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Space Studies Board: Annual Report 2007 5.7 Exploring Organic Environments in the Solar System A Report of the Ad Hoc Task Group on Organic Environments in the Solar System Executive Summary The sources, distributions, and transformations of organic compounds throughout the solar system are being studied actively. The results can provide information about the evolution of the solar system and about possibilities for life elsewhere in the universe. All life on Earth is based on the complex interplay of diverse carbon compounds. In short, the chemistry of carbon is the chemistry of life. But carbon is extremely versatile. Its compounds can be synthesized in many different ways and from a wide variety of starting materials, the vast majority of which have nothing to do with biology. The chemical reactions involving carbon can be driven by many different sources of energy and can occur in diverse environments, many of which are inimical to life as we understand it. Many carbon compounds are extremely hardy, and their preservation in the geological record can tell researchers much about processes and environmental conditions in the distant past. Similarly, other carbon compounds are extremely fragile. The presence of organic compounds in various astronomical environments can tell researchers much about the conditions that prevail today. The discovery of a single drop of oily residue on Mars, for example, would be enormously informative even if the residue were irrefutably abiotic in origin. Some might argue that such a discovery would be like finding an encyclopedia from a Mars library, which would tell linguists so much about the inhabitants even if they could not translate it. To recover the information carried by extraterrestrial carbon compounds, researchers must improve their ability to recognize the signals that point to specific syntheses and conditions. PURPOSE AND APPROACH OF THIS REPORT The purpose of this report is to tell the story of carbon: to follow carbon through a variety of terrestrial and extraterrestrial environments, to track its changes as it is subjected to a variety of physical and chemical processes, and to attempt to convey what the study of carbon and its compounds tells us about the origin and evolution of the solar system. In particular, the Task Group on Organic Environments in the Solar System surveys what is known about the sources of reduced carbon compounds throughout the solar system and examines how planetary exploration can improve our understanding. It is not the purpose of this report to recommend expensive new research activities and propose costly new initiatives. Rather, the task group’s goal is to place a variety of disparate activities in a unified context. As part of this process, the task group considers a number of closely related questions, including the following: 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 NOTE: “Executive Summary” reprinted from Exploring Organic Environments in the Solar System, The National Academies Press, Washington, D.C., 2007, pp. 1-8.
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Space Studies Board: Annual Report 2007 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. RECOMMENDED RESEARCH 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. The recommendations and a brief rationale are given below. A detailed discussion is presented in Chapters 2 through 6 of this report. Overall Approach to Research Two recommendations are better characterized as general strategies rather than specific opportunities: 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., aliphatic, aromatic, acetylenic); Individual compounds (e.g., methane/ethane); 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.
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Space Studies Board: Annual Report 2007 These objectives are 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 below (in “Selected Opportunities for Research”) will move NASA more smoothly toward ultimate success. For example, the task group proposes 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. 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 parts per million 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 The selected research opportunities were divided by the task group into three general categories based on the cost of the research and the time frame in which it could be undertaken. The recommended research is given by category below. Near-Term Opportunities The first category of research—near-term opportunities—includes ground-based studies that can be carried out in the very near term and for a minimal cost relative to the other recommended research activities. Chondritic and Mars Meteorites. Carbonaceous meteorites are an important source of abiotic, extraterrestrial carbon that is delivered to Earth at no cost. Together with the unequilibrated ordinary chondrites, a few martian meteorites, and fragments of crust from the earliest Earth, they represent immediately available samples of great relevance to studies of organic material in the solar system. New analyses of carbonaceous chondrites would benefit from modern analytical methods (e.g., compound-specific isotopic analysis) that allow the separation of signals from terrestrial contamination and indigenous extraterrestrial organic matter, thus overcoming a problem that severely hindered analyses throughout the 1960s and 1970s. A more sensitive and detailed analysis of carbonaceous chondrites is a cost-effective step that would be of great value in enhancing understanding of the formation of these organic materials and, therefore, yielding new information about organic-chemical processes in the early solar system. The results would provide reference points for comparison with the organics in samples returned by missions to other bodies in the solar system. Analyses should examine the following: The location and relative abundances of the organic molecules within the mineral matrices and on mineral surfaces; The structural composition of all organic phases including, to the greatest extent possible, any macromolecular material; The isotopic compositions of all molecules and other definable subfractions; and The nature of contaminants and the mechanisms by which samples can become contaminated, both before and after collection.
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Space Studies Board: Annual Report 2007 Recommendation: Plans should be developed for the establishment of an informal, community-based forum— modeled on the highly successful Mars Exploration Program Analysis Group (MEPAG)—charged to coordinate plans and develop priorities for the intensive investigation of the composition of organic materials in carbonaceous chondrites, SNC meteorites, and ordinary chondrites containing volatiles (including rare gases) that suggest relationships to the carbonaceous chondrites. The existing Curation and Analysis Planning Team for Extraterrestrial Materials (CAPTEM) may provide the seed from which such a community-based forum can be nurtured. To provide comparability and to bring the best techniques to bear on each object, samples should be shared extensively between laboratories. Martian Regolith Simulation. It has been proposed that any organic matter in the martian regolith will have been modified via reaction with strong oxidants present in the soil. Carefully designed laboratory experiments will allow an assessment of this problem and will point to the most effective strategies for direct analysis of organic materials by future Mars landers such as the Mars Science Laboratory. Regolith simulations may help address issues related to, for example, optimal minimum drilling depths for future Mars lander missions. Recommendation: Laboratory models of Mars soil chemistry should be used to study plausible mechanisms for the oxidative alteration of organic materials in the martian regolith and to evaluate their integrated effects. Materials studied should include likely exogenous products (organic compounds like those found in meteorites) as well as conceivable martian prebiotic and biotic products. Increasing the Supply of Meteorites Available for Study. The ready availability of and access to meteorites for laboratory studies, particularly the rare carbonaceous chondrites, is a key facet of the exploration of organic environments in the solar system. The preferred means for acquiring samples—collecting them in the field—has led to major searches in those places where meteorites are most likely to be spotted, the hot and cold deserts of the world. Both locations have their advantages and disadvantages; a detailed cost-benefit analysis of all of the relevant factors is beyond the scope of this report. There is, however, another approach to increasing the supply of meteorites: the selective purchase or exchange of important samples. Indeed, the task group suggests that the greatest near-term scientific impact from a given expenditure of funds will result not from the enhancement of meteorite collecting programs but rather from the acquisition by purchase or exchange of a significant piece of the Tagish Lake meteorite. Recommendation: The scientific significance of the Tagish Lake meteorite is such that NASA, the National Science Foundation, the Smithsonian Institution, and other relevant organizations and agencies in the United States and their counterparts in Canada should examine the means by which a significant portion of this fall can be acquired, by purchase, exchange, or some other mechanism, so that samples can be made more widely available for study by the scientific community. Laboratory Studies to Support Observations of Primitive Bodies. Laboratory studies are a prerequisite for all observational studies and are an essential precursor to the design of inherently expensive spacecraft instrumentation. At present, the relevant optical constants have been measured for only a few of the organic and inorganic compounds that are likely to be present in primitive bodies of interest. Without a suite of materials with known constants to incorporate in the spectral models, the identification of many of the observed spectral features remains challenging. With modest support for laboratory work of this kind, great progress could be made in understanding the organic component in these bodies. Recommendation: The physical, chemical, and spectroscopic properties of ices of potential hydrocarbon species should be studied to facilitate the detection of organic materials. Support for Telescope Studies of Organic Materials in the Solar System. Access to a small number of unique, publicly available, ground-based infrared astronomical facilities has enhanced and will continue to advance understanding of the organic constituents of various solar system bodies through direct observations and through observations conducted in support of spacecraft missions. Activities that would significantly enhance ground-based
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Space Studies Board: Annual Report 2007 observations of organic materials in the solar system include increasing NASA’s share of the observing time on the Keck telescope and replacing the NASA Infrared Telescope Facility with a larger instrument capable of making these observations. Recommendation: The task group reiterates the call made in the 2003 report of the National Research Council’s Solar System Exploration Decadal Survey Committee, New Frontiers in the Solar System, that NASA’s support for planetary observations with ground-based astronomical instruments, such as the Infrared Telescope Facility and the Keck telescopes, be continued and upgraded as appropriate, for as long as they provide significant scientific return and/or mission-critical support services.1 Interplanetary Dust and Molecules. Present particle-collection programs utilize aircraft and flights with other primary missions, and schedules are controlled by factors other than the timing of meteor showers. Quantitative yields and the ranges of materials sampled could be greatly improved if flights were timed to utilize these opportunities. Recommendation: A program specifically designed to collect dust in the stratosphere during meteor showers should be implemented. Relatively Near-Term Missions Consistent with Previous Decadal Strategy Study Recommendations in the category of relatively near-term missions are for research that can be implemented or carried out in 5 to 10 years and that is also supported by the findings and recommendations of the 2003 solar system exploration decadal survey report, New Frontiers in the Solar System.2 Mars. On Earth, the most suitable lithologies for the preservation and accumulation of organic matter are sedimentary rocks that are typically fine-grained and are characterized by well-defined, aqueously derived mineral assemblages. Thus, it may be possible to obtain additional information about the associated organic matter present in these mineral assemblages in a single measurement of the organic and inorganic material present. Recommendation: Currently planned missions to Mars should seek to identify silicified martian terrains associated with ancient low-temperature hot springs in concert with a high probability of ground ice deposits to locate organic materials formed on Mars. Similarly, the identification of shallow marine and/or lacustrine sediments would provide another terrain well worth exploring in future missions as sites for martian endogenous organosynthesis. As instrument development continues for future robotic missions to Mars, it is important that such missions be designed so that they are capable of assessing as fully as possible the inventory of organic matter there. Clearly such development should be strongly guided by the information provided by the Mars Exploration Rovers, Spirit and Opportunity. Although future robotic missions will be equipped with instrumentation to analyze samples, these analyses will never be able to achieve the capabilities of Earth-based laboratories. The discovery by the Mars Exploration Rovers of unambiguous sedimentary outcrops greatly increases the impetus for a martian sample-return mission. Similarly, the discovery of the halogens bromine and chlorine in abundance at the location of the Spirit rover landing site strongly suggests the former presence of surface water. Samples from either location might very well contain organic matter derived from extinct (or perhaps even extant) life. The successes of Spirit and Opportunity further validate the need to implement the 2003 solar system exploration decadal survey’s recommendation for a flagship mission to Mars—that is, to begin the developments necessary so that martian samples can be brought back to Earth for study in terrestrial laboratories as early as possible in the next decade.3 Far-Term Research Opportunities The far-term research recommended by the task group 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
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Space Studies Board: Annual Report 2007 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. Titan. Titan is believed to be a major reservoir of organic materials in the solar system, and the dynamic processes of Titan’s atmospheric chemistry provide an ongoing example of the abiotic formation of complex organics from methane. This satellite merits close scrutiny by continued ground-based observation and computer and laboratory modeling of its atmospheric chemistry. Recommendation: Planning should start now for a follow-up of the Cassini mission to Titan that would include a lander sent to sample its surface, since the complexity of the organics there is expected to be much greater than that of the organics in its atmosphere. The lander should have the capability of sampling organic materials that are solids at 96 K as well as those that are liquids. The Titan Explorer mission considered by the solar system exploration decadal survey is a good starting point for this planning. Primitive Bodies. The successful landing of the NEAR spacecraft on the asteroid Eros has demonstrated the feasibility of sending a probe to an asteroid. The solar system exploration decadal survey report recommended in situ and sample-return missions to asteroids and comets to provide direct information on the structures of the organic compounds present in comets and asteroids and to provide information about whether or not the asteroids are the sources of meteorites and dust reaching Earth. Recommendation: In situ analyses as well as sample-return missions should be performed for both asteroids and comets. The task group points to the solar system exploration decadal survey report’s recommended New Frontiers-class Comet Surface Sample Return mission4 as an example of an activity that would greatly enhance understanding of the organic constituents of the solar system’s primitive bodies. Current and upcoming missions are targeted to active comets; Pluto/Charon and perhaps one or two Kuiper Belt objects; and asteroids of spectral classes S, G, and V. Most of these missions have not been optimized for the study of organic materials even though the population of primitive small bodies may preserve organic materials from a wide range of nebular heliocentric distances. A rich research opportunity exists to explore these different chemical and thermal regimes, thus enabling an understanding of the distribution and history of organic materials in the solar system. Recommendation: Every opportunity should be taken to direct space missions to small bodies to do infrared spectral studies of these targets, especially a D- or P-type asteroid, to determine if these dark bodies contain an appreciable amount of carbon compounds and, if so, whether they are the sources of the carbonaceous meteorites and dust reaching Earth. In this regard, a possible opportunity is conducting such studies as an adjunct to the Trojan Asteroid/Centaur Reconnaissance flyby mission described in the solar system exploration decadal survey.5 Although this mission was not ranked in the survey’s final list of priorities, the possibility of using a single spacecraft to make a sequential flyby of three different classes of primitive bodies—i.e., a D- or P-type main-belt asteroid, a jovian Trojan asteroid, and a Centaur—has sufficient merit to warrant additional study for possible implementation as a New Frontiers mission at some time in the future. Europa, Callisto, and Ganymede. In the early 2000s, NASA’s solar system exploration plans included a Europa Orbiter mission that would undertake flyby observations of Callisto and Ganymede prior to entering orbit about Europa. Although excessive cost growth led to the cancellation of this mission, scientific interest in the study of Jupiter’s large, icy satellites continues to be strong. The Europa Geophysical Explorer, a somewhat more elaborate version of the Europa Orbiter, was the highest-priority large mission recommended by the 2003 solar system exploration decadal survey.6 NASA responded to the survey’s recommendation by initiating the development of the Jupiter Icy Moons Orbiter (JIMO) mission, the first of a line of advanced-technology spacecraft with significantly expanded science capabilities compared to previous concepts for missions to Europa. JIMO would have conducted
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Space Studies Board: Annual Report 2007 global mapping of all three icy satellites, at resolutions of 10 m or better, and might have included a small Europa lander. Organic materials can be studied by making provisions for high-signal-to-noise-ratio spectroscopy at resolutions adequate to discriminate potential carbon-bearing species in both high- and low-albedo regions. JIMO was indefinitely deferred in 2005, and NASA and the planetary science community are currently assessing plans for a more conventional and very much less expensive alternative.7 Recommendation: The task group reiterates the solar system exploration decadal survey’s findings and conclusions with respect to the exploration of Europa and recommends that NASA and the space science community develop a strategy for the development of a capable Europa orbiter mission and that such a mission be launched as soon as it is financially and programmatically feasible. Any future Europa lander mission should be equipped with a mass spectrometer capable of identifying simple organic materials in a background of water and hydrated silicates. NOTES 1. National Research Council (NRC), New Frontiers in the Solar System: An Integrated Exploration Strategy, The National Academies Press, Washington, D.C., 2003, pp. 206-207. 2. NRC, New Frontiers in the Solar System: An Integrated Exploration Strategy, 2003. 3. NRC, New Frontiers in the Solar System: An Integrated Exploration Strategy, 2003, pp. 198-200. 4. NRC, New Frontiers in the Solar System: An Integrated Exploration Strategy, 2003, p. 195. 5. NRC, New Frontiers in the Solar System: An Integrated Exploration Strategy, 2003, p. 25. 6. NRC, New Frontiers in the Solar System: An Integrated Exploration Strategy, 2003, p. 4. 7. NRC, Priorities in Space Science Enabled by Nuclear Power and Propulsion, The National Academies Press, Washington, D.C., 2006, pp. 17-20.