Ancient effects played a powerful role in the complex history of the earth. On other Solar System bodies like the Moon or Mercury, the impact history persists on their surfaces because there is nothing to erase it. But Earth’s geological activity has obliterated evidence of impact craters over time, with some help from erosion.
Earth’s complex history has elevated her status among her siblings in the solar system, creating a world teeming with life. Ancient asteroid impacts have played a role in this story, bringing cataclysms and disturbances and irrevocably changing the course of events.
It is difficult to decipher the role played by these giant impacts because the evidence is missing or badly compromised. How are scientists tackling this problem?
One crater after the other.
The best-known giant asteroid impact on Earth is the Chicxulub impactor, which wiped out dinosaurs and paved the way for mammalian dominance about 65 million years ago. But there have been other huge impacts, including one in South Africa. It’s called Vredefort Crater and is the largest confirmed impact crater on Earth.
The Vredefort impactor struck Earth about 2 billion years ago during the Paleoproterozoic Era and is now in South Africa. Previous research found the Vredefort impactor to be between 6 and 9 miles in diameter, and the crater — or impact structure as scientists call it — was about 100 to 200 miles across when it formed.
However, erosion has reduced its size in the intervening 2 billion years, making it difficult to accurately estimate the type of impact, the size of the crater, and the impact of the impact.
But a new study published in the Journal of Geophysical Research: Planets comes to a different size and impact velocity for the Vredefort impactor. The study’s authors say the impactor was larger than thought, hit the Earth at a greater speed than thought, and had devastating and far-reaching consequences.
The study is A revision of the formation conditions of the Vredefort crater. The lead author is Natalie Allen, a Ph.D. Student in the Department of Physics and Astronomy at Johns Hopkins University.
“It is crucial to understand the largest impact structure we have on Earth,” Allen said in a press release. “Having access to the information provided by a structure like Vredefort Crater is a great opportunity to test our model and understanding of the geological evidence so we can better understand the impacts on Earth and beyond.”
The size of the impact
More recent impacts such as the Chicxulub event have also had far-reaching and catastrophic consequences. Chicxulub caused megatsunamis, violent earthquakes, firestorms that reduced forests to ash and cinders, atmospheric dust build-ups that caused global temperatures to drop for an extended period of time, and of course, the extinction of dinosaurs.
But Earth was very different when the Vredefort impact event took place in the Paleoproterozoic. There were no animals and no forests.
Previous estimates for the Vredefort impactor are about 9 miles in diameter and an impact velocity of 9 miles/s. That would excavate a crater about 107 miles across. The crater has been heavily eroded over the past 2 billion years, so previous geological evidence supported the 15 × 15 estimate.
But the problem is that the crater is much bigger now. The most reliable modern estimate for the crater is between 155 and 174 miles. To resolve the discrepancy, the authors of this study brought new tools in the form of computer simulations to the Vredefort event impact.
Researchers ran simulations using the impact code iSALE2D (impact Simplified Arbitrary Lagrangian Eulerian) tool. It is a shock physics simulation tool that helps researchers understand impact events. Scientists use it to simulate impacts and reproduce their effects. Their simulations resulted in an impactor size and velocity that more accurately reflects the modern evidence. The researchers say that the Vredefort impactor was actually either a 9-mile diameter body moving at 9 miles/s or a 12-mile diameter body moving at 12 miles/s.
The size of the crater is not the only evidence consistent with the revised impactor diameter and velocity. Certain features in the rock beneath the impact site also point to a larger impactor than previously thought. Researchers have found shock metamorphic features in the Vredefort impact structure, including “breccia, spall cones, planar strain features in quartz and zircon, and melt,” the authors write. Their position indicates that the impact was stronger than expected.
Underestimate the power
These revised results for the Vredefort impact mean that the impact was more violent than previously thought. The impact dwarfs that of the dinosaur-killing Chicxulub impactor.
Chicxulub was catastrophic to life on Earth at the time, so the Vredefort impact would have been a mega-catastrophe. However, it left no record of a mass extinction and no consistent ash sheet around the globe like Chicxulub did. What devastation did the Vredefort impactor wreak on Earth?
“Unlike the Chicxulub impact, the Vredefort impact left no record of mass extinctions or wildfires because 2 billion years ago there were only unicellular life forms and no trees,” said Professor Miki Nakajima, one of the study’s authors, also of Johns Hopkins. “However, the impact would have potentially had a greater impact on global climate than the Chicxulub impact.”
The impact thickened the atmosphere with dust and aerosols, blocking sunlight and causing temperatures to plummet. At that time, oxygen was accumulating in the atmosphere and photosynthetic organisms were widespread and had existed for a billion years. what happened to you
“This could have had devastating effects on photosynthetic organisms,” Nakajima said. “After the dust and aerosols settled — which could have taken anywhere from hours to a decade — greenhouse gases such as carbon dioxide released by the impact could have raised global temperatures by several degrees, potentially over a long period of time.”
As we see all around us, a temperature rise of just a few degrees has a powerful impact on global climate. Floods, hurricanes, droughts and other phenomena are becoming more common in our warming world.
Geological evidence is hard to find, but 2 billion years ago Earth was just emerging from the Huron Ice Age, so there was likely a lot of ice on the planet’s surface. If the Vredefort impact increased global temperatures, the melting could have raised sea levels significantly. The impact could also have heralded a period of violent storms, although there’s no way of knowing for sure.
The authors do without a detailed explanation of the consequences for life on earth at that time. But they come to several conclusions.
What happened to life on earth?
Previous estimates of crater size and impactor size are inconsistent with geological evidence. The weaker impact from previous surveys may not generate enough pressure to create the geological features at the Vredefort impact site.
Impacts of this size also create a layer of melt beneath the point of impact. While much of this would have eroded in the 2 billion years, the team’s models show that some should still exist below the center where they are found today.
Another of their conclusions concerns the locations of the landmasses 2 billion years ago. We know that the continents have shifted widely and even connected in the past, but pinpointing their exact locations at specific times is difficult. Scientists have found ejecta from the Vredefort impact in various locations around the world, most notably in Karelia, Russia, and they know how far ejecta can travel from an impact of a given energy.
With a more accurate understanding of the energy of Vredefort’s impact, the authors were able to determine the location of Karelia at the time of impact. When the impact on Vredefort took place, Karelia was between 1,242 and 2,553 miles from the impact site. Now they are four times as far apart.
“It’s incredibly difficult to constrain the location of land masses long ago,” Allen says. “The current best simulations are mapped back about a billion years, and the uncertainties get bigger the further back you go. Clarifying evidence like this ejecta layer mapping can allow researchers to test their models and help complete the look into the past.”
It will be difficult for scientists to ever understand what happened to life on Earth when the Vredefort impactor struck. The massive amounts of gases released, along with all that dust, may have rendered photosynthesis ineffective for large parts of the world. It may have taken 10 years for all that dust to settle and for the gases to leave the atmosphere. It was a disaster in every way.
Life on Earth has run the gauntlet of catastrophic impacts, extinctions and global climate variability. The dinosaur-killing Chicxulub impactor places great value on this. But this study shows that massive impacts could have altered the course of life on Earth, even when that life was unicellular. What specific impact did Vredefort’s influence have on life’s long evolutionary journey?
“The global impact of this impact would have been far-reaching,” the authors write. “The release of greenhouse gases would have changed the global climate, but predictions beyond that require investigations beyond the scope of this work.”
This article was originally published on universe today by Evan Gough. Read the original article here.
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