite in the collection.


More than twenty years passed between the launch of the highly productive Viking mission and another successful mission to Mars. In November 1996 the Mars Global Surveyor (MGS) took off from the Cape Canaveral Air Station in Florida atop a Delta II rocket. The spacecraft arrived near the planet ten months later. To save on fuel, the MGS was put into its final Martian orbit very slowly through a process called aerobraking.

During aerobraking a spacecraft is repeatedly skimmed through the thin upper atmosphere surrounding a planet. Each skim reduces the speed of the craft due to frictional drag. Aerobraking eliminates the need for extra fuel to do a retro-burn to slow down a spacecraft.

The MGS was put through a long series of gentle skims for a year and a half. Generally, aerobraking does not take this long. However, flight controllers were extremely careful with the MGS because one of its solar panels did not fully deploy during flight. Scientists were afraid that aggressive skimming might put too much stress on the panel.

In March 1999 the spacecraft began its mapping mission. This continued for one Martian year (687 days). The most significant finding during mapping was images of gullies and other flow features that scientists believe may have been formed by flowing water. The MGS also captured close-up photographs of Phobos. The images reveal that the moon is covered with at least three feet of powdery material.

In April 2002 the MGS began performing data relay and imaging services for other NASA spacecraft carrying out missions at Mars. In November 2006 NASA lost contact with the MGS and assumed that its batteries had finally failed. The spacecraft operated for nine years and fifty-two days, the longest Mars mission to record.


Mars Pathfinder was a mission conducted as part of NASAs Discovery Program. This was the agencys faster, better, cheaper approach to space science. The mission was developed in only three years and cost $265 million. On December 4, 1996, the spacecraft launched atop a Delta II rocket from the Cape Canaveral Air Station in Florida. The Pathfinder traveled for seven months before entering into the gravitational influence of Mars.

On July 4, 1997, the spacecraft was ordered to begin its descent to the planets surface. The landing craft separated from the spacecraft shell and began to drop. A giant parachute released to slow its fall. Eight seconds before hitting the ground the landers air bags deployed around it like a cocoon to cushion its impact on the surface. The lander ball bounced and rolled for several minutes before coming to a stop more than half a mile from where it first impacted. It was in the rocky flood plain Ares Vallis (Valley of Ares).

After the successful landing, NASA renamed the lander the Carl Sagan Memorial Station, in memory of the astronomer Carl Sagan (1934-1996). He died while Pathfinder was en route to Mars. The lander unfolded three hinged solar panels onto the ground. (See Figure 7.1.) It released a small six-wheeled rover named Sojourner that began exploring the nearby area. The name resulted from a NASA contest in which schoolchildren proposed names of historical heroines for the mission. The winning entry suggested Sojourner Truth (1797?-1883), an African-American woman who crusaded for human rights during the 1800s.

For two and a half months the Sojourner collected data about Martian soil, radiation levels, and rocks. The rover weighed twenty-three pounds and could move at a

top speed of two feet per minute. It was powered by a flat solar panel that rested atop its frame. (See Figure 7.2.)

Meanwhile, the lander collected images and relayed data back to Earth. It also measured the amount of dust and water vapor in the atmosphere. The landers forty-inch mast held little wind socks at different heights to determine variations in wind speed near the planets surface. Magnets were mounted along the lander to collect dust particles for analysis. Scientists learned that airborne Martian dust is magnetic and may contain the mineral maghemite, a form of iron oxide.

The Pathfinder returned more than seventeen thousand images and performed fifteen chemical analyses. Scientists studying this data concluded that Mars might have been warm and wet sometime in the past with a thicker, wetter atmosphere. In late September 1997 Pathfinder sent its last message home.


In Mars. I. Atmosphere (Atlantic Monthly, vol. 75, May 1895), Lowell said, If Mars be capable of supporting life, there must be water upon his surface; for to all

forms of life water is as vital a matter as air. To all organisms water is absolutely essential. On the question of habitability, therefore, it becomes all-important to know whether there be water on Mars.

A century later this same issue drove NASA to conduct it most extensive program of missions to the Red Planet: the Mars Exploration Program. This is a long-term program in which robotic explorers are used to investigate Mars in support of four science objectives:

  • Determining whether life ever existed on Mars
  • Characterizing the climate of Mars
  • Characterizing the geology of Mars
  • Preparing for future human exploration of Mars

The overall motto of the Mars Exploration Program (March 22, 2006, is Follow the Water. In other words, the mission scientists hope that the discovery of liquid water on Mars will lead them to any microscopic life forms that exist on the planet or were ever present.

The 2001 Mars Odyssey mission falls under NASAs Mars Exploration Program. The mission was named after the hit 1968 movie 2001: A Space Odyssey, based on a short story by the science-fiction writer Arthur C. Clarke (1917-2008).

On April 7, 2001, Odyssey was launched toward Mars atop a Delta II rocket. The spacecraft reached Mars six months later. To conserve fuel Odyssey was placed in Martian orbit via aerobraking.

In February 2002 Odyssey reached its final orbit and began mapping the planets surface. The mission was intended to last for at least one Mars year. In early January 2004 Odyssey completed one Mars year in service. As of March 2008, NASA ( reported that the spacecraft was still operational and functioning well.

A schematic of the spacecraft is shown in Figure 7.3. It includes three scientific instruments: a thermal imaging system, a gamma-ray spectrometer, and the Mars Radiation Environment Experiment (MARIE).

The thermal imaging system collects surface images in the infrared portion of the electromagnetic spectrum. Everything that has a temperature above zero kelvin (the lowest possible temperature in the universe, at which all atomic activity ceases; equivalent to 459.7 degrees Fahrenheit) emits infrared radiation. Scientists use Odysseys images to identify and map minerals in the surface soils and rocks. This work is being coordinated with the mineral mapping being performed by the Mars Global Surveyor.

Odyssey s gamma-ray spectrometer can detect the presence of various chemical elements on the planets surface. This is particularly useful for finding water ice buried beneath the surface and for detecting salty minerals. Odyssey data indicate the presence of large amounts of water ice just beneath the surface in the polar regions. The MARIE instrument collects radiation data that will be useful to planning any future Mars expeditions by humans.

Odyssey s telecommunications system performs a dual role. It transmits to NASA data collected by the spacecraft itself and data collected by other NASA spacecraft conducting Mars missions.


Scientists knew that 2003 was going to be a good year to go to Mars, because Mars would be in opposition to Earth. On August 28, 2003, the Sun, Earth, and Mars were going to line up in a row. This happens every twenty-six months.

The opposition of 2003 was special, because it was going to occur while Mars was at its closest point to the Sun. This configuration is known as a perihelic opposition. When Mars is in perihelic opposition, it is much closer to Earth than usual. This means that less fuel and flight time are required to send a spacecraft from Earth to Mars near the time of a perihelic opposition.

Perihelic oppositions happen every fifteen to seven-teen years. During the late 1990s the Japan Aerospace

Exploration Agency, the European Space Agency (ESA), and NASA began planning Mars missions to coincide with the perihelic opposition of 2003. The Japanese Nozomi spacecraft suffered radiation damage during its flight and never made it to Mars.


The Mars Express mission is the first mission to Mars by the ESA. It was timed to put the spacecraft in flight near the time of Marss perihelic opposition.

In June 2003 the spacecraft was launched toward Mars from the Baikonur launch pad in Kazakhstan. A Russian Soyuz-Fregat rocket was used as the launch vehicle. The spacecraft included two parts: an orbiter and a lander named Beagle 2. The lander name was chosen in honor of the ship on which Charles Darwin (1809-1892) traveled during the 1830s while exploring South America and the Pacific region.

In late November 2003 the Mars Express reached the planets vicinity and prepared to go into orbit. On December 19, 2003, the Beagle 2 was released from the orbiter. Six days later the lander entered the Martian atmosphere on its way to a landing site at Isidis Planitia (Plains of Isis). The ESA lost contact with Beagle 2 as it descended toward the planet. Repeated attempts to reestablish contact were made over the next few months, but were not successful.

The Mars Express orbiter achieved orbit to begin its mission of collecting planetary data. As of March 2008, the ESA ( reported that the orbiter was still operating. The orbiter carries seven instruments designed to investigate the Martian atmosphere and geological structure and to search for subsurface water. One of the instruments (ASPERA-3 ) was supplied by NASA.


Another Mars mission began in 2003 with the launch of NASAs twin Mars Exploration Rovers (MERs). Each spacecraft carried a lander to Mars. Inside each lander was a golf cartsized rover that was designed to explore the Martian surface.

The rovers were named Spirit and Opportunity. The names were the winning entries in a naming contest NASA held in 2002. The winning entry came from a third-grade student living in Scottsdale, Arizona. She was born in Russia and adopted by an American family. She chose the names to honor her feelings about the United States.

NASA selected seven specific objectives for the MER missions:

  • Find and sample rocks and soils that could reveal evidence of past water on the planet
  • Characterize the composition of rocks, soils, and minerals near the landing sites
  • Look for evidence of geological processes (such as erosion or volcanic activity) that could have shaped the Martian surface
  • Use the rovers to verify data reported by the orbiters regarding Martian geology
  • Probe for minerals containing iron or water or minerals known to form in water
  • Analyze rocks and soils to characterize their mineral content and morphology (form and structure)
  • Seek out clues about the geological history of the planet to determine whether watery conditions could have supported life

The Launches

Spirit launched first on June 10, 2003. Opportunity launched several weeks later on July 7, 2003. The launch dates were chosen to put the spacecraft in flight near the time of Marss perihelic opposition.

Both spacecraft were launched atop Delta II rockets from the Cape Canaveral Air Station in Florida. Figure 7.4 shows a drawing of a rover spacecraft being released by its rocket to make the journey to Mars.

Landing on Mars

Figure 7.5 shows the various parts of the spacecraft that traveled to Mars. Each rover was nestled inside a landing vehicle protected by an aeroshell connected to the cruise stage of the spacecraft. The cruise stage contained fuel tanks, solar panels, and the propulsion system for trajectory corrections during flight. The aeroshell included two parts: a back shell and a heat shield. The back shell carried a deceleration instrument to ensure that the parachute was deployed at the right altitude above the Martian surface. It also had some small rockets to stabilize the spacecraft as it fell. The heat shield protected the lander-rover package from the heat generated by entering the Martian atmosphere.

The stages of entry, descent, and landing are shown in Figure 7.5. At twenty-one minutes before landing, the cruise stage separated from the rest of the spacecraft. Fifteen minutes later the spacecraft entered the atmosphere about seventy-four miles above the surface. The parachute deployed at an altitude of five miles when the craft was traveling nearly three hundred miles per hour. Seconds later the heat shield was jettisoned away. Eight seconds before hitting the ground the spacecraft deployed its air bags to cushion its impact with the ground. Retrorockets were fired to slow its descent. Three seconds later the parachute line was cut. The spacecraft ball bounced and rolled until it finally came to a stop. About an hour after landing the airbags were deflated and retracted so the lander could open its petal and release the rover.

On January 4, 2004, the Spirit MER landed on Mars. It was just after 8:30 p.m. at the mission control center in California. The landing site was in Gusev Crater, which was named in honor of the Russian astronomer Matvei Gusev (1826-1866). The crater is about one hundred miles in diameter and lies at the end of a long valley known as Ma'adim Vallis. This translates as Mars Valley, because Ma'adim is Hebrew for Mars. Major valleys on the Red Planet are named after Mars in different Earth languages.

On January 25, 2004, the Opportunity MER set down near Marss equator in an area called Meridiani Planum, which is considered the site of zero longitude on Mars. This is the longitude arbitrarily selected by astrogeolo-gists to be the prime meridian for the rest of the planet. Opportunitys landing site was nearly half way around Mars from Gusev Crater.

Both landing sites were chosen for their flat terrain. Gusev Crater is of interest to scientists because they believe it could be a dried-up lakebed. The Meridiani Planum is thought to contain a layer of hematite beneath the surface. Hematite is a gray iron-ore mineral similar to red rust that on Earth usually only forms in a wet environment. Both landing sites were considered prime locations to look for evidence of ancient water.

Roving Spirit and Opportunity

The components of an MER are labeled in Figure 7.6. The rovers are just over five feet long and weighed about 380 pounds on Earth. The panoramic cameras sit about five feet above the ground atop a mast.

Each rover carries a package of science instruments called an Athena science payload. Each payload includes two survey instruments, three instruments for close-up investigation of rocks, and a tool for scraping off the outer layer of rocks. The rovers were designed to move at a top speed of two inches per second. An average speed of 0.4 inches per second was expected when a rover was traveling over rougher terrain.

The rovers were designed to operate independently of their landers. Each rover carries its own telecommunications equipment, camera, and computer. The electronic equipment receives power from batteries that are repeatedly recharged by solar arrays. It was late summer on Mars when the rovers began their mission. Scientists expected that power generation would taper off after about ninety sols (or ninety-two Earth days) and eventually stop as the arrays became too dust-coated to harness solar power. However, scientists were pleasantly

surprised when dust devils kept sweeping by the rovers and blowing the dust off the arrays. These periodic cleanings have allowed the rovers to keep operating for much longer than expected.

The rovers completed their prime missions in April 2004. Since that time they have investigated dozens of additional sites. In May 2005 Opportunity became stuck in a small sand dune when its wheels sank into soft sand

and could not gain traction. Scientists worked for nearly five weeks to maneuver the rover back onto more solid ground. In September 2005 NASA reported that Spirit had reached the summit of a Martian hill nearly 350 feet higher than where the rover landed. The hill was tentatively named Husband Hill in honor of Rick D. Husband (19572003), the commander of the doomed space shuttle Columbia. Scientists used the panoramic pictures captured from this vantage point to map out future exploration routes for the rover.

In January 2008 both rovers reached their four-Earth-year anniversary on Mars. At that time, Spirit had traveled more than 4.5 miles and Opportunity just over 7 miles from their respective landing site. NASA scientists believe the rovers will operate indefinitely as long as their solar arrays continue to be cleaned by Marss dust devils. Thus far, the planet has not experienced a global-wide dust storm during the MER missions. However, such storms do occur on Mars and could render the rovers inoperable.

The Name Game

Only the IAU has the authority to assign official names to planetary features. Major features, such as mountains, valleys, and large craters, have already been named. The IAU naming process can take many months and even years to accomplish. NASA scientists handling images from the MERs have to quickly assign temporary working names to the many new smaller features being revealed. The evolution of this process is described in the article Naming Mars: Youre in Charge (Astrobiology Magazine, June, 20, 2004).

Most of the names are picked arbitrarily by whatever scientist first views an incoming image. Features are named after people, places, sailing ships, or other things the scientist fancies. Opportunity landed within a tiny crater dubbed Eagle Crater in honor of the Apollo 11 spacecraft that carried the first men to Earths Moon. When Spirit landed in January 2004, it captured images of seven hilltops about two miles in the distance. Scientists dubbed them the Columbia Hills in honor of the seven shuttle Columbia astronauts who perished during 2003. Each hill was named after one of the astronauts. NASA hopes that the IAU will choose to make these names official.

Water and Blueberries

On March 2, 2004, NASA scientists announced that Opportunity had uncovered strong evidence that the Mer-idiani Planum had been soaking wet in the past.

The claim was based on examination of the chemical composition and structure of rocks found in an outcrop in the area. The rocks contained minerals, such as sulfate salts, that are known to form in watery areas on Earth. The rocks also had niches in which crystals appear to have grown in the past. These empty niches are called vugs and are a strong indicator that the rocks sat in water for some time. Finally, there are round particles embedded in the rock that are about the size of ball bearings. Scientists have nicknamed them blueberries. The iron-rich composition of the blueberries and the way they are embedded in the rocks hints that water acted against the rocks in the past.

In 2005 NASA published a series of reports in Earth and Planetary Science Letters (vol. 240, no. 1, November 2005) detailing the latest findings from Opportunity. Scientists believe that ancient conditions in the Meridiani Planum region were strongly acidic, oxidizing, and sometimes wet. These harsh conditions are considered unlikely to have allowed Martian life to develop at that time in the planets history.

Mission Costs

The total cost of the MER missions has been estimated at $825 million. Each spacecraft cost about $325 million to develop, build, and equip with scientific instruments. Another $100 million was spent launching the spacecraft, and $75 million was devoted to operations and science costs.


On August 12, 2005, NASA launched the Mars Reconnaissance Orbiter (MRO) toward the Red Planet. The spacecraft was approximately twenty-one feet by forty-five feet in size and weighed more than two tons. A powerful Atlas V two-stage rocket was used to hoist the heavy MRO into space.

The MRO includes sophisticated radar, mineralogy, and atmospheric probes designed to investigate the atmosphere, terrain, and subsurface of the planet. (See Figure 7.7.) It also carries a high-resolution camera to provide detailed images of the Martian surface. NASA calls the spacecraft its eyes in the sky. The MRO entered Mars orbit on March 10, 2006, and, following several months of aerobraking, took up an orbiting position 160 to 190 miles from the surface of the planet to begin its science mission. That mission is scheduled to last for one Martian year. In May 2007 NASA announced that the MRO had returned eleven terabits of scientific data about Mars. The orbiter is expected to operate at least through 2010 and return a total of thirty-four terabits of scientific data.

Beginning in late 2008 the MRO will act as a communications relay satellite for future Mars missions. The total price of the MRO mission has been estimated at approximately $720 million.


On August 4, 2007, NASA launched the Phoenix Mars Lander aboard a Delta II rocket. (See Figure 7.8.) The Lander will set down on Mars northern polar region in May 2008. The planned landing site is in Vastitas Borealis (Northern Plains), a relatively flat landscape believed to contain water ice close to the surface.

The Lander carries seven science instruments, including a robotic arm for digging and collecting soil and ice samples. (See Figure 7.9.) Samples will be analyzed by onboard instruments for water and carbon-containing compounds. The Lander also includes a stereoscopic imager to record full-color panoramic views of the environment, gas and soil analyzers, a meteorological station to track daily and seasonal weather changes, and a descent imager that will photograph Mars during the spacecrafts descent. The primary mission duration is projected to be 90 to 150 sols (approximately 92 to 154 Earth days). The Lander will cease operations when win-ter sets in, because there will be no sunlight to capture on the solar arrays and recharge the spacecrafts batteries. The Lander is expected to be buried by ice during the polar Martian winter.


NASA plans to launch the Mars Science Laboratory during the Mars opposition of 2009. This rover will collect soil and rock samples and subject them to detailed chemical analysis using onboard instruments. Both NASA and the ESA had considered launching robotic Mars missions during the Mars opposition of 2011, but as of March 2008, both missions had been postponed until at least 2013.

In November 2007 the International Mars Architecture for Return Samples (IMARS) committee met in Washington, D.C., to discuss preliminary plans for an international mission to Mars to collect and return Martian soil samples to Earth. Representatives from NASA, the ESA, the Canadian Space Agency, and the Japan Aerospace Exploration Agency attended the meeting. According to Ker Than, in Global Group Aims to Return Martial Soil to Earth (New Scientist, December 11, 2007), the committee believes multiple spacecraft will be involved in the mission, which is tentatively scheduled to take place in the late 2010s. A Martian sample return mission is expected to be extremely expensive, but is considered a key precursor to any human missions to Mars.

Human Missions to Mars

Human exploration missions will probably not be possible until the 2030s. There are several major obstacles that must be overcome to make these missions feasible. Most of the problems lie within bioastronautics (the field of biology concerned with the effects of space travel on humans).

Scientists worry that radiation exposure poses a major health risk to astronauts traveling in deep space (beyond Earths magnetosphere). A solar flare while they are in flight or on Mars could be particularly hazardous. Mars has no magnetosphere of its own, and its atmosphere is thin, with little shielding effect. The radiation levels around Mars are two to three times higher than around Earth. Special protective clothing and materials will have to be developed to protect the astronauts from the radiation hazards.

Bone loss due to long-term weightlessness is also a major concern. A trip to Mars takes about six months with current propulsion technology. They might have to spend a long time on the planet. It is considered likely that the astronauts would make their flights to and from Mars near the times of Mars oppositions, which occur twenty-six months apart. Thus, it is possible that an entire Mars mission could last around two years. Scientists know that humans lose 1% to 2% of their bone mass per month while in space. This bone loss would pose a serious health threat to the astronauts during such a long mission.

The psychological pressures of long space missions have not been well studied. A trip to Mars would require astronauts to live and work in tight quarters and under stressful con-ditions for one to two years. The psychological strain could prove to be a major problem during such a long journey.

Another obstacle facing astronauts on a Mars mission would be access to medical care. On such a long flight the astronauts would have to have doctors aboard and some means of performing remote diagnosis and treatment of any medical problems that arose.

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Mars has fascinated humans throughout history. It appears as a blood-red star in the sky, which led the Romans to name it after their war god. Its motions across the sky helped German astronomer Johannes Kepler (1571-1630) derive his laws of planetary motion, which dictate how celestial bodies move. Two small moons, Phobos and Deimos, were discovered orbiting Mars in 1877. But it is primarily the question of life that has driven scientists to study Mars.

Basic Physical and Orbital Properties

Mars displays a number of Earth-like properties, including a similar rotation period, seasons, polar caps, and an atmosphere. In the 1800s astronomers also noted seasonal changes in surface brightness, which they attributed to vegetation. In 1877 Italian astronomer Giovanni Schiaparelli reported the detection of thin lines crossing the planet, which he called canali, Italian for "channels." But the term was mistranslated into English as "canals," which implies waterways constructed by intelligent beings. American astronomer Percival Lowell (1855-1916) popularized the idea of canals as evidence of a Martian civilization, although most of his colleagues believed these features were optical illusions. This controversy continued until the 1960s when spacecraft exploration of the planet showed no evidence of the canals.

Telescopic observations revealed the basic physical and orbital properties of Mars, as well as the presence of clouds and dust storms, which indicated the presence of an atmosphere. Dust storms can be regional or global in extent and can last for months. Global dust storms typically begin in the southern hemisphere around summer solstice because this is also when Mars is closest to the Sun and heating is the greatest. Temperature differences cause strong winds, which pick up the dust and move it around. Astronomers now know that the seasonal variations in surface brightness are caused by a similar movement of dust and not by vegetation.

Spectroscopic analysis suggested that the Martian atmosphere is composed primarily of carbon dioxide (CO2), and this was confirmed by measurements made by the Mariner 4 spacecraft in 1965. The atmosphere is 96 percent carbon dioxide, 3 percent nitrogen, and about 1 percent argon, with minor amounts of water vapor, oxygen, ozone, and other substances. The atmosphere is very thinthe pressure exerted by the atmosphere on the surface is only 0.006 bar (the atmospheric pressure at sea level on Earth is 1 bar). This thin atmosphere is unable to retain much heat; hence the Martian surface temperature is always very cold (averaging -63°C [-81°F]). This thin atmosphere also is unable to sustain liquid water on the surface of Marsany liquid water immediately evaporates into the atmosphere or freezes into ice. Geologic evidence suggests, however, that surface conditions have been warmer and wetter in the past.

A Geologically Diverse Planet

The geologic diversity of Mars was first realized from pictures taken by the Mariner 9 spacecraft in 1971-1972. Three earlier spacecraft (Mariner 4 in 1965 and Mariner 6 and Mariner 7 in 1969) had returned only a few images of the planet as they flew past. These images primarily revealed a heavily cratered surface, similar to the lunar highlands. Mariner 9, however, orbited Mars and provided pictures of the entire planet. Mariner 9 revealed that while 60 percent of the planet consists of ancient, heavily cratered terrain, the other 40 percent (mostly found in the northern hemisphere) is younger. Mariner 9 revealed the existence of the largest volcano in the solar system (Olympus Mons, which is about three times higher than Mt. Everest), a huge canyon system (Valles Marineris) that stretches the distance of the continental United States and is seven times deeper than the Grand Canyon, and a variety of channels formed by flowing water. These channels are not the same thing as the canalsno evidence of engineered waterways has been found on Mars, indicating that the canals are optical illusions. The discovery of channels formed by flowing water, however, reignited the question of whether life may have existed on Mars.

Findings of the Viking Missions

The Viking missions were designed to determine if life currently exists on Mars. Viking 1 and Viking 2 were each composed of an orbiter and a lander. Viking 1's lander set down in the Chryse Planitia region of Mars on July 20, 1976. Viking 2's lander followed on September 4, 1976, in the Utopia Planitia region to the northeast of where the first lander set down. Both landers were equipped with experiments to look for microbial life in the Martian soil as well as cameras to search for any movement of larger life-forms. All the experiments produced negative results, which together with the lack of organic material in the soil led scientists to conclude that no life currently exists on Mars.

The Viking orbiters, meanwhile, were providing the best information of the Martian surface and atmosphere to date. Scientists discovered that seasonal changes in the polar cap sizes are major drivers of the atmospheric circulation. They also discovered that the polar caps are primarily composed of carbon dioxide ice, but that the residual cap that remained at the North Pole even at the height of summer is probably composed of water ice. The frequency, locations, and extents of dust storms were studied in better detail than what Earth-based telescopes could do, providing new information on the characteristics of these events.

Is There Water on Mars?

The surface also continued to reveal new surprises. Fresh impact craters are surrounded by fluidized ejecta patterns, likely produced by impact into subsurface water and ice. Detailed views of the volcanoes, channels, and canyons provided improved understanding of how these features formed and how long they were active. But most intriguing was the accumulating evidence that liquid water has played a major role in sculpting the Martian surface. Curvilinear features interpreted as shorelines were found along the boundary between the lower northern plains and the higher southern highlands, leading to suggestions that the northern plains were filled with an ocean at least once in Martian history.

Smooth-floored craters whose rims are cut by channels suggest that lakes collected in these natural depressions. The appearance of degraded craters in old regions of the planet suggests erosion by rainfall. Spectroscopic data from Earth-based telescopes as well as the Russian Phobos mission in 1989 indicate that water has affected the mineralogy of the surface materials over much of the planet.

Clearly Mars has been warmer and wetter in the past. Where did all that water go? Some water can be found as vapor in the thin Martian atmosphere and some is locked up as ice in the polar regions. But these two reservoirs contain a small percentage of the total amount of water that scientists believe existed on the planet. Some of the water likely has escaped to space because of Mars' small size and low gravity. But scientists now believe that a large amount of the water is stored in underground ice and water reservoirs. Liquid water, derived from these underground reservoirs, may exist again on the Martian surface in the future because of episodic changes in atmospheric thickness. Scientists now know that the amount of tilt of Mars's rotation axis changes on about a million-year cycle because of gravitational influences from other planets. When the Martian poles are tipped more towards the Sun, the poles are exposed to more sunlight and the ices contained in these regions can vaporize to create a thicker atmosphere, which can cause higher surface temperatures by greenhouse warming.

Martian Meteorites

The Viking exploration of Mars ended in 1982, and few spacecraft provided information for the next fifteen years. The United States and Russia launched many spacecraft, but these missions were either failures or only partial successes. Nevertheless, new details were obtained during this time from a different sourcemeteorites. As early as the 1960s some scientists proposed that some unusual meteorites might be from Mars. These meteorites were volcanic rocks with younger formation ages (about 1 billion years) than typical meteorites (about 4 billion years). There are three major groups of these unusual meteorites: the shergottites, nakhlites, and chassignites (collectively called the SNC meteorites). In 1982 scientists discovered gas trapped in one of these SNC meteorites. When the gas was analyzed it was found to have isotopic ratios identical to those found in the Martian atmosphere. This discovery clinched the Martian origin for these meteorites. Scientists believe the meteorites are blasted off the surface of Mars during energetic impact events. The SNCs provide the only samples of the Martian surface that scientists can analyze in their laboratories because none of the Mars missions have yet returned surface material to Earth.

The only Martian meteorite with an ancient formation age (4.5 billion years) was discovered in Antarctica in 1984. Analyses of carbonate minerals in the meteorite in 1996 revealed chemical residues that some scientists interpret as evidence of ancient bacteria on Mars. This discovery is still very controversial among scientists but it has raised the question of whether conditions on early Mars were conducive to the development of primitive life. This is a question that many future missions hope to address.

Recent and Future Missions to Mars

Since 1997, spacecraft missions have made several new discoveries about Mars that have continued to support the hypothesis that the planet was warmer, wetter, and more active at times in the past. In 1997 the Mars Pathfinder mission landed on the surface of Mars in the mouth of one of the channels. The mission included a small rover called Sojourner, which was able to analyze a variety of rocks near the landing site. Sojourner revealed that the rocks display a variety of compositions, some of which suggest much more complicated geologic processes than scientists previously believed occurred on Mars. Images from the Mars Pathfinder cameras also suggest that more water flowed through this area than previously believed, increasing the estimates for the amount of water that has existed on the surface of the planet.

In late 1997 the Mars Global Surveyor (MGS) spacecraft began orbiting Mars. This mission is providing new information about atmospheric circulation, dust storm occurrence, and surface properties. MGS has provided scientists with the first detailed topography map of the planet. One of the major results of the topography map is that the northern plains are extremely smooth, a condition encountered on Earth only on sediment-covered ocean floors. This smooth surface, together with better definition of the previously proposed shorelines, lends further support to the idea that an ocean existed in the northern plains. A spectrometer on MGS revealed a large deposit of hematite in the heavily cratered highlands. Hematite is a mineral that is commonly formed by chemical reactions in hot, water-rich areas. Other instruments on MGS have determined that although Mars does not have an active magnetic field today, there was one in the past, as indicated by the remnant magnetization of some ancient rocks. This ancient magnetic field could have protected the early atmosphere from erosion by solar wind particles. Finally, the MGS cameras are revealing evidence of sedimentary materials in the centers of old craters and have found gullies formed by recent seepage of groundwater along the slopes of canyons and craters. Crater evidence suggests that some of the volcanoes have been active to more recent times than previously thought, suggesting that heat may be interacting with subsurface water even today. Such hydrothermal regions are known to be areas where life tends to congregate on Earthcould Martian biota have migrated underground and formed colonies around similar hydrothermal areas? Scientists do not know but there is much speculation about such a scenario.

The Mars Odyssey spacecraft successfully arrived at Mars in October 2001 and by January 2002 the spacecraft had settled into its final orbit. Its instruments are reporting strong spectroscopic evidence of near-surface ice across most of the planet.

Our view of Mars has changed dramatically from that of a cold, dry, geologically dead world to a warm, wet, oasis where life may have arisen and may yet thrive in certain locations. Several missions are planned in the next few years by the United States, the European Space Agency, Russia, and Japan to further explore Mars. These missions include a variety of orbiters, landers, rovers, and sample-return missions, which will allow scientists to answer additional questions about the history and future of Mars. Eventually humans will likely become directly involved in the exploration of Mars, and colonies may be established so that Mars can become our stepping-stone to further exploration of the universe.

see also Exploration Programs (volume 2); Government Space Programs (volume 2); Kepler, Johannes (volume 2); Life in the Universe, Search for (volume 2); NASA (volume 3); Planetary Protection (volume 4); Planetary Exploration, Future of (volume 2); Robotic Exploration of Space (volume 2); Sagan, Carl (volume 2).

Nadine G. Barlow


Kieffer, Hugh H., Bruce M. Jakosky, Conway W. Snyder, and Mildred S. Matthews. Mars. Tucson: University of Arizona Press, 1992.

Raeburn, Paul Mars: Uncovering the Secrets of the Red Planet. Washington, DC: National Geographic Society, 1998.

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Mars, the fourth planet from the Sun, was named for the Roman god of war. It is a barren, desolate, crater-covered world prone to frequent, violent dust storms. It has little oxygen, no liquid water, and ultraviolet radiation that would kill any known life-form. Temperatures range from about 80°F (27°C) at midday to about 100°F (73°C) at midnight. Because of its striking red appearance in the sky, Mars is known as the "red planet."

Mars is roughly 140 million miles (225 million kilometers) away from the Sun. It has a diameter of 4,200 miles (6,800 kilometers), just over half the diameter of Earth. Its rotation on its axis is slightly longer than one Earth day. Since it takes Mars 687 (Earth) days to orbit the Sun, its seasons are about twice as long as those on Earth.

Physical properties of Mars

Mars has numerous Earthlike features. The two distinguishing features mark the planet's northern hemisphere. The first is a 15-mile-high (24-kilometer-high) volcano called Olympus Mons. Measuring 375 miles (600 kilometers) across, it is larger than any other in the solar system.

The second is a 2,500-mile-long (4,000-kilometer-long) canyon called Valle Marineris, eleven-and-a-half times as long and twice as deep as the Grand Canyon. The southern hemisphere is noteworthy for Hellas, an ancient canyon that was long ago filled with lava and is now a large, light area covered with dust.

Mars is also marked by what appear to be dried riverbeds and flash-flood channels. These features could mean that ice below the surface melts and is brought above ground by occasional volcanic activity. The water may temporarily flood the landscape before boiling away in the low atmospheric pressure. Another theory is that these eroded areas could be left over from a warmer, wetter period in Martian history. Mars has two polar caps. The northern one is larger and colder than the southern. Two small moons, Phobos and Deimos, orbit the planet.

Exploration of Mars

Beginning in the early 1960s, both the former Soviet Union and the United States sent unmanned spacecraft to Mars in an attempt to learn more about the planet. Although some of those missions were unsuccessful, others were able to transmit data back to Earth. In 1965, the U.S. probe Mariner 4 flew past Mars, sending back 22 pictures of the planet's cratered surface. It also revealed that Mars has a thin atmosphere composed mostly of carbon dioxide and that the atmospheric pressure at the surface of Mars is less than 1 percent of that on Earth.

The 1969 fly-by flights of Mariner 6 and Mariner 7 produced 201 new images of Mars, as well as more detailed measurements of the structure and composition of its atmosphere and surface. From these measurements, scientists determined that the polar ice caps are made of haze, dry ice, and clouds.

Two years later, Mariner 9 became the first spacecraft to orbit Mars. During its year in orbit, Mariner 9 's two television cameras sent back pictures of an intense Martian dust storm as well as images of 90 percent of the planet's surface and the two Martian moons. It observed an older, cratered surface on Mars's southern hemisphere and younger surface features on the northern hemisphere.

Viking probes. In 1976, the United States launched the Viking 1 and Viking 2 space probes. Each Viking spacecraft consisted of both an orbiter and a lander. Viking 1 made the first successful soft landing on Mars on July 20, 1976. A soft landing is one in which the spacecraft is intact and functional on landing. Soon after, Viking 2 landed on the other side of the planet. Cameras from both landers showed rust-colored rocks and boulders with a reddish sky above. The rust color is due to the presence of iron oxide in the Martian soil.

The Viking orbiters sent back weather reports and pictures of almost the entire surface of the planet. They found that although the Martian atmosphere contains low levels of nitrogen, oxygen, carbon, and argon, it is made primarily of carbon dioxide and thus cannot support human life. The soil samples collected by the landers show no sign of past or present life on the planet.

Possible life?

In August 1996, scientists announced they had found possible traces of early Martian life in a potato-sized igneous rock. The small meteorite had been flung into space by the impact of a huge asteroid or comet 15 million years ago. It then wandered about space until it fell on the Antarctic ice sheet about 13,000 years ago. Geologists discovered the meteorite (along with more than a dozen others) in buried ice in 1984. Upon examining the rock, scientists found what they believe are fossilized remains of microorganisms that might have existed on Mars during an early part of its history when it was warmer and wetter.

New era in exploration

In 1996, the National Aeronautics and Space Administration (NASA) marked a new era in exploration when it began a ten-year campaign to explore various regions of Mars. The goal of the campaign is to discover whether life in any form ever existedor still existson the red planet.

Mars Global Surveyor. The campaign began with the launch of the Mars Global Surveyor on November 7, 1996. The Surveyor established an orbit 250 miles (400 kilometers) above the surface of the planet in September 1997. The spacecraft's two-year mission was to map systematically the surface of the planet. To do so, it used a laser altimeter to map mountains and valleys; a camera system to record land forms and clouds; and detectors to measure atmospheric composition, radiation, and surface minerals.

The Surveyor 's first major discovery was to solve one of the greatest mysteries surrounding Mars: the planet does possess a magnetic field. A magnetic field is usually generated by molten metal at a planet's core. On the surface, the field shields a planet and life on it from cosmic and solar radiation. Although Mars's field is weak, its existence adds evidence to the possibility that life may have existed on the planet long ago.

In April 1999, the Surveyor sent back to Earth some astonishing information: the crust of Mars's surface has alternating layers of magnetic fields. Scientists theorize that the magnetic bands are formed when magma from far below the surface of Mars is forced to the surface by plate tectonics. (Plate tectonics is a geological theory that Earth's surface is composed of rigid plates or sections that move about the surface in response to internal pressure.) As the magma cools and hardens into a new layer of crust, the iron in the magma is magnetized towards the current magnetic field. This discovery could point to a past of geologic activity similar to that of the Earth and possibly very early on in its history supported simple life forms.

In June 2000, scientists studying pictures sent back by Surveyor announced that the standard description of Mars as cold, desolate, and dry would have to be changed. The pictures clearly showed channels and grooves on the steep, inside walls of craters that indicate the downward flow of water. These surface features appear to be evidence of water in the upper crust of Mars that had seeped through and run down the channels. Scientists suggested that these water flows happened in recent geological timeperhaps just a few hundreds, thousands, or millions of years ago.

In December 2000, after further analysis of pictures sent back by Surveyor, scientists announced that in its earlier history, Mars was a

warmer world with a denser atmosphere, and its surface was covered with lakes and shallow seas. They based these assumptions on evidence of distinct, thick layers of rock within craters and other depressions on the surface of the planet.

After having gathered tens of thousands of images of Mars, the Mars Global Surveyor completed its mapping mission in early 2001. Its main mission accomplished, the probe was given additional scientific work to complete, including scouting out landing sites for future spacecraft. NASA engineers hope to use Surveyor to relay commands to twin rovers slated to land on the planet in early 2004.

Mars Pathfinder and the Sojourner rover. On December 4, 1996, less than a month after the launch of the Mars Global Surveyor, NASA launched the Mars Pathfinder. Six months later, on July 4, 1997, the Mars Pathfinder landed successfully on Mars in the plain of Ares Vallis and released the Sojourner rover.

From pictures sent back by the Mars Pathfinder, scientists deduced the plain where the spacecraft landed had once been reshaped by colossal floods. The tilt of rocks and the tails of debris behind pebbles in the area led scientists to estimate that the main flood was hundreds of miles wide, hundreds of feet deep, and flowed for thousands of miles. Scientists could not answer the question of where the water went.

Part of the mission of the rover was to record the chemical makeup of rocks and the soil. The instruments on Sojourner revealed that Mars has a history of repeated cycles of internal melting, cooling, and remelting. The rocks analyzed contained large amounts of the mineral quartz, which is produced when the material is melted and remelted many times. Sojourner's examination also revealed that Mars seems much more like Earth geologically than the Moon does. The Martian rocks analyzed resemble a common Earth volcanic rock named andesite.

These findings support scientific theories that Mars has been convulsed (literally turned inside out) by internal heat through much of its 4.6-billion-year history.

Lost missions. In 1999, NASA suffered a double blow when two spacecraft, each on a mission to Mars, were lost. In September of that year, the Mars Climate Orbiter was to have reached Mars, settled into an orbit, explored the Martian atmosphere, and acted as a communications relay station. However, because technicians failed to convert metric and English measurements in navigational instructions sent to the spacecraft, it flew in too close to the planet and most likely burned in the atmosphere before crashing. It was never heard from again. Just three months later, in December, the Mars Polar Lander was scheduled to have landed on Mars to begin prospecting the landscape of dirt and ice for traces of water and evidence of the planet's climatic history. However, scientists for the project never heard from the 1,200-pound (545-kilogram) robotic spacecraft after it was supposed to have landed. They speculate that a software glitch in the spacecraft's program caused it to crash just moments before its projected landing.

Future expeditions. In October 2000, NASA unveiled an ambitious plan to send eight or more probes to Mars over the next two decades to search for evidence of water or life. The first of these, Mars Odyssey, was launched in the spring of 2001, with a planned arrival in the fall. Once in orbit, the spacecraft will try to determine the composition of the planet's surface, to detect water and shallow buried ice, and to study the radiation environment. In mid-2003, in a mission planned by the European Space Agency and the Italian Space Agency, NASA will launch the Mars Express. This spacecraft's main mission will be to search for subsurface water from orbit and to deliver a lander to the Martian surface. That lander, the Beagle 2, will sniff air, dig dirt, and bake rock samples for evidence of past or present life.

Also in 2003, NASA will send two powerful rovers to Mars that will be identical to each other, but will land at different regions of the planet. These robotic explorers will be able to trek up to 328 feet (100 meters) across the Martian surface each day in search of evidence of liquid water that may have been present in the planet's past.

In 2005, NASA plans to launch a powerful scientific orbiter, the Mars Reconnaissance Orbiter. The orbiter will map the Martian surface with an eagle-eyed camera, trying to bridge the gap between surface observations and measurements taken from orbit. The camera will have an unprecedented 8-inch (20-centimeter) resolution, allowing it to record features as small as a license plate. In 2007, NASA plans to launch a roving long-range, long-duration science laboratory that will provide extensive surface measurements and pave the way for a future sample return mission.

[See also Solar system ]

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Mars (in astronomy)

Mars, in astronomy, 4th planet from the sun, with an orbit next in order beyond that of the earth.

Physical Characteristics

Mars has a striking red appearance, and in its most favorable position for viewing, when it is opposite the sun, it is twice as bright as Sirius, the brightest star. Mars has a diameter of 4,200 mi (6,800 km), just over half the diameter of the earth, and its mass is only 11% of the earth's mass. The planet has a very thin atmosphere consisting mainly of carbon dioxide (95%) with some nitrogen, argon, oxygen, and other gases. Mars has an extreme day-to-night temperature range, resulting from its thin atmosphere, from about 80°F (27°C) at noon to about -100°F (-73°C) at midnight; however, the high daytime temperatures are confined to less than 3 ft (1 m) above the surface.

Surface Features

A network of linelike markings first studied in detail (1877) by G. V. Schiaparelli was referred to by him as canali, the Italian word meaning "channels" or "grooves." Percival Lowell, then a leading authority on Mars, created a long-lasting controversy by accepting these "canals" to be the work of intelligent beings. Under the best viewing conditions, however, these features are seen to be smaller, unconnected features. The greater part of the surface area of Mars appears to be a vast desert, dull red or orange in color. This color may be due to various oxides in the surface composition, particularly those of iron. About one fourth to one third of the surface is composed of darker areas whose nature is still uncertain. Shortly after its perihelion Mars has planetwide dust storms that can obscure all its surface details.

Photographs sent back by the Mariner 4 space probe show the surface of Mars to be pitted with a number of large craters, much like the surface of Earth's moon. In 1971 the Mariner 9 space probe discovered a huge canyon, Valles Marineris. Completely dwarfing the Grand Canyon in Arizona, this canyon stretches for 2,500 mi (4,000 km) and at some places is 125 mi (200 km) across and 2 mi (3 km) deep. Mars also has numerous enormous volcanoes—including Olympus Mons (c.370 mi/600 km in diameter and 16 mi/26 km tall), the largest in the solar system—and lava plains. In 1976 the Viking spacecraft landed on Mars and studied sites at Chryse and Utopia. They recorded a desert environment with a reddish surface and a reddish atmosphere. Experiments analyzed soil samples for evidence of microorganisms or other forms of life; none was found, but a reinterpretation (2010) of the results in light of data collected later suggests that organic compounds may have been present. In 1997, Mars Pathfinder landed on Mars and sent a small rover, Sojourner, to take soil samples and pictures. Among the data returned were more than 16,000 images from the lander and 550 images from the rover, as well as more than 15 chemical analyses of rocks and extensive data on winds and other weather factors. Mars Global Surveyor, which also reached Mars in 1997 and remained operational until 2006, returned images produced by its systematic mapping of the surface. The European Space Agency's Mars Express space probe went into orbit around Mars in late 2003 and sent the Beagle 2 lander to the surface, but contact was not established with the lander. In addition to studying Mars itself, the orbiter has also studied Mars's moons. The American rovers Spirit and Opportunity landed successfully in early 2004 and have explored the Martian landscape (Spirit's last transmission was in 2010). In 2008 NASA's Phoenix lander touched down in the planet's north polar region; it conducted studies for five months. Curiosity, another NASA rover, landed on Mars near its equator in 2012.

Analysis of space probes' data indicates that Mars appears to lack active plate tectonics at present; there is no evidence of recent lateral motion of the surface. With no plate motion, hot spots under the crust stay in a fixed position relative to the surface; this, along with the lower surface gravity, may be the explanation for the giant volcanoes. However, there is no evidence of current volcanic activity.

There is evidence of erosion caused by floods and small river systems as well as evidence of ancient lakebeds. The possible identification of rounded pebbles and cobbles on the ground, and sockets and pebbles in some rocks, suggests conglomerates that formed in running water during a warmer past some 2–4 billion years ago, when liquid water was stable and there was water on the surface, possibly even large lakes or an ocean. Rovers have identified minerals and rocks believed to have formed in the presence of liquid water. There is also evidence of flooding that occurred less than several million years ago, most likely as the result of the release of water from aquifers deep underground or the melting of ice. However, a study of ancient Martian impact craters suggests that the atmospheric pressure on early Mars was consistent with a thin atmosphere, and that the warm conditions required for liquid water may have been rare or intermittent. Data received beginning in 2002 from the Mars Odyssey space probe suggests that there is water in sand dunes found in the northern hemisphere, and the Mars Reconnaissance Orbiter, which went into orbit around the planet in 2006, collected radar data that indicates the presence of large subsurface ice deposits in the mid-northern latitudes of Mars. Most of the known water on Mars, however, lies in a frozen layer under the planet's large polar ice caps, which themselves consist of water ice and dry ice (frozen carbon dioxide); the lander Phoenix found and observed frozen water beneath the soil surface in the north polar region in 2008.

Seasonal Changes

Because the axis of rotation is tilted about 25° to the plane of revolution, Mars experiences seasons somewhat similar to those of the earth. One of the most apparent seasonal changes is the growing or shrinking of white areas near the poles known as polar caps. These polar caps, which are are composed of water ice and dry ice (frozen carbon dioxide). During the Martian summer the polar cap in that hemisphere shrinks and the dark regions grow darker; in winter the polar cap grows again and the dark regions become paler. The seasonal portion of the ice cap is dry ice. When the ice cap is seasonally warmed, geyserlike jets of carbon dioxide gas mixed with dust and sand erupt from the ice.

Astronomical Characteristics

The mean distance of Mars from the sun is about 141 million mi (228 million km); its period of revolution is about 687 days, almost twice that of the earth. At those times when the sun, earth, and Mars are aligned (i.e., in opposition) and Mars is at its closest point to the sun (perihelion), its distance from the earth is about 35 million mi (56 million km); this occurs every 15 to 17 years. At oppositions when Mars is at its greatest distance from the sun (aphelion) it is about 63 million mi (101 million km) from the earth. It rotates on its axis with a period of about 24 hr 37 min, a little more than one earth day.

Satellites of Mars

Mars has two natural satellites, discovered by Asaph Hall in 1877. The innermost of these, Phobos, is about 7 mi (11 km) in diameter and orbits the planet with a period far less than Mars's period of rotation (7 hr 39 min), causing it to rise in the west and set in the east. The outer satellite, Deimos, is about 4 mi (6 km) in diameter.


See J. K. Beatty and A. Chaikin, ed., The New Solar System (3d ed. 1991); F. W. Taylor, The Scientific Exploration of Mars (2010); W. J. Clancey, Working On Mars: Voyages of Scientific Discovery with the Mars Exploration Rovers (2012).

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Since ancient times, the planet Mars has been seen as both a bright red light in the sky and an inspirer of aggressive behavior. Mars is the next planet out in the solar system from Earth, and its relative brightness in the night sky varies more than any other planet. Every two years and seven weeks, Mars changes its appearance from a dim spark to a bright red star. For this reason, Mars has represented a duality to humanity: It is both an inspiring astronomical object and the symbol of the God of War. It was Marss variability and red hue which inspired, in the astronomers Nicolaus Copernicus and Johannes Kepler, an intense curiosity concerning its orbit, thus helping spawn the Scientific Revolution in the sixteenth century.

As the Scientific Revolution blossomed, this fascination with Mars continued and changed. Giovanni Schiaparellis sighting of canali, or canals, in the Martian landscape in 1877, followed by further reports of this phenomena by Percival Lowell, meant that Mars was the possible abode of extraterrestrial life and intelligence. The Lowellian concept of Mars as a dying planet was formed, and it came to be seen by some as a desert planet where a dead or dying civilization might be found. This spawned a host of fictional accounts of Martians, such as the War of the Worlds by H. G. Wells and the tales of John Carter of Barsoom (Mars) by Edgar Rice Burroughs.

Mars also inspired rocket developers such as Robert H. Goddard, and the planet was finally reached successfully by probes, beginning in 1964 with the American Mariner 4, which found a frozen desert covered with water channels, suggesting a previous Earthlike epoch.

In 1976, in a place on Mars called Cydonia, what appeared to be a massive archeological complex was discovered. On two separate orbits of the Viking A probe, images showed kilometer-sized objects resembling a pyramid near what looked like a carved humanoid face. NASA dismissed the images as illusions, but two engineers, Vincent DiPietro and Gregory Molenaar, investigated the images digitally. One person intrigued by these images was Richard Hoagland, a science reporter formerly with CBS News who organized a team of scientists and engineers called the Independent Mars Investigation Team, which validated the provocative nature of the images. However, Hoagland was criticized by other scientists for sensationalizing the results of the investigation. This effort was motivated by both the compelling nature of the images and the Lowellian folklore of Mars, but it was also a product of the tense cold war atmosphere of the early 1980s. This tension made researchers sensitive to any suggestion of a dead humanoid civilization, fearing the same fate might befall the inhabitants of Earth.

Another aspect of interest in the Cydonia objects was the very humanoid form suggested by the images, which recalled the fascination with the human form of previous epochs. For this reason, the Cydonia images were not only disturbing for their implications but also reassuring in their validation of the human experience, suggesting it is part of something cosmic. Since the cold war ended, Cydonia has become a favorite target of satellite images, which show that the objects are highly eroded, and thus very difficult to characterize. However, despite their eroded characterand continued efforts to dismiss them as merely geological formationsthe objects still provoke mystery.

Mars as a whole continues to be an object of intense scientific investigation, with strong suggestions of primitive microbial biology, past and present, and past Earthlike conditions. Thus Mars may yet provide the answer to the age-old question of whether or not humanity is alone in the cosmos. Mars has also become the stated target of human exploration and settlement. Therefore, it can be said that Mars has provoked more human intellectual activity than any other planet and may be the setting for its greatest advances in the future.

SEE ALSO Space Exploration


McDaniel, Stanley V., and Monical Rix Paxson, eds. 1998. The Case for the Face: Scientists Examine the Evidence for Alien Artifacts on Mars. Kempton, IL: Adventures Unlimited Press.

Wallbank, T. Walter, Alastair Taylor, and Nels Bailkey. 1967. Civilization Past and Present. 3rd ed. Glenview, IL: Scott, Foresman.

John Brandenburg

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Mars Fourth major planet from the Sun. Mars appears red to the naked eye because of the high iron content of its surface crust, and is known as the Red Planet. The atmosphere consists mainly of 95% carbon dioxide, 2.5% nitrogen, and 1.5% argon, with smaller quantities of oxygen, carbon monoxide and water vapour. Its axial tilt is similar to the Earth's, so it passes through a similar cycle of seasons. The surface temperature on Mars varies between extremes of 130K and 290K. Its surface reveals a long and complex history of geological activity. The major difference in terrain is between the largely smooth, lowland volcanic plains of the northern hemisphere, and the heavily cratered uplands of the south. The biggest volcanic structure on Mars is Olympus Mons, which is hundreds of kilometres across and 27km (17mi) high. Other geographical features, such as giant canyons, are channels in which rivers once flowed. The variable polar ice caps appear to be composed of solid carbon dioxide with underlying caps of water ice. Mars has two tiny satellites in very close orbits, Phobos and Deimos. In 1996, scientists investigating a meteorite, thought to have originated on Mars, found fossilized microorganisms that some believe indicate the presence of primitive life on the planet. In 2003, two separate missions to Mars were launched. The British-built probe Beagle 2 failed to make contact with Earth. In 2004, the NASA funded Opportunity probe found evidence that water suitable for supporting life, once lay on the surface of Mars.;

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Mars (adj. martian) The fourth planet in the solar system, 1.524 AU from the Sun. Its radius is 3390 km, its density 3940 kg/m3, and the inclination of equator to orbit is 25.1°. It has a small atmosphere (7 mb) of CO2. The polar caps are of water ice with seasonal solid CO2. The northern-hemisphere crust is mainly basaltic plains and volcanoes; the southern an ancient cratered terrain (see CRATER). The Tharsis Bulge is an uplifted or volcanic plateau. Large canyons exist and there is evidence of former water erosion. Some basaltic meteorites are derived from Mars. It has two small satellites, Phobos and Deimos, probably captured asteroids.

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MARS (Modern Architectural Research) Group. Group of architects (including Arup, Coates, and Lubetkin) founded in 1933 to promote International Modernism and Rationalism in the United Kingdom (it was really the UK branch of CIAM). Taking its cue from Le Corbusier's Plan Voisin and other theoretical ideas, it proposed (1942) widespread destruction and rebuilding of London. It was disbanded in 1957.


Korn (1953);
Reading (1986);
Tafuri & and Dal Co (1986).

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259. Mars

See also 25. ASTRONOMY ;100. COSMOLOGY ;318. PLANETS .

Astronomy. a topographical description of the planet Mars.
Astronomy. the observation and study of the planet Mars. areologist , n. areologic, areological , adj.

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Marsaides-mémoires, Lamaze, Lars, Mars, parse, Paz, Stars and Bars, vase, vichyssoise

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MARS meteorological automatic reporting station (or system)

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