Sulfur: Mars's Ancient Mystery Ingredient

The CO₂ that made up much of Mars's ancient atmosphere, along with a water ocean that likely once covered the planet's surface, should have been a climatic recipe for the formation of carbonates. Yet, Mars doesn't seem to have much of them. Researchers at Harvard University and MIT suggest that SO₂ and H₂S, ejected into the atmosphere from volcanoes, may have produced the conditions necessary to sustain a liquid ocean and prevent carbonaceous rock formation (Science 2007, 318, 1903). Without the substantial atmospheric O₂ that's found on Earth, martian SO₂ wouldn't have been oxidized to SO₄^2- and would have remained in the atmosphere as a greenhouse gas, warming the planet and allowing liquid water to exist. The sulfur compounds would also have slightly acidified the ocean, suppressing the precipitation of carbonates in favor of sulfite materials. This sulfur-influenced chemistry could also explain the scarcity of carbonate sediments produced on Earth several billion years ago.