Carbon extraterrestrial origin

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. 

Gooding, J. L., Wentworth, S. J. Zolensky, M. E. (1988) Calcium carbonate and sulfate of possible extraterrestrial origin in the EETA 79001 meteorite. Geochim. Cosmochim. Acta, 52,909-15. 

An alternative to the terrestrial synthesis of the nucleobases is to invoke interstellar chemistry. Martins has shown, using an analysis of the isotopic abundance of 13C, that a sample of the 4.6 billion year old Murchison meteorite which fell in Australia in 1969 contains traces of uracil and a pyrimidine derivative, xanthine. Samples of soil that surrounded the meteor when it was retrieved were also analyzed. They gave completely different results for uracil, consistent with its expected terrestrial origin, and xanthine was undetectable [48], The isotopic distributions of carbon clearly ruled out terrestrial contamination as a source of the organic compounds present in the meteorite. At 0°C and neutral pH cytosine slowly decomposes to uracil and guanine decomposes to xanthine so both compounds could be the decomposition products of DNA or RNA nucleobases. They must have either travelled with the meteorite from its extraterrestrial origin or been formed from components present in the meteorite and others encountered on its journey to Earth. Either way, delivery of nucleobases to a prebiotic Earth could plausibly have been undertaken by meteors. The conditions that formed the bases need not have been those of an early Earth at all but of a far more hostile environment elsewhere in the Solar System. That environment may have been conducive to the production of individual bases but they may never have been able to form stable DNA or RNA polymers this development may have required the less extreme conditions prevalent on Earth. 

These four nonnatural amino acids are of extraterrestrial origin, since isotopic distributions of carbon and nitrogen are different from those of terrestrial amino acids. In addition, pristine samples of a meteorite found in South Pole regions collected under controlled conditions showed similar ee of a-methylated amino acids. The natural amino acids in the meteorites are racemic due to the ionization of their a-proton. In contrast, a-alkylated amino acids cannot completely racemize since they do not have a proton but a methyl group at the a-position. Terrestrial chirality likely originated from distinct meteorites since the a-methyl-amino acids of the Murchison meteorite occur in enantiomeric excesses. It is unknown whether the meteorites contained most of the standard amino acids during about 4.5 billion years the earth and the sun travelled through space. 

Bomstein, Max P, Scott A., and Louis I. AUamandola. Molecules from Space. Scientific American 281 (fuly 1999) 41-49. This article considers the possible extraterrestrial origin of some fairly sizable carbon-containing molecules. 

Ashworth, and Hutchison, 1975 [11] made electron microscopic observations of the hydrous alteration products of olivine in an achondrite and in an ordinary chondrite. Their conclusion was that the Nakhla achondrite, and possibly the Weston chondrite, contain water of extraterrestrial origin which was mobilized by mild shock deformation. Carbonaceous chondrites are believed to be unaltered material left over from the formation of the solar system. They contain substantial amounts of reduced carbon and of water in the form of hydroxyl ions. The oxidation state of iron in some carbonaceous chondrites has been determined by means of Moess-bauer spectroscopy, and it is demonstrated that there is a correlation between the oxidation state of iron and the content of water and reduced carbon in the meteorites (Roy-Poulsen et al., 1981 [284]). 

The Apollo lunar samples have provided the first opportunity for the analysis of material collected from an extraterrestrial body under carefully controlled conditions. No definitive evidence about the concentration and nature of lunar carbon was provided by the a back-scattering experiments carried out by the Sureveyor unmanned landers. However, these remarkably successful experiments did indicate a magmatic origin for the samples analysed1"3. This was confirmed by the Apollo missions, three distinct samples types being collected from the lunar regolith ... 

Cl chondrites contain 6% of their cosmic complement of carbon, mainly in the form of organic matter. The intense controversy that once surrounded the origin of this organic matter has subsided. Most authors now agree that this material represents primitive prebiotic matter, not vestiges of extraterrestrial life. The principal questions remaining are what abiotic processes formed the organic matter, and to what extent these processes took place in locales other than the solar nebula interstellar clouds or meteorite parent bodies. 

Soot condensed in a carbon vapor produced in an electric arc technique contained Cgo and C70 fullerene and linear carbon crystals, (110) = 0.435 nm, with 60 to 80 (probably 70) carbon atoms [96]. The new carbyne model suggests they are hexagonal crystals with six Cn or C12 chains but unfortunately there are no TEM images of these crystals [96]. Amorphous carbon was produced containing both sp and sp bonds [77]. While the VIS-IR spectrum of carbonaceous matter containing carbyne did not resemble Astronomical carbon spectra [97], the notion of carbyne-containing amorphous carbons as precursors in extraterrestrial environments is supported by the meteorite data [67] and laboratory experiments [4]. The original... 

At the present time the balance of evidence is against a cometary origin for prebiotic carbon on Earth, for the same comets would also have delivered water to the Earth and yet the D/H ratio of the terrestrial oceans is different from that in comets. The more likely extraterrestrial input is from asteroids and meteorites, for there is evidence from both lunar and terrestrial samples that the late heavy bombardment event at 3.9 Ga (Section 6.4.1) contributed meteoritic material to the Earth at this time. 

The generation of carbon onions in space has not yet been fuUy elucidated. However, it seems reasonable to assume that they originate from nanoscopic diamond particles. These may be converted into carbon onions upon heating, electron bombardment, or intensive irradiation (Section 4.3.5.4). The existence of nanodiamonds in extraterrestrial material could be confirmed by analyses on different meteorites. Especially the AUende meteorite contains significant amounts of tiny diamond particles (Section 5.1.2). 

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