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

TL;DR

This study measures the oxygen isotope ratio in water from comet 67P/Churyumov-Gerasimenko using Rosetta's mass spectrometer, revealing an enrichment of $^{18}$O that supports the idea of primordial cometary water.

Contribution

First in-situ measurement of $^{16}$O/$^{18}$O ratio in cometary water, confirming models of primordial water enrichment in comets.

Findings

$^{16}$O/$^{18}$O ratio is 445 ± 35, indicating 11% enrichment.

Results support models of primordial water formation with 5-20% enrichment.

Consistent with the comet containing ancient, unaltered water.

Abstract

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.