Evidence from Curiosity Rover Shows Mars Once Had Oxygen-Rich Atmosphere
http://www.americaspace.com/?p=93145
It had already been assumed that there must have once been more oxygen (O2) than there is now, to account for all of the reddish iron oxide
(rust) on the surface, which would require both water and oxygen. Other studies have also supported this hypothesis, and suggested that there
used to be more oxygen, but now the new data from Curiosity indicates that there was probably even more oxygen than first thought. That clue
comes from the high levels of manganese oxide found in some rocks by the rover. It is the most direct evidence found so far for a previous
oxygen-rich atmosphere earlier in Mars’ history.