Meteorites and Comets: Organic Matter and Exobiological Hypotheses, 1834-Present by Norman Redington and Karen Ræ Keck
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Citing the abundant presence of polycyclic aromatic hydrocarbons, carbonate globules similar to terrestrial fossil bacteria, and oxidized sulfide compounds, Mc Kay et al., argue that ALH84001, an SNC meteorite, may contain fossil micro-organisms and point to the possibilty of life on Mars, if not now, certainly at some time in the past. Mc Kay and his colleagues are entering a debate that is over a century old and has resumed about every thirty years in the XX Century.
Scientists of the XIX Century discovered that some meteorites have a carbonaceous nature, and several reported finding organic material in meteorites or extracting organic substances from them. J.J. Berzelius, for example, reported in 1834 that he had found humic acid in the Alais meteorite (a carbonaceous chondrite); he did not believe that it was biological. A number of researchers found coal-like and petroleum-like substances in meteorites. Since terrestrial coal and petroleum are thought to form from compression of decomposed biological materials, the presence of similar substances in meteorites gave rise to speculation that life existed elsewhere in the solar system. The possibility that samples were contaminated by terrestrial substances was considered, but most researchers concluded that meteoritic compounds exist and, although similar to biological terrestrial matter, are abiotic. The debate about the nature of seemingly biological structures in meteorites, however, continues.
The most widely-accepted interpretation of organic compounds in meteorites is that carbon compounds are probably abiotic but have the potential to become life in the right conditions. Since many meteorites are believed to be at least as old as the solar nebula, these prebiotic molecules could be the precursors of life as we know it.
Most of the earlier explanations for the origin of organic molecules in meteorites (other than those assuming all such molecules to be either contaminants or artifacts of the extraction process) involved processes taking place during the accretion process which formed the meteorite parent body. Several pathways have been suggested. One of the most popular is the Fischer-Tropsch reaction in which metal catalyses the formation of complex organic compounds (amino acids, for example) from inorganic gases. Another is the Miller-Urey reaction, by which methane is evolved into more complex molecules with energy supplied by ultraviolet light or electric discharges. More recently, however, opinion has shifted; it is thought that many organic molecules formed, perhaps by ion-molecule reactions, in the pre-accretion gas cloud or even in interstellar space, or alternately that they resulted from reactions taking place on the parent body between liquid water (the presence of which seems increasingly certain) and anhydrous minerals.
Another possible explanation of the organic substances found in meteorites is that they are produced by recently or presently living organisms. Out of this interpretation come the possibility that life does not arise separately on celestial bodies but has moved from one place to another. The Panspermia debate is nearly as old as the life on meteorites controversy and has found expression in this century in the work of Sir Fred Hoyle and Chandra Wickramasinghe.
Originally, Panspermia was an anti-Darwinian argument. Hermann von Helmholtz and William Thomson (Lord Kelvin) proposed that life arrived on earth from the heavens. One critic remarked that one might as well have God breathing life onto the waters as to rely on meteorites bearing life. Svante Arrhenius took up the idea, but it has been outside mainstream scientific thought. In the 1930's, Charles Lipman claimed to have cultured living organisms from a meteorite, but no one was able to replicate or substantiate his claim.
More recently, Hoyle and Wickramasinghe have revived the concept of Panspermia. They believe that molecular clouds contain interstellar grains coated with complex polymers, grains which may even be organisms. They have also proposed that life may have evolved in the (hypothetical) radioactive liquid cores of early comets and that life could be present in comets even now. They suggest that epidemics are the result of comet-borne viruses. In later work, they propose that viruses are continually being diffused throughout the universe and that this diffusion may be planned.
These hypotheses created much controversy and gained few followers. However, the ideas of prebiotic organic grains in interstellar clouds and organic ices in comets have gained some acceptance. The suggestion that comets bearing prebiotic organic matter contributed to the origin of life has many adherents, although it is unclear how much of this material could survive an impact.
Most scientists in meteorite oganic chemistry focus on the study of carbonaceous chondrites and the organic material found in them. The composition of these meteorites is similar to the composition of the sun. As a result, they are considered to be ancient objects, relics of the early solar system. Some believe that these chondrites and the organic compounds in them formed in the solar nebula, while others believe that the chondrites are the remnants of a parent body that broke up. The latter view is currently the more accepted hypothesis.
For a long time, scientists questioned whether organic compounds in meteorites were indigenous or were earthly contaminants. The former idea has gained acceptance today because analyses have shown that the isotopic signatures of meteoritic and terrestrial organic compounds are different; that amino acids and other complex biomolecules in meteorites are racemic (that is, in contrast to biological specimens, right-handed and left-handed forms are equally represented); and that even and odd chains occur equally in meteoritic but not in terrestrial (biological) monocarboxylic acids. These findings show meteoritic organic compounds are abiotic and, therefore. assumed to be of extraterrestrial origin.
However, not all scientists accept the abiotic assumption. In 1961, B. Nagy reported finding non-racemic indigenous meteoritic amino acids and actual microfossils in carbonaceous chondrites. H.C. Urey, a strong supporter of Nagy, proposed in the early '60's that the moon is the source of carbonaceous chondrites. He believed that the earth had captured the moon and in the process had splashed it with water, whence the organic compounds came. Evidence from the Apollo missions failed to support Urey's hypotheses. Most scientists have since rejected Nagy's claims and consider so-called microfossils to be something else, such as mineral inclusions or contaminants.
Carbonaceous chondrites are not the only type of meteorites said by some to bear evidence of life. In the 1880's, O. Hahn and D.F. Weinland claimed to have found fossil sponges and other multi-celled organisms in ordinary chondritic meteorites. They claimed, in addition, that Widdstätten patterns were evidence of algal growth. No one took these ideas seriously, and they survive as scientific curiosities. Many scientists have, however, found organic compounds in ordinary chondrites and in iron meteorites, but this work has generated only a small literature compared to the study of carbonaceous chondrites.
In the 1990's, Z.A. Mushina and E.A. Glebovskaya, Bulgarian researchers, have proposed that the Sikhote-Alin iron meteorite is similar to a Silurian fossiliferous rock that has been metamorphosed. Their paper, published in Russian, has not generated the flurry of interest that McKay, et al.'s has.
From the beginning of the study of organic compounds in meteorites in the XIX Century, scientists have debated their nature and origin. Although a vast literature has grown in the last 150 years, the debate recurs in the XX Century about once every thirty years. In the 1930's, Lipman's claim generated debate, which ended when people failed to culture organisms from meteorite samples. In the 1960's, Nagy and Urey were chief figures in a controversy that made the pages of the New York Times. In the 1990's, the discovery of a possible microfossil in a Martian meteorite has begun a new round in the debate. Perhaps as technology advances, we will see a definite answer to these questions.