The clue detected by Mariner 9, according to Gil Levin, was that the level of the gas, carbon dioxide (CO₂), was so high, according to the Mariner 9 instruments, that it could not be maintained at such a level, unless continuously replenished by a chemical source on Mars. Levin suggested that the chemical source might be life –not animal life but perhaps single-celled life forms, similar to Earth’s bacteria.

But other investigators had a different explanation: perhaps a simple chemical that was not organic –something like hydrogen peroxide (H₂O₂)– was present in the atmosphere and at work converting organic material into CO₂, thus imitating the effects of living chemical activity that scientists call metabolism. Specifically, the chemical activity wherein organic matter is converted to CO₂ is known as oxidation and indeed it is a process that occurs through both living and non-living phenomena. The opposite of oxidation is called reduction, a process that we shall discuss later in connection with the Viking mission.

A problem with the H₂O₂ hypothesis was that Mariner 9 detected neither H₂O₂ nor any other oxidizing agent in the atmosphere. The question of life on Mars was still open and would require a more aggressive search, with instruments designed for the direct detection of microorganisms.

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