The Viking missions to Mars were part of a series of U.S. efforts to explore and better understand the red planet. Each Viking spacecraft consisted of an orbiter and lander. The landers were sterilized before launch to prevent contamination of Mars with organisms from Earth. Their objectives were to obtain highly detailed pictures of the Martian surface, learn about its composition, and search for life.
Viking 1 was launched on Aug. 20, 1975, followed later that year on Sep. 9 by Viking 2. The spacecraft deployed their landers using parachutes and retro-thrusters which prevented a dangerous high-speed impact. Viking 1 landed on the western slope of Chryse Planitia (the Plains of Gold) at 22.3 degrees N latitude, 48.0 degrees longitude. Viking 2 landed on Utopia Planitia; 47.6 degrees N latitude, 225.7 degrees W. longitude. Over the next four years, the orbiters mapped 97% of the Martian surface and relayed information from the landers back to Earth. They also collected data which suggests that moons Phobos and Demios may have originally been asteroids captured by Mars' gravity.
The Viking landers monitored Martian weather, including regular dust storms and temperature changes, and returned high-resolution color photos which revealed interesting geologic features such as volcanoes and canyons. Their analysis of Mars' soil found no evidence of life, instead showing that organic molecules are even more scarce there than on our Moon. The Viking missions did discover water in both solid and vapor form, suggesting that life may have existed on the planet many years ago.
Some of the data returned from these two spacecraft are shown in the image archive below. With this data, scientists began to make educated guesses about what the interior, surface history, and evolution of Mars must be.