The heart of the Viking Program was the two landers, which constituted about 80 to 90 percent of the program. The primary mission of the Viking landers was to determine, if possible, whether there was any life on the surface of Mars. "The lander should include an ensemble of complementing experiments relevant to the possible existence of life on Mars, since no single experiment is either completely definitive or unambiguous.'' Coupled but dissimilar experiments would be one satisfactory approach, such as a mass spectrometer that could detect carbon-containing compounds and a life detector that could search for signs of grossing organisms with a carbon base. Other lander experiments the panel suggested included mass spectrometry for determining atmospheric composition, x-ray fluorescent examination of soil composition, and determination of subsurface water vapor, among others.
Once the lander's mission and its method of landing was settled (a soft landing was chosen), weight and volume constraints began to affect the construction of the lander and its various subsystems. After the Voyager Program with plans for an 11,500 kg spacecraft was abandoned in 1967, a follow-on study concluded that a spacecraft weighing 3,700 kg could he transported to Mars by a Titan-Centaur-class launch vehicle. The lander and its flight capsule would account for more than a third of this weight (1,195 kg). At the start of the mission, the orbiter and lander would be housed in a 4.3-meter shroud atop the Titan-Centaur. The landed spacecraft would be 3 meters at its widest point and 2 meters tall from the footpads to the tip of the large disk S-band high-gain antenna.
Instrumentation included two 360° cylindrical scan cameras mounted near one long side of the base. From the center of this side extended the sampler arm, with a collector head, temperature sensor, and magnet on the end. A meteorology boom, holding temperature, wind direction, and wind velocity sensors extended out and up from the top of one of the lander legs. A seismometer, magnet and camera test targets, and magnifying mirror were mounted opposite the cameras, near the high-gain antenna. An interior environmentally controlled compartment held the biology experiment and the gas chromatograph mass spectrometer. The X-ray fluorescence spectrometer was also mounted within the structure. A pressure sensor was attached under the lander body. The scientific payload had a total mass of approximately 91 kg. The lander was then enclosed into a bioshield and baked for decontamination so as to prevent the contamination of the Martian surface by terrestrial organisms.
The above photograph is a model of the Viking Lander.