Ancient influences played out a powerful role in Earth’s complex history. On other Solar System bodies, such as the moon or Mercury, the impact history is preserved on their surfaces because there is nothing to erase it. But Earth’s geological activity has erased evidence of impact craters over time, with some help from erosion.
Earth’s complex history has elevated it to its place among its brethren in the Solar System and created a world teeming with life. Ancient asteroid impacts played a role in this history, bringing destruction and disruption and irrevocably changing the course of events.
Deciphering the role these giant impacts played is difficult, as evidence is missing or severely degraded. So how do scientists approach this problem?
One crater at a time.
The most famous giant asteroid impact on Earth is the Chicxulub impactor, which wiped out the dinosaurs about 65 million years ago and paved the way for mammals to become dominant. But there have been other giant impacts, including one in South Africa. It’s called Vredefort Crater and it’s the largest confirmed impact crater on Earth.
The Vredefort impactor struck Earth about 2 billion years ago during the Paleoproterozoic Era and is now located in South Africa. Previous research has shown the Vredefort impactor to be between 6 and 9 miles in diameter, and the crater — or impact structure as scientists call them — was about 100 to 200 miles wide when it formed.
However, erosion has reduced its size in the intervening 2 billion years, making the nature of the impactor, the size of the crater, and the effects of the impact difficult to measure precisely.
But a new study published in Journal of Geophysical Research: Planets reaches a different impact size and velocity for the Vredefort impactor. The study’s authors say the impactor was larger than we thought, hit Earth faster than we thought, and had devastating and far-reaching consequences.
The study is A review of the formation conditions of the Vredefort crater. Lead author is Natalie Allen, Ph.D. student in the physics and astronomy department at Johns Hopkins University.
“Understanding the largest impact structure we have on Earth is critical,” Allen said in a press release. “Accessing the information provided by a structure like Vredefort Crater is a great opportunity to test our model and our understanding of the geological evidence to better understand the effects on Earth and beyond.”
The size of the impact
More recent impacts such as the Chicxulub event have also had widespread and devastating consequences. Chicxulub caused mega-tsunami, violent earthquakes, fires that turned forests into ashes and cinders, accumulations of atmospheric dust that caused the global temperature to drop for an extended period of time, and of course the extinction of the dinosaurs.
But Earth was very different when the Vredefort impact event occurred in the Paleoproterozoic Era. There were no animals and no forests.
Previous estimates of the Vredefort impactor place it at about 9 miles in diameter with an impact velocity of 9 miles/second. This will excavate a crater about 107 miles in diameter. The crater has eroded extensively in the intervening 2 billion years, so previous geological evidence supported the “15×15” estimate.
But the problem is that the crater is now understood to be much larger. The most based modern estimate for the crater is between 155 and 174 miles. To help resolve the discrepancy, the authors of this study brought new tools to the Vredefort impact event in the form of computer simulations.
The researchers conducted simulations with the iSALE2D (Simplified Arbitrary Lagrangian Eulerian) shock code tool. It is a shock physics simulation tool that helps researchers understand impact events. Scientists use it to simulate the effects and reproduce their results. Their simulations resulted in an impactor size and velocity that more accurately reflects modern evidence. Researchers say that the Vredefort impactor was actually either a 9-mile-diameter body traveling at 9 miles/second or a 12-mile-diameter body traveling at 12 miles/second.
Crater size is not the only evidence that aligns with the revised impactor diameter and velocity. Some features in the bedrock below the impact point also suggest a larger impactor than previously thought. The researchers found shock metamorphic features in the Vredefort impact structure, including “breccias, fracture cones, planar deformation features in quartz and zircon, and melting,” the authors write. Their location suggests the impact was stronger than we thought.
These revised results for the Vredefort impact mean that the impact was more energetic than previously thought. The impact dwarfs that of the dinosaur-killing Chicxulub impactor.
Chicxulub was destructive to life on Earth at the time, so the Vredefort impact would be a major disaster. However, it left no record of mass extinction and no consistent ash layer around the world like Chicxulub did. What havoc did the Vredefort strike aircraft wreak on Earth?
“Unlike the Chicxulub impact, the Vredefort impact left no record of mass extinction or forest fires, since there were only single-celled life forms and no trees 2 billion years ago,” said Professor Miki Nakajima, one of the authors work. , also from Johns Hopkins. “However, the impact would have affected global climate potentially more widely than the Chicxulub impact.”
The impact thickened the atmosphere with dust and aerosols, blocking sunlight and causing temperatures to drop. At that time, oxygen was accumulating in the atmosphere and photosynthetic organisms were widespread and had already existed for a billion years. What happened to them?
“This could have had a devastating effect on photosynthetic organisms,” Nakajima said. “After the dust and aerosols settled – which could take anywhere from hours to a decade – greenhouse gases such as carbon dioxide emitted by the impact would likely have raised global temperatures by several degrees over a long period of time ».
As we see all around us, a rise in temperature of just a few degrees has a powerful effect on the global climate. Floods, hurricanes, droughts and other phenomena are occurring with greater frequency in our warming world.
Geological evidence is hard to come by, but 2 billion years ago, Earth was just emerging from the Huronian Ice Age, so there was probably a lot of ice on the planet’s surface. If the Vredefort impact increased global temperatures, the melting may have raised ocean levels significantly. The impact may also have heralded a period of violent storms, though there’s no way to know for sure.
The authors refrain from a precise explanation of the consequences for life on Earth at that time. But they come to many conclusions.
What happened to life on Earth?
Previous estimates of the size of the crater and the size of the impactor do not match the geological evidence. The weaker impact from previous surveys is not able to generate enough pressure to create the geological features at the Vredefort impact site.
Impacts of this size also create a sheet of melt below the point of impact. While much of it would have eroded over the course of 2 billion years, the team’s models show that some should still exist beneath the center, where it lies today.
Another of their conclusions concerns the positions of the land masses 2 billion years ago. We know that the continents have drifted quite a bit and even joined together in the past, but it is difficult to determine their exact locations at specific times. Scientists have found ejecta from the Vredefort impact at different locations around the world, especially in Karelia, Russia, and know how far the ejecta from an impact of a given energy can travel.
Thus, with a more precise understanding of the energy of the Vredefort impact, the authors were able to narrow down the position of Karelia at the time of impact. When it hit Vredefort, Karelia was between 1,242 and 2,553 miles from the point of impact. They are now four times as far apart.
“It’s been incredibly difficult to pin down the position of land masses for a long time,” says Allen. “The current best simulations have been mapped back about a billion years, and the uncertainties get bigger the further back you go. Clarifying evidence such as this mapping of the ejecta layer can allow researchers to test their models and help complete the view of the past.”
It will be difficult for scientists to ever understand what happened to life on Earth when the Vredefort impactor hit. The huge amounts of gases released, along with all the dust, may have made photosynthesis inefficient for large parts of the planet. It may have taken 10 years for all that dust to settle and the gases to leave the atmosphere. It was a disaster any way you cut it.
Life on Earth has run a gauntlet of catastrophic impacts, extinctions and global climate spins. The dinosaur-killing percussive Chicxulub puts a lot of emphasis on this. But this study shows that massive impacts may have shaped the course of life on Earth, even when that life was only single-celled. What specific effects did Vredefort’s impact have on life’s long evolutionary journey?
“The global implications of this impact would be far-reaching,” the authors write. “The release of climate gases would have changed the global climate, but predictions beyond that require study beyond the scope of this paper.”
This article was originally published on Universe today by Evan Gough. Read the original article here.