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The Cosmic History of the Biogenic Elements and Compounds INTRODUCTION From our terrestrial perspective it is difficult to conceive of life forms in which the elements hydrogen, carbon, oxygen, nitrogen, sulfur, and phos- phorus do not play a predominant role. That they do indeed play such a role throughout the universe seems highly probable, in part because (apart from phosphorus) these are the most abundant elements throughout the cos- mos and they occur in significant quantities among the building blocks of terrestrial planets as represented by the primitive chondrites and comets. Moreover, their chemistry is particularly well suited to the development of the complex structures and functions characteristic of living systems. Since the Sun and planets formed only some 4.6 billion years ago in a universe whose age is perhaps 15 billion years, it is clear that these "biogenic ele- ments" experienced a long and complex chemical history before being in- corporated into terrestrial biochemistry. At present it is not known whether this prior history played a direct role in the origin of life on Earth. What is clear is that astrochemistry is to a large extent the chemistry of the biogenic elements and that understanding the nature and evolution of chemical com- plexity throughout the universe is crucial to understanding both the early chemical state of our own solar system and the frequency with which simi- lar or related conditions exist elsewhere in our galaxy and other galaxies. There is, in addition, increasingly suggestive evidence for the survival of interstellar molecular material within objects present in the solar system today. Such evidence comes from studies of the isotopic compositions of the carbonaceous components of certain meteorites and from the inferred chemical composition of cometary nuclei, the latter supported by models derived from recent spacecraft encounters. Moreover, some current models 21
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22 THE SEARCH FOR LIFE'S ORIGINS of the solar nebula suggest that the bulk of the Earth's volatiles would not have condensed at 1 AU from the Sun, implying that they were provided by a bombardment of the Earth by volatile-rich cometary and meteoroidal de- bris, which may well have contained interstellar components. At the least, these ideas imply links between the chemistry of primitive objects in the solar system and the interstellar environment in which the Sun and planets formed and that such links involve the chemistry of the elements necessary for the origin of life on Earth. Certainly, a knowledge of the chemistry and physics of both interstellar clouds and the solar nebula will provide crucial information on how and from what materials the solar system was formed. The cosmic history of the biogenic elements and their compounds thus becomes a critical field of study for exobiologists. Apart from hydrogen, which is for all essential purposes primordial, these elements are formed in the interiors of stars and returned to the interstellar medium either in the violent events accompanying the late stages of evolution of a massive star (supernova explosions) or in the even larger amounts of processed material expelled continuously or episodically from stars in late stages of their life cycles. The subsequent of chemical complexity is a complicated and still poorly understood story, involving condensation of particulate material ("dust") in the outflowing envelopes around evolved stars, gas-phase reac- tions that build complex organic molecules in dense interstellar clouds of gas and dust, and interaction of the particulate and gaseous phases with the interstellar radiation field and cosmic rays. In circumstellar and interstellar regions, astronomers have unequivocally identified gaseous organic molecules with up to 13 atoms and molecular weights twice that of glycine, the simplest amino acid. Although the pres- ence of an interstellar "dust" component has been known for more than 50 years, its composition, structure, and special variations are still subjects of heated controversy. Evidence is accumulating, however, that the size of these dust particles may well overlap that of large molecules, and their composition, in terms of biogenic compounds, quite likely ranges from water and amorphous carbon or graphite to complex, heterocyclic organic poly- mers. It is within the denser interstellar clouds that new stars and planetary systems form. The details of this process are not well understood. None- theless, it is generally accepted that the physical and chemical properties of the biogenic compounds play a crucial role in the thermodynamics of star formation, because radiative energy loss from these molecules allows the cloud to cool and hence to collapse. Moreover, the trace biogenic constitu- ents provide critical probes of the physical, chemical, and kinematic states of both interstellar clouds and protostellar systems, by way of their rotational and vibrational transitions observable at radio and infrared wavelengths.
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AD COMPOUNDS 23 How much of this interstellar chemistry is preserved as the parent mo- lecular cloud collapses to yield the protosun, as the accretion disk develops into the solar nebula, and as the building blocks of the planets accrete, is uncertain. The investigation of possible links between the chemistries of these stages in solar-system formation, and the determination of physical and chemical conditions during this process by studying both primitive objects and molecular clouds, are fascinating and crucial areas to be ex- plored in coming years. It is clear that the solar nebula was not in chemical equilibrium. Can local kinetic processes mimic those that occurred under interstellar conditions? What new organic compounds might have formed in the solar nebula or on the primitive bodies of the solar system? Can the processes operating on primitive bodies give insight into chemical evolu- tion on Earth? Detailed analysis of the chemistry and structure of com- pounds and phases containing the biogenic elements in surviving primitive material, including comets, can probe such questions. The single overriding goal of this phase of evolutionary history is stated below. Five major objectives contributing to the achievement of this goal follow. GOAL: To understand the history of physical and chemical transforma- tions undergone by the biogenic elements and compounds, from nucleosyn- thesis to their incorporation and subsequent modification in preplanetary bodies. OBJECTIVE 1: To determine the extent and the evolution of molecular complexity in interstellar and circumstellar environments. Almost 20 years have passed since the first gaseous polyatomic mole- cules ammonia (NH3) and water (H2O) were discovered in the interstel- lar medium via the technique of radio astronomy. Since that time, more than 80 different molecular species and numerous isotopic modifications have been identified unambiguously in the gas phase. Most of these species have been detected through their rotational transition frequencies at radio wavelengths (the term "radio" is often used to apply to wavelengths of 1 mm or less), although a few molecules, especially in circumstellar sources, have been characterized by their vibrational spectra in the infrared. The detected molecules range in complexity from diatomics such as hydrogen (H2) and carbon monoxide (CO) to a 13-atom unsaturated linear nitrite HEWN and include many simple organic molecules (Table 2.1~. Typically, molecules involving the biogenic elements carbon, nitrogen, and oxygen are trace constituents of a gas dominated by molecular hydrogen. Nevertheless, the large mass of "dense" interstellar clouds implies that there is substan- tially more organic matter in a typical cloud than on the Earth. In addition to the existence of organic molecules, dense interstellar clouds have other
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24 TABLE 2.1 Identified Interstellar Molecules THE SEARCH FOR LIFE'S ORIGINS Simple Hydrides, Oxides, Sulfides, Halides, and Related Molecules H2 CO NH3 CS HC1 SiO SiH4X SiS H2O so2 CC H2S OCS CH4X PN HNO (?) SiCX Naclx Alclx KClx AlFX Nitrites, Acetylene Derivatives, and Related Molecules HCN HC_C CN H3C C_C CN H3C CH2 CN H2C=CH2X H3CCN H(C_C)2 CN H3C C-CH H2C=CH CN HC_CHX CCO(?) H(C-C)3 CN H3C- (C_C)2 H HNC CCCO H(C_C)4 CN H3C (C_C)2óCN? HN=C=0 CCCS H(C_C)s CN HN=C=S HC_CCHO H3CNC Aldehydes, Alcohols, Ethers, Ketones, Amides, and Related Molecules H2C=0 H3COH HO CH=0 H2C=S H3CóCH2 OH H3C O CH=0 H3C CH=0 H3CSH H3C O CH3 NH2 CH=0 (CH312CO (?) H2C=C=0 Cyclic Molecules C3H2 SiC2 C3H ions CH+ HCO+ H3O+ (?) H2D+ (?) HOCO+ HCNH+ HN2+ HCS+ SO+ HOC+ (?) Radicals CH C3H CN HCO C2S OH C4H C3N NO SO C2H CsH H2CCN NS C6H H2CNH H3CNH2 H2NCN NOTE: The superscript x indicates detection only in the envelopes around evolved stars. A question mark (?) indicates molecules claimed but not yet confirmed. features in common, such as a preponderance of gas-phase matter (with perhaps 1 percent of the material in the form of solid dust grains), tempera- tures well below those on Earth (10 to 100 K), gas densities quite low by terrestrial standards (103 to 106 molecules/cm3), and chemically "reducing" environments in which H2 is the dominant molecular species but in which oxygenated molecules also exist. The most massive objects, "giant" mo- lecular clouds, are larger, hotter (100 K versus 10 K), and show more evidence of past and present massive star formations than the much smaller
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 25 "dark clouds." Circumstellar sources appear to exhibit a significant organic chemistry only if, unlike most objects, they are carbon- rather than oxygen- rich. In such stellar envelopes, the gas density and temperature are severe functions of the distance from the center of the star. Although the spectra of many gaseous molecules have been detected in interstellar and circumstellar sources, most astronomers have been less inter- ested in the chemical composition of these sources than in utilizing the spectra of abundant species such as CO and NH3 to probe the prevailing physical conditions. High priority in the next decade should therefore be accorded to a systematic study of the chemical composition of interstellar and circumstellar clouds. Of particular interest in the context of exobiology are studies of the degree of molecular complexity that can be attained and of the diversity of chemical compositions that are produced as a result of evolutionary effects and different physical conditions. SYSTEMATIC STUDIES OF INTERSTELLAR CLOUDS Although much has been learned about individual interstellar and cir- cumstellar sources, a systematic study of the gas-phase chemistry of any of these sources has not yet been achieved, even though portions of such studies are available. A systematic study would entail determination of the following: the chemical state of the major elements, including the biogenic ones; isotopic abundances and isotopic fractionation effects; the way in which abundances of major constituents vary as functions of position and physical conditions within the cloud and possible cloud history; and the extent of molecular complexity (see below). Consider oxygen as an example of how little is known about the domi- nant repositories of the major elements. It is argued indirectly that oxygen (O2) and water (H2O) are probably the most abundant oxygen-containing species after CO in the gas phase of interstellar and circumstellar clouds, and yet it is difficult to study these species from the ground or even from aircraft because of atmospheric absorption. A strategy for determining the abundances of these important gas-phase species via their millimeter and submillimeter transitions requires space-based instrumentation, perhaps ini- tially of the Explorer class, but ultimately employing the higher angular resolution of the proposed Large Deployable Reflector (LDR) and the Space Infrared Telescope Facility (SIRTF) spacecraft (Space Science in the Twenty- First Century, SSB, 1988a,c). Consider, as a second example, the case of carbon. A significant fraction of carbon abundance in the gas phase is in the form of CO. However, it is unclear how much is in the form of carbon dioxide (CO2) or simple hydrocarbons such as methane (CH4) and acetylene (C2H2), because these nonpolar species do not possess strong rotational spectra. To determine their importance, infrared techniques will have to be
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26 THE SEARCH FOR LIFE'S ORIGINS utilized to observe vibrational transitions. It is clear from atmospheric ab- sorption at these wavelengths and from the currently limited sensitivity of ground-based infrared telescopes that significant progress will be made by high-spectral-resolution detectors in space, including those employing hetero- dyne techniques. Another subject of considerable interest is that of isotopic fractionation, which may provide the most accurate "fingerprints" of interstellar processes that are preserved in comets and primitive asteroids. Low-temperature inter- stellar clouds lead to strong fractionation effects, especially with regard to deuterium/hydrogen (HD/H2) abundance ratios in trace species. For ex- ample, although the interstellar abundance ratio HD/H2 is approximately 1-2 x 10-5, the abundance ratio between other deuterated species and their hydrogen analog can be higher than 0.01 in cold clouds. This effect is un- derstood theoretically and occurs because the reactions between molecular ions and neutral molecules that dominate the chemistry at low temperature (ion-molecule reactions) can only proceed rapidly in exothermic directions. More systematic observations of selected fractionation ratios as functions of cloud temperature and density are required to refine current theories fur- ther. Once these theories have become more quantitative, theoretical treat- ments of how these isotopic ratios can be preserved as the interstellar cloud becomes a protosolar nebula will be most useful. Some studies of the variations in abundance of selected species as func- tions of position and physical conditions within clouds are under way. For example, radio astronomers have begun to probe selected regions in the Orion nebula, a prototype giant molecular cloud. A variety of chemically unusual regions, associated to a greater or lesser extent with star formation, have already been delineated. It would be of interest to devote similar attention to lower-mass clouds such as TMC-1 and L183, because such smaller and colder regions may lead upon collapse to solar-type stars. Complementary to the studies discussed above are broad surveys of the radio line spectra of interstellar sources: knowledge of the radio frequency spectra of most molecular clouds is extremely patchy. Systematic maps of the spectra have thus far been partially accomplished for only two giant interstellar clouds, that in Orion and one near the galactic center. These surveys, which detected on the order of 1000 emission lines, have resulted in a significant increase in the amount of chemical information available (see Figure 2.1~. A similar survey of the dark cloud TMC-1 would be most worthwhile because it is a precursor of solar-type stars. Because clouds such as TMC-1 are so cool (10 K) and have little turbulence, spectral line widths are very narrow, making a survey much more difficult than in the giant clouds. To survey TMC-1 over a wide range of frequencies, a broad- band high-resolution spectroscopic capability is required. Some of the in- strumentation being developed in the SETI program may be useful here
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS ED COMPOUNDS 27 1 1 1 1 1 1 1 1 1 1 1 Orion Molecular Cloud 2.0 1.0 - ~: o.o 0.6 0.4 0.2 0.0 SOo _ CH3OH ~ CH3OH _~` _c ~ ~ 1 IRC + 1 0216 C4H C3H w~,/~ 1 1 1 1 1 1 1 1 1 1 1 76000 76100 76200 76300 Frequency (MHz) 76400 76500 FIGURE 2.1 Portions of the millimeter wavelength spectra of a dense interstellar cloud (Orion) and the envelope around an evolved star (IRC+10216~. (Astronomy and Astrophysics for the 1980s, National Research Council, 1982, 1983a,b). In the long run it is essential to broaden such studies to include a large sample of clouds. Will other abundance patterns be found? Will evolution- ary effects on chemical composition emerge? Then, eventually, can the analogous chemistry be probed in external galaxies? COMPLEX MOLECULES Biochemistry is clearly the chemistry of large, complex, organic mole- cules. The largest molecule unambiguously observed in the gas phase of interstellar and circumstellar clouds is PECAN. Although infrared spectra provide evidence for far larger species (such as polycyclic aromatic hydro- carbons; see discussion following objective 2), specific molecules have not been identified from the existing low-resolution spectra. To extend gas- phase high-resolution radio astronomical methods to search for molecules considerably larger than 13 atoms will require continually improving elec- tronics and a strategy involving laboratory studies. The laboratory work is necessary because many species more complex than 13 atoms have not been
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28 TlIE SEARCH FOR LIFE'S ORIGINS studied spectroscopically in the gas phase, especially in the radio and milli- meter wave regions where their characteristic rotational transitions lie. The larger a molecule, the higher is its density of rotational levels, so that the intensity available in a single transition diminishes. Thus, to observe single rotational transitions of complex molecules in interstellar sources will re- quire more sensitive instrumentation and large amounts of searching time. In addition, as molecules grow in complexity, their most intense spectral lines shift toward lower frequencies. Current plans to enhance the capabili- ties of the large (300 m), low-frequency radio telescope at Arecibo to in- clude the 1- to 8-GHz frequency range would seem to be a boon for com- plex molecule studies. Another important component of a strategy for determining the extent of molecular complexity in interstellar and circumstellar sources involves chemi- cal modeling. Such modeling can tell astronomers what likely molecules may be found in a given environment and what intensities can be expected. From successful models involving smaller molecules, it is safe to say that much of the chemistry of the cold interstellar gas is accounted for by schemes based on gas-phase ion-molecule reactions which, because they typically possess no activation energy, can occur rapidly even at low temperature. Although models of interstellar clouds involving small gas-phase mole- cules are in good agreement among themselves and with observation, they differ significantly in their predictions of complex molecule abundances. These differences derive at least in part from lack of laboratory data on important ion-molecule reactions. Thus, an additional component of the strategy emerges the need for laboratory work on important ion-molecule reactions to aid modelers in calculating the expected abundances of com- plex molecules. Nor is this the final element of such a strategy: chemical models cannot be based entirely on laboratory studies of relevant reaction rates. Reaction systems with rate coefficients that are highly temperature dependent, which have only been studied in the laboratory at approximately room temperature, may occur at unsuspected rates under interstellar condi- tions. In addition, some classes of reactions are not easily studied in the laboratory. An example is the low-pressure process called radiative asso- ciation, which is thought to be critical in gas-phase syntheses of complex molecules. To examine this and other processes requires theoretical studies of rate coefficients. Such studies are then another integral part of a strategy aimed at determining the limits of molecular complexity in interstellar and circumstellar sources. Although the tenor of the discussion on interstellar chemistry has been concentrated on gas-phase processes, the influence of dust particles cannot be ignored. These particles are sites of molecular adsorption, Resorption, and possible reactions, and they can protect complex molecules from stellar ultraviolet radiation. Further discussion of particulate matter is given after objective 2 (see below).
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 29 In general, modeling of circumstellar sources has lagged somewhat be- hind that of interstellar ones. However, within the last few years, several circumstellar models have become available. The picture of carbon-rich circumstellar sources such as IRC+10216 that emerges is one in which chemical equilibrium at high temperature is achieved as material is ejected from the star, only to be reprocessed by an active photochemistry and ion- molecule reactions as the material proceeds further from the stellar photo- sphere. Significant amounts of complex molecules may be produced by these processes. STAR-FORMING REGIONS AND THE SUBMILLIMETER SPECTRAL RANGE How is the chemistry of an interstellar cloud affected by the process of star formation? Virtually nothing is known in this regard for isolated solar- mass stars. For more massive stars, however, some evidence has been obtained from study of the Orion nebula. Astronomers have thus far de- tected at least three unusual regions in which the abundances of gas-phase molecules are quite different from more normal values. Suggested causes include chemical reactions driven by shock waves, molecules desorbed from the interstellar grains by temperatures exceeding 100 K, and interactions between species so produced and the "normal" constituents of the ambient cloud. As more information becomes available concerning the unique chem- istry of star-forming regions, it should be possible to develop models of their chemistries with some predictive power. Indeed, primitive models of the star-forming regions in Orion are currently being formulated. Detailed observational studies of small regions warmer than the ambient interstellar medium will require very high angular resolution, which must be obtained by interferometric techniques. Expansion of existing facilities and eventual construction of instruments such as the Millimeter Array and the Submillimeter Array Telescope being discussed by the National Radio Astronomy Observatory (NRAO) and the Smithsonian Astrophysical Obser- vatory (SAO), respectively, will be required. In addition, frequencies higher than those normally used, particularly in the submillimeter region, are im- portant. Because, as the temperature rises, the dominant rotational line emission of most smaller molecules shifts into this wavelength region. Moreover, some light molecules such as simple hydrides can be observed only in the submillimeter spectral region; some of these species are critical to a quantitative understanding of chemical processes in interstellar clouds (e.g., the ion H3+ and metal hydrides such as MgH). Unfortunately, severe problems are associated with submillimeter observations. Ground-based observation is extremely difficult because of atmospheric water. Although a first generation of ground-based submillimeter telescopes is currently being constructed in high, dry locations, the advantages to observing this spectral
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30 THE SEARCH FOR LIFE'S ORIGINS region from space are enormous (e.g., with LDR and SIRTF). An equally important problem, however, is the small laboratory data base on which submillimeter astronomy can draw. Very few gas-phase molecules have been examined in the submillimeter region; many more studies are needed. Thus, a necessary component of a strategy aimed at using this spectral range to study star-forming regions involves laboratory spectroscopy. To achieve the recommendations listed below, it will be necessary for exobiologists to interact closely with the astronomical and planetary science communities. The committee supports the major recommendations of the Astronomy Survey Committee (Astronomy and Astrophysics for the 1980s, Volume 1, National Research Council, 1982) to construct an LDR in space to carry out spectroscopic and imaging observations in the far-infrared and submillimeter wavelength regions of the spectrum that are inaccessible to study from the ground. Such an instrument, in the 10-m class, will offer unprecedented opportunities for studying the molecular and atomic pro- cesses that accompany the formation of stars and planetary systems. The committee also concurs with the recommendation from A Strategy for Space Astronomy and Astrophysics for the 1980s (SSB, 1979) that development of a meter-class, cryogenically cooled, infrared telescope be actively contin- ued, with the option of its construction as a free-flying spacecraft being retained until the Shuttle environment has been demonstrated to be suffi- ciently free of contaminants (SIRTF). OBJECTIVE 2: To determine the composition, structure, and interrela- tionships among circumstellar, interstellar, and interplanetary dust. Interstellar grains constitute an important component of the interstellar medium. They play a crucial role in the heating and cooling of interstellar clouds through the absorption of visible and ultraviolet photons and the ejection of energetic photoelectrons. They also influence the gas-phase composition of molecular clouds directly by providing surfaces for reac- tions and indirectly by locking up some elements, as well as by shielding molecules from the dissociative ultraviolet interstellar radiation field. Ob- servations have shown that these dust grains have a size distribution rang- ing from approximately 3000 ~ down to perhaps molecular sizes and that they lock up a large fraction (290 percent) of some heavier elements such as silicon, iron, calcium, and aluminum, as well as a substantial fraction of the available carbon, nitrogen, and oxygen. The life history of interstellar grains is a complex interplay of many different competing processes, including nucleation and condensation around stars and accretion, chemical modification, and shock processing in the interstellar medium. Some interstellar grains ("star dust") originally con- densed in the high-density, high-temperature environment (n ~ 108 cm~3; To 1000 K) of the circumstellar envelopes of red giants, planetary nebulae, and
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 31 novae and have subsequently been expelled into the interstellar medium along with gaseous species. Other possible dust components may originate in the interstellar medium itself by accretion, reaction, and photolysis of gaseous species on preexisting grain cores. Laboratory experiments suggest that a C60 spherical molecular species "fullerene" may also be a component of interstellar dust. Table 2.2 contains a summary of current knowledge of the composition of interstellar dust. There is at least some evidence for all of the dust components shown in circumstellar or interstellar environments, mainly through low-resolution infrared spectroscopy (see below). Silicate grains are a ubiquitous component of the dust in the diffuse (n < 102 cm~3) interstellar medium, and this star dust component may actually make up about half the interstellar dust volume (cf. Table 2.2~. On the basis of elemental abundances and stability, the remainder of the dust vol- ume has to consist of species containing predominantly carbon. However, it is still an open question whether this carbon is in the form of graphite, which would likely be a star dust component, or in the form of refractory organic grain mantles, which might form via processes in the interstellar medium. Although small graphite particles (~200 A' may be the carriers of the ubiquitous 2200-A bump in the ultraviolet spectra seen toward stars, large graphite grains (1000 ~) do not possess any currently detectable infra- red or ultraviolet absorption features; thus, we can only guess at their con- tribution to the interstellar dust volume. Icy grain mantles, consisting of simple molecules such as H2O, CO, and perhaps NH3 and CH3OH (methanol), are an important component of inter- stellar dust inside dense molecular clouds, but they have never been ob- served in the diffuse interstellar medium. Traces of more complex organic molecules (e.g., aldehydes, ketones, and nitrites) have also been reported in some objects. Icy grain mantles are presumably formed by the accretion of gas-phase species onto preexisting cores inside molecular clouds. In the less dense interstellar medium, these volatile materials would be efficiently destroyed by photodesorption and subsequent photodestruction in the inter- stellar ultraviolet radiation field and by shock waves. Inside dense molecu- lar clouds the much lower ultraviolet flux from embedded newly formed stars or from cosmic-ray excitation of molecular hydrogen may be suffi- ciently high to transform the simple icy molecules into more complex mole- cules, which are more refractory. This process may also be the source of the more refractory grain mantles possibly observed in the diffuse interstel- lar medium. GRAIN INTERRELATIONSHIPS The evolution of biogenic elements in the interstellar medium prior to the formation of the solar system has gained additional interest with the
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 45 organic compounds would have been destroyed and carbonaceous grains would have been converted to graphite. Indeed, these expectations are largely borne out: metamorphosed ordinary chondrites lack organic com- pounds but do contain graphite. Perhaps the most important outcome of internal metamorphisms in the parent bodies would have been the expulsion and delivery of volatiles through overlying layers to near-surface regions by diffusion or volcanic activity. Such outgassing or transport of fluids could have been accompanied by mineral-catalyzed synthesis of organic com- pounds. Future Investigations Little is known about how the physical structure and chemical composi- tion of individual grains influence their growth under putative nebular con- ditions. To fill this knowledge gap, several types of investigations should be carried out. Calculations should be conducted to determine how the rates of formation or destruction of grain aggregates vary with particle hardness, porosity, and composition for metallic, silicate, organic, and icy grains. Theoretical studies should be complemented by laboratory experi- ments, some of which might be appropriate to carry out under microgravity conditions on the Space Station. From simulations of grain collisions under nebular conditions it should be possible to determine the relative "sticking efficiencies" of materials composed of the biogenic elements as compared with those of the rock-forming elements. The structures of aggregates pro- duced in these investigations will provide useful models against which to compare grain aggregates obtained from meteorites, IDPs, and comets. For the experimental studies, facilities capable of accelerating small particles to a range of pertinent velocities would be very valuable. Experiments should be conducted in which organic compounds and grains within inorganic matrices are subjected to laboratory simulations of phe- nomena presumed to have occurred on planetoids. For the biogenic com- pounds and phases used as starting materials in these experiments, modifi- cations of physical, chemical, and isotopic properties as a function of envi- ronmental conditions must be determined. Deeper understanding of the conditions of aqueous alteration, the identi- ties of the precursor phases, and the nature of the resulting hydrous phases should be sought in petrographic and mineral-chemical studies of carbona- ceous chondrites and IDPs. The fact that prebiotic compounds such as carboxylic acids and amino acids appear to occur only in these altered objects is particularly noteworthy, and elucidating the relationship between the origins of these inorganic and organic components is a research problem of high priority.
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46 THE SEARCH FOR f IFE' S ORIGINS OBJECTIVE 5: To determine the distribution, structure, and composi- tion of presolar and nebular products in existing primitive materials in the solar system. Previous sections have considered, more or less chronologically, the evolution of chemical complexity in interstellar clouds, in the solar nebula that resulted from the collapse of such a cloud, and in solid objects that were formed in this nebula. Some end products of this evolution continue to exist today in asteroids, meteorites, comets, and IDPs and may be studied to elucidate this overall process. ASTEROIDS The asteroids are a large collection of small bodies that orbit the Sun, predominantly at distances of 2 to 3.5 AU in the "main belt" between Mars and Jupiter, residing in a transition region between the rocky terrestrial planets and the gas-rich outer planets. Dynamical calculations of asteroid orbits suggest that most of the asteroids have remained near their present relative positions in the solar system since their formation. Thus, one of the most important reasons for studying the asteroids is that they might pre- serve valuable information about the chemical and physical processes (e.g., condensation and accretion) operating in this transition region during the formation and early evolution of the solar system. Our present knowledge of asteroids is based primarily on determination of their orbits and study of the temporal variability and spectral distribution of the reflected and emitted radiation from unresolved starlike images. Spec- troscopic observations show that the asteroids vary in their surface mineral- ogical compositions and fall into broad classes that parallel, in a general fashion, some of the meteorite classes. The primitive nature of the bulk of asteroidal material is reflected by the predominance (by mass) of dark car- bonaceous material (C-type asteroids) in the main belt. Likewise, Ceres, which is the largest asteroid and contains approximately one-third of the total mass in the main belt, has a density of 2.6 + 0.7 g/cm3. This low density suggests that Ceres is far more volatile-rich than any of the terrestrial planets. Similar densities are in fact observed for the CI and CM2 types of carbonaceous chondrites; these meteorites are generally thought to be some of the most primitive early solar-system materials for which we have samples. The density of Ceres is also consistent with the predicted density of nebular condensates forming in the region of 300 K. The study of primitive material in meteorites has provided valuable in- formation about the chemical composition of the solar system and the chemi- cal and physical processes operating in the solar nebula and early solar system. However, the enormous advantage in studying primitive asteroidal
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 47 materials is that the observed properties can be identified with a specific location in the solar system. Future Investigations Determination of the chemical composition of primitive asteroidal mate- rial with sufficient accuracy to make meaningful comparisons with the chemi- cal composition of meteoritic material is of prime importance. If such a direct link can be made, then the large number of meteorite samples can be used as probes of the main belt region and of specific locations in the solar nebula. To this end the committee endorses the recommendation made by COMPLEX (Committee on Planetary and Lunar Exploration, Strategy for the Exploration of Primitive Solar-System Bodies Asteroids, Comets, and Meteoroids: 1980-1990, SSB, 1980) that "the principal chemical elements present in asteroids to more than 1 percent abundance by atom be measured to an accuracy of about 0.5 atom percent. It is expected that these will include the elements H. C, O. Na, Al, Si, S. Ca, Ti, Fe, and Nil" The recommended measurement accuracies should be sufficient to permit infor- mative comparisons with the known meteorite classes, to determine the oxidation state of major elements and to assess the degree of hydration of surface minerals. Measurement of these elements should be made at one location on the surface at least; however, it is very desirable to make measurements at different locations to determine the scale and extent of surficial heterogene- ity. Similarly it is also of interest to determine the scale and extent of radial heterogeneity by making measurements at depth or around craters where samples of the interior may have been exposed. Another important endeavor is determination of the bulk content and the chemical form of the major biogenic elements (H. C, N. O. P. and S). These may be present in a variety of molecular components that would be diagnostic of the primitive nature and degree of subsequent alteration of the asteroid. The distribution of carbon among various carbon-bearing volatiles (CO, CO2, CH4), carbon- ates, graphite, and organic polymers is of particular interest in these mea- surements. A third area of investigation, which may take a longer time for imple- mentation, is the measurement of the D/H, i3C/~2C, ~sN/~4N, 180/~60, and 170/ i60 isotopic ratios on at least one sample of an asteroid. The carbon iso- topic ratios are of interest because of the carbonaceous nature of many asteroids and the observed variability of ~3C/~2C ratios in primitive meteorite components, whereas the oxygen isotopic ratios are important for compari- son with the ratios in various meteorite classes. Different types of scientific instrumentation and different means of in- vestigation and research will have to be involved in these investigations.
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48 THE SEARCH FOR LIFE'S ORIGINS Two useful techniques are X-ray fluorescence and gamma-ray spectros- copy. X rays are excited in surficial materials by solar radiation and pro- vide information on the light elements (e.g., Mg, Al, and Si) in the topmost few micrometers of a surface. Gamma rays are emitted by long-lived natu- ral radionuclides such as potassium, thorium, and uranium and also by shorter- lived nuclides formed by cosmic-ray and solar particle interactions with the surface. Both X and gamma rays can provide qualitative and semiquantita- tive analyses for a large number of elements. Both nondestructive mapping techniques and destructive analytical tech- niques may be required to measure the abundances and chemical forms of the major biogenic elements. Spectral reflectance measurements in the ultraviolet, visible, and near-infrared region can be used to determine the mineralogy and composition of surficial materials and to map the spatial extent of different classes of materials (e.g., carbonaceous matter, hydrated phases). Thermal emission spectroscopy in the mid-range of the infrared region has similar applications. Because these two techniques are sensitive to different mineral phases present on the surface, they provide information complementary to the elemental analysis techniques, which are not sensi- tive to different phases. Detailed characterization of the various molecular components in which the biogenic elements might be present will be considerably more difficult. Pyrolysis or combustion of carbonaceous material with analysis of the evolved vapors by gas chromatography/mass spectrometry has been used for meteor- ite samples and may also be used on a soft-lander. Morphological charac- terization of carbonaceous phases can be made by scanning electron micros- copy; this would be possible on a returned sample or in situ by using a specially developed instrument for spaceflight. Finally, the committee notes the suggestion that the Martian moons Pho- bos and Deimos may be captured asteroids, so their characterization is di- rectly relevant to this objective. METEORITES Meteorites are interplanetary objects that survive passage through the terrestrial atmosphere as discrete bodies or associated fragments. Ranging in size from a few grams to several tons, meteorites are grouped into two broad categories, depending on whether they are undifferentiated or differ- entiated. The undifferentiated meteorites, or chondrites, have generally not been melted; consist of a mixture of small spheroidal objects (chondrules) and finer-grained, heterogeneous material (matrix); and have close resem- blance in elemental abundances to the Sun. In fact, the nonvolatile ele- ments are generally present in solar proportions, whereas the volatile ele-
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 49 meets are depleted to variable extents. The most primitive of the chondrites are the carbonaceous chondrites. On the other hand, the differentiated meteorites, which include the irons, stony-irons, and chondrites, have been subjected to melting and fractiona- tion events, do not consist of the simple chondrule-matrix duplex structure, and do not closely resemble the chemistry of the undifferentiated meteorites or the Sun. In many instances, the compositions of the differentiated mete- orites (e.g., the chondrites) suggest chemical fractionations similar to those produced by igneous activity on the Earth and Moon. However, unlike the continuing igneous activity on the Earth, isotopic dating shows that most of the igneous fractionations reflected in the differentiated meteorites occurred 4.5 billion years ago, shortly after the formation of the solar system. A1- though the differentiated meteorites are important sources of information about the thermal histories of small planetesimals in the early solar system, they provide much less information than do the chondrites on presolar and nebular phases in primitive materials. The carbonaceous chondrites, which are generally thought to be among the most primitive early solar-system materials for which samples exist, are the best candidates for preserving presolar and nebular phases or their sig- natures (e.g., "fossil" elemental abundance patterns or isotopic anomalies). Indeed, rubidium-strontium (Rb-Sr) dating of the CAIs in the allende carbo- naceous chondrite has identified some of these inclusions as the oldest known solids in the solar system. The antiquity of the CAIs, and their resemblance (at least to a first approximation) to the chemistry and mineral- ogy of solid assemblages predicted as vapor-solid condensates at high tem- peratures from a solar composition gas, have led to intensive study of CAIs in the allende and other carbonaceous chondrites. However, to date no pristine nebular phases (i.e., vapor-solid condensates) have been identified unambiguously in any components of the chondritic meteorites. A similar situation prevails in the search for presolar phases in primitive meteorites. The canonical model for the formation of the solar nebula envisioned a homogeneous, totally vaporized swirling cloud of gas that became a mixture of gas and dust upon cooling. In this scenario, a well- defined sequence of mineral phases, which became progressively more vola- tile-rich, formed from this homogeneous cloud with decreasing temperature. The end products of this sequence were postulated to be the oxidized iron- and H2O-rich minerals observed in the carbonaceous chondrites. However, the discovery of non-mass-dependent isotopic anomalies for oxygen and subsequently for titanium in CAIs showed that this viewpoint was fundamentally incorrect. A wide range of other non-mass-dependent and mass-dependent isotopic anomalies in refractory elements (Mg, Si, Ca, Cr. Ba, Nd, Sm) have since been observed in CAIs. Although the non-
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so THE SEARCH FOR LIFE'S ORIGINS mass-dependent isotopic anomalies have been interpreted in terms of mate- rial from different nucleosynthetic sources, no presolar grains have been unambiguously identified in the CAIs. Widespread isotopic anomalies are also observed in the biogenic elements hydrogen, carbon, and nitrogen and in the noble gases neon, krypton, and xenon. The observed isotopic anomalies in the biogenic elements reinforce the notion from the refractory element isotopic anomalies that presolar material from a variety of environments was incorporated into chondritic meteorites relatively unaltered and without being thoroughly homogenized in the solar nebula. Large deuterium enrichments are observed in the insoluble organic matter that forms the bulk (70 to 80 percent) of all carbon in the CI and CM2 carbonaceous chondrites. These enrichments, which cannot plausibly be explained by mass fractionation in the solar nebula, are believed to indicate that these meteorites contain remnants of material from dark inter- stellar clouds. Isotopically heavy carbon found in CM2 chondrites may indicate the incorporation of carbon grains from red giant stars into these meteorites. Isotopically light nitrogen in components of the Allende mete- orite may indicate incorporation of almost pure ON into this meteorite. At present, the complex picture described by the collective isotopic variations is incompletely understood but strongly suggests the preservation of pre- solar material from different nucleosynthetic sources and a variety of astro- physical environments. Future Investigations Observational studies of meteorites can be expected to continue to yield important results and to influence thinking on the chemical and physical processes responsible for shaping our solar system. In its 1980 report Strategy for the Exploration of Primitive Solar-System Bodies Asteroids, Comets, and Meteoroids: 1980-1990 (SSB, 1980), COMPLEX recommended that "a vigorous program of laboratory and theoretical investigations of meteorites be maintained" and also stated that "to realize the full promise of meteorite research it is necessary to maintain laboratory capabilities at the highest level of evolving technology and to encourage the development of even more sophisticated analytical methods." The committee endorses both these statements. In addition, a range of complementary studies should be pursued. Some topics are exceedingly important. Laboratory studies are required of the molecular and isotopic compositions and yields of organic molecules produced by ion-molecule reactions, ultraviolet-pumped photochemical re- actions, and high-temperature nucleation-condensation processes in a vari- ety of astrophysical environments such as dark molecular clouds and cool stellar outflows. It is of utmost importance to conduct simulation experi-
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 51 meets under realistic conditions of pressure, temperature, composition, and energy flux or to perform the experiments in such a fashion as to permit meaningful extrapolations to these conditions. Laboratory studies should also be made of the survivability of artificially induced and natural isotopic anomalies in refractory carbonaceous materials such as the insoluble organic polymer found in carbonaceous chondrites. Of particular interest is the change in a deuterium-enriched sample during heating in a solar composition gas for varying time periods. The resistance of t3C-enriched graphitic grains to pyrolysis and isotopic exchange during heating in Hz-CO gas mixtures with solar '3C/~2C ratios is also of interest. Such studies should be designed to provide kinetic data that can be applied to solar nebular models of the survivability of infalling interstellar grains. Concerted observational studies of primitive meteorites should be made to determine unambiguously the nature, amount, and distribution of deu- terium-enriched carrier phases. The use of in situ techniques such as the ion microprobe should be exploited fully in these efforts. Although the selective chemical dissolution techniques used in studies of noble gas and deuterium, TIC, or ON carrier phases have provided invaluable information, these techniques are ultimately limited by their destructive nature, which renders observation of the carrier phases in the host meteorite impossible. COMETS AND INTERPLANETARY DUST PARTICLES Comets Comets occupy a special place in the cosmic history of the biogenic elements and compounds: they hold promise of containing the most vola- tile-rich relics of processes that occurred in stars, interstellar clouds, and the protosolar nebula, while at the same time bearing evidence of their own formation and evolution as building blocks of planetary materials. Not only are they thought to contain grains and gas inherited from the interstellar cloud that spawned the solar system, they are also expected to have ac- creted both refractory and volatile material formed in cold regions of the protosolar nebula. In addition, a role as carriers of volatile and biogenic elements to the terrestrial planets where, at least on Earth, life arose and evolved is attributed to them. These expectations arise from theories that comets formed by cold accre- tion of interstellar dust and gas or solar nebular condensates, or mixtures of these materials, into small planetesimals whose size, composition, and or- bital distance from the Sun precluded subsequent differentiation. In turn, the theories are based on estimates of the relative abundances of the major elements in comets as inferred from a long history of ground-based and airborne observations of species in their comae and tails and from in situ
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52 TlIE SEARCH FOR LIFE'S ORIGINS studies of Comet Halley. The relatively high ratio of volatile (e.g., water) to involatile (e.g., silicates) substances observed in comets signifies that they were accreted at great distances from the Sun and then never heated for long to temperatures much above the sublimation point of water ice in space. Although resembling primitive carbonaceous chondrites in exhibit- ing approximately solar atomic ratios of the metallic elements, comets more closely approximate the Sun and interstellar frost in the relative abundances of the volatile elements. These abundances, coupled with the putative lack of internal differentiation, place comets among the most primitive solid objects in the solar system and the likeliest to have preserved intact the gases and grains that accreted to form them. For these reasons, comets assume the highest priority among solar-system objects for study of the cosmic evolution of compounds and phases containing the biogenic ele- ments. Many major scientific questions can be addressed by the study of com- ets. These questions should be kept in mind during present and future inves- tigations. They include the following: possible relationships among bio- genic compounds and phases in cometary, meteoritic, and interstellar mat- ter; similarities between cometary and interstellar organic chemistry; and the insertion into, and stability of, interstellar material in cometary nuclei. Prior to the return of Comet Halley, the nucleus of a comet had never been directly observed, and inferences about its composition relied on re- constructions based on the abundances of radicals, ions, and atoms ob- served in the coma and tail. Reconstructions of unobservable "parent" molecules in the nucleus from observable "daughter" species are fraught with uncertainties. Nonetheless, H2O and HCN had previously been identi- fied as parent molecules. Exciting new data pertinent to the gas phase have been obtained from Comet Halley by the Giotto and Vega spacecraft, as well as from related ground-based and airborne observations. Some of these new findings point to CO, CO2, and perhaps H2CO as additional parent molecules. In particu- lar, the gases released from the nucleus were composed of about 80 percent water, 10 to 20 percent CO, a few percent CO2, and smaller amounts of other gases. In addition, analyses of the coma gas phase by neutral and ion mass spectrometers revealed a surprising abundance of peaks attributable to hydrocarbons and other organic compounds. Although the identities of these compounds are presently controversial, their occurrence strongly underscores the complexity of the organic chemical content of comets. Other especially noteworthy findings were the discovery of jets of cyanide associated with the emission of dust from active regions of the nucleus and the observation that the source of much of the CO was extended in the coma. This raises the novel possibility that the dust may contribute species directly to the gas phase.
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AD COMPOUNDS 53 Perhaps the most significant and exciting new insights into comets arose from direct observations of the nucleus of Comet Halley and its solid dust component. Fine-grained dust composed of dark, apparently carbonaceous, matter was found covering inactive regions of the comet surface and ejected in plumes from active regions into the coma. Spacecraft analyses of dust grains in the coma by impact mass spectrometry revealed a variety of com- positional types. In addition to silicatelike particles and inorganic grains of chondritic composition, several populations were found to be composed of various combinations of the biogenic elements carbon, hydrogen, oxygen, and nitrogen exclusively, as well as mixed with inorganic elements. The size and composition of the particles are consistent with our knowledge of interstellar dust, but no conclusions can yet be drawn about their origin. Clearly these particles and their counterparts or analogues in meteorites and IDPs provide fascinating new targets for study. In principle, the isotopic composition of hydrogen, carbon, nitrogen, sul- fur, and other elements in comets could provide clues to their origin. Iso- topic measurements obtained at Comet Halley for carbon, nitrogen, and sulfur, although still imprecise, appear to fall within the range of solar- system materials. Similarly, the bulk ratio of deuterium to hydrogen is compatible with that of terrestrial materials and bulk meteorites. A detailed study of dust at Comet Halley to probe the possible existence of inclusions with large D/H ratios was not possible. Interplanetary Dust Particles Interplanetary dust particles (IDPs) are extraterrestrial particles, typi- cally less than 1 mm in diameter, that survive entry into the upper atmos- phere and are currently collected by high-flying aircraft. Their contents of solar wind noble gases and cosmic-ray tracks attest to their extraterrestrial origin. Among the variety of particle types that have been identified, the most common ones exhibit the solar pattern of relative abundances for ma- jor and minor elements that typifies primitive, chemically unfractionated, chondritic materials. Often called "cosmic dust," these IDPs constitute a unique collection of samples that complement meteorites as "fossils" of the earliest history of the solar system. Some may be of interstellar origin, but the bulk are presumably cometary or asteroidal. Most of the chondritic particles that have been examined are in the 5- to 50-,um size range. They are typically black, and semiquantitative analyses show them to contain 2 to 5 weight percent, or higher, of carbon. Abun- dances of hydrogen and nitrogen have not yet been measured. Two popula- tions make up these particles: one contains only anhydrous phases; the other is composed largely of hydrated minerals, among which the most abundant are layer lattice silicates (clays).
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54 THE SEARCH FOR LIFE'S ORIGINS The anhydrous particles are unique in several distinctive ways. They contain much larger amounts of carbon than comparably anhydrous meteor- ite samples. Their extreme porosity suggests previous filling by ice and structural fragility, the latter being consistent with physical properties of materials in cometary meteors. They are composed of extremely small grains, ranging from micrometers down to tens of angstroms in size, as- sembled in a highly porous, three-dimensional structure. Especially note- worthy among the minerals found as grains are carbides, graphite, and sul- fides, along with olivine and enstatite. Carbonaceous material appears to be ubiquitous as amorphous coatings and clumps and as a medium for the embedment of other inorganic grains. The lack of any counterpart for materials with these characteristics in the meteorite collections argues strongly for a different, probably cometary, . . Orlgln. Recent measurements performed on individual IDPs with the ion micro- probe revealed anomalously high D/H ratios associated with organic carbo- naceous material. Similar findings were obtained on both anhydrous and hydrous IDPs. In the case of carbonaceous chondrites, such high ratios have been interpreted as indicating the presence of interstellar organic matter. The commonality of this organic matter among several types of primitive materials may reflect a common interstellar source. Some of the IDPs composed of hydrous phases may also be related to comets. Although the clay minerals in some cases closely resemble those of carbonaceous chondrites, in other cases they are distinctly different. The degree of compactness exhibited by these particles may reflect the influence of liquid water on the origin of the hydrous phases. If such were the case, and if the particles were determined to be cometary based on other criteria, the implications for cometary thermal evolution, physical properties, and solution-phase organic chemistry would be far-reaching. Future Investigations For the foreseeable future, IDPs will provide the only prospect for direct study of comet samples. Therefore, a vigorous program of ground-based studies should be pursued to characterize them according to physical prop- erties and chemical, isotopic, and mineralogical composition, with primary emphasis on the phases and structures containing the biogenic elements. Furthermore, to expand the size of the existing inventory and perhaps ob- tain particles not captured in the stratosphere, opportunities should be ex- ploited to collect IDPs in relatively unaltered form in low Earth orbit, as for example on the Space Station. The so-called primitive IDPs appear to have no analogues in the meteor- ite collections and, therefore, are most likely to be cometary in origin. In
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THE COSMIC HISTORY OF THE BIOGENIC ELEMENTS AND COMPOUNDS 55 contrast, those "chondritic" particles that contain layer lattice silicates grossly resemble samples of carbonaceous chondrites whose chemistry and mineral- ogy have been altered by liquid water, but the detailed similarities that would confirm a meteoritic rather than a cometary origin remain to be established. For this reason, the latter particles must be included along with the "primitive" ones in future investigations, and parallel studies at very high spatial resolution of the finest-grained material in carbonaceous and unequilibrated ordinary chondrites must be exploited to establish the neces- sary comparative data base. For comets, an approach is needed to address via in situ investiga- tions scientific questions about the elemental, isotopic, molecular, and mineral composition of the comet nucleus, as well as its physical properties and geological characteristics. Included in any mission package should be instruments designed to determine (1) the identities and abundances of the volatile organic compounds at depth in the nucleus as well as in the gas and dust of the coma, (2) the physical structure of the coma dust, (3) the abun- dances of the biogenic elements in the dust, and (4) the isotopic composi- tions of the biogenic elements in the gas phase. The proposed Comet Rendezvous Asteroid Flyby (CRAP) mission would provide such an instru- mental complement and would thus be the next major advance in our scien- tific understanding of comets. Furthermore, this mission would serve as a necessary precursor to a comet nucleus sample return mission (Strategy for the Exploration of Primitive Solar-System BodiesóAsteroids, Comets, and Meteoroids: 1980-1990, SSB, 1980; A Strategy for Exploration of the Outer Planets: 1986-1996, SSB, 1986b). Over this same time frame, returning short-period comets and new com- ets will provide occasions for ground-based and airborne observations. These opportunities should be exploited to address new questions raised by the recent studies of Comet Halley. For the longer term, however, highest priority for the study of the cosmic evolution of the biogenic compounds and phases must be given to the return of a comet nucleus sample. Under carefully controlled laboratory condi- tions, the full range of state-of-the-art analytical instruments and techniques could be brought to bear. Perhaps most important, the ingenuity of an international community of scientists would be released from the constraints of preprogrammed experimental approaches imposed by the requirements of remote analyses. With the expectation that such a sample will be available some time near the turn of the century, it is timely now to begin developing the analytical and sample manipulation techniques required to operate at subambient temperatures on a micrometer scale on samples likely to be dominated by ices and volatile components. The committee strongly en- dorses the recommendation for a comet sample return mission in Space Science in the Twenty-First Century (SSB, 1988a,b).
Representative terms from entire chapter: