Ancient Earth’s upper atmosphere contained about the same amount of oxygen as today, according to a new study that challenges the accepted view of our planet’s atmosphere.
Using the oldest fossil micrometeorites – space dust – ever found, the new study has made a surprising discovery about the chemistry of Earth’s atmosphere 2.7 billion years ago.
The findings show that the ancient Earth’s upper atmosphere 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.
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Researchers from the Monash University, the Australian Synchrotron and Imperial College London extracted micrometeorites from samples of ancient limestone collected in Western Australia.
“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 Andrew Tomkins, from the Monash University.
“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,” Tomkins said.
Researchers 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,” said Matthew Genge, from Imperial College London.
The 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 carbon dioxide (CO2) by ultraviolet light.
“A possible explanation for this layered atmosphere might have involved a methane haze layer at middle levels of the atmosphere,” Tomkins said.
“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,” he said.
“It is incredible to think that by studying fossilised particles of space dust the width of a human hair, we can gain new insights into the chemical makeup of Earth’s upper atmosphere, billions of years ago,” he added.
The study was published in the journal Nature.