DISCREPANCIES BETWEEN MEASURED AND CALCULATED ABUNDANCE OF DICARBOXYLIC ACIDS IN CARBONACEOUS CHONDRITES

Organic compounds within carbonaceous chondrites may represent some of the most primitive reduced-carbon species known. These compounds are considered to have been synthesized during water-rock interaction accompanying Fischer-Tropsch synthesis of hydrocarbons or Strecker synthesis of amino acids. Polyfunctional carboxylic acids are among the most abundant compounds present in carbonaceous chondrites. Although a number of researchers have examined the abundance of polyfunctional carboxylic acids in carbonaceous chondrites, less progress has been made in identifying a mechanism for synthesis of these compounds. Following the efforts of Shock and Schulte (1990, GCA 54: 3159-3173) for amino acids, we have calculated the values of ΔG°_f for selected dicarboxylic acids to test whether abundances of these species match abundances measured in meteorites.

Values of ΔG°_f for the aqueous species at infinite dilution were calculated using the model of Yezdimer et al. (2000, Chem. Geol. 164: 259-280). Activities of the compounds of interest were determined through equilibrium mass action equations. Temperature was assigned to 100ºC (Cohen and Coker 2000, Icarus. 145: 369-381); the relative abundances of organic molecules in the study are compared as ratios to malonic acid.

If published measurements are accurate indicators of the actual abundance of species in the Murchison and Yamato-791198 meteorites (Shimoyama and Shigematsu 1994, Chem Lett. (3): 523-526), then the suite of dicarboxylic acids present was not in metastable equilibrium. Along the FMQ buffer, for example, the ratio oxalic:malonic (C2:C3) in the meteorite is too high by ~5 orders of magnitude according to prediction; whereas the ratio succinic:malonic (C4:C3) compares well, and C5:C3 and C6:C3 compare reasonably between predicted and measured ratios. However, as chain length increases (C7, C8, C9), measured dicarboxylic acid ratios deviate from those predicted to lie on the FMQ buffer.

Discrepancies between predicted and measured ratios may imply alteration after departure from the parent body, or alteration during or after entry into the earth's atmosphere; alternatively, extraction techniques could serve to bias results (Shock and Schulte, 1990, GCA 54: 3159-3173).