Nestled within the desolate landscape of Western Australia’s Pilbara region lies a geological marvel known as the North Pole Dome.
This striking ensemble of rocks rises from the earth resembling an “upside-down ice cream cone.” While some rocks are merely a few centimeters tall, others tower several meters high, all draped in the familiar rust hue of the surrounding terrain. Scientists assert that these formations contain evidence of the planet’s oldest recorded asteroid impact.
“Upon inspecting the rock, we discovered older grains and minute crystals nestled within,” explained Professor Chris Kirkland. “These crystals provide insight into when the original rock was formed, approximately 3.5 billion years ago.”
Kirkland spearheaded a research effort at Curtin University, focusing on the chronology of minerals, particularly zircon crystals no larger than grains of sand. He described the undertaking as both “extremely challenging” and “meticulous”.
“The process involves sifting through microscopic rock slices to locate those crystals that reveal various geological processes,” he shared. “Then, by directing laser beams into these crystals, we assess their chemical and isotopic compositions, which allows us to ascertain their age.”
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.”