Lost sulfur in the universe may reside in salt on dust and pebbles

An international team led by astronomers at Leiden University has shown in laboratory experiments that sulfur can bind with ammonium under icy cosmic conditions and form a salt that sticks to dust and pebbles. The resulting sulfur salt not only helps to explain the mystery of the missing sulfur gas, but also a puzzling peak in data from the James Webb Space Telescope’s MIRI instrument and other telescopes.

The findings appear in the journal Astronomy & Astrophysics.

For the past two decades, astrochemists and astronomers have been puzzled by two seemingly inexplicable mysteries. The first was that the amount of volatile sulfur in dense clouds and star-forming regions is much lower than in the more tenuous regions between stars. The sulfur seemed to be disappearing. The second was that the spectrum of infrared light from star-forming regions contains a striking but unexplained peak.

The team led by researchers from Leiden University in the Netherlands proposes a solution to both mysteries at once: ammonium hydrosulfide salt. The researchers support their solution with laboratory experiments that simulate cosmic conditions. These involve extremely cold conditions in which dust, ice and pebbles are present, and relatively few molecules can react.

The experiments showed that volatile NH₃ (ammonia, well-known from detergents) and volatile H₂S (hydrogen sulfide, the smell of rotten eggs) react rapidly to form NH₄SH (ammonium hydrosulfide salt) when they join in ices around dust particles. This suggests that in dense star-forming regions, some of the volatile sulfur is trapped in dust and pebbles. As a result, the sulfur seems to have disappeared.

In addition, the experiments showed that the ammonium hydrosulfide salt produces a peak at the exact location of the previously unexplained peak in data from—among others—the MIRI instrument on the James Webb Space Telescope. This peak allowed the astronomers to calculate that up to approximately 20% of the missing sulfur could be in the form of this sulfur salt in dust and pebbles.

Two birds with one stone

“I think it is great that we are finally unraveling both mysteries,” says Katie Slavicinska. She is a Ph.D. student at Leiden University and the first author of the scientific paper. “With our research, we are killing two birds with one stone.”

The research was triggered by results from ESA’s Rosetta mission. During this mission, a spacecraft orbited comet 67P between 2014 and 2016. Analyses published in late 2022 showed that the comet’s dust particles contained unexpectedly high levels of ammonium hydrosulfide.

Slavicinska explains, “And since we suspect that comets contain a lot of pristine icy material from the early days of our solar system, looking for ammonium hydrosulfide in the ice of star-forming regions was the logical next step.”

Second author Adwin Boogert, a Dutch scientist working at the University of Hawaii at Manoa says, “It’s exciting to see how we can increasingly follow chemical traces back from our current solar system to the origin of new solar systems.”

In the future, the researchers plan to make more observations with the MIRI instrument on the James Webb Space Telescope to confirm the theory of the infrared peak. They also hope to find the remaining eighty percent of the missing sulfur. Previous research suggests that metallic sulfides and sulfur allotropes could play a role.