Pathways to Meteoritic Glycine and Methylamine

Glycine and methylamine are meteoritic water-soluble organic compounds that provide insights into the processes that occurred before, during, and after the formation of the Solar System. Both glycine and methylamine and many of their potential synthetic precursors have been studied in astrophysical environments via observations, laboratory experiments, and modeling. In spite of these studies, the synthetic mechanisms for their formation leading to their occurrence in meteorites remain poorly understood. Typical 13C-isotopic values (δ13C) of meteoritic glycine and methylamine are 13C-enriched relative to their terrestrial counterparts; thus, analyses of their stable carbon isotopic compositions (13C/12C) may be used not only to assess terrestrial contamination in meteorites, but also to provide information about their synthetic routes inside the parent body. Here, we examine potential synthetic routes of glycine and methylamine from a common set of precursors present in carbonaceous chondrite meteorites, using data from laboratory analyses of the well-studied CM2 meteorite Murchison. Several synthetic mechanisms for the origins of glycine and methylamine found in carbonaceous chondrites may be possible, and the prevalence of these mechanisms will largely depend on (a) the molecular abundance of the precursor molecules and (b) the levels of processing (aqueous and thermal) that occurred inside the parent body. In this work, we also aim to contextualize the current knowledge about gas-phase reactions and irradiated ice grain chemistry for the synthesis of these species through parent body processes. Our evaluation of various mechanisms for the origins of meteoritic glycine and methylamine from simple species shows what work is still needed to evaluate both, the abundances and isotopic compositions of simpler precursor molecules from carbonaceous chondrites, as well as the effects of parent body processes on those abundances and isotopic compositions. The analyses presented here combined with the indicated measurements will aid a better interpretation of quantitative analysis of reaction rates, molecular stability, and distribution of organic products from laboratory simulations of interstellar ices, astronomical observations, and theoretical modeling.

Solid-State Methylamine VUV Irradiation Study Using Carbon Monoxide as an H Radical Scavenger

Solid-phase methylamine (CH3NH2) was vacuum ultraviolet (VUV) photoprocessed at low temperature (20 K) using a hydrogen flow discharge lamp, which allows irradiation down to 120 nm. Methanimine (CH2=NH), the methylammonium cation (CH3NH3+) and the counterion CN–, as well as the amino radical (NH2), methane (CH4) and ammonia (NH3), were identified as the photoproducts by using FTIR spectroscopy. So far, the branching ratios of the photodissociation pathways of methylamine in the solid phase remain unknown. The methylamine molecule holds two non-equivalent hydrogen atoms on the methyl and the amino group, so we can expect the formation of two distinct radicals via a carbon–hydrogen or a nitrogen–hydrogen bond cleavage, namely CH2NH2 and CH3NH. These radicals are highly reactive and may reform methylamine with hydrogen atom recombination. Their direct infrared spectroscopic detection is therefore tricky. To solve that problem, we use carbon monoxide (CO) as an H radical scavenger, forming the intermediate species HCO. After the irradiation of a CH3NH2 : CO binary ice mixture, formamide (NH2CHO) and N-methylformamide (CH3NHCHO) were identified as the main photoproducts using both infrared and mass spectrometry. We give a rough approximation of the branching ratios, which are in agreement with previous studies in the gas phase.