THE BIG RED PLANIT
Mars, is the fourth planet from the Sun and orbits the Sun at a distance of about 141 million miles. Mars is named for the Roman god of war because it appears fiery red in Earth's night sky.
Mars is a relatively small planet, with about half the diameter of Earth and about one-tenth Earth's mass. The force of gravity on the surface of Mars is about one-third of that on Earth. Mars has twice the diameter and twice the surface gravity of Earth's Moon. The surface area of Mars is almost exactly the same as the surface area of the dry land on Earth.
The Martian day, or the time it takes Mars to rotate once on its axis, is about a half an hour longer than an Earth day. Its year, or the time it takes to revolve once around the Sun, is about two Earth years long. Mars has two moons, Phobos and Deimos, which are named after the dogs of the Roman god Mars. These tiny bodies are heavily cratered dark chunks of rock and may be asteroids captured by the gravitational pull of Mars. Phobos orbits Mars once in less than one Martian day, so it appears to rise in the west and set in the east, usually twice each day. Deimos has the more ordinary habit of rising in the east and setting in the west.
Mars appears as a fairly bright, starlike object in the night sky of Earth. It moves through Earth's sky fairly rapidly, on a timescale of months. Because of the relative movements of Earth and Mars around the Sun, Mars appears to move backward in the sky for a short time around opposition, when the two planets are closest. As Mars and Earth orbit the Sun, the distance between them varies from about 47 million miles at opposition to about 233 million miles when the planets are on opposite sides of the Sun. This change in distance causes the apparent size of Mars to vary by a factor of 5 and its brightness to vary by a factor of 25.
When Mars is viewed through a telescope, it looks like a red and orange disk. An observer can easily see white ice caps at the north and south poles of Mars. These caps grow and shrink throughout the Martian year, just as the polar caps of Earth do. The darker areas of Mars's surface may look greenish to the observer, but this is an optical illusion caused by the contrast in color between the dark patches and brighter areas. Scientists believe that the dark areas are regions of relatively unweathered bare rock, while the bright areas are regions with deposits of weathered material, especially fine dust.
At certain times of the year, usually the southern Martian spring and summer when Mars is closest to the Sun, great dust storms appear as yellow clouds. The largest of these storms can cover the globe of Mars and last for months. At other times white clouds of water vapor are visible. Scientists now believe that the "canals" people observed on Mars during the 19th century are actually another optical illusion, caused by the mind's tendency to draw connections between irregular patches in a fuzzy image.
The Hubble Space Telescope (HST) provides the clearest Earth-based views of Mars, and astronomers use it to study the weather on the planet. The HST has provided images of dust storms with such detail that scientists can pinpoint the areas on the planet in which the storms started. The telescope also makes general studies of the atmosphere possible. Using HST images, astronomers have determined that the atmosphere of Mars is cooler, clearer, and drier than it was in the mid-1970s, the last time scientists were able to monitor the atmosphere closely.
Scientists believe that Mars's interior consists of a crust, mantle, and core like Earth's interior, but they do not know the relative sizes of these components. Because no spacecraft has ever brought instruments that can study Mars's interior to the planet, the only real data that scientists have about the planet's structure are its mass, size, and the structure of the gravity field. From that data scientists can learn some things about density in different parts of the planet.
Compared to Earth, Mars probably has a relatively thick crust. Beneath the Tharsis bulge, an area of volcanic activity in the northern hemisphere, it may be as thick as 80 miles. Beneath the landing site of the United States spacecraft Viking 2, it may be as thin as 9 miles.
The core is probably mostly iron, with a small amount of nickel. Other light elements, particularly sulfur, could exist in the core as well. If so, the core may be quite large. From studying Earth's magnetic field and core, scientists theorize that the motions of the liquid rock in Earth's core generate its magnetic field. Mars does not have a significant magnetic field, so scientists believe that Mars's core is probably solid.
Mars does not, and probably did not ever, have active plate tectonics, or a crust made up of separate sections that move about and sometimes crash into each other. Because Mars is so much smaller than Earth, it cooled quickly after formation and the crust thickened, forming one solid piece and eliminating any possibility of plate tectonics as is seen on Earth. Though the Martian crust is not broken into separate plates, Mars's liquid mantle has sculpted the planet's surface. The molten rock has broken through the crust to form volcanoes and its motion has cracked the crust to form large rifts.
The surface of Mars would be a harsh place for humans, but it is more like the surface of Earth than any other planet. The temperature on Mars does not get much cooler than the temperature at Antarctica. At the surface it ranges from about -140° C to 15° C (about -225° F to 60° F). During most of the year wind speeds are fairly low-about 4.5 mph-but during dust storms they can approach 40 to 50 mph. These winds often originate in large basins in the southern hemisphere and carry large volumes of dust from the basins to other regions, sometimes covering the entire planet in the storm. The dust is not sandy, as in a sandstorm on Earth, but has the consistency of flour.
The northern and southern hemispheres of Mars have different characteristics. The southern hemisphere has many impact craters and has a generally much higher elevation than the northern hemisphere. The southern highlands are probably the oldest terrain on Mars. The northern hemisphere of Mars contains a much wider variety of geologic features, including large volcanoes, a great rift valley, and a variety of channels. The northern hemisphere also contains large expanses of relatively featureless plains.
Mars has the largest volcano in the solar system, Olympus Mons. It is 16 miles high, almost twice as high as Mount Everest, and covers an area comparable to the state of Arizona. Near it, three other volcanoes almost as large-Arsia Mons, Pavonis Mons, and Ascraeus Mons-form a line running from southwest to northeast. These four volcanoes are the most noticeable features of a large bulge in the surface of Mars, called Tharsis. Another volcano, Alba Patera, is also part of the Tharsis bulge, but is quite different in appearance. It is probably less than 4 miles high, but has a diameter of 1000 miles. None of Mars's volcanoes appear to be active.
The Tharsis bulge has had a profound effect on the appearance of the surface of Mars. The Tharsis bulge includes many smaller volcanoes and stress fractures, in addition to the large volcanoes. Its presence affects the weather on Mars and may have changed the climate by changing the rotation of the planet. Valles Marineris (named for the U.S. Mariner spacecraft that discovered it) is the most notable stress feature associated with the Tharsis bulge. It is a great rift valley extending from the Tharsis region away to the east-southeast. It is about the same length as the distance from New York to California. This canyon system reaches widths of 440 miles and depths of 4 miles.
Hellas Planitia is a giant impact basin in the southern hemisphere. The impact of a large meteorite formed the basin long ago. With a diameter of about 1250 miles, it is the largest such basin on Mars.
Three types of channels on Mars were probably formed by the action of water. These channels are unrelated to the "canals" thought to be seen in early telescopic views of Mars. Channel networks are similar in appearance to streambeds on Earth and occur in the southern highlands. These channels may date from a time early in Mars's history when the atmosphere was thicker and liquid water could flow on the surface. Outflow channels, which giant floods may have formed, occur on the boundary between the southern highlands and the northern plains regions. Ares Vallis, where the Mars Pathfinder spacecraft landed, is one of these outflow channels. Landslides and other erosion probably formed fretted channels by enlarging preexisting channels. The Mars Pathfinder spacecraft found minerals in Ares Vallis that are similar to minerals that form near water on Earth, supporting the theory that Mars had liquid water at some point in its history. Mars has small, permanent ice caps at its north and south poles. The caps increase in size in the winter of each hemisphere. The caps in the north and south are quite different from one another. The northern permanent cap is composed of water ice and is about 620 miles across. A seasonal cap of frozen carbon dioxide adds to the northern ice cap in the northern winter. The southern permanent cap is one-third the diameter of the northern cap because summer in the southern hemisphere is warmer than in the north. The southern seasonal cap is larger than the northern cap-more carbon dioxide is frozen out in the south than the north because Mars is farthest from the Sun, and therefore coldest, in the southern winter. Carbon dioxide may also make up the southern permanent cap. Regions of striped-looking terrain, probably formed of layers of dust and ice, occur at the edges of both polar caps. Climate cycles almost like the ice ages on Earth may have caused this layering.
The atmosphere of Mars is 95 percent carbon dioxide, nearly 3 percent nitrogen, and nearly 2 percent argon with tiny amounts of oxygen, carbon monoxide, water vapor, and other gases. Earth's atmosphere is mostly nitrogen and oxygen, with only 0.03 percent carbon dioxide. The pressure of Mars's atmosphere varies with the season, ranging from 6 to 10 millibars. The variation in pressure is caused by carbon dioxide freezing out at the poles of the planet in fall and winter. The pressure also varies with altitude and is about a factor of ten less on the top of Olympus Mons than on the floor of Hellas Planitia.
The atmosphere of Mars contains very little water vapor. The level of water vapor averages about 0.016 percent, compared to Earth's average level of about 2 percent. The water content of the atmosphere on Mars varies seasonally and by location. This water can form clouds and even frost. Six major types of clouds form in Mars's atmosphere. The polar hood is a haze of water and perhaps carbon dioxide ice that forms over the polar regions in the fall and can cover much of the northern plains. Wave clouds form on the sheltered side of large obstacles, such as craters, and have very distinct ridges. Convective clouds form in high areas at midday. Orographic clouds form when air lifts over large-scale objects like Olympus Mons, and are most common in spring and summer when the water vapor content of the air is highest. Ground hazes occur in low areas at dawn and dusk and probably consist of water ice. Wispy high-altitude clouds sometimes occur just at dawn and dusk. The Viking 2 lander recorded images of water-ice frost during the winter.
Space probes have provided the most detailed information about Mars. Exploration of Mars began in 1960, when the former USSR launched its first probe to Mars. The United States National Aeronautics and Space Administration (NASA) launched the Mariner and Viking programs in the 1960s and 1970s. The programs returned vast amounts of data about the chemical and physical characteristics of Mars and a large number of photographs of its surface.
The Soviet program to explore Mars suffered many setbacks. The USSR sent 12 probes to Mars before their first partial successes with missions Mars 4, 5, and 6 in 1973. The Soviets then waited 15 years to again explore Mars, until the Phobos missions in 1988. The Phobos probes primarily studied Mars's moon Phobos. Phobos 1 was lost on its way to Mars, but Phobos 2 sent back information on the composition of both Phobos and Mars for a limited time.
Russia has continued scientific study of Mars after the dissolution of the Soviet Union, though on a more modest scale than the Soviet space program. In November 1996 the Russian spacecraft Mars 96 suffered an unsuccessful launch and crashed back to Earth.
The U.S. exploration of Mars began with the Mariner program. The spacecraft Mariner 4, launched in 1963, took the first close-up pictures of Mars. These pictures had a powerful impact because they only showed impact craters on Mars like those on Earth's Moon. They did not reveal any evidence of the advanced civilizations that people in the 19th century imagined might exist on Mars. Mariners 6 and 7 took much more detailed pictures of the Martian surface and took measurements of the force of gravity and the atmosphere.
Mariner 9, launched in 1971, revealed the volcanoes, canyons, and channels of Mars, showing that the planet was much more like Earth than like the Moon. NASA launched another series of probes to Mars, Viking 1 and 2, in 1975. These spacecraft provided scientists with most of the current data about Mars. The Viking probes included orbiters, which mapped Mars and made global studies of its geology and chemistry, and landers, which measured the composition of the surface and searched for life. These landers were the first probes to land successfully on Mars. Mars Observer, launched in 1992, was lost just before it reached Mars. Its near-twin, Mars Global Surveyor 96, was launched in 1996 and went into orbit around the planet in 1997. It carries instruments to measure the composition and topography of the surface and to monitor the weather conditions in much more detail than scientists can from Earth. Mars Global Surveyor 96, which has a high elliptical orbit that brings the spacecraft within 75 miles of the planet, has provided scientists with detailed images of Mars. Some of these images reveal what appear to be erosion patterns on the planet's surface, strengthening the belief that Mars once had flowing water. These images show meandering valleys and channels that appear to have been cut by flowing water, and smoothed surface rocks and crater rims that have been worn or washed away. Astronomers believe that if Mars had surface water, its source may have been watery comets that bombarded the young planets of the solar system as it was forming.
Mars Global Surveyor 96 has also captured images of windblown dunes, massive plates of solid lava, and an extensive canyon system on Mars. The vast areas of windblown dunes formed from dust in the Martian landscape. Some of these dunes are white, while dunes in other areas are darker. Scientists believe the brighter sand dunes consist of sulfates, possibly gypsum, and that the darker dunes are composed of fine grains of lava. These dunes are swept along the now-dry surface of Mars by seasonal, high-velocity winds blowing through the thin atmosphere. Scientists believe this dust may have been formed by the mechanical weathering action of rainfall on rocks and soil during the planet's early history. This belief lends additional support to the theory that Mars once had surface water and possibly had a full hydrologic cycle.
The plates of solid lava on Mars are located in the planet's northern lowlands in a geologic formation known as the Elysium Basin. Scientists believe these huge plates were formed by ancient volcanic activity that spewed a thick lava crust, which slowly cracked as it hardened. When more hot lava erupted under this crust and began to flow, the overlaying crust of solid lava broke apart, and the massive pieces were transported and deposited inside the basin.
The Martian canyon system called Valles Marineris is the largest known canyon in the solar system. Global Surveyor 96 images show horizontal layers of rock that extend up to 8 km (5 mi) into the planet's crust. These horizontal layers could be sedimentary rock (rock formed by the accumulation of mineral matter deposited by the action of water, wind, or glacial ice). However, scientists believe these horizontal rock layers are ancient lava flows because of the coloring and thickness of these layers. If these are lava deposits, scientists say that volcanic activity on Mars was much more extensive than previously believed. In addition, if sufficient carbonates exist in Mars's crust, scientists theorize that such extensive volcanic activity could have ejected enough carbon dioxide into the atmosphere to sustain a wet, warm climate on early Mars.
Mars Pathfinder, also launched in 1996, consisted of a lander and a small rover. The lander studied weather conditions on Mars. The rover, called Sojourner, explored away from the lander and studied surface materials near the landing site. The Pathfinder spacecraft arrived on Mars on July 4, 1997, and operated until October 7, 1997. The lander sent back many photographs of its landing site, and Sojourner visited and analyzed 15 nearby rocks.
NASA launched two spacecraft to Mars under the Mars Surveyor 98 program. The first spacecraft, Mars Climate Orbiter, was launched in December 1998 and reached Mars in September 1999, but NASA lost contact with the spacecraft, apparently due to navigational errors. The orbiter contained a camera as well as other instruments designed to gather information about the Martian atmosphere and weather patterns. Specifically, the orbiter was intended to monitor the movement of atmospheric dust and water vapor and to track seasonal variations in these constituents.
The second spacecraft, Mars Polar Lander, was launched on January 1999, and was scheduled to reach Mars in December 1999. It will land on Mars's south pole during the southern hemisphere's springtime, after the ice cap has retreated. Before landing, two probes attached to Mars Polar Lander will separate and land within about 60 miles of the main spacecraft. Once on the ground, Mars Polar Lander will use a robotic arm to probe the surface to image the layers of soil and rock and to find out if Martian soil contains any water. The two separate probes will gather similar data from their locations.
INTERMARSNET is a program being planned by NASA and the Euopean Space Agency (ESA) that will consist of a network of landers to study the surface and interior, complemented by an orbiter. Long-term plans for the exploration of Mars include a mission that will return samples from the planet and eventually a piloted mission.
In July 1998 a Japanese spacecraft called Nozomi was launched to Mars. Nozomi carries a Canadian atmospheric probe called the Canadian Thermal Plasma Analyzer and is scheduled to reach Mars in 2003. Once Nozomi reaches Mars, a boom will unfold from the probe and a sensor will measure atmospheric particles and gases. With this data, scientists hope to learn more about the composition and origin of the Martian atmosphere. Nozomi is the first Japanese spacecraft to go to Mars and carries the first Canadian probe to travel to another planet.
Humans have looked for signs of life on Mars for over a hundred years. Italian astronomer Giovonni Schiaparelli and others in the middle and late 19th century believed they saw seasonal color changes that indicated a wave of greening in spring. Some people also believed that they saw canali (Italian for "channels"), or straight lines crisscrossing Mars's surface. Scientists now know that windblown dust causes the color changes and that the channels are an optical illusion.
Astronomers of the early 20th century saw what they believed were more and more canals as interest in them grew. American astronomer Percivil lowell was the most vocal proponent of these canals. He believed they indicated the existence of an advanced civilization on Mars and wrote several books on the subject in the early 20th century. He proposed that the canals carried water from the wet polar regions to the dry equatorial deserts. As telescopes improved, however, astronomers found it more and more difficult to see Lowell's canals. Images of Mars's surface from the Mariner spacecraft finally proved that the canals did not exist.
Lowell's beliefs about civilization on Mars have had a powerful effect on human perception of the planet. British author H. G. Wells's The War of the Worlds (1898) and American author Edgar Rice Burroughs's series of Mars books, starting with A Princess of Mars (1912), clearly show the influence of Lowell's ideas.
Some people believe in the existence of a civilization on Mars even today. The so-called Face on Mars is thought by some people to be evidence of intelligent life on Mars. This feature, shown in Viking images of the Cydonia region of Mars, looks vaguely like a humanoid face. Geologists' views of these features are quite different. The Cydonia region lies near the boundary of the southern highlands and the northern plains and contains a large number of isolated hills, the eroded remnants of the edge of the highland terrain. These hills resemble the mesas of the U.S. Southwest. Their resemblance to human faces or pyramids is due to the low resolution and dark areas of missing data in the Viking images and to tricks of light and shadow. The camera on Mars Global Surveyor 96 took pictures of the region in 1998. These images revealed that the "face" was just a hill.
One part of the Viking landers' mission was to search for evidence of life. One instrument was designed to detect organic molecules in the soil. It found no evidence of organic molecules. Three biological experiments tested soil samples for evidence of metabolism, growth, or photosynthesis. None of these experiments showed substantial evidence for the presence of life.
Most scientists today believe that there is no life on Mars. Conditions there are extremely hostile to life as we know it. Because of the thin atmosphere, ultraviolet radiation that can destroy living matter reaches the surface. It is so cold that liquid water cannot exist on the surface during the night. If life ever had established a foothold on Mars, it would likely have affected the environment there in some way, but no evidence of this exists. However, it is not impossible that some form of life could exist on Mars today. Inside pores and cracks in rocks, where there is protection from the extreme conditions of the surface, and where liquid water can exist even at very low temperatures, it is theoretically possible that living organisms could survive.
In August 1996 NASA scientists announced that a meteorite consisting of Martian rock contained possible fossil evidence of bacteria-like life. Further study of the chemical components of the rock offer little support for this theory, but the possibility that the rock contains fossils has not been eliminated. If confirmed, this discovery would be the first evidence for life outside Earth. It would increase the likelihood of life elsewhere in the universe, because it would mean that the conditions for the development of life are not specific to Earth.
By Jordan Wood