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Mars

Mars

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.

EARLY TELESCOPIC VIEWS OF 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.

GIOVANNI SCHIAPARELLI

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.

ASAPH HALL

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

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.

INHABITED OR NOT?

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.

MARS IN SCIENCE FICTION

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.

SCIENTIFIC FACTS ABOUT MARS

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.

MISSIONS 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, http://marsprogram.jpl.nasa.gov/missions/log/ (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

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.

THE VIKING MISSION

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.

ALH84001

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, http://curator.jsc.nasa.gov/antmet/marsmets/index.cfm) that there have been thirty-one known Martian meteorites found on Earth since 1815. ALH84001 is the oldest meteorite in the collection.

MARS GLOBAL SURVEYOR

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

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.

2001 MARS ODYSSEY

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, http://mars.jpl.nasa.gov/overview/) 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 (http://marsprogram.jpl.nasa.gov/odyssey/) 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.

THE PERIHELIC OPPOSITION OF 2003

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.

MARS EXPRESS

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 (http://www.esa.int/esaMI/Mars_Express/index.html) 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.

MARS EXPLORATION ROVERS

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.

MARS RECONNAISSANCE ORBITERR

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.

PHOENIX MARS LANDER

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.

THE FUTURE OF MARS EXPLORATION

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

Mars

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

Bibliography

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

Mars

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.

Bibliography

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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Mars

Mars

BIBLIOGRAPHY

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

BIBLIOGRAPHY

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

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.

http://lpl.arizona.edu/nineplanets/nineplanets/mars.html; http://wr.usgs.gov

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Mars

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

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.

Bibliography

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

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Mars

259. Mars

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

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

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Mars

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

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MARS

MARS meteorological automatic reporting station (or system)

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Mars

Mars in Roman mythology, the god of war and the most important Roman god after Jupiter. He was probably originally an agricultural god, and the month of March is named after him. His Greek equivalent is Ares. Legends of Mars include the story of his love for Venus, and the trap made for them by her husband Vulcan, and from early times this became a favourite subject for artists. The wolf and the woodpecker were regarded as sacred to Mars, and the field of Mars is another name for the Campus Martius of ancient Rome. The hill of Mars is the Areopagus (‘hill of Ares’) of ancient Athens.

Mars is the name given to a small reddish planet which is the fourth in order from the sun and is periodically visible to the naked eye. From its distinctive colour, which comes from its iron-rich minerals, it is known informally as the red planet. In astrological belief, the influence of the planet is associated with combative, aggressive, or masculine qualities.

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Mars

Mars

Mars was a major Roman deity, second only to Jupiter* in the Roman pantheon. He began as a protector of agriculture but later became the god of war, honored throughout the realm of the conquering Romans. The Romans admired Greek culture and absorbed Greek deities into their own. They came to identify their own war god, Mars, with the Greek war god, Ares, but Mars was a more dignified and popular figure.

According to legend, Juno, the queen of the gods, gave birth to Mars after being touched by a magic plant. He was originally associated with vegetation and fertility. As the Romans became increasingly warlike, Mars gradually developed into a god of war, but he never lost his connection with agriculture and the plant world entirely. The Romans honored him with festivals in his month, March, which occurs at a time of the year when new growth begins in the fields and military campaigns resume after a winter break.

Mars's high place of honor in the Roman pantheon comes in part from his role as an ancestor of Rome. According to the story of the founding of Rome, Mars was the father of Romulus and Remus, twin boys born to a human priestess and raised by a wolf. Romulus later founded the city of Rome, and the Romans believed that Romulus's divine father would come to their aid in times of crisis or disaster. The wolf and the woodpecker, animals involved in the saving of the twins, were sacred to Mars. Picus, a Roman god who took the form of a woodpecker, was Mars's companion.

One story about Mars relates that the god's sacred shield had fallen from the sky in the time of the early Roman king Numa Pompilius. Believing that the shield was vital to the well-being of Rome, Numa had 11 identical shields made and hung all 12 of them in a shrine to confuse any thief who might try to steal Mars's shield. Numa also established an order of priests called the Salii to guard the shields. For many years, Roman priests continued to wear the old-fashioned armor and to perform ritual war dances during the March festivals of Mars.

deity god or goddess

pantheon all the gods of a particular culture

ritual ceremony that follows a set pattern

Soldiers throughout the empire offered sacrifices to Mars before and after battles. They also honored the goddess Bellona, who appeared as Mars's sister, wife, and daughter in various myths. The Campus Martius, a large field outside Rome where soldiers exercised, was sacred to Mars.

See also Ares; Roman Mythology; Romulus and Remus.

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Mars

Mars Ancient Roman god of war, often depicted as an armed warrior; one of the three protector-deities of the city of Rome itself (with Jupiter and Quirinus). He was originally associated with agriculture but later took on his dominant military aspects; the wolf and woodpecker were sacred to him.

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Mars

Mars / märz/ 1. Roman Mythol. the god of war and the most important Roman god after Jupiter.The month of March is named after him. Greek equivalent Ares. 2. Astron. a small, reddish planet that is the fourth in order from the sun and is periodically visible to the naked eye.

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Mars

MARS

MARS . The Latin name Mars, found throughout Italy, lacks any Indo-European etymology. It appears in both a simple form and in doubled form. The Latin Mars coexists with an ancient form, Mavors (kept in use by poets), as well as a contracted form, Maurs (see Corpus inscriptiorum Latinarum, Berlin, 1863, vol. 1, no. 49). As for the doubled form, Marmar, it very likely stems from *Mar-mart-s; it is found in the Carmen Arvale along with Marmor, which seems an odd form. Mamers, which the ancients identified as an Oscan term (Paulus-Festus, ed. Lindsay, 1913, p. 150 L.), derived from *Mamars by apophony of the second vowel. A god Maris is known among the Etruscan gods, but the identification with Mars is doubtful, since Laran is the Etruscan god of war. The Umbrian ritual of the seven Iguvine Tables (from ancient Iguvium, modern Gubbio) attests to the worship of Mars in that region: Table VIb1 mentions the sacrifice of three oxen to Mars Grabovius. With Jupiter and Vofionus, Mars receives the epithet "Grabovius" (the link with Latin Gradivus, epithet of Mars, is uncertain) and is the second god of the so-called "Grabovian trinity." Mars is the god to whom is dedicated the ver sacrum, an Italic ritual, originally Sabin (Strabo 5.4.12), in which the god is offered all that is born and produced during the spring. This evidence indicates that the Mamertini (whose name is linked with Mamars), when a plague spread in the Samnium, entered Bruttium in 288 bce and then settled in Messana (Festus, p. 150 L). Such a ritual attests to the Italic dimension of Mars. The dative Mamartei, read in an inscription at Satricum dating from the sixth century bce, implies the existence of a nominative, *Mamars. The Lapis Satricanus, found in the foundation of a temple of Mater Matuta, mentions, in an archaic genitive form, the name of Valerius Publicola (consul in Rome at the beginning of the Republic), and a dedication or offering by his sodales to Mars (popliosio valesiosio suodales mamartei ).

Mars is the Roman god of power, particularly of war. He held the second position in the archaic triad of Jupiter, Mars, and Quirinus, which, according to Georges Dumézil, pre-existed the Capitoline triad. He received the second share of the spolia opima in the threefold distribution of the highest military spoils established by the law of Numa (Festus, p. 204 L.). He had not only a particular priest, the flamen Martialis, but also a specific kind of offering: the suovetaurilia, a set of three victims (boar, ram, and bull) sacrificed as part of the Capitoline triad, a purification ritual called the lustratio populi celebrated by the Roman censors at the closing of the lustrum in the Campus Martius. The old brotherhood of the Salii, created by Numa (Livy, 1.20. 4), was specially concerned with the war rituals and the god of war. The Salii, divided into two twelve-member groups called the Salii Palatini and the Salii Collini, were under the protection of Jupiter, Mars, and Quirinus (Servius, Ad Aeneidem 8.663). It is probable that the opening service for the military season in spring was handled by the Salii Palatini, with Mars as their patron, and the closing service by the Salii Collini, with Quirinus as their patron.

Two sets of feasts, in March and October, correspond to the opening and closing of the military season. The first, in the spring, began on March 1, when the Salii Palatini went out of their Curia on the Palatine and the Salii Collini left the Sacrarium on the Quirinal to reach the Regia and offer a sacrifice to Mars. This cycle comprised the following feasts: the horse races on March 14 for the Equirria on the Field of Mars; on the same day, an old man would be expelled from the city in the Mamuralia, a reenactment of the legend of Mamurius Veturius (Mamurius Veturius, whose name recalled the god, was the smith who fabricated the shields, ancilia, of the Salii. One of the shields was said to have dropped from the skysee Ovid, Fasti 3.369 ff.); a sacrifice called Agonium Martiale was celebrated on March 17; the lustration of arms took place at the Quinquatrus on March 19 and that of battle trumpets at the Tubilustrium on March 23. Before beginning operations, a Roman general entered the Sacrarium Martis in the Regia and exclaimed, "Mars uigila!" ("Mars, wake up!") If the lances of the god vibrated, it was a good omen and the war could begin.

The second cycle, in autumn, included the rite of purification at the Tigillum Sororium on October 1 (purification rites in memory of Horatius, who killed the Curatii and his own sister); the sacrifice of a war horse during the rites of the Equus October (October Horse), on October 15; and the lustration of arms, Armilustrium, on October 19. In spring, as in autumn, the priestly brotherhood of the Salii danced at the feasts (a dance called tripudium ) while brandishing lances and shields. In ordinary times these arms were kept in the sacrarium of Mars within the Regia.

The god's military character was well established, but scholars of the predeistic shool, such as Herbert Jenkins Rose or Gustav Hermansen, who thought that Roman religion was based on numen (something like the Melanesian mana ) and not on anthropomorphism, developed the theory of an agrarian Mars. This opinion seems to be based on a confusion between the god's intrinsic nature and the range of applications for his intervention. His power could be employed not only in warfare but also in agriculture. At an Eitrem Conference held in Oslo in 1955 Herbert Jenkins Rose insisted that the Equus October was sacrified ob frugum euentum (for the gathering of crops, but Georges Dumézil demonstrated that with this ritual the Romans intended to thank the god of war for preserving the harvest from enemies and allowing them to gather in their crops. If the lance of Mars is important in the ritual (Arnobius, adv. nationes 6.11, said that according to Varro the ancient Romans had pro Marte hastam ) and can give a good omen with its vibrations, we cannot come to the conclusion that the Romans first worshiped the hasta and only later conceived a god with the human features of a warrior.

The name of the god does not appear in the fragments of the Carmina Saliorum, but Gellius (13. 23. 2) mentions it in an old prayer associated with Nerio and the Moles, two aspects of his power. In the Carmen Arvale, the Arval brothers prayed to "fierce Mars" (fere Mars) to protect Roman territory by "leaping to the border" (limen sali). Likewise, Cato the Elder's peasant, celebrating a private sacrifice of a pig, a sheep, and a bull, called suovetaurile for a lustratio agri, invoked Mars "to halt, rebuff, and cast away visible and invisible maladies" (De agricultura 141).

The god's most ancient place of worship was situated on the Field of Mars at the ara Martis, the altar near which D. Junius Brutus Callaicus erected a temple in 138 bce. The most important sanctuary, outside the Porta Collina, near the Via Appia, had been dedicated on June 1, 338 bce, and was the starting point for the annual cavalry parade (Dionysius of Halicarnassus, 6.13.4).

At the beginning of the Second Punic War, in 217 bce, Mars was associated in a lectisternium with Venus, after the pattern of Ares and Aphrodite, in order to exalt the connection between Romulus, son of Mars, and Venus, ancestor of the Aeneades. Later, Augustus created the cult of Mars the avenger (Mars Ultor)avenger of the Roman disaster suffered by M. Licinius Crassus at Carrhae in 53 bce, and also avenger of the assassination of Julius Caesar (the victory at Philippi in 42 bce). In 20 bce, a round temple was erected upon the Capitoline in honor of Mars Ultor and, in 2 bce, the great temple, situated in its own forum, was built (Dio Cassius, 54.8.3 and 60.5.3). Thus Mars enjoyed new prestige.

See Also

Flamen; Lustratio; Roman Religion, article on The Early Period.

Bibliography

Dumézil, Georges. Archaic Roman Religion. 2 vols. Translated by Philip Krapp. Chicago, 1970. Discusses the theory of the agrarian Mars.

Dumézil, Georges. Fêtes romaines d'été et d'automne. Paris, 1975. Pages 139156 and 177219 treat the Equus October.

Hermansen, Gustav. Studien über den italischen und den rö-mischen Mars. Copenhagen, 1940. Supports the theory of the agrarian Mars.

Heurgon, Jacques. Trois études sur le "ver sacrum." Brussels, 1957. See pages 2035.

Poultney, James Wilson. The Bronze Tables of Iguvium. Baltimore, 1959.

Ramat, Anna Giscalone. "Studi intorno ai nomi del dio Marte." Archivio glottologico italiano 47 (1962): 112142.

Rose, Herbert Jenkins. Ancient Roman Religion. London, 1948.

Rose, Herbert Jenkins. "Some Problems of Classical Religion." The Eitrem lectures delivered at the University of Oslo, March 1955. Oslo, 1958, pp. 117.

Schilling, Robert. La religion romaine de Vénus. 2nd ed. Paris, 1982. Pages 107 and following treat the association between Mars and Venus in the lectisternium of 217 bce.

Scholz, U. W. Studien zum altitalischen und altrömischen Marskult und Marsmythos. Heidelberg, 1970.

Stibbe, C. M., et al. Lapis Satricanus: Archaeological, Epigraphical, Linguistic, and Historical Aspects of the New Inscription from Satricum. The Hague, 1980.

Versnel, H. S. "Die neue Inschrift von Satricum in historischer Sicht." Gymnasium 89 no. 3 (1982): 193235.

Versnel, H. S. "Transition and Reversal in Myth and Ritual" and "Apollo and Mars One Hundred Years after Roscher." In Inconsistencies in Greek and Roman Religion. Leiden and New York, 19901993.

Wagenvoort, Henobrik. Roman Dynamism. Oxford, 1947.

Wissowa, Georg. Religion und Kultus der Römer. 2d ed. Munich, 1912. See pages 141 and following.

Robert Schilling (1987)

Charles Guittard (2005)

Translated from French by Paul C. Duggan

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Mars

Mars ★½ 1996

Fast-paced space western finds independent lawman Caution Templar (Gruner) receiving a frantic message from his brother to return to the wild mining town of Alpha City, Mars. Only by the time he arrives his brother is dead and Templar is determined to find out what happened. 92m/C VHS, DVD . Olivier Gruner, Shari Belafonte, Scott Valentine, Amber Smith, Alex Hyde-White, Lee DeBroux, Gabriel Dell; D: Jon Hess; W: Patrick Highsmith, Steven Hartov. VIDEO

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Mars

Mars

The red planet

Physical properties of Mars

The history of Mars

A requiem for Percival Lowell

Martian satellites

Resources

Mars is the fourth planet from the sun, orbiting the sun once every 687 (Earth) days at a mean distance of 141 million mi (227 million km). Called the red planet for its distinct orange-red color, Mars has been the object of intense interest for over a century. Long popularly regarded as a possible haven for extraterrestrial life, Mars was thought to be barren after the Viking spacecraft landed on it in 1976 and found no unequivocal evidence of living organisms. But interest in Mars as at least a possible ancient host of life resurged in the 1990s with the claim of fossilized microbes in meteorites from Marsalthough the scientific consensus is against the microbial interpretation of those microfossilsand evidence from orbital and lander missions in the 2000s that suggested a warm, wet past for Mars.

Mars has numerous earthlike features. There are large, extinct volcanoes dotting its surface, eroded channels where water once flowed freely, and ice caps covering its poles that look very much like Earths polar regions. However, unlike Earths, the thin Martian atmosphere is made mainly of carbon dioxide, and there is no evidence that life presently exists on or under the Martian surface. Although

Mars may now be a cold, dead world, the variety of features on its surface suggests a complex and fascinating present and history.

The red planet

There are four planets in the inner solar system. The innermost is Mercury: tiny, barren, and hard to observe as it is located near the sun. Next comes Venus, the planet nearest in size and mass to Earth, but swathed in clouds; a bland, featureless ball with surface temperatures hot enough to melt lead. The third is Earth, most complex of all planets in the solar system and the only one known to harbor life. Mars, half again as far from the sun as Earth, is also distinct. Intriguing features are distinguishable on its surface even from earthbound telescopes, and it bears polar ice caps that sometimes look much like Earths (although they are composed partly of solid carbon dioxide, unlike Earths caps, which are composed entirely of frozen water).

Nineteenth-century observations of Mars by Giovanni Schiaparelli (18351910) showed the existence of what Schiaparelli called canali, meaning channels, not canals (which would have implied deliberate construction). The existence of somewhat linear, light and dark channel-like features on Mars is affirmed by many other scientists, but the Italian word canali quickly acquired its popular and inaccurate English translation, canals.

The excitement of this discovery spurred a man named Percival Lowell in 1894 to leave his Boston home for Flagstaff, Arizona, where he founded the

observatory that bears his name. Lowell spent the rest of his life studying Mars through the 24 in (61 cm) refracting telescope on Mars Hill above Flagstaff, and became convinced that intelligent life existed on the red planet. Lowells drawings became increasingly complex as he observed and reobserved the planet, and he devoted himself to convincing the public that Mars was indeed inhabited.

Lowell Observatory soon became the site of fundamental advances in astronomy, such as the 1930 discovery of the solar systems outermost planet, Plutodemoted officially to dwarf planet status by the International Astronomical Union in 2006. However, Percival Lowell was wrong about Mars: it was not covered by a network of striking straight canals designed to water a dying planet with melting polar-cap water. In 1976, twin robotic spacecraft, Viking 1 and Viking 2, landed at different points on Marss northern hemisphere. They carried experiments designed to test the Martian soil for the presence of microorganisms, and found no results that unequivocally pointed to life chemistry. A few scientists still contend that the results might have indicated life, but the consensus is that they did not. The expedition had initially looked promising as one experiment yielded reactions suggestive of life forms, but further analysis revealed that the reactions were not biological.

Viking revealed that the Martian terrain bears an eerie resemblance to some of the desert landscapes not so far from the hill where Lowell spent so many nights at his telescope. The pictures revealed, in the words of mission scientist and science popularizer Carl Sagan, that there were rocks and a distant eminence, as natural and unselfconscious as any landscape on Earth. Mars was a place.

Physical properties of Mars

Mars is rusty; iron oxides are responsible for its orange hue. It is smaller than Earth. Its diameter of about 2,111 mi (3,397 km) is a little over half that of Earth, and it is only 10% as massive as our planet. Mars has seasons because the tilt of its axis relative to the plane of its orbit is nearly the same as Earths. It rotates on its axis once every 24 hours and 40 minutes,

so a Martian day is just a little longer than one of ours. The sun would appear larger in the Martian sky because Mars is half as far from the sun as Earth, and its year is 687 (Earth) days long.

Marss gravity is weaker than Earths (38% of Earths, on the surface) and the planet has been unable to retain much of an atmosphere. The Martian atmosphere is less than 1% as dense as Earths and is made mostly of carbon dioxide, with trace amounts of nitrogen and argon.

Atmospheric carbon dioxide is the source of Marss polar ice caps. Carbon dioxide in the atmosphere acts like a giant insulator for a planet, preventing heat from radiating away to space. Marss atmosphere is mostly carbon dioxide yet is so thin that it holds little heata blazing summer day on Mars might get up to the freezing point of water 32°F (0°C), but at night the temperature plummets well back below 0°F (18°C). At the poles, temperatures drop well below100°F (73°C), sufficiently cold for the carbon dioxide in the atmosphere to freeze. Marss polar ice caps consist of frozen carbon dioxide with an underlayer of ice.

Marss surface does have some very Earthlike features. There are enormous volcanoes, the largest, Olympus Mons, is almost the size of the entire state of Arizona and is three times taller than Mt. Everest, Earths tallest mountain. Also there are long, eroded channels telling us that at some time in the past water flowed freely on the Martian surface.

Mars surface terrain can be divided into two main areas, the southern highlands (the older part of Mars) and the northern plains (a lower, younger region). Dividing these areas is a planet-encircling feature called the global escarpment. The southern highlands are densely cratered and there are two very large impact crater basins there called Hellas and Argyre. There is abundant evidence of river systems draining the southern highlands, and the drainage is mainly toward the northern plains (or lowlands) across the global escarpment. On of the largest valleys in the solar system, Valles Marinaris, cuts across this escarpment, showing where water drained from south to north during a period in Mars history when abundant water was present. The northern lowlands are about 1.6 mi (2.5 km) below the mean radius of Mars and contain evidence of extensive flood-type volcanic flows, as well as river systems and wind-blown dust layers. In the northern plains, two continent-sized upwarped areas occur (Tharsis and Elysium). These are volcanic areas, home of giant shield volcanoes, including the largest known volcano in the solar system, Olympus Mons.

The history of Mars

The surface features of Mars show that the planet has had an exciting history. Long ago, the surface was volcanically active. Early in the planets history, it probably had crustal plates moving about as is the case on Earth, but as Mars cooled and its crust thickened, tectonic activity ceased. The enormous size of Olympus Mons supports this idea. On Earth, moving crust slides over a hot spot or upwelling of molten rock, forming a series of volcanic mountains (the Hawaiian Islands are a classic example). On Mars, with no plate motion, the lava simply piles up in one spot. There are several volcanoes on Mars larger than any on Earth, suggesting the planet has long had a thick, immobile crust.

Erosion channels on Marss surface show that the planet once had running water. On Mars today, water would boil immediately even at the low Martian temperatures, because the atmospheric pressure is so low. (Water boils at progressively lower temperatures as one goes to higher altitudes because the atmospheric pressure is lower. at lower pressures it is easier for molecules to escape the surface of a liquid.) This suggests that the Martian atmosphere was once much

denser than it is now. Otherwise, water could never have flowed on the planets surface.

Photographic evidence from NASAs Mars Odyssey orbit mission (2001) suggests that liquid water may occasionally still erupt today from high up the interior walls of certain craters, boiling away as it surges downhill.

Some of the eroded channels on Mars resemble terrestrial riverbeds, more or less gradually carved, but others show evidence of a violent past. They seem to have been formed by enormous flash floods, perhaps caused when a Martian lake broke through a collapsing natural feature such as a rock wall and cascaded across the land. Several such incidents have been documented in the geologic record on Earth.

Some scientists theorize that over many millions of years, Marss atmosphere thinned, and, as the planet cooled, its water boiled away. Some of the water stills remain on the planet, permanently frozen in the ice caps and in the soil. How much is hidden underground may be determined by ground-penetrating radar aboard NASAs Mars Reconnaissance Orbiter (2006) and the European space Agencys Mars Express orbiter (2003).

Although Martian tectonic activity has ceased and the atmosphere has largely dissipated, dust storms still rage across its surface. The Mariner and Viking orbiters observed giant dust storms sweeping across the Martian land. The largest of these storms can sweep dust particles around the entire planet. One of the greatest dust storms ever observed on Mars occurred in 1971, when the entire planet was shrouded just as one of the earliest Mars orbiters,Mariner 9, arrived to take pictures. No pictures could be obtained until the end of the storm in 1972.

A requiem for Percival Lowell

Until very recently it appeared that Percival Lowell was wrong about the existence of life on Mars. However, in August 1996, a team of scientists at the national Aeronautics and Space Administrations (NASA) Johnson Space Center and at Stanford University announced the discovery of evidence that strongly suggests primitive life may have existed on Mars over 3.6 billion years ago. This evidence is contained in a 4.2lb, potato-sized meteorite discovered in Antarctica in 1984, named ALH84001. This meteorite is one of twelve found on Earth to date that match the unique Martian chemistry measured by the Viking spacecraft when they landed on Mars in 1976. The meteorite contains detectable amounts of polycyclic aromatic hydrocarbons (PAHs), the first organic molecules thought to be of Martian origin; several mineral features (i.e., carbonates) characteristic of biological activity; and possible microscopic fossils of bacterialike organisms.

Mars is now a cold, dry, almost airless world, but between 3.6 and 4 billion years ago water flowed across the Martian landscape. The planet had a thicker atmosphere and was also much warmer than it is today. The rock that eventually fell to earth as a meteorite was located underneath the Martian surface and fractures in the rock were penetrated by water and carbon dioxide from the planets atmosphere. (The rock is debris from an asteroid that collided with Mars millions of years ago, scattered into space before eventually falling to Earth about 13,000 years ago.) Carbonate minerals were deposited in the meteorites fractures.

Scientists studying the meteorite initially argued that living organisms may have assisted in the formation of the carbonate, organisms that were eventually fossilized much like fossils are formed in limestone rock on Earth. The largest of these possible fossils are less that 0.01, the diameter of a human hair. In appearance and size, these structures are quite similar to microscopic fossils of the tiniest bacteria found on Earth. The presence of PAHs in the meteorite provides further evidence that life may have existed on Mars because PAHs are frequently formed by the degradation of the complex organic molecules contained in microorganisms after the microorganisms die.

However, debate over the interpretation of the tiny structures in ALH84001 has been active since the initial announcement. No similar markings have appeared in other meteorites known to be from Mars. Two independent chemical studies in 1998 gave evidence that at least some signs in the rock are because of contamination from Earth. One study looked for amino acids, the building blocks of proteins that play an essential role in biology, and were detected in only small amounts that appeared to be terrestrial in origin. Another found no sign of organic matter in the tiny globs of carbonate on the meteorite, but did not look for PAHs. Given the intense interest in finding life outside Earth, together with the rigorous scientific demands of any such remarkable claim, the controversy is sure to continue for years. Most planetary scientists argue that the evidence is, at best, inconclusive with regard to supporting evidence of biological processes.

In the last half of the 1990s, NASA sent several probes to Mars that were unlike anything seen before. The Mars Pathfinder mission landed on the red planet

on July 4, 1997, on the rocky flood plain Ares Vallis. After landing the Pathfinder craft unfurled and a 23pound, six-wheeled remote roving vehicle, named Sojourner, crawled off the platform and onto the planets surface, while Pathfinder itself raised a camera arm to a height of five feet.

Pathfinder returned over 10,000 color pictures from Mars, painting a picture of the surface as one that over a billion years ago had once been scoured by huge floods of liquid waters, with salty residues left from puddles that once slowly evaporated. At the same time, Pathfinder took pictures of Sojourner roaming about the planets surface, sometimes nestling against rocks for analysis. Sojourner was equipped to chemically analyze the rocks it encounteredthe first two of which were nicknamed Barnacle Bill and Yogiwith an alpha proton x-ray spectrometer that bounces particles or x rays off rocks and analyzes what returns. Barnacle Bill was quartz-like, indicating it had been heated and reheated somewhere in the planets crust. The more-primitive Yogi was most likely of volcanic origin. Pathfinder also found wild fluctuations of temperaturesas much as 20 degrees up or down in a few secondsand evidence of towering dust devils up to a half-mile high winding across the desert plain. And it found evidence that, like Earth, Mars is not merely a solid rock ball, but has a crust, a mantle, and an iron core.

Later in 1997, the Mars global survey or went into orbit around Mars and began a mapping survey of the planet. It found that Mars had a weak magnetic field, about 1/800 that of Earth, but one stronger than scientists had expected. (By comparison, Jupiters magnetic field is 10,000 times that of Mars.) This field is important in the geological history of Mars, and helps determine the nature of its rock below the surface. At some points in the orbit, the spacecraft was able to descend to between 105 and 75 mi (170120 km) in altitude, beneath the ionosphere, low enough to detect remnant magnetic fields of material on the surface. These results suggested that Mars once had a magnetic field comparable to that on Earth, which would have protected the surface from the cosmic rays and energetic particles from the sun. The global surveyor also found evidence of hematite, an iron-bearing mineral that forms only in high-temperature aqueous systemscompelling evidence for hydrothermal vents on Mars. Magnetic stripes discovered on the surface in 1999 hint that early-on the surface of Mars may have been formed by tectonic plates, much like that on Earth.

In some quarters, there is intense interest in Mars as a site for eventual human exploration and settlement. Official US space policy has included the goal of human exploration since 2004. Many scientists and other parties, however, view such schemes as likely to starve purely scientific explorations because of their immense cost. Also, though there is a strong popular movement favoring eventual Mars settlement, the goal of settling Mars is by no means universally viewed as either technically feasible or humanly desirable. No human flights to Mars had been scheduled as of 2006. A continuing series of robotic flights was scheduled for the coming years.

Our knowledge of Mars grew explosively starting in early 2004, when the twin Mars exploration rovers landed successfully on opposite sides of the planet. The six-wheeled, solar-powered rovers are equipped with several cameras, high-speed data links to Earth, and grinding tools for examining rocks. (Unlike Viking, they did not contain experiments designed to look for life chemistry.) The rovers, nominally designed to operate for only 90 days (the primary mission), were still exploring almost three years later. Driving for milesclimbing hills, traversing plains, and descending into cratersreturning a wealth of panoramic, microscopic, and three-dimensional images and geological and meteorological data, deepening our scientific knowledge of Mars and confirming a warmer, wetter phase in its early history. The duration of that period remained a point of scientific dispute as of 2006. In late 2006, NASAs Mars reconnaisance orbiter mission began photographing the planet from above in unprecedented detail, resolving objects as small as three feet across. One of the orbiters earlier pictures showed one of the twin Mars exploration rovers perched on the edge of a large crater, wheel-tracks and the shadow cast by its camera mast clearly visible.

Martian satellites

Mars has two tiny satellites, Phobos (17 × 12 mi/ 27 × 19 km) and Deimos (9 × 7mi/15 × 11 km). Studies of both show that they are chondritic asteroids that have been captured by Mars gravity (but were originally formed in the main belt of asteroids located beyond Mars). Both are in 1:1 spin orbit couples with Mars, meaning that the same face of these small moons faces Mars all the time. Both satellites have densities of about 2 gm/cm3, indicating that they are internally fractured as well as being made of rather light mineral and organic compounds.

The most common superficial form on these satellites is the impact crater. carter Stickney on Phobos is the largest such feature, about 6 mi (10 km) in diameter. Crater Hall, also on Phobos, is the second largest. A ridge between these craters is named Kepler. Both satellites are covered by a thick layer of broken debris

KEY TERMS

Deimos The smaller of Marss two satellites, only 3.7 mi (6.0 km) across its smaller dimension.

Olympus Mons The largest Martian volcano, about 370 mi (600 km) across at its base. The existence of such large volcanoes suggests that Mars has a thick, tectonically inactive crust.

Phobos The larger of Marss two satellites.

Polar caps The deposits of frozen carbon dioxide at Marss poles. The ice caps advance and recede with the changing Martian seasons, and bear a strong resemblance to earths polar regions.

Valles Marineris The giant Martian canyon, located in a place of numerous rifts and faults in the Martian crust. This canyon would stretch across the entire continental United States.

called regolith (or soil). Regolith on the sides of impact craters can be seen in layers, suggesting the regolith is impact related (ejected fragments).

Boulders can be seen lying on the side of Deimos. Linear grooves occur in regolith on both satellites, and the linear grooves are beaded (a feature that suggests finer soil particles are draining down into open spaces in the underlying fractures within the satellites). Both satellites are inside the roche limit of stable orbit, meaning that eventually both satellites will impact on the surface of Mars. Apparently, this sort of thing has happened before as there are now some well documented elongate (low angle) impact craters on Mars that appear to have been formed by objects in orbits similar to Phobos and Deimos.

See also Planetary atmospheres.

Resources

BOOKS

de Pater, Imke, and Jack J. Lissauer. Planetary Sciences Cambridge, UK: Cambridge University Press, 2001.

Kargel, Jeffrey S. Mars: A Warmer, Wetter Planet. New York: Springer, 2004.

Squyres, Steve. Roving Mars: Spirit, Opportunity, and the Exploration of the Red Planet. New York: Hyperion, 2005.

PERIODICALS

Kerr, Richard A. On Mars, a Second Chance for Life. Science 306 (2004): 2010-2012.

Squyres, S.W., et al. The Opportunity Rovers Athena Science Investigation at Meridiani Planum, Mars.Science. 306 (2004): 1698-1703.

Squyres, S.W., et al. The Spirit Rovers Athena Science Investigation at Gusev Crater, Mars. Science. 305 (2004): 794-799.

OTHER

National Aeronautics and Space Administration (NASA ). NASA Mars Exploration Program. November 1, 2006. <http://mars.jpl.nasa.gov/> (accessed November 3, 2006).

Jeffrey C. Hall

David T. King, Jr.

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Mars

Mars

Mars is the fourth planet from the center of the solar system , orbiting the Sun once every 687 (Earth ) days at a mean distance of 141 million mi (227 million km). Called the "red planet" for its distinct orange-red color , Mars has been the object of intense interest for over a century. Popularly regarded as a possible source of life, Mars was thought to be barren after the Viking spacecraft landed on it in 1976 and found no evidence of living organisms. But interest in Mars as at least an ancient host of life resurged in the 1990s with the claim of fossilized microbes in meteorites from Mars, and pictures from the 1997 Pathfinder mission that suggested water once swept across the Martian surface.

Mars has numerous earthlike features. There are large, extinct volcanoes dotting its surface, eroded channels where water once flowed freely, and ice caps covering its poles that look very much like Earth's polar regions. But, the thin Martian atmosphere is made mainly of carbon dioxide . Although Mars may now be a cold, dead world, the variety of features on its surface suggests a complex and fascinating past.

Perhaps we will learn more about this fascinating planet when the new NASA Odyssey mission and the two planned NASA Mars Exploration Rover missions launch and reach Mars in the next few years.


The red planet

There are three planets other than Earth in the inner solar system. The innermost is Mercury: tiny, barren, and hard to observe as it is located near the Sun. Next comes Venus , the planet nearest in size and mass to Earth, but swathed in clouds ; a bland, featureless ball through the small telescope . Mars, half again as far from the Sun as Earth, is different. Features are distinguishable on its surface, and it sometimes shows polar ice caps that look much like Earth's.

Early observations of Mars by Giovanni Schiaparelli showed the existence of what Schiaparelli called canali, meaning channels. The existence of somewhat linear, light and dark channel-like features on Mars is affirmed by many other scientists, but the Italian word canali quickly acquired its popular and inaccurate meaning: canals. Water can cut a channel, but only intelligent life can build a canal .

The excitement of this discovery spurred a man named Percival Lowell in 1894 to leave his Boston home for Flagstaff, Arizona, where he founded the observatory that bears his name. Lowell spent the rest of his life studying Mars through the 24 in (61 cm) refracting telescope on Mars Hill above Flagstaff, and became convinced that intelligent life existed on the red planet. Lowell's drawings became increasingly complex as he observed and reobserved the planet, and he devoted himself to convincing the public that Mars was indeed inhabited.

Although Lowell Observatory soon became the site of fundamental advances in astronomy , such as the 1930 discovery of the solar system's outermost planet, Pluto , Percival Lowell was wrong about Mars. In 1976 two unmanned spacecraft, Viking 1 and Viking 2, landed at different points on Mars's northern hemisphere. They carried experiments designed to test the Martian soil for the presence of microorganisms , and ultimately found nothing. The expedition had initially looked promising as one experiment yielded reactions suggestive of life forms, but further analysis revealed that the reactions were not biological. The Martian terrain bears an eerie resemblance to some of the desert landscapes not so far from the hill where Lowell spent so many nights at his telescope.


Physical properties of Mars

The "red planet" is so named because of the color of its surface, which indeed is strikingly red. Simply put, Mars has rusted—iron oxides are responsible for its orange hue.

Mars is smaller than Earth. Its diameter of about 2,111 mi (3,397 km) is a little over half that of Earth, and it is only 10% as massive as our planet. Mars has seasons because the tilt of its axis relative to the plane of its orbit is nearly the same as Earth's. It rotates on its axis once every 24 hours and 40 minutes, so a Martian day is just a little longer than one of ours. The Sun would appear larger in the Martian sky because Mars is half as far from the Sun as the Earth, and its year is 687 (Earth) days long.

Mars's gravity is weaker than Earth's, and the planet has been unable to retain much of an atmosphere. The Martian atmosphere is less than 1% as dense as Earth's, and is made mostly of carbon dioxide, with trace amounts of nitrogen and argon.

Atmospheric carbon dioxide is the source of Mars's polar ice caps. Atmospheres act like giant insulators for planets, preventing heat from radiating away to space . Mars's thin atmosphere holds very little heat—a blazing summer day on Mars might get up to the freezing point of water 32°F (0°C), but at night the temperature plummets well back below 0°F (-18°C). At the poles, temperatures drop well below -100°F (-73°C), sufficiently cold for the carbon dioxide in the atmosphere to freeze. Mars's polar ice caps consist of frozen carbon dioxide with an underlayer of ice.

Although no life has been found on Mars, the planet's surface does have some very Earth-like features. There are enormous volcanoes, the largest of which, Olympus Mons, is almost the size of the entire state of Arizona. Elsewhere are long, eroded channels telling us that at some time in the past water flowed freely on the Martian surface.

Mars surface terrain can be divided into two main areas, the southern highlands (the older part of Mars) and the northern plains, a lower younger region. Dividing these areas is a planet-encircling feature called the global escarpment. The southern highlands are densely cratered and there are two very large impact crater basins there called Hellas and Argyre. There is abundant evidence of river systems draining the southern highlands, and the drainage is mainly toward the northern plains (or lowlands) across the global escarpment. On of the largest vallies in the solar system, Valles Marinaris, cuts across this escarpment, showing where water drained from south to north during a period in Mars history when abundant water was present. The northern lowlands are about 1.6 mi (2.5 km) below the mean radius of Mars and contain evidence of extensive flood-type volcanic flows, as well as river systems and wind-blown dust layers. In the northern plains, two continent-sized upwarped areas occur (Tharsis and Elysium). These are volcanic areas, home of giant shield volcanoes, including the largest volcano in the solar system, Olympus Mons.


The history of Mars

The surface features of Mars show that the planet has had an exciting history. Long ago, the surface was volcanically active. Early in the planet's history, it probably had crustal plates moving about as is the case on Earth, but as Mars cooled and its crust thickened, the tectonic activity ceased. The enormous size of Olympus Mons supports this idea. The crust slides over a hot spot , and lava coming up forms a series of mountains . On Mars, with no plate motion , the lava simply piles up in one spot. There are several volcanoes on Mars larger than any on Earth, suggesting the planet has a thick, inactive crust.

The eroded channels on Mars's surface show that the planet once had running water. Water boils at progressively lower temperatures as one goes to higher altitudes because the atmospheric pressure is lower. (At lower pressures it is easier for molecules to escape the surface of a liquid.) On Mars today, water would boil immediately even at the low Martian temperatures, because the atmospheric pressure is so low. This suggests that the Martian atmosphere was once much denser than it is now. Otherwise, water could never have flowed on the planet's surface.

Some of the eroded channels on Mars resemble terrestrial riverbeds, but some show evidence of a violent past. They seem to have been formed by enormous flash floods, perhaps caused when a Martian lake broke through a collapsing natural feature such as a rock wall and cascaded across the land. Several such incidents are documented in the geologic record on Earth.

Many scientists theorize that Mars's atmosphere thinned, and, as the planet cooled, the water boiled away. Some of the water may still remain on the planet, permanently frozen in the ice caps or in the soil. Much of it was probably lost when the Sun's ultraviolet radiation dissociated the water molecules into their hydrogen and oxygen atoms .

Although Martian tectonic activity has ceased and the atmosphere has largely dissipated, storms still rage across its surface. The Viking orbiters observed giant dust storms sweeping across the Martian land. The largest of these storms can sweep dust particles around the entire planet, rushing past the streambeds, ancient craters, volcanoes, and canyons, obscuring everything in their path. One of the greatest dust storms ever observed on Mars occurred in 1971, when the entire planet was shrouded just as one of the earliest Mars orbiters, Mariner 9, arrived to take pictures. No pictures could be obtained until the end of the storm in 1972.


A requiem for Percival Lowell

Until very recently it appeared that Percival Lowell was wrong about the existence of life on Mars. However, in August 1996, a team of scientists at the National Aeronautics and Space Administration's (NASA) Johnson Space Center and at Stanford University announced the discovery of evidence that strongly suggests primitive life may have existed on Mars over 3.6 billion years ago. This evidence is contained in a 4.2–lb, potato-sized meteorite discovered in Antarctica in 1984, named ALH84001. This meteorite is one of twelve found on Earth to date that match the unique Martian chemistry measured by the Viking spacecraft when they landed on Mars in 1976. The meteorite contains detectable amounts of polycyclic aromatic hydrocarbons (PAHs), the first organic molecules thought to be of Martian origin; several mineral features (i.e. carbonates) characteristic of biological activity; and possible microscopic fossils of bacteria-like organisms.

Mars is now a cold, dry, almost airless world, but between 3.6 and 4 billion years ago water flowed across the Martian landscape. The planet had a thicker atmosphere and was also much warmer than it is today. The rock that eventually fell to Earth as a meteorite was located underneath the Martian surface and fractures in the rock were penetrated by water and carbon dioxide from the planet's atmosphere. (The rock is debris from an asteroid that collided with Mars millions of years ago, scattered into space before eventually falling on Earth about 13,000 years ago.) Carbonate minerals were deposited in the meteorite's fractures.

Scientists studying the meteorite initially argued that living organisms may have assisted in the formation of the carbonate, organisms that were eventually fossilized much like fossils are formed in limestone rock on Earth. The largest of these possible fossils are less that 0.01 the diameter of a human hair. In appearance and size, these structures are quite similar to microscopic fossils of the tiniest bacteria found on Earth. The presence of PAHs in the meteorite provides further evidence that life may have existed on Mars because PAHs are frequently formed by the degradation of the complex organic molecules contained in microorganisms after the microorganisms die.

However, debate over the interpretation of the tiny structures in ALH84001 has been active since the initial announcement. No similar markings have appeared in other meteorites known to be from Mars. Two independent chemical studies in 1998 gave evidence that at least some signs in the rock are the of contamination from Earth. One study looked for amino acids, the building blocks of proteins that play an essential role in biology , and were detected in only small amounts that appeared to be terrestrial in origin. Another found no sign of organic matter in the tiny globs of carbonate on the meteorite, but did not look for PAHs. Given the intense interest in finding life outside the Earth, together with the rigorous scientific demands of any such remarkable claim, the controversy is sure to continue for years. As of 2003, most planetary scientists argue that the evidence is, at best, inconclusive with regard to supporting evidence of biological processes.

In the last half of the 1990s, NASA sent several probes to Mars that were unlike anything seen before. The Mars Pathfinder mission landed on the red planet on July 4, 1997, on the rocky flood plain Ares Vallis. After landing the Pathfinder craft unfurled and a 23-pound, six-wheeled remote roving vehicle, named Sojourner, crawled off the platform and onto the planet's surface, while Pathfinder itself raised a camera arm to a height of five feet.

Pathfinder returned over 10,000 color pictures from Mars, painting a picture of the surface as one that over a billion years ago had once been scoured by huge floods of liquid waters, with salty residues left from puddles that once slowly evaporated. At the same time, Pathfinder took pictures of Sojourner roaming about the planet's surface, sometimes nestling against rocks for analysis. Sojourner was equipped to chemically analyze the rocks it encountered—the first two of which were nicknamed Barnacle Bill and Yogi—with an alpha proton x-ray spectrometer that bounces particles or x rays off rocks and analyzes what returns. Barnacle Bill was quartz-like, indicating it had been heated and reheated somewhere in the planet's crust. The more-primitive Yogi was most likely of volcanic origin. Pathfinder also found wild fluctuations of temperatures—as much as 20 degrees up or down in a few seconds—and evidence of towering dust devils up to a half-mile high winding across the desert plain. And it found evidence that, like Earth, Mars is not merely a solid rock ball, but has a crust, a mantle, and an iron core.

Later in 1997 the Mars Global Surveyor went into orbit around Mars and began a mapping survey of the planet. It found that Mars had a weak magnetic field, about 1/800 that of Earth, but one stronger than scientists had expected. (By comparison, Jupiter's magnetic field is 10,000 times that of Mars.) This field is important in the geological history of Mars, and helps determine the nature of its rock below the surface. At some points in the orbit, the spacecraft was able to descend to between 105 and 75 mi (170-120 km) in altitude, beneath the ionosphere, low enough to detect remnant magnetic fields of material on the surface. These results suggested that Mars once had a magnetic field comparable to that on Earth, which would have protected the surface from the cosmic rays and energetic particles from the Sun. The Global Surveyor also found evidence of hematite, an iron-bearing mineral that forms only in high-temperature aqueous systems—compelling evidence for hydrothermal vents on Mars. Magnetic stripes discovered on the surface in 1999 hint that early-on the surface of Mars may have been formed by tectonic plates, much like that on Earth.

There is intensive interest in Mars as a site for eventual human exploration. Several robotic missions are planned for the near future, and public debate concerning manned space exploration as well as long-term goals for NASA has renewed in the wake of the tragic loss of the space shuttle Columbia in February 2003.


Martian satellites

Mars has two tiny satellites, Phobos (27 x 19 km) and Deimos (15 x 11 km). Studies of both show that they are chondritic asteroids that have been captured by Mars gravity (but were originally formed in the main belt of asteroids located beyond Mars). Both are in 1:1 spin orbit couples with Mars, meaning that the same face of these small moons faces Mars all the time. Both satellites have densities of about 2 gm/cm3, indicating that they are internally fractured as well as being made of rather light mineral and organic compounds.

The most common superficial form on these satellites is the impact crater. Carter Stickney on Phobos is the largest such feature, about 6 mi (10 km) in diameter. Crater Hall, also on Phobos, is the second largest. A ridge between these craters is named Kepler. Both satellites are covered by a thick layer of broken debris called regolith (or "soil"). Regolith on the sides of impact craters can be seen in layers, suggesting the regolith is impact related (ejected fragments).

Boulders can be seen lying on the side of Deimos. Linear grooves occur in regolith on both satellites, and the linear grooves are "beaded" (a feature that suggests finer soil particles are draining down into open spaces in the underlying fractures within the satellites). Both satellites are inside the Roche limit of stable orbit, meaning that eventually both satellites will impact on the surface of Mars. Apparently, this sort of thing has happened before as there are now some well documented elongate (low angle ) impact craters on Mars that appear to have been formed by objects in orbits similar to Phobos and Deimos.

See also Planetary atmospheres.


Resources

books

Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew L. Chaikin. The New Solar System. Cambridge: Cambridge Univ. Press, 1999.
de Pater, Imke, and Jack J. Lissauer. Planetary Sciences Cambridge, UK: Cambridge University Press, 2001.

Morrison, D., and Tobias Owen. The Planetary System. 3rd ed. Addison-Wesley Publishing, 2002.

Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge University Press, 2002.

periodicals

Chaikin, A. "Four Faces of Mars." Sky & Telescope (Jul 1992): 18.

Haberle, R.M. "The Climate of Mars." Scientific American (Aug 1978): 6.

other

Arnett, B. SEDS, University of Arizona. "The Nine Planets, a Multimedia Tour of the Solar System." November 6, 2002 [cited February 8, 2002]. <http://seds.lpl.arizona.edu/nine planets/nineplanets/nineplanets.html>.


Jeffrey C. Hall

David T. King, Jr.

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Deimos

—The smaller of Mars's two satellites, only 3.7 mi (6.0 km) across its smaller dimension.

Olympus Mons

—The largest Martian volcano, about 370 mi (600 km) across at its base. The existence of such large volcanoes suggests that Mars has a thick, tectonically inactive crust.

Phobos

—The larger of Mars's two satellites.

Polar caps

—The deposits of frozen carbon dioxide at Mars's poles. The ice caps advance and recede with the changing Martian seasons, and bear a strong resemblance to Earth's polar regions.

Valles Marineris

—The giant Martian canyon, located in a place of numerous rifts and faults in the Martian crust. This canyon would stretch across the entire continental United States.

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Mars

CHAPTER 7
MARS

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

—Astronomer Carl Sagan, 1967

Enigmatic means mysterious. Mars has been a mystery to humans for thousands of years. Although we know much about it now, 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 who 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 minus 64 degrees Fahrenheit. Rays of ultraviolet radiation beat down on the surface continuously. There is no oxygen to breathe, only 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 planet's poles. Where there is water, there is 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 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 tantalizing—humans standing on another planet. Finally, there would be some life on Mars.

EARLY TELESCOPIC VIEWS OF 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 Mars' 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 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 figured out that the Martian day lasts about twenty-four hours. The same conclusion was reached independently by the French astronomer Giovanni Cassini (1625–1712).

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

GIOVANNI SCHIAPARELLI

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 an Italian astronomer named Giovanni Schiaparelli (1835–1910). He created some of the first maps of the planet and assigned names to prominent features. Schiaparelli's 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 could 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 dark and light patches on the planet. This reinforced the mistaken idea that the patches were areas of land and water.

ASAPH HALL

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

Hall reported that the moons were very small and orbited close to the planet's surface. This had made them impossible to see before. Hall 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 myth as being servants to the god Mars.

PERCIVAL LOWELL

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 7,000 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 cap toward desert regions near the planet's equator.

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

In a series of articles published during 1895 by The Atlantic Monthly, Lowell laid out 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.

INHABITED OR NOT?

Lowell's 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 planet's surface. Agnes Clerk (1842–1907) was a science writer trained in astronomy who wrote well-respected books about astronomical discoveries. She called Lowell's 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 natural scientist Alfred Russel Wallace (1823–1913) wrote a book titled Is Mars Habitable? that examined Lowell's 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 outside the 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. Astronomer W. H. Pickering (1858–1938) believed the lines to be cracks in Mars' 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 Lowell's explanation.

MARS IN SCIENCE FICTION

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 Jonathan Swift (1667–1745). Gulliver's 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 written.

After Schiaparelli and Lowell popularized the idea of intelligent life on Mars, science fiction writers gleefully embraced the notion. In 1898 H.G. Wells (1866–1946) portrayed Martians as lethal invaders in War of the Worlds. The insect-like 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.

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. Some of them are green.

In 1924 the motion picture Aelita: Queen of Mars debuted in the Soviet Union. It features an engineer who takes a spaceship to Mars and falls in love with the planet's 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 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 H.G. Wells a half century before.

Evil invaders from Mars were common villains in low-budget horror movies of the 1950s. Historians now believe that 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.

SCIENTIFIC FACTS ABOUT MARS

Mars is a small planet. Its diameter is about half that of Earth. Mars is twice as large as Earth's moon.

A Martian day lasts twenty-four hours and thirty-seven 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, at the poles the temperature can drop to minus 200 degrees Fahrenheit. At the equator during the summer, the temperature can reach 80 degrees Fahrenheit during the day.

The force of gravity is much weaker on Mars than it is on Earth. An astronaut standing on Mars would feel only thirty-eight percent 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 upon them. Only the IAU has that authority.

There are two particularly prominent features on Mars. One is a volcano called 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 a canyon called Valles Marineris (Mariner Valleys in English). This enormous canyon is 2,500 miles long by 60 miles wide and up to 6 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 planet's surface. The Martian atmosphere is thin and contains more than ninety-five percent 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 poles of Mars are covered by solid carbon dioxide (dry ice) layered with dust and water ice. These polar caps change in size as the seasons changes. Sometimes

Mission Country Launch date Purpose Results
[Unnamed]USSR10/10/1960Mars flybyDid not reach Earth orbit
[Unnamed]USSR10/14/1960Mars flybyDid not reach Earth orbit
[Unnamed]USSR10/24/1962Mars flybyAchieved Earth orbit only
Mars 1USSR11/1/1962Mars flybyRadio failed at 65.9 million miles (106 million km)
[Unnamed]USSR11/4/1962Mars flybyAchieved Earth orbit only
Mariner 3U.S.11/5/1964Mars flybyShroud failed to jettison
Mariner 4U.S.11/28/1964first successful Mars flyby 7/14/65Returned 21 photos
Zond 2USSR11/30/1964Mars flybyPassed Mars but radio failed, returned no planetary data
Mariner 6U.S.2/24/1969Mars flyby 7/31/69Returned 75 photos
Mariner 7U.S.3/27/1969Mars flyby 8/5/69Returned 126 photos
Mariner 8U.S.5/8/1971Mars orbiterFailed during launch
Kosmos 419USSR5/10/1971Mars landerAchieved Earth orbit only
Mars 2USSR5/19/1971Mars orbiter/lander arrived 11/27/71No useful data, lander destroyed
Mars 3USSR5/28/1971Mars orbiter/lander, arrived 12/3/71Some data and few photos
Mariner 9U.S.5/30/1971Mars orbiter, in orbit 11/13/71 to 10/27/72Returned 7,329 photos
Mars 4USSR7/21/1973failed Mars orbiterFlew past Mars 2/10/74
Mars 5USSR7/25/1973Mars orbiter, arrived 2/12/74Lasted a few days
Mars 6USSR8/5/1973Mars orbiter/lander, arrived 3/12/74Little data return
Mars 7USSR8/9/1973Mars orbiter/lander, arrived 3/9/74Little data return
Viking 1U.S.8/20/1975Mars orbiter/lander, orbit 6/19/76–1980, lander 7/20/76–1982Combined, the Viking orbiters and landers returned 50,000+ photos
Viking 2U.S.9/9/1975Mars orbiter/lander, orbit 8/7/76–1987, lander 9/3/76–1980Combined, the Viking orbiters and landers returned 50,000+ photos
Phobos 1USSR7/7/1988Mars/Phobos orbiter/landerLost 8/88 en route to Mars
Phobos 2USSR7/12/1988Mars/Phobos orbiter/landerLost 3/89 near Phobos
Mars ObserverU.S.9/25/1992orbiterLost just before Mars arrival 8/21/93
Mars Global SurveyorU.S.11/7/1996orbiter, arrived 9/12/97Currently conducting prime mission of science mapping
Mars 96Russia11/16/1996orbiter and landersLaunch vehicle failed
Mars PathfinderU.S.12/4/1996Mars lander and rover, landed 7/4/97Last transmission 9/27/97
Nozomi (Planet-B)Japan7/4/1998Mars orbiterCould not achieve Martian orbit due to propulsion problem
Mars ClimateU.S.12/11/1998Orbiterlost on arrival at Mars 9/23/99
Orbiter
Mars PolarU.S.1/3/1999lander/descent probes to explore Martian south polelost on arrival 12/3/99
Lander/Deep Space 2
Mars OdysseyU.S.4/7/2001Orbitercurrently conducting prime mission of science mapping
Mars ExpressEurope6/2/2003Orbiter and Beagle 2 LanderOrbiter currently collecting planetary data. Radio contact lost with Beagle 2 during descent
Mars ExplorationU.S.6/10/03 (Spirit) and 7/7/03 (Opportunity)Two Rovers: Spirit and OpportunityRovers landed in January 2004. Currently exploring planet surface
source: Adapted from "Historical Log," in NASA's Mars Exploration Program, National Aeronautics and Space Administration, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, July 24, 2003 [Online] http://marsprogram.jpl.nasa.gov/missions/log/ [accessed January 14, 2004]

during the summer the uppermost dry ice evaporates away, only to reform again when the weather turns cold.

Martian Moons

The two Martian moons Phobos and Deimos are not round spheres like Earth's Moon. They are shaped like lopsided potatoes. Phobos is approximately 5,800 miles from Mars, while Deimos is nearly 15,000 miles away.

The Martian moons are very 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, the 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 35 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 that close again until the year 2287.

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 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 35 million miles from Earth during that opposition.

The most recent Mars opposition occurred in August 2003. The next one will be in November 2005. Oppositions are the best times to send spacecraft to Mars.

MISSIONS 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 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. As shown in Table 7.1, more than half of the missions intended for Mars have failed for one reason or another. Some were plagued by launch problems, while 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. Although 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 Union, most NASA Mars missions conducted during the 1960s achieved their objectives. There was also notable success over the next decade with the Viking spacecraft. There was a long lull after that in NASA's Mars exploration program.

During the 1990s NASA launched five separate missions to Mars. Their names were Mars Observer (1992), Mars Global Surveyor (1996), Mars Pathfinder (1996), Mars Climate Orbiter (1998), and Mars Polar Lander (1999). Only two of the missions 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 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 NASA's failed Mars missions.

In June 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 system 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.

NASA's 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 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 Polar Lander also carried a $30 million technology package known as Deep Space 2. Deep Space missions are part of a JPL program called New Millennium. The program was started during the mid-1990s to flight-test new technologies for possible use in future missions. For example, the Deep Space 1 mission conducted in 1998 included the test of an ion engine while a spacecraft was orbiting the sun. In-flight testing of experimental devices in space is preferable to conventional laboratory testing because space conditions are difficult to simulate on the ground.

The Deep Space 2 package was comprised of two small probes, each weighing about ten pounds. The probes were packed with miniaturized robotic instruments that were going to be tested under the harsh conditions on Mars. The probes were designed to impact the ground at a high rate of speed and penetrate beneath the sandy surface. Tiny sampling instruments were to gather soil for analysis of water content. The probes were equipped with data relays to transmit the results to the computer aboard the Global Surveyor already in orbit around the planet.

NASA hoped that successful operation of the micro-instrument packages would lead to development of micro-spacecraft. These small compact spacecraft would be much cheaper to build than conventional spacecraft and could be sent out in fleets to conduct scientific investigations. The loss of Deep Space 2 was a setback for the program. However, in 2004 NASA intends to test micro-components as part of another New Millennium project called Space Technology 5.

THE MARINER PROGRAM

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 400 and 2,200 pounds. They were designed to operate for several years and collect specific scientific data about Earth's nearest planetary neighbors.

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

Mariner 4

In July 1965 Mariner 4 was launched into a solar orbit and 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 planet's gravity.

During its flyby, Mariner 4 took nearly two dozen photos, the first close-ups ever obtained of Mars. They showed a world pock-marked 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 more than 200 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 telescopes from Earth. 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 more than 7,000 photos of the planet's surface. For the first time scientists got a good look at Mar's 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 the Martian moons Phobos and Deimos.

Scientists learned from Mariner data that Mars had virtually no magnetic field and was bombarded with ultra-violet radiation. Earth's extensive magnetic field (or magnetosphere) helps protect our 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.

THE VIKING MISSION

Within only four years NASA went from orbiting Mars to a 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 planet's surface.

The spacecraft launched only weeks apart during late summer 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 (the Plains of Gold). On September 3, 1976, the Viking 2 lander set down at Utopia Planitia (the 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 minus 184 degrees Fahrenheit in the winter to 7 degrees Fahrenheit in the summer at the lander locations.

The orbiters mapped 97 percent 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 1978. The Viking 1 orbiter lasted another two years. The Viking1 lander continued to make transmissions to Earth until 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 an 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 can not 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 was not universally accepted, however, and remains controversial.

ALH84001

During the 1980s NASA was busy running the space shuttle program. There was no money to send spacecraft to Mars. Luckily a piece of Mars turned up on Earth. On December 27, 1984, a meteorite hunter found a 4-pound rock on the Allan Hills ice field in Antarctica (the South Pole). 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 stand for the year of the find (1984). The last digits indicate the order in which rocks were 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 person who found it wrote "Yowza-Yowza" across the sample 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 16 million years ago an asteroid probably slammed into the planet and sent the rock hurtling through space. Scientists believe that the rock arrived on Earth 13,000 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. 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 Institute. In all there have been twelve known Martian meteorites found on Earth since 1815. ALH84001 is the oldest meteorite in the collection.

MARS GLOBAL SURVEYOR

More than twenty years passed between the launch of the highly productive Viking missions and another successful mission to Mars. In November 1996 the Mars Global Surveyor took off from Cape Canaveral, Florida, atop a Delta II rocket. This is the first of a series of Surveyor missions intended to map Mars. The spacecraft arrived near the planet ten months later. To save on fuel the Global Surveyor 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 Global Surveyor was put through a very long series of gentle skims for a year and a half. Generally aerobraking does not take this long. Flight controllers were extremely careful with the Surveyor, 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 Mars 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. Surveyor also captured close-up photographs of Mars' moon Phobos. The images reveal that the moon is covered with at least three feet of powdery material.

In April 2002 Global Surveyor began performing data relay and imaging services for other NASA spacecraft carrying out missions at Mars. As of March 2004 Surveyor is still in orbit and functioning properly.

MARS PATHFINDER

Mars Pathfinder was a mission conducted as part of NASA's Discovery program. This was the Agency's "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. 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 planet's surface. A giant parachute released to slow its fall. A landing craft separated from the spacecraft shell and began to drop. Eight seconds before hitting the ground the lander's 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 a rocky flood plain known as Ares Vallis (Valley of Ares).

After the successful landing NASA renamed the lander the Carl Sagan Memorial Station, in memory of the late astronomer Carl Sagan (1934–1996). He died while Pathfinder was en route to Mars. The lander unfolded three hinged solar panels onto the ground as shown in 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, an African-American woman who crusaded for civil rights during the 1800s.

For two and a half months the Sojourner rover collected data about Martian soil, radiation levels, and rocks. The robotic machine 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 lander's 40-inch mast held little wind socks at different heights to determine variations in wind speed near the planet's surface. Magnets were mounted along the lander to collect dust particles for analysis. Scientists learned that airborne Martian dust is very magnetic and may contain the mineral maghemite, a form of iron oxide.

The Pathfinder returned more than 17,000 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.

2001 MARS ODYSSEY

In 1895 Percival 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 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 is "Follow the Water." In other words, the mission scientists hope that 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 NASA's Mars Exploration Program. The mission was named after the hit 1968 movie 2001: A Space Odyssey, based on a short story by science fiction author Arthur C. Clarke.

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. The spacecraft skimmed against the upper edge of the Martian atmosphere hundreds of times over a 3-month period to slow itself down.

In February 2002 Odyssey reached its final orbit and began mapping the planet's 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 2004 the spacecraft is 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 greater than absolute zero emits infrared radiation. Scientists use Odyssey's 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 planet's 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 is performing a dual role. It transmits to NASA data collected by the spacecraft itself and also data collected by other NASA spacecraft conducting Mars missions.

THE PERIHELIC OPPOSITION OF 2003

Scientists all over the world 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 also 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 only happen every fifteen to seventeen years. During the late 1990s, both the European Space Agency and NASA began planning Mars missions to coincide with the perihelic opposition of 2003.

MARS EXPRESS

The Mars Express mission is the first mission to Mars by the European Space Agency (ESA). It was timed to put the spacecraft in flight near the time of Mars' 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 used during the 1800s by Charles Darwin to explore uncharted seas on Earth.

In late November 2003 the Mars Express reached the planet's 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). Isis was the Egyptian goddess of heaven and fertility. 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 fate of the lander is unknown.

The Mars Express orbiter achieved orbit to begin its mission of collecting planetary data. As of May 2004 it was still undergoing deployment of its sub-surface sounding radar. It was expected to be fully operational by June 2004. The orbiter carries seven instruments designed to investigate the Martian atmosphere and geological structure and to search for subsurface water.

MARS EXPLORATION ROVERS

Another Mars mission began in 2003 with the launch of NASA's twin Mars Exploration Rovers (MERs). Each spacecraft carried an orbiter and a lander to Mars. Inside each lander was a rover about the size of a golf cart, 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 girl living in Scottsdale, Arizona. She was born in Russia and adopted by an American family. She chose the names to honor her feelings about America.

NASA adopted 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 Mars' perihelic opposition.

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

The Flight Trajectories

The flight trajectories for the MERs were chosen to take advantage of the perihelic opposition configuration of the Sun, Earth, and Mars.

Figure 7.5 shows the flight trajectory for the Spirit spacecraft. The Sun is pictured at the middle of the diagram. Earth's orbit is the innermost circle, while Mars' orbit is the outer circle. Both planets travel in a counterclockwise direction around the Sun. The figure shows the position of the Earth and Mars at the time of Spirit's launch and its arrival at Mars.

The Earth's path around the Sun is almost a perfect circle. Mars' path is more elliptical. The Sun does not sit at the center of the ellipse, but is offset to one side. The Earth travels around the Sun in 365.25 days. It takes Mars nearly twice that long to make an orbit (687 days). This is why Mars did not move as far around the Sun as the Earth during Spirit's journey.

Figure 7.6 shows the flight trajectory for the Opportunity spacecraft. Both spacecraft were subjected to occasional flight maneuvers along the way to keep them on their path to intercept Mars in January 2004.

Notice that the MER missions took place when the paths of Mars and the Earth were relatively close to each other. The spacecraft would have taken longer and had to travel farther and use more fuel if scientists had timed them to occur when the orbital paths of Mars and Earth were farther apart.

Landing on Mars

Figure 7.7 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 7.8. At twenty-one minutes before landing (L-21 min) 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 300 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. Retro-rockets 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 a crater named Gusev Crater in honor of the Russian astronomer Matvei Gusev (1826–1866). The crater is about 100 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 the Hebrew word for Mars. Major valleys on the Red Planet are named for the word Mars in different Earth languages.

On January 25, 2004, Opportunity set down near Mars' equator in an area called Meridiani Planum. Planum means plateau or high plains. The Meridiani Planum is considered the site of Mars zero longitude. This is the longitude arbitrarily selected by astrogeologists to be the prime meridian for the rest of the planet. Opportunity's landing site was nearly half way around Mars from Gusev Crater.

Both landing sites were chosen for their very 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 grey 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 rover are labeled in Figure 7.9. The rovers are just over five feet long. The panoramic cameras sit about five feet above the ground atop a mast.

Each rover weighed about 380 pounds on Earth and carried 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 operate independently of their landers. Each rover carries its own telecommunications equipment, camera, and computer. The electronic equipment received power from batteries that were repeatedly recharged by solar arrays. It was late summer on Mars when the rovers began their mission. Scientists expected that power generation would continue for at least 90 sols (or 92 Earth days) before the arrays became too dust-coated to harness solar power. A seasonal change (from summer to autumn) was also expected to limit the effectiveness of the solar arrays as time went by. 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.

Water and Blueberries

On March 2, 2004, NASA scientists announced that the Opportunity rover had uncovered strong evidence that its landing area Meridiani Planum was "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, 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 way that the "blueberries" are embedded in the rocks hints that water acted against the rocks in the past.

Mission Costs

The total cost of the MER missions was 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. About $75 million was devoted to operations and science costs.

THE FUTURE OF MARS EXPLORATION

NASA plans to send robotic spacecraft to Mars during the oppositions of 2005, 2007, and 2009.

The first, in 2005, will be the Mars Reconnaissance Orbiter. It will carry a high-resolution camera to provide very detailed images of the Martian surface. Radar, mineralogy, and atmospheric probes are also planned for the mission. In 2007 a spacecraft named Phoenix is scheduled to land on the Martian surface to investigate the water ice near one of the polar regions. It will also search for organic molecules in the soil. Two years later a rover named the Mars Science Laboratory will be sent to conduct detailed chemical analysis of the Martian surface. NASA's long-term strategy calls for a robotic sample return mission sometime in the 2010s.

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 Earth's magnetosphere). A solar flare while they are in flight or on Mars could be particularly hazardous. Mars has no magnetosphere of its own. Its atmosphere is very thin, with little shielding effect against radiation. 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. This would mean a minimum of one year in space for the astronauts. 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. These 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 percent 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 very stressful conditions for a year or two. 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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