Shock processing of interstellar nitrogen compounds in the solar nebula

Some organic material in chondrites (primitive meteorites) exhibits a very low ¹⁴ N/ ¹⁵ N, 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 N ₂ or NH ₃ 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 N ₂ and all carbon is locked in CO. Only when NH ₃ or CH ₄ (or both) were present in the initial pre-shock nebula gas do CN and HCN form. We also find that C ₂ H ₂ (acetylene) and C ₂ H form in low abundances if the carbon is all locked in CO in the pre-shock gas. The presence of CH ₄ facilitates the formation of acetylene and related compounds. In the absence of CH ₄ or NH ₃ , only negligible amounts of species containing CΞC or CΞN 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 ¹⁴ N/ ¹⁵ N signature can be preserved if the ¹⁵ N is carried by CΞN-bearing interstellar compounds.