Meteorites, individual Murchison

Nicolussi GK, Pellin MJ, Lewis RS, Davis AM, Amari S, Clayton RN (1998a) Molybdenum isotopic composition of individual presolar silicon carbide grains from the Murchison meteorite. Geochim Cosmochim Acta 62 1093-1104... 

EmUiani C (1966) Paleotemperature analysis of Caribbean core P6304-8 and P6304-9 and a generalized temperature curve for the past 425000 years. J Geol 74 109-126 Emiich K, Ehhalt DH, Vogel JC (1970) Carbon isotope fractionation during the precipitation of calcium carbonate. Earth Planet Sci Lett 8 363-371 Engel MH, Macko SA, SUfer JA (1990) Carbon isotope composition of individual amino acids in the Murchison meteorite. Nature 348 47-49... 

Nicolussi, G. K., Pellin, M. J., Lewis, R. S. et al. (1998) Molybdenum isotopic compositions of individual presolar silicon carbide grains from the Murchison meteorite. Geochimica et Cosmochimica Acta, 62, 1093-1104. 

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. 

Savina M. R., Davis A. M., Tripa C. E., Pellin M. J., Clayton R. N., Lewis R. S., Amari S., Gallino R., and Lugaro M. (2003a) Barium isotopes in individual presolar siUcon carbide grains from the Murchison meteorite. Geochim. Cosmochim. Acta 67, 3201-3214. 

Engel M. H., Macko S. A., and Silfer J. A. (1990) Carbon isotope composition of individual amino acids in the Murchison meteorite. Nature 348, 47-49. 

Gilmour I. and Pillinger C. T. (1994) Isotopic compositions of individual polycyclic aromatic hydrocarbons from the Murchison meteorite. More Not. Roy. Astron. Soc. 269, 235-240. 

Murchison and Allende meteorites contained a substantial number of individual crystalline carbyne grains [21] was almost certainly incorrect. In yet another ultrathin section and acid-resistant residue of the Murchison meteorite C=C functional groups were identified but they were linked to aromatic carbons [61]. The only reliable evidence for carbyne (i.e. chaoite) in meteorites [20] suggests it formed in situ by solid-state carbon annealing that is an acceptable geological process. A tenet of cosmochemistry is that all solids initially formed by condensation from a cooling vapor phase. Whether condensation proceeded at (near) thermodynamic equilibrium, kinetically controlled, metastable equilibrium, or a combination, remains open to debate. Kinetically controlled, metastable condensation is likely for silicates [6,79] and was proposed for carbyne condensation from interstellar polycyanoacetylenes [80]. 

Further examples of lanthanide patterns showing the effects of volatility are found in the highly refractory mineral hibonite (CaAl 20,9). A veritable zoo of patterns is found in individual hibonite grains from the Murchison meteorite (fig. 6). Patterns representing Groups I, II and III are all present and clearly indicate multiple stages of evaporation and recondensation. 

Fig. 6, A wide variety of lanthanide patterns for individual hibonite (CaAli Oig) grains from the Murchison meteorite, which reflect multiple episodes of evaporation and condensation. [Data are from Crozaz and Zinner (1986) and from Zinner and Crozaz (1986). ...

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