One of astronomy’s most persistent chemical mysteries is why a major part of the sulfur reservoir appears to be missing from dense interstellar clouds. In a new study led by the Center for Astrochemical Studies (CAS) and conducted in collaboration with the Centro de Astrobiología in Madrid, MPE scientists combined laboratory experiments and advanced computer modeling to investigate how sulfur-bearing molecules evolve on icy grains in interstellar space. Their findings suggest that current theories of sulfur chemistry in the cosmos remain incomplete — but also point toward new ways of closing the gap.
Astronomers have long known that sulfur should be far more abundant in dense interstellar clouds than observations indicate. This implies that most of the sulfur reservoir is in a form that is difficult to detect, highly likely residing in the ice covering interstellar dust grains. To shed more light on this “missing sulfur problem”, MPE researchers simulated the irradiation of frozen mixtures of carbon dioxide (CO2) and carbon disulfide (CS2) at temperatures near absolute zero, mimicking conditions inside dark molecular clouds where stars and planets form. Using the pyRate astrochemical code developed at CAS, adapted specifically for the experiment, the team tracked how ultraviolet radiation transforms sulfurbearing ices over time.
The simulations successfully reproduced several key chemical processes seen in the laboratory. But the model also exposed major uncertainties in current understanding of sulfur chemistry. Some compounds — including OCS, CS, and SO — formed too efficiently in the simulations, while others, such as sulfur dioxide and sulfur allotropes, were underproduced. “The discrepancy between the simulations and experiments highlights how limited our knowledge of the evolution of sulfur-bearing compounds under interstellar conditions still is”, says Olli Sipilä, a postdoctoral researcher at MPE who led the study. “However, performing simulations tailored to mimic experiments helps us understand the experimental results better, and also makes it possible to constrain effects that occurred during the experiment but which could not be directly detected.”
Toward Uncovering the Hidden Sulfur Reservoir
Another major finding of the work was that nondiffusive chemistry — chemical reactions occurring without the need for molecules to migrate across the ice surface — is essential for reproducing many of the sulfurbearing compounds observed experimentally. “It is clear that customary models where reactivity is limited by the reactants diffusing on the ice simply cannot reproduce the experimental findings”, says Wiebke Riedel, a postdoctoral researcher and recent CAS graduate who developed the implementation of nondiffusive chemistry in pyRate.
The work represents the first attempt to model a complex, multicomponent interstellar ice experiment using a rate-equation astrochemical code, marking an important milestone for the field. By combining experimental and theoretical approaches, the study offers a new framework for investigating how sulfur is stored and transformed in space — a question closely tied to the chemistry that shapes emerging planetary systems and, ultimately, the ingredients available for life.
The work represents the first attempt to model a complex, multicomponent interstellar ice experiment using a rate-equation astrochemical code, marking an important milestone for the field. By combining experimental and theoretical approaches, the study offers a new framework for investigating how sulfur is stored and transformed in space — a question closely tied to the chemistry that shapes emerging planetary systems and, ultimately, the ingredients available for life.
Contacts:
Dr. Olli Sipilä
Postdoc at Center for Astrochemical Studies
Tel: +49 89 30000-3646
Email: osipila@mpe.mpg.de
Max-Planck-Institut für extraterrestrische Physik, Garching
Dr. Wiebke Riedel
Postdoc at Center for Astrochemical Studies
Tel: +49 89 30000-3007
Email: riedel@mpe.mpg.de
Max-Planck-Institut für extraterrestrische Physik, Garching
Original Publication
O. Sipilä, R. Martín-Doménech, W. Riedel, D. Navarro-Almaida, A. Fuente, A. Taillard, G.M. Muñoz Caro
Modeling the UV-photon irradiation of CS2-bearing ices in the laboratory with the pyRate gas-grain astrochemical code
Astronomy & Astrophysics
Source | DOI
Further Information
January 23, 2026
Astrophysicists Discover Largest Sulfur-Containing Molecular Compound in Space
July 04, 2024
Using the JWST, a team of researchers including Paola Caselli and Michela Giuliano from MPE, have probed deep into dense cloud cores, revealing details of interstellar ice that were previously unobservable. The study focuses on the Chamaeleon I region, using JWST’s NIRCam to measure spectroscopic lines towards hundreds of stars behind the cloud.
