Neutral-Neutral Synthesis of Organic Molecules in Cometary Comae

TL;DR

This study demonstrates that complex organic molecules in cometary comae can be formed through neutral-neutral gas-phase reactions, challenging the assumption that they originate solely from cometary ices.

Contribution

The paper introduces chemical-hydrodynamic models showing efficient formation of organic molecules in cometary comae via neutral-neutral reactions, supported by updated reaction networks.

Findings

HC3N and NH2CHO can be produced in comae matching observed abundances.

Neutral-neutral reactions are sufficient for complex organic molecule synthesis.

Model results align with observations from comets like 67P.

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 kilometers 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 gases. 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 HC$_3$N and NH2CHO can be synthesized to match the abundances of these molecules in previous observations of Oort Cloud comets. The precise origin of these (and other) complex organic molecules in cometary comae can be verified through interferometric mapping observations, for example, using the Atacama Large Millimeter/submillimeter Array (ALMA).