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ESA's Rosetta spacecraft at comet 67P/Churyumov-Gerasimenko (67P) was thefirst mission that accompanied a comet over a substantial fraction of itsorbit. On board was the ROSINA mass spectrometer suite to measure the localdensities of the volatile species sublimating from the ices inside the comet'snucleus. Understanding the nature of these ices was a key goal of Rosetta. Weanalyzed the primary cometary molecules at 67P, namely H$_2$O and CO$_2$,together with a suite of minor species for almost the entire mission. Ourinvestigation reveals that the local abundances of highly volatile species,such as CH$_4$ and CO, are reproduced by a linear combination of both H$_2$Oand CO$_2$ densities. These findings bear similarities to laboratory-basedtemperature programmed desorption experiments of amorphous ices and imply thathighly volatile species are trapped in H$_2$O and CO$_2$ ices. Our results donot show the presence of ices dominated by these highly volatile molecules.Most likely, they were lost due to thermal processing of 67P's interior priorto its deflection to the inner solar system. Deviations in the proportionsco-released with H$_2$O and CO$_2$ can only be observed before the inboundequinox, when the comet was still far from the sun and the abundance of highlyvolatile molecules associated with CO$_2$ outgassing were lower. Thecorresponding CO$_2$ is likely seasonal frost, which sublimated and lost itstrapped highly volatile species before re-freezing during the previousapparition. CO, on the other hand, was elevated during the same time andrequires further investigation.

Volatiles in the H$_2$O and CO$_2$ ices of comet 67P/Churyumov-Gerasimenko