Early Mars atmosphere 'oxygen-rich'
The study dealt with differences between rocks from Gusev Crater on Mars (above) and meteorites
Mars' atmosphere could have been rich in oxygen four billion years ago - well before Earth's air became augmented with the gas.
That is the suggestion put forward by the author of a study in Nature journal, which outlines an explanation for differences between Mars meteorites and rocks examined by a robot rover.
Dr Bernard Wood said the idea fits with the picture of a planet that was once warm, wet and habitable.
But other scientists were sceptical.
While the rise of atmospheric oxygen on Earth was probably mediated by life, Martian oxygen could have been produced through the chemical "splitting" of water.
Prof Wood and his colleagues from Oxford University looked at the chemical composition of Martian meteorites found on Earth and data from Nasa's Spirit rover, which examined surface rocks at Gusev Crater on Mars.
Both are igneous rocks (of volcanic origin), but they show major geochemical differences. For example, the Gusev Crater rocks are five times richer in nickel than the meteorites.
This had posed something of a puzzle, casting doubt on whether the meteorites were typical volcanic products of the Red Planet.
Young and old
"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," Prof Wood explained.
"This result is surprising because while the meteorites are geologically 'young', around 180 million to 1.4 billion years old, the Spirit rover was analysing a very old part of Mars, more than 3.7 billion years old."
Whilst the researchers conceded that large regional variations in the geological composition of Mars could not be ruled out, they argue in their paper that these differences arose via subduction - in which rocks are recycled in the planet's interior.
Dr Wood, James Tuff and Jon Wade from Oxford propose that the Martian surface became "oxidised" early in its history, and that these surface rocks were drawn into the shallow interior and recycled back to the surface during volcanic eruptions around four billion years ago.
The meteorites, by contrast, are much younger volcanic rocks that emerged from deeper within Mars and so were less influenced by this process.
Oxidation and reduction
Redox (reduction-oxidation) reactions involve the transfer of electrons between molecules, atoms, or ions
Oxidation involves losing electrons (an increase in the oxidation state)
Reduction involves gaining electrons (a decrease in the oxidation state)
Although material can become oxidised in the presence of free oxygen gas - it is not essential for oxidation reactions to occur.
But Dr Francis McCubbin, from the University of New Mexico, who was not involved with the Nature study, told BBC News: "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."
"I agree with the overarching conclusions of this work that there are substantial redox gradients with depth on Mars, and this could be potentially very important for Mars' habitability because some organisms can take advantage of redox (reduction-oxidation) reactions and use them as an energy/food source.
He added: "Although not implicitly stated, the early oxidized magmatism would also favour the production of water, another ingredient that is key to habitability."
On alternative possibilities to atmospheric oxygen, Prof Wood told BBC News: "One is that Mars was an initially oxidised planet - that's pretty unlikely. There aren't any meteorites or other bodies in the Solar System that show this high state of oxidation.
"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."
Prof Wood explained that, as oxidation was what gave Mars its distinctive colour, it is likely that the planet was "warm, wet and rusty" billions of years before Earth's atmosphere became oxygen-rich.
He added: "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.
"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.
"But the gravity on Mars is one third of that on Earth, so hydrogen would be lost more easily. So the oxygen build-up could be enhanced on Mars relative to Earth."