The ancient history of our planet’s impact events is coming into focus, thanks to researchers who have triangulated similarly aged impacts on Earth, the Moon, and the asteroid 4 Vesta in a new study.
The lunar meteorite NWA 12593 correlates not only with impact events observed across three locations in our solar system but also contains evidence of three separate impact events that occurred at different times.
The findings, revealed in a recent paper published in Geology, show that scientists are using this new impact data to fill in the volatile early-Earth period when life began.
Early Earth Impacts
The Archean period, roughly 4 to 2.5 billion years ago, was a consequential time from which our earliest fossil records date, corresponding to around 3.5 billion years ago. Researchers believe that life likely first arose at this time, yet it may have faced adversity from bombardment by space rocks between 3.2 and 3.5 billion years ago, although scientists disagree on how intense this impact activity was.
Our planet’s geological activity eventually destroys evidence of the ancient past through erosion, subduction, and burial, continually reshaping the surface. Yet those early years are essential to understanding the origins of water, atmospheres, and life on Earth.
“On Earth, the first fossil evidence of life shows up around 3.5 billion years ago, meaning that life is emerging and evolving before then. The question that we often have, even going back further, is what was the impact record when life was emerging?” said lead author Carolyn Crow, a planetary scientist at the University of Colorado Boulder.
“It is important for understanding how life is taking hold, how life is emerging,” Crow added. “The cadence of these catastrophic events is an important part of the equation.”
A Neighbor Remembers
While Earth’s surface is constantly renewed by geological processes, our nearest significant neighbor, the Moon, is much more stable, retaining its geological record for longer periods. Because of this, lunar meteorites provide a resource for scientists to understand the impact of Earth and the inner solar system by matching them to known impacts on our planet and in the asteroid belt.
For the recent study, the team focused on a lunar meteorite discovered in northwest Africa, designated NWA 12593, and identified three separate impact events in a single sample.
Radiometric dating traced the oldest to 3.5 million years ago, intriguingly close to the time of the earliest known fossils. Extraordinary in power, this impact produced the mineral zirconia by melting the lunar surface into a lava-like flow. Without the precise laboratory conditions required to produce stable cubic zirconia, only traces of mineral remained in the sample.
According to the team, the first impact aligns with previously known impact events on Earth, and the fourth-largest object in the asteroid belt, 4 Vesta. It’s extremely rare to find such corresponding evidence between three bodies, providing important data on this time of frequent collisions.
“It’s not very common, which is why we’re very excited about it,” says Crow. “It’s pretty rare to have all three records line up like this.”
Later Impacts
Evidence of later impacts included breccia composed of large, angular, broken mineral fragments cemented together. The team believes the breccia resulted from a second impact that struck the melt sheet formed by the first.
“Breccias are similar to what you would see if you went and chipped out a chunk of concrete. You would see all these little rocks, and then it’s fused together by the cement,” says Crow. “But the meteorite is fused together by the impact process. You get all these chunks of different kinds of rocks that the impact hit into. These all get mixed up, and then it gets fused together like your concrete sidewalk.”
As for the third impact, the evidence for this is the meteorite’s mere presence on Earth; an event that occurred some time after the first would have ejected the lunar breccia, the researchers say, allowing it to eventually land on our planet.
In total, the sample reveals an intriguing impact history, preserving a record of bombardment that provides solid evidence for a flurry of activity in the inner solar system during a violent chapter in the early history of our Solar System.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.
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