  The ratios of the three stable oxygen isotopes 16O, 17O and 18O on Earth and,
as far as we know in the solar system, show variations on the order of a few
percent at most, with a few outliers in meteorites. However, in the
interstellar medium there are some highly fractionated oxygen isotopic ratios
in some specific molecules. The goal of this work is to investigate the oxygen
isotopic ratios in different volatile molecules found in the coma of comet
67P/Churyumov-Gerasimenko and compare them with findings from interstellar
clouds in order to assess commonalities and differences. To accomplish this
goal, we analyzed data from the ROSINA instrument on Rosetta during its mission
around the comet. 16O/18O ratios could be determined for O2, methanol,
formaldehyde, carbonyl sulfide and sulfur monoxide/dioxide. For O2 the 16O/17O
ratio is also available. Some ratios are strongly enriched in the heavy
isotopes, especially for sulfur bearing molecules and formaldehyde, whereas for
methanol the ratios are compatible with the ones in the solar system. O2 falls
in-between, but its oxygen isotopic ratios clearly differ from water, which
likely rules out an origin of O2 from water, be it by radiolysis, dismutation
during sublimation or the Eley-Rideal process from water ions hitting the
nucleus as postulated in the literature.
