July 20 marked the 45th anniversary of the historic Viking mission landing on Mars. Days later, Gilbert Levin, one of the mission’s principal investigators, passed away. Despite the elapsed time, and (in my case) very tangentially, we were still able to participate in the discussion of one of the most controversial aspects of the mission: the evidence of life on Mars. This is the story.
Towards 2007, at the Madrid Astrobiology Center we were starting a line of research on the molecular remains that living beings leave in geological materials (organic biosignatures or biomarkers). We had also just proposed instrumentation to study the organic composition of Titan and Enceladus, which would travel on the future TandEM mission.
Our work caught the attention of Christopher McKay of NASA’s Ames Research Center, who sent us a sample of soil number 726. It was the latest in 30 years of scientific discussion of the results of the Viking mission to Mars.
The Viking mission consisted of two ships, each consisting of an orbiter and a lander. Viking 1 landed on Mars on July 20, 1976 in Chryse Planitia. Viking 2 did it on September 3 in Utopia Planitia, on the other side of the planet. The landers transmitted data until November 1982. The objectives of the mission were to obtain images, environmental, geophysical and composition data of the atmosphere and soil. It was also trying to answer the big question: is there or was there life on Mars?
Viking in search of life
The Vikings conducted, among others, two experiments:
- The LR experiment (labeled release or radiorespirometry), developed by the teams of Harold Klein, director of the Biology Team, and Gilbert Levin.
- The molecular analysis experiment (experiment molecular analysis) using the GCMS instrument developed by MIT chemist Klaus Biemann.
For the LR experiment, soil was incubated with organic nutrients labeled with carbon-14, measuring the emission of radioactive CO₂, which would suggest the presence of metabolic activity.
The objective of the GCMS instrument was to identify organic compounds present in the soil or air. For the analysis, a soil sample was heated to high temperature, to volatilize possible organic compounds. The volatilized organic compounds would enter the analyzer to identify them.
The method is intuitive: if we heat any soil sample to a high temperature, it will emit organic compounds from microscopic life and plant debris present in the soil. On Earth, it is not even necessary to heat, since the petricor, which makes summer storms so pleasant, is already a biomarker.
On Mars, the LR experiment gave a positive result. However, the GCMS instrument did not detect any organic compounds in the Martian soil. What did it mean? There was life in the ground, but its components could not be detected? Was there no life and was the result due to a chemical effect?
Soil number 726
The data from the Viking mission did not answer the question of whether there was life on Mars, but they did not deny it either. Rather, they opened up new questions. Before launching the mission, the instruments were tested with various types of soils, to obtain a reference library that would help interpret the data from Mars.
Gil Levin’s team conducted the experiments with soil 726. It was collected in 1966 in Antarctica, in a remote nunatak called Coalsack Bluff, in the Transantarctic Mountains. In tests, soil 726 gave a similar result to Martian soil: positive in the LR experiment and no organic compounds detected with the GCMS instrument.
Soil 726 contained traces of carbon and kerogen particles, a material of biological origin common in rocks. Did this mean that there was some life on Mars? Why couldn’t GCMS detect organic compounds in soil 726? Was it because the design of the instrument was not suitable? It had to be discussed, as we cannot afford to send a questionable instrument to detect traces of life.
For Levin, the Viking data suggested the presence of life. For others, the result of the LR experiment was explainable if the soil is chemically oxidizing. The ability of the GCMS instrument to detect small amounts of organic components was discussed.
It was suggested that the problem was not the instrument, a technical prodigy, but that the treatment of the sample was not adequate for the mission. As designed, it could only detect certain organic compounds present in high amounts. For this reason, McKay sent us soil samples 726 to several involved in the study of biomarkers. The objective was to check if organic components could be seen in the soil, and to decide if the Viking sample treatment was adequate or not.
Perchlorate and the sterility of the soil of Mars
The discussion ended in 2008, when the Phoenix Mars Lander discovered perchlorate in the soil of Mars. One explanation for the Viking results was the oxidizing character of the soil. Perchlorate salts are oxidizing and decompose forming hypochlorite (bleach), among other things, due to the radiation that devastates the Martian surface. The combination of oxidants and radiation suggested that Martian soil is sterile and incompatible with organic matter.
Thus, the surface is not the ideal place to search for life or biomarkers. Even if there were them in the ground, stable thanks to the low temperature and dryness of the planet, at the moment in which they were tried to analyze they would be destroyed by perchlorate. Therefore, Viking could not identify traces of life in any case. It was necessary to change the strategy: to look for traces of life in the subsoil or trapped in clays, as well as indirect evidence through the study of minerals and rocks.
And the 726 floor? Yes, it contained biomarkers. McKay and his colleagues were right: the molecular analysis experiment, as configured at Viking, was not sensitive enough, but it didn’t matter. We now know that the Viking experiments were designed from too geocentric a perspective. Another approach was needed, which applies to the Curiosity and Perseverance rovers.
An instrument similar to Viking’s GCMS, however, can be important in exploring other places, such as Europa or Titan, where organic compounds are key. It will be fascinating to see the data from ongoing and future missions.
César Menor-Salván, Assistant Professor Doctor. Biochemistry and Astrobiology. Systems Biology Department, University of Alcalá
This article was originally published on The Conversation. Read the original.
