Sulphur isotopic composition of comet 67P/Churyumov-Gerasimenko: Constraints from C34S2 measured by ROSINA

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. This study aims to determine the 34 S/³2S isotopic ratios of CS₂ in comet 67P/Churyumov–Gerasimenko and to investigate if the CS₂ isotopologue double ratio, ¹2C³4S₂/¹2C³4S³2S-to-¹2C³4S³2S/¹2C³2S₂, is consistent with the statistically expected value. 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 CS₂ signal was high. Three CS₂ isotopologues, 12 C³2S₂, ¹2C³4S³2S, and ¹2C³4S₂, were used to derive three ³4S/^ 32 S isotopic ratios. We report on the first detection of the doubly heavy isotopologue 12 C³4S₂ in a comet. All three derived ³4S/³2S ratios yield consistent results, with δ³4S values ranging from -69. 91 to -5. 49 S. In contrast to the strong mass-dependent fractionation in water isotopologues measured on 67P -- a D₂O/HDO-to-HDO/H₂O ratio of 17, which is far above the equilibrium value of 0. 25 -- our CS₂ isotopologue double ratio yields 0. 2377 These are within 1 to 2σ of the Vienna-Canyon Diablo Troilite (V-CDT) standard, indicating at most a minor depletion in 34 consistent within 1σ of the statistically expected value of 0. 25.