2018ApJ...869...98M

Cometary ices are rich sources of organic molecules relevant to prebiotic chemistry and may have played a key role in the delivery of the building blocks of life to Earth. Comets endure thermal processing of their surfaces during their orbits around the Sun. Furthermore, icy grain precursors of cometary materials may have experienced heating in the solar nebula. Despite the important role that this thermal processing could play in the chemistry of ices, little effort has been devoted to understanding the purely thermal reactivity involving electronically stable molecules. Both methylamine (CH₃NH₂) and carbonyl sulfide (OCS) have been detected in the cometary coma of comets, particularly in 67P-Churyumov-Geramenisko (67P) visited by the Rosetta spacecraft. We present here the first laboratory study addressing the reactivity induced by thermal processing of methylamine and OCS mixed ice. Mimicking a realistic scenario within which the surface of the comet is thermally processed as a result of its path close to the Sun, we demonstrate that the methylthiocarbamate anion in salt with methylammonium, [CH₃NHOCS]^– [CH₃NH₃]⁺, and thiocarbamic acid dimer are formed by nucleophilic addition of OCS on methylamine. This study shows that a thermally driven reaction chemically analogous to the one proposed as a condensing pathway of amino acids in the early Earth may take place in cometary ice. The formed thiocarbamate is a known intermediate in the formation of peptides. Furthermore, the consumption of OCS by this reaction with methylamine may account for the missing sulfur in dense clouds and protostars.