Asteroid That Formed Largest And Oldest Impact Crater On Earth Was Bigger Than Previously Believed

About 2 billion years ago, an impactor hurtled toward Earth, crashing into the planet in an area near present-day Johannesburg, South Africa, and forming the Vredefort crater – the biggest and oldest terrestrial impact crater known so far.

Scientists have widely accepted, based on previous research, that the crater was formed by an asteroid about 15 kilometers (approximately 9.3 miles) in diameter that was traveling at a velocity of 15 kilometers per second.

But according to new research from the University of Rochester, the impactor may have been much bigger—and would have had devastating consequences across the planet.

“Understanding the largest impact structure that we have on Earth is critical,” says Natalie Allen, now a Ph.D. student at John Hopkins University. Allen is the first author of the paper, based on research she conducted as an undergraduate at Rochester University. “Having access to the information provided by a structure like the Vredefort crater is a great opportunity to test our model and our understanding of the geologic evidence so we can better understand impacts on Earth and beyond.”

Over the course of 2 billion years, the Vredefort crater has been partially buried by younger sediments. This makes it difficult for scientists to directly estimate the size of the crater at the time of the original impact, and therefore the size and velocity of the impactor that formed the crater.

An object that is 15 kilometers in size and traveling at a velocity of 15 kilometers per second would produce a crater about 172 kilometers (approximately 107 miles) in diameter. However, this is much smaller than current estimates for the Vredefort crater. These current estimates are based on new geological evidence and measurements estimating that the structure’s original diameter would have been between 250 and 280 kilometers (approximately 155 and 174 miles) during the time of the impact.

The researchers conducted simulations to match the updated size of the crater. Their results showed that an impactor would have to be much larger—about 20 to 25 kilometers—and traveling at a velocity of 15 to 20 kilometers per second to explain the observed crater size.

This means the impactor that formed the Vredefort crater would have been double the size than the asteroid that killed off the dinosaurs 66 million years ago, forming the Chicxulub crater. That impact had damaging effects globally, including greenhouse heating, widespread forest fires, acid rain, and destruction of the ozone layer, in addition to causing the Cretaceous-Paleogene extinction event that killed the dinosaurs.

If the Vredefort crater was even larger and the impact more energetic than that which formed the Chicxulub crater, the Vredefort impact may have caused even more catastrophic global consequences.

“Unlike the Chicxulub impact, the Vredefort impact did not leave a record of mass extinction, given that there were only single-cell lifeforms 2 billion years ago,” Nakajima says. “However, the impact would have affected the global climate potentially more extensively than the Chicxulub impact did.”

Dust and aerosols from the Vredefort impact would have spread across the planet and blocked sunlight, cooling the Earth’s surface, she says. “This could have had a devastating effect on photosynthetic organisms. After the dust and aerosols settled – which could have taken anywhere from hours to a decade – greenhouse gases such as carbon-dioxide that were emitted from the impact would have raised the global temperature potentially by several degrees for a long period of time.”

A study published in 2019 found a widespread and dramatic drop – from 80 to 99.5 percent – in microbial activity around 2 billion years ago. Scientists weren’t sure what caused this drop, but in light of the new findings, maybe the Vredefort impact played a role.

The simulations also allowed the researchers to study the material ejected by the impact and the distance the material traveled from the crater. They can use this information to determine the geographic locations of land masses billions of years ago.

For instance, previous research determined material from the impactor was ejected to present-day Karelia, Russia. Using their model, they found that 2 billion years ago, the distance of the land mass containing Karelia would have been only 2,000 to 2,500 kilometers from the crater in South Africa—much closer than the two areas are today.

“It is incredibly difficult to constrain the location of landmasses long ago,” Allen says. “The current best simulations have mapped back about a billion years, and uncertainties grow larger the further back you go. Clarifying evidence such as this ejecta layer mapping may allow researchers to test their models and help complete the view into the past.”

The paper “A Revision of the Formation Conditions of the Vredefort Crater” is published in the Journal of Geophysical Research: Planets (2022). Material provided by the University of Rochester.