By looking at the Moon, the most complete and accessible chronicle of the asteroid crashes that formed our solar system, a multinational team of scientists is questioning our view of a chunk of Earth’s history.
The research indicates that the number of asteroid impacts on the Moon and Earth increased by two to three times starting around 290 million years ago, according to a study published today in Science.
“Our research provides evidence for a dramatic change in the rate of asteroid impacts on both Earth and the Moon that occurred around the end of the Paleozoic era,” said lead author Sara Mazrouei, who recently earned her Ph.D. in the Department of Earth Sciences in the Faculty of Arts & Science at the University of Toronto (U of T). “The implication is that since that time we have been in a period of relatively high rate of asteroid impacts that is 2.6 times higher than it was prior to 290 million years ago.”
Most of Earth’s older craters created by asteroid impacts were thought to have been obliterated by erosion and other natural processes. However, a new study suggests otherwise.
“The relative rarity of large craters on Earth older than 290 million years and younger than 650 million years is not because we lost the craters, but because the impact rate during that time was lower than it is now,” said Rebecca Ghent, an associate professor in U of T’s Department of Earth Sciences and one of the paper’s co-authors. “We expect this to be of interest to anyone interested in the impact history of both Earth and the Moon, and the role that it might have played in the history of life on Earth.”
For decades, scientists have attempted to understand the rate at which asteroids collide with Earth by using radiometric dating to calculate the ages of the rocks that surround them. However, because some craters were thought to have vanished due to erosion, it was difficult to calculate an accurate impact rate and assess whether it had altered over time.
Examining the Moon, which is hit by asteroids in the same quantities as Earth over time, is one approach to get around this problem. However, until NASA’s Lunar Reconnaissance Orbiter (LRO) began circling the Moon and examining its surface a decade ago, there was no method to establish the ages of lunar craters.
“The LRO’s instruments have allowed scientists to peer back in time at the forces that shaped the Moon,” said Noah Petro, an LRO project scientist based at NASA Goddard Space Flight Center.
The team was able to compile a list of the ages of all lunar craters younger than a billion years using LRO data. They did so by monitoring the rate of degradation of youthful craters using data from the LRO’s Diviner instrument, a radiometer that monitors the heat radiating from the Moon’s surface.
Our research provides evidence for a dramatic change in the rate of asteroid impacts on both Earth and the Moon that occurred around the end of the Paleozoic era. The implication is that since that time we have been in a period of relatively high rate of asteroid impacts that is 2.6 times higher than it was prior to 290 million years ago.
Sara Mazrouei
During the lunar night, rocks radiate far more heat than regolith, a fine-grained soil. In thermal pictures, this allows scientists to differentiate boulders from small particles.
Ghent has previously used this knowledge to determine the rate at which huge rocks blasted onto the Moon’s surface upon asteroid impact break down into soil over tens of millions of years as a result of a continual rain of microscopic meteorites.
The researchers was able to calculate ages for hitherto un-dated lunar craters using this concept. They discovered that the two bodies had recorded the same history of asteroid bombardment when compared to a similar timeframe of Earth’s craters.
“It became clear that the reason why Earth has fewer older craters on its most stable regions is because the impact rate was lower up until about 290 million years ago,” said William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado and another of the paper’s co-authors. “The answer to Earth’s impact rate was staring everyone right in the face.”
The cause of the increase in impact rates is unknown, but astronomers believe it may be tied to huge collisions that occurred more than 300 million years ago in the main asteroid belt between Mars and Jupiter’s orbits. Debris from such occurrences has the potential to reach the inner solar system.
Through a partnership with Thomas Gernon, an Earth scientist at the University of Southampton in England who studies kimberlite pipes, Ghent and her colleagues discovered substantial supporting evidence for their conclusions.
These subterranean pipes are long-extinct volcanoes that stretch a few kilometers below the surface in a carrot shape and are located in some of the least eroded parts of Earth, near surviving impact craters.
“The Canadian shield hosts some of the best-preserved and best-studied of this terrain and also some of the best-studied large impact craters,” said Mazrouei.
Gernon demonstrated that kimberlite pipes built around 650 million years ago had little erosion, implying that massive impact craters formed on stable terrains younger than this must likewise be intact.
“This is how we know those craters represent a near-complete record,” Ghent said.
While the researchers are not the first to suggest that the rate of asteroid attacks on Earth has fluctuated over billions of years, they are the first to demonstrate it scientifically and quantify it.
“The findings may also have implications for the history of life on Earth, which is punctuated by extinction events and rapid evolution of new species,” said Ghent. “Though the forces driving these events are complicated and may include other geologic causes, such as large volcanic eruptions, combined with biological factors, asteroid impacts have surely played a role in this ongoing saga.”
The question is whether the expected change in asteroid collisions can be traced back to activities that took place on Earth many years ago. The findings were published in Science in the paper “Earth and Moon impact flux increased at the end of the Paleozoic.”
Support for the research was provided by the National Science and Engineering Research Council of Canada, NASA’s Solar System Exploration Research Virtual Institute, and the Natural Environment Research Council of the United Kingdom.
