About two billion years ago, an impactor hurtled toward Earth, crashing into the planet in an area near present-day Johannesburg, South Africa. The impactor-most likely an asteroid-formed what is today the biggest crater on our planet.

Scientists have widely accepted, based on previous research, that the impact structure, known as the Vredefort crater, was formed by an object about 15 kilometers (approximately 9.3 miles) in diameter that was travelling 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. This research, published in the Journal of Geophysical Research, provides a more accurate understanding of the large impact and will allow researchers to better simulate impact events on Earth and other planets, both in the past and the future.

"Understanding the largest impact structure that we have on Earth is critical," says Natalie Allen '20, now a PhD student at John Hopkins University. Allen is the first author of the paper, based on research she conducted as an undergraduate at Rochester with Miki Nakajima, an assistant professor of Earth and environmental sciences.

"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."

Updated simulations suggest 'devastating' consequences
Over the course of two billion years, the Vredefort crater has eroded. 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 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.

Allen, Nakajima, and their colleagues 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 a crater 250 kilometers in size.

This means the impactor that formed the Vredefort crater would have been larger 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 or forest fires given that there were only single-cell lifeforms and no trees existed two 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 cardon dioxide that were emitted from the impact would have raised the global temperature potentially by several degrees for a long period of time."

A multi-faceted model of Vredefort crater
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, Allen, Nakajima, and their colleagues found that, two 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."

Undergraduate research leads to publication
The idea for this paper arose as part of a final for the course Planetary Interiors (now named Physics of Planetary Interiors), taught by Nakajima, which Allen took as a junior.

Allen says the experience of having undergraduate work result in a peer-reviewed journal article was very rewarding and helped her when applying for graduate school.

"When Professor Nakajima approached me and asked if I wanted to work together to turn it into a publishable work, it was really gratifying and validating," Allen says. "I had formulated my own research idea, and it was seen as compelling enough to another scientist that they thought it was worth publishing!"

She adds, "This project was way outside of my usual research comfort zone, but I thought it would be a great learning experience and would force me to apply my skills in a new way. It gave me a lot of confidence in my research abilities as I prepared to go to graduate school."

Research Report:A Revision of the Formation Conditions of the Vredefort Crater

Related Links
University of Rochester
Asteroid and Comet Mission News, Science and Technology

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