Sulfur is one of the most abundant elements in the universe. If you peer into a diffuse interstellar cloud, you find loads of it—about the amount expected based on fusion patterns in the stars it was born in. However, if you look at a dense, cold molecular cloud—the kind where those stars actually form—it seems like 99% of the sulfur expected to be there is missing. Scientists have puzzled over this “missing sulfur problem” for decades, though a leading theory is that the element hides in icy dust grains, making it hard to detect.
A new paper published in Astronomy & Astrophysics from the Max Planck Institute for Extraterrestrial Physics and the Centro de Astrobiologia describes a new computer simulation model aimed at supporting the interpretation of laboratory results and testing our current understanding of sulfur evolution in interstellar ices.
The simulation was written in pyRate—a Python-based application that calculates how chemicals interact, especially between ice and gas phases. The paper marks the first successful model of the chemistry of a multicomponent interstellar ice analog with a rate-equation simulation. Scientists love “firsts,” but what does that actually mean in practice in this case?
The authors focused on simulating the results of one particular lab experiment on sulfur that was performed in 2024. During this experiment, a mixture of carbon dioxide (CO2) and carbon disulfide (CS2) was cooled to 10 K and then blasted with vacuum-ultraviolet (VUV) photons. During the physical experiment, this processing broke the molecules apart and created a mishmash of new sulfur-bearing chemicals such as sulfur dioxide, carbonyl sulfide and even pure sulfur chains known as allotropes. Critically, a significant amount of the sulfur “disappeared” from the experiment—likely locked up in long sulfur chains that were invisible to the instrumentation used to monitor them.
Mimicking this experiment in simulation was the goal of the current paper, and it yielded some interesting breakthroughs. First was how the molecules actually move. Most astrochemists simply assume that molecules move via thermal diffusion—they wander around a surface until bumping into another molecule. But when the team ran the simulation with only standard diffusion occurring, the reaction that produced such a plethora of sulfur-containing compounds ground to a halt. Enabling “non-diffusive chemistry”—where atoms can interact with their neighbors immediately upon breaking off from their host molecule—was the key to getting the reaction to proceed, likely because 10 K doesn’t really provide a lot of thermal impetus.
Another breakthrough came in understanding how thick an ice layer a VUV photon can penetrate. It turns out the answer is about 100 “monolayers,” or single sheets of ice molecules. This can be added as a feature to future iterations of these astrochemical codes, as there had been some debate about a VUV photon’s ability to penetrate deep into icy formations.
However, there were some discrepancies between the simulation and the actual experimental data from the 2024 experiment. Experimentally, the main compound found when all was said and done was sulfur dioxide, along with high levels of sulfur allotropes. However, the simulation predicted low amounts of both molecules. Additionally, the simulation predicted high concentrations of carbonyl sulfide, sulfur monoxide and carbon monosulfide. While these initially were not reported, further analysis of the infrared spectra revealed that the experimental data are actually compatible with the presence of some carbon monosulfide and sulfur monoxide molecules, as their chemical signatures were likely obscured by overlap with the dominant sulfur dioxide features.
The authors took these discrepancies as a clue, showing that our current understanding of interstellar chemical interactions is lacking at best. But they also showed that the original experiment might have missed something—the chemical signatures for carbon monosulfide and sulfur monoxide heavily overlapped with the dominant sulfur dioxide signal, so some of those concentrations might have been misinterpreted.
Either way, this is a step forward in understanding how chemistry in the galaxy at large works. Such work will allow the authors to update pyRate to more accurately match the laboratory experiment and even inform future observational campaigns for the likes of the James Webb Space Telescope. Slowly but surely, scientists are working to get to the bottom of the missing sulfur mystery, no matter how many monolayers of ice they have to dig through.
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