Thermally irreversible sulfur chemisorption on silicate grains as a major sulfur reservoir in molecular clouds

Sulfur (S) chemistry in the interstellar medium remains poorly constrained due to the longstanding "missing sulfur" problem, where a significant fraction of cosmic sulfur in molecular clouds remains unidentified. To gain a comprehensive understanding of S chemistry in the interstellar medium, its behavior on interstellar dust grains must be clarified. Here, we experimentally investigated the surface elementary processes of S atoms under conditions mimicking interstellar molecular clouds. Our in-situ laser-based detection method enabled the direct tracking of S atoms adsorbed on astrophysically relevant silicate substrates and amorphous solid water (ASW). We found that sulfur atoms strongly chemisorbs onto silicate surfaces and remains bound even after ASW sublimation, rendering it observationally invisible. An astrochemical model coupled with these results suggests that at least 10 percent of the total sulfur budget in molecular clouds must be locked onto silicate dust grains. Our results identify sulfur chemisorption on silicate as a key mechanism driving sulfur depletion and provide a physical explanation for the missing sulfur problem.