Simulations of ice chemistry in cometary nuclei

TL;DR

This paper introduces a novel computational model simulating solid-phase chemistry in cometary nuclei over billions of years, revealing insights into complex molecule formation and the origins of observed molecular abundances.

Contribution

It presents the first astrochemical kinetics model for solid-phase chemistry in cometary ices, including photochemistry and cosmic-ray effects, over geological timescales.

Findings

Significant formation of complex organic molecules in cometary ice layers.

High abundances of O₂ and H₂O₂ can result from cosmic-ray processing over 1 Gyr.

Model suggests in situ processing explains observed molecular compositions.

Abstract

The first computational model of solid-phase chemistry in cometary nuclear ices is presented. An astrochemical kinetics model, MAGICKAL, is adapted to trace the chemical evolution in multiple layers of cometary ice, over a representative period of 5 Gyr. Physical conditions are chosen appropriate for "cold storage" of the cometary nucleus in the outer Solar System, prior to any active phase. The chemistry is simulated at a selection of static temperatures in the range 5 - 60 K, while the ice is exposed to the interstellar radiation field, inducing a photochemistry in the outer ice layers that produces significant formation of complex organic molecules. A treatment for the chemistry resulting from cosmic-ray bombardment of the ices is also introduced into the model, along with a new formulation for low-temperature photochemistry. Production of simple and complex molecules to depth on the order of 10~m or more is achieved, with local fractional abundances comparable to observed values in many cases. The production of substantial amounts of O$_2$ (and H$_2$O$_2$) is found, suggesting that long-term processing by high-energy cosmic rays of cometary ices in situ, over a period on the order of 1 Gyr, may be sufficient to explain the large observed abundances of O$_2$, if the overall loss of material from the comet is limited to a depth on the order of 10 m. Entry into the inner solar system could produce a further enhancement in the molecular content of the nuclear ices that may be quantifiable using this modeling approach.