  Mars today has no active volcanism and its atmosphere is oxidizing, dominated
by the photochemistry of CO2 and H2O. Using a one-dimensional photochemical
model, we consider whether plausible volcanic gas fluxes could have switched
the redox-state of the past martian atmosphere to reducing conditions. In our
model, the total quantity and proportions of volcanic gases depend on the water
content, outgassing pressure, and oxygen fugacity of the source melt. We find
that with reasonable melt parameters the past martian atmosphere (~3.5 Gyr to
present) could have easily reached reducing and anoxic conditions with modest
levels of volcanism, >0.14 km^3/yr, well within the range of prior estimates.
Counter-intuitively we also find that more reducing melts with lower oxygen
fugacity require greater amounts of volcanism to switch a paleo-atmosphere from
oxidizing to reducing. The reason is that sulfur is more stable in such melts
and lower absolute fluxes of sulfur-bearing gases more than compensate for
increases in the proportions of H2 and CO. These results imply that ancient
Mars should have experienced periods with anoxic and reducing atmospheres even
through the mid-Amazonian whenever volcanic outgassing was sustained at
sufficient levels. Reducing anoxic conditions are potentially conducive to the
synthesis of prebiotic organic compounds, such as amino acids, and are
therefore relevant to the possibility of life on Mars. Also, anoxic reducing
conditions should have influenced the type of minerals that were formed on the
surface or deposited from the atmosphere such as elemental polysulfur (S8) as a
signature of past reducing atmospheres. Finally, our models allow us to
estimate the amount of volcanically sourced atmospheric sulfate deposited over
Mars' history, approximately 10^6 to 10^9 Tmol, with a spread depending on
assumed outgassing rate history and magmatic source conditions.
