The remains of 60 micrometeorites have survived 2.7 billion years in the limestone Tumbiana Formation of Western Australia. They are the oldest space rocks discovered on Earth. The fact that the meteorites contain oxides of iron show that the upper part of the atmosphere back then must have contained oxygen.
“We were very surprised to find micrometeorites at all, let alone those with iron oxides,” says Matthew Genge of Imperial College London. “It was incredible, these tiny spherules had trapped ancient atmosphere, storing it away like little treasure chests.”
The biggest surprise was the presence of oxygen, says lead author Andrew Tomkins of Monash University in Australia. “As geologists, we are taught the Earth had no oxygen in its atmosphere before 2.3 to 2.4 billion years ago.”
Several lines of evidence back the idea that Earth’s air only contained minute amounts of oxygen before the so-called great oxidation event some 2.4 million years ago, but crucially they are all based on the composition of the lower atmosphere.
Since the meteorites contain oxygen, it must have been present in the upper atmosphere, some 75 kilometres up. And from the types of oxide minerals in the meteorites, the researchers estimate that levels back then would have rivalled those found in the atmosphere today, roughly 20 per cent.
In fact, the team discovered, atmospheric chemists had predicted that the upper atmosphere on a low-oxygen early Earth would contain a lot of oxygen.
That’s because solar ultraviolet radiation would have split molecules such as water, carbon dioxide and sulphur dioxide, freeing oxygen at high altitudes. The hydrogen released from such reactions would have ended up in space, while elemental sulphur would have fallen to the ground.
Tomkins believes that a methane-rich inversion layer in the middle atmosphere would have inhibited vertical circulation, separating the vast bulk of the anoxic air below from the tenuous oxygen-rich upper atmosphere.
Tomkins hopes to find more meteorite samples from across Earth’s history, to see how the upper atmosphere may have changed, and maybe even when oxygen first appeared.
But this might be a long shot. “It is remarkable that objects as small as the micrometeorites survived intact for 2.7 billion years,” Kevin Zahnle of the NASA Ames Research Centre writes in an accompanying article in Nature.
The survival of iron oxides is particularly unusual, and it may only have happened because of unusually fortuitous circumstances. The lake into which the micrometeorites fell was highly alkaline, with its deepest layers totally anoxic. This is probably what prevented the minerals from dissolving.
“Such conditions are rarely encountered in the geological record, which means that the Pilbara micrometeorites might be a one-off discovery,” Zahnle writes. “But one can wish upon a shooting star.”