“If we find so much as a microbe independently evolved on Mars it will mean that the galaxy literary teems with life”
Dr. Carl Sagan, 1971
NASA’s Viking mission was the first time any space agency had landed a spacecraft on the planet Mars. The first images returned from the surface of Mars by the two landers are striking and in comparison to images returned from the surface of Mars by later mission – almost unique.
The first image returned showed a rocky, sandy terrain with a pinkish yellow hue to the sky. With low levels of red coloration in the sky, it could have been a photograph of many terrestrial rocky deserts at dawn or dusk. Other images from Viking 1 and Viking 2 Lander sites showed the same rocky desert terrain but often with blue Earth-like hue to the martian sky. This of course was in stark contrast to images that would be presented to the public by later missions.
From the perspective of the biology mission, both Viking Landers were equipped with three experiments designed to test for microbial life. The exobiologists who designed the Viking biology experiments had made a number of assumptions. They had assumed that life on Mars would be carbon based with similar chemistry to life on Earth. Martian microbes would be widespread across the surface of Mars and those microbes would elicit chemical evidence for metabolism. Their detection needs to be performed by well understood analytical chemistry techniques.
The acceptance criteria for the discovery of life on Mars was that “a positive response from a single experiment indicates presence of active biology”.
The experiments selected for inclusion in the mission ultimately were limited to three and had a total weight of 15.5 kg (34 lbs) and fit into a small part of the spacecraft only about 30 cm (12 inches) square.
The Gas Exchange (GEx) Experiment
Vance Oyama from NASA Ames Research Center ran the gas exchange experiment. This was an experiment specifically designed to look for microbes called heterotrophs which are organisms that feed on organic compounds.
In the experiment he did this by feeding a nutrient solution to a gathered soil sample. The microbes, if present in the soil, would be detected by action of their metabolism consuming the organic chemical in the nutrient solution and giving off certain gases associated with metabolism. It looked for the gases H2 (hydrogen), N2 (nitrogen), O2 (oxygen), CH4 (methane) and CO2 (carbon dioxide) by sampling the artificial atmosphere containing water vapour and CO2 above the soil sample after addition of the nutrient solution. The sampled gases were then separated using a gas chromatograph before passing into the detector.
The result of the experiment was negative.
Although the experiment resulted in a rush of gas being given off, the speed of the reaction, and the fact that reaction was stable even after sterilization of the soil sample at 145°C, indicated that the response was chemical in origin and not biological. As a general rule of thumb abiotic chemical reactions are faster than biological chemical reactions. This meant that confidence that the reaction was due to microbes was low.
Even if microbes had been present the speed of the reaction suggested the cause was chemistry rather than biology.
The availability of nitrogen in the Martian soil had been unknown before the mission and this experiment showed that nitrogen was available on Mars for biology to use. The reactions that had occurred were suspected to have been caused by the presence of hydrogen peroxide (H2O2) in the soil although an exotic blend of three different unusual superoxides were needed to fully explain the results.
The Labelled Release (LR) Experiment
Dr Gil Levin of NASA contractor Biospherics Inc. ran the labelled release experiment that, like the GEx experiment, was designed to look for heterotrophs by detecting their metabolism.
The experiment used a radiolabeled nutrient solution to feed any microbes that may have been present in the soil sample. The microbes would then be expected to metabolise the radiolabeled organic compounds in the nutrient solution and their metabolism would result in the release of a radioactive gas that could be detected using a radioactivity counter.
The radiolabeled nutrient solution contained 14C (carbon 14) labelled formic acid, glycine, glycolic acid, D and L-lactic acid and both D and L-alanine. Metabolism was expected to liberate CO2 (carbon dioxide), CO (carbon monoxide) or CH4 (methane) labelled with 14C (carbon 14).
The result of the test was positive.
It showed a rush of 14CO2 gas when the sample of soil was wetted with the nutrient solution indicating the microbes have converted the radiolabeled organic compounds into the radiolabeled metabolite gas.
There was no correlation found between O2 released in the GEx experiment with fresh soil samples. The labelled release response was additionally found to be lost upon storage of the sample and it was also shown to be sensitive to prior heating, unlike what had been seen in the GEx experiment.
This result of the experiment however was ultimately rejected because the results found were not consistent with the Gas Chromatography Mass Spectrometer (GCMS) data. The GCMS had, later, failed to identify any organic components in soil samples. However, by the original experimental design the LR experiment was a positive result. NASA’s own acceptance criteria for life had been met.
Pyrolytic Release (PR) Experiment
Norman Horowitz of Caltech ran the pyrolytic release experiment that was designed to detect phototrophs. These are organisms that require only sunlight, carbon dioxide and water to survive.
Using an artificial atmosphere with radiolabeled 14CO2 microbes would be expected to take in the 14C isotope into their biomass. Afterwards, baking the sample at a high temperature would be expected to release the 14C isotope allowing its detection using a radioactivity counter.
The result of the experiment was positive.
Although the response had been weak it was found to be statistically significant. The main reason for rejecting this particular result was its weak response. However, heating the samples at 90°C had been found to not reduce the reaction and heating a sample to 175°C did not completely stop the reaction. The reaction was also inhibited by the presence of water. As with the positive result of the LR experiment, that GCMS had not detected organics was ultimately used to write off the positive result. The PR experiment response could just possibly have been biological in origin however it appeared to be a mixture of responses that included chemical reactions.
The Controversial GCMS
The Gas Chromatography Mass Spectra (GCMS) instrument was not part of the biology experiment. It was run by the Molecular Analysis Team and it looked for the presence of organic molecules in the soil. The instrument was not able to differentiate between organic molecules that were biological in origin or abiotic. However, it was hoped that, if life was found by the biology experiments, then this could be corroborated by the Molecular Analysis Team’s GCMS.
It was later disclosed that the GCMS instrument used on Mars was not as sensitive as the biology experiments. In trials the instrument even failed to detect organic material in antarctic soil that was known to contain microbes. The instrument was also unable to detect certain organic compounds at all. The sensitivity of the GCMS was several orders of magnitude lower than had originally been believed. Yet, its failure to detect organics was the predominant reason to argue that the biology experiments had failed to detect life.
Notes on Wikipedia
Strangely, at the time of writing, the wikipedia page for the Viking Lander Biology Experiments claims that the GCMS was one of four biology experiments on the Viking Landers. It lists the GCMS results at the top of the page before giving details of the actual biology experiments. This is an error; one of numerous errors on this wikipedia entry. Some writers have not been so generous as to call these errors. All I will say with regards to using wikipedia is proceed with caution.
The Wolf Trap
Wolf Vishniac was commissioned to design an experiment to detect life on Mars and he designed the Wolf Trap Experiment. Unfortunately, his design used a relatively large amount of water and with limitations on space and weight in addition to Viking mission due to budget cuts his experiment was dropped from the biology experiments sent to Mars. However, it still had a part to play on Earth.
Three years before Viking, in the Dry Valleys of Antarctica, which were thought to be the most Mars-like terrain on Earth, Vishniac used his wolf trap experiment to attempt to detect microbes in Antarctic soil that had been declared sterile. Norman Horowitz had used his pyrolytic release experiment on samples and did not find life in the antarctic dry valley soils. From this Horowitz was inclined to think that Mars would be similarly devoid of life. However, Vishniac, using his Wolf Trap experiment, did find microbes in these Antarctic soils that the pyrolytic release experiment had failed to find. He had shown that the sensitivity of the pyrolytic release experiment has simply been too low for the microbes that were present to be detected.
From this Vishniac believed that the problem affecting the pyrolytic release experiment was that the nutrient solution that was being used. Both the labelled release experiment and the pyrolytic release experiment were, he believed, using a nutrient solution as a food source that was potentially toxic to the microbes in the soil. These microbes after all are in extreme conditions – what we would now call extremophiles – and as such they may require extreme conditions for their survival. Vishniac, by using a more nutrient starved solution was able to obtain great success in detecting microbes in these environments.
Summary of Viking Search for Life
The Viking Mission was a huge success on many levels however it is often seen that the biology experiment was a failure – somewhat unfairly I should add. Gilbert Levin, principal scientist for the labelled release experiment, has been an outspoken critic of NASA in this regard and he maintains that the biology experiment did indeed find microbial life on Mars.
What was clear though was that the response occurring the biology experiments was being attenuated by highly reactive chemicals in the soil. Some of the results obtained in the experiments were hard to explain without invoking life. Occam’s razor is not part of scientific methodology but it is clear that the simplest explanations in this case was being rejected in favour of complex abiotic reactions.
During control runs of the Labellel Release experiment it was found that oxidation (addition of oxygen) of the nutrient solution components to CO2 was occurring. However, during the pyrolytic release experiment control runs reduction (removal of oxygen) from CO2 (carbon dioxide) to CO (carbon monoxide) was observed.
That two opposite reactions were occuring in the soil during these control runs is very difficult to account for without invoking some exotic effect beyond the realm of known chemistry. Could the new realm to consider be biology? It is known that microbes can produce both of these two reactions. The two effects, seen in the control runs of the two experiments, are indicative of biological action.
Mars tested positive for life in 1975 according to NASA’s own experimental criteria.
© Anthony Beckett 2021
