The effect of CO2on gases trapping in cometary ices

Abstract

Water vapour and CO2 have been detected as the major volatile constituents in the coma of comet 67P/Churyumov-Gerasimenko, by ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) onboard the Rosetta spacecraft. In this paper, we describe experimental results showing the influence of CO2 trapping in amorphous water ice on the trapping efficiency of other gases, in particular, N2 and Ar. These gases were detected by ROSINA in situ measurements for the first time in comets. In the present experiments, gas mixtures with CO2 and water vapour were deposited on a cold surface at temperatures ranging between 30 and 60 K, in a thoroughly evacuated vacuum chamber. At these temperatures, the water ice is formed in amorphous state, and it is highly porous with gases trapped in the lattice. Upon heating, changes occur in the ice, and it passes from the amorphous to the crystalline structure. The gases trapped in the ice are released both during these structural changes and upon the sublimation of the ice. The trapping efficiency is determined by monitoring the fluxes of the released gases relative to water vapour during these stages. Here, we examine the behaviour of gas mixtures trapped in amorphous ice, in order to test whether CO2 inhibits or enhances the trapping efficiency of other volatiles. We find various behaviour patterns and present the results in the light of the in situ Rosetta measurements.