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6 July 1998 Stable isotopes as probes of terrestrial and extraterrestrial biotic and abiotic synthesis of organic matter
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Determining how, where and when life as we know it originated are three of the most challenging questions confronting scientists. The fact that the Earth has likely been infected with life as far back in time as the rock record extends makes it almost futile to search for life's precursors on this planet. There is no definitive way to distinguish structures of the key biomonomers essential for life's origin from the remnants of once living organisms that have accumulated or recycled through the geosphere for approximately the past 4 billion years. While extraterrestrial materials that have fallen to Earth have been probed for clues to life's origin elsewhere in the solar system, these materials also, upon impact, become part of the Earth system. Thus, their subsequent contamination via exchange with terrestrial biota is always a realistic possibility. Exploring other planetary bodies is the third possible way to trace life's origin or the origin of organic compounds that preceded life. However, it is almost impossible to expect that manned or unmanned probes will not to some extent infect the surfaces of these bodies during contact. We propose that one possible way to begin to distinguish abiotic compounds from biotic compounds as well as terrestrial vs. extraterrestrial compounds is an assessment of the stable isotope compositions of the light elements that comprise them relative to those of the substrates from which they were formed.
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Michael H. Engel and Stephen A. Macko "Stable isotopes as probes of terrestrial and extraterrestrial biotic and abiotic synthesis of organic matter", Proc. SPIE 3441, Instruments, Methods, and Missions for Astrobiology, (6 July 1998);

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