Around 2 billion years ago when Earth was covered in ice, a meteorite slammed into what is now outback Western Australia.

Key points: Precise dating of melted crystals indicate a meteorite slammed into the Yarrabubba crater 2.229 billion years ago

Precise dating of melted crystals indicate a meteorite slammed into the Yarrabubba crater 2.229 billion years ago This time coincided with the end of the first 'snowball' Earth

This time coincided with the end of the first 'snowball' Earth Modelling by the scientists suggests this may have helped end the deep freeze, but other experts are sceptical

The impact left a 70-kilometre-wide scar on the land known as Yarrabubba impact crater.

"The age we've got for the Yarrabubba impact structure makes it the oldest impact structure on the planet," said Chris Kirkland, a geologist at Curtin University.

Very little is left of the impact today, bar a small red hill at the centre called Barlangi Rock.

"You could easily drive past it and not recognise that you're actually driving through a really big impact structure," Professor Kirkland said.

The 70km-wide Yarrabubba crater sits on top of an ancient piece of Earth's crust known as the Yilgarn Craton. ( Supplied: Chris Kirkland )

But proof of the crater's violent past and age is imprinted in crystals within its distinctive rocks.

Precise dating of these crystals shows the cosmic collision occurred 2.229 billion years ago — give or take 5 million years — according to new research published today in the journal Nature Communications.

The team also propose the timing raises the possibility that the meteorite strike helped lift Earth out of its deep freeze.

"The impact event itself might not have been the full reason for a global climate shift, but if we are in a dynamic period of Earth's history when other things are happening this might have been the straw that broke the camel's back," Professor Kirkland said.

But while other experts applauded the dating of such an old structure, they are sceptical that the cosmic collision shook Earth out of its snowball state.

How do you date a crater?

The Yarrabubba impact crater, which stretches from Meekatharra to Sandstone, sits within one of Earth's oldest surviving pieces of crust known as the Yilgarn Craton.

While it had always been assumed the crater was ancient, no-one had pinned down an exact age, said the study's lead author Timmons Erickson.

Dr Erickson, now based at NASA's Johnson Space Center, was driving through the basin on the way to another location when he decided to stop off at Barlangi Rock.

The small hill is made up of granite that was snap melted when the meteorite hit the Earth.

"When [a meteorite] hits the ground it creates intense pressures and heat and causes very unique circumstances in the Earth's crust we don't usually see."

Close-up of a zircon crystal showing the original structure (blue) surrounded by the area snap heated by the meteorite strike (pink). ( Supplied: Chris Kirkland )

After returning to the lab, he sifted through kilograms of rock picking out tiny crystals of zircon and monazite just the width of a hair "with a steady hand and a good set of tweezers".

The team then examined the texture of the grains and used a mass spectrometer to fire beams of ions into the melted part of the crystals to determine the ratio of uranium to iron.

The researchers used a mass spectrometer to measure the ratio of uranium to lead in the pink area of the crystals above. ( Supplied: Chris Kirkland )

"Mineral grains that trap uranium turn into clocks because uranium over time turns to lead and we know the rate of that change," Professor Kirkland explained.

"By being able to precisely target these shocked grains, we've been able to get a much more precise handle on when this event occurred."

Dating an impact using zircon is extremely challenging, said Tim Barrows, a geologist from the University of Wollongong who has used different techniques to date much younger craters such as Wolfe Creek.

"It's sorely needed information on those very early events. It is really difficult to date those impacts," Dr Barrows commented.

Oldest recognised impact structure

Until now, the Vredefort Dome, a 2.02-billion-year-old crater in South Africa, which at 250 km across also happens to be the world's biggest, was widely accepted as the world's oldest known impact crater.

"This [discovery] is pushing it back another 200 million years," Professor Kirkland said.

"Of course, you would predict there would be older craters, it's just we haven't found them and we certainly haven't been able to date them."

Looking for signs of early Earth in its ancient rocks can also be controversial.

Andrew Glikson, an Earth and palaeoclimate scientist at the Australian National University, said the dating work by the team was "excellent".

But, he said, the honour of "oldest-known impact structure" should go to the Maniisoq formation in southwest Greenland, which is around 3 billion years old.

Professor Kirkland, however, said there was not enough evidence Maniisoq was created by an impact.

"The community currently does not regard Maniitsoq as an impact structure and in my view it is more likely related to tectonic deformation," he said.

Snowball Earth "speculation"

The dating of the crystals places the impact right towards the end of the very first "snowball Earth" known as the Huronian glaciation.

Deposits in the youngest remnants of ancient crust such as in South Africa that are the same age as Yarrabubba indicate glaciers extended across much of the Earth.

If Yarrabubba had been covered by a 5km-deep glacier at the time, a huge amount of water vapour would have been instantly flung into the stratosphere when the meteorite struck, according to modelling by the researchers.

"If you throw water vapour up into the upper atmosphere it becomes a greenhouse gas," Professor Kirkland said.

"We need to think about these extreme events where we might have had some extraterrestrial body or some non-Earth material making a big change to our Earth system."

The Earth has been almost completely covered in ice at least three times throughout its 4.5 billion year history. ( Getty Images: Phillip Tur )

Dr Barrows said the modelling was "speculative".

"The climatic effects from meteorite events don't last very long," he said.

"It's hard to imagine it having a long-lasting effect like terminating a global glaciation when the short-term effects are likely to be a nuclear winter."

Still, he said, the "experiment" was very interesting given the enormous size of this crater, which is half the size of the impact that wiped out the dinosaurs.

Dr Glikson also disputed the modelling.

"Water vapour would be there for weeks, maybe months, but it doesn't last," he said.

Carbon dioxide, on the other hand, which would also be released during a meteorite impact could last for thousands of years and could contribute to warming.

"There is always some CO2 in the rocks and this would've warmed the region of the Earth for a period, but we don't have the evidence of warming at that time."

Even if warming is a possibility, not much is known about this early snowball Earth and dating around the end of the ice age is "rubbery".

The other flaw in the modelling pointed out by both Dr Barrows and Dr Glikson is that we also don't know if Yarrabubba itself was covered in ice at the time as any evidence has eroded away.

Dr Erickson agreed the modelling was a "working hypothesis".

"One of the things that would be great is doing a little more fieldwork."

In fact, there are glacial deposits further north in the Hammersley region of the Pilbara.

"It would be really interesting to look in these ancient basins for [evidence of the Yarrabubba impact] and see how closely it correlates with these glacial deposits," he said.