A new computer simulation model helps explain the universe's "missing sulfur" problem. Scientists have long observed a discrepancy in sulfur levels between diffuse interstellar clouds and dense, cold molecular clouds. While diffuse clouds show expected sulfur amounts, dense clouds appear to have 99 percent less sulfur than anticipated.
Researchers from the Max Planck Institute for Extraterrestrial Physics and the Centro de Astrobiologia published their findings in *Astronomy & Astrophysics*. Their model, written in the Python-based application pyRate, simulates chemical interactions between ice and gas phases. This marks the first successful rate-equation simulation of multicomponent interstellar ice analog chemistry.
The simulation replicated a 2024 laboratory experiment involving carbon dioxide and carbon disulfide cooled to ten Kelvin. The mixture was exposed to vacuum-ultraviolet (VUV) photons. This process broke molecules apart, creating new sulfur-bearing chemicals like sulfur dioxide and carbonyl sulfide. A significant amount of sulfur disappeared during the experiment, likely forming long, undetectable sulfur chains.
Key breakthroughs in the simulation included understanding molecular movement and VUV photon penetration. Standard thermal diffusion alone did not account for the observed reactions. Enabling "non-diffusive chemistry," where atoms interact immediately upon breaking off, was crucial. The model also determined that VUV photons can penetrate about 100 monolayers of ice.
Discrepancies between the simulation and experimental data provided new insights. The experiment showed high levels of sulfur dioxide and sulfur allotropes, while the simulation predicted low amounts. Conversely, the simulation predicted high concentrations of carbonyl sulfide, sulfur monoxide, and carbon monosulfide. Further analysis of experimental infrared spectra revealed that the data was compatible with the presence of some carbon monosulfide and sulfur monoxide, whose signatures were initially obscured.
These findings suggest that current understanding of interstellar chemical interactions may be incomplete. The work will inform updates to pyRate and guide future observations by instruments like the James Webb Space Telescope. Scientists continue to work towards solving the missing sulfur mystery.
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