Mars had an oxygen-rich atmosphere more than a billion years before Earth

22 June Daily Mail

Mars had an oxygen-rich atmosphere more than a billion years before the Earth, scientists believe.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 1,500 million years.

Scientists compared Martian meteorites that have crashed onto the Earth and data from rocks examined by the American space agency Nasa's Spirit rover.Differences in their composition can best be explained by an abundance of oxygen early in Martian history.Spirit was exploring a very ancient part of Mars containing rocks more than 3,700 million 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.Martian 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 (GOE).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, from 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.'

In May, scientists revealed that they had recently discovered pebbles on Mars, showing that a stream has flowed on the planet - and giving more weight to the theory that it was once able to support life.The rounded pebbles were found by the team from Nasa's Curiosity rover mission and published in journal Science. Pebbles are only formed when they are carried through water over long distances, according to researchers.It is the first time that evidence of sustained water flows on Mars has been uncovered.

The discovery of pebbles came just before an announcement eight days ago that powerful evidence that water good enough to drink had been found by Opportunity, one of Nasa's aging rover vehicles.

Evidence of water of Mars has been indicated by a succession of discoveries on Mars but until now they have all suggested flows of sulphuric acid. This is the first time water has been found in a form that is likely to be drinkable.The discovery boosts the chances that one of life's most important building blocks was once present on the planet, and might even still be there.