Carbonaceous chondrites organic compounds

The authors chose pyruvic acid as their model compound this C3 molecule plays a central role in the metabolism of living cells. It was recently synthesized for the first time under hydrothermal conditions (Cody et al., 2000). Hazen and Deamer carried out their experiments at pressures and temperatures similar to those in hydrothermal systems (but not chosen to simulate such systems). The non-enzymatic reactions, which took place in relatively concentrated aqueous solutions, were intended to identify the subsequent self-selection and self-organisation potential of prebiotic molecular species. A considerable series of complex organic molecules was tentatively identified, such as methoxy- or methyl-substituted methyl benzoates or 2, 3, 4-trimethyl-2-cyclopenten-l-one, to name only a few. In particular, polymerisation products of pyruvic acid, and products of consecutive reactions such as decarboxylation and cycloaddition, were observed the expected tar fraction was not found, but water-soluble components were found as well as a chloroform-soluble fraction. The latter showed similarities to chloroform-soluble compounds from the Murchison carbonaceous chondrite (Hazen and Deamer, 2007). 

Table 10.1 Major groups of soluble organic compounds identified in carbonaceous chondritic meteorites...

Structures of some organic compounds found in carbonaceous chondrites. 

Studier MH, Hayatsu R, Anders E (1965) Organic Compounds in Carbonaceous Chondrites. Science 149 1455... 

At the end of this section dealing with the question What are carbonaceous chondrites and before trying to answer the question Where do they come from and what is their age , it seems necessary to emphasise the fact that the carbon present in the Cl and CM and to a lesser extent in the CO and CV chondrites exists as elemental carbon (Cl, CM, CO, CV), organic molecules (Cl and CM) and an ill-defined macromolecular compound (Cl, CM, CO, CV). The exact composition of this complex mixture will be discussed below. The only point that we would like to stress in this section is that organic matter is always found intimately mixed with mineral matrices. 

Chemical state of carbon in carbonaceous chondrites agrees with that predicted from their formation conditions (indicated by boxes), as inferred from isotopic fractionation of O and C, or abundanc es of volatile metals (Table 1 and Fig. 11 Onuma et al., 1972, 1974 Anders et al., 1976). Cl and C2 chondrites, having formed between 360 and 400 K, contain mainly organic compounds with only traces of carbynes (Whittaker et al., 1980). C3 chondrites contain mainly elemental carbon, which, at least in the case of Allende, is present as carbynes rather than graphite... 

The most ancient organic molecules available for study in the laboratory are those carried to Earth by infalling carbonaceous chondrite meteorites. All the classes of compounds normally considered to be of biological origin are represented in carbonaceous meteorites and, aside from some terrestrial contamination it is safe to assume that these organic species were produced by nonbiolo-gical methods of synthesis. In effect, carbonaceous chondrites are a namral laboratory containing... 

Figure 1 Representative structures of classes of organic compounds identified in carbonaceous chondrites.

Table 1 Abundances of major classes of organic compounds in the Murchison (CM2) carbonaceous chondrite.

Cronin J. R. (1989) Origin of organic compounds in carbonaceous chondrites. Adv. Space. Res. 9, 59-64. 

Sephton M. A. and Gilmour I. (2001b) Compound specific isotope analysis of the organic constituents in carbonaceous chondrites. Mass Spec. Rev. 20, 111-120. 

Studier M. H., Hayatsu R., and Anders E. (1965) Organic compounds in carbonaceous chondrites. Science 149, 1455-1459. 

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. 

The pyrolytic studies on meteorites are commonly done at different temperatures. A preheating step is utilized to insure that any possible adsorbed gases on the surface of the meteorite from the terrestrial environment are eliminated. Several organic compounds are monitored in pyrolysates such as polycyclic aromatic compounds. As an example, the results on naphthalene production upon pyrolysis from several carbonaceous chondrites, normalized by the total carbon content before pyrolysis, are shown in Figure 17.2.1 [76],... 

Based on information from these sources, scientists estimate that the nucleus of a comet consists of about 42 percent volatile compounds (about 80 percent of which is water) and 58 percent solid particles, generally described as "dust." About 45 percent of this nuclear dust is similar in chemical composition to the carbonaceous chondrites found in meteorites that is, they are primarily silicate in composition. (Carbonaceous chondrites are discussed at greater length in the next section.) Another 40 percent of the nuclear dust is organic in nature, that is, composed of carbon compounds of varying degrees of complexity. The final 15 percent of nuclear dust is composed of very small particles with masses of only a few atto-grams (10 l8g). 

Meteorites are the grains of meteoroids or meteors that survive the journey through the Earth s atmosphere and reach the surface. Some are almost pure iron-nickel alloy, whereas others contain silicates and sulphides, and yet others (the carbonaceous chondrites) contain organic compounds. 

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