Viking probes

Viking probes

(vÿ -king) Two identical American spacecraft, each comprising an Orbiter and a Lander, that were launched in 1975 on a mission to search for life on Mars. Each Orbiter weighed 2325 kg and carried two television cameras to photograph the surfaces of Mars and its satellites, and instruments to map atmospheric water vapor and surface temperature variations; 52 000 pictures were relayed to Earth. After detaching from its Orbiter, each Lander, weighing 1067 kg, made a successful landing using a combination of aerodynamic braking, parachute descent, and retro-assisted touchdown. The Viking 1 Lander set down on July 20 1976 in Chryse Planitia after a delay while Orbiter photographs were searched for a smooth landing area. Viking 2 landed on Sept. 3 1976 in Utopia Planitia, 7420 km northeast of the Viking 1 Lander.

Each Lander carried two television cameras, a meteorological station, a seismometer, and a set of soil-analysis experiments serviced by a sampling arm. Only the seismometer carried by Viking 1 failed to function; that on Viking 2 registered mainly wind vibrations and a few minor Martian ‘earth’ tremors. The television cameras returned views of a red rock-strewn surface under a pink dusty sky. Sand or dust dunes were evident at the Viking 1 site but no life forms were seen. The meteorology instruments reported temperatures varying between 190 K and 240 K with mainly light winds gusting to 50 km per hour.

X-ray analysis of soil samples showed a high proportion of silicon and iron, with smaller amounts of magnesium, aluminum, sulfur, cerium, calcium, and titanium. Gas chromatograph mass spectrometers carried by the Landers failed to detect any biological compounds in the Martian soil, but more ambiguous results were returned by three experiments designed to test for microorganisms in various soil samples.

A labeled-release experiment tested for radioactive gases that might have been expelled from organisms in a soil sample fed with a radioactive nutrient. In a pyrolytic-release experiment soil was placed with gases containing the radioactive isotope carbon–14. The idea was that Martian organisms might assimilate the isotope into their cell structure in a process similar to earthly photosynthesis. Later the soil was baked and a test made for the carbon isotope in gases driven off. Finally, a gas exchange experiment monitored the composition of gases above a soil sample, which might or might not have been fed with a nutrient. If Martian organisms were breathing the gases, a change in composition might be detected.

In practice, the pyrolytic release experiment produced negative results, while the gas exchange experiment gave a marginally positive response, which could be explained in terms of a chemical reaction involving the soil and nutrient. The positive response of the labeled-release experiment was marked but disappeared when the soil was sterilized by preheating.

Detailed analysis of the Viking Lander experiments led to the conclusion that there is no form of life on Mars.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
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