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

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 ³³/³²S 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 2014 October and 2016 May. Bulk isotopic ³³S/³²S and ³⁴S/³²S ratios derived from these three species yield δ³³S = (−302 ± 29)‰ and δ³⁴S = (−41 ± 17)‰, respectively. The observed isotopic fractionation is significantly different from the assumed Solar system standard [Vienna-Canyon Diablo Troilite (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 photodissociation 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 versus HDO/H₂O and non-solar isotopic ratio for xenon as well as for Si point towards a non-homogeneously mixed protosolar nebula.

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