Mars, the fourth planet in order of increasing distance from the sun and the first beyond the earth’s orbit. Under favorable conditions, it appears in the night sky as a yellowish red object (hence the name “red planet”) of the first magnitude. Mars has long fascinated us because of its many similarities to the earth and because of the possibility that life might exist there. The flyby of the crewless spacecraft Mariner 4 past the planet in 1965 started an era of intense exploration that still continues. Following several crewless flybys and orbiters launched by the United States (Mariners 4, 6, 7, and 9) and by the Soviet Union (Mars 3, 4, 5, and 6), the first successful soft landing of a spacecraft on another planet was achieved on July 20, 1976, when the U.S. spacecraft Viking 1 landed on the surface of Mars. Since then, Mars has been visited by several unpiloted craft, including the Mars Pathfinder spacecraft in 1997 and the Mars Global Surveyor from 1997 to 2006. (Squyres, 12) When images from these two probes were compared, scientists began to suspect that water had once flowed at several locations. Since 2004, NASA’s (National Aeronautics and Space Administration’s) Mars Exploration Rovers twin robot-geologists Spirit and Opportunity have explored the harsh Martian environment in search of water. The Phoenix Mars Lander, which safely reached the planet’s the North Pole in 2008, will analyze the icy soil for evidence of past microbial life. Mars is now perceived as a planet of spectacularly diverse topography with huge volcanoes, deep canyons, dry riverbeds, and extensive sand seas. While evidence of life there continues to be elusive, Mars remains interesting for geologic, chemical, and meteorological comparisons with the earth (Paolo, p.89).
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Following the first telescopic observations of Mars by Galileo in 1610, the planet has been observed continually, with changes in its appearance noted and mapped. Mars is too distant for any surface relief to be discerned through the telescope. All that is seen are bright and dark markings, which may be in the atmosphere or on the surface. Most surface markings are in the equatorial regions, where various dark features contrast with the light areas or “deserts.” The shape and size of most markings change both seasonally and, slowly, over many years. Despite the many changes, the most prominent features are recognizable even on the earliest maps. The markings show poor correlation with topographic features revealed by spacecraft observations. Most are thought to result from thin deposits of windblown debris whose distribution changes with time. Bright polar caps are clearly visible through the telescope, and the larger size of the northern polar cap long has been recognized.
Most of the changes in appearance through the telescope are due to atmospheric effects of various kinds. Large arrays of white clouds commonly form in the middle latitudes and may persist for weeks. Those around the volcanic centers of Tharsis and Elysium most likely form when the air cools as it rises over the high volcanic regions. Other white clouds probably are caused by the daily recycling of water between the soil and the lower atmosphere. Frontal clouds and standing-wave clouds, seen clearly in spacecraft pictures, are probably not visible from the earth. During the fall thick clouds gather in the high latitudes to form polar hoods, which mask the growth of the polar caps. Brightening at the poles in this season is probably the result of both these clouds and the cap itself. Brightening in low areas such as Hellas and Argyre may also result from a combination of surface frost and clouds.
Whereas white clouds generally are brightest when observed in blue light, yellow clouds are brightest in yellow and orange. Yellow clouds occur mostly in the mid-southern latitudes at midsummer when large lateral and vertical temperature gradients cause extreme turbulence, which lifts large amounts of dust into the atmosphere. Activity generally starts in the region 320° W to 30° W and 30° S to 60° S and in most years spreads widely, so that ultimately the whole planet is engulfed in a gigantic dust storm. After the midsummer turbulence declines, dust slowly settles out of the atmosphere and the surface markings reappear. Global dust storms of this type were observed close up in 1971 by the Mariner 9 flyby space probe and in 1977 by the Viking orbiters (Squyres, p. 32).
No other aspect of Mars has aroused such widespread interest and controversy as the so-called canals. The controversy started in 1877 with the Italian astronomer Giovanni Schiaparelli’s publication of a map of Mars that showed many dark lines on the surface. These he called Canali, the Italian word for both “canal” and “channel.” In the ensuing decades, Mars observers were divided between those who claimed the canals existed and those who claimed they did not. The strongest proponent of the canals was the American astronomer Percival Lowell, who produced ever more intricate maps of linear markings based on observations at the observatory he founded in Flagstaff, Ariz. In a book published in 1908, he aroused considerable popular attention by suggesting that the markings were irrigation canals built by an advanced civilization. As better telescopes were built and instrumental measurements failed to confirm their existence, belief in the canals declined (Furniss, p. 68). The various space probes that have since visited Mars found no evidence for most of the lines on the early maps, with the result that most are now regarded as optical illusions.
Most current knowledge of Mars is derived from space-probe observations, initially from the Mariner 9 and Viking missions. In 1965 the U.S. Mariner 4 flyby space probe returned the first close-up pictures of the planet, followed in 1969 by two additional flyby missions, Mariners 6 and 7. All three probes flew over the parts of the planet that most resemble the moon and presented a rather deceptive view of the planet as a moonlike body. The diverse geologic character of the Martian surface was not fully recognized until 1971. During that year Mariner 9 and two Soviet spacecraft, Mars 2 and 3, were placed in orbit around Mars. The Soviet spacecraft was short-lived, and their accompanying Landers failed to return useful data from the surface, but Mariner 9 continued to operate for a year, returning more than 7,000 pictures of the planet. (Paolo, 45) Additional photographs were obtained in 1974 by the Soviet vehicles Mars 4, 5, and 6. In 1976 two Viking spacecraft were placed in orbit around Mars, and two additional spacecraft landed on the surface. The Viking 2 and 1 orbiters continued to function, respectively, until 1978 and 1980, by which time they had taken over 50,000 pictures of the planet and returned a wealth of other data. Contact with the Viking 2 and 1 Landers was lost, respectively, in 1980 and 1982 (Squyres, p. 57).
After a 17-year hiatus in Mars exploration, the United States launched Mars Observer in 1992. Mission objectives were to study geology, geophysics, and climate of the red planet, but it ended in disappointment when contact was lost with the craft just before it entered Martian orbit. In 1996 Mars Pathfinder was launched to demonstrate that an unpiloted spacecraft could deliver and deploy a robotic rover. Not only was the mission a success, but also Pathfinder and its rover, Sojourner, returned unprecedented amounts of information including images, soil analyses, and wind measurements before their final data transmission in September 1997. The next two missions to Mars failed: Climate Orbiter burned up on entering Mars’s atmosphere in September 1999; and three months later Polar Lander and Deep Space 2 were lost on arrival.
These disappointments were followed by a spate of successes, beginning in 1997 when Mars Global Surveyor slipped into Martian orbit. For nine years the probe mapped the red planet returning dramatic evidence of hillside water flows before succumbing to battery failure in 2006. The Mars Odyssey spacecraft, launched in 2001, has captured more than 130,000 images and continues to transmit information about Martian geology, climate, and mineralogy. NASA joined with the European Space Agency and the Italian Space Agency for the Mars Express mission in 2003 (Paolo, p. 23). Despite losing Beagle 2, its land rover, Mars Express has provided information about various surface features, including buried impact craters, evidence of glacial activity, and the presence of methane gas. The pursuit of geological clues to the possibility of life on Mars continued with NASA’s land rovers Spirit and Opportunity, twin robotic vehicles that rolled off their airbag-encased Landers on opposite sides of Mars in 2004 (Furniss, p. 102). Sporting names selected from more than 10,000 entries in a student essay contest, the two solar-powered rovers have outlived their intended three-month mission and continue to transmit high-resolution, full-color images of Martian terrain, soil surfaces, and rocks. The Mars Reconnaissance Orbiter, launched in 2005, currently orbits high above the red planet, using a sounding device to search for subsurface water.
In May 2008 the Mars Reconnaissance Orbiter relayed photographs of the safe descent of the Phoenix Mars Lander as it parachuted onto the planet’s frozen North Pole. Daily instructions were sent from the earthbound control center, directing the Lander to collect soil samples from the icy surface. Phoenix used its robotic arm to deliver soil and ice samples to its onboard experiment platforms. The samples are to be analyzed in hopes of determining whether the location could have supported microbial life during the planet’s past.
Mars is markedly asymmetrical in the distribution of its surface features. Much of the Southern Hemisphere is heavily cratered like the lunar highlands and includes two large impact basins, Hellas and Argyre. In contrast, much of the Northern Hemisphere is covered with sparsely cratered plains. The planet has two major volcanic regions, the Tharsis region centered at 110° W on the equator and the Elysium region centered at 25° N, 212° W. Extending eastward from Tharsis are several large canyons that together makeup Valles Marineris, while east and north of the canyons are several huge dry riverbeds. The poles are distinctly different from the rest of the planet and appear to have thick deposits of layered sedimentary rocks exposed at the surface. The North Pole is also surrounded by extensive sand dunes.
Densely Cratered Terrain
This terrain is characterized by many large, relatively shallow craters; smooth intercrater plains; and a relatively small number of smaller craters (those less than 30 miles, or 50 km, in diameter). The terrain probably dates from very early in the planet’s history, possibly from 4 billion years ago, when the impact rate was higher than at present. The most extensive cratered areas are in the Southern Hemisphere. Fresh Martian craters differ markedly in appearance from those on the moon and Mercury. Most lunar craters are surrounded by disordered rubble-like ejecta that appears to have been deposited from ballistic trajectories. In contrast, the Martian craters are surrounded by ejecta that appears to have flowed along the ground. The fluid properties of the ejecta have been attributed to the presence in the Martian surface of large amounts of ground ice, which melts on impact and is incorporated into the ejecta. Crater examination by the Opportunity probe has revealed evidence of a watery and possibly habitable past on Mars.
Sparsely Cratered Plains
Plains cover much of the Northern Hemisphere and also occur within large impact basins such as Hellas and Argyre in the south. They are distinguished from the densely cratered terrain by the almost total absence of impact craters larger than 30 miles (50 km) in diameter. The plains have a different appearance in different areas. Around the large volcanoes in Tharsis and Elysium, the plains appear to be a thick succession of lava flows. In other areas, such as Chryse Planitia, where the Viking 1 spacecraft landed, the plains resemble those of the lunar maria, being relatively featureless except for impact craters and low winding ridges. These plains are probably also volcanic. The plains in the high northern latitudes have a variety of poorly understood features. Extensive areas have a polygonal fracture pattern, with individual polygons averaging 6 miles (10 km) across. In other areas are parallel linear markings, low escarpments, and intricate patterns of light and dark. Many of the unique characteristics of the northern plains have been attributed to repeated deposition and removal by the wind of layers of ice-rich debris. The number of impact craters on most of the plains, while considerably smaller than on the densely cratered terrain, is still sufficiently large to indicate an old age, probably in the range of 1 to 4 billion years. The only possible exceptions are the plains around the large volcanoes, which appear younger.
Volcanic and Tectonic Features
The large volcanoes of Tharsis are among the most spectacular features of the planet. The largest, Olympus Mons, is 15.5 miles (25 km) high and more than 340 miles (550 km) across at its base. Three other volcanoes in Tharsis reach approximately the same height. Each is topped by a central caldera, or crater, 50 to 75 miles (80 to 120 km) across, and on the flanks are numerous lava channels, lava tubes, flow fronts, and other features indicative of very fluid lava. Analysis of photographs transmitted by Odyssey in 2007 of the massive Arsia Mons volcano reveals seven black spots that scientists suspect are caves the size of football fields. If so, they would shield their contents from surface radiation and could potentially shelter life (Squyres, p. 78).
The style of volcanism on Mars is similar to that in Hawaii, except that the Martian features are ten times larger. The volcanoes are relatively young and may still be active. Tharsis is also the center of a set of fractures that occur over almost an entire hemisphere of the planet. They appear to have formed as a result of the loading of the crust by the Tharsis bulge.
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Volcanoes also occur elsewhere on Mars, but they tend to be older and smaller than those in Tharsis. In 2007 the Spirit rover rolled onto evidence of an ancient volcanic explosion near its landing site dubbed “Home Plate” in Gusev Crater. Analysis revealed high chlorine content in the 2-meter- (6-foot) thick plateau of bedrock, suggesting that fluid basalt lava had come in contact with brine, indicating that water had been involved (Paolo, p. 59).
To the east of Tharsis and aligned along with the radial fractures are several enormous canyons. They stretch from the summit of the Tharsis bulge eastward for approximately 3,000 miles (5,000 km). Individual canyons are up to 125 miles (200 km) across and 1 to 4 miles (2 to 7 km) deep. The walls are steep and in many sections deeply gullied. In some parts, the walls have collapsed to form gigantic landslides several tens of miles across that have traveled more than 60 miles (100 km) along the canyon floor. The canyons are believed to have formed mainly by down faulting, followed by slumping and gullying of the walls (Furniss, p. 62).
Channels pose some of the more puzzling problems of Martian geology. Much of the densely cratered terrain is dissected by small channels that form drainage networks much like terrestrial river valleys. Liquid water, however,
- Furniss, Tim. The History of Space Exploration: And Its Future… Mercury Books London: 2005
- Paolo, Ulivi & Harland, David. Robotic Exploration of the Solar System. Springer Praxis Books: 2008
- Squyres, Steve. Roving Mars: Spirit, Opportunity, and the Exploration of the Red Planet Hyperion; Reprint edition: 2007