Meteorite organic compounds

In several previous chapters, we have discussed element volatility. Here we focus on some of the most volatile constituents in meteorites - organic compounds, noble gases, and ices. Each of these actually constitutes a voluminous subject of its own in cosmochemistry, and we can only provide overviews of these interesting components. 

It appears that conditions in the solar nebula were appropriate for the FTT but not the Miller-Urey reaction. Kinetic calculations (Lewis and Prinn, 1980) as well as observations on comets (Delsemme, 1977) show that CO and COj, not CH, were the principal forms of carbon. And the dust-laden solar nebula was opaque to UV, precluding any photochemical reactions. It seems best, however, to approach the problem empirically, by examining the meteoritic organic compounds themselves for clues to their formation. We shall review these compounds class by class, looking for the signatures of the FTT or Miller-Urey reactions. 

Vdovykin, G. P. Carbonaceous Matter of Meteorites (Organic Compounds, Diamonds, Graphite), Nauka Publishing Office, Moscow (1967) English translation, NASA TT F-582, Washington, D.C., (1970). 

The organic material in carbonaceous chondrites is of more direct interest with regard to the problem of the origin of life on the Earth, not only because of the possible participation of similar material in the formation of the Earth, but because of the possibility that the mechanisms of formation of terrestrial and meteoritic organic compounds may be related. It has been suggested by Anders et al. (1974) that many of the organic compounds in meteorites may have been formed by Fischer—Tropsch-type reactions. There is reasonable agreement, for example, between certain hydrocarbon isomers... 

Thus, the question is whether such classes of molecules were present on the young Earth. The only witnesses capable of giving an answer to this question are meteorites (Deamer, 1988). The group of David Deamer studied Murchison material after extraction and hydropyrolysis (at 370-570 K, with reaction times of several hours or days). GC and MS analyses showed the presence of a series of organic compounds, including significant amounts of amphiphilic molecules such as octanoic (C ) and nonanoic acids (C9) as well as polar aromatic hydrocarbons. 

D. W. Deamer and J. P. Dworkin have reported in detail on the contribution of chemistry and physics to the formation of the first primitive membranes during the emergence of precursors to life the authors discussion ranges from sources of amphiphilic compounds, growth processes in protocells, self-organisation mechanisms in mixtures of prebiotic organic compounds (e.g., from extracts of the Murchison meteorite) all the way to model systems for primitive cells (Deamer and Dworkin, 2005). 

Observations from meteorite falls suggest that organic compounds that may be prebiotic are available in a concentrated form within meteorite samples. Hot-water washes and other extraction processes show the production of many of the compounds required for life. Most importantly from the ALH84001 find is that material can be moved between planetary bodies and organic molecules can be transported... 

Star formation and the formation of star systems with planets around them, constantly takes place in dense interstellar clouds. The material present in these clouds is incorporated into the objects that are formed during this process. Pristine or slightly altered organic matter from the cloud from which our solar-system was formed is therefore present in the most primitive objects in the solar system comets, asteroids, and outer solar-system satellites. Pieces of asteroids (and perhaps comets) can be investigated with regards to these components through the analyses of meteorites (and eventually in samples returned from these bodies by spacecraft) in laboratories on Earth. The infall of asteroid and comet material from space may have contributed to the inventory of organic compounds on primordial Earth. 

As rich as the Murchinson and other meteorites are, comets are perhaps the most generous source of organic compounds. They are particularly rich in HCN (Miller, 1998) which appears to be very common in space and it has for this reason been investigated by several authors (Ord, 1961 Ferris and coworkers, 1973,1974, and 1978 Matthews, 1975). 

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

The energy from the decay of radioactive elements was probably not an important energy source for the synthesis of organic compounds on the primitive earth since most of the ionization would have taken place in silicate rocks rather than in the reducing atmosphere. The shock wave energy from the impact of meteorites on the earth s atmosphere and sur-... 

Our ideas on the prebiotic synthesis of organic compounds are based largely on the results of experiments in model systems. So it is extremely gratifying to see that such synthesis really did take place on the parent body of the meteorite, and so it becomes quite plausible that they took place on the primitive earth. 

Anders, Edward, Hayatsu, Ryoichi, Studier, Martin H., Organic Compounds in Meteorites, Science (1973) 182, 781. 

On the other hand, the IOM samples from which several percent amounts of organic compounds had been removed by hydrothermolytic treatment (IOM-H) gave results that are in sharp contrast to the above-mentioned meteoritic sample. Here, both (R)- and (S)-pyrimidyl alkanol 12 were obtained equally and indicate the absence of chiral factors in the IOM-H sample, i.e., the results are stochastic. Similar stochastic results were obtained on conducting the asymmetric autocatalysis in the presence of Murchison powders from which all the organic material had been removed by exposure to oxygen plasma. 

Highly sensitive chiral discrimination of amino acids with low ee was described. Amino acids with low ee act as a chiral initiator of asymmetric autocatalysis. In the presence of amino acids with low ee, pyrimidine-5-carbaldehyde was treated with z-P Zn to produce chiral pyrimidyl alkanol with the absolute configuration correlated with that of the amino acid by the consecutive asymmetric autocatalysis with amplification of ee. In addition, direct examination of extraterrestrial chirality was performed using meteorites by applying the asymmetric autocatalysis as the chiral sensor. The results indicated the presence of some chiral factor in the meteorites other than known organic compounds such as amino acids. 

Is this a plausible premise In order to approach this question, we can assume that the mixture of organic compounds in carbonaceous meteorites such as the Murchison meteorite resembles components available on the early Earth through extraterrestrial infall. A series of organic acids represents the most abundant water-soluble fraction in carbonaceous meteorites [ 15,67,68]. Samples of the Murchison meteorite were extracted in an organic solvent commonly used to extract membrane lipids from biological sources [69,70]. When this material was allowed to interact with aqueous phases, one class of compounds with acidic properties was clearly capable of forming membrane-bounded vesicles (Fig. 7). 

Fig. 7 Membranes can be formed by components of carbonaceous meteorites [69,70]. a The Murchison meteorite contains approximately 2% organic carbon by weight, b Organic compounds can be extracted from the meteorite by a lipid solvent system (chloroform-methanol), then separated by two-dimensional chromatography. Polycyclic compounds in the mixture produce fluorescent spots, c The organic acid fraction from the TLC plate readily assembles into membranous vesicles when exposed to dilute aqueous solutions buffered at pH 8-9. The vesicles were photographed by their autofluorescence. Scale bar shows 20 im...

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