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This review discusses the organic content of primitive meteorites and their relevance …

Biology Articles » Astrobiology » Organic matter in meteorites » The organic richness of carbonaceous chondrites

The organic richness of carbonaceous chondrites
- Organic matter in meteorites

Organic compounds in carbonaceous chondrites were first observed in 1834 by Berzelius in the Alais carbonaceous chondrite, which fell in France in 1806 [5]. In 1868, Berthelot conducted the first analysis with a sample of Orgueil [4]. Since then, hundreds of investigations have been performed with increasingly sophisticated techniques on several meteorites [45]. Qualitatively and quantitatively analyzing organic molecules in meteorites is an extremely difficult task, as the amounts of some of these compounds are similar to those that would be transferred to the meteorite by just a few fingerprints [40]. Chromatographic and mass spectrometric techniques have, therefore, been widely used with great success. The most valuable information has come from the accurate analysis of the interior of carbonaceous chondrites that were recovered shortly after their fall, that is, that did not experienced extensive terrestrial weathering. In that context, “organized elements” described in the early 1960s in the carbonaceous chondrites Orgueil and Ivuna were initially considered to be evidence that primitive microorganisms were carried to the Earth by meteorites, but were later shown to be contamination products [13]. Also, it has been highly debated whether the Martian meteorite ALH84001, found in Antarctica (there are about 30 Martian meteorites, recognized by their distinctive oxygen isotopic composition and their content of Martian atmospheric gases, trapped in shockproduced glass pockets [34]), contains fossil remains of a past Martian biota or not.

Mixtures of clays, or phyllosilicates, comprise the dominant minerals (50 to 80%) in CI and CM chondrites, and a minor proportion of some CV carbonaceous chondrites. These hydrous minerals resemble some terrestrial clays and were mostly formed as a result of low-temperature aqueous alteration of pre-existing assemblages of anhydrous minerals in asteroids [19]. As an example, Fig. 3 shows the interior of the ALH84034 CM chondrite. Its petrologic analysis indicates that it suffered from severe aqueous alteration on its parent body; both matrix and chondrules had been transformed into clays. Matrix is possibly the least-understood major component of carbonaceous chondrites, but organic matter, which has been intensely studied, is most concentrated in those meteorites, and especially in those portions of the meteorites that experienced the most intense aqueous alteration. In situ textural studies aimed at determining the exact location of the organic matter are still difficult [41]; therefore, its chemical composition is usually studied after chemical extraction from the meteorite.

Refractory organic material

The organic matter in carbonaceous chondrites occurs in multiple forms. A macromolecular material that is extremely fine in grain size, centered at about 10 nm, constitutes the major organic-carbon-bearing phase in all carbonaceous chondrites, representing more than 80% of the total carbon. This complex material is virtually insoluble in solvents and most acids, and appears to be composed of both amorphous and poorly crystalline components, the latter consisting of turbostratic carbon or highly disordered graphite. In the Allende meteorite, a wide range of polycyclic aromatic hydrocarbons (PAHs) with extensively alkylated rings have been observed to be heterogeneously distributed within the matrix [11]. Major species include naphthalene, phenanthrene, anthracene, and their alkyl-substituted derivatives, as well as a wide range of heavier PAHs [48]. The elemental composition of the macromolecular material is about C100H60N7O12S2 [22]. Some fractions of this insoluble material serve as hosts for a variety of distinct noble gases, including several with isotopic compositions that can only be explained in terms of interstellar processes.

Well-defined organic molecules

Solvent-extractable organic molecules in Murchison are distributed among a large variety of compound families with characteristic functional groups (Table 2). Hundreds of individual organic molecules have been found, and surely many more will be identified in the future. Although these organic compounds comprise less than 0.1% of the total mass of Murchison, their structural and functional diversity are particularly intriguing. Amino acids, the building blocks of proteins, and all of the organic bases in nucleic acids, the fundamental carriers of hereditary information for life as we know it, have been found, for example, in various carbonaceous chondrites. Even if life forms were not carried into the Earth in meteorites, the basic building blocks of life might have been. Some molecules can assume two mirror image forms, the so-called optical enantiomers. Abiotically produced substances contain, in most cases, equal amounts of the two forms, which results in optical inactivity, whereas in biological synthesis one form is preferred over the other. While most amino acids and carboxylic acids have optical enantiomers, in Murchison, the majority of these substances do not have optical activity, indicating an abiotic mechanism for their formation. However, the search for homochiral substances in branched α-amino acids, which are not present in terrestrial proteins, revealed that L-enantiomers predominate slightly over D-enantiomers [12]. The origin of optical activity in biological organisms has, in fact, originated a vigorous scientific debate. During the last decade, a series of rather simple experiments have demonstrated the feasibility of producing optically active compounds from achiral materials [3]. Space technology in Earth orbit is being currently used to investigate whether amino acids and peptides required for the emergence of life can be safely transported to Earth vicinity without chemical degradation and racemization.

Some of the organic molecules discovered in Murchison have been identified in other carbonaceous chondrites as well. Among the meteorites for which data are available, the extractable organic molecules have different relative abundances for the various classes of compounds, suggesting that their occurrence may be related to different processes in their individual parent bodies.

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