Sublimation of volatiles from H₂O:CO₂ bulk ices in the context of comet 67P/Churyumov- Gerasimenko

Abstract

Context. The ROSINA instrument on board the Rosetta spacecraft measured, among others, the outgassing of noble gases from comet 67P/Churyumov- Gerasimenko. The interpretation of this dataset and unravelling underlying desorption mechanisms requires detailed laboratory studies. Aims. We aim to improve our understanding of the desorption patterns, trapping, and fractionation of noble gases released from the H₂O:CO₂-dominated ice of comet 67P. Methods. In the laboratory, ice films of neon, argon, krypton, or xenon (Ne, Ar, Kr, and Xe) mixed in CO₂:H₂O were prepared at 15 K. Temperature-programmed desorption mass spectrometry is employed to analyse the desorption behaviour of the noble gases. Mass spectrometric ROSINA data of 67P were analysed to determine the fraction of argon associated with CO₂ and H₂O, respectively. Results. CO₂ has a significant effect on noble gas desorption behaviour, resulting in the co-release of noble gases with CO₂, decreasing the amount of noble gas trapped within water, shifting the pure phase noble gas peak desorption temperature to lower temperatures, and prolonging the trapping of neon. These effects are linked to competition for binding sites in the water ice and the formation of crystalline CO₂. Desorption energies of the pure phase noble gas release were determined and found to be higher than those previously reported in the literature. Enhancement of the Ar/Kr and Ar/Xe ratios are at best 40% and not significantly influenced by the addition of CO₂. Analysis of ROSINA mass spectrometric data shows that the fraction of argon associated with H₂O is 0.53 ± 0.30, which cannot be explained by our laboratory results. Conclusions. Multicomponent ice mixtures affect the desorption behaviour of volatiles compared to simple binary mixtures and experiments on realistic cometary ice analogues are vital to understanding comet outgassing.