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Viking biological experiments

Each NASA Viking Lander carried three biological experiments to the surface of Mars in the late 1970s. These were the first experiments used specifically to look for biosignatures on another planet.

The three experiments all looked for changes in chemical composition of the atmospheric gasses trapped over a sample of Martian soil as it was exposed to different temperatures, chemical substances, and other conditions.

In general, the results were negative for signs of life, although one of the three experiments gave an initial positive reading which was later interpreted by most scientists as a false positive caused by an abiotic (non-biological) chemical reaction.

Contents

1 Background

2 The Experiments

2.1 Pyrolytic Release
2.2 Labeled Release
2.3 Gas Exchange
2.4 Gas Chromatograph - Mass Spectrometer

3 The Results

4 Scientific Conclusions and Ongoing Debate

5 Related articles

6 See also

Background

It is worth noting that in the 1970s when these experiments were designed, very little was known about the surface of Mars. Many researchers believed that microbial life, if present, might be abundant in Martian surface materials just as it is on Earth. The experiments were generally designed to detect life if it was present in fairly large quantities and was actively metabolizing.

The Experiments

The design package included three biological experiments plus a fourth which was not limited to biological experiments but served dual purposes. The Biology payload weighed only 15.5 kg (34.2 lb) and consumed on average only 15 watts of power.

Pyrolytic Release

The Pyrolytic Release or PR experiment looked for evidence of photosynthesis by incubating Martian soil with light, water, and a carbon-containing atmosphere of either CO or CO2. The carbon-bearing gases were made with carbon-14, a heavy, radioactive isotope of carbon. If there were photosynthetic organisms present, it was believed that they would incorporate some of the carbon as biomass through the process of carbon fixation, just as plants and cyanobacteria on earth do. After several days of incubation, the experiment removed the gasses, baked the remaining soil at 650 C (1200 F), and collected the products in a device which counted radioactivity. If any of the C14 had been converted to biomass, it would be vaporized during heating and the radioactivity counter would detect it.

Labeled Release

The LR experiment was in many ways the reverse experiment of the PR test. A sample of Martian soil was inoculated with a water and nutrient solution, where the nutrients (like glucose) were manufactured with radioactive C14. If any respiring organisms (like animals and non-photosynthetic bacteria) were present, they would consume the nutrients and release C14-containing CO2, which would in turn be detected by a radioactivity counter.

Gas Exchange

The GEX experiment did not involve radioactive labeling of this sort. Instead, it carefully cleansed soil samples of Martian gasses with a flow of helium, then incubated them with CO2, inert gasses, and water. Periodically, the instrument sampled the atmosphere of the incubation chamber and used a gas chromatograph to measure the concentrations of several gasses, including oxygen, CO2, nitrogen, hydrogen, and methane. The scientists hypothesized that metabolizing organisms would either consume or release at least one of the gasses being measured.

Gas Chromatograph - Mass Spectrometer

The GCMS is a device which separates vapor components chemically via a gas chromatograph and then feeds the result into a mass spectrometer, which measures the molecular weight of each chemical. As a result, it can separate, identify, and quantify a large number of different chemicals. The GCMS was used to analyze the components of untreated Martian soil, and particularly those components that are released as the soil is heated to different temperatures.

The Results

The initial results from both landers were very exciting, but also confusing. All three experiments observed chemical changes that indicated the possible presence of life, though in general the signals were small. The Labeled Release experiment, however, showed a fairly strong production of radioactive gas when C14-labeled nutrients were added. To a lesser degree, the GEX measured the production of oxygen when water was added to a sample.

However, the GCMS measured no significant amount of organic molecules in the Martian soil, in fact the strongest organic concentrations it measured were minute trace contaminants brought from Earth, left over from the assembly and cleaning of the sample chambers and instruments. This result was difficult to explain if Martian bacterial metabolism was responsible for the positive results seen by LR and GEX. Also, the production of radioactivity in LR and oxygen in GEX tapered off and died out after a short time, and when more water or nutrients was applied, the gas production didn't return.

Finally, control experiments done by heat-sterilizing soil samples before adding the water showed very similar results to fresh samples. If living organisms really had been responsible for the result shown, heat sterilization should have killed them and prevented a positive signal.

Scientific Conclusions and Ongoing Debate

Most scientists ultimately concluded that the results seen were best explained by a chemical reaction with some substance in the soil, which reacted with the water and nutrients to release oxygen and breakdown products from the LR nutrient solution. This explained the tapering-off effect and the lack of a second response to additional liquids -- the reactant in the soil was simply used up. It also explained the positive response of a fresh sample after heat sterilization, as the chemical responsible wasn't destroyed by heating as bacteria would have been.

One often-touted explanation is that the Martian soil, being continuously exposed to UV light from the Sun (Mars has no protective ozone layer), has built up a thin layer of a very strong oxidant. A sufficiently strong oxidizing molecule would react with the added water to produce oxygen and hydrogen, and with the nutrients to produce carbon dioxide. However, it would be a chemical we are unfamiliar with on Earth because the presence of humidity in our atmosphere would immediately destroy any oxidant strong enough to react with water. The exact nature of the hypothetical oxidant is a subject of ongoing debate today, and some experiments on existing and planned missions are designed to look for it.

However, not all scientists who have studied the matter are convinced that the initial Viking results were caused by a chemical reaction. In particular, one of the designers of the LR experiment, Gilbert Levin, believes the results are diagnostic for life. He and others have conducted ongoing experiments attempting to reproduce exactly the Viking data, either with biological or non-biological materials on Earth. At least some of their results support his position.

While the majority of astrobiologists still believe that the Viking biological experiments were negative, Levin is not alone, either. The matter is still under debate and still receives attention in both the popular press and the scientific literature into the 21st century.

The question will probably not be resolved entirely until future missions to Mars conclusively demonstrate the presence of life on the planet, identify the chemical(s) responsible for the Viking results, or both.