Neutral-neutral synthesis of organic molecules in cometary comae

Abstract

Remote and in situ observations of cometary gases have revealed the presence of a wealth of complex organic molecules, including carbon chains, alcohols, imines, and the amino acid glycine. Such chemical complexity in cometary material implies that impacts by comets could have supplied reagents for prebiotic chemistry to young planetary surfaces. However, the assumption that some of the molecules observed in cometary comae at millimetre wavelengths originate from ices stored inside the nucleus has not yet been proven. In fact, the comae of moderately-active comets reach sufficient densities within a few thousand kilometres of the nucleus for an active (solar radiation-driven) photochemistry to ensue. Here, we present results from our latest chemical-hydrodynamic models incorporating an updated reaction network, and show that the commonly-observed HC₃N (cyanoacetylene) and NH₂CHO (formamide) molecules can be efficiently produced in cometary comae as a result of two-body, neutral-neutral, gas-phase reactions involving well-known coma species. In the presence of a near-nucleus distributed source of CN (similar to that observed by the Rosetta spacecraft at comet 67P), we find that sufficient HC₃N and NH₂CHO can be synthesized to match the abundances of these molecules observed previously in Oort cloud comets. The precise coma origins of these (and other) complex organic molecules can be verified through radio interferometric mapping observations, for example using the Atacama Large Millimeter/submillimeter Array.