> Yeah, but most of that was waiting around for oxygen-releasing life to evolve. No, it wasn't. Photosynthesis that released O2 evolved relatively quickly, and

> Yeah, but most of that was waiting around for oxygen-releasing life to evolve. No, it wasn't. Photosynthesis that released O2 evolved relatively quickly, and was more efficient than other forms of photosynthesis. Then it took another billion years for oxygen to accumulate in the atmosphere. Mind you, we had an entire planet with oceans teeming with photosynthetic life. That will be next to impossible to replicate on Mars. There are two oxygen sinks on Earth that reacted with oxygen released from photosynthesis: the first was the ocean, the other was minerals on land. This is the reason that it took so long for atmospheric oxygen to accumulate. Mars will have a lot of oxidizable minerals and compounds that will react with oxygen before it can accumulate in its atmosphere. Even then, it will escape into space without a sufficient magnetosphere to trap an atmosphere.