Viking biological experiments
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The two Viking spacecraft each carried four biological experiments to the surface of Mars in the late 1970s. These were the first Mars landers to carry out experiments to look for biosignatures of life on Mars. The landers used a robotic arm to put soil samples into sealed test containers on the craft. The two landers were identical, so the same tests were carried out at two places on Mars' surface, Viking 1 near the equator and Viking 2 far enough north to see frost in winter[1] .
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[edit] The Experiments
The four experiments are presented here in the order in which they were carried out by the Viking craft.
[edit] 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. It could measure molecules present at a level of only a few parts per billion.
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 (see below).
[edit] Gas Exchange
The GEX experiment looked for gasses given off by an incubated soil sample by first replacing the Martian atmosphere with the inert gas Helium. It applied a liquid complex of organic and inorganic nutrients and supplements to a soil sample, first with just nutrients added, then with water added too.[1] 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. Such changes in the atmosphere in the sample chamber were to be evidence for life. A positive result was to be followed by the control part of the experiment as described for the PR below.
[edit] Labeled Release
The Labeled Release experiment is the one that gave the most promising for the exobiologists. In the LR experiment, a sample of Martian soil was inoculated with a drop of very dilute aqueous nutrient solution The nutrients (7 molecules that were Miller-Urey products, and that were metabolized by all microorganisms tested) were tagged with radioactive 14C. The air above the soil was monitored for the evolution of radioactive gas as evidence that microorganisms in the soil had metabolized one or more of the nutrients. Such a result was to be followed with the control part of the experiment as described for the PR below. The result was quite a surprise following the negative results of the first two tests, with a steady stream of radioactive gasses being given off by the soil.[1]
[edit] Pyrolytic Release
Light, water, and a carbon-containing atmosphere of carbon monoxide (CO) and carbon dioxide (CO2), simulating that on Mars. The carbon-bearing gases were made with carbon-14 (14C), 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 14C had been converted to biomass, it would be vaporized during heating and the radioactivity counter would detect it as evidence for life. Should a positive response be obtained, a duplicate sample of the same soil would be heated to "sterilize" it. It would then be tested as a control - always the most important part of any experiment - and should it still show activity similar to the first response, that was evidence that the activity was chemical in nature. However, a nil, or greatly diminished response, was evidence for biology. This same control was to be used for any of the three life detection experiments that showed a positive initial result.
[edit] Scientific conclusions and ongoing debate
Despite the positive result from the labeled release experiment NASA ultimately concluded that the results seen in the four experiments were best explained by a chemical reaction with some substance in the soil. The currently held belief is that the Martian soil, being continuously exposed to UV light from the Sun (Mars has no protective ozone layer) and strong winds 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 superoxidant is a subject of ongoing debate today, and some experiments on existing and planned missions are designed to look for it[citation needed].
However it has also been argued that the labelled release experiment detected that there were so few metabilising organisms in the martian soil that it would have been impossible for the gas chromatograph to detect them.[1] This view is put forward by one of the designers of the LR experiment, Gilbert Levin, who believes the results of the Viking landers are diagnostic for life on Mars. He and others have conducted ongoing experiments attempting to reproduce exactly the Viking data, either with biological or non-biological materials on Earth. While no experiment has ever precisely duplicated the Mars LR test and control results, experiments with hydrogen peroxide-saturated titanium dioxide have produced similar results[citation needed].[2]
While the majority of astrobiologists still believe that the Viking biological experiments were negative[citation needed], 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 either conclusively demonstrate the presence of life on the planet, identify the chemical(s) responsible for the Viking results, or both.
[edit] Criticism
James Lovelock argued that the Viking mission would have done better to examine the Martian atmosphere than look at the soil. He theorised that all life tends to expel waste gasses into the atmosphere, and as such it would be possible to theorise the existence of life on a planet by detecting an atmosphere that was not in chemical equilibrium[1]. He concluded that there was enough information about Mars' atmosphere at that time to discount the possibility of life there.
[edit] See also
[edit] External links
- Viking Lander LR Data Set from NASA
- Viking Biology from the NASA NSSDC Master Catalog of Experiments
- 2001 Space.com article about the debate over the LR results
- Life on Mars: Viking and the Biology Experiments
- Circadian rhythms Speculation Study
- Mars: The Living Planet. Barry E. DiGregorio, with additional contributions by Gilbert V. Levin and Patricia Ann Straat. North Atlantic Books, Berkeley, CA, 365 pages, 1997.
[edit] References
- ^ a b c d Chambers, Paul (1999), Life on Mars; The Complete Story, London: Blandford, ISBN 0713727470
- ^ R. C. Quinn, A. P. Zent (1999). "Peroxide-Modified Titanium Dioxide: a Chemical Analog of Putative Martian Soil Oxidants". [[Journal Origins of Life and Evolution of Biospheres]] 29 (1): 59-72. DOI:10.1023/A:1006506022182.
- F. S. Brown, H. E. Adelson, M. C. Chapman, O. W. Clausen, A. J. Cole, J. T. Cragin, R. J. Day, C. H. Debenham, R. E. Fortney, R. I. Gilje, D. W. Harvey, J. L. Kropp, S. J. Loer, J. L. Logan, Jr., W. D. Potter, and G. T. Rosiak (1978). "The biology instrument for the Viking Mars mission". Review of Scientific Instruments 49: 139-182. DOI:10.1063/1.1135378.
- H. P. Klein, J. Lederberg, A. Rich, N. H. Horowitz, V. I. Oyama, G. V. Levin (1976). "The Viking Mission Search For Life On Mars". Nature 262: 24-27. DOI:10.1038/262024a0.
- H. P. Klein (1999). "Did Viking Discover Life on Mars?". Journal Origins of Life and Evolution of Biospheres 29: 1573-0875. DOI:10.1023/A:1006514327249.
- H. P. Klein (1992). "The Viking biology experiments: Epilogue and prologue". Journal Origins of Life and Evolution of Biospheres 21: 1573-0875. DOI:10.1007/BF01809861.
