A bunch of 60 micrometeorites discovered embedded in limestone in Australia have a value far beyond their being the oldest micrometeorites ever found. Scientists now say these 2.7 billion year-old spheres the size of dust grains challenge prevailing theories about the oxygen content of early Earth's atmosphere.

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In addition, this discovery also stands to impact the parameters in the search for extraterrestrial life that rely a lot on the amount of oxygen present in the atmosphere of exoplanets.

The conventional wisdom is the Earth had no oxygen in its atmosphere before 2.3 to 2.4 billion years ago. Scientists agree the Earth before 2.5 billion years ago had an atmospheric oxygen level less than 0.001 percent that of today's.

At that early time, the Earth hadn't been flooded by the oxygen created by plants and algae as a byproduct of photosynthesis. This atmospheric revolution occurred some 2.5 to 2.4 billion years ago and is known as the "Great Oxidation Event."

Analysis of these 60 micrometeorites disputes this theory in a big way.

As explained by Andrew Tomkins, a geologist at Monash University in Melbourne, Australia, the chemical makeup of the Earth's atmosphere 2.7 billion years ago would have affected how the micrometeorites melted and what molecules the cooling meteorites formed.

Scientists can, therefore, use these micrometeorites to deduce the composition of the atmosphere at the time since they know micrometeorites melt and quench-cool about two seconds after they start burning as they enter the Earth's atmosphere.

Tomkins discovered these micrometeorites traveled though an atmosphere that was some 20 percent oxygen. He and his team came to this stunning conclusion by studying the exact composition of meteorites' metal oxides.

An atmosphere that was 20 percent oxygen is only possible if the far reaches of the upper atmosphere some 50 to 100 kilometers above the surface were inundated by oxygen. In contrast, the lower atmosphere was almost certainly devoid of oxygen, said Tomkins.

Oxygen in the upper atmosphere would have been formed by ultraviolet light from the sun splitting water vapor and carbon dioxide.

Tomkin's findings, however, create a massive conundrum for astronomers and planetary scientists.

If the Earth's upper atmosphere in the past did indeed have massive amounts of oxygen (a condition necessary to evolve and sustain life) then the atmospheres of the many exoplanets being discovered could appear more oxygen-rich than they actually are. This, in turn, will lead to misleading conclusions about the habitability of these exoplanets.

Tagsmicrometeorites, atmosphere, Andrew Tomkins, Monash University, Oxygen, Exoplanets