Ancient space dust revealed new hints on the oxygen level of our planet's early atmosphere, based on the findings of a new study. The analysis suggested that the Earth's ancient upper atmosphere contained about the same amount of oxygen then as it does at present, about 20 percent.

Roughly the width of a human hair, these ancient space dust particles made their way into Earth approximately 2.7 billion years ago. They were discovered on prehistoric sedimentary rocks found in the Pilbara region of Western Australia.

Also called "micrometeorites," they are made of iron oxide minerals. When they entered the Earth's ancient atmosphere at an altitude of up to 60 miles, they melted, reacted with oxygen, and transformed.

A team of British and Australian researchers looked into these micrometeorites and found that they were once metallic iron particles.

After being exposed to oxygen in the upper atmosphere on their way to the planet, they transformed into iron oxide minerals.

"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 lead author Andrew Tomkins from Melbourne's Monash University.

The researchers said that a much higher oxygen level in the upper atmosphere was needed to coax the chemical transformation. This suggested that the oxygen level in the Earth's ancient upper atmosphere must have been higher 3.9 to 2.5 billion years ago than previously thought.

Imperial College London's cosmic dust expert and study co-author Matthew Genge calculated that the oxygen levels in the upper atmosphere must be at least 20 percent to make the transformation possible. This level is roughly the same as the oxygen level in our upper atmosphere today.

Genge added that the findings were quite a surprise since it was long established that the planet's lower atmosphere had poor oxygen levels 2.7 billion years ago.

The recent discovery can lead to new investigations into our planet's atmospheric evolutions. Tomkins added that micrometeorites have been discovered in ancient rock samples before; however, they weren't used to study the chemistry of our planet prior to this research.

The new findings were published in the Nature journal on May 11.

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