Thus far, these chemical analyses point to an abiogenic origin for the organic matter, and no conclusive evidence exists of biological activity on the meteorite parent body. Microbiological investigations of samples of the carbonaceous chondrites have yielded only inconclusive evidence on the problem of "organized elements."
Several of these microstructures from different carbonaceous chondrites are illustrated in a paper by Mamikunian and Briggs ([ref.30]). It has been difficult to identify the organized structures, and most do not have morphologies identical to known terrestrial micro-organisms. However, they may prove to be a variety of mineral grains, droplets of organic matter and sulfur, as well as a small amount of contaminating terrestrial debris.
A comparison between the photographs of the organized elements observed in the Orgueil and Ivuna meteorites and the synthetic proteinoid microspheres observed by Fox ([ref.25]) point to similarities between the two. One inference from this finding is that the organized elements in carbonaceous chondrites were never alive but, rather, should be considered as natural experiments in molecular evolution. Also, these similarities strengthen the belief that the laboratory experiments are similar to the natural experiments in space.
In cooperation with the Smithsonian Astrophysical Observatory, NASA has a network to track meteors in the Midwest (South Dakota, Nebraska, Kansas, Oklahoma, Iowa, Missouri, and Illinois). Photographs of meteor trails are used for scientific study, and attempts are made to track and recover meteorites for examination for traces of organic material of extraterrestrial origin.
Fundamental research in terrestrial organic geochemistry has shown that ancient sediments and drill core samples subjected to organic analysis contain certain stable biochemical components of past life. This preserved record is significant not only in studies of early-life chemical pathways but also in studies of the interaction of organic matter with the geological factors. Since life on any planetary body will interact with the soil, or surface material, it is of interest to understand the relationship.
CONCEPTS FOR DETECTION OF EXTRATERRESTRIAL LIFE
It is not possible to present completely convincing evidence for the existence of extraterrestrial life. The problem often reduces to probabilities and to estimates of observational reliability. In almost all cases the evidence is optimistically considered strongly suggestive of—or, at the worst, not inconsistent with—the existence of extraterrestrial life. Alternatively, there is a pessimistic view that the evidence advanced for extraterrestrial life is unconvincing, irrelevant, or has another, nonbiological explanation.
In studies of the laboratory synthesis of life-related compounds and its significance concerning the origin of life, several results seem to suggest that organochemical synthesis is a general process, occurring perhaps on all planets which retain a reducing atmosphere. The temperature ranges must be such that precursors and reaction products are not thermally dissociated. The reaction rates for the synthesis of more complex organic molecules diminish to a negligible value when the temperature range is below 100° C.
Besides the planetary parameter of temperature, an even more fundamental necessity for a living state exists—a liquid solvent system. For terrestrial life forms, water serves this purpose. Water has this and other properties of biological significance because of hydrogen bonding between adjacent molecules in the liquid state.
Ultraviolet radiation could serve as an extraterrestrial energy source for organic synthesis. Research shows that, while an atmosphere is important, living systems can survive a wide range of ambient pressures and are little affected by a wide range of magnetic field strengths.