Scientist’s understanding of the evolution of the ancient Earth’s atmosphere to the one that supports not only humans but life in general, has been a mystery for some time. However, a new study is removing the veil of uncertainty and providing a unique look into what was 2.7 billion years ago.
Australian spacedust providing answers
In a new study published in the journal Nature this week, researchers believe that they have been gifted an understanding of how the Earth’s atmosphere may have evolved in order to support life. Scientist’s had, for years, nearly assumed that the lower levels of oxygen in the lower atmosphere of Earth meant that Earth’s upper atmosphere would have also have had lower levels oxygen than it does today.
That, however, doesn’t seem to be the case much to the surprise of the researchers that found that the spacedust studied showed that oxygen makeup in the upper atmosphere from 2.7 billion years ago was about the same as today’s 20%.
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“The evolving atmosphere changed the chemistry of a large range of geological processes, some of which are responsible for forming gigantic mineral resources,” says lead study author Andrew Tomkins of Monash University in Melbourne, Australia. In fact this research “helps us think about biosphere-hydrosphere-geosphere interactions and how they’ve changed over time,” he continued.
The researchers in this study prefer the more scientific “micrometeorites” rather than spacedust that science writers bandy about when writing about the study. The micrometeorites used in the study came from limestone deposits in the Pilbara region of Western Australia.
Tomkins explains that when this cosmic material entered Earth’s atmosphere, it melted at about 50 miles above the surface of the Earth. They then reformed as they approached the Earth’s surface and were transformed. It’s this melting and reforming that the researchers looked at for their study.
“People have found micrometeorites in rocks before, but nobody had thought to use them to investigate atmospheric chemistry,” Tomkins says.
Iron turns to iron oxide
The micrometeorites were comprised of iron when they entered the Earth’s atmosphere, upon contact with oxygen in the Earth’s atmosphere they became iron oxide minerals. This understanding came through the use of powerful microscopes by the team and this transformation would not have been possible if the Earth’s (upper) atmosphere didn’t contain more oxygen than was previously believed.
The study’s co-author, Matthew Genge at Imperial College London was responsible for determining that the oxygen levels of the upper atmosphere hasn’t changed in 2.7 billion years.
This strengthens the theory that the Earth’s atmosphere was distinctly “stacked” with a layer of methane lying between the oxygen “rich” lower and upper atmospheres that kept the two from mingling.
This layer of methane likely existed until 2.4 billion years ago when a “great oxidation event” occurred where the methane was eliminated following a massive oxygen rise based on the actions of photosynthesizing cyanobacteria.
“Oxygen and methane don’t go well together, so this rise in oxygen would have eventually reacted the methane out of the system,” Tomkins says. “Removal of methane would allow more effective mixing of the upper and lower atmospheres.”
While this idea is supported by the new research, this elimination of methane remains a hypothesis.
“We have taken a sample of the upper atmosphere at only a single point in time,” Tomkins says. “The next step is to extract micrometeorites from rocks covering a broad range of geological time, and to look at broad changes in the chemistry of the upper atmosphere.”