Sulfur, one of the universe's most abundant elements, appears to vanish in dense, cold molecular clouds—the very regions where stars form. While diffuse interstellar clouds contain the expected amount, dense clouds show up to 99% less sulfur than predicted, a puzzle that has persisted for decades.
Scientists have long suspected the missing sulfur might be trapped in icy dust grains, making it invisible to telescopes. This new astrochemical model lends fresh support to that theory, simulating how sulfur atoms bind to grain surfaces and become incorporated into ice mantles.
The model explains why sulfur is undetectable in gas-phase observations of dense clouds. It accounts for the chemical pathways that pull sulfur from the gas and lock it into solids, a process that could be happening across the galaxy.
If confirmed by future observations with telescopes like the James Webb Space Telescope, the finding would reshape our understanding of how elements move through star-forming regions. It also has implications for the chemical makeup of planets that form in such clouds.
Some researchers caution that laboratory experiments are still needed to verify the model's assumptions about sulfur binding on ice surfaces at extreme cold temperatures.