A nearly terrestrial D/H for comet 67P/Churyumov-Gerasimenko

Isotopic ratios of water can be used to trace the origin of water in our oceans, and comets have been proposed as a potential source. But cometary comae are a mixture of gas and ice-covered dust, and processing on the surface and in the coma change the composition of ice on dust grains relative to that of the nucleus. As the ice on dust grains sublimates, the local coma composition changes. Rosetta coma composition observations of 67P/Churyumov-Gerasimenko (67P/C-G) are local coma measurements, and thus are highly sensitive to being influenced by ice sublimating from dust near the spacecraft.

Previously, the D/H of 67P/C-G was reported to be one of the highest D/H values for a comet. Such a high D/H would require that 67P/C-G formed very far away from the sun. However, this does not agree with all measurements made in other Jupiter Family Comets (JFCs) which have lower D/H ratios. Also, the comet should also have a lot more CO and N₂ than has been observed because these ices also form at large distances from the Sun. Although attempts have been made to address these discrepancy by arguing that comet 67P/C-G is made up of primordial materials from before the solar system formed, questions continued to arise about how a JFC could have such a high D/H. We have developed a new technique for evaluating the Rosetta observations that provides greater reliability in isolating the signal of HDO while providing more accurate estimates of the uncertainties. This method is also faster to apply than linear least squares methods, allowing us to evaluate more than 16,000 measurements of D/H throughout the Rosetta mission. Of these measurements, more than 4000 water isotope measurements had sufficient signal to noise to study variation of D/H over the full mission. These data show that dust dramatically increases local D/H. The isotope ratio measured at a distance from the nucleus where the spacecraft is away from any ice sublimating from dust is close to terrestrial, like that of other JFCs. This lower D/H has implications for understanding comet formation and the role of comets in delivering water to Earth.