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

• Published in: Monthly notices of the Royal Astronomical Society 2017-07, Vol.469 (Suppl_2), p.S787-S803
• Main Authors: Calmonte, U, Altwegg, K, Balsiger, H, Berthelier, J-J, Bieler, A, De Keyser, J, Fiethe, B, Fuselier, S A, Gasc, S, Gombosi, T I, Le Roy, L, Rubin, M, Sémon, T, Tzou, C-Y, Wampfler, S F
• Format: Article
• Language: English
• Subjects: Sciences of the Universe
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stx2534

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.