How to link the relative abundances of gas species in coma of comets to their initial chemical composition ?

TL;DR

This study develops a quasi 3D model of cometary nuclei to link observed coma compositions with internal structures, revealing how outgassing profiles reflect physical and thermal properties of comets, especially water ice structures.

Contribution

The paper introduces a comprehensive 3D model that accounts for phase changes and water ice structures, providing new insights into the relationship between coma composition and nucleus properties.

Findings

Outgassing profiles indicate physical and thermal properties of the nucleus.

Day/night variations help distinguish water ice structures.

Relative abundances vary significantly with solar distance and ice structure.

Abstract

The chemical composition of comets is frequently assumed to be directly provided by the observations of the abundances of volatile molecules in the coma. The present work aims to determine the relationship between the chemical composition of the coma, the outgassing profile of volatile molecules and the internal chemical composition, and water ice structure of the nucleus, and physical assumptions on comets. To do this, we have developed a quasi 3D model of a cometary nucleus which takes into account all phase changes and water ice structures (amorphous, crystalline, clathrate, and a mixture of them); we have applied this model to the comet 67P/Churyumov-Gerasimenko, the target of the Rosetta mission. We find that the outgassing profile of volatile molecules is a strong indicator of the physical and thermal properties (water ice structure, thermal inertia, abundances, distribution, physical differentiation) of the solid nucleus. Day/night variations of the rate of production of species helps to distinguish the clathrate structure from other water ice structures in nuclei, implying different thermodynamic conditions of cometary ice formation in the protoplanetary disc. The relative abundance (to H2O) of volatile molecules released from the nucleus interior varies by some orders of magnitude as a function of the distance to the sun, the volatility of species, their abundance and distribution between the trapped and condensed states, the structure of water ice, and the thermal inertia and other physical assumptions (dust mantle, ...) on the nucleus.