Fate of SO2 in the ancient Martian atmosphere: Implications for transient greenhouse warming

There is increasing evidence that sulfur played an important role on early Mars. Sulfur is distributed ubiquitously on the Martian surface, and sulfur in Martian meteorites carries the signature of atmospheric interactions. Recent work suggests that the radiative properties of sulfur volatiles that were degassed into the Martian atmosphere may have caused a greenhouse effect early in the planet's history. It remains unclear, however, over what timescales warming from sulfur volatiles would have persisted, and consequently how significant this warming may have been. While most photochemistry research to date has concentrated on current Martian conditions, the ancient Martian atmosphere was thicker, warmer, and more reducing than the current regime. Here we investigate sulfur photochemistry in a 500 mb ancient Martian atmosphere. After adapting a model used to study sulfur photochemistry on Earth during the Archean, we find a short lifetime for SO2 in the current Martian atmosphere, similar to results of other photochemical studies. However, our simulations suggest that moderate mixing ratios of SO2 (10(-8) <= f(SO2) <= 10(-6)) could have persisted in the ancient Martian atmosphere for hundreds of years, generating short but potent warming events following episodes of volcanic activity.