On July 20, 1976, NASA's Viking 1 made history when it became the first American spacecraft to successfully touch down on the surface of Mars and return images and scientific data for six years.
Two identical spacecraft, Viking 1 and Viking 2, each consisting of a lander and an orbiter, were launched a year earlier in 1975 from Cape Canaveral, Fla. The Viking 1 lander touched down on the western slope of Chryse Planitia (the Plains of Gold) on July 20, while the Viking 2 lander settled down at Utopia Planitia on Sept. 3.
The orbiters imaged the entire surface of Mars at a resolution of 150 to 300 meters and selected areas at 8 meters. The Viking landers transmitted images of the surface, took surface samples and analyzed them for composition and signs of life. They also studied the Martian meteorology and atmosphere and deployed seismometers.
Viking orbiter images revealed volcanoes, lava plains, immense canyons, cratered areas, wind-formed features, and evidence of past surface water. The planet appeared to be divisible into two main regions, the northern low plains and southern cratered highlands. Superimposed on these regions are the Tharsis and Elysium bulges, which are high-standing volcanic areas, and Valles Marineris, a system of giant canyons near the equator.
The landers' biology experiments were designed to look for possible signs of life. They discovered unexpected and mysterious chemical activity in Martian soil, but they provided no clear evidence for the presence of living microorganisms in soil near the landing sites. Scientists believe the combination of solar ultraviolet radiation that saturates the surface, the extreme dryness of the soil and the oxidizing nature of the soil chemistry prevent the formation of living organisms in Martian soil.
The last data from Viking Lander 2 arrived at Earth on April 11, 1980. Viking Lander 1 made its final transmission on Nov. 11, 1982.