Carbonaceous Chondrites and Their Organic Content

Carbonaceous chondrite (CC) meteorites are a subgroup of the stony meteorites that take their name from round glassy inclusions, named chondrules and the distinction ofcontaining organic carbon in various percent abundances (about 1-4%). CC have a primitive elemental composition that is similar to that of the Sun as well as that of the Cosmos overall, since the Sun is a rather average star. They have the appearance ofaggregate rocks (Fig. 6.2), i.e., ofmate-rial packed together without signs of having been transformed by extreme heat or pressure. Their major constituent is a fine-grained silicate matrix, which contains larger mineral inclusions and, interspersed within it, a complex and heterogeneous organic material.2

Because ofthe particularly pristine nature of CC and the general availability ofmeteorites, their organic material offers to direct analysis a unique natural sample of abiotic organic chemistry as it came to be in the Solar System shortly before the onset of life.

Table 6.1 shows the general organic composition ofthe Murchison meteorite, which fell in a large size (over 100 kg) in 1969a and has been extensively studied since. The larger portion of meteoritic organic carbon is sometimes described as kerogen-like because, like terrestrial kerogens, it is an insoluble macromolecular material made up of N, O and S besides C and H and has a complex composition that is not known in much molecular detail. The bulk of this material has been inferred mainly from spectroscopic analysesb and decomposition studies, where it is pyrolyzed by heat or oxidized into its fragments. It is believed to have the hypothetical structure shown in Figure 6.3 and to be composed ofclusters ofaromatic rings, bridged by aliphatic chains containing S, N, O, with peripheral branching and functional groups. The insoluble carbon is not homogeneous, however and ~ 10% is found as self-contained nanostructures, spheres and nanotubes of diverse elemental composition; also found within it are minute amounts of "exotic" carbon, so called because it was likely formed in the envelopes ofstars (i.e., prior to the formation ofSolar System). In addition, it was discovered recently that the material is not completely

*Sandra Pizzarello—Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604 USA. Email: [email protected]

Figure 6.2. An interior fragment of the Murchison meteorite and a microscopic view of its matrix and inclusions. Courtesy of Dr. Michael E. Zolensky, NASA Johnson Space Center.

insoluble after all and can release some individual compounds under conditions ofhigh T and P similar to those ofterrestrial hydrothermal vents (300°C, 100 MPa).

For many carbonaceous meteorites ofthe same type as Murchison as well as others, the remaining portion ofthe organic carbon is made up of soluble compounds that are released from meteorite powders by extraction with water and solvents (Table 6.2).3 As it can be seen by their names and structures, many of the types of individual compounds known to us throughout the Biosphere are found present in meteorites.

Table 6.1. The organic composition of the Murchison

meteorite

A. Carbonate minerals (CO32-)

2-10%

B. Insoluble organic carbon

65-80%

C. Soluble organic compound

10-20%

Meteoritic organic compounds, although in amounts of only parts per million (ppm, micrograms/gram of meteorite), are found in a large variety offorms, which often contain all the possible structures for a given chemical group and up to the limits of their solubility.

For example, while the whole ofterrestrial proteins is made up of just twenty amino acids, all of the a-amino type, over one hundred kinds of meteoritic amino acids are found, which vary over many, sometimes all, of the possible chemical structures involving an alkyl chain, one or two carboxyl- and one or two amino-groups (Scheme 6.1). Ten amino acids are found in both CC and terrestrial proteins: glycine, alanine, serine, threonine, valine, leucine, isoleucine, proline, aspartic acid and glutamic acid (serine and threonine have been detected only recently in the GRA 95229 Antartica meteorite from the Johnson Space Center).

Other examples of organic compounds distributed in both meteorites and the biosphere are nicotinic acid (which, in the form of

Figure 6.3. Make-up of the insoluble organic carbon in the Murchison meteorite.

Table 6.2. The soluble organic compounds of the murchison meteorite

Compound

Typical Molecule and Structure Typical Molecule

Carboxylic acids

Amino acids

Hydroxy acids

Ketoacids

Dicarboxylic acids

Sugar alcohols and acids

Alcohols, aldehydes and ketones

Amines and amides

Pyridine carboxylic acids

Purines and pyrimidines

Hydrocarbons: Alyphatic

Aromatic Polar

Acetic acid Alanine Lactic acid Pyruvic acid Succinic acid Glyceric acid Ethanol Ethyl amine Nicotinic acid

Adenine

Propane Naphthalene

Isoquinoline nicotinamide, is the co-enzyme ofa large group ofoxido-reductases), several monocarboxylic acids (such as formic, acetic and butyric acids that take their names from being found in ants, vinegar and spoiled butter, respectively), sugar-like compounds (components ofmany terrestrial polysaccharides) and a complex amphiphilic material (having both polar and hydrophobic moieties) that is extracted from meteorite powders by chloroform and forms membrane-like vesicles in water.4

The most interesting similarity between meteoritic and biological compounds is that some of the chiral amino acids of meteorites display L-enantiomeric excesses (ee) that, ifnot as large, are similar to the ones found in the amino acids of terrestrial proteins.

So far, the ee have been determined unequivocally for the subgroup of the a-amino-a-alkyl amino acids.5 They vary from 0-15% and represent the only example of molecular asymmetry outside the biosphere (see Chapter 7 for details on molecular chirality and its distribution in the biosphere). The finding ofee in meteoritic amino acids seems to suggest that the trait of chiral asymmetry was at least in part available to prebiotic chemistry. The origin of these ee is not known and could have been the result ofasymmetric influences either during or after chiral amino acid formation. For example, circularly polarized light (CPL) is a form of radiation where the electric and magnetic vectors rotate along the axis of transmission. This rotation can take a clockwise or counter-clockwise turn; therefore there will be a right- and left-CPL that are mirror images of each other, chiral and will each interact differently with the two enantiomers ofa chiral molecule (Chapter 7). If this radiation is "one-handed" and has high enough energy to be destructive of the molecule (as in the case of UV CPL), the two enantiomers will be destroyed at different rates and may leave one enantiomer in a larger amount than the other after exposure.c It has been proposed that such an effect could have taken place during the formation of meteoritic amino acids, possibly at the stage ofprecursor molecules and before reaching the shielding of the asteroidal parent body (see following).

Scheme 6.1. Diverse molecular composition of the organic compounds in meteorites.
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