ESA's Rosetta spacecraft at comet 67P/Churyumov-Gerasimenko (67P) was the
first mission that accompanied a comet over a substantial fraction of its
orbit. On board was the ROSINA mass spectrometer suite to measure the local
densities of the volatile species sublimating from the ices inside the comet's
nucleus. Understanding the nature of these ices was a key goal of Rosetta. We
analyzed the primary cometary molecules at 67P, namely H2O and CO2,
together with a suite of minor species for almost the entire mission. Our
investigation reveals that the local abundances of highly volatile species,
such as CH4 and CO, are reproduced by a linear combination of both H2O
and CO2 densities. These findings bear similarities to laboratory-based
temperature programmed desorption experiments of amorphous ices and imply that
highly volatile species are trapped in H2O and CO2 ices. Our results do
not show the presence of ices dominated by these highly volatile molecules.
Most likely, they were lost due to thermal processing of 67P's interior prior
to its deflection to the inner solar system. Deviations in the proportions
co-released with H2O and CO2 can only be observed before the inbound
equinox, when the comet was still far from the sun and the abundance of highly
volatile molecules associated with CO2 outgassing were lower. The
corresponding CO2 is likely seasonal frost, which sublimated and lost its
trapped highly volatile species before re-freezing during the previous
apparition. CO, on the other hand, was elevated during the same time and
requires further investigation.Comment: This is a pre-copyedited, author-produced PDF of an article accepted
for publication in Monthly Notices of the Royal Astronomical Society
following peer review. The version of record is available online at:
https://doi.org/10.1093/mnras/stad300