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

Abstract

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.   ReferencesAltwegg, K., Balsiger, H., Berthelier, J. J., et al. 2017, Philos. Trans. R. Soc., A, 375, 20160253Altwegg, K., Balsiger, H., & Fuselier, S. A. 2019, Annu. Rev. Astron. Astrophys., 57, 113Balsiger, H., Altwegg, K., Bochsler, P., et al. 2007, Space Sci. Rev., 128, 745Calmonte, U., Altwegg, K., Balsiger, H., et al. 2017, MNRAS, 469, S787Ding, T., Valkiers, S., Kipphardt, H., et al. 2001, Geochim. Cosmochim. Acta, 65, 2433Rubin, M., Bekaert, D. V., Broadley, M. W., Drozdovskaya, M. N., & Wampfler, S. F. 2019, ACS Earth Space Chem., 3, 1792