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Research says Mars had oxygen-rich atmosphere 4000 million years ago

Differences between Martian meteorites and rocks examined by a NASA rover can be explained if Mars had an oxygen-rich atmosphere 4000 million years ago—well before the rise of atmospheric oxygen on Earth 2500 million years ago.

Scientists from Oxford University investigated the compositions of Martian meteorites found on Earth and data from NASA's 'Spirit' rover that examined surface rocks in the Gusev crater on Mars. The fact that the surface rocks are five times richer in nickel than the meteorites was puzzling and had cast doubt on whether the meteorites are typical volcanic products of the red planet.

'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 Bernard Wood, of Oxford University's Department of Earth Sciences, who led the research reported in this week's Nature.

'This result is surprising because while the meteorites are geologically 'young', around 180 million to 1400 million years old, the Spirit rover was analysing a very old part of Mars, more than 3700 million years old.'

Whilst it is possible that the geological composition of Mars varies immensely from region to region the researchers believe that it is more likely that the differences arise through a process known as subduction – in which material is recycled into the interior. They suggest that the Martian surface was oxidised very early in the history of the planet and that, through subduction, this oxygen-rich material was drawn into the shallow interior and recycled back to the surface during eruptions 4000 million years ago. The meteorites, by contrast, are much younger volcanic rocks that emerged from deeper within the planet and so were less influenced by this process.

Professor Wood said: '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. 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.'

The Nature letter: Volcanism on Mars controlled by early oxidation of the upper mantle

Detailed information about the chemical composition and evolution of Mars has been derived principally from the SNC (shergottite–nakhlite–chassignite) meteorites, which are genetically related igneous rocks of Martian origin. They are chemically and texturally similar to terrestrial basalts and cumulates, except that they have higher concentrations of iron and volatile elements such as phosphorus and chlorine and lower concentrations of nickel and other chalcophile (sulphur-loving) elements. Most Martian meteorites have relatively young crystallization ages (1.4 billion years to 180 million years ago) and are considered to be derived from young, lightly cratered volcanic regions, such as the Tharsis plateau. Surface rocks from the Gusev crater analysed by the Spirit rover are much older (about 3.7 billion years old) and exhibit marked compositional differences from the meteorites7. Although also basaltic in composition, the surface rocks are richer in nickel and sulphur and have lower manganese/iron ratios than the meteorites. This has led to doubts that Mars can be described adequately using the ‘SNC model’. Here we show, however, that the differences between the compositions of meteorites and surface rocks can be explained by differences in the oxygen fugacity during melting of the same sulphur-rich mantle. This ties the sources of Martian meteorites to those of the surface rocks through an early (>3.7 billion years ago) oxidation of the uppermost mantle that had less influence on the deeper regions, which produce the more recent volcanic rocks.

This would be incredible if true. Any opinions?
The only caveat I don't see them making is that an oxidizing environment doesn't necessarily require an atmosphere rich in 'Oxygen'

The atmosphere of Venus contains a powerful oxidizer (H2SO4) Even the current martian atmosphere (CO2) is a mild oxidizer.

"Professor Wood said: 'The implication is that Mars had an oxygen-rich atmosphere" - that's the link they've not proven unless I'm missing something. To be fair - it's been 15 years since I wandered into anything even slightly resembling a chemistry class.

Even if they did mean oxygen itself I don't think that's particularly surprising. Some of Saturn's icy moons have thin atmospheres of oxygen produced by dissociation of water molecules (presumably accompanied by hydrogen escape). If the early atmosphere of Mars contained abundant water vapour and was unprotected from ionising radiation then it's easy to think of free oxygen being produced there by a similar process.
Along with the magnetic striping and apparent tectonic boundary evidence by displacement along Valles Marineris, this provides a tick on the positive side for some form of plate tectonics on early Mars. This would also tie in with the late stage crustal realignment raised by Kite et al supported by a rather compelling constant-scale natural boundary global map of Mars developed by Chuck Clark, edge defined by late-stage volcanism. If there was significant subduction early on before the crust thickened then this could have had the effect of cooling the core. Once the surface froze the core would have heated up over of few billion years, providing a mechanism for the late stage volcanism that gave rise to the Tharsis Bulge. I have to agree with djellison in that oxidised material being subducted does not necessarily imply a free oxygen rich atmosphere.
QUOTE (ngunn @ Jun 19 2013, 01:13 PM) *
... If the early atmosphere of Mars contained abundant water vapour and was unprotected from ionising radiation then it's easy to think of free oxygen being produced there by a similar process.

Agree. Note that the original Nature article says nothing about an oxygen-rich atmosphere for early Mars. They talk only about a relatively oxidized upper mantle (similar to that of Earth, in this regard), as deduced from the relatively Ni-rich basaltic magma produced by early partial melting, as measured by Spirit in Gusev Crater. They note that one way to produce this result would be via subduction of oxidized ("rusty") surface material (as on Earth).

Hydrogen loss through water dissociation can produce such "rust" and is also believed (according to the 1985 article by Dreibus and Wanke cited in this Nature article) to be responsible for the extremely iron oxide-rich nature of the martian mantle, and of its melts. If hydrogen loss were more pronounced near the surface, as one might expect given the great volatility of hydrogen, you could preferentially oxidize the early hydrous upper mantle without needing to invoke Earth-style subduction there. Breathable oxygen, or an oxygen-rich atmosphere of any sort, is not implied in any case.

One of the authors is quoted in the news story that early Mars seemingly had "an oxygen-rich atmosphere" and a surface that was "wet, warm, and rusty". The second statement is probably more correct than the first, although impacts and/or volcanism could have produced warmth only sporadically.

Oxford University press release 19/6 2013
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