Interstellar organic matter in meteorites

Abstract

Hydrogen which is highly enriched in deuterium is present in organic matter in a variety of meteorites including non-carbonaceous chondrites. The concentrations of this hydrogen are quite large. For example Renazzo contains 140 μmoles/g of the 10,000‰ δD hydrogen. The D/H ratios of hydrogen in the organic matter vary from 8 × 10^(−5) to 170 × 10^(−5) (δD ranges from − 500‰ to 10,000‰) as compared to 16 × 10^(−5) for terrestrial hydrogen and 2 × 10^(−5) for cosmic hydrogen. The majority of the unequilibrated primitive meteorites contain hydrogen whose D/H ratios are greater than 30 × 10^(−5). If the D/H ratios in these compounds were due to enrichment relative to cosmic hydrogen by isotope exchange reactions, it would require that these reactions take place below 150 K. In addition the organic compounds having D/H ratios above 50 × 10^(−5) would require temperatures of formation of < 120 K. These types of deuterium enrichments must take place by ion-molecule reactions in interstellar clouds where both ionization and low temperatures exist. Astronomically observed D/H ratios in organic compounds in interstellar clouds are typically 180 × 10^(−5) and range between about 40 × 10^(−50 and 5000 × 10^(−5). The D/H values we have determined are the lower limits for the organic compounds derived from interstellar molecules because all processes subsequent to their formation, including terrestrial contamination, decrease their D/H ratios. In contrast, the D/H ratios of hydrogen associated with hydrated silicates are relatively uniform for the meteorites we have analyzed with an average value of 14 × 10^(−5); very similar to the terrestrial value. These phyllosilicates values suggest equilibration of H_2O with H_2 in the solar nebula at temperatures of about 200 K and higher. The ^(13)C/^(12)C ratios of organic matter, irrespective its D/H ratio, lie well within those observed for the earth. If organic matter originated in the interstellar medium, our data would indicate that the ^(13)C/^(12)C ratio of interstellar carbon five billion years ago was similar to the present terrestrial value. Our findings suggest that other interstellar material, representing various inputs from various stars, in addition to the organic matter is preserved and is present in the meteorites which contain the high D/H ratios. We feel that some elements existing in trace quantities which possess isotopic anomalies in the meteorites may very well be such materials.

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