Organic compounds of carbonaceous chondrites from Antarctica and their implication to a primitive condensate

Abstract

Carbonaceous chondrites contain extraterrestrial organic compounds. Because of little contamination by terrestrial organic matter, we focused our analyses on Antarctic carbonaceous chondrites, Carbon vs nitrogen contents of 10 carbonaceous chondrites yielded a straight line relation in a log-log diagram. It seems that the two contents change almost continuously from CI to C3 chondrites rather than discrete distribution on the types of the chondrites. This straight line relation indicates that the ratio is not constant but increases as the contents of organic compounds decreases. Our estimate shows the ratio is about 11 at the boundary between CI abd C2 chondrites and about 25 between C2 and C3. Extrapolation of the relation toward more primitive than CI chondrites enables to estimate the contents of carbon and nitrogen in such a condensate. Analyses of the Yamato-791198(C2) showed that this chondrite yielded the most abundant amino acids analyzed so far. On the other hand, the Yamato-793321 and Belgica7904 contained no detectable amount of amino acids, in spite of the appropriate abundances of carbon and nitrogen as C2 chondrites. Although there remains a possibility that the depletion of amino acids was due to Antarctic-water washing, it might be caused during the organic formation in the early nebular process. The Yamato-791198 was further analyzed for other organic compounds. Analyses of carboxylic acids showed the presence of acetic to C12 acid in the aliphatic portion with various structural isomers of C 4 to C 7 acids. Some of these organic acids were found at the same level of the concentration as amino acids. Aliphatic and aromatic hydrocarbons were also present in the chondrite. These results of analyses of organic compounds will be discussed in detail together with our postulate of the primitive condensate.