An uncrewed U.S. exploratory mission to Mars.

Prior to the establishment of the Viking Program, four Mariner missions had been sent to Mars. The first, Mariner 4, was launched in 1964. It returned twenty-one images as it flew to within 10,000 kilometers of Mars. Five years later, Mariner 6 and Mariner 7 flew past Mars and sent back 201 pictures. Mariner 9 reached Mars in 1971. Because it was an orbiter rather than a flyby, Mariner 9 returned 7,300 images during its one-year lifetime. This mission discovered the volcanoes and Valles Marineris, which were explored in more detail by the Viking Program. In addition to the American Mariner probes, the Soviet Union launched four largely unsuccessful probes to Mars in 1973, including one that attempted to land on the surface.

Although plans to conduct uncrewed missions to Mars were initiated shortly after the National Aeronautics and Space Administration (NASA) was established in 1958, the Viking Program was not approved until 1968. Its primary scientific goal was to search for evidence of life on Mars. Other goals were to land on the surface of Mars and return scientific data about the planet. After the project’s original launch date was delayed from 1973, the Vikings were finally launched in 1975 from Cape Canaveral, Florida, atop Titan III launch vehicles. They spent one year traveling to Mars and arrived in the summer of 1976.

Each Viking craft consisted of both an orbiter and a lander. Both Viking orbiters orbited the planet for a few years before being powered down. After 706 orbits, the Viking 2 orbiter was turned off first on July 25, 1978. The Viking 1 orbiter completed more than 1,400 orbits before being turned off on August 17, 1980. The two orbiters had different orbital inclinations, so that Viking 1 could study the lower latitudes, while Viking 2 could study the polar regions. Both orbiters returned what at the time were the highest-resolution images available of the entire Martian surface. They also made various atmospheric measurements. Each of the orbiters also flew near one of the two moons of Mars, returning close-up images.

Shortly after arrival at Mars, each lander separated from its orbiter to land on the surface of Mars. Viking 1 landed on the Chryse Planitia, or plain, at 22 degrees north latitude and 47 degrees west longitude on July 20, 1976. Viking 2 landed on the Utopia Planitia at 48 degrees north latitude and 226 degrees west longitude on September 3, 1976. Plains or relatively flat areas were chosen for both landing sites in order to minimize the risk of a crash landing. Despite this strategy, Viking 1 missed a large boulder by only 7.5 meters (25 feet). These two Viking landers were the first craft to successfully land on the surface of Mars. The Viking 2 lander continued to return data until the end of its mission on April 11, 1980. The Viking 1 lander continued to return data until the end of its mission on November 13, 1982. The data included detailed images of the surface, analysis of the soil and atmosphere, and a negative search for evidence of life.

Each Viking orbiter contained two cameras used to obtain detailed images of the entire Martian surface. The two Viking orbiters took a combined total of more than 46,000 photographs of the Martian surface. The average resolution was from 150 to 300 meters. In March, 1977, the Viking 1 orbit was adjusted so that its closest approach to Mars was 300 kilometers. Viking 2 followed on October 23, 1977. At this distance, the Viking orbiters could resolve surface features as small as 20 meters, although they did not map the entire surface with this resolution.

After performing their initial task of locating safe landing sites for the landers, the orbiter cameras revealed details of both previously known and previously undiscovered features, including global dust storms; the Olympus Mons and Tharsis Ridge volcanoes, the largest known volcanic mountains in the solar system; the Valles Marineris, a canyon that would stretch across the entire United States; and a number of arroyos, or dry river beds, and other features indicating the past presence of large amounts of liquid water. The orbiters also contained infrared spectrometers and radiometers to map Mars’s atmospheric water vapor content and thermal properties. The Viking 1 orbiter took pictures of Phobos, the larger of Mars’s two moons, from a closest approach of 90 kilometers. The Viking 2 orbiter photographed Deimos, the smaller of Mars’s two moons, from a closest approach of 22 kilometers.

During the landers’ descents, instruments on the landers performed analysis of the Martian atmospheric properties at various levels to determine the atmospheric structure. These properties included composition, pressure, and temperature. Scientists learned that the atmosphere of Mars is primarily (95 percent) carbon dioxide. At about 0.5 percent of Earth’s surface atmospheric pressure, the surface pressure of Mars is too low for water to exist in a liquid state. Coupled with the evidence of large amounts of liquid water in the past, this information provided evidence for major global climate changes on Mars. After landing, these atmospheric instruments continued to provide data on the Martian weather and climate.

Each Viking lander contained two cameras to provide direct close-up images of the Martian surface. Over the course of the Viking mission, a total of more than 1,400 images were sent back from the Martian surface. They revealed a boulder-strewn reddish surface, which gives Mars the red color that inspired the ancients to name the planet for their god of war. Because Mars’s atmosphere is too thin to scatter blue light the way Earth’s atmosphere does and because the airborne dust particles are red in color, the images also show a pink sky. The images also show early morning surface frost and structures like sand dunes that result from the global dust storms.

To analyze the composition of this red surface, each lander was equipped with a scoop that could dig a few centimeters into the surface and place a surface sample into an X-ray fluorescence spectrometer. Analysis of these samples showed that the Martian surface contains a fairly large percentage of both iron and oxygen. The red color is iron oxide or rust.

Each lander also contained a seismometer to measure earthquake, or marsquake, activity. The Viking 1 seismometer failed, the only instrument to fail in the entire Viking mission. The other seismometer revealed that the few quakes that occurred were very weak.

Scientists integrated the results of these various experiments to deduce the geological and atmospheric history of Mars. They determined that whereas Mars had been geologically active early in its history, that geologic activity had ceased. Although the planet’s volcanoes were no longer active, they had at one time released carbon dioxide, water vapor, and other gases. During this time, the thicker atmosphere had allowed water to exist in liquid form. As the atmosphere gradually escaped into space, water could no longer exist as a liquid and is now present as ice and vapor in thin clouds. Carbon dioxide is still present in the atmosphere. Solar ultraviolet rays split much of the water into hydrogen and oxygen. The lighter hydrogen escaped into space. The oxygen combined with the iron on the surface to produce the rust and the red color.

Perhaps the most important experiments performed by the Viking landers were the three experiments designed to search for signs of life on Mars. Although cameras had already observed no obvious signs of large life forms, these experiments—the gas-exchange, labeled-release, and pyrolytic-release experiments—were designed to look for evidence of microscopic life.

The gas-exchange experiment involved placing a soil sample into an aqueous nutrient solution that was dubbed “chicken soup” by the experimenters. Any primitive life forms using the nutrients should release gas, which could be detected by looking for changes in the atmospheric composition of the test chamber. This experiment found no evidence of biological activity.

The labeled-release experiment used carbon-14, a radioactive isotope of carbon, in the nutrient solution. Primitive microscopic organisms absorbing the nutrient would eventually release gas containing the carbon-14, which could be detected by its radioactivity. The pyrolitic-release experiment also used carbon-14, but it used it in the atmosphere rather than in the nutrients. After an incubation period, the sample was analyzed to see if it had absorbed any of the carbon-14. The results of these experiments were somewhat ambiguous, but could be explained by chemical rather than biological reactions. Hence, the Viking mission, which was the first specific attempt to find evidence of life on another planet, found no solid evidence of life on Mars.

Prior to the inception of the U.S. space program, many people had believed that Mars might contain life. This idea resulted largely, but not entirely, from the efforts of Percival Lowell who, in the early twentieth century, had popularized the idea that Mars contained canals. Lowell made detailed maps of these alleged canals, asserting that they had been built by a race of Martians to transport water from the pole caps to the warmer equatorial regions on their dry world. Although most astronomers of the time disputed Lowell’s work, the idea of life on Mars had been firmly planted in the public imagination. In addition, other scientific studies of Mars had shown that although conditions there might be harsh, life on the planet was at least a possibility. After the Viking Program found no evidence of life on Mars, most scientists assumed that Mars was lifeless. However, the question was reopened two decades later by the announcement of possible evidence for primitive fossilized bacteria in a Martian meteorite that had been discovered in Antarctica.

After the Viking Program ended, there was relatively little exploration of Mars until the 1990’s, when exploration of Mars resumed. The next successful landing on Mars was that of the Pathfinder, which landed in July, 1996, twenty years after the Vikings had landed. For two decades, the Viking Program had provided humankind’s most detailed knowledge of the planet Mars.

• Hartmann, William K. Moons and Planets. 3d ed. Belmont, Calif.: Wadsworth, 1993. Written from a comparative planetology perspective, this book contains information on Mars integrated throughout the text and organized by specific topics, such as atmospheres and interiors. Written between the Viking and more recent Mars missions, the information on Mars is primarily that of the Viking mission.
• Moore, Patrick, and Garry Hunt. Atlas of the Solar System. Chicago: Rand McNally, 1983. The chapter on Mars contains a brief history of Martian observations prior to Viking, a summary of the Viking mission, and a summary of the mission’s results. The book also contains a large number of photographs and maps of the Martian surface.
• Morrison, David, and Tobias Owen. The Planetary System. Reading, Mass.: Addison-Wesley, 1988. Chapters 9 and 10, on Mars, concentrate on the scientific knowledge gained from the Viking mission.
• Raeburn, Paul. Mars: Uncovering the Secrets of the Red Planet. Washington, D.C.: National Geographic Society, 1998. Filled with high-quality photographs, this book tells the story of Mars exploration. Chapter 3 concentrates on the Viking mission.

National Aeronautics and Space Administration

Spaceflight

Uncrewed spaceflight