Before 2.5 to 2Gya, Earth's surface was a chemically reducing environment (evidence includes reduced forms of iron, sulfur, and uranium in rocks before that time). It is believed to have become oxidized as a direct result of photosynthetic life, leading to the precipitation of oxidized iron in BIFs, sulfate deposition, etc., etc.
The surface of Mars is now oxidized (evidence includes sulfates and iron-oxides).
What is the mechanism? Why doesn't the oxidized state require life?