The exogenous delivery of amino acids to the modern Earth: An evaluation from polar ices

Abstract

Exogenous sources have been suggested as an alternative for the Earth-based synthesis of the organic compounds necessary for the origin of life (Chyba and Sagan, 1992). If extraterrestrial sources were significant on the early Earth, they should still be supplying the Earth today with organic material, although at a significantly reduced rate. Based on theoretical considerations, micrometeorites and interplanetary dust particles (IDPs) would be the most important sources of intact organics to the Earth's surface (Anders, 1989: Chyba and Sagan, 1992). Any extraterrestrial organic compounds delivered to the surface of the Earth today would be minuscule in comparison to terrestrial biogenic organic material. The most pristine environment on Earth with respect to terrestrially derived organic components is polar ice. We have searched for the amino acid a-aminoisobutyric acid (AIB) in Greenland ice core samples in order to evaluate the delivery of extraterrestrial organics to the modem Earth (Bada, et al., 1996). This amino acid was selected because, with the exception of a few fungal peptides, it rarely occurs in the terrestrial biosphere. However, it is the most abundant amino acid in many carbonaceous chondrites (Cronin and Pizzarello, 1983). Ice core samples from DYE 3 covering the calendar year period 1860 to 1930, and samples from GISP II from 3000 to 6000 years BP, were analyzed. The ice samples were melted, evaporated to dryness and analyzed as described elsewhere (Bada, et al., 1996). The amount of AIB in the various samples was determined using high performance liquid chromatography (HPLC) with fluorescence detection; to enhance the detection of AIB, the protein amino acids were selectively removed using the ninhydrin reaction. With one exception, the amount of AIB found in all the ice core samples was <l-3x10 -12 g per gram of ice. These are upper limits for the AIB ice content based on the detection limits of the method utilized in the analyses. The one exception in which AIB was detected was in 4270-4440 year old ice from the GISP II site. In this sample AIB was