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Exploring Organic Environments in the Solar System 3 Meteorites THE ORIGIN OF METEORITES Meteorites are products of collisions that occur within the asteroid belt. As such, they sample remnants of accretionary products of the early solar system. It has also been proposed that a few may be the remains of cometary nuclei, i.e., derived from outside the asteroid belt. A small number of meteorites are recognized to be samples of debris from the surfaces of the Moon and Mars, ejected by impacts. Meteorites are divided into three main groups: irons, stony irons, and stony meteorites. The irons contain at least 90 percent metal, predominantly iron and nickel. The stony irons are further divided into pallasites, composed of crystalline minerals embedded in a metallic matrix, and mesosiderites, which contain a more fine-grained and intimate mixture of minerals and metal. The stony meteorites consist predominantly of silicates and are subclassified as either chondrites (materials that have never experienced igneous processing) or achondrites (lavas, cumulates, or residues from partial melting). Many stony meteorites contain spherules of rock ranging in size from a few millimeters to one centimeter across. These so-called chondrules are apparently the crystallization products of silicate melt droplets, whose melting history remains a subject of intense investigation. Indeed, the presence or absence of chondrules was once used as the defining characteristic of chondrites and achondrites, respectively. The chondrites are further subdivided according to their composition: Ordinary chondrites, Enstatite (magnesium silicate-containing) chondrites, and Carbonaceous chondrites. All three groups of chondritic meteorites contain carbon that has been variously modified by parent body processes. Note that this carbon includes both inorganic and organic phases. It was the presence of organic matter identified in the carbonaceous chondrites that captured the interest of chemists in the 1800s.1,2 Notwithstanding this interest, analytical techniques capable of describing the organic constituents of these meteorites were not available until the 1950s, when a rising interest in space science coupled with advances in analytical chemistry led to a rejuvenation of efforts to characterize the organic matter in carbonaceous chondrites.
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Exploring Organic Environments in the Solar System CARBONACEOUS CHONDRITES: A RECORD OF THE ORGANIC CHEMICAL EVOLUTION OF THE EARLY SOLAR SYSTEM Studies of carbonaceous chondrites have provided evidence that interstellar organic matter has survived processes associated with the formation of the solar system and has been incorporated into grains and larger objects. Within the asteroidal parent bodies, the presumably simpler interstellar organic compounds were transformed into more complex organic compounds by aqueous and thermal processing.3-6 A remarkably broad range of organic compounds has been identified in carbonaceous chondrites.7,8 This organic ensemble includes amino acids, purines, and pyrimidines. The presence of these particular compounds demonstrates that the solar system provided at least one environment in which recognizable biomolecules were synthesized abiotically. Furthermore, the presence of such compounds in meteorites indicates that impacts by meteorites, comets, and dust must have delivered potentially biologically useful organic compounds to early Earth and the other terrestrial planets. Carbonaceous chondrites thus currently provide a potentially rich and accessible source of information regarding the organic compounds naturally present during prebiotic evolution of the terrestrial planets as well as other solid bodies such as the moons of Jupiter and Saturn (e.g., Europa, Enceladus, or Titan). Murchison Meteorite: A Timely Gift from the Solar System In September 1969, just as many chemists were beginning the search for organic compounds in lunar samples, a new carbonaceous chondrite fell near Murchison, Victoria, Australia. Rich in volatiles, it contains more than 10 percent water and about 2.2 percent carbon by weight. Initial research was directed toward determining the constituents of the aqueous and solvent-soluble fractions of this meteorite. It was soon determined that Murchison contained a spectacularly complex suite of small molecules (Table 3.1). For example, to date, more than 70 different amino acids have been identified in this meteorite. The distribution of amino acids is similar to that produced in certain abiotic syntheses and, where mirror-image structures (i.e., stereoisomers) are anticipated, both chiral forms are found to be nearly equal in abundance. This balance is one signature of abiotic synthesis. Moreover, the amino acids extracted from the Murchison meteorite contain distinctly higher levels of 13C than do any terrestrial amino acids. Thus, even though many of the soluble compounds in Murchison are recognizable as biochemically significant, e.g., the amino acids, their stereochemical and isotopic characteristics clearly identify them as both extraterrestrial and nonbiological.9 The classes of soluble meteoritic organic compounds that have familiar biochemical counterparts include the amino acids, fatty acids, purines, pyrimidines, and sugars.10,11 Additional soluble constituents include alcohols, aldehydes, amides, amines, mono- and dicarboxylic acids, aliphatic and aromatic hydrocarbons, heterocyclic aromatics, hydroxy acids, ketones, phosphonic and sulfonic acids, sulfides and ethers.12-14 Concentrations of the major representatives of these classes vary widely from less than 10 parts per million (amines) to tens of parts per million (amino acids) to hundreds of parts per million (carboxylic acids).15 Cooper et al.16 identified a complex suite of sugars and sugar derivatives present in the soluble fraction of Murchison at concentrations comparable to that of the amino acids (~60 ppm) in the Murchison and Murray meteorites. Chromatographic analyses of virtually all classes of acyclic compounds reveal complex molecular assemblages containing homologous series of compounds up to C12 in some cases (carboxylic acids). The relative abundances of the heavy stable isotopes of carbon and hydrogen (13C and 2H) in these compounds differ significantly from those of terrestrial materials and provide decisive evidence that these materials are not terrestrial contaminants. Distinctive patterns of structural variation are seen in the Murchison organics but they differ from those found in living systems.17 As a class, the amino acids exemplify this contrast. Specifically: The abundances of amino acids decrease with increasing carbon number,18 The abundances of branched-chain isomers far exceed those of the straight-chain isomers, and Structural diversity dominates at the lower carbon numbers (e.g., acyclic, monoamino acids).
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Exploring Organic Environments in the Solar System TABLE 3.1 Distribution of Carbon in the Murchison Meteorite Form Amounta Total carbon 2.12% Interstellar grains Diamond 400 ppm Silicon carbide 7 ppm Graphite <2 ppm Carbonates 2-10% total C Macromolecular material 70-80% total C Small organics Aliphatic hydrocarbons •• Aromatic hydrocarbons •• Polar hydrocarbons ••• Volatile hydrocarbons • Aldehydes and ketones •• Alcohols •• Amines • Monocarboxylic acids ••• Dicarboxylic acids •• Sulfonic acids ••• Phosphonic acids • N-heterocycles • Purines and pyrimidines • Carboxamides •• Hydroxy acids •• Amino acids •• Sugars and sugar derivatives •• a••• >100 ppm; •• >10 ppm; • >1 ppm. SOURCE: Modified after J.R. Cronin, “Clues from the Origin of the Solar System: Meteorites,” pp. 119-146 in The Molecular Origins of Life: Assembling Pieces of the Puzzle, A. Brack A. (ed.), Cambridge University Press, Cambridge, U.K., 1998. Overall, approximately 70 amino acids have been identified to date among the 159 possible C2 to C7 isomers.19,20 Living systems use a highly restricted number of amino acid isomers to fulfill requirements for protein structure and function. At present, life is known to encode only 20 protein α-amino acids, all of which have at least one hydrogen atom attached to the α carbon. Only eight out of 20 of these terrestrial, biological amino acids have been identified in meteorite extracts. Mechanism of Formation of Organic Compounds in Carbonaceous Chondrites The observed patterns of variation in molecular structure and abundance in carbonaceous chondrites suggest initial synthesis routes involving the formation and random recombination of small, free radicals.21,22 Such reactions tend to produce the greatest variation in possible structural isomers at lower carbon numbers. It is generally accepted that ion-molecule and related reactions within interstellar clouds produce mixtures of nitriles and other highly reactive organic compounds (e.g., polyacetylenes). These compounds, when exposed to liquid water in the meteorite parent body, will hydrolyze to form many compounds superficially similar to those identified in the
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Exploring Organic Environments in the Solar System TABLE 3.2 Soluble Organic Compounds in the Tagish Lake and Murchison Meteorites Tagish Lake Murchison Class Concentration (ppm)a Compounds Identifiedb Concentration (ppm) Compounds Identified Aliphatic hydrocarbons 5 12 >35 140 Aromatic hydrocarbons ≥1 13 15-28 87 Dicarboxylic acids 17.5 18 >30 17 Carboxylic acids 40.0 7 >300 20 Amino acids <0.1 4 60 74 Hydroxy acids b.d 0 15 7 Pyridine carboxylic acids 7.5 7 >7 7 Sulfonic acids ≥20 1 67 4 Nitrogen heterocycles n.d. n.d. 7 31 Amines <0.1 3 8 10 Amides <0.1 1 n.d. 4 Dicarboximides 5.5 4 >50 3 NOTE: n.d., not determined. aConcentrations are based on chromatographic peak intensities and include compounds identifed by reference standards and mass spectra. Variability was not estimated at this time, as measurements were obtained by analyses of one meteorite stone. bCompounds identifed with reference standards. SOURCE: Modified from S. Pizzarello, Y. Huang, L. Becker, R.J. Poreda, R.A. Nieman, G. Cooper, and M. Williams, “The Organic Content of the Tagish Lake Meteorite,” Science 293: 2236-2239, 2001, Table 1. Murchison extracts. For example, nitriles will hydrolyze to yield carboxylic acids. Amino acids may have formed directly from a series of reactions referred to as the Strecker synthesis. In this reaction cyanohydrins and aminonitriles form from condensation of HCN and aldehydes or ketones. Subsequent hydrolysis yields α-amino and α-hydroxy acids. Amino acids substituted at positions more than one carbon atom away from the carboxyl group (common in the Murchison organic solubles) indicate alternative synthetic pathways. The cosmochemical community has also benefited from another large meteorite fall, the Tagish Lake carbonaceous chondrite recovered in January 2000 on the border between British Columbia and the Yukon Territory, Canada. This meteorite fortuitously fell in the winter on snow and ice and was recovered frozen. Obtained in this frozen state, the Tagish Lake meteorite holds the promise of revealing considerably more about the inventory of small volatile compounds that may have been lost from Murchison and other falls. Analyses of a soluble fraction of the Tagish Lake stone reveal a suite of soluble organic compounds including mono- and dicarboxylic acids, dicarboximines, pyridine carboxylic acids, sulfonic acids, and aliphatic and aromatic hydrocarbons (Table 3.2). However, in contrast to carbonaceous chondrites such as Murchison, Orgueil, and Ivuna,23,24 virtually no amino acids are detected in extractions obtained from the Tagish Lake meteorite when it was treated with water or organic solvents. One investigator has proposed that the parent body of Tagish Lake did not support formation of amino acids via the Strecker-cyanohydrin reaction due to a lack of liquid water (key to the hydrolysis step). An alternative postulate is that the parent body of Tagish Lake was formed closer to Jupiter where the composition of starting materials was different from that of other primitive bodies. These suggestions led to the speculation that Tagish Lake may represent cometary rather than asteroidal material, employing the same arguments for the designation of anhydrous versus hydrous IDPs. Clearly, considerably more analytical work is required before the sources and history of these materials can be outlined in detail. Nitrogen Heterocycles in Carbonaceous Chondrites The essential biological role of nucleobases, e.g., purines and pyrimidines, as base-pairing complements in RNA and DNA has led to interest in the isolation and characterization of such compounds and related derivatives
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Exploring Organic Environments in the Solar System in meteorites.25-31 Although cosmochemical studies of meteorites, in particular Murchison, have provided clues about the mechanism of formation of amino acids via the Strecker synthesis,32 the mechanisms of formation for purines, pyrimidines, and other nitrogen heterocycles remain obscure. Stoks and Schwartz33 noted that alkyl pyridines can be produced from aldehydes and ammonia by a reaction that is catalyzed under conditions that support Fischer-Tropsch (FT) type reactions. A similar mechanism has been postulated to be responsible for the formation of organic molecules in a cooling gaseous solar nebula, where dust grains may have provided catalytic surfaces.34 Many of the Murchison N-heterocycles may have been produced by HCN polymerization followed by hydrolysis.35 Notably, HCN is a critical intermediate in Miller-Urey (MU) type reactions.36 It is present in the gas phase of the ISM and is thought to have condensed into the icy mantle of dust grains. However, key issues remain. Kung et al.37 have shown that neither FT nor MU systems could yield N-isotopic ranges comparable to those observed in meteoritic organic matter. The N-heterocycles identified in Murchison include: Purines (xanthine, hypoxanthine, guanine, and adenine), Pyrimidines (uracil), Quinolines/isoquinolines, and Alkyl pyridines. The purines and pyrimidines (1.3 ppm, total) identified so far are restricted to biologically utilized entities, whereas the quinolines and pyridines are structurally diverse; i.e., they include significant quantities of isomeric, alkyl derivatives, most of which are not similar to biosynthetic products.38 Finally, carboxylated pyridines have been detected in hot-water extracts of the Tagish Lake carbonaceous chondrite.39 These include isomers of nicotinic acid and 12 methyl- and dimethyl-substituted homologs. Isotopic analyses of these N-heterocycles reveal enrichment in D, 13C, and 15N at levels outside the terrestrial range.40 Similar enrichments in heavy isotopes are signatures of low-temperature reactions in interstellar clouds and support an extraterrestrial source for these molecules. Macromolecular Organic Matter in Carbonaceous Chondritic Meteorites The majority of organic matter contained within carbonaceous chondrites is in the form of nonextractable, presumably macromolecular carbon. The meteoritic insoluble organic matter (IOM) is usually isolated as the insoluble residue that remains after solvent extractions and dissolution of minerals by treatment with HF and HCl. There has been an unfortunate tendency to refer to this insoluble organic matter as “kerogen-like.”41 However, the term “kerogen” refers to insoluble organic matter derived from biomacromolecular precursors found in ancient terrestrial sediments; kerogen therefore represents a mixture of resistant biopolymers and condensation products resulting from reactions between lipids, proteins, and carbohydrates. As there clearly is no biological connection between meteoritic IOM and terrestrial kerogen, the term “kerogen” should not be applied to the IOM of carbonaceous chondrites. The structure of the macromolecular component in carbonaceous chondrites has proven to be difficult to ascertain. Based on early pyrolytic (thermal degradation) studies, a calculated elemental formula of C100H48N1.8O12S2 has been proposed for the IOM in the Murchison meteorite.42 Given its insoluble nature, analysis of this material is restricted either to nondestructive molecular spectroscopic methods such as FTIR and solid-state nuclear magnetic resonance (NMR) spectroscopy or to destructive methods such as pyrolysis gas chromatography–mass spectrometry and chemical degradation (e.g., oxidation with CuO, KMnO4, or RuO4). In general, the destructive analyses of IOM yield dominantly 1- and 2-ring aromatic products, generally highly alkylated, and commonly oxygen-substituted. The significant abundances of alkylated phenols in both pyrolysis and chemical degradation products has led to the conclusion that alkyl-aryl ether linkages in the IOM macromolecule are common.43 Heteroatoms, N and S, are accounted for mostly by alkyl pyridines, benzonitrile, and alkyl thiophenes and benzothiophenes. Hydrous pyrolysis (i.e., heating in water at high temperatures and pressures) has been used to degrade and solubilize the IOM in a number of carbonaceous chondrites.44 In addition to aromatic hydrocarbons, alkyl-phenols
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Exploring Organic Environments in the Solar System are observed in significant abundance, lending support to previous conclusions that alkyl-aromatic ethers are predominant covalent linkages in the IOM macromolecule. The most striking observation based on hydrous pyrolysis experiments is the abundance of small aromatic molecules (i.e., one- and two-ring compounds) over larger polycyclic aromatics, e.g., coronene (seven rings) and larger. The preponderance of evidence based on the degradative studies indicates that the majority of molecular units that constitute the IOM macromolecule are small, functionalized aromatic moieties. The first, nondestructive, solid-state NMR analyses of meteoritic IOM were performed by Cronin et al.,45 wherein IOM isolated from Orgueil, Murchison, and Allende were analyzed using 13C NMR spectroscopy. This early analysis led to the conclusion that the fraction of aromatic carbon in IOM was on the order of 40 to 50 percent of the total, a value that Cronin believed was too low. More recent analyses by Gardinier et al.46 and Cody et al.47 supported Cronin et al.’s earlier concern and revealed that in the case of Murchison, the aromatic content is ~63 percent of the total carbon. Solid-state 13C NMR analysis by Pizzarello et al. 48 of the Tagish Lake meteorite suggested that virtually 100 percent of the carbon in this meteorite’s IOM is aromatic. However, subsequent analysis of the Tagish IOM (a different sample than that analyzed by Pizzarello) by Cody et al.49 showed a yield of aromatic carbon of ~83 percent. Notwithstanding this variation, it is now universally accepted that the structure of meteoritic IOM is highly complex. Furthermore, the relative contributions of different functional groups vary enormously across meteorite groups.50 To date, the IOM has been extensively characterized only for Murchison,51 wherein eight independent solid-state NMR experiments were used to provide a self-consistent assessment of chemical characteristics. First, these analyses showed that although the fraction of aromatic carbon is high (~60 percent), the average size of aromatic moieties is not large. Significant quantities (>1 to 5 percent) of graphite and/or carbon-rich domains (aside from nanodiamond that is readily observed using NMR) are absent. Second, the average fraction of aromatic carbon bonded to hydrogen is low (~30 percent for Murchison), indicating that substitution and cross-linking of the aromatic moieties are common. Third, the aliphatic carbon is highly branched [i.e., there is a significant fraction of methine (CH) carbon as opposed to methylene (CH2)]. The majority of aliphatic carbon may be methyl-substituted alicyclic moieties. Finally, the oxygen content as determined from NMR is very high and, given the constraints of the elemental analyses, requires that most of the oxygen occurs in linkages such as ethers or esters rather than phenols or carboxyl groups. Finally, traces of various inorganic carbon species including diamond, graphite, fullerene, and silicon carbide are present in the IOM. Within these inorganic carbon phases, many individual constituents (e.g., nanodiamond grains and SiC grains) carry large isotopic anomalies that point to specific stellar origins such as supernovas for the nanodiamonds or carbon stars and novas for SiC, fullerene, and graphite.52-54 Isotopic Characteristics of Insoluble Organic Matter The bulk isotopic compositions of IOM fractions of numerous carbonaceous chondrites are well established. In general, the carbon isotopic abundance does not vary significantly in comparisons of IOM derived from various carbonaceous groups; where bulk carbon isotopic values fall within the terrestrial range of δ13C at ~ −17‰. The stable isotopic composition of nitrogen, however, varies more substantially, with values ranging from those that appear clearly extraterrestrial (δ15N of 200‰ to 300‰) down to values indistinguishable from terrestrial (δ15N ~ –5‰). It has been proposed that the range in nitrogen isotopic abundance reflects variation in parent body processing, where the high abundances of 15N correspond to more pristine (less altered) IOM.55 Abundances of deuterium almost always deviate significantly from terrestrial levels (e.g., ~ +600‰ up to ~ +3,500‰). These suggest a chemical connection between insoluble organic matter in meteorites and ion-molecule reactions in the interstellar medium. The carbon isotopic compositions of discrete molecules derived from degradative chemistry, e.g., hydrous-pyrolysis,56 have also been examined. Significant variations have been observed. The abundance of 13C in ethylbenzene, for example, varies by more than 20‰ between the Orgueil, Cold Bokkeveld, and Murchison carbonaceous chondrites.57 The origin of this intermeteorite variability is not known and certainly requires further investigation. Finally, step-combustion with simultaneous isotope-ratio monitoring also indicates nonhomogeneous isotopic distributions (e.g., C, N, and H) in the bulk insoluble organic matter of carbonaceous chondrites. The pattern of
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Exploring Organic Environments in the Solar System release of 13C and 15N is complex and varies substantially in intermeteorite comparisons. The observed variation has been interpreted as resulting from the extent of parent body processing.58 Chirality: Enantiomeric Excesses Observed in Stereoisomers from Carbonaceous Chondrites A fundamental characteristic of life is the homochirality of most of its building blocks. Various theories have been proposed to explain the origin of this property. Most require a chemical-amplification scheme but differ in the origin of the initial imbalance. Experimental investigations into the abiotic syntheses of organic compounds do not produce chiral products.59,60 Moreover, recent evaluations of abiotic mechanisms proposed for the origin of chiral molecules on the primitive Earth have concluded that such processes are not likely to occur naturally.61 Enantiomeric excesses have been observed among amino acids extracted from meteoritic matter62-65 and have been sought in micrometeorites66 and comets (i.e., the Cometary Sampling and Composition (COSAC) experiment aboard the European Space Agency mission Rosetta67). In 1997, Cronin and Pizzarello68 reported modest L-enantiomeric excesses of 2 to 9 percent in some amino acids in the Murchison meteorite. They avoided the problems of contamination by making measurements on 2-amino-2,3-dimethylpentanoic acid, α-methyl norvaline, and isovaline. All three of these compounds are α-methyl substituted; the first two have no known biological counterparts and the third has a highly restricted distribution in fungal antibiotics. The α-methyl substituents are significant in that they block the known racemization of chiral amino acids. This behavior suggests that the enantiomeric excess may have occurred when these amino acids formed. Bailey and colleagues suggested that the observed enantiomeric excesses could have been induced by circularly polarized light scattered from dust in regions of high-mass-star formation.69 These sources occur more widely than do the supernova remnants or pulsars that were first proposed by Rubenstein et al. as sources of circularly polarized synchrotron radiation.70 However, both theories do not take into account that amino acids are very fragile compounds, which are easily destroyed by particle radiation and even by low-energy ultraviolet photons.71 ORGANIC CARBON IN UNEQUILIBRATED ORDINARY CHONDRITES AND ENSTATITE CHONDRITES Most analyses of organic material pertain to Murchison and a few other carbonaceous chondrites. The unequilibrated ordinary chondrites (UOCs) are generally recognized as the most primitive ordinary chondrites. They also contain organic carbon (in excess of that contained in Murchison72 when normalized to matrix content, although considerably less on a total meteorite basis). Historically, these interesting stones have received considerably less attention than have the carbonaceous chondrites. This disparity is due at least in part to the fortuitous fall of the large and organic-rich Murchison chondrite in 1969 and in part due to the fear of terrestrial contamination that leads most researchers to restrict their analytical studies to this most recent fall. Claims that biogenic compounds are present in UOCs span more than a century. Kaplan et al. reported the detection of amino acids and sugars in UOCs,73 although this observation has been generally discounted as due to terrestrial contamination. Alkanes were detected in one UOC;74 however, an analysis using compound-specific isotopic analysis of the Bishunpur UOC clearly indicates a terrestrial source of contamination.75 Clemett et al.76 and Kovalenko et al.77 reported polycylic aromatic hydrocarbons in a UOC using two-stage, laser-desorption, resonant-ionization, time-of-flight mass spectrometry. Later, Sephton et al.78 detected toluene and dimethyl ethyl naphthalene in a hydrous pyrolysate obtained from the Bishunpur UOC; the fact that these compounds were only detected upon destruction of the macromolecular matrix supports indigeneity as opposed to terrestrial contamination. Notwithstanding these later data, the organic matter contained within UOCs remains largely unexplored. The enstatite chondrites also contain relatively large amounts of carbon. These meteorites, however, may have been subjected to metamorphic temperatures high enough to convert all of the organic carbon to graphitic or graphite-like phases. Notably, the high content of 15N suggests that the carbon in the enstatite chondrites was derived from an organic precursor. But there is no a priori reason to believe that E3 chondrites experienced any
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Exploring Organic Environments in the Solar System higher degree of metamorphism than did H, L, or LL3 chondrites. What is clear, however, is that enstatite chondrites have a much lower percentage of matrix and that the dominant chondrules did experience igneous temperatures during their history. MARTIAN METEORITES Meteorites designated by the abbreviation SNC (Shergottite-Nakhlite-Chassignite) are generally accepted as martian in origin. This assignment is based on the isotopic compositions and relative abundances of argon and other noble gases within the SNC meteorites.79 To date, all that is known about the organic matter on Mars comes from the examination of martian meteorites. Studies of examples of the 35 or so cataloged martian meteorites80 have revealed much about their organic constituents. For example, stepped-combustion studies of the Shergotty meteorite and other known shergottites indicate that they contain complex organic molecules and components of both low and high thermal stability.81 Similar studies of the shergottite known as EET A79001 revealed unexpectedly high concentrations of organic materials. However, because the abundances of the carbon isotopes are identical to that of terrestrial biogenic compounds, contamination cannot be excluded.82 Complex organic materials with a high molecular weight also turned up in a more recent analysis of both EET A79001 and the Nakhla meteorite. On pyrolysis, the major components of this complex organic matter found in both meteorites are aromatic and alkylaromatic hydrocarbons, phenol, and benzonitrile. Analysis of individual molecules in the Nakhla pyrolysate showed that they were similar to materials found in carbonaceous chondrites. Nevertheless, terrestrial contamination still cannot be ruled out entirely.83 Perhaps the most famous martian meteorite is ALH 84001, which was reported to contain both “nanofossiles” and polycyclic aromatic hydrocarbons.84 The announcement of these results has led to a reassessment of how to search for life on Mars and elsewhere. Much of the controversy surrounding ALH 84001 centers on stable carbon isotope studies of the carbonate and the associated organic matter. Thus, subsequent studies of the ALH 84001 organic matter have focused on determining the δ13C values for specific organic compounds isolated from various mineral phases. Two independent investigations of the organics in ALH 84001, stepped combustion experiments to measure the 13C/12C compositions for the organic matter, indicated that a small portion (~50 ppm out of 250 ppm) of this material yielded a δ13C value of −15‰.85,86 A δ13C value of –15‰ would be unusual for indigenous martian organics based on 13C/12C measurements of trapped gases in some martian meteorites that revealed two distinct carbon reservoirs on Mars: an isotopically heavy component (atmosphere) enriched in δ13C (+36‰) and a high-temperature igneous (i.e., mantle) component (δ13C −20 to −30‰). On the other hand, a δ13C value of −15‰ is consistent with the isotopic composition of the IOM component in carbonaceous chondrites. Thus, it is possible that some portion of the organic matter in ALH 84001 may be derived from chondritic or cometary debris exogenously delivered to the surface of Mars, and subsequently transported to Earth via the SNC carrier. An important question, therefore, remains as to whether any of the organic matter detected in ALH 84001 is indigenous, having formed as a result of biogenic or abiogenic processes on the surface of Mars. Recent measurements of the oxygen isotopes in ALH 84001 carbonate-rich material revealed a δ17O anomaly that can only be explained by a thin atmosphere and ozone production leading to a highly oxidized surface at the time that the ALH 84001 carbonates formed. The presence of highly oxidizing surface conditions was also used to explain the lack of organic compounds detected during the Viking missions. These new findings suggest that similar conditions may have existed early on in the development of the martian atmosphere. If these conditions did, in fact, occur, then the organic matter found in ALH 84001 may not have formed on the surface of the planet because the environment would have been unfavorable for abiotic synthesis and subsequent accumulation of organic compounds. This suggestion does not discount the possibility, however, that the organic matter was formed abiologically deeper within the crust and that the 17O anomalous carbonate-bearing fluids did not percolate down from the surface; clearly, considerably more work needs to be done to clarify the provenance of the organic matter detected in ALH 84001 and perhaps other martian meteorites.
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Exploring Organic Environments in the Solar System ORGANIC MATTER IN METEORITES: RECOMMENDATIONS 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. The ancient terrestrial materials have been subjected to extensive post-depositional alteration and, as far as can be discerned, already bore the imprint of biogenicity at the time of their burial. New analyses of carbonaceous chondrites would benefit from modern analytical methods (e.g., compound-specific isotopic analysis) that allow the separation of signals from contamination by terrestrial organics and signals from indigenous extraterrestrial organic matter, thus overcoming a problem that severely hindered analyses throughout the 1960s and 1970s. For example, the Murchison meteorite has for more than 30 years been the subject of numerous detailed analyses of the organic and inorganic compounds present in it. The continuing analysis of the Murchison meteorite has revealed the presence of a diverse array of different classes of organic compounds as well as distinctive isotopic compositions (see Table 3.1). A more sensitive and detailed analysis of the other 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 spacecraft missions to other bodies in the solar system. The detection of any organic material in SNC meteorites is of considerable interest. Such studies must address issues of indigeneity before focusing on biogenicity and martian history. For broadly significant studies of organic material in the cosmos, the return to Earth of carbonaceous cometary or asteroidal material is an objective of the highest order. How should plans be developed and proposals solicited for the curation and coordinated, intensive investigation of the composition of organic materials in carbonaceous chondrites, unequilibrated ordinary chondrites, and SNC meteorites? While the available samples are not improving on the shelf, significant developments in analytical technology have occurred in the past few decades, and so the time is ripe for investment of significant portions of the available stocks in a new round of analyses. Analyses should focus on 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 preand post-collection. Very specifically, these investigations would require the coordination of analyses in multiple laboratories, the development of new procedures, and the upgrading of existing facilities. The effort envisioned would be on a scale completely different from that of prior analyses of carbonaceous chondrites. To provide comparability and to bring the best techniques to bear on each object, samples should be shared extensively between laboratories. The planning of analyses, allocation of and/or access to samples, and, possibly, provision of funding could be managed by a committee of appropriately selected experts. Two extreme models of how such a committee might function are offered: At the centralized, highly focused extreme is the approach used for the coordinated and very intensive investigations of the Apollo lunar samples conducted between 1969 and 1985. The role of such a committee would, ideally, extend beyond the planning of analyses to include allocation and, perhaps, the provision of funding. At the decentralized, more informal extreme is a community-based group modeled on the Mars Exploration Program Analysis Group (MEPAG) that has successfully provided science input for the planning and
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Exploring Organic Environments in the Solar System prioritization of NASA’s Mars exploration activities.87 The role of such a group is likely to be limited to that of a forum for the planning of analyses. The existing Curation and Analysis Planning Team for Extraterrestrial Materials (CAPTEM) might provide the nucleus around which an inclusive, community-based forum might be accreted. Although the former model is likely to be the most direct approach for obtaining information about organic materials from elsewhere in the solar system, it is also the most likely to run afoul of the realities of the operation of the curatorial community. In the case of lunar samples, all materials—or most, if the samples from the former Soviet Union’s three lunar sample-return missions are included—were under the control of a single organization, NASA. That is far from the case where meteorites are concerned. The world’s meteorite collections are, for all practical purposes, managed by the U.S., Japanese, and European (EUROMET) Antarctic meteorite programs; a handful of large museums—principally those in Washington, Chicago, New York, London, Paris, and Vienna—and private dealers. It is far from clear if any of these groups would cede control of their collections or any portion of their collections for distribution by a group interested in only one facet of the meteorite (e.g., organics, hydrous alteration, chondrule formation, and so on). While it is true that all of these organizations welcome research requests and have distributed material for organic analyses, the meteorites of greatest interest to the organics community (e.g., carbonaceous chondrites, unequilibrated ordinary chondrites, and SNC meteorites) are, unfortunately, among the most valuable for other types of studies. Moreover, the curators and committees that are, in effect, the caretakers of these collections have to balance the interests of individual groups with the long-term preservation of material for future studies. Thus, of the two models suggested for the coordination of studies of organic materials in meteorites, the more informal, community-based forum designed to set a consensus research agenda is likely to be more appropriate. 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) might provide the seed from which such a community-based forum might be nurtured. To provide comparability and to bring the best techniques to bear on each object, samples should be shared extensively between laboratories. 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. There are really only two ways to acquire meteorites: Collect them from areas of concentration, or Buy or trade for them from those who are lucky enough to see a fall or find an isolated specimen. The pros and cons of these two strategies are considered below. Collecting Meteorites For researchers engaged in meteorite studies, the preferred means for acquiring samples is to collect them in the field. This approach has been far more productive in terms of numbers of meteorites and has led to major searches in those places in the world where meteorites are most likely to be found. Although meteorites fall to Earth in a random fashion, they are more likely to be spotted and collected if they fall in locations where they stand
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Exploring Organic Environments in the Solar System out against their background. In other words, a meteorite that falls in rocky terrain or dense vegetation is less conspicuous than one falling on a featureless ice sheet or salt flat. Thus, the hot and cold deserts of the world are the prime collecting areas. Cold Deserts Antarctica has been the world’s most productive hunting ground for meteorites ever since 1969, when Japanese scientists discovered meteorite concentrations on bare ice stranding sites.88 Since then some 35,000 meteorite specimens have been recovered by the combined activities of expeditions from the United States, Japan, China, the European EUROMET consortium, and other national programs. These meteorites are collected in probably the most sterile environment on Earth, and great pains are taken to minimize contamination in both their collection and curation. The U.S. Antarctic Meteorite program is currently supported by the National Science Foundation’s (NSF’s) Office of Polar Programs, NASA, and the Smithsonian Institution. The field component of the program, the Antarctic Search for Meteorites (ANSMET), is currently supported by NSF and NASA. Initial examination and curation of samples are undertaken at the astromaterials curation facility at NASA’s Johnson Space Center, and initial characterization and long-term curation are the responsibility of the Smithsonian’s National Museum of Natural History. The first ANSMET expedition (a joint U.S.-Japanese effort) discovered what turned out to be a significant concentration of meteorites at the Allan Hills in southern Victoria Land, a region that was to eventually reveal the famous martian meteorite, ALH 84001. In addition to ANSMET’s long-standing NSF sponsorship, NASA has funded an expanded field effort, and this paid off with the U.S. team’s recovery in December 2003 of the first martian meteorite in 9 years, as well as the collection of large numbers of carbonaceous chondrites. Antarctica is such a productive source of meteorites not simply because rocks are easily spotted on icy surfaces, but also because the Antarctic environment actively concentrates meteorites in particular regions. That is, meteorites fall onto the ice and eventually become incorporated into the coastward flowing ice sheets. In those regions where ice flow is impeded by mountain ranges and subsurface obstructions, old deep ice can be forced to the surface. If this occurs in locations where strong winds ablate the surface ice, then meteorites that fell to Earth tens of thousands to millions of years ago will accumulate as a lag deposit. Meteorite densities of as much as one per square meter can be found in some locales.89 Hot Deserts However, at the same time as the potential of Antarctica’s cold deserts as a source of meteorites was being realized, other, smaller groups were exploring the potential of the world’s hot deserts.90 Although these environments lack the active concentration mechanism provided by ice flow and ablation in Antarctica, access to such regions is far less logistically challenging than mounting expeditions to Antarctica. The Sahara has become a major source for meteorites. Many in the U.S. meteorite curation community have avoided dealing in these meteorites because of issues of unknown provenance and likely exportation from Libya. Some of these problems could be overcome with field searches, and the idea has been discussed (e.g., a joint Moroccan-U.S. field party). However, virtually all Saharan meteorites have been shown to have suffered considerable terrestrial weathering and have experienced much higher levels of contamination, and in many cases meteorite components have been replaced by carbon compounds (in this case, carbonates). In general there have been ample opportunities for the introduction or destruction of organic matter. Hot Versus Cold Deserts In considering collection of meteorites in hot as opposed to cold deserts, several factors have to be weighed. These include the relative cost per meteorite of collecting programs in different regions and the scientific utility of the samples collected. As productive as ANSMET is, the cost of supporting large collecting teams in remote locations is considerable in terms of both expense and utilization of scarce logistical resources (e.g., the limited
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Exploring Organic Environments in the Solar System supply of LC-135 transport aircraft that are required to support a broad range of Antarctic research activities, not just meteorite collecting). On the other hand, a typical desert collecting program, such as the highly successful Omani-Swiss program organized by Bern University and the Natural History Museum Bern,91 requires logistical support in the form of four-wheel drive vehicles rather than specially equipped aircraft. Some might argue that it is worthwhile to augment hot-desert collecting programs as an adjunct to the existing activities in Antarctica. Small-scale activities in new locales not only may result in an enhanced supply of scientifically interesting specimens, but also may well have collateral benefits such as jump-starting space-science-related activities in regions and nations not traditionally involved in such endeavors. Others will counter that contamination issues render meteorites collected in hot deserts less scientifically interesting, particularly for organic studies, than their antarctic counterparts. Purchasing Meteorites The attitudes of scientists toward the commercial meteorite trade are quite mixed. Many researchers believe that meteorites are a purely scientific legacy and that it is ethically no different to collect and trade in meteorites than it is to collect and trade in artifacts plundered from archeological sites. Some countries (but not the United States) have laws that protect meteorites from both commercial trade and export. Many scientists favor the adoption of similar legislation in the United States. On the other hand, most researchers have neither the time nor the financial resources to find new meteorite collection areas and systematically search them for specimens (the exception being Antarctica). Thousands of meteorites available to researchers today would not have been found were it not for profit-driven collectors. The archeological-artifact analogy breaks down because, unlike, for example, an Anasazi pot, a meteorite can be divided into pieces without completely destroying its scientific value. Moreover, it can still be studied even completely out of the context of the location where it was originally found. Thus, as a practical matter, most meteorite researchers recognize the reality of the commercial meteorite trade and have no choice but to rely on nonscientists who buy, sell, collect, and search for meteorites in order to obtain research material. Similarly, curators of collections in virtually every museum and university engage in commerce and trade with private meteorite dealers in order to obtain new material. Therefore, many meteorite researchers believe that a strong relationship between the commercial and scientific communities is an appropriate mechanism to maximize the return on the research-dollar. The task group believes that serious consideration should be given to the selective acquisition of scientifically important meteorites. Much of the U.S. supply of the Murchison meteorite owes it existence to NASA funds made available to the Smithsonian Institution to buy a large, private collection shortly after the fall of Murchison. An interesting analogy exists today in Tagish Lake. Although small amounts of material have been made available to the scientific community through the Johnson Space Center, the bulk of the Tagish Lake fall remains in the hands of its finder, who has offered it for sale. 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 of a significant piece of the Tagish Lake meteorite so that it can be made available for study by the broader scientific community. 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. NOTES 1. J.J. Berzelius, “Found Humic Acid in Alais Carbonaceous Chondrite; Decided a Biological Origin Unlikely,” Annalen der Physik und Chemie 33: 113, 1834.
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Exploring Organic Environments in the Solar System 2. M. Berthelot, “A Theoretical Paper Seeking to Explain Presence of Petroleumlike Hydrocarbons in Meteorites in Terms of a Reaction Between Metal Carbides and Water,” Comptes Rendus 67: 849, 1868. 3. O. Botta and J.L. Bada, “Extraterrestrial Organic Compounds in Meteorites,” Surveys in Geophysics 23: 411-467, 2002. 4. L. Becker, R.J. Poreda, and J.L. Bada, “Extraterrestrial Helium Trapped in Fullerenes in the Sudbury Impact Structure,” Science 272: 249-252, 1996. 5. J.R. Cronin, S. Pizzarello, and D.P. Cruikshank, “Organic Matter in Carbonaceous Chondrites, Planetary Satellites, Asteroids and Comets,” pp. 819-857 in Meteorites and the Early Solar System (J.F. Kerridge and M.S. Matthews, eds.), University of Arizona Press, Tucson, Ariz., 1988. 6. J.R. Cronin and S. Chang, “Organic Matter in Meteorites: Molecular and Isotopic Analyses of the Murchison Meteorite,” pp. 209-258 in The Chemistry of Life’s Origin (J.M. Greenberg, C.X. Mendoza-Gómez, and V. Pirronello, eds.), Kluwer Academic Press, The Netherlands, 1993. 7. P.G. Stoks and A.W. Schwartz, “Basic Nitrogen-heterocyclic Compounds in the Murchison Meteorite,” Geochimica et Cosmochimica Acta 46: 309-315, 1982. 8. J.R. Cronin, S. Pizzarello, and D.P. Cruikshank, “Organic Matter in Carbonaceous Chondrites, Planetary Satellites, Asteroids and Comets,” pp. 819-857 in Meteorites and the Early Solar System (J.F. Kerridge and M.S. Matthews, eds.), University of Arizona Press, Tucson, Ariz., 1988. 9. Some nonbiological amino acids that have a small excess of one stereoisomer have been detected in the Murchison meteorite. These are believed to have been formed abiotically. See the section “Chirality: Enantiomeric Excesses Observed in Stereoisomers from Carbonaceous Chondrites” in this chapter. 10. O. Botta and J.L. Bada, “Extraterrestrial Organic Compounds in Meteorites,” Surveys in Geophysics 23: 411-467, 2002. 11. 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Representative terms from entire chapter: