Scientists say the planet’s oldest asteroid strike In Australia 2.2 billion years ago

To place the Yarrabubba event in a geologic context, we had to find its age. To find the age, we had to look carefully at minerals in the rocks shocked by the impact. (The National Interest)

The world’s oldest remaining asteroid crater is at a place called Yarrabubba, southeast of the town of Meekatharra in Western Australia.

Our new study puts a precise age on the cataclysmic impact – showing Yarrabubba is the oldest known crater and dating it at the right time to trigger the end of an ancient glacial period and the warming of the entire planet.

What we found at Yarrabubba

Yarrrabubba holds the eroded remnants of a crater 70 kilometres wide that was first described in 2003, based on minerals at the site that showed unique signs of impact. But its true age was not known.

We studied tiny “impact-shocked” crystals found at the site, which show the crater formed 2.229 billion years ago (give or take 5 million years).

This new, precise date establishes Yarrabubba as the oldest recognised impact structure on Earth. It is some 200 million years older than the next oldest, the Vredefort impact in South Africa.

More intriguing, the geological record shows the Earth had glacial ice before the time of the impact – but afterwards, ice disappeared for hundreds of millions of years. Was the Yarrabubba impact a trigger for global climate change?

How to date an asteroid hit

An asteroid strike is one of the most violent geologic events. In an instant, Earth’s crust is squeezed to unimaginable pressures, before exploding and ejecting carnage across the landscape. Large impacts leave behind scars the size of a small city.

The basin formed by an impact will partly fill with molten and pulverised rock from the Earth and from the asteroid itself. The edge of the crater forms a ring of mountains; over time erosion gradually erases the story.

Today, Yarrabubba has been worn down into a minor feature on a barren landscape.

To place the Yarrabubba event in a geologic context, we had to find its age. To find the age, we had to look carefully at minerals in the rocks shocked by the impact.

Geologists date events using “isotopic clocks” in minerals like zircon and monazite. These minerals contain small amounts of uranium, which gradually decays into lead at a known rate.

Asteroid strikes raise the temperature in rocks they hit, causing minerals to lose their accumulated lead, which resets the clock. After impact, the isotopic clocks start ticking again as new lead accumulates.

So by measuring the isotopes of uranium and lead in these minerals, we can calculate how much time has passed since the impact.

At Yarrabubba, we identified tiny crystals of zircon and monazite – each about the width of human hair – with textures that show they had been heated by a massive impact.

We analyzed the amounts of lead and uranium isotopes in these crystals using mass spectrometry, and found their clocks had been reset 2.229 billion years ago (give or take five million years). That’s when we realised Yarrabubba coincided with a major change in Earth’s climate.




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