Shock processing of interstellar nitrogen compounds in the solar nebula Original Research Article

Some organic material in chondrites (primitive meteorites) exhibits a very low 14N/15N, suggesting that the compounds that carry this heavy nitrogen signature formed in the interstellar medium. Other organic components of the same chondrites show a more solar isotopic signature, suggesting they derive from an isotopically solar reservoir of nitrogen such as N2 or NH3 in the solar nebula. In this work, we model the chemistry of the shocks that have been hypothesized as the mechanism to melt chondrules. We find that such shocks (≈ 8 km/s) do not produce significant amounts of HCN and CN if all nitrogen is initially locked in N2 and all carbon is locked in CO. Only when NH3 or CH4 (or both) were present in the initial pre-shock nebula gas do CN and HCN form. We also find that C2H2 (acetylene) and C2H form in low abundances if the carbon is all locked in CO in the pre-shock gas. The presence of CH4 facilitates the formation of acetylene and related compounds. In the absence of CH4 or NH3, only negligible amounts of species containing CC or CN bonds form. Acetylene and cyanide-related compounds may be precursors to the organics that condensed into meteorites about 4.5 billion years ago. We find that CN bonds largely survive these shocks; thus, the very low interstellar 14N/15N signature can be preserved if the 15N is carried by CN-bearing interstellar compounds.