Molecule dependent oxygen isotopic ratios in the coma of comet 67P/Churyumov-Gerasimenko

TL;DR

This study investigates oxygen isotopic ratios in molecules from comet 67P's coma, revealing molecule-dependent variations that differ from Earth's and solar system values, providing insights into cometary and interstellar chemistry.

Contribution

It provides the first detailed analysis of oxygen isotopic ratios in multiple volatile molecules in a comet's coma, comparing them with interstellar and solar system data.

Findings

Oxygen isotopic ratios vary significantly among molecules.

Sulfur-bearing molecules and formaldehyde are enriched in heavy isotopes.

Oxygen isotopic ratios in O2 differ from water, suggesting different origins.

Abstract

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.