Vredefort Asteroid – the biggest known crater on the planet
JOHANNESBURG would have gone first, then Pretoria, followed by the Limpopo town of
Bela Bela, all smashed by a mega explosion like nothing seen before.
Fortunately, this mega explosion happened long before humans and their cities arose and even before complex life began. It happened 2 billion years ago when an asteroid slammed into what is now Vredefort in the Free State.
What it left is the biggest known crater on the planet.
For a long time scientists have known that the crater was big, stretching to more than 100 kilometres, but now it has got even bigger.
And from new research, scientists are hoping that the Vredefort impactor might one day help in spotting and preventing the next big asteroid strike.
It was thought that the object that struck the Vredefort Dome area was probably about 15km in diameter and hit the Earth travelling at 15km a second. What it left was a crater 175km in diameter.
Now, through simulations, researchers from the University of Rochester in the US believe the impactor was 20km in diameter and the crater 250 to 280km in size. This is far larger than the asteroid that killed off the dinosaurs 66 million years ago and formed the Chicxulub crater, off the coast of Mexico.
“Unlike the Chicxulub impact, the Vredefort impact did not leave a record of mass extinction or forest fires, given that there were only single-cell lifeforms and no trees existed 2 billion years ago,” said Miki Nakajima, an assistant professor of Earth and environmental sciences at the University of Rochester.
“However, the impact would have affected the global climate potentially more extensively than the Chicxulub impact did.”
The research was recently published in the Journal of Geophysical Research and the main author was then undergraduate student Natalie Allen. She quickly discovered she had her work cut out. The problem was the amount of erosion that had happened over the last 2 billion years, which made defining the size of the crater difficult.
“We actually used a lot of evidence from inside the crater itself. Teams for over the last 100 years or so have collected rock samples from inside the crater and these are very interesting,” Allen says. “We call them metamorphic features and they are evidence of really high pressures and temperatures.
“Things like shatter cones are a famous example of them and they are large radial structures that go out from the source of a large pressure event like what happens with nuclear explosions or impacts like this.”
With this new geological evidence and measurements, the researchers could through simulations work out the size of the asteroid that made the crater.
“Part of the work for me was working out how large an asteroid would need to be to cause these catastrophic effects. Because we don’t entirely understand this,” says Allen.
Increasingly, efforts have been made to understand killer asteroids and find them before they strike earth. Just in the last month Nasa flew a spacecraft into an asteroid to see if it could alter its course.
The Near-earth Object Observations Programme is an early warning system that uses ground and space based telescopes to search for asteroids that may be heading to Earth. The problem, says Allen, is that an object like the one that smashed into Vredefort would be easier to spot. The smaller asteroids are harder to see as they are dark and don't reflect light.
Today the epicentre of the Vredefort impact is a far more serene place than it was 2 billion years ago. Hills born from that blast now flank the Vaal river that meanders through what is now called the Vredefort Dome.
It has become a spot for hikers and fishermen escaping Joburg. Now it has another purpose; in future the Vredefort Dome might help in our very survival, by helping us understand a menace hurtling through space on its way to annihilate our race and planet.
“Understanding the largest impact structure we have on Earth is critical,” says Allen “Access to the information provided by a structure like the Vredefort crater is a great opportunity to test our model.”