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Exploring Organic Environments in the Solar System III —Exploration: Where to Go and What to Study
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Exploring Organic Environments in the Solar System 7 Approaches to Research Intensive, programmatic studies of missions being directed to specific locations in the solar system can seek to identify the most efficient means of exploration, the pathway that would provide the greatest amount of decisive information in return for the simplest, quickest, and cheapest measurements and analyses. Such studies inevitably reflect diverse factors. The task group’s purposes in this study are to focus on organic matter rather than on specific missions or locations, to call attention to objectives of particular importance, and to consider issues that might cut across otherwise-separate programs. To accomplish this, the task group set out to address the following questions: What are the sources of reactants and energy that lead to abiotic synthesis of organic compounds and to their alteration in diverse solar system environments? What are the distribution and history of reduced carbon compounds in the solar system, and which features of that distribution and history, or of the compounds themselves, can be used to discriminate among synthesis and alteration processes? What are the criteria that distinguish abiotic from biotic organic compounds? What aspects of the study of organic compounds in the solar system can be accomplished from ground-based studies (theoretical, laboratory, and astronomical), Earth orbit, and planetary missions (orbiters, landers, and sample return), and which new capabilities might have the greatest impact on each? With these questions in mind, the task group sought to identify reservoirs of organic material in the solar system and to consider what is known about their history as well as their present composition. Two broad questions can be identified: What are the relationships between organic materials in diverse extraterrestrial settings such as planetary and satellite regoliths, asteroids, comets, and meteorites? What processes produced the organic materials? Much can be inferred from compositional and isotopic data. For example, a particular set of organic compounds might be found in interstellar media. The same materials might occur in comets, and plausibly related materials could turn up in meteorites and on asteroids. The pattern would indicate a possible line of inheritance, showing that, at the time of its origin, the solar system incorporated interstellar organic material. The hypothesis could be
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Exploring Organic Environments in the Solar System reinforced or even confirmed decisively if the ratio of 13C to 12C were the same in all of the materials studied. A finding of shared origins would be profoundly significant. The specific roster of identified compounds would be interesting, but the more general point that the organic chemistry of the solar system was connected to that of the broader cosmos would have many ramifications. If one set of compounds survived, what about others? If the solar system contained significant quantities of organic material from the outset, what consequences followed? Clearly, ongoing broad surveys of organic materials, particularly those that provide data that can establish relationships between diverse locales, should be encouraged. In such work, breadth, i.e., the examination of materials from the widest possible range of settings, could be as important as detail. A second aspect of molecular history concerns synthetic processes. Wherever some organic material is found, and however it might be related to similar materials elsewhere, by exactly what process was it made? Were the atoms drawn from gaseous precursors? Did they combine on a surface? Can patterns of repetition (polymerization) be recognized? Were the chemical reactions highly selective, leading to only a few molecular structures, or was the range of products diverse? Were living organisms involved? For answering such questions, detail is essential. Investigators would like to determine precisely the structure, abundance, and isotopic composition of every compound. It would be better still to determine the distributions of isotopes within each compound (i.e., intramolecular patterns of isotopic order that could reveal how the components of the molecule were assembled). Since living organisms often utilize minerals in their metabolic processes, it will be important also to investigate the inorganic phases associated with the organic materials. Such thorough analysis will require samples large enough to sustain extensive dissection. GENERAL STRATEGIES Two general strategies for approaching both ground-based research and research carried out by flight missions are as follows: Recommendation: Strategy 1—Every opportunity should be seized to increase the breadth and detail in inventories of organic material in the solar system. As results accumulate, each succeeding investigation should be structured to provide information that will allow improved comparisons between environments. Analyses should determine abundance ratios for the following: Compound classes (e.g., acetylenic, aliphatic, aromatic); Individual compounds (e.g., methane/ethane, HCN/HNC); Elements in organic material (e.g., C, H, N, O, S); and The isotopes of elements such as C, H, N, and O. Investigators should strive to interpret these results in terms of precursor-product relationships. These objectives are rudimentary compared to studies of, for example, the chirality of amino acids. They are, however, broadly applicable and represent systematic steps toward addressing questions of biogenicity, lines of inheritance of organic material, and mechanisms of synthesis. With limited funds, returns from investigations like those proposed as opportunities for research in Chapters 2 through 6 will move more smoothly toward ultimate success. For example, it is proposed that newer, more sensitive and specific analytical methods be used for the analysis and reanalysis of carbonaceous chondrites. As these studies proceed and the results from flight experiments are obtained, it will become apparent which of these new techniques should be adapted to flight experiments. Moreover, the ground-based investigations of chondrites will pave the way for better analyses of returned samples, whenever they become available.
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Exploring Organic Environments in the Solar System Recommendation: Strategy 2—Organic-carbon-related flight objectives should be coordinated across missions and structured to provide a stepwise accumulation of basic results. Some of the objectives that should be included in such missions are as follows: Quantitation of the amount of organic carbon present to ± 30 percent precision and accuracy over a range of 0.1 ppm to 1 percent; Repetitive analyses of diverse samples at each landing site; Comparability so that relatable data are obtained from a wide range of sites; and Elemental and isotopic analyses so that the composition (H/C, N/C, O/C, and S/C) is obtained together with the isotope ratios of all the carbon-bearing phases. These recommended approaches to research will allow scientists to build an overview of the distribution of organic carbon in the solar system; provide information about heterogeneity at each location studied; and support preliminary estimates of relationships, if any, between organic materials at diverse sites. SELECTED OPPORTUNITIES FOR RESEARCH Chapters 2 through 6 discuss a large number of research investigations that have the potential to significantly increase knowledge about the sources and history of carbon in the solar system. From those, the task group selected the research that seemed to promise the greatest return on investment. The selected research opportunities were then divided into three general categories based on the cost of the research and the time frame in which it could be carried out: Ground-based studies that can be carried out in the very near term and for a minimal cost relative to the other recommended research; Studies that can be carried out in the relatively near term—5 to 10 years—and are also supported by the findings and recommendations of the NRC’s 2003 solar system exploration decadal strategy report,1 which surveyed the broad community of scientists studying various aspects of the solar system, and, through a series of workshops and meetings, developed a roadmap of prioritized research for the next decade; and Far-term research recommended for its potential to expand knowledge of carbon compounds in the solar system and that would probably be carried out 10 years or more in the future but might require some near-term planning. This recommended research is ranked in terms of its potential for expanding knowledge of carbon compounds in the solar system and for its close relationship to research and missions currently in progress or recently completed. The recommended research is presented in Chapters 2 through 6 and is summarized in the Executive Summary. NOTE 1. National Research Council, New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003.
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Representative terms from entire chapter: