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]

Schematic of the Viking Lander Biological Experiment System
Schematic of the Viking Lander Biological Experiment System

Contents

[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 (PI: Klaus Biemann, MIT)

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, martian soils were found to contain less carbon than lifeless lunar soils returned by the Apollo Program. 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 (PI: Vance Oyama, NASA Ames)

The GEX experiment looked for gases 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 gases, including oxygen, CO2, nitrogen, hydrogen, and methane. The scientists hypothesized that metabolizing organisms would either consume or release at least one of the gases 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 (PI: Gilbert Levin, Biospherics Inc.)

The Labeled Release experiment is the one that gave the most promise 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 gases being given off by the soil immediately following the first injection.[1] Subsequent injections did not, however, elicit the same reaction, and the experiment remains enigmatic.

[edit] Pyrolytic Release (PI: Norman Horowitz, CalTech)

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 gases, 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 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, the general conclusion is that the results seen in the four experiments were best explained by chemical reactions with the Martian 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 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.[2]

However it has also been argued that the labelled release experiment detected that there were so few metabolising organisms in the Martian soil that it would have been impossible for the gas chromatograph to detect them.[1] This view has been 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.[3]

While the majority of astrobiologists still believe that the Viking biological experiments were negative,[citation needed] Levin is not alone in believing otherwise. The matter is still under debate and still receives attention in both the popular press and the scientific literature into the 21st century. Despite the lack of a clear yes or no answer to the question of bacterial life on Mars, the Viking results will continue to be instrumental in planning for the future directions of Mars biology investigations.

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 gases 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.

There are some that now argue that, if there was life at the Viking lander sites, it may have been killed by the exhaust from the landing rockets.[4] That is not a problem for missions which land via an airbag-protected capsule, slowed by parachutes and retrorockets, and dropped from a height that allows rocket exhaust to avoid the surface. Mars Pathfinder's Sojourner rover and the Mars Exploration Rovers each used this landing technique successfully. The Phoenix Scout lander, however, lands with rockets, and may have issues similar to Viking's.

By the end of 2006, Mario Crocco, neurobiologist at the Neuropsychiatric Hospital Borda (Buenos Aires, Argentina), had re-evaluated the data collected by that mission, adding data from recent investigations. The article was published in Electroneurobiología, a State journal published by the same Neuropsychiatric Hospital Borda; that devotes a section to basic concepts in theoretical biology. They postulate that the results are consistent with the presence of hypothetical martian microbial life in the surface of the planet, and have proposed the informal name Gillevinia straata.[5] as characterized by the observed activity, inasmuch as the new data, together with concepts discussed in the author's tradition regarding what life is, allow to consider such activity as metabolic.

[edit] See also

[edit] External links

[edit] References

  1. ^ a b c d Chambers, Paul (1999), Life on Mars; The Complete Story, London: Blandford, ISBN 0713727470 
  2. ^ See http://mepag.jpl.nasa.gov/goals/MEPAGgoals-approved071604.pdf
  3. ^ 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. 
  4. ^ "Did probes find Martian life ... or kill it off?". Associated Press via MSNBC (2007-01-07). Retrieved on 2007-05-31.
  5. ^ Crocco, M. (2007), Los taxones mayores de la vida orgánica y la nomenclatura de la vida en Marte: primera clasificación biológica de un organismo marciano (ubicación de los agentes activos de la Misión Vikingo de 1976 en la taxonomía y sistemática biológica). Electroneurobiología 15 (2), 1-34; http://electroneubio.secyt.gov.ar/First_biological_classification_Martian_organism.pdf
  • 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. 
  • K. Biemann, J. ORO , P. Toulmin III, L. E. Orgel, A. O. Nier, D. M. Anderson, P. G. Simmons, D. Florey, A. V. Diaz, D. R. Rushneck, and J. A. Biller (1976). "Search for Organic and Volatile Inorganic Compounds in Two Surface Samples from the Chryse Planitia Region of Mars". Science 194 (4260): 72 - 76. doi:10.1126/science.194.4260.72. 
  • Klaus Biemann (2007). "On the ability of the Viking gas chromatograph–mass spectrometer to detect organic matter". Proc Natl Acad Sci 104 (25): 10310-10313. doi:10.1073/pnas.0703732104. 
  • Rafael Navarro-González, Karina F. Navarro, José de la Rosa, Enrique Iñiguez, Paola Molina, Luis D. Miranda, Pedro Morales, Edith Cienfuegos, Patrice Coll, François Raulin, Ricardo Amils, Christopher P. McKay (2006). "The limitations on organic detection in Mars-like soils by thermal volatilization–gas chromatography–MS and their implications for the Viking results". Proc Natl Acad Sci 103 (44): 16089-16094. doi:10.1073/pnas.0604210103.