The ${}^{16}$O/${}^{18}$O ratio in Water in the Coma of Comet 67P / Churyumov-Gerasimenko measured with the Rosetta / ROSINA Double-Focusing Mass Spectrometer

The European Space Agency (ESA) spacecraft Rosetta accompanied the Jupiter-family comet (JFC) 67P / Churyumov-Gerasimenko for over two years along its trajectory through the inner solar system. Between 2014 and 2016, it performed almost continuous in-situ measurements of the comet’s gaseous atmosphere in close proximity to its nucleus. In this study, the ${}^{16}$O/${}^{18}$O ratio of H${}_2$O in the coma of 67P / Churyumov-Gerasimenko, as measured by the ROSINA DFMS mass spectrometer on board Rosetta, was determined from the ratio of H${{}_2}^{16}$O / H${{}_2}^{18}$O and ${}^{16}$OH / ${}^{18}$OH. The value of 445 $\pm$ 35 represents an $\sim$ 11% enrichment of ${}^{18}$O compared with the terrestrial ratio of 498.7 $\pm$ 0.1. This cometary value is consistent with the comet containing primordial water, in accordance with leading self-shielding models. These models predict primordial water to be between 5% to 20% enriched in heavier oxygen isotopes compared to terrestrial water. Addendum: The ${}^{16}$O/${}^{17}$O ratio of H${}_2$O in the coma of 67P / Churyumov-Gerasimenko.