Extract from ABC News
The oldest known asteroid impact is now believed to be the Pilbara. (Supplied: Curtin University)
In short:
Scientists have confirmed that an asteroid crater in Western Australia's East Pilbara is the oldest in the world.
It is the second time the ancient crater's age has been pinpointed after a false start last year.
What's next?
Some scientists are still disputing the findings, saying there are inaccuracies in the study.
Scientists have dated a crater in the Australian outback at 3 billion years old, making it the world's oldest known site of an asteroid impact.
The North Pole Dome, about 1,600 kilometres north of Perth, was the source of controversy last year when it was reported as the oldest in the world, a claim debunked by another group of scientists a month later.
The debate continues as advanced dating techniques used a second time removed the uncertainty, while another expert still disputes the claim.
Advanced techniques were used to calculate the time of asteroid impact. (Supplied: Curtin University)
Curtin University lead author Chris Kirkland said the first study found the site was an ancient impact but the exact age was uncertain.
"[The second time] we went and looked into the rocks themselves, and looked at the individual little crystals within the rock, and we found these really unusual skeletal grains," he said.
"We were able to date the age of those very special grains that relate to the impact event."
The key evidence to track the time of impact came from zircon, a tiny mineral capable of keeping geological time for billions of years.
Chris Kirkland was the lead author of the study. (Supplied: Curtin University)
"Some zircons at North Pole Dome have unusual branching, skeletal shapes," Professor Kirkland said.
"We interpret these as impact-modified crystals, formed when older zircon was disrupted, partly recrystallised, and in places regrown during the intense heating caused by the impact."
Professor Kirkland said to confirm the result the team analysed a second mineral, apatite, which formed as hot fluids moved through the shock-damaged rocks.
This independent dating method produced the same age.
Academics dispute the study
Last year, when the crater was discovered and scientists from Curtin University found it to be the oldest documented crater, other experts disputed the findings.
Harvard University postdoctoral fellow Alec Brenner was one, and said the latest study also did not properly date the North Pole Dome impact structure.
"We've already documented shatter cones in nearby 2.77-billion-year-old rocks," Mr Brenner said.
"That alone requires the impact happened after 2.77 billion years ago.
"While the new study dismisses this observation because these rocks 'have not been dated', they are straightforwardly correlated to nearby rocks that have been dated."
Tiny zircon crystals were used for the study. (Supplied: Curtin University)
Mr Brenner said the long history of the Pilbara region had no shortage of complexity, including cryptic hydrothermal events that could form the zircon used to determine the time of impact.
"Seeing an unknown fluid flow event does not mean it resulted from an impact," he said.
"The lead author has also worked on other craters where similar dated fluid flow events were clearly non-impact-related. Most are not.
"So I'd suggest they've dated an undocumented hydrothermal episode in the area."
Crater history in WA
The previous record for the world's oldest crater was also in WA at Yarrabubba, in the Midwest.
This crater was said to be an impact 2.2 to 2.3 billion years ago.
"So we are doing another 750 million years older," Professor Kirkland said of the North Pole Dome crater's age.
"We can put these ages about and say the numbers, but if you stop to think about it, it is truly mind-boggling."
Rock formations grown on the asteroid impact. (Supplied: Curtin University)
He said the rocks they studied are similar to story books that tell the story of the planet's past.
"It is quite enthralling, and the fact that you can look at the chemistry of these mineral grains to say something about an event that happened in a split second three billion years ago is really amazing," he said.
The possibility of even older craters remaining undocumented was not written off by Professor Kirkland.
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