Scientists have gleaned fresh insights into the chemistry of early Earth's atmosphere 2.7 billion years ago by analysing fossilised micrometeorites.

The study, published today in the journal Nature, indicates that ancient Earth’s upper atmosphere contained about the same amount of oxygen as today, and that a methane haze layer separated it from an oxygen-starved lower atmosphere. These findings challenge the accepted view that Earth’s ancient atmosphere was oxygen poor 2.7 billion years ago.

The researchers suggest their results can be explained if Earth at that time had an atmosphere that mixed very little between the different layers. A possible explanation for this may be that the methane haze layer at middle levels would have absorbed UV light from the Sun, which would have released heat and created a warm zone that would inhibit mixing between layers. The team suggest the upper oxygen layer may have been produced when CO? was broken down by ultraviolent light from the Sun.

The research was carried out by a team from Imperial College London, the Australian Synchrotron and led by Monash University.

Micrometeorites, the width of a human hair, are constantly raining down on Earth. The team analysed ancient micrometeorite samples that had fallen on Earth billions of years ago, which were eventually encased in limestone deposits. They were collected in the Pilbara region in Western Australia.

Dr Matthew Genge, co-author from Imperial College London, said: “These micrometeorites act like little treasure chests storing information about our ancient atmosphere. Unlocking their secrets revealed a surprise because it has been firmly established that the Earth’s lower atmosphere was very poor in oxygen 2.7 billion years ago. So how the upper atmosphere could contain so much oxygen before the appearance of photosynthetic organisms was a real puzzle to us. We now think Earth’s methane haze may have prevented mixing between the levels, creating an ancient atmospheric layer-cake effect where oxygen was abundant on the outer layer.”

The team examined the micrometeorites using cutting-edge microscopes from Monash Centre for Electron Microscopy (MCEM) and the Australian Synchrotron. They found that the meteorites had once been particles of metallic iron, which had been turned into iron oxide minerals when exposed to oxygen and heated up by friction in the upper atmosphere. Dr Genge carried out calculations that determined that micrometeorites could have only been turned into iron oxide minerals if oxygen levels in the upper atmosphere were similar to present day levels.

Dr Andrew Tomkins, lead author from Monash University, adds: “It is incredible to think that by studying ancient space dust particles the width of a human hair, we can gain new insights into the chemical makeup of Earth’s upper atmosphere, billions of years ago.”

The next step will see the team looking for micrometeorites within rocks of different ages to search for changes in the ancient atmosphere when oxygen started to be produced by photosynthetic organisms 2.5 billion years ago in the great oxidation event.

Dr Genge and his research group at Imperial are now focussing on studying modern micrometeorites from Antarctica as well as those collected from rocks throughout the last 2.7 billion years of Earth history to learn more about how Earth’s climate has evolved. Dr Genge’s work on this project is supported by the UK’s Science and Technology Facilities Council.