Origin of sulfur allotropes on the carbonaceous asteroid Ryugu and implications to the sulfur chemistry in the interstellar medium.

The unexpected detection of the sulfur allotropes (S8, S7, and S6) on the carbonaceous asteroid Ryugu following the Hayabusa2 mission has offered insights into the unconventional sulfur chemistry in space. These molecules can be traced to processes occurring in the early stages of the evolution of cold molecular clouds, where interstellar icy mantles facilitate the formation of complex sulfur-bearing species by energetic processing. Exploiting interstellar ice analogs, the present study affords compelling evidence on the production of sulfur allotropes (S8, S7, and S6) in sulfur-doped interstellar analog ices composed of sulfur dioxide (SO2), hydrogen sulfide (H2S), and water (H2O) using soft photoionization reflectron time-of-flight mass spectrometry. The synthesis of these allotropes offers fundamental insights into their possible origin on Ryugu while also presenting a plausible mechanism for the sulfur depletion in cold molecular clouds. Notably, the oxidation of sulfur (-II) in hydrogen sulfide (H2S) represents the driving force in sulfur allotrope formation.