Sulphur isotope mass-independent fractionation observed in comet 67P/Churyumov–Gerasimenko by Rosetta/ROSINA

Abstract : The knowledge about sulphur isotopic fractionation in volatile cometary species is limited as only measurements in five comets have been done and only for 34S/32S. The lack of information about the fractionation of ³³S/³²S makes it impossible to compare them with what is known from refractories. We present results of ³⁴S/³²S and for the first time ³³S/³²S isotopic ratio in H₂S, OCS, and CS₂ in the coma of comet 67P/Churyumov-Gerasimenko. Observations used for this study were performed with Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Double Focusing Mass Spectrometer during October 2014 and May 2016. Bulk isotopic ³³S/³²S and ³⁴S/³²S ratio derived from these three species yield δ³³S = (-302 ± 29)‰ and δ³⁴S = (-41 ± 17)‰ respectively. The observed isotopic fractionation is significantly different from the solar system standard (V-CDT) and all other reported values for solar system objects, except other comets. Furthermore we show that neither mass dependent nor mass-independent fractionation due to photo dissociation as it has been observed in recent laboratory studies can be the cause of the significant depletion compared to solar system standard. In addition, we conclude that there seems to be an intrinsic difference in sulphur isotopic fractionation in cometary volatiles and refractories while the difference between molecules is most likely due to different chemical pathways. The significant fractionation of sulphur isotopes together with a high D₂O/HDO vs. HDO/H₂O and non-solar isotopic ratio for xenon (Marty 2017) as well as for Si (Marty 2017) point towards a non-homogeneously mixed protosolar nebula.