At 10 Kelvin, there is barely enough thermal energy to make molecules wander
For decades, astronomers have watched sulfur vanish — abundant in the thin clouds between stars, yet nearly absent in the cold, dense nurseries where new stars are born. A team at the Max Planck Institute for Extraterrestrial Physics and Spain's Centro de Astrobiologia has now built a computational model suggesting that this missing sulfur hides within icy dust grains, locked in chemical forms that telescopes cannot easily see. Their work, built around a simulation called pyRate, reveals that at temperatures near absolute zero, chemistry does not wait for molecules to wander — atoms react the instant they are freed, neighbor to neighbor. In tracing where sulfur disappears, these researchers are quietly mapping the hidden architecture of star formation itself.
- Ninety-nine percent of the universe's sulfur goes missing in the very places where stars are born, a decades-long mystery that has resisted every attempt at explanation.
- The new pyRate simulation exposed a fundamental flaw in standard astrochemical assumptions — at 10 Kelvin, thermal diffusion essentially halts, and only direct, neighbor-to-neighbor atomic reactions keep chemistry alive.
- When the model's predictions clashed with a 2024 laboratory experiment, researchers turned the mismatch into a discovery, finding that key compounds like carbon monosulfide and sulfur monoxide had been hiding beneath stronger spectral signals all along.
- The simulation also resolved a standing debate by showing that ultraviolet photons penetrate approximately 100 monolayers deep into interstellar ice — a number that can now anchor future models.
- The findings are being refined to guide James Webb Space Telescope observations, bringing astronomers measurably closer to reading the full chemical story written in the ice of stellar nurseries.
Sulfur is abundant in the wispy, diffuse clouds scattered between stars — present in roughly the amounts that stellar fusion theory predicts. But step into a dense molecular cloud, the cold dark nursery where new stars actually ignite, and nearly 99 percent of it disappears. For decades, astronomers have stared at this absence. The leading hypothesis holds that sulfur becomes trapped inside icy dust grains, locked away beyond the reach of telescopes. Now a team at the Max Planck Institute for Extraterrestrial Physics and Spain's Centro de Astrobiologia has built a computer model that may finally illuminate what happens to all that missing sulfur.
Their tool, pyRate, is a Python-based simulation designed to track how chemicals shift between ice and gas phases — the first of its kind to model the chemistry of a complex, multicomponent interstellar ice using rate-equation methods. The team set out to replicate a 2024 laboratory experiment in which a mixture of carbon dioxide and carbon disulfide was cooled to 10 Kelvin and bombarded with vacuum-ultraviolet photons. The radiation shattered molecules and forged new sulfur-bearing compounds, while a significant portion of sulfur seemed to vanish into undetectable sulfur chain structures.
What the simulation revealed was unexpected. Standard astrochemical models assume molecules drift across icy surfaces through thermal diffusion, wandering until they collide. But at 10 Kelvin, there is barely enough energy for that wandering to occur. When the team ran the model using only thermal diffusion, the chemistry simply stopped. The breakthrough came by enabling what they call non-diffusive chemistry — atoms reacting directly with their immediate neighbors the moment they break free from a parent molecule. The model also settled a lingering debate by showing that ultraviolet photons penetrate roughly 100 monolayers deep into interstellar ice.
The simulation did not perfectly match the experiment. The lab found sulfur dioxide and sulfur allotropes as dominant products; the model predicted low concentrations of both, while suggesting high levels of carbonyl sulfide, sulfur monoxide, and carbon monosulfide. Rather than treating this as failure, the researchers used the mismatch as a diagnostic. Returning to the original infrared spectra, they found that carbon monosulfide and sulfur monoxide had been present all along — their signatures buried beneath the overwhelming signal from sulfur dioxide. The data had obscured them, not erased them.
The team plans to refine pyRate further, feeding improved models into future James Webb Space Telescope observational campaigns. Each iteration edges astronomers closer to understanding where sulfur goes when stars are born — and what chemistry is quietly unfolding beneath the ice of the cosmos.
Sulfur is everywhere in space. Look at a diffuse interstellar cloud—the thin, wispy kind scattered between stars—and you find roughly as much sulfur as stellar fusion theory predicts should be there. But look at a dense molecular cloud, the cold, dark nurseries where new stars actually ignite, and something strange happens: about 99 percent of the sulfur vanishes. For decades, astronomers have stared at this absence and wondered where it went. The leading hypothesis is that sulfur gets trapped inside icy dust grains, locked away where telescopes cannot see it. Now a team at the Max Planck Institute for Extraterrestrial Physics and Spain's Centro de Astrobiologia has built a computer model that may finally help explain what happens to all that missing sulfur.
The researchers created a simulation using pyRate, a Python-based tool designed to calculate how chemicals behave when they shift between ice and gas phases. What makes this work significant is that it represents the first successful attempt to model the chemistry of a complex, multicomponent interstellar ice using rate-equation simulation—the kind of detailed mathematical approach that can track individual molecular interactions. The team focused on replicating a laboratory experiment conducted in 2024, one that offered a window into how sulfur actually behaves in the extreme cold of space.
In that experiment, scientists cooled a mixture of carbon dioxide and carbon disulfide to 10 Kelvin—colder than the coldest place on Earth—and bombarded it with vacuum-ultraviolet photons. The radiation shattered the molecules and forged new sulfur-bearing compounds: sulfur dioxide, carbonyl sulfide, and long chains of pure sulfur called allotropes. Crucially, a significant amount of sulfur seemed to disappear during the process, likely locked inside those sulfur chains in ways the lab instruments could not detect. The simulation's job was to recreate this messy chemistry step by step.
What emerged from the model revealed something unexpected about how molecules move at such extreme temperatures. Most astrochemists have long assumed that molecules drift across icy surfaces through thermal diffusion—they wander randomly until colliding with a neighbor. But when the researchers ran the simulation using only this standard diffusion, the chemical reactions simply stopped. The breakthrough came when they enabled what they call "non-diffusive chemistry," where atoms can interact with their immediate neighbors the moment they break free from their parent molecule. At 10 Kelvin, there is barely enough thermal energy to make molecules wander, so direct contact becomes the dominant pathway for chemical change.
The model also revealed something practical about how deep ultraviolet light can penetrate into ice. The answer turned out to be roughly 100 monolayers—single sheets of ice molecules stacked atop one another. This finding can now be incorporated into future astrochemical models, settling a debate that had lingered in the field about just how far photons could reach into icy formations.
Yet the simulation did not perfectly match the 2024 experiment. The lab found large amounts of sulfur dioxide and sulfur allotropes as the dominant products, but the model predicted low concentrations of both. Conversely, the simulation suggested high levels of carbonyl sulfide, sulfur monoxide, and carbon monosulfide—compounds that initially seemed absent from the experimental results. The researchers treated this mismatch not as failure but as a diagnostic tool. When they reexamined the infrared spectra from the original experiment, they discovered that the signatures of carbon monosulfide and sulfur monoxide were actually present all along, hidden beneath the overwhelming signal from sulfur dioxide. The experiment had not missed them; the data had simply obscured them.
This discrepancy points to a larger truth: our understanding of how chemistry unfolds in the interstellar medium remains incomplete. But it also suggests that both the simulation and the experiment have room to improve. The researchers plan to refine pyRate to better match laboratory results, work that will eventually feed into observational campaigns using instruments like the James Webb Space Telescope. Each iteration brings astronomers closer to solving a puzzle that has haunted the field for decades—understanding where sulfur goes when stars are born, and how the chemistry of the cosmos actually works beneath the ice.
Citações Notáveis
Our current understanding of interstellar chemical interactions is lacking at best— Max Planck Institute and Centro de Astrobiologia research team
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Why does sulfur vanishing in molecular clouds matter? It's just one element.
Because it tells us we don't understand how chemistry works where stars form. If we can't account for 99 percent of something as common as sulfur, we're missing something fundamental about the universe.
So the icy dust grain theory—that's been around for a while?
Yes, but it's been hard to test. You can't easily see what's locked inside ice from Earth. That's why this simulation is useful. It lets you run the experiment over and over, change variables, see what actually happens at temperatures we can barely imagine.
The non-diffusive chemistry finding—that atoms interact directly instead of wandering around. Does that change how we think about cold chemistry everywhere?
It should. We've been assuming molecules move by thermal diffusion for a long time. But at 10 Kelvin, there's almost no thermal energy. Atoms have to react with whatever is right next to them. It's a different game entirely.
What about the discrepancies between the simulation and the lab? That seems like a problem.
It is, but it's also useful. It tells us where our models are weak and where the experiments might be hiding information. Sometimes a mismatch is more valuable than a perfect match.
Where does this lead?
Better models, better observations with the James Webb Space Telescope, and eventually a real answer to where all that sulfur goes. We're not there yet, but we're closer.