The two Viking spacecraft each carried four types of 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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The mission looked for microorganisms, rather than larger creatures, because it is known that the favorable conditions for the evolution of multicellular organisms ceased some four billion years ago on Mars.[2] The four experiments are presented here in the order in which they were carried out by the two Viking landers.
(PI: Klaus Biemann, MIT) The Gas Chromatograph — Mass Spectrometer (GCMS) is a device that 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.[3]
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. It was previously believed that 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 the Labeled Release experiment (see below). The Phoenix mission provided the missing information to explain Viking's results. Perchlorate ions were found in the Martian soil which, when heated, act as a strong oxidant that can destroy organic molecules, and produce the organic molecules chloromethane and dichloromethane, which can be easily mistaken for cleaning products. If there were organic molecules in the Martian soil, the Viking's method of testing, which involved heating the sample, could have destroyed those organics.[4]
(PI: Vance Oyama, NASA Ames) The Gas Exchange (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. The result was negative.
(PI: Gilbert Levin, Biospherics Inc.) The Labeled Release (LR) 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) were tagged with radioactive 14C. The air above the soil was monitored for the evolution of radioactive 14CO2 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. The experiment was done by both Viking probes the first using a sample from the surface exposed to sunlight and the second probe taking the sample from underneath a rock both initial injections came back positive.[1] Subsequent injections a week later did not, however, elicit the same reaction, and the result remains inconclusive.[5]
(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.[6]
Organic compounds seem to be common, for example, on asteroids, meteorites, comets and the icy bodies orbiting the Sun, so detecting no trace of any organic compound on the surface of Mars came as a surprise. The GC-MS was definitely working, because the controls were fine and it was able to detect traces of the cleaning solvents that had been used to sterilize it prior to launch.[7] At the time, the total absence of organic material on the surface made the results of the biology experiments moot, since metabolism involving organic compounds were what those experiments were designed to detect. However, the general scientific community surmise that the Viking's biological tests remain inconclusive.[1][8][9][10] Most researchers surmise that the results of the Viking biology experiments can be explained by purely chemical processes that do not require the presence of life, and the GC-MS results rule out life.
Despite the positive result from the Labeled Release experiment, a general assessment is that the results seen in the four experiments are best explained by oxidative chemical reactions with the Martian soil. One of the current conclusions 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 (CO2).
On August 2008, the Phoenix lander detected perchlorate, a strong oxidizer when heated above 200°C. This was initially thought to be the cause of a false positive LR result.[11][12] However, results of experiments published in December 2010[13] propose that organic compounds "could have been present" in the soil analyzed by both Viking 1 and 2, since NASA's Phoenix lander in 2008 detected perchlorate, which can break down organic compounds. The study's authors found that perchlorate can destroy organics when heated and produce chloromethane and dichloromethane as byproduct, the identical chlorine compounds discovered by both Viking landers when they performed the same tests on Mars. Because perchlorate would have broken down any Martian organics, the question of whether or not Viking found organic compounds is still wide open.[14]
Before the discovery of the oxidizer perchlorate on Mars in 2008, some theories remained opposed to the general scientific conclusion. An investigator suggested that the biological explanation of the lack of detected organics by GC-MS could be that the oxidizing inventory of the H2O2-H2O solvent well exceeded the reducing power of the organic compounds of the organisms.[15]
It has also been argued that the Labeled Release (LR) experiment detected 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 positive LR results are enough diagnostic for life on Mars. He and others have conducted ongoing experiments attempting to reproduce 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.[16]
While the majority of astrobiologists still conclude that the Viking biological experiments were inconclusive or negative, Gilbert Levin is not alone in believing otherwise. The current claim for life on Mars is grounded on old evidence reinterpreted in the light of recent developments.[17][18][19] On 2006, Mario Crocco went as far as proposing the creation of a new nomenclatural rank that classified some Viking results as 'metabolic' and therefore representative of a new form of life.[20] The taxonomy proposed by Crocco has not been accepted by the scientific community, and the validity of Crocco's interpretation hinged entirely on the absence of an oxidative agent in the Martian soil.
On a paper published in December 2010,[13] the scientists suggest that if organics were present, they would not have been detected because when the soil is heated to check for organics, perchlorate destroys them rapidly producing chloromethane and dichloromethane, which is what the Viking landers found. This team also notes that this is not a proof of life but it could make a difference in how scientists look for organic biomarkers in the future.[4][21] It is expected that the results from the upcoming ExoMars and Mars Science Laboratory missions will help settle this controversy.[21]
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.[22] He concluded that there was enough information about Mars' atmosphere at that time to discount the possibility of life there. Since then, methane has been discovered in Mars' atmosphere at 10ppb, thus reopening this debate.
A press commentary argued that, if there was life at the Viking lander sites, it may have been killed by the exhaust from the landing rockets.[23] 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 descended to the surface with retro-rockets, however, their fuel was hydrazine, and the end products of the plume (water, nitrogen, and ammonia) were not found to have affected the soils at the landing site.
The question of life on Mars 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. About thirty three years after the Viking program, the Beagle 2, a British robotic lander spacecraft, was sent to Mars in 2003 to specifically assess possible chemical biosignatures of life, but the spacecraft was destroyed on landing. The Mars Science Laboratory rover is scheduled to launch in 2011 and will determine the nature and inventory of organic carbon compounds in the soil and atmosphere of Mars. Astrobiology research on Mars will continue with the Mars Trace Gas Mission orbiter in 2016 and ExoMars and MAX-C rovers in 2018.