Scientists May Have Solved the Missing Sulfur Mystery in Star-Forming Clouds

Sulfur is the tenth most abundant element in the universe, yet in the dense molecular clouds where stars are born, scientists find only about one percent of the expected amount. This decades-old riddle, the missing sulfur problem, has baffled astrochemists for generations. A leading theory holds that sulfur lies frozen within dust grain ice mantles, invisible to telescopes. A new computer simulation now brings scientists closer to solving it.

Simulating Frozen Chemistry with pyRate

Published in the journal Astronomy & Astrophysics, the study from the Max Planck Institute for Extraterrestrial Physics and Spain's Centro de Astrobiología, led by Olli Sipilä, used the pyRate code to model a 2024 laboratory experiment. Researchers cooled carbon dioxide and carbon disulfide ice to 10 Kelvin and irradiated it with ultraviolet photons. When the simulation used standard diffusive chemistry, where molecules wander until colliding, almost no reactions occurred. Only non-diffusive chemistry, allowing freshly released atoms to react instantly, reproduced the observed sulfur compounds and confirmed UV radiation penetrates roughly 100 ice monolayers deep.

Why Sulfur's Cosmic Address Matters

This problem extends beyond the field of chemistry. Sulfur is necessary for biological processes because it forms amino acids and proteins, making its existence in space of immense astrobiological importance. Nonetheless, out of all known sulfur compounds in interstellar ices, there are only carbonyl sulfide and sulfur dioxide, comprising less than five percent of the expected amount of sulfur. Allotropes, long chains of sulfur molecules, are most likely. Even though pyRate simulations point out some deficiencies of current knowledge on interstellar ice chemistry, it lays out a promising route for further observations with the help of the James Webb Telescope.