World's oldest asteroid crater revealed

We’re going way back in time - to what’s just been confirmed as the oldest asteroid crater on Earth. It’s called the Yarrabubba crater, in the centre of Western Australia, it was created when something large smashed into the planet over two and a quarter billion years ago. By looking at rocks that came from the impact - called shocked crystals - a team of Australian geologists figured out when it happened. And because that date corresponds with some big changes in the Earth’s climate, the next step was to build a computer simulation to see what might have happened. Thomas Davison at Imperial College London created that simulation - and he told the whole story to Phil Sansom...

Thomas - We went to a crater in Western Australia called the Yarrabubba crater, collected some samples, and dated those samples; and found that the age of the crater is older than the previously known oldest crater. So this crater is 2.229 billion years old.

Phil - What was it that hit the earth 2.2 billion years ago that made this crater?

Thomas - Good question. So we don't know exactly what it was. Either an asteroid or a comet, the most likely scenario would be an asteroid hitting Earth. If it was an asteroid, we've run some models of this and found that the most likely scenario would be a seven kilometre asteroid hitting at around 17 kilometres a second.

Phil - Wow.

Thomas - My colleague Timmons Erickson, he went out and collected some samples from within the crater, picked up some bags of rocks and brought them back to the lab; and then they sorted these samples, they used some special techniques to pull out some very small grains that are called zircons and monazites. Now these grains that they were analysing were about the width of a human hair.

Phil - And those are things that can only come from something like an asteroid or a meteor?

Thomas - The features seen in those grains - they have shock features in them, and these are only found in impact events; so when an asteroid has hit the earth at many kilometres per second, formed a shockwave which has squeezed the rocks under really high pressures and temperatures, and changed the crystal structure of the minerals.

Phil - And how do you tell how old those are?

Thomas - So they looked at these two minerals, monazite and zircon, and they looked at the amounts of lead and uranium in those grains. Now uranium naturally decays to lead over time at a known rate. So we know that if you look at the ratio between the uranium and the lead, we can say how old a sample is. When you have an impact that squeezes the rocks under these really high pressures, that removes the lead from these grains, and you reset that clock.

Phil - Oh, it's like the asteroid took all the sand out the bottom of the hourglass - all the lead...

Thomas - Exactly.

Phil - And now the uranium's turning into lead again, you can see how much it's changed to.

Thomas - That's exactly right, yeah.

Phil - What did the world look like back then?

Thomas - So we think that when the asteroid struck the earth and formed the Yarrabubba crater, we think the earth was under a global ice age condition. So this is sometimes called the Snowball Earth scenario. We think there would have been maybe a couple of kilometres of ice, possibly globally. So I then ran some computer simulations using some shock physics models to find out what would happen if an asteroid of this size hit the earth with an ice sheet on top.

Phil - What does that look like to the area and the ice around it?

Thomas - So the ice itself immediately under the asteroid is going to vaporise almost immediately. The ice is then pushed up high into the atmosphere in this... what we call a vapour plume, going up into the upper atmosphere. And then that would then spread out through the atmosphere once it's up there.

Phil - And then what does that do to the atmosphere?

Thomas - Well, water vapour is a greenhouse gas. So if we can get enough of this up into the upper atmosphere, this could then start forcing the climate change, warming up the atmosphere and causing the ice age to thaw.

Phil - And did it cause a big climate change?

Thomas - Well, there was a big climate change at the time of the impact. Now normally we can attribute global trying to change to perhaps some volcanic activity; that wasn't the case at the time that the Yarrabubba impact happened. So we then think that maybe this could be the driving force behind that climate change.

Phil - And how big of a change was that?

Thomas - Before the impact event there was this - perhaps global - glaciation. After the impact, in the rock record, those glacial deposits are not present for up to 400 million years after the impact. So whatever thawed out the earth, it had lasting effects and lasting changes on the climate.