World’s oldest meteorites unveils ancient atmosphere

May 13, 2016

Australian and British researchers have proposed in a letter published in the journal Nature, that the micrometeorites consist of iron oxide minerals, which suggested they formed when dust particles of meteoritic iron metal were exposed to oxygen as they blazed through the Earth’s upper atmosphere.They uncovered the micrometeorites which are barely the width of a human hair from ancient sedimentary rocks in Western Australia’s Pilbara region.It is the oldest fossils of cosmic dust ever discovered which provide a glimpse into atmospheric conditions above the Earth more than 2.7 billion years ago and could do the same on other planets.This finding challenges previous notions that the upper atmosphere at the time was as low in oxygen as the lower atmosphere, said lead author and geoscientist Dr Andrew Tomkin from Monash University.”All the previous studies have looked at the chemistry of the lower atmosphere but nobody had ever actually figured out a way to look at the chemistry of the upper atmosphere before,” Dr Tomkin said.”Imagine picking a dust grain the width of a human hair out of a 2.7-billion-year-old rock and using that to figure out the chemical structure of the ancient atmosphere.” Dr Tomkin and colleagues were originally on the hunt for the world’s oldest micrometeorites, and reasoned that the best place to find these would be some of the world’s oldest sedimentary rocks, located 65 kilometres east of Marble Bar.The Pilbara rocks are not only ancient but are made of sediments that would have accumulated very slowly over a long period of time without much disturbance, which increases the likelihood of the sand grain-sized micrometeorites being found.”If you look up any night of the week you’ll see shooting stars, if you wait long enough, they’re literally all over the surface of the Earth” Dr Tomkin said.

One of 60 micrometeorites extracted in the Pilbara region.Image by Andrew Tomkins
One of 60 micrometeorites extracted in the Pilbara region.Image by Andrew Tomkins

Extracting them from the limestone simply involves dissolving the softer rock with mild acid to reveal the iron-based micrometeorites. However, when the researchers examined the micrometeorites, they realised the particles were oxidised in such a way that could only have happened during their high-speed, high-temperature flight through the upper atmosphere, Dr Tomkin said.”That’s because chemical reactions happen fast at high temperatures and barely happen at all at low temperatures, once they cool down and have been slowed down,” he said.”They’ve stopped reacting with the atmosphere as soon as they’ve stopped that bright flight you see in a shooting star, so they’re basically sampling a specific height in the atmosphere.”When these micrometeorites landed on Earth 2.7 billion years ago, the lower atmosphere was believed to be very low in oxygen and the assumption was that the same probably applied to the upper atmosphere.But, Dr Tomkin said recent modelling by atmospheric chemists had pointed to the possibility that oxygen may have formed in the upper atmosphere as ultraviolet light split atmospheric carbon dioxide into oxygen and carbon monoxide.”That’s a prediction that they had made and we were suggesting only a relatively small difference from their prediction, which is pretty interesting,” he said. Dr Tomkin said the technique of studying the surface of micrometeorites could be applied not only across other periods in Earth’s history, but also to study the atmospheric history of other planets.”One of the things that would be cool is to think about using micrometeorites on the surface of Mars to look at the atmosphere there and how that might have changed with time,” he said.

Contador Harrison