Volcanic emission of reduced sulfur species shaped the climate of early Mars.

Sulfur and other volatiles could be transported from the martian interior to surface through magmatic processes, including mantle melting, magma differentiation, and degassing. However, these processes were not fully integrated in past sulfur cycling models because of complexity from the gas-melt interactions in chemically and dynamically evolving magmatic systems with multicomponent volatiles. Here, we incorporate these processes to simulate how sulfur, carbon, and hydrogen degas from martian melts. We find that reduced sulfur species, HS and S, are dominantly emitted through degassing at crustal to surficial pressures. These sulfur species could condense as sulfide and elemental sulfur, potentially yielding the sulfate deposits observed on the martian surface through secondary oxidation. Our models also show that evolved magmas reach graphite and sulfide saturation at crustal pressures and thus may establish sulfur and carbon reservoirs in the martian crust. The degassed HS and S may form a hazy atmosphere with SF, a potent greenhouse gas, to shape the paleoclimate of Mars.