Stable isotopic compositions of organic compounds in carbonaceous chondrites and their fractionation processes

Abstract

A complex mixture of organic compounds, such as amino acids, polycyclic aromatic hydrocarbons, and insoluble organic matter, has been found in carbonaceous chondrites. Extensive studies have revealed that these organic compounds are typically enriched in D, ¹³C, ¹⁵N, and ¹⁸O compared to terrestrial counterparts. These isotopic characteristics are generally regarded as the evidence for their interstellar origin where organic compounds are extremely enriched in heavy isotopes, especially in D. However, because the degree of the D-enrichment significantly differs between interstellar and meteoritic organic compounds, if the latter do have an interstellar origin, some physical/chemical processes which they have experienced after their formation should lower the original enrichments in heavy isotopes for organic compounds. Several laboratory studies have been performed to investigate isotopic fractionations brought by various processes which meteoritic organic compounds may have experienced in space, such as interstellar environments and meteorite parent body. In this paper, I will review the previous laboratory experiments as to isotope fractionation of organic compounds in extraterrestrial environments such as molecular clouds and meteorite parent bodies.