Variability of hypervolatile abundances and isotope ratios in the coma of 67P/Churyumov-Gerasimenko

In recently published work we found that because Rosetta coma composition observations of 67P/Churyumov-Gerasimenko (67P/C-G) are local coma measurements, they are highly sensitive to being influenced by ice sublimating from dust near the spacecraft. This means that the local dust cycle on the comet should be considered when driving values meant to reflect the nucleus abundances and isotope ratios. Previous analysis of the Rosetta mass spectrometer (ROSINA DFMS) data took a least squares approach to fitting that stepped through each unknown parameter one parameter at a time. However, each DFMS spectrum has at least six and as many as fifteen free parameters. By fitting one parameter at a time, the uncertainty in the fit of each parameter propagates through the fit creating much greater uncertainties than represented by any difference between the fit and the data. We have developed a new technique for evaluating the Rosetta observations that fits all unknown parameters of a single spectrum. Through this approach we derive more accurate estimates of the true uncertainty in the fit and are able to reliably fit a much larger number of spectra. When we applied this method to derive D/H we found that the previously reported anomalously high D/H was a result of enrichment in D/H on dust grains and was not inherent to the comet. By evaluating measurements in the context of the well-established dust cycle we determined that the nucleus D/H is closer to terrestrial values. These values are more reasonable allowing 67P/C-G to agree with other Jupiter Family Comets (JFCs) as well as the abundances of CO and N₂ measured by DFMS. In this presentation we provide an update to the abundances of hypervolatiles and other isotope ratios base on our new method for fitting the observations and on our understanding of the comet dust cycle.