A British scientist says new findings suggest Mars had an oxygen-rich atmosphere more than a billion years before Earth developed its own.

An examination of meteorites on Earth and rocks on Mars suggests oxygen was affecting the martian surface 4 billion years ago, at least 1,500 million years before oxygen built up in appreciable quantities in Earth's atmosphere, Bernard Wood of Oxford University said.

The evidence comes from a comparison of Martian meteorites that have crashed onto Earth and data from rocks examined by NASA's Spirit rover at a very ancient part of Mars containing rocks more than 3,700 million years old, researchers said.

Differences in composition can best be explained by an abundance of oxygen early in martian history, they 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," Wood told the Irish Times.

"As oxidation is what gives Mars its distinctive color, it is likely that the 'red planet' was wet, warm and rusty billions of years before Earth's atmosphere became oxygen rich," he said.