Sulfides and hollows formed on Mercury's surface by reactions with reducing S-rich gases

The surface of Mercury is enriched in sulfur, with up to 4 wt.% detected by the NASA MESSENGER mission, and has been challenging to understand in the context of other terrestrial planets. We posit, that magmatic S was mobilized as a gas phase in volcanic and impact processes near the surface, exposing silicates to a hot S-rich gas at reducing conditions and allowing conditions for rapid gas-solid reactions. Here, we present novel experiments on the reaction of Mercury composition glasses with reduced S-rich gas, forming Ca- and Mg-sulfides. The reaction products provide porous and fragile materials that create previously enigmatic hollows on Mercury. Our model predicts that the gas-solid reaction forms Ca-Mg-Fe-Ti-sulfide assemblages with SiO2 and aluminosilicates, distinct from formation as magmatic minerals. The ESA/JAXA BepiColombo mission to Mercury will allow this hypothesis to be tested. •Reduced S-rich gas reacts rapidly to form sulfides at the surface of Mercury.•Sulfidation of forsterite and diopside forms MgS, CaS, and quartz.•Volume changes in the regolith due to sulfidation affect hollow formation.