Measuring the 34S/32S isotopic ratio in CS2 at comet 67P/Churyumov–Gerasimenko

Comets are thought to be relatively unaltered remnants from the protosolar nebula. Studying their composition may therefore give valuable clues about the processes that governed the formation and evolution of our Solar System. Since the 1980’s, several missions were launched with the objective to study comets in situ, the most recent one being ESA’s Rosetta mission in March 2004. Rosetta underwent a decade-long journey in space before reaching its target comet 67P/Churyumov–Gerasimenko (hereafter 67P) in early August 2014, accompanying it for two more years until end of September 2016 on its orbit around the Sun while continuously taking measurements providing an extensive amount of scientific data. One of the instruments onboard Rosetta was DFMS (Double Focusing Mass Spectrometer) of the ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) suite (Balsiger et al. 2007), utilized to measure in situ volatile gases in 67P’s coma (cf. Rubin et al. (2019) for a review).

A notable result from the Rosetta mission is the measurement by Altwegg et al. (2017) of a D₂O/HDO-to-HDO/H₂O ratio much higher than expected from statistics at thermal equilibrium (17 versus 0.25), suggesting that the ice of 67P formed at very low temperatures and remained cold. Another implication is that species embedded within the ice retain their presolar abundance. In addition, various isotopic ratios of volatiles were found to be non-solar, supporting the hypothesis of a heterogeneous protoplanetary nebula (cf. Altwegg et al. (2019) and references therein). Sulphur, the tenth most abundant element in the Universe, has four stable isotopes: ³²S, ³³S, ³⁴S and ³⁶S, with relative abundances 94.93%, 0.76%, 4.29%, and 0.02%, respectively, according to the V-CDT standard (Ding et al. 2001). For comet 67P, Calmonte et al. (2017) determined the isotopic ratios ³⁴S/³²S and ³³S/³²S for three sulphur-bearing molecules in the coma, namely H₂S, OCS and CS₂, and found that the isotopic abundances for ³³S and ³⁴S, relative to the most abundant one, ³²S, are depleted in the coma compared to the V-CDT standard. Their relative difference to the V-CDT standard value is δ³³S = (-302 ± 29)‰ and δ³⁴S = (-41± 17)‰, respectively.

 

In this work, we extract the ³⁴S/³²S isotopic ratio in CS₂in the coma of 67P using in situ measurements from DFMS. We analyze data during equinox in March 2016, when carbon disulphide had a high production rate and Rosetta was within 10 km from the nucleus centre. Complementary to Calmonte et al. (2017), we include the CS₂isotopologue containing the same heavy sulphur isotope twice, namely C³⁴S₂, in the analysis and deduce ³⁴S/³²S from both the ratio of C³⁴S₂/C³⁴S³²S and C³⁴S³²S/C³²S₂ for comparison to the expected statistical values at thermal equilibrium.

 

References
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Altwegg, K., Balsiger, H., & Fuselier, S. A. 2019, Annu. Rev. Astron. Astrophys., 57, 113
Balsiger, H., Altwegg, K., Bochsler, P., et al. 2007, Space Sci. Rev., 128, 745
Calmonte, U., Altwegg, K., Balsiger, H., et al. 2017, MNRAS, 469, S787
Ding, T., Valkiers, S., Kipphardt, H., et al. 2001, Geochim. Cosmochim. Acta, 65, 2433
Rubin, M., Bekaert, D. V., Broadley, M. W., Drozdovskaya, M. N., & Wampfler, S. F. 2019, ACS Earth Space Chem., 3, 1792