21 May 2004

Calling All Martians! The Viking Mission Labeled Release Experiment and the Search for Martian Life. Part III

Reginald Smith

Hello fellow amateur scientists. Welcome to the final installment in the analysis of the data from the Mars Viking Lander Labeled Release (LR) Experiment. If this is your first time reading this series, I would recommend you look at Part 1 and Part 2 for the appropriate background.

We have already obtained and analyzed the data from the Labeled Release Experiment on the Viking landers. In this article, I will explain how the scientific community reacted to the results. In particular, we will look at how and why many scientists believe that these data either indicate or do not indicate life on Mars. I will summarize the main arguments from each group.

Viewpoint 1: The experiments do not prove the existence of life

This is the current mainstream view of the scientific community regarding the results from the Viking biological payload experiments. One misunderstanding, however, should be cleared up from the start. Adhering to this opinion does not mean these scientists think there is absolutely no life on Mars now or in the past. In fact, some are still open to the idea. These scientists are just concluding that Viking's experiments did not present a "smoking gun" case for life on Mars. Whether or not there is life elsewhere on Mars cannot be answered conclusively by these experiments.

As you know from part II, there was a positive response from the Labeled Release Experiment. The evolution of carbon dioxide (CO₂) means that there is some biological or non-biological agent or process that caused the nutrient solution to release CO₂ gas. Whether this agent is biological or non-biological is the crux of the debate. Below are the arguments that the response is non-biological in origin.

•  The Martian environment is absolutely hostile to life

Mars, despite the presence of CO₂ and inorganics essential for life, is hardly a paradise. Sunlight and ultraviolet radiation pass to the Martian surface with almost no attenuation due to the lack of any significant ozone layer. In addition, Martian surface temperatures are very cold and only reach a high of around 17 degrees C in the summer. Also, oxidant compounds on the Martian surface, such as iron oxide, are chemically hostile to many organic compounds. Combine these with no known current presence of liquid water on the Martian surface, and you have an environment that is not appealing to any known microorganism.

•  The other biological payload experiments generated negative results

The Labeled Release Experiment was only one of three biological payload experiments. The other two, the Pyrolytic Release and Gas Exchange experiments (see first part), gave results that indicated chemical activity but were not favorable to the presence of life. The Pyrolytic Relase Experiment did detect organic compounds but only about one-tenth of what had been obtained from a similar analysis with frozen soil from Antarctica. The Gas Exchange Experiment detected a rapid evolution of oxygen after the sample was humidified. However, the oxygen evolution was not repeated after addition of more water and nutrients. In fact, the gas production decreased, leading to the conclusion by most authorities that the results were by chemical processes and not life. Finally, a sensitive gas chromatograph mass spectrometer on board Viking that tested for organic compounds in Martian soil gave negative results. The fact that these experiments gave negative results for life dampen the success of the Labeled Release Experiment. (1)

•  Gas evolution of Labeled Release Experiment was not exponential

This is one of the first criticisms of the LR experimental results. The growth curves for the evolution of CO₂ do not show an exponential curve that would have been predicted if the production of CO₂ was due to the exponential reproduction of a population of asexual microorganisms. How do we know the curves are not exponential since they seem to rise so fast? The key is the curvature.

Notice that the rise of gas production simulated in Fig. 1 has a negative curvature. All exponential curves have a positive curvature. This created doubt in some that the response was an authentic biological response.

•  An oxidizing agent could produce a similar result in the LR Experiment

In order to explain the results of the LR experiment (as well as the Gas Exchange Experiment) without involving life, one would have to believe a chemical compound in the Martian soil reacted to produce the gas evolved from the interaction with the aqueous nutrient solution. The chemistry of such a reaction would require a strong oxidizing agent. Many oxidants and complexes have been presented as the possible culprit. It has been hypothesized that the incidence of UV radiation may help create such compounds. Chief among these was hydrogen peroxide (H₂0₂). Experiments with hydrogen peroxide have yielded results with similar response to the LR results, but at H₂0₂ concentrations that are unlikely to exist on the Martian surface. One cannot rule out the possibility of a reaction catalyst, however, to enhance the reactivity of H₂0₂ (2).

•  Poor results from second and third nutrient injections

Universally, second or third nutrient injections in the LR experiment did not elicit a more elevated response. In fact, they were usually associated with a decrease of CO₂ production. That would seemingly fly in the face of results that indicate the presence of a continuously growing culture of microbial life (1).

Viewpoint 2: The experiments prove (or make likely) the presence of life

This viewpoint is held by some scientists who believe the above criticisms of the Viking experiments are either not valid or do not rule out the possibility of a biologically produced reading. Chief among these proponents is Gilbert Levin, one of the scientists on the Viking missions. They believe a biological interpretation for the LR experiments is most appropriate. Their main arguments can be classified as follows:

•  Earth microbe conditions need not dictate Martian microbe conditions

Many scientists point out that even though most Earth microbes could not exist on Mars. This does not necessarily exclude the possibility of stronger, native Martian strains. Granted, in the 1970s and 1980s when this research was done, the existence of extremophile organisms such as bacteria in deep sea vents, etc. was not well known. Our current understanding of microbes shows they can tolerate stresses previously thought impossible.

•  Negative results from the gas chromatograph mass spectrometer are not necessarily conclusive

The negative detection of organic compounds by the gas chromatograph mass spectrometer are disputed by Levin in (2). In particular, he raises the point that theoretical considerations can show that the gas chromatograph mass spectrometer would give a negative reading for a soil sample with very low microbial concentrations. These same microbial concentrations gave a positive response in lab tests with an identical LR type experiment,

•  LR Experiment results are not inconsistent with the presence of life

Growth experiment with Earth microbes have been shown to also have non-exponential curves for gas production (2). Also, if Martian life only showed affinity for some of the substrates in the LR medium, growth may not be at expected rates (3).

•  Though oxidants cannot completely be ruled out, some are not as likely as thought

Though a non-biological response explanation cannot yet be completely ruled out, many have argued that some of the mysterious oxidation compounds that have been suggested are not likely. Hydrogen peroxide, an early favorite, is extremely susceptible to decomposition from sunlight and UV radiation. This is why it is sold in a dark, opaque bottle in drug stores. Given the strength of sunlight and UV on the Martian surface, it is unlikely hydrogen peroxide could survive unless it was in some sort of exotic chemical complex. (2)

•  Gas was produced in oscillations that corresponded to one Martian day (circadian rhythm) in Viking Lander 2 Cycle 3

Recent research by Gilbert Levin and others has discovered that a large part of data in Viking Lander 2 Cycle 3 has oscillations with a period of one Martian day. This is strikingly similar to circadian rhythm activity known in all Earth organisms. One could argue that this could be induced by changes in the sample temperature that also oscillate with the Martian day. However, the gas cycles do not exactly correspond with the temperature change cycles. In addition, the cyclical behavior gradually dampened in amplitude about 40 sols after the second nutrient injection. This is completely covered in a paper by Gilbert Levin, Patricia Straat, and Joseph Miller. It is available free on-line (4).

Conclusive?

As mentioned earlier, most scientists currently view the Viking experimental results as not suppporting the presence of life. However, it is questionable whether full conclusions for either side can be drawn without further experiments. There are two issues that both sides agree on, however, in terms of the nature of the active agent in the Martian soil. First, the agent was present at two geographically distant locations that corresponded to the two landers. One may then surmise this agent is very common on the Martian landscape. Second, the agent is deactivated at high temperatures. The sterilization of the LR Experiments showed this conclusively.

Though the question of Martian life is hundreds of years old, we will likely see it resolved in the next 25 years. The British/ESA Beagle 2 Mars Lander was supposed to conduct experiments to search for evidence of life. Unfortunately, it never made it to the surface intact.

The questions we answer at Mars will help us to search more effectively for life in other parts of our solar system such as Europa and Titan. Even if life is only microbial, it will make a huge impact on our view of ourselves and the universe that surrounds us. If only for this, more research is worth the expense.

References

1. Margulis, L. et. al., The Viking Mission: Implication for Life on Mars, Journal of Molecular Evolution 14, 223-232, 1979.

2. Levin, G. V., and P. A. Straat, A search for a nonbiological explanation of the Viking labeled release life detection experiment, Icarus 45, 494-516, 1981.

3. Klein, Harold et. al. "The Viking Biological Investigation: Preliminary Results." Science 194, 1976 99-105.

4. Miller, Joseph, et. al., A Periodic Analysis of the Viking Labeled Release Experiment, Instruments, Methods, and Missions for Astrobiology, SPIE Proceedings 4495, 96-107, 2001. www.biospherics.com/mars/spie2/Miller-Straat-Levin_FINAL.htm