Nature

Ancient Space Dust Is Challenging Established Views on Earth’s Early Atmosphere

May 12, 2016 | Joanne Kennell

One of 60 micrometeorites extracted from 2.7 billion year old limestone in Western Australia
Photo credit: Andrew Tomkins

“This was a surprise.”

Using the oldest fossil micrometeorites — also known as space dust — ever found, researchers have made a very surprising discovery that challenges the currently accepted view about Earth’s early atmosphere.

The findings of a new study published yesterday (May 11) in the journal Nature, show that the chemistry of Earth’s atmosphere 2.7 billion years ago was not oxygen-poor afterall. Rather, it’s upper atmosphere contained about the same amount of oxygen as today, and a methane haze separated this oxygen-rich layer from the oxygen-deficient lower atmosphere.

The experiment, led by Andrew Tomkins and a team from the School of Earth, Atmosphere and Environment at Monash, along with scientists from the Australian Synchrotron and Imperial College, London, extracted 60 micrometeorites the width of human hair from samples of ancient limestone collected in the Pilbara region in Western Australia, and examined them at the Monash Centre for Electron Microscopy (MCEM) and the Australian Synchrotron.

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"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 Tomkins in a Monash University news release.

This is the first time anyone has found a way to sample the chemistry of ancient Earth’s upper atmosphere, Tomkins went on to explain.

Matthew Genge, an expert in modern cosmic dust, performed calculations showing that 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," Genge said in the release.

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However, Tomkins suggests that 2.7 billion years ago, Earth may have had a layered atmosphere with very little vertical mixing.

"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," Tomkins said.

Higher levels of oxygen in the upper atmosphere were likely produced by the breakdown of carbon dioxide (CO2) by ultraviolet light.

The next stage of the team’s research will be to extract micrometeorites from a series of rocks covering over a billion years of Earth’s history in order to learn about the changes in its atmospheric chemistry and structure. Particularly, they will focus on the period known as the great oxidation event, when there was a sudden jump in oxygen concentration in the lower atmosphere 2.4 billion years ago.

You can watch a video about Andrew Tomkins, who is nicknamed the Meteorite Hunter, and his research below.

 

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