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Project Progress

Analysis of Organic Compounds in Meteorites and Antarctic Ice

1) Amino Acids in CM1 Carbonaceous Chondrites

Collaborators: Zita Martins, Pascale Ehrenfreund (Leiden Institute of Chemistry, NL)

CM2 carbonaceous chondrites contain a wide variety of complex amino acids, while CI1 types Orgueil and Ivuna display a much simpler composition, with only glycine and β-alanine present in significant abundances. CM1 carbonaceous chondrites show a higher degree of aqueous alteration than CM2-types and may therefore provide information about the influence of aqueous alteration on the amino acid composition. Relative amino acid concentrations have been shown to be indicative for parent body processes with respect to the formation of this class of compounds [1]. In order to understand the relationship of the amino acid composition between these three types of meteorites, we have analyzed three Antarctic CM1 chondrites, MET01070, ALH88045, and LAP02277, and compared their amino acid composition with that of the CM2s Murchison, Murray, LEW90500, ALH83100, as well as the CI1s Orgueil and Ivuna.

The total amino acid concentration in CM1 carbonaceous chondrites was found to be much lower than the CM2s Murchison and LEW90500. This finding alone suggests that amino acids are decomposed during extended aqueous alteration on the parent body, although it can not excluded that leaching in the Antarctic ice sheet may have been a factor as well. The relative amino acid abundances were compared in order to identify trends between the compositions in these types of meteorites. It was found that the three classes of meteorites (CM2, CM1, CI) have very distinct relative amino acid compositions (Figure 1). While in the CM2 and CM1-type meteorites the AIB concentration is as high or even higher than the glycine concentration, AIB is only detected in trace amounts in the CIs. In contrast to the CM2s and the CIs, the CM1s contain large relative abundances of alanine. Finally, the dominating amino acids in the CIs are glycine and β-alanine [2].

Overall, these results support the hypothesis that amino acids in CM and CI-type meteorites were synthesized under different physical and chemical conditions. The differences in the amino acid distributions in the three classes of carbonaceous chondrites may best be explained with differences in the abundances of interstellar precursor compounds in the source regions of their parent bodies in combination with the decomposition of amino acids during extended aqueous alteration.

2) Systematic Study of the Organic Contamination of Antarctic Ice and Meteorites

Collaborators: Daniel P. Glavin, Jason P. Dworkin (GSFC); Zita Martins, Pascale Ehrenfreund (Leiden Institute of Chemistry, NL); Christian Emmenegger, Renato Zenobi (ETH Zurich, CH); Ralph P. Harvey (Case Western Reserve University, Cleveland, USA).
Over the last couple of decades, a large number of meteorites have been recovered from stranding surfaces in Antarctica. Among those were several meteorites from Mars, and ongoing debates focus on the issue if signs of life found in these meteorites [3] are extraterrestrial in nature, or if they are simply terrestrial contamination. In an earlier study, ice samples from regions unrelated to the stranding surfaces have been used as comparison with the meteorite samples, which could lead to possibility that misleading results in the interpretation of these data. This study concentrated on PAHs, and only a small ice sample (150g) from an unspecified location of the Allan Hills Ice sheet was analyzed [4]. On the other hand, studies of EETA79001 and ALH84001 have shown that the relative abundances of amino acids [5,6] and PAHs [7] in the druse (carbonate) material resembled those in an Antarctic ice sample from the Allan Hills region, suggesting that the source of these amino acids may be the ice meltwater. The aim of his project has been to recover ice samples from the immediate proximity of the meteorites and analyze both the ice and meteorite samples with regard to abundances of indigenous organic compounds such as amino acids and polycyclic aromatic hydrocarbons (PAHs).
The PAH studies were carried out at the Dept. for Chemistry and Biochemistry at the ETH in Zurich, Switzerland, using laser desorption/laser ionization mass spectroscopy (L2MS). The amino acid analyses were carried out at the Laboratory for Analytical Chemistry at the Goddard Center for Astrobiology using OPAN/NAC derivatization follwed by analysis using the Liquid Chromatograph-Time of Flight-Mass Spectrometer (LC-ToF-MS) Instrument.

For the ice samples, the PAHs were extracted from the ice water using solid-phase extraction into PVC membranes, which was analyzed using L2MS. The concentration levels of dissolved PAHs were found to be below 10-80 pg/l. The amino acid concentrations in the ice samples were found to be near blank level. However, some of the ices samples were found to contain α-aminoisobutyric acid (AIB) at concentration levels of up to 33±11 ppt.

The PAH composition of the meteorites was found to be variable between and within the specimens. A direct comparison with the soluble PAHs in the ice was not possible due to the low levels of these compounds in the PVC membrane extracts of the ice samples. However, the detection of alkylated homologs of skeletal PAHs in the meteorites indicates that some of these compounds could have a terrestrial origin. The amino acid analysis of the meteorites is in progress.

3) Nucleobases in the Murchison Meteorite

Collaborators: Zita Martins, Pascale Ehrenfreund (Leiden Institute of Chemistry, NL); Daniel P. Glavin, Jason P. Dworkin (GSFC); Marilyn Fogel (Carnegie Institution of Washington, USA); Mark A. Sephton (Imperial College, London, GB); Jonathan Watson (The Open University, Milton Keynes, GB).

We have examined formic acid extracts of two separate fragments of the CM2 chondrite Murchison for the presence of nucleobases using sensitive chromatographic techniques such as high-performance liquid chromatography with ultraviolet spectroscopy (HPLC-UVS, Leiden Institute of Chemistry) and gas chromatography-mass spectrometry (GC-MS, Open University and NASA GSFC). The formic acid extracts were fractionated to limit the presence of interfering compounds during chromatographic analysis. The recoveries of nucleobases were determined in great detail. Xanthine and uracil were found to be the most abundant nucleobases as detected by HPLC-UVS and GC-MS. In contrast to previously published data [8,9], low purine concentrations in the Murchison meteorite extracts were found. Compound specific carbon isotope ratio measurements by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS, Carnegie Institution) revealed non-terrestrial values for xanthine and uracil. Interferences that could compromise these isotope data from a) dicarboxylic acids indigenous to the meteorite, b) co-eluting compounds, and c) nucleobases from the soil at the fall site, could be excluded. Our results indicate that at least one purine and one pyrimidine are indigenous to the Murchison meteorite and therefore advance proposals that life’s raw materials were delivered to the early Earth and other planetary bodies by exogenous sources, including carbonaceous meteorites.

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References

¹ O. Botta, D. P. Glavin, G. Kminek, J. L. Bada, “Relative Amino Acid Concentrations as a Signature for Parent Body Processes of Carbonaceous Chondrites”, Origins Life Evol. Biosphere 32, 143-163 (2002).

² P. Ehrenfreund, D. P. Glavin, O. Botta, G. W. Cooper, J. L. Bada, “Extraterrestrial Amino Acids in Orgueil and Ivuna: Tracing the Parent Body of CI Type Carbonaceous Chondrites”, Proc. Natl. Acad. Sci. USA 98, 2138-2141 (2001).

[3] D. S. McKay, E. K. Gibson, K. L. Thomas-Keprta, H. Vali, C. S. Romanek, S. J. Clemett, X. D. F. Chillier, C. R. Maechling, R. N. Zare, “Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001”, Science 273, 924-930 (1996).

⁴ S. J. Clemett, M. T. Dulay, J. S. Gillette, X. D. F. Chillier, T. B. Mahajan, R. N. Zare, “Evidence for the Extraterrestrial Origin of Polycyclic Aromatic Hydrocarbons in the Martian Meteorite ALH84001”, Faraday Discuss. 109, 417-436 (1998).

⁵ G. D. McDonald and J. L. Bada, “A Search for Endogenous Amino Acids in the Martian Meteorite EETA79001”, Geochim. Cosmochim. Acta 59, 1179-1184 (1995).

⁶ J. L. Bada, D. P. Glavin, G. D. McDonald, L. Becker, “A Search for Endogenous Amino Acids in Martian Meteorite ALH84001”, Science 279, 362-365 (1998).

⁷ L. Becker, D. P. Glavin, J. L. Bada, “Polycyclic Aromatic Hydrocarbons (PAHs) in Antarctic Martian Meteorites, Carbonaceous Chondrites, and Polar Ice”, Geochim. Cosmochim. Acta 61, 475-481 (1997).

⁸ P. G. Stoks, A. W. Schwartz “Uracil in carbonaceous meteorites”, Nature 282, 709-710 (1979).

⁹ P. G. Stoks, A. W. Schwartz, “Nitrogen-heterocyclic compounds in meteorites: significance and mechanisms of formation”, Geochim. Cosmochim. Acta 45, 563-569 (1981).