Sulphur isotopic composition of comet 67P/Churyumov-Gerasimenko: Constraints from C ³⁴ S ₂ measured by ROSINA

Context . Sulphur is chemically versatile and ubiquitous, and important in both planetary and astrochemical processes. Compared to its cosmic abundance, it appears to be significantly depleted in dense interstellar regions - a phenomenon that is the subject of ongoing research. Comets, which retain material from the early Solar System, provide a valuable record for assessing sulphur reservoirs and their isotopic compositions. Comet 67P/Churyumov-Gerasimenko was particularly suitable for in situ investigation. Aims . This study aims to determine the 34 S/ 32 S isotopic ratios of CS 2 in comet 67P/Churyumov-Gerasimenko and to investigate if the CS 2 isotopologue double ratio, 12 C 34 S 2 / 12 C 34 S 32 S-to- 12 C 34 S 32 S/ 12 C 32 S 2 , is consistent with the statistically expected value. Methods . We analysed high-resolution spectra acquired by the ROSINA Double Focusing Mass Spectrometer (ROSINA/DFMS) in March 2016, when the Rosetta spacecraft was within 17 km of the nucleus and the CS 2 signal was high. Three CS 2 isotopologues, 12 C 32 S 2 , 12 C 34 S 32 S, and 12 C 34 S 2 , were used to derive three 34 S/ 32 S isotopic ratios. Results . We report on the first detection of the doubly heavy isotopologue 12 C 34 S 2 in a comet. All three derived 34 S/ 32 S ratios yield consistent results, with δ 34 S values ranging from −69.91‰ ± 42.75‰ to −5.49‰ ± 51.43‰. These are within 1 to 2σ of the ViennaCanyon Diablo Troilite (V-CDT) standard, indicating at most a minor depletion in 34 S. In contrast to the strong mass-dependent fractionation in water isotopologues measured on 67P - a D 2 O/HDO-to-HDO/H 2 O ratio of 17, which is far above the equilibrium value of 0.25 - our CS 2 isotopologue double ratio yields 0.2377 ± 0.0139, consistent within 1σ of the statistically expected value of 0.25.