Compounds In Carbonaceous Chondrites Research Articles

Deuterium Isotope Fractionation of Polycyclic Aromatic Hydrocarbons in Meteorites as an Indicator of Interstellar/Protosolar Processing History.

The stable isotope composition of soluble and insoluble organic compounds in carbonaceous chondrites can be used to determine the provenance of organic molecules in space. Deuterium enrichment in meteoritic organics could be a residual signal of synthetic reactions occurring in the cold interstellar medium or an indicator of hydrothermal parent-body reactions. δD values have been measured in grains and bulk samples for a wide range of meteorites; however, these reservoirs are highly variable and may have experienced fractionation during thermal and/or aqueous alteration. Among the plethora of organic compounds in meteorites are polycyclic aromatic hydrocarbons (PAHs), which are stable and abundant in carbonaceous chondrites, and their δD ratio may preserve evidence about their formation environment as well as the influence of parent-body processes. This study tests hypotheses about the potential links between PAHs-deuteration concentrations and their formation conditions by examining the δD ratio of PAHs in three CM carbonaceous chondrites representing an aqueous alteration gradient. We use deuterium enrichments in soluble 2–5-ring PAHs as an indicator of either photon-driven deuteration due to unimolecular photodissociation in warm regions of space, gas-phase ion–molecule reactions in cold interstellar regions of space, or UV photolysis in ices. We also test hypothesized reaction pathways during parent-body processing that differ between partially and fully aromatized PAHs. New methodological approaches were developed to extract small, volatile PAHs without fractionation. Our results suggest that meteoritic PAHs could have formed through reactions in cold regions, with possible overprinting of deuterium enrichment during aqueous parent-body alteration, but the data could not rule out PAH alteration in icy mantles as well.

Speciation of organosulfur compounds in carbonaceous chondrites

Despite broad application of different analytical techniques for studies on organic matter of chondrite meteorites, information about composition and structure of individual compounds is still very limited due to extreme molecular diversity of extraterrestrial organic matter. Here we present the first application of isotopic exchange assisted Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) for analysis of alkali extractable fraction of insoluble organic matter (IOM) of the Murchison and Allende meteorites. This allowed us to determine the individual S-containing ions with different types of sulfur atoms in IOM. Thiols, thiophenes, sulfoxides, sulfonyls and sulfonates were identified in both samples but with different proportions, which contribution corroborated with the hydrothermal and thermal history of the meteorites. The results were supported by XPS and thermogravimetric analysis coupled to FTICR MS. The latter was applied for the first time for analysis of chondritic IOM. To emphasize the peculiar extraterrestrial origin of IOM we have compared it with coal kerogen, which is characterized by the comparable complexity of molecular composition but its aromatic nature and low oxygen content can be ascribed almost exclusively to degradation of biomacromolecules.

Hydrogen and carbon isotopic ratios of polycyclic aromatic compounds in two CM2 carbonaceous chondrites and implications for prebiotic organic synthesis

Study of meteoritic organic compounds offers a unique opportunity to understand the origins of the organic matter in the early Solar System. Meteoritic polycyclic aromatic hydrocarbons (PAHs) and heteropolycyclic aromatic compounds (HACs) have been studied for over fifty years, however; their hydrogen stable isotopic ratios (δD) have never been reported. Compound-specific δD measurements of PAHs and HACs are important, in part because the carbon isotopic ratios (δC13) of various meteoritic PAHs cannot be readily distinguished from their terrestrial counterparts and it is difficult to rule out terrestrial contamination based on carbon isotopic ratios alone. In this study, we have extracted and identified more than sixty PAHs and HACs present in two CM2 carbonaceous chondrites Murchison and LON 94101. Their carbon and hydrogen stable isotopic ratios (δC13 and δD) were measured and used to discuss about their synthetic environments and formation mechanisms. The concentration of aromatic compounds is ∼30% higher in Murchison than in the Antarctic meteorite LON 94101, but both samples contained similar suites of PAHs and HACs. All PAHs and HACs found exhibited positive δD values (up to 1100‰) consistent with an extraterrestrial origin, indicating the relatively low δC13 values are indeed an inherent feature of the meteoritic aromatic compounds. The hydrogen isotopic data suggest aromatic compounds in carbonaceous chondrites were mainly formed in the cold interstellar environments. Molecular level variations in hydrogen and carbon isotopic values offer new insights to the formation pathways for the aromatic compounds in carbonaceous chondrites.

THE PHOTOCHEMISTRY OF PYRIMIDINE IN REALISTIC ASTROPHYSICAL ICES AND THE PRODUCTION OF NUCLEOBASES

Nucleobases, together with deoxyribose/ribose and phosphoric acid, are the building blocks of DNA and RNA for all known life. The presence of nucleobase-like compounds in carbonaceous chondrites delivered to the Earth raises the question of an extraterrestrial origin for the molecules that triggered life on our planet. Whether these molecules are formed in interstellar/protostellar environments, in small parent bodies in the solar system, or both, is currently unclear. Recent experiments show that the UV irradiation of pyrimidine (C4H4N2) in H2O-rich ice mixtures that contain NH3, CH3OH, or CH4 leads to the formation of the pyrimidine-based nucleobases uracil, cytosine, and thymine. In this work, we discuss the low-temperature UV irradiation of pyrimidine in realistic astrophysical ice mixtures containing H2O, CH3OH, and NH3, with or without CH4, to search for the production of nucleobases and other prebiotic compounds. These experiments show the presence of uracil, urea, glycerol, hexamethylenetetramine, small amino acids, and small carboxylic acids in all samples. Cytosine was only found in one sample produced from ices irradiated with a higher UV dose, while thymine was not found in any sample, even after irradiation with a higher UV dose. Results are discussed to evaluate the role of the photochemistry of pyrimidine in the inventory of organic molecules detected in meteorites and their astrophysical/astrobiological implications.

Soluble organic compounds in the Tagish Lake meteorite

AbstractThe C2 ungrouped Tagish Lake meteorite preserves a range of lithologies, reflecting variable degrees of parent‐body aqueous alteration. Here, we report on soluble organic compounds, including aliphatic and aromatic hydrocarbons, monocarboxylic acids, and amino acids, found within specimens representative of the range of aqueous alteration. We find that differences in soluble organic compounds among the lithologies may be explained by oxidative, fluid‐assisted alteration, primarily involving the derivation of soluble organic compounds from macromolecular material. In contrast, amino acids probably evolved from precursor molecules, albeit in parallel with other soluble organic compounds. Our results demonstrate the role of parent‐body alteration in the modification of organic matter and generation of prebiotic compounds in the early solar system, and have implications for interpretation of the complement of soluble organic compounds in carbonaceous chondrites.

Irradiated benzene ice provides clues to meteoritic organic chemistry

Aromatic hydrocarbons account for a significant portion of the organic matter in carbonaceous chondrite meteorites, as a component of both the low molecular weight, solvent-extractable compounds and the insoluble organic macromolecular material. Previous work has suggested that the aromatic compounds in carbonaceous chondrites may have originated in the radiation-processed icy mantles of interstellar dust grains. Here we report new studies of the organic residue made from benzene irradiated at 19K by 0.8MeV protons. Polyphenyls with up to four rings were unambiguously identified in the residue by gas chromatography–mass spectrometry. Atmospheric pressure photoionization Fourier transform mass spectrometry was used to determine molecular composition, and accurate mass measurements suggested the presence of polyphenyls, partially hydrogenated polyphenyls, and other complex aromatic compounds. The profile of low molecular weight compounds in the residue compared well with extracts from the Murchison and Orgueil meteorites. These results are consistent with the possibility that solid phase radiation chemistry of benzene produced some of the complex aromatics found in meteorites.

Effects of secondary alteration on the composition of free and IOM-derived monocarboxylic acids in carbonaceous chondrites

Monocarboxylic acids (MCAs) are important astrobiologically because they are often the most abundant soluble compounds in carbonaceous chondrites (CCs) and are potential synthetic end products for many biologically important compounds. However, there has been no systematic study on the effect of parent body alteration on molecular and isotopic variability of MCAs. Since MCAs in meteorites are dominated by low molecular weight (C1–C8), highly volatile compounds, their distributions are likely to be particularly sensitive to secondary alteration processes. In contrast, the aliphatic side chains of insoluble organic matter (IOM) in CCs, whose composition has been shown to be closely related to the MCAs, may be far more resistant to secondary alteration. In the present study, we determined the distributions and isotopic ratios of free and IOM-derived MCAs in six carbonaceous chondrites with a range of classifications: Murchison (CM2), EET 87770 (CR2), ALH 83034 (CM1), ALH 83033 (CM2), MET 00430 (CV3) and WIS 91600 (C2). We compare mineralogical and petrological characteristics to the MCAs distributions to better define the processes leading to the synthesis and alteration of meteoritic MCAs. Our results show that aqueous and especially thermal alteration in the parent bodies led to major loss of free MCAs and depletion of straight relative to branched chain compounds. However, the MCAs derived from aliphatic side chains of IOM are well preserved despite of secondary alterations. The molecular and isotopic similarities of IOM-derived MCAs in different chondrite samples indicate very similar synthetic histories for organic matter in different meteorites.

Stable isotopic compositions of organic compounds in carbonaceous chondrites and their fractionation processes

Stable isotopic compositions of organic compounds in carbonaceous chondrites and their fractionation processes

Asteroids and the origin of life—two steps of chemical evolution on the surface of these objects

Abstract It is now well-known that carbonaceous chondrites contain large quantities of prebiotic molecules, including amino acids, carbohydrates, and heterocyclic bases of nucleic acids. It has become evident that the chemical evolution proceeds under conditions of open space, which serves as a huge chemical reactor. The extraterrestrial origin of these prebiotic molecules has been established on the basis of diversity, molecular structure, and isotopic composition. The molecular composition of a meteorite’s organics resembles abiotic processes governed by physico-chemical processes that occurred on the parent bodies, including asteroids. The synthesis of soluble compounds in carbonaceous chondrites consists of a multi-step process in which organic precursors were formed in the interstellar cloud, incorporated with many volatiles into icy planetesimals and, upon aqueous processing produced the variety of compounds which can form complexes under space energy sources. We show experimentally that the solid mixtures of biologically significant compounds (nucleosides and amino acids) produce more complex compounds when they are exposed to either vacuum UV photons or ionizing radiation.

Carbon and hydrogen isotope fractionation of acetic acid during degradation by ultraviolet light

Low molecular-weight carboxylic acids, such as acetic and propionic acids, are abundant organic compounds in carbonaceous chondrites, and are generally enriched in heavy stable isotopes (i.e., 13C and D) relative to terrestrial organic compounds. In this study, we have determined carbon and hydrogen isotopic fractionation of acetic acid, and site-specific carbon isotopic fractionation of acetic acid, during degradation by ultraviolet (UV) light. Acetic acid became enriched in 13C and D with increased UV exposure times. The isotopic fractionation factors (α) of total, methyl and carboxyl carbon of acetic acid were 0.9922, 1.0022 and 0.9823 respectively, and the α value for hydrogen was 0.9875. These results suggest that UV degradation could be a process for yielding 13C- and D-enrichment of acetic acid in natural environments.

Carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter from the Murchison meteorite

Abstract— Low molecular weight monocarboxylic acids, including acetic acid, are some of the most abundant organic compounds in carbonaceous chondrites. So far, the 13C- and D-enriched signature of water-extractable carboxylic acids has implied an interstellar contribution to their origin. However, it also has been proposed that monocarboxylic acids could be formed by aqueous reaction on the meteorite parent body. In this study, we conducted hydrous pyrolysis of macromolecular organic matter purified from the Murchison meteorite (CM2) to examine the generation of monocarboxylic acids with their stable carbon isotope measurement. During hydrous pyrolysis of macromolecular organic matter at 270–330 °C, monocarboxylic acids with carbon numbers ranging from 2 (C2) to 5 (C5) were detected, acetic acid (CH3COOH; C2) being the most abundant. The concentration of the generated acetic acid increased with increasing reaction temperature; up to 0.48 mmol acetic acid/g macromolecular organic matter at 330 °C. This result indicates that the Murchison macromolecule has a potential to generate at least ˜0.4 mg acetic acid/g meteorite, which is about four times higher than the amount of water-extractable acetic acid reported from Murchison. The carbon isotopic composition of acetic acid generated by hydrous pyrolysis of macromolecular organic matter is ˜-27‰ (versus PDB), which is much more depleted in 13C than the water-extractable acetic acid reported from Murchison. Intramolecular carbon isotope distribution shows that methyl (CH3-)-C is more enriched in 13C relative to carboxyl (-COOH)-C, indicating a kinetic process for this formation. Although the experimental condition of this study (i.e., 270–330 °C for 72 h) may not simulate a reaction condition on parent bodies of carbonaceous chondrite, it may be possible to generate monocarboxylic acids at lower temperatures for a longer period of time.

Quantifying hydrogen‐deuterium exchange of meteoritic dicarboxylic acids during aqueous extraction

Abstract— Hydrogen isotope ratios of organic compounds in carbonaceous chondrites provide critical information about their origins and evolutionary history. However, because many of these compounds are obtained by aqueous extraction, the degree of hydrogen‐deuterium (H/D) exchange that occurs during the process needs to be quantitatively evaluated. This study uses compound‐specific hydrogen isotopic analysis to quantify the H/D exchange during aqueous extraction. Three common meteoritic dicarboxylic acids (succinic, glutaric, and 2‐methyl glutaric acids) were refluxed under conditions simulating the extraction process. Changes in δD values of the dicarboxylic acids were measured following the reflux experiments. A pseudo‐first order rate law was used to model the H/D exchange rates which were then used to calculate the isotope exchange resulting from aqueous extraction. The degree of H/D exchange varies as a result of differences in molecular structure, the alkalinity of the extraction solution and presence/absence of meteorite powder. However, our model indicates that succinic, glutaric, and 2‐methyl glutaric acids with a δD of 1800%***o would experience isotope changes of 38o, 10o, and 6o, respectively during the extraction process. Therefore, the overall change in δD values of the dicarboxylic acids during the aqueous extraction process is negligible. We also demonstrate that H/D exchange occurs on the chiral α‐carbon in 2‐methyl glutaric acid. The results suggest that the racemic mixture of 2‐methyl glutaric acid in the Tagish Lake meteorite could result from post‐synthesis aqueous alteration. The approach employed in this study can also be used to quantify H/D exchange for other important meteoritic compounds such as amino acids.

Primitive Grain Clumps and Organic Compounds in Carbonaceous Chondrites

Refractory interstellar grains acquire tarry polymeric coatings in dense protostellar molecular clouds. Collisions between polymer-coated grains lead to the formation of micron sized grain clumps that are subsequently expelled into the diffuse interstellar medium. Such grains could contain the building blocks of life such as amino acids in their interiors protected from dissociative ultraviolet radiation.

Chemical synthesis of lipids and the origin of life

Keeping in mind the importance of amphiphilic lipids for the formation of semipermeable membranes, a review summary of the sources of appropriate precursors, and chemical reactions for the abiotic synthesis of lipids is presented here within the framework of the theory of chemical evolution. It covers the presence in different cosmic environments of precursors for the formation of the biochemical molecules necessary for the emergence of life on Earth. It starts (1) with a short introduction. Then the following matters are briefly reviewed: (2) The circumstellar and interstellar molecules, some of which, could generate straight chain fatty acids through C9. (3) The possible reactions of hydrogenation and hydrolysis of cyanopolyynes which in the presence first of hydrogen and then liquid water could lead to the formation of aliphatic acids. (4) The composition of comets, where the preliminary analysis by mass spectrometry indicate straight chain hydrocarbons through only C5. (5) The organic compounds in carbonaceous chondrites where aliphatic acids through C12 have been identified, although the branched chain isomers are abundant. (6) The synthesis of some biochemical compounds, such as amino acids present in carbonaceous chondrites, which were probably formed by condensation of presolar precursors, aldehydes and ketones, with HCN in the presence of ammonia and liquid water in the meteorite parent body. The isotopic evidence seems to support this interpretation. (7) The formation of the Earth-Moon system by the catastrophic impact of a Mars-size body with the proto-Earth. (8) The subsequent capture of cometary water, organic and inorganic compounds, which must have led to a very reactive primitive Earth's atmospheric environment. The cometary iron-nickel grains could have catalyzed the formation of fatty acids by Fischer-Tropsch reactions. (9) The laboratory synthesis of straight chain fatty acids from C5 through C20 by Fischer-Tropsch processes. The amounts are usually in excess of the yields of aliphatic hydrocarbons. The chemical synthesis of glycerophospholipids including phosphatidylcholine. (10) The formation of liposomes, primarily, from phosphatidylcholine and the encapsulation within them of biopolymers. (11) Speculations on protocellular models of increasing complexity based on liposomes enclosing catalytic biomolecules. (12) Finally, some of the important problems remaining to be solved concerning the experimental approach to the study of the origin of life are briefly considered. It is hoped that in the next century, significant advances will be made in our understanding of the origin of life on Earth.

Origin of organic compounds in carbonaceous chondrites

Carbonaceous chondrites, a class of primitive meteorite, have long been known to contain their complement of carbon largely in the form of organic, i.e., hydrocarbon-related, matter. Both discrete organic compounds and an insoluble, macromolecular material are present. Several characteristics of these materials provide evidence for their abiotic origin. The principal formation hypotheses have invoked chemistry occurring either in the solar nebula or on the parent body. However, recent stable isotope analyses of the meteorite carboxylic acids and amino acids indicate that they may be related to interstellar cloud compounds. These results suggest a formation scheme in which interstellar compounds were incorporated into the parent body and subsequently converted to the present suite of meteorite organics by the hydrothermal process believed to have formed the clay minerals of the meteorite matrix.

Organic compounds in carbonaceous chondrites and chemical evolution

Organic compounds in carbonaceous chondrites and chemical evolution

Unusual stable isotope ratios in amino acid and carboxylic acid extracts from the Murchison meteorite.

Much effort has been directed to analyses of organic compounds in carbonaceous chondrites because of their implications for organic chemical evolution and the origin of life. We have determined the isotopic composition of hydrogen, nitrogen and carbon in amino acid and monocarboxylic acid extracts from the Murchison meteorite. The unusually high D/H and 15N/14N ratios in the amino acid fraction (delta D = 1,370% after correction for isotope exchange; delta 15N = 90) are uniquely characteristic of known interstellar organic materials. The delta D value of the monocarboxylic acid fraction is lower (377%), but still consistent with an interstellar origin. These results confirm the extraterrestrial origin of both classes of compound, and provide the first evidence suggesting a direct relationship between the massive organo-synthesis occurring in interstellar clouds and the presence of pre-biotic compounds in primitive planetary bodies. The isotope data also bear on the historical problem of distinguishing indigenous material from terrestrial contaminants.

Quantification of monocarboxylic acids in the Murchison carbonaceous meteorite

Although the nature and origin of carbon and its compounds in meteorites have been reviewed by Hayes1 and Nagy2, much of our understanding of carbon and its compounds in carbonaceous chondrites remains speculative. Evidence for the extraterrestrial nature of the solvent-soluble organic matter was first found in the Murchison carbonaceous chondrite3; this material in the Murchison meteorite and in other meteorites has been identified as amino acids and precursors3–10, monocarboxylic acids11, dicarboxylic acids12, hydrocarbons6,13,14, amines15, and other organic compounds16–20. The data suggest a chemical synthesis from simple precursors. Although the mechanisms by which these organic compounds were formed are unknown, theories (such as Fischer-Tropsch Type (FTT)21 and discharge reactions22) have been proposed to account for their presence. Knowledge of the distribution of isomers in these chondrites is essential for understanding the mechanism of synthesis, and we report here the quantification of some of the branched- and straight-chain isomers of the monocarboxylic acids extracted from the Murchison meteorite.

Primitive grain clumps and organic compounds in carbonaceous chondrites

WE show here that the physical conditions in prestellar molecular clouds favour the condensation of complex organic polymers, including amino acids, within a matrix of smaller refractory particles. Such composite grain clumps with dimensions exceeding 1 µm could be expelled along with gaseous material in protostellar cocoons, causing the widespread dispersal of biological activity in the Galaxy. We argue that grain clumps of the type considered here may be identified with μm-sized inclusions in carbonaceous chondrites.

Thermodynamic Equilibrium and the Inorganic Origin of Organic Compounds

Theoretical and experimental support is presented for the hypothesis that many organic compounds may form under conditions of thermodynamic equilibrium. This possibility must be considered along with special effects of selective catalysts, radiation, and degradation from biological matter, in explaining the origin of organic compounds in carbonaceous chondrites. Similar considerations may apply to solar nebulas and planetary atmospheres. The equilibrium distribution of organic compounds at temperatures between 300 degrees K and 1000 degrees K and pressures of 10-(6) to 50 atm for the C-H-O system have been computed. At moderate temperatures and low pressures, conditions where graphite production is inhibited, aromatic compounds may form even in the presence of large excesses of hydrogen. Such conditions exist in the solar nebula and in the atmospheres of some of the major planets. Equilibrium concentrations of a large number of compounds at 1000 degrees K with nitrogen, sulfur, and chlorine added to the system have also been determined. In some cases, a limited equilibrium method is employed in which those few compounds which form with the most difficulty are excluded from the computations, while representatives of all other families of compounds are included. This approach is shown to be useful in the interpretation of certain experimental data in which complete equilibrium has not been attained. We have also found that gases, activated to the plasma state by a high-energy radio frequency field, recombine on cooling to yield product mixtures which are in qualitative agreement with those predicted by the equilibrium computations. We believe that such products can be profitably studied as if at a metastable limited equilibrium.