Atmospheric injection of sulfur from the Medusae Fossae forming events

A plethora of in-situ bulk chemical and mineralogical analyses, remote sensing data, and geochemical models suggest that the Martian crust is sulfur(S)-rich, exceeding the S content of terrestrial basalts by almost an order of magnitude. The main source of crustal S on Mars is the volcanic exhalation of SO2 and H2S gases. Volcanic ash, expelled along with gases, may be a sink for up to 30% of the exhaled S gas species. We analyze the elemental composition of the Martian shallow sub-surface using the Mars Odyssey Gamma Ray Spectrometer to find areas that are simultaneously enriched in constituents of major volcanic gases such as S, Cl, and H2O. A large sedimentary deposit of likely pyroclastic origin called the Medusae Fossae Formation (MFF) is located within this region of possibly extensive alteration by volcanic gases. Based on reasonable terrestrial analog estimates that the MFF scavenged, at maximum, 30% of the exhaled S gas species, we find that a significant amount of S (>1017 kg) would have been delivered to the atmosphere over the time it took for the MFF to be deposited on Mars. The mass of the S emitted from the MFF-forming event(s) is up to 8 orders of magnitude higher than the mass of S emitted from the largest Quaternary volcanic eruption on Earth. Thus, volcanic degassing from the MFF forming events could have significantly affected surface geological processes, climate evolution, and habitability of Mars. •The Medusae Fossae Formation (MFF) is simultaneously enriched in S, Cl, and H2O.•The enrichment of S, Cl, and H2O at the MFF could be due to volcanic degassing.•The S/Cl molar ratio of the MFF corroborates enrichment by volcanic degassing.•The MFF forming events could have supplied a huge amount of atmospheric S.