The Viking mission and the search for life on Mars

Modern scientific hypotheses concerning the origin of life generally have as their central theme the idea that living systems arise through "chemical evolution," a process in which simple compounds are generally transformed under the influence of various energy sources into more and more complex molecules, ultimately resulting in a system of replicating molecules [Buvet and Ponnamperuma, 1971; Miller and Orgel, 1974]. Based upon comparisons between the chemical composition of terrestrial living systems and cosmic abundances [Frieden, 1972] as well as upon theoretical considerations [Wald, 1962], it is further assumed that the key substances in this evolution are all carbon‐based compounds. Given a planet with a "hospitable" environment, these processes continue until the stage is set for biological infestation of the planet and further biological evolution follows. As it became clear that direct experimentation on other planets was technically feasible with the advent of spacecraft technology, attention turned to Mars as the most promising extraterrestrial object upon which to search for evidence of chemical evolution [Pittendrigh et al., 1966]. This view was reached despite significant gaps in our knowledge of that planet, most notably about the properties and composition of its surface. Even after the encounters by Mariners 4, 6, and 7 and the more encompassing Mariner 9 mission, virtually nothing was known about the chemistry of the Martian surface. Under these circumstances, the status of evolution of carbon compounds and particularly the question of the existence of life on Mars were entirely speculative.