Prediction of forbidden ultraviolet and visible emissions in comet 67P/Churyumov-Gerasimenko

TL;DR

This study models the excitation processes of ultraviolet and visible emissions in comet 67P to help interpret spectroscopic data for determining its neutral gas composition, especially CO, CO2, and H2O.

Contribution

It provides the first detailed coupled-chemistry emission model for comet 67P, quantifying excitation processes and emission intensities at different distances and compositions.

Findings

Quantified contributions of excitation processes to emissions.

Calculated emission intensities for various compositions.

Provided baseline data for low out-gassing comets.

Abstract

Remote observation of spectroscopic emissions is a potential tool for the identification and quantification of various species in comets. CO Cameron band (to trace \cod) and atomic oxygen emissions (to trace H$_2$O and/or CO$_2$, CO) have been used to probe neutral composition in the cometary coma. Using a coupled-chemistry emission model, various excitation processes controlling CO Cameron band and different atomic oxygen and atomic carbon have been modelled in comet 67P-Churyumov-Gerasimenko at 1.29~AU (perihelion) and at 3~AU heliocentric distances, which is being explored by ESA's Rosetta mission. The intensities of CO Cameron band, atomic oxygen and atomic carbon emission lines as a function of projected distance are calculated for different CO and CO$_2$ volume mixing ratios relative to water. Contributions of different excitation processes controlling these emissions are quantified. We assess how CO$_2$ and/or CO volume mixing ratios with respect to H$_2$O can be derived based on the observed intensities of CO Cameron band, atomic oxygen, and atomic carbon emission lines.The results presented in this work serve as base line calculations to understand the behaviour of low out-gassing cometary coma and compare them with the higher gas production rate cases (e.g. comet Halley). Quantitative analysis of different excitation processes governing the spectroscopic emissions is essential to study the chemistry of inner coma and to derive neutral gas composition.