The sulfur content of volcanic gases on Mars

Both high sulfur contents of the martian regolith and lack of detection of extensive carbonate deposits suggest that the latest geological events that shaped the landscapes of Mars were dominated by acidic waters possibly related to appreciable SO2 concentrations in the atmosphere. On the basis of fundamental thermochemical principles, we model here the likely sulfur contents of (1) the martian and terrestrial mantles and (2) the volcanic gases delivered by the corresponding basaltic magmas. We find that the martian mantle contains at least 3–4 times as much sulfur as its terrestrial counterpart, yielding basaltic melts richer in sulfur than those on Earth. Such an S-enrichment is explained by contrasted redox conditions prevailing during magma ocean equilibration, which lead to distinct iron contents of the martian and terrestrial mantles and of their basaltic derivatives. Calculated volcanic gas compositions in equilibrium with a magma ocean sustaining a denser atmosphere are shown to be dominated by CO ± CO2 and H2 ± H2O species, depending on fO2, sulfur species amounting to only ~ 1%. In contrast, volcanic gases supplied at later stages of Mars evolution, such as during the building of the Tharsis province, are shown to be significantly richer in sulfur, with S contents on average 10–100 times that of gases emitted by magmas on Earth. If degassing during such a period occurred in a tenuous atmosphere (1 bar or less), volcanic gases were dominated by SO2 rather than by H2S, which should have favored the acidification of any persistent water layer. The calculated amounts of S emitted by the Tharsis volcanic region turn out to be equivalent to a 20–60 m thick layer of sulfate minerals if uniformly covering the martian surface, in qualitative agreement with remote sensing of the martian regolith.