Sunday , June 20 2021

Scientists led by U of T identify period of increase of asteroid impacts on ancient Earth by studying the moon

An international team of scientists is challenging our understanding of a part of Earth's history by looking at the moon, the most complete and accessible chronicle of the asteroid collisions that have sculpted our solar system.

In a study published today in ScienceThe team shows that the number of asteroid impacts on the Moon and Earth has increased from two to three times, beginning about 290 million years ago.

"Our research provides evidence of a dramatic change in the rate of asteroid impacts on Earth and the Moon that occurred around the end of the Paleozoic era," said the lead author. Sara Mazrouei, who recently earned his PhD from the Department of Earth Sciences at the University of Toronto School of Arts and Sciences.

"The implication is that since that time we have been in a relatively high period of asteroid impacts, which is 2.6 times larger than it was before 290 million years ago."

Previously, most of the asteroid-produced craters on Earth, over 290 million years old, had been presumed to have been erased by erosion and other geological processes. But the new research shows otherwise.

"The relative rarity of large craters on Earth over 290 million years and less than 650 million years is not because we lost the craters, but because the impact rate during that period was less than it is now," he said. Rebecca Ghent, an associate professor in the U of T Department of Earth Sciences and one of the co-authors of the paper. "We hope this is of interest to anyone interested in the history of the impact of Earth and the Moon, and the role it could have played in the history of life on Earth."

Scientists have been trying for decades to understand the rate at which asteroids hit the Earth using radiometric dating of rocks around craters to determine their ages. But since erosion was believed to cause some craters to disappear, it was difficult to find a precise impact rate and determine if it had changed over time.

This image shows the method by which the Diviner instrument of the Lunar Reconnaissance Orbiter detects the heat emanating from the lunar surface, which allows scientists to map the abundance of rocks around young impact craters. The team used this information to estimate the ages of lunar craters over 10 kilometers in diameter. Young craters have many rocks near their edges, and these rocks break into smaller particles over time, disappearing from the thermal data of the Diviner (data from Rebecca Ghent; illustration by Thomas Gernon).

One way around this problem is to examine the moon, which is hit by asteroids in the same proportions over time as Earth. But there was no way to determine the ages of the lunar craters until NASA's Lunar Reconnaissance Orbiter (LRO) began circling the moon a decade ago and studied its surface.

"LRO instruments have allowed scientists to look back at the forces that shaped the Moon in the past," said Noah Petro, an LRO project scientist based at NASA's Goddard Space Flight Center.

Using LRO data, the team was able to assemble a list of the ages of all lunar craters less than a billion years old. They did so using data from LRO's Diviner instrument, a radiometer that measures the heat radiating from the surface of the moon to monitor the rate of degradation of young craters.

During the lunar night, the rocks radiate much more heat than the fine-grained soil called regolith. This allows scientists to distinguish rocks from fine particles in thermal images. Ghent had already used this information to calculate the rate at which large rocks around the moon's young craters – ejected from the surface during the impact of an asteroid – break in the ground for tens of millions of years. In applying this idea, the team was able to calculate the ages of the previously undetermined lunar craters.

When compared to a similar chronology of Earth craters, they found that the two bodies recorded the same asteroid bombing story.

"It became clear that the reason the Earth has fewer ancient craters in its more stable regions is because the impact rate was lower until about 290 million years ago," said William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado. ., and another of the co-authors of the article. "The response to Earth's impact rate was looking at everyone in the face."

The reason for the jump in impact rate is unknown, though researchers speculate that this may be related to large collisions occurring more than 300 million years ago in the main asteroid belt between the orbits of Mars and Jupiter. Such events can create debris that can reach the inner solar system.

Ghent and his colleagues found strong evidence of support for their findings through a collaboration with Thomas Gernon, an Earth scientist based at the University of Southampton, England, who works on a land resource called kimberlite tubes. These subterranean tubes are long extinct volcanoes that extend into a carrot some kilometers below the surface and are found in some of the Earth's less eroded regions in the same places where preserved impact craters are found.

"The Canadian Shield houses some of the best preserved and best-studied of this terrain – and also some of the best-studied high impact craters," said Mazrouei.

Gernon has shown that kimberlite tubes formed in the last 650 million years have not suffered much erosion, indicating that larger impact craters than this on stable terrains should also be intact.

"This is how we know these craters represent an almost complete record," Ghent said.

Although researchers have not been the first to propose that the rate of asteroids on Earth has fluctuated over the last few billion years, they are the first to show it statistically and quantify the rate.

"The findings may also have implications for the history of life on Earth, which is punctuated by events of extinction and rapid evolution of new species," Ghent said. "Although the forces that drive these events are complicated and may include other geological causes, such as large volcanic eruptions combined with biological factors, asteroid impacts certainly played a role in this ongoing saga.

"The question is whether the predicted change in asteroid impacts could be directly linked to events that occurred long ago on Earth."

The results are described in the study "The impact flow of Earth and Moon increased at the end of the Paleozoic", published in Science. Research support was provided by the National Council for Scientific and Scientific Research of Canada, the Virtual Institute for Exploration of Solar Systems of NASA and the Natural Environment Research Council of the United Kingdom.

With NASA files

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