The oxygen was either produced by life forms or by a chemical reaction in the atmosphere of MarsMars had an oxygen-wealthy 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 considerable quantities in the atmosphere for at least another 1.5bn years.
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Source: theguardian.com