The cold dusty clouds from which stars form in the Carina Nebula. Credit: ESO/APEX/T. Preibisch et al. (Submillimetre); N. Smith, University of Minnesota/NOAO/AURA/NSF (Optical)
Sulphur is the 10th most abundant element in our solar system and the local interstellar medium. But when astronomers turn their high-powered telescopes to the molecular clouds and star-forming regions of the wider cosmos, they find gaseous sulphur strangely lacking.
“If you use, for instance, the James Webb Space Telescope, you get a specific signature at specific wavelengths for oxygen and carbon and nitrogen and so forth,” says astrochemist Ryan Fortenberry, an associate professor at the University of Mississippi, USA.
“But when you do that for sulphur, it’s out of whack, and we don’t know why there isn’t enough molecular sulphur.”
Ralf Kaiser, a professor of chemistry at the University of Hawai‘i at Mānoa, USA, explains: “The observed amount of sulphur in dense molecular clouds is less – compared to predicted gas-phase abundances – by 3 orders of magnitude.”
This fundamental dilemma in the field of astrochemistry – known as the “sulphur depletion problem” – may finally be explained by the findings of a new study published in Nature Communications.
The study shows that sulphur is probably hiding in “the most common forms of sulphur that we already know about”, says Fortenberry.
Molecular clouds composed of gas and dust are found in the interstellar medium, the regions between star systems.
When Fortenberry, Kaiser and their collaborators simulated the temperature and pressure conditions of interstellar space in the laboratory, they found that 2 stable forms of molecular sulphur can form on these icy dust grains.
They show that hydrogen sulphide (H2S) on interstellar dust can be converted by cosmic radiation to octasulphur crowns – a group of 8 sulphur atoms configured in ring-like crowns – and polysulphanes – chains of sulphur atoms bonded by hydrogen.
This process locks the sulphur into solid forms.
“Since interstellar dust grains and cold molecular clouds provide essentially the raw material for protoplanetary disks and resulting solar systems … interstellar grains and sulphur-carrying molecules are at least partially incorporated into comets and other planetary bodies as the molecular clouds transform into star-forming regions,” the authors write.
They suggest that “octasulphur on comets and meteorites might originate from interstellar icy mantles”.
The findings explain why sulphur has been so difficult to detect in cold molecular clouds, since the bonds it forms are always changing – from crowns to chains to a variety of other configurations. “It never maintains the same shape,” says Fortenberry.
The results point astronomers searching for interstellar sulphur towards warmer star-forming regions. Here, the molecules should become observable via radio or infrared telescopes once they ‘sublimate’ – transition from solid to gas – as the temperature increases.
The pungent odour of rotten eggs is a telltale sign that colourless hydrogen sulphide gas is hanging around, even at low levels, on Earth.
“Hydrogen sulphide is everywhere: it’s a product of coal-fired power plants, it has an effect on acid rain, it changes the pH levels of oceans, and it comes out of volcanoes,” says Fortenberry.
“If we gain a better understanding of what the chemistry of sulphur can do, the technological commercialisation that can come from that can only be realised with a foundation of fundamental knowledge.”