G. Schlesinger and S. L. Miller
Department of Chemistry
University of California, San Diego
La Jolla, CA 92093, USA.
Advances in Space Research (1984) 4: 69-74.
Amino and hydroxy acids have been identified in the Murchison meteorite. Their presence is consistent with a synthetic pathway involving aldehydes, hydrogen cyanide and ammonia in an aqueous environment (Strecker-cyanohydrin synthesis). From the various equilibrium and rate constants involved in this synthesis, four independent estimates of the ammonium ion concentration on the parent body at the time of compound synthesis are obtained; all values are about 2 milli-Molar. Succinic acid and beta-alanine have also been detected in the Murchison meteorite. Their presence is consistent with a synthesis from acrylonitrile, hydrogen cyanide and ammonia. Using the equilibrium and rate constants for this synthetic pathway, and the succinic acid/beta-alanine ratio measured in the Murchison meteorite, an estimate of the hydrogen cyanide concentration of 1 milli-M to 10 milli-M is obtained. Since hydrogen cyanide hydrolyzes relatively rapidly in an aqueous environment (t-1/2 < 10,000 years) this high concentration implies a period of synthesis of organic compounds as short as 10,000 years on the Murchison meteorite parent body.
The concentration of ammonia which we have calculated from the hyroxy/amino acid ratios of approximately 2 milli-M seems to be reasonable within the framework of an aqueous model for organic compound synthesis on the parent body. The occurrence of an aqueous environment on the parent body has been proposed before based on the mineralogy of the meteorite but does not mean large bodies of water. The Strecker-cyanohydrin synthesis could have occurred in water droplets on the surface of the parent body or more likely in internal pore water beneath a layer of permafrost.
The estimated concentration of 5 milli-M for total cyanide is more difficult to rationalize. Unlike ammonia, HCN is thermodynamically and kinectically unstable in an aqueous environment. The maximum half-life for the hydrolysis of HCN to formic acid at pH 8 and 0°C is approximately 10,000 years. Polymerization reactions and other sinks for HCN would give an even shorter effective half-life. In order to maintain this concentration of HCN for long periods of time, a substantial synthesis is required. As a consequence, there should be an extensive accumulation of organic compounds in the meteorite. Yet this is not observed. The synthesis of amino and hydroxy acids is also rapid. The half-life of the rate limiting step (the hydrolysis to the amides) is less than 2000 years at 0°C. Both of these factors suggest that a rapid synthesis of the organic compounds took place on the parent body.
This is the first example of applying physical organic chemical considerations to the synthesis and decomposition of organic compounds on the parent body of the carbonaceous chondrites. These considerations may also apply to the primitive earth and the synthesis of organic compounds that formed the first living organism.