MELBOURNE, Australia - A recent study of the oldest micrometeorites known to exist shows that Earth's ancient atmosphere was rich in oxygen, challenging the accepted view that Earth's atmosphere was oxygen-poor billions of years ago.
By examining micrometeorites - space dust - that fell to Earth 2.7 billion years ago, researchers found that ancient Earth's upper atmosphere during the Archean eon contained about the same amount of oxygen as today, and that a methane haze layer separated this oxygen-rich upper layer from the oxygen-starved lower atmosphere.
Scientists extracted the micrometeorites from samples of ancient limestone collected in the Pilbara region in Western Australia and later examined them at the Monash Centre for Electron Microscopy and the Australian Synchrotron.
"Using cutting-edge microscopes we found that most of the micrometeorites had once been particles of metallic iron - common in meteorites - that had been turned into iron oxide minerals in the upper atmosphere, indicating higher concentrations of oxygen than expected," said Dr. Andrew Tomkins of Monash Univesity. "This was an exciting result because it is the first time anyone has found a way to sample the chemistry of the ancient Earth's upper atmosphere."
Imperial College researcher Dr. Matthew Genge, an expert in cosmic dust, performed calculations that showed oxygen concentrations in the upper atmosphere would need to be close to modern day levels to explain the observations.
"This was a surprise because it has been firmly established that the Earth's lower atmosphere was very poor in oxygen 2.7 billion years ago; how the upper atmosphere could contain so much oxygen before the appearance of photosynthetic organisms was a real puzzle," Dr. Genge said.
The new results suggest the Earth at this time may have had a layered atmosphere with little vertical mixing, and higher levels of oxygen in the upper atmosphere produced by the breakdown of CO2 by ultraviolet light.
"A possible explanation for this layered atmosphere might have involved a methane haze layer at middle levels of the atmosphere. The methane in such a layer would absorb UV light, releasing heat and creating a warm zone in the atmosphere that would inhibit vertical mixing," Dr. Tomkins said.