For a many years, researchers have been studying Mars and wondering if it may have ever supported life, or still supports it now. Numerous research projects have been built specifically to answer those questions, and now we may have an answer few would expect. According to researchers at the University of Oxford, Mars may have had an oxygen-rich atmosphere roughly 4 billion years ago, about 1.5 billion years prior to Earth developing one.
Often called the 'Red Planet,' Mars earned that name a long time ago when its surface oxidized, or rusted, to its current color. As it does not have plate tectonics like Earth does, its crust has remained largely as it has been for billions of years, which allows us to peer back in time, by looking at the right places. NASA's Spirit rover was sent to analyze rocks in an area that dates back some 3.7 billion years, and found them to be heavily oxidized. The Oxford researchers however have analyzed Martian meteorites found on Earth that date back only 180 million to 1.4 billion years ago and they are not oxidized, despite both rocks having similar origins. The researchers believe the reason for the difference is that the older rocks Spirit examined were exposed to an oxygen-rich atmosphere, temporarily drawn into the planet's shallow interior, and were then erupted back to the surface. The meteorites however came from deeper in the planet and were never exposed to high levels of oxygen.
While it is still possible that the area Spirit examined has a very different geological composition than where the meteorites came from, the researchers believe an oxygen-rich atmosphere is more likely the cause. If correct, this would suggest that 4 billion years ago, just half a billion years after the Earth formed, Mars was warm, wet, and already oxidizing.
Source: University of Oxford