About 2 billion years ago, an impactor hurtled towards Earth, crashing into the planet in an area near present-day Johannesburg, South Africa. The impactor – most likely an asteroid – formed what is now the largest 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 in diameter moving at a speed of 15 kilometers per second.
But according to new research from the University of Rochester, the impactor could have been much larger, with devastating consequences across the planet. This research, published in the Journal of Geophysical Researchprovides a more accurate understanding of the big impact and will allow researchers to better simulate impact events on Earth and other planets, both in the past and in the future.
Understanding the largest impact structure we have on Earth is essentialsays Natalie Allen, now a doctoral student at Johns Hopkins University. Allen is the first author of the paper, based on research she conducted as a student in 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 geological evidence so that we can better understand impacts on Earth and beyond..
Updated simulations suggest ‘devastating’ consequences
Over 2 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 speed of the impactor that formed it.
An object 15 kilometers in size traveling at a speed 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 that estimate the original diameter of the structure would have been between 250 and 280 kilometers at the time of impact.
Allen, Nakajima, and their colleagues ran simulations to adjust for the updated crater size. Their results showed that an impactor would have to be much larger – about 20 to 25 kilometers – and travel at a speed of 15 to 20 kilometers per second to explain a crater 250 kilometers in size.
This means that the impactor that formed the Vredefort crater would have been larger than the asteroid that wiped out the dinosaurs 66 million years ago, forming the Chicxulub crater. That impact had detrimental effects globally, including greenhouse warming, widespread wildfires, acid rain, and ozone layer destruction, as well as causing the Cretaceous-Paleogene extinction event that wiped out the dinosaurs.
If the Vredefort crater was even larger and the impact more energetic than the one that formed the Chicxulub crater, the Vredefort impact could have had even more catastrophic global consequences.
Unlike the Chicxulub impact, the Vredefort impact did not record a mass extinction or forest fires, since 2 billion years ago there were only unicellular life forms and no trees.says Nakajima. However, the impact would have affected global climate potentially more extensively than the Chicxulub impact..
Dust and aerosols from the Vredefort impact would have spread across the planet and blocked out sunlight, cooling Earth’s surface, he 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 emitted from the impact would have raised global temperatures by potentially several degrees for a long period of time.
A multifaceted model of the 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 location of land masses from billions of years ago. For example, previous investigations determined that the impactor material was expelled as far as present-day Karelia, in Russia. Using their model, Allen, Nakajima and their colleagues found that, 2 billion years ago, the distance of the landmass containing Karelia would have been only 2,000 to 2,500 kilometers from the crater in South Africa, much closer than they are now. both areas today.
It is incredibly difficult to determine the location of land masses long ago.says Allen. The best current simulations go back about a billion years, and uncertainties grow the further back you go.. Clarifying evidence such as this ejecta layer mapping may allow researchers to test their models and help fill out the picture of the past..
University of Rochester | Allen, NH, Nakajima, M., Wünnemann, K., Helhoski, S., & Trail, D. (2022). A review of the formation conditions of the Vredefort crater. Journal of Geophysical Research: Planets, 127, e2022JE007186. doi.org/10.1029/2022JE007186