An international team of astronomers led by Leiden University, and including scientists from UK ATC, has found the missing sulfur in the Universe. For years, scientists have been puzzled by the low levels of sulfur gas in dense star-forming regions compared to the more tenuous areas between stars. The sulfur seemed to be disappearing. 

Using laboratory experiments that simulate icy cosmic conditions, researchers discovered that sulfur can bind with ammonium to form ammonium hydrosulfide salt. This salt sticks to dust and pebbles, effectively hiding the sulfur from detection. This finding not only explains the missing sulfur but also a previously unexplained peak in data from the James Webb Space Telescope's (JWST) MIRI instrument​.

The experiments showed that volatile ammonia (NH₃ ) and hydrogen sulfide (H₂S) react rapidly to form ammonium hydrosulfide (NH₂SH) when they join in ices around dust particles. This reaction occurs under extremely cold conditions. The resulting salt produces an emission peak at the exact wavelength of the previously unexplained peak in JWST's data, allowing astronomers to calculate that up to 20% of missing sulfur could be in this form.

This discovery is significant for understanding the chemical composition of star-forming regions and the early stages of planetary system formation. The research was inspired by results from ESA's Rosetta mission, which found high levels of ammonium hydrosulfide in the dust particles of comet 67P. Since comets are believed to contain pristine material from the early solar system, looking for ammonium hydrosulfide in star-forming regions was a logical next step.

Pamela Klaassen, astronomer at the UK ATC and co-author on this new paper in Astronomy & Astrophysics, said: "This discovery not only solves a long-standing mystery but also opens up new avenues for exploring the chemical processes in star-forming regions."

Find out more.

Read the Astronomy & Astrophysics paper.