Sulfur is one of the most abundant elements in the universe. In diffuse interstellar clouds the amount of sulfur observed matches the expected value based on stellar nucleosynthesis. However, in dense, cold molecular clouds—where new stars actually form—almost 99 % of the expected sulfur appears to be missing.
This long‑standing issue, known as the “missing sulfur problem,” has led scientists to propose that sulfur is sequestered on icy dust grains, making it difficult to detect.
A recent paper in Astronomy & Astrophysics presents a new computer‑simulation model developed by researchers at the Max Planck Institute for Extraterrestrial Physics and the Centro de Astrobiologia. The model aims to support laboratory results and test current theories of sulfur evolution in interstellar ices.
The simulation incorporates surface reactions on dust grains and the interactions between sulfur and other molecules under the cold conditions of molecular clouds. Findings indicate that sulfur can be bound in sulfur compounds on ice crystals, which hampers its detection in astronomical observations.
Authors stress that further experiments and observations are essential to pinpoint the exact sulfur species and to better understand the processes that lead to ice accumulation. Nonetheless, the model represents a significant step toward resolving the sulfur deficit and offers new avenues for studying star‑forming regions.