The oldest known meteorite crashed into the Earth some 3.47 billion years ago, leaving its mark in some of the world's most ancient rock formations, scientists have discovered.
Although there is now no trace of the massive extraterrestrial object or its crater, geologists have identified its telltale fallout in rocks in South Africa and Australia.
The researchers estimate the meteorite was 12 miles (19 kms) wide – roughly twice the size of the one that probably killed off the dinosaurs 65 million years ago – and caused shock waves around the planet.
To pinpoint the date of the collision a team led by Donald Lowe, professor of geological and environmental sciences at Stanford University, California, analysed two ancient rock formations, the Barberton greenstone belt in South Africa and the Pilbara block in Australia.
Both formations are older than 3 billion years, when the Earth was about 1 billion years old, and simple, single-celled bacteria were the most complex life forms on a planet that was mostly covered in water and where average temperatures were about 85C (185F).
In layers dated to about 3.5 billion years ago the scientists found tiny spherical particles, called "spherules", formed by the intense events caused by the meteorite's impact.
A study published in the journal Science found these sperules to be 3.47 billion years old. "We are reporting on a single meteorite impact ... We have no idea where actual impact might have been," Professor Lowe said.
"A meteorite passes through the atmosphere in about one second, leaving a hole, a vacuum, behind it, but air can't move in fast enough to fill that hole," he said.
"When the meteorite hits the surface, it instantaneously melts and vaporises rock, and that rock vapour is sucked right back up the hole into the atmosphere. It spreads around the Earth as a rock vapour cloud that eventually condenses and forms droplets that solidify into spherules, which rain back down onto the surface," Professor Lowe said.
The impact of a 12-mile-wide meteorite would have generated tsunamis – giant waves – miles high, and might even have cracked the Earth's crust in such a way that helped the first continents to form.
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