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

¹ Space Research and Planetary Sciences, Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
² Department of Climate, Space Sciences, and Engineering, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109, USA
³ Space Science Directorate, Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78228, USA
⁴ Department of Physics and Astronomy, The University of Texas at San Antonio, San Antonio, TX 78249, USA
⁵ Belgian Institute for Space Aeronomy, BIRA-IASB, Ringlaan 3, 1180 Brussels, Belgium
⁶ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 25 September 2025
Accepted: 4 January 2026

Abstract

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 ³⁴S/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.

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 CS₂ signal was high. Three CS₂ isotopologues, ¹²C³²S₂, ¹²C³⁴S³²S, and ¹²C³⁴S₂, were used to derive three ³⁴S/³²S isotopic ratios.

Results. We report on the first detection of the doubly heavy isotopologue ¹²C³⁴S₂ in a comet. All three derived ³⁴S/³²S ratios yield consistent results, with δ³⁴S 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 ³⁴S. 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 ± 0.0139, consistent within 1σ of the statistically expected value of 0.25.