Chondrite, carbonaceous

The chondrules contained in the chondrites contain olivine, pyroxene, plagiok-lase, troilite and nickel-iron they can make up 40-90% of the chondrites. Chondrules are silicate spheroids, fused drops from the primeval solar nebula. Because of their differing constitution, chondrites are further subdivided one group in particular is important for the question of the origin of life, and has thus been intensively studied—that of the carbonaceous chondrites. 

Carbonaceous chondrites (C-chondrites) account for only 2-3% of the meteorites so far found, but the amount of research carried out on them is considerable. C-chondrites contain carbon both in elemental form and as compounds. They are without doubt the oldest relicts of primeval solar matter, which has been changed only slightly or not at all by metamorphosis. C-chondrites contain all the components of the primeval solar nebula, apart from those which are volatile they are often referred to as primitive meteorites . 

The pulverized meteorite material is extracted using a series of solvents of differing polarity. The extracts contain mixtures of discrete compounds, such as amino 

All meteorite analyses are made more difficult because of the problem of contamination. Thus one group (Kvenholden, 1970) reported the presence of polycyclic aliphatic compounds, while a second (Studier, 1972) found straight-chain alkanes to be the dominant species. The latter result was often cited and taken as evidence that processes similar to the Fischer-Tropsch synthesis must have occurred in nebula regions of the cosmos. 

Aromatic hydrocarbons were found in more recent analyses pyrene, fluoran-threne, phenanthrene and naphthalene in the ratio of 10 10 5 1 (Cronin, 1998). The majority (around 70%) of the hydrocarbons extracted from the Murchison meteorite are polar compounds such as  

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Water and carbon play critical roles in many of the Earth s chemical and physical cycles and yet their origin on the Earth is somewhat mysterious. Carbon and water could easily form solid compounds in the outer regions of the solar nebula, and accordingly the outer planets and many of their satellites contain abundant water and carbon. The type I carbonaceous chondrites, meteorites that presumably formed in the asteroid belt between the terrestrial and outer planets, contain up to 5% (m/m) carbon and up to 20% (m/m) water of hydration. Comets may contain up to 50% water ice and 25% carbon. The terrestrial planets are comparatively depleted in carbon and water by orders of magnitude. The concentration of water for the whole Earth is less that 0.1 wt% and carbon is less than 500 ppm. Actually, it is remarkable that the Earth contains any of these compounds at all. As an example of how depleted in carbon and water the Earth could have been, consider the moon, where indigenous carbon and water are undetectable. Looking at Fig. 2-4 it can be seen that no water- or carbon-bearing solids should have condensed by equilibrium processes at the temperatures and pressures that probably were typical in the zone of fhe solar... 

Mullie, F. and Reisse, J. Organic Matter in Carbonaceous Chondrites, 139, 83-117 (1986). Murakami, Y. Functionalited Cyclophanes as Catalysts and Enzyme Models. 115, 103-151 (1983). Mutter, M., and Pillai, V. N. R. New Perspectives in Polymer-Supported Peptide Synthesis. 106, 119-175 (1982). 

New computer simulations of the accretion process of the protoearth indicate that only a few large bodies with a high water concentration collided with the Earth during the later bombardment. They apparently came from the same region of the asteroid belt as the carbonaceous chondrites. 

Table 3.2 The commonness of elements (log n) in the solar system, in the sun and in carbonaceous chondrites of type Cl, with respect to hydrogen log n(H) = 12, i.e., n( ) = 1012 (Unsold and...

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). 

UV radiation hypothetical, but so is the transport of molecules from outer space to Earth. Recent analyses of the Murchison meteorite by two scientists from the University of Arizona, Tucson (Cronin and Pizzarello, 1997 Cronin, 1998) have shown it to contain the four stereoisomeric amino acids DL-a-methylisoleucine and DL-a-methylalloisoleucine. In both cases, the L-enantiomer is present in a clear excess (7.0 and 9.1%). Similar results were obtained for two other a-methyl amino acids, isovaline and a-methylvaline. Contamination by terrestrial proteins can be ruled out, since these amino acids are either not found in nature or are present in only very small amounts. Since the carbonaceous chondrites are thought to have been formed around 4.5 billion years ago (see Sect. 3.3.2), the amino acids referred to above must have been subject to one or more asymmetric effects prior to biogenesis. 

Now, apart from the planets, many meteorites were formed, moving in quite different orbits and of quite different chemical composition. In particular, the so-called C-l meteorites composed of carbonaceous chondrites have a composition of elements much closer to that of the Sun. It is proposed (see for example Harder and also Robert in Further Reading) that many of these meteorites collided with very early Earth and became incorporated in it, so that eventually some 15% of Earth came from this material (see Section 1.11). Other planets such as Mars and the Moon could have had similar histories, but the remote planets and Venus are very different. 

The largest class of meteorite finds is stony meteorites, made principally of stone. The general stony classification is divided into three subclasses called chondrites, carbonaceous chondrites and achondrites, and it is at this level of distinction at which we will stop. Before looking at their mineral and isotopic structure in more detail, it is useful to hold the composition of the Earth s crust in mind here for comparison. The Earth s crust is 49 per cent oxygen, 26 per cent silicon, 7.5 per cent aluminium, 4.7 per cent iron, 3.4 per cent calcium, 2.6 per cent sodium, 2.4 per cent potassium and 1.9 per cent magnesium, which must have formed from the common origin of the solar system. 

The Murchison meteorite shown in Figure 6.7, like all meteorites, is named after the place from which it was recovered and in this case it is the town of Murchison, Victoria in Australia about 100 km north of Melbourne. The fall occurred in 1969 and was followed by an analysis of the chemical composition in some considerable detail. The Murchison meteorite is a carbonaceous chondrite containing about 2 per cent carbon, some as inorganic carbonates, and some as soluble compounds such as amino acids but the bulk as a macromolecular heterogeneous material referred to as kerogen. 

Figure 6.9 Laser desorption mass spectra of extracts of the Murchison and Allende carbonaceous chondrites and an extraction from the K/T boundary. (Adapted from Becker L, Poreda R. J. and Bunch T. E., 2000 by permission of the Proceedings of the National Academy of Sciences 97 2979)...

Meteorites General classification into stony, stony-iron and iron, each with an interesting mineralogy, notably the carbonaceous chondrites... 

Carbonaceous chondrite A meteorite containing once-molten globules of rock called chondrules that are surrounded by carbon-containing species. 

Murchison meteorite A carbonaceous chondrite meteorite landing 100 miles north of Melbourne in a town called Murchison. 

Essentially the same amino acids, and nearly equal quantities of D and L enantiomers, were detected in the Murray meteorite, another type II carbonaceous chondrite [6]. Recent expeditions to Antarctica have returned with a large number of meteorites, many of which are carbonaceous chondrites. These may have been protected from terrestrial contamination by the pristine Antarctic ice. Careful analysis of two of these, the Yamato (74662) and the Allan Hills (77306), both type II carbonaceous chondrites, by ion exchange chromatography, gas chromatography, and GC/MS, have detected a wide variety of both protein and non-protein amino acids in approximately equal D and L abundances [9,10]. Fifteen amino acids were detected in the Yamato meteorite and twenty in the Allan Hills, the most abundant being glycine and alanine. The amino acid content of the Yamato meteorite is comparable with that of the Murchison and Murray, but the Allan Hills contains 1/5 to 1/10 that quantity. Unlike earlier meteorites from other locations, the quantities of amino acids in the exterior and interior portions of the Yamato and Allan Hills meteorites are almost identical [9,10]. Thus, these samples may have been preserved without contamination since their fall in the blue ice of Antarctica, which js 250,000 years old in the region of collection. 

Several factors indicate that the amino acids detected in all of these carbonaceous chondrites are indigenous and that they must have originated abiotically. First, the presence of protein and non-protein amino acids, with approximately equal quantities of D and L enantiomers points to a nonbiological origin and precludes terrestrial contamination. In addition, the non-extractable fraction of the Murchison is significantly heavier in 13C than terrestrial samples. Finally, the relative abundances of some compounds detected resemble those of products formed in prebiotic synthesis experiments. The aliphatic hydrocarbons are randomly distributed in chain length, and the C2, C3, and C4 amino acids have the highest concentrations (i.e., the most easily synthesized amino acids with the least number of possible structures are most abundant) [4]. 

Hydroxypyrimidines have been detected in the Murchison, Murray, and Orgueil carbonaceous chondrites in abundances similar to those of amino acids [7]. Earlier analyses of the Orgueil meteorite By thin layer chromatography of organic extracts indicated the presence of melamine, ammeline, adenine, and guanine [8]. Although these could not be confirmed by Folsome, et al., [7] using GC/MS, recent studies by Schwartz [11] and by Hayatsu, et al., [12] have shown that these constituents of the nucleic acids may indeed exist in the carbonaceous chondrites. 

The careful study of at least five different carbonaceous chondrites establishes the fact that these meteorites contain carbon compounds of extraterrestrial origin and of great significance in chemical evolution. Their presence confirms that the chemical reaction paths producing biologically important monomer molecules occur in the far reaches of our solar system. 

Each abundance was divided by the abundance of that element (except for Rh) in Type / carbonaceous chondrites. Rh abundances were divided by Rh abundances in other types of chondrites as Cl values were not available. Errors in the LBL measurements reflect 1 a values of the counting errors, except for the Au error. The latter is the root-mean-square deviation of six measurements, because the six values were not consistent within counting errors. The Os measurement was on a HNO,-insoluble residue that had been fired to 800°C. Key , this work and O, previous work of Ganapathy. 

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