[url=http://www.guardian.co.uk/science/2013/jun/19/mars-oxygen-rich-atmosphere]Article:[/url] [quote]Mars had an oxygen-rich atmosphere more than a billion years before the Earth, say scientists. An examination of meteorites and rocks on the planet suggests that oxygen was affecting the Martian surface four billion years ago. On Earth, oxygen did not build up to appreciable quantities in the atmosphere for at least another 1.5bn years. The researchers compared Martian meteorites that have crashed onto the Earth with data from rocks examined by Nasa's Spirit Mars rover. Differences in their composition can best be explained by an abundance of oxygen early in Martian history. Spirit was exploring an ancient part of Mars containing rocks more than 3.7bn years old. The rocks bear the hallmarks of early exposure to oxygen before being "recycled" – drawn into shallow regions of the planet's interior and then spewed out in volcanic eruptions. Volcanic Martian meteorites, on the other hand, originate from deeper within the planet where they would be less affected by oxygen. The meteorites travel to Earth after being flung into space by massive eruptions or impacts. The new research, published in the journal Nature, has implications for the possibility of past life on Mars. On early Earth, the atmosphere was gradually filled with free oxygen by photosynthesising microbes. Scientists call this the Great Oxygenation Event. The link between oxygen and life on Mars is less certain. Oxygen could have been produced biologically, or by a chemical reaction in the atmosphere. Lead scientist Professor Bernard Wood of Oxford University said: "The implication is that Mars had an oxygen-rich atmosphere at a time, about 4,000 million years ago, well before the rise of atmospheric oxygen on Earth around 2,500 million years ago. "As oxidation is what gives Mars its distinctive colour, it is likely that the 'red planet' was wet, warm and rusty billions of years before Earth's atmosphere became oxygen-rich."[/quote]