The Gas Chromatograph Mass Spectrometer (GCMS) experiment was carried aboard each of the Viking spacecraft landers that reached the red planet in 1976. It was intended to detect evidence of bological material in the Martian soil. It took a small soil sample, separated volatile elements using a gas chromatograph, and analyzed their composition with a mass spectrometer. It provided a way of resolving any ambiguities that might arise from the three biology experiments - the gas exchange (GEX) experiment, the labeled release experiment, and the pyrolitic release experiment.
While the GCMS found no trace of biological processes on the surface of Mars, scientist Gilbert Levin believes that the GCMS instrument sent to Mars could easily have missed biologically significant amounts of organic matter in the soil, as it had in a number of tests on Earth.
This Viking Gas Chronometer Mass Spectrometer is identical to the instruments on Mars that were used to measure the composition of the atmosphere and search for organic compounds in the Martian soil.
To determine the composition of the atmosphere, and whether organic molecules exist in the surface samples, a Gas Chromatograph Mass Spectrometer measured the atomic composition of gas samples from heated soil and the atmosphere. No organic molecules were detected at either landing site, but the abundance of the gas argon (36AR) was found to be less than that expected. This difference suggests that during the early evolution of the planet, the amount of gas released to the atmosphere of Mars was less than that released to the atmosphere of the early Earth.
The composition of heavier elements, those that are common in rocks and soils, is determined by means of an X-ray Fluorescence Spectrometer. Samples of Mars are dropped into the instrument in the Lander body, where they are exposed to high-energy x-rays. Each element then produces its own characteristic x-rays, which are counted and recorded. Following the analysis, the sample is dropped out of the bottom of the test container so a new sample can be measured.
Analysis of the surface soil indicates that both landing sites are similar in composition and primarily composed of the elements silicon and iron. The sulfur content is unexpectedly high, and might be caused by water-soluble minerals deposited on the surface. Lander soils could be derived from weathering of an iron-rich igneous rock, such as basalt. Consistent with these analyses, magnetic particles were photographed clinging to the magnets on the sampling arm and on the camera reference chart.