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The ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instrument aboard the Rosetta mission revolutionized our understanding of cometary material composition. One of Rosetta's key findings is the complexity of the composition of comet 67P/Churyumov-Gerasimenko. Here, we used ROSINA data to analyze dust particles that were volatilized during a dust event in September 2016 and report the detection of large organosulfur species and an increase in the abundances of sulfurous species previously detected in the coma. Our data support the presence of complex sulfur-bearing organics on the surface of the comet. In addition, we conducted laboratory simulations that show that this material may have formed from chemical reactions that were initiated by the irradiation of mixed ices containing H2S. Our findings highlight the importance of sulfur chemistry in cometary and precometary materials and the possibility of characterizing organosulfur materials in other comets and small icy bodies using the James Webb Space Telescope.

On 2019 January 1, the New Horizons spacecraft flew by (486958) Arrokoth, a small body in the Kuiper Belt that is the most distant object ever visited by a spacecraft. A strong unidentified absorption band was observed in the spectrum of Arrokoth at 1.8 μm. We report here experimental evidence suggesting that the near-infrared spectrum of Arrokoth is indicative of sulfur-rich, tholin-like organic residue. The spectra of organic residues produced by irradiating ice mixtures "with H₂S" CH₃OH:NH₃:H₂S:H₂O (3:3:3:1) and "without H₂S" CH₃OH:NH₃:H₂O (3:3:1) were measured to study the effect of H₂S. The "with H₂S" sulfur-rich laboratory-synthesized organic residue displays an absorption band at 1.8 μm that is absent in the spectrum of "without H₂S" sample. This feature matches the Arrokoth spectrum better than any other expected material. This suggests the past presence of H₂S ice on the surface of Arrokoth and a role for Kuiper Belt objects as a key S reservoir in the early solar system.