Mars could have been rich with oxygen long before the gas was found in any concentration on Earth. The idea of ancient oxygen on Mars could explain some differences between Martian meteorites recovered on Earth compared to rocks sampled by robot laboratories on the Red Planet.

This theory, proposed by Bernard Wood of Oxford University, however, has met with some skepticism.

Looking at data from samples of igneous (volcanic) taken on Mars by the Spirit rover, Wood noticed that those rocks were five times higher in nickel than similar material from Mars which fell as asteroids. One way to explain the difference between surface rocks on the planet today and those that came here via meteorite impact is through oxidation of the surface. Crust would be driven slightly underground by the process of subduction, then sent back to the surface by volcanoes four billion years ago. Wood theorizes that for oxidation to occur by that method, Mars would have had to possess an oxygen-rich atmosphere when that process took place. On contrast, the meteorites which reach Earth from Mars come from younger material which existed deep below the crust of the planet, where any oxygen would have been much rarer.

"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. 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," Wood said.

On Earth, the oxygen in our atmosphere was generated by early life. On Mars, this production was likely created by the splitting of water into its component ingredients of oxygen and hydrogen. Due to the low gravity on Mars, much of that hydrogen would have escaped the planet, leaving oxygen behind.

The bright red color of the planet is due to a 25 percent concentration of iron oxide, commonly known as rust, on its surface.

"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," Wood said. 

The Earth would not form its oxygen-rich atmosphere for over a billion years later.

This new finding was profiled in the journal Nature.