Synthesis and Delivery of Peptides by Comet Impacts: A Possibility of Chemical Evolution in Enceladus’s Subsurface Sea

Abstract:

Comet impacts are ubiquitous phenomena in the Solar system and the frequency was extremely high in the early history. Comets contain abundant organic materials, including bio-essential amino acids. Thus, it is proposed that comets have delivered organic molecules, which are important for the origins of life, to the outer solar bodies as well as the Earth.

In the study, we conducted shock experiments on icy mixtures of amino acids (glycine and alanine), water ice, and forsterite at cryogenic condition (77 K), simulating comet impacts. The purpose of the study is to examine the effect of impact shock on amino acids in comets. We also focused on the oligomerization of amino acids to produce peptides. Peptides are important molecules for the origins of life, because they are building block of proteins and also act as catalysts to form other biomolecules. In the experiments, the starting mixture was kept frozen in a reaction capsule placed in liquid nitrogen (77K) and was impacted by a vertical propellant gun. The shock pressure range achieved in the experiments was 4.8-26.3 GPa.

The results showed that amino acids were oligomerized into peptides up to tripeptides by impact shock at cryogenic condition. In addition, the yields of linear dipeptides were higher than those of cyclic diketopiperazines. These results are in contrast to the results of shock experiments to amino acid solutions at room temperature, which resulted in the synthesis of comparable amount of diketopiperazines to that of linear peptides (Blank et al., 2001). Thus, the cryogenic condition at impact shock would be a key to facilitate the formation of linear peptides. If we apply our results to the event of icy satellite formation, comet impacts should play an important role to supply linear peptides to the satellites. The shock-synthesized peptides would have been spread over the subsurface seas of the icy satellites (e.g. Enceladus) and might have been an important source for chemical evolution on the satellites.