Extraterrestrial origin

It is interesting to note that recent evidence shows that even extra-terrestrially formed hydrocarbons can reach the Earth. The Earth continues to receive some 40,000 tons of interplanetary dust every year. Mass-spectrometric analysis has revealed the presence of hydrocarbons attached to these dust particles, including polycyclic aromatics such as phenanthrene, chrysene, pyrene, benzopyrene, and pentacene of extraterrestrial origin indicated by anomalous isotopic ratios. 

New analyses of material from the interior of the Orgeuil and Ivona meteorites show the presence of P-alanine, glycine and y-amino-n-butyric acid as the main components (0.6-2.0ppm) traces of other amino acids were also detected. The amino acids were present as racemic mixtures, i.e., d/l = 1, so that an extraterrestrial origin can be assumed (Ehrenfreund et al., 2001). 

The question also arises as to where the chiral molecules came from. Were the L-amino acids or the D-sugars selected on the primeval Earth, or are exuaterresuial sources responsible for the homochirality This second possibility is dealt with by hypotheses on the effect of circularly polarised light, of extraterrestrial origin, on chiral molecules in the molecular clouds from which the solar system was formed. One such hypothesis was proposed by Rubenstein et al. (1983) and developed further by others, particularly A. W. Bonner (Bonner and Rubenstein, 1987) both scientists worked at Stanford University. The authors believe that the actual radiation source was synchrotron radiation from supernovae. The excess of one enantiomeric form generated by this irradiation process would have needed to be transported to Earth by comets and meteorites, probably during the bombardment phase around 4.2-3.8 billion years ago. 

Extraterrestrial origins of life Terrestrial origins of life Life was delivered to the Earth (or any planet) by meteorites of cometary material, leading to the idea of panspermia The molecules of life were built on Earth, perhaps in the primordial soup or little warm pool... 

Stony principally silicates or rocky meteorites. It is harder to determine the extraterrestrial origin of these meteorites and it usually requires careful laboratory analysis. 

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. 

In the next four years we hope to have tested the concept of worldwide distribution of the Ir anomaly, tested the premise of an extraordinary extraterrestrial origin of the anomaly, attempted... 

A large number of successful experimental studies which tried to work out plausible chemical scenarios for the origin of life have been conducted in the past (Mason, 1991). A sketch of a possible sequence of events in prebiotic evolution is shown in Figure 3. Most of the building blocks of present day biomolecules are available from different prebiotic sources, from extraterrestrial origins as well as from processes taking place in the primordial atmosphere or near hot vents in deep oceans. Condensation reactions and polymerization reactions formed non-in-structed polymers, for example random oligopeptides of the protenoid type (Fox... 

Hudson, B., Flynn, G. J., Fraundorf, P., Hohenberg, C. M., Shirck, J. (1981) Noble gases in stratospheric dust particles Confirmation of extraterrestrial origin. Science, 211, 383-6. 

Olinger, C. T., Maurette, M., Walker, R. M., Hohenberg, C. M. (1990) Neon measurements of individual Greenland sediment particles proof of an extraterrestrial origin and comparison with EDX and morphological analyses, Earth Planet. Sci. Lett., 100, 77-93. 

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. 

Some N15 enrichment in individual Murchison amino acids (versus terrestrial counterparts) suggests an extraterrestrial origin for an L-enantiomer excess-Engel, M. H., and Macko S. A. Nature, 1997, 389,265. 

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. 

Lawless J. G., Kvenvolden K. A., Peterson E., Ponnamperuma C., and Jarosewich E. (1972) Evidence for amino-acids of extraterrestrial origin in the Orgueil meteorite. Nature 236, 66-67. 

Rajan R. S., Brownlee D. E., Tomandl D., Hodge P. W., Farrar H., and Britten R. A. (1977) Detection of He in stratospheric particles gives evidence of extraterrestrial origin. Nature 267, 133-134. 

In 1794 the German physicist Chladni published a small book in which he suggested the extraterrestrial origin of meteorites. The response was skepticism and disbelief. Only after additional witnessed falls of meteorites did... 

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