Mars' atmosphere could have been oxygen rich four billion years ago, long before the earth was lung-friendly.
The study hopes to explain the difference between the meteorites and rocks examined by the mars curiosity rover. The findings could fit in with the belief that the planet once possessed a livable climate, according to the BBC.
Other scientists aren't as enthusiastic about the idea, claiming the "splitting" of water as opposed to life forms could have formed the oxygen.
The Oxford University research team studied volcanic (igneous) rocks found on Mars, and similar Martian meteors discovered on Earth. They noticed there was five times more mercury in the Mars rocks.
"What we have shown is that both meteorites and surface volcanic rocks are consistent with similar origins in the deep interior of Mars, but that the surface rocks come from a more oxygen-rich environment, probably caused by recycling of oxygen-rich materials into the interior," said Professor Wood, who led the study.
The rocks are older than the meteors found on Earth, Wood believes this shows the planet once contained an contained oxygen.
"This result is surprising because while the meteorites are geologically 'young', around 180 million to 1.4 billion years old, the Spirit rover was analyzing a very old part of Mars, more than 3.7 billion years old," he said.
The team believes the rocks went through a process of subduction, where rocks are "recycled in the planet's interior."
Early Mars' surface could have become oxidized, causing the rocks to be drawn back into the planet and later spit out by volcanoes. This would explain why the younger meteors do not show signs of oxygen.
Dr Francis McCubbin, from the University of New Mexico, who did not participate in the study does not agree with the research team's findings.
"I did not reach the conclusion that their results imply an early oxygen-rich atmosphere on Mars, only that the upper mantle was more oxidized than the deep interior, which does not actually require any oxygen gas to accomplish," he said. "Materials can be oxidized in an oxygenated environment, but it is not necessary for the process to occur. Although not implicitly stated, the early oxidized magmatism would also favor the production of water, another ingredient that is key to habitability."
An alternative theory, according to Wood, was that Mars was intitially oxidized, even though he believes this is unlikely.
"You don't need a lot of oxygen to cause this - you don't need to be at 20% concentration. It would depend on temperature and how much water was around. But you need free oxygen to do it. And the process didn't take place to any great extent on Earth at that time - which is interesting," he said.
Wood believes oxidation is what caused the red color of Mars, which suggests the planet was "warm, wet, and rusty" way before the Earth.
"The principal way we would expect to get oxygen is through photolysis of water - water vapour in Mars' atmosphere interacting with radiation from the Sun breaks down to form hydrogen and oxygen," he said. Most of that hydrogen and oxygen recombines back to water. But a small fraction of the hydrogen is energetic enough to escape from the planet. A small amount of hydrogen is lost leaving an oxygen excess."
Wood explained the gravity of Mars is much weaker than on Earth, so hydrogen would escape more easily.
