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On Mars: Exploration of the Red Planet. 1958-1978

[51] The search for extraterrestrial life was a direct by-product of 20th century biochemists' quest for the origins of life on Earth. Instruments proposed by scientists to determine if there were detectable life forms or the organic matter necessary for such life forms to exist elsewhere in the solar system were based on the assumption that the laws governing the evolution of life on Earth are universally applicable, as are the laws of physics. When the Viking spacecraft was launched to Mars in 1975, they carried three biological experiments and a gas chromatograph-mass spectrometer, instruments with an intellectual and technological history reaching back to the early days of American space science. In fact, the development of life-detecting devices predates the availability of both spacecraft and launch vehicles.
As with many aspects of modern biology, the search for extraterrestrial life begins with Charles Darwin. His classic work, On the Origin of Species (1859), sparked considerable discussion of evolution, but it also led to speculation over the original source of life. In the 1860s, Louis Pasteur concluded that the spontaneous generation of microbes was not possible; all life on Earth came from preexisting life. What was the origin of those life forms? The Darwinian theory led many scientists to believe that the multiplicity of plant and animal species had a common source. In an 1871 letter, Darwin suggested that perhaps Earth's atmosphere, too, had evolved.
It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we would conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed. 1
In his speculation, Darwin rejected the premise that Earth's environment had always been static.
[52] Most scientists disagreed with the theory that life on Earth had its beginnings in a prebiotic environment, until the idea was simultaneously revived in the 1920s by two biochemists, J.B.S. Haldane of Great Britain and Aleksandr Ivanovich Oparin of the Soviet Union. Halden and Oparin independently asserted that although it was very unlikely for life forms or organic molecules to appear biologically in an oxygen-rich atmosphere, such compounds could have appeared millions of years ago in a very different environment. They postulated that in a prebiotic, sterile era organic compounds of ever-increasing complexity, accumulated in the seas and eventually by random combinations produced a listing molecule. On the nature of that prehistoric atmosphere, Haldane and Oparin disagreed.
Haldane favored a combination of ammonia, carbon dioxide, water vapor, and little or no oxygen. Organic compounds were synthesized by energy front ultraviolet light. Gradually the evolutionary process produced more complex molecules capable of self-duplication. Oparin's primordial atmosphere consisted of methane, ammonia, water vapor, and hydrogen. According to his theory, an abundance of organic compounds in the seas, given enough time, would permit the formation of organic molecules that would be the foundation for yet more complex life forms. Despite their work, most other biochemists through the 1910s insisted on attempting to synthesize organic compounds in oxygen-rich environments. In the 1950s, the focus shifted to the production of amino acids.
As with improved astronomical instruments, new biochemical techniques, such as paper chromatography,* opened new doors. One door led to the study of amino acids, the building blocks of protein. Biochemists believed that amino acids might hold clues to the origin of life, since primeval forms of life were assumedly protein-centered. Melvin Calvin commented on the logic behind these early studies: "We had every reason to suppose that the primitive Earth had on its surface organic molecules.'' If one went further and postulated a "reducing," or oxygen-poor atmosphere, "most of the carbon was very largely in the form of methane or carbon monoxide,....the nitrogen was mostly in the form of ammonia, there was lots of hydrogen, and oxygen was all...in the form of water." Given these simple molecules, was it possible to create more complex ones in the laboratory? Calvin and several other scientists began to experiment with reduced atmospheres containing primarily carbon compounds.2
Stanley L. Miller, while pursuing his doctoral-studies at the University of Chicago, was the first to produce amino acids in a reducing atmosphere. Working under Harold Urey, he developed a closed-system apparatus into which he introduced a mixture of methane, ammonia, water, and hydrogen. When subjected to a high-frequency spark for a week, milligram-quantities of glycine, alanine, and alpha-amino-n-butyric acid were produced. [53] Apparently, he was on the right track. Miller reported his early results in Science magazine in May 1953. 3 Norman Horowitz, a biologist from the California Institute of Technology, commented: "This experiment on organic synthesis on simulated primitive Earth atmosphere is the most convincing of all the experiments that have been done in this field." 4
Six years later Miller and Urey reported further on the implications of their research. The absence of hydrogen in Earth's present atmosphere was a clue. They had begun their study assuming that cosmic dust clouds, from which presumably the planets had been formed, contained a great excess of hydrogen. "The planets Jupiter, Saturn, Uranus and Neptune are known to have atmospheres of methane and ammonia,'' they noted, similar to primitive Earth's atmosphere. Given the lower temperatures and higher gravitational fields of these outer planets, time had not been sufficient for the excess hydrogen to escape. Miller and Urey held that Earth and the inner planets had "also started out with reducing atmospheres and that these atmospheres became oxydizing, due to the escape of hydrogen." Their production of amino acids in the laboratory indicated that before the development of an oxygen-rich atmosphere (the result of biological activity), the primitive environment was conducive to the formation of many different complex organic compounds. As soon as oxygen began to replace the hydrogen, experiments indicated that the spontaneous production of those compounds (amino acids) ceased. 5
Miller's experience in the laboratory spurred further research, and with it speculation reappeared about the presence of life on other planets. As Miller and Urey pointed out in 1959, living matter does not require oxygen to grow and flourish; it was "possible for life to exist on the earth and grow actively at temperatures ranging from 0°C, or perhaps a little lower, to about 70°C....Only Mars, Earth, and Venus conform to the general requirements so far as temperatures are concerned." 6 Because of the opacity of the heavy clouds on Venus, little could be deduced about the planet. Mars, on the other hand, had a clear atmosphere. Seasonal changes observed on the Martian surface suggested the possibility of vegetation.
The Red Planet became very important to the scientists searching for the origins of earthly forms of life. "If we find life on Mars, for example, and if see find that it is very similar to life on earth yet arose independently of terrestrial life, then we will be more convinced that our theories are right.'' Miller went on to argue:
The atmosphere of Mars would have been reducing when this planet was first formed, and the same organic compounds would have been synthesized in its atmosphere. Provided there were sufficient time and appropriate conditions of temperature, it seems likely that life arose on his planet. This is one of the important reasons for the tremendous interest in finding out if living organisms are on Mars and why most of all we want to examine these organisms. We want to examine them in biochemical detail, and this would involve bringing a sample back to the [54] earth. What are the basic components of these organisms? Do they have proteins, nucleic acids, sugar? If they are completely different, then our theories about the primitive earth and the results of this experiment seem not at all convincing. If Martian organisms are identical to the earth's organisms in basic components, then there seems to be the possibility that some cross-contamination occurred between the earth and Mars. But, if Martian organisms have small but significant difference, then it would seem that theirs was probably an independent evolution, under the kind of conditions that we envision as those of the primitive earth. 7
In 1959, Miller and Urey concluded, "Surely one of the most marvelous feats of the 20th-century would be the firm proof that life exists on another planet." They could have been addressing NASA when they added, "All the projected space flights and the high costs of such developments would be fully justified if they were able to establish the existence of life on either Mars or Venus." 8
Especially significant for the search for extraterrestrial life were developments in the field of comparative biochemistry. Nobel-Prize-winning geneticist Joshua Lederberg told a Stockholm audience in the spring of 1959 that "comparative biochemistry has consummated the unification of biology revitalized by Darwin one hundred years ago." For many years, Lederberg noted, there had been a "pedagogic cleavage of academic biology from medical education." Lederberg cited two other specialists in the field in making his point: "Since Pasteur's startling discoveries of the important role played by microbes in human affairs, microbiology as a science has always suffered from its eminent practical applications. By far the majority of the microbiological studies were undertaken to answer questions connected with the well-being of mankind." By the late 1950s, however, research into the chemical and genetic aspects of the microbiological world led medical and biological investigators to realize that their work had much in common. "Throughout the living world we see a common set of structural units-amino acids, coenzymes, nucleins, carbohydrates and so forth-from which every organism builds itself. The same holds for the fundamental processes of biosynthesis and of energy metabolism," 9 This global perspective on the underlying unity of life on Earth, together with the common chemical origin of the planets, made it not unreasonable to postulate the possibility of life on other bodies in the solar system. Furthermore, the discovery of life elsewhere would give biological theory a long-sought universality. The origin of life studies and the work in comparative biochemistry formed the intellectual foundation trial permitted respectable scientists to discuss the possible existence of extraterrestrial life.

* The process of separating a solution of closely related compounds by allowing a solution to seep through an absorbent paper so that each compounds becomes absorbed in a separate zone.