DO and HDS in the coma of 67P/Churyumov-Gerasimenko.

The European Rosetta mission has been following comet 67P/Churyumov-Gerasimenko for 2 years, studying the nucleus and coma in great detail. For most of these 2 years the Rosetta Orbiter Sensor for Ion and Neutral Analysis (ROSINA) has analysed the volatile part of the coma. With its high mass resolution and sensitivity it was able to not only detect deuterated water HDO, but also doubly deuterated water, DO and deuterated hydrogen sulfide HDS. The ratios for [HDO]/[HO], [DO]/[HDO] and [HDS]/[HS] derived from our measurements are (1.05 ± 0.14) × 10, (1.80 ± 0.9) × 10 and (1.2 ± 0.3) × 10, respectively. These results yield a very high ratio of 17 for [DO]/[HDO] relative to [HDO]/[HO]. Statistically one would expect just 1/4. Such a high value can be explained by cometary water coming unprocessed from the presolar cloud, where water is formed on grains, leading to high deuterium fractionation. The high [HDS]/[HS] ratio is compatible with upper limits determined in low-mass star-forming regions and also points to a direct correlation of cometary HS with presolar grain surface chemistry.This article is part of the themed issue 'Cometary science after Rosetta'.