So, a few weeks ago, there was a report out that suggested Mars may have had a more oxygen rich environment in its early history than previously suspected. While it’s not clear that this means Mars had an oxygenated atmosphere- or that if oxygen was present, it was in significant concentrations- it is thought provoking.
For one thing, if I’m interpreting the paper correctly, it does suggest that early Mars may have had some tectonic activity, which is a pretty big thing- for a long time it was assumed Mars never really was tectonically active (it certainly isn’t today).
If there was in fact an oxygen atmosphere- probably generated from the photolysis of H2O, rather than photosynthesis- it could have implications for early life arising on Mars. On one hand, oxidizing environments tend to be less than ideal for prebiotic chemistry- this is one of the reasons that the atmosphere of the early Earth was thought to be reducing up until relatively recently. After all, oxygen was toxic to most of the early anaerobic organisms.
On the other hand, assuming that life developed on Mars, there would have been a considerable evolutionary pressure to develop an aerobic metabolism, which in turn could have allowed more complex forms of life to develop, potentially much earlier than happened on Earth.
So, depending on how you spin it, this discovery could lessen the likelihood that life developed on Mars, or it could increase the likelihood that somewhat more complex life could have evolved during Mars’ early warm, wet period. As always, more information is needed, but the possibilities are rather tantalizing…