Origin of organic matter in early solar system—V. Further studies of meteoritic hydrocarbons and a discussion of their origin

Abstract

The Murchison meteorite contains aliphatic and aromatic hydrocarbons similar to those made in static Fischer-Tropsch-type syntheses. Principal compound classes above C 8 are n-alkanes, mono- and dimethylalkanes, alkenes, alkylbenzenes and -naphthalenes. Below C 8, n-alkanes are virtually absent; instead, benzene, toluene, branched alkanes dominate. The CH 4/C 2H 6 ratio is greater than 30, possibly greater than 700. Isoprenoids from C 17 to C 20 occur in a surface rinse but not in subsequent extracts and appear to be terrestrial contaminants. Thiophenes, porphyrin-like pigments and chlorobenzenes were also found; the latter appear to be contaminants. In the Allende meteorite, only methane, benzene, toluene and an aromatic polymer seem to be indigenous. A comprehensive review of current evidence shows that Fischer-Tropsch-type reactions can account for most principal features of meteorite organic matter: hydrocarbons, nitrogen bases, amino acids, carbon-isotope fractionations and possibly pigments. Some features (aromatic hydrocarbons) require a reheating stage; others such as S- and Cl-compounds and the aromatic ‘polymer’ remain unexplained and need to be investigated. No other process is known at present that accounts for an equally wide range of evidence.