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?
The task group found it most convenient and logical to address the third question first. The reason for this approach is simple. The principal features that distinguish biotic and abiotic carbon compounds are closely related to the physical and chemical characteristics of organic compounds. Thus, these distinguishing criteria are elaborated in the context of a general introduction to organic chemistry in Chapter 1. With regard to the indicators that might differentiate between a biotic and an abiotic origin for particular organic compounds, the task group found that the most compelling indicators of an abiotic origin include the following:
The presence of a smooth distribution of organic compounds in a sample, e.g., a balance of even versus odd numbers of carbon atoms in alkanes;
The presence of all possible structures, patterns, isomers, and stereoisomers in a subset of compounds such as amino acids;
A balance of observed entantiomers; and
The lack of depletions or enrichments of certain isotopes with respect to the isotopic ratio normally expected.
Likewise, the converse of the above items is an indicator of possible biotic synthesis. Thus, for example, an imbalance of even versus odd numbers of carbon atoms in, for example, alkanes or the presence of only a small subset of all possible structures, patterns, isomers, and stereoisomers is an indicator of possible biotic origin. However, some abiotic processes can mimic biotic ones and vice versa, and inferences will necessarily be based on several indicators and will of course be probabilistic.
The answers to the first two questions—sources of reactants and energy that lead to abiotic synthesis and the distribution of organic compounds in the solar system—depend strongly on what part of the solar system is being considered. This report therefore deals separately with the various solar system environments—which range from the surfaces of cold, dark asteroids in remote, eccentric orbits to the hot, turbulent atmospheres of the giant gas planets. It considers what is known about the origins and histories of the organic materials in each setting. This discussion is contained in Chapters 2 through 6 of this report.
The fourth question, research opportunities, is addressed in each of those chapters as well. In addition, Chapter 7 outlines two general strategies recommended by the task group as integral to a planned approach to searching for and understanding organic material in the solar system.
In selecting the best research opportunities for enhancing understanding of organic material in the solar system, the task group considered the following factors:
The likelihood that significant organic material would be found;
The feasibility of the investigation; and
The likely impact or significance of the results.