For years, astronomers and astrochemists have puzzled over two big mysteries in space.
First, they noticed that dense clouds and star-forming regions had much less sulfur gas compared to the empty spaces between stars.
Where was the sulfur disappearing to?
Second, telescopes like the James Webb Space Telescope (JWST) detected a strange peak in infrared light coming from star-forming regions, but no one knew what caused it.
Now, an international team of researchers led by Leiden University in the Netherlands believes they’ve solved both mysteries at once.
Their experiments suggest that the missing sulfur is hiding in a special salt called ammonium hydrosulfide, which forms under icy, cosmic conditions and sticks to dust and pebbles.
To test their idea, the researchers recreated the extreme conditions found in space.
At incredibly cold temperatures, they combined ammonia (NH3), commonly found in detergents, with hydrogen sulfide (H2S), the gas that smells like rotten eggs.
They discovered that these two gases react quickly to form ammonium hydrosulfide (NH4SH) salt when frozen around tiny dust particles.
This reaction explains why sulfur gas seems to disappear in dense star-forming regions—it gets trapped as salt on dust and pebbles.
Their lab experiments also showed that ammonium hydrosulfide salt produces a specific infrared light pattern, matching the mysterious peak seen by the JWST and other telescopes.
Based on their calculations, the researchers estimate that about 20% of the missing sulfur in star-forming regions could be trapped in this salt form on cosmic dust.
The idea for this research came from the European Space Agency’s Rosetta mission, which orbited comet 67P from 2014 to 2016. Scientists found high amounts of ammonium hydrosulfide in the comet’s dust.
Since comets are thought to contain ancient material from the early days of the solar system, the team suspected this salt might also exist in other parts of space, like star-forming regions.
Ph.D. student and lead researcher Katie Slavicinska said, “It’s amazing to solve both of these mysteries at the same time. We now know where some of the sulfur is hiding and why the strange infrared peak appears.”
The team plans to make more observations with the JWST to confirm their findings. They’re also working to uncover the remaining 80% of the missing sulfur. Previous studies suggest it might exist in metallic sulfides or other sulfur-rich materials.
“This research connects our solar system to the origins of new ones,” said co-author Adwin Boogert from the University of Hawaii. “It’s exciting to trace these chemical clues back to the beginning.”
This discovery not only helps us understand space chemistry but also sheds light on how materials form in the universe.
