An asteroid hit deep in the Australian outback. What it left may have broken a record

IN BRIEF

A study has determined the "most precise age yet" for the oldest known impact crater in the world.

It's located in Western Australia and researchers say it could provide a clue into the origins of early life on Earth.

In the heart of Western Australia's arid Pilbara region stands a collection of rocks dubbed the North Pole Dome.

They stick out of the earth like an "upside-down ice cream cone": some rocks just a few centimetres in size, others several metres high. They're rust-coloured, like much of this region — but researchers say the formation holds evidence of the oldest known asteroid impact on Earth.

"When we looked at the rock itself, we found older grains, older little crystals inside. Those crystals tell us about when the original rock formed about 3.5 billion years ago," said Professor Chris Kirkland.

Kirkland led a Curtin University study that examined the age of minerals, including zircon crystals about the size of a grain of sand. He described this work as "extremely challenging" and "painstaking".

"You're hunting through microscopic slices of rock to find those crystals that tell you about the various processes that have happened. Then ... you're firing laser beams into them to determine their chemistry and isotopic signatures, which is how you can work out the age."

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The study builds upon previous research which analysed the stratigraphy — or rock layers — of these structures. Kirkland explained in the past, the exact timing of asteroid strikes remained somewhat of a mystery.

"It's like having a book, you know roughly where you are in the book — if you're at the front, back or middle," Kirkland said.

"If you've got a page ripped out you can make a guess where it sits but you don't absolutely know.

He said the latest findings, which use isotopic dating, "gives us a page number and then we can fit it back into the book, in its proper place".

Craig O'Neill is a professor of planetary geophysics at the Queensland University of Technology and acknowledged that research in this field has sparked prior "debate" and "controversy".

"I think any area where you have such little evidence ends up quite controversial and dominated by very small observations and a discussion over what they mean. That's fine, that's how we get ahead, but it's worth pointing out," O'Neill said.

Professor Tim Johnson from Curtin University also co-authored the Curtin University studies. He told SBS News in the past, "we possibly got a little carried away with ourselves".

Johnson now believes: "The data is complicated, and there are other explanations but our best estimate for the age of this impact is about three billion years ago, which makes it by far the oldest impact on Earth.

"The only one in the so-called Archean eon, where we know impacts would have been much more common than they are today."

Earlier evidence suggested the Earth's oldest asteroid strike occurred at Yarrabubba (also in the Pilbara region) roughly 2.2 billion years ago.

Western Australia's 'deep time record'

The Pilbara region of WA has long fascinated scientists. It's home to some of the oldest and best-preserved fragments of continental crust, even attracting attention from the NASA space agency.

"These [asteroid] impacts were actually hitting everywhere but we're just very lucky in West Australia to have a deep time record", Kirkland said.

While planets including Mercury, Venus and Mars have craters or asteroid belts pointing to the impacts of collisions, evidence on Earth is more "difficult to find" according to Johnson.

Approximately three billion years ago, the planet was covered by water, with few land masses rising above the surface — save "a few volcanic islands, a bit like Hawaii".

The Earth was hotter, with an atmosphere rich in carbon dioxide. Even the moon, which has asteroid craters still visible to the human eye, was closer to Earth, bringing more intense tides.

"In almost every possible way you can conceive of, the Earth would have looked like a very different place than it does today."

For O'Neill from QUT, recent evidence in the Pilbara raises one of the ultimate scientific questions: what was the origin of early life on Earth?

"It [the asteroid strike] would have caused a nuclear winter from all the vaporised rock and dust that it threw up. We're talking about really, really violent events and it had to have an effect on what was around at the time."

While Johnson from Curtin University admits his research raises questions, he hopes it also sparks opportunities.

There's evidence of ancient asteroids in South Africa and Canada, both dating back billions of years. In south-west Greenland, rocks are thought to be around 3.8 billion years old.

Johnson believes evidence of asteroid impacts is "absolutely everywhere".

"It [may be] difficult to find, but it's good fun looking for it."

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