A recent study by researchers at Oxford University has shown that the planet Mars may hav had an oxygen-rich atmosphere 4 billion years ago.

Using meteorites identified as coming from Mars, as well as data from Mars rover Spirit, researchers found that rocks currently found on the surface of Mars have much more nickel than the meteorites. This suggests, researchers say, that the early Martian surface was full of oxygen. The findings have been published in the journal Nature.

“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 Bernard Wood, a co-author of the study and a professor in Oxford University’s Department of Earth Sciences. “This result is surprising because while the meteorites are geologically ‘young’, around 180 million to 1,400 million years old, the Spirit rover was analysing a very old part of Mars, more than 3,700 million years old.”

Wood and his colleagues believe that the different compositions of the Martian material may be due to subduction, the process by which surface material is recycled into a planet’s interior. The hypothesis is that Mars was very oxidized early in its history, and that subduction has brought early surface material to where Spirit found it, while the meteorites are younger Martian material from deeper within the planet.

“The implication is that Mars had an oxygen-rich atmosphere at a time, about 4000 million years ago, well before the rise of atmospheric oxygen on earth around 2500 million years ago,” said Wood. “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.”