Mars could have had an oxygen-rich atmosphere more than a billion years before the Earth, scientists suggest.
Researchers examining meteorites and rocks on the planet say that oxygen was plentiful on the Martian surface four billion years ago.
On Earth, oxygen did not build up to similar levels in the atmosphere for at least another 1,500 million years.
If true it is an exciting development but some scientists are sceptical
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. But other scientists have voiced scepticism over the idea. Dr Francis McCubbin, from the University of New Mexico, told the BBC: "I did not reach the conclusion that their results imply an early oxygen-rich atmosphere on Mars, only that the upper mantle was more oxidised than the deep interior, which does not actually require any oxygen gas to accomplish."
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."
