Carbonaceous chondrites organic material

Piazzarello, S., Cooper, G. W. and Flynn, G. J. (2006) The nature and distribution of the organic material in carbonaceous chondrites and interplanetary dust particles. In Meteorites and the Early Solar System II, eds. Lauretta, D. S. and McSween, H. Y., Jr. Tucson University of Arizona Press, pp. 625-651. A comprehensive, up-to-date review of organic matter in carbonaceous chondrites, but not for the faint-hearted. 

The presence of organic molecules in samples of extraterrestrial matter has been known for more than a century. Some of the greatest chemists of the nineteenth century were involved in the analysis of samples of meteoritic material. They were able to show that carbonaceous chondrites (as they are now named) contain organic molecules. The first to detect carbon in a meteoritic sample was Thenard, in 1806, by analysis of a sample of the Alais meteorite. This result was confirmed in 1834 by Berzelius, who was also the first to detect the presence of water of crystallisation. Working on a sample of the Kaba meteorite, Wohler (1858) confirmed the presence of organic matter, and in a paper dated 1859 said, I am still convinced that besides free carbon this meteorite contains a low-melting point, carbon containing substance which seems to be similar to certain fossil hydrocarbon-like substances... . 

Carbon in the carbonaceous chondrites does not exist as polymer or organic molecules alone. Carbonates are also present in relatively small amounts 20,23) and the same is true for elemental carbon. Elemental carbon seems to exist as carbynes (triple-bonded allotropes of carbon). At least three types of carbynes have been described in Murchison 341 but these results were questioned in 1982 by Smith and Buseck63). According to these authors, sheet silicates mixed with elemental carbon could be misidentified as carbynes in X-ray diffraction patterns. These particular carbonaceous phases (carbynes or otherwise) and other carbonaceous phases (polymer and amorphous carbon phases called C-oe and C- 3) are carriers of noble gases trapped in the chondritic material. Some of these carbynes seem to be condensates from the protosolar nebula while others are probably of presolar origin34 >. 

Carbonaceous grains. Carbonaceous material in chondritic meteorites is highly complex and has diverse origins. Most studies of carbonaceous material have focused on the analysis of residues of insoluble organic material extracted from... 

The materials with highest 13C concentrations are several carbonates from carbonaceous chondrites. Carbonates from the cap rock of salt domes, formed bacterially from CaS04 and oxidized organic substances, show strong 13C... 

About 70-95% of the organic matter in carbonaceous chondrites consists of an ill-defined, insoluble macromolecular material, often referred to as polymer or kerogen . A typical elemental composition for Murchison polymer (Hayatsu et al., 1980a), on a dry, ash-free basis, is C 76.5%, H 4.5%, N 2.4%, S 4.3%, and O 12.4% (by difference). 

Sephton et al. (1998, 1999, 2000) used hydrous pyrolysis followed by supercritical extraction to examine insoluble organic matter in Orgueil (CIl), Murchison (CM2), and Cold Bokkeveld (CM2). The hydrous pyrolysates obtained for the three meteorites displayed a remarkable degree of qualitative similarity suggesting that the macro-molecular materials in different carbonaceous chondrites are apparently composed of essentially the same aromatic structural units, predominantly one to three ring alkyl-substituted aromatic structures. Significant quantitative differences were observed, however, and these were interpreted as indications of the different parent body histories of the three meteorites (Sephton et al., 2000). 

Carbonaceous material is widespread throughout CP IDPs both as discrete inclusions of noncrystalline material and as a semicontinuous matrix with embedded mineral grains. Often it has a vesiculated appearance consistent with an organic component (Figure 7(a)). The bulk abundance of carbon in CP IDPs varies from —4% to 45% with an average of 13% (Keller et al., 1994). In contrast to the fine-grained matrices of carbonaceous chondrites, ordered... 

The Murchison carbonaceous chondite (CM2) provides the most reliable data relevant to organic materials in meteorites [31,32], because tire Murchison is one of the most famous carbonaceous chondrites and preserves many kinds of organic materials. The featiues of the organic materials are summarized as follows. 

Meteorites, especially carbonaceous chondrites, contain a substantial amount of organic material. The organic materials can be divided into two types. One type... 

PAHs found in carbonaceous chondrites show a compositional difference from the sliock-synthesized PAHs in this study. The former are predominantly condensed ring compormds, whereas the latter iu-e dominated by polyphenyl compounds. However, many PAHs reported to be present in carbonaceous chondrites could be produced by the shock reaction from benzene. Major species of PAHs in carbonaceous chondrites such as naphthalene, biphenyl, and phenanthrene were formed abundantly in this study. Fruthermore, the mutual ratios of structural isomers in tire Miuehison meteorite ]149], the Yamato-791198 meteorite [150], and the Yamato-74662 meteorite [151] resemble those of the shock products in our experiments in particular, the coincidence in the ratios of 2-MeNap/l-MeNap and fluorantliene/pyrene is striking. Tliis implies a genetic connection between the shock products and the organic materials in carbonaceous chondrites. 

An alternative source of prebiotic molecules on the early Earth is from meteorites. Carbonaceous chondrite meteorites contain abundant carbon, present in a variety of molecular forms, including amino acids, polyols such as sugars and sugar-alcohols. These molecules are all important to life and form key components of nucleic acids and cell membranes (Cooper et al., 2001). Similar molecular evidence for amino acids is found in even more ancient fragments of the early Universe -IDPs. These molecules formed either in the solar nebula or in an interstellar environment (Flynn et al., 2003). Comets have also been considered a viable potential source of prebiotic molecules although currently the view is that most organic material was delivered by asteroids rather than by comets (Dauphas Marty, 2002). 

Carbonaceous chondrites generally include many kinds of organics, which may have been abiotically synthesized and may record the early thermal histoiy of the solar system. Predominant organic materials detected in carbonaceous chondrites are aromatic polymers—two-, three-, and four-ring PAHs such as naphthalene, phenanthrene, pyrene, and chrysene [149]. Moreover, carbonaceous chondrites include volatile aromatic hydrocarbons such as benzene [42]. 

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