An international team of scientists are challenging the theory that ancient Earth’s atmosphere was low in oxygen. 

By extracting micrometeorites from Australian limestone, Dr Andrew Tomkins, from Monash University and other scientists from Imperial College, London and the Australian Synchrotron think that ancient Earth’s upper atmosphere contained the same amount of oxygen as it does now. They believe a methane haze layer separated the oxygen-rich upper layer from the oxygen-starved lower atmosphere.

Dr Tomkins said: “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.

“This was exciting because it is the first time anyone has found a way to sample the chemistry of the ancient Earth’s upper atmosphere,” he said.

Dr Michael Genge, from Imperial College, who carried out tests to confirm the oxygen concentrations of the atmosphere, said it was a surprise the upper atmosphere could hold so much oxygen before the appearance of photosynthetic organisms. Dr Tomkins said: “a possible explanation for this may 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.”

The researchers’ next step will be to extract micrometeorites from rocks spanning one billion years of the Earth’s history to learn more about atmospheric chemistry changes. Of particular focus will be an event almost two and a half billion years ago when there was a sudden jump in the lower atmosphere’s oxygen concentration.