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 carved our solar system.
Over the last 290 million years, asteroid collisions with Earth and the moon have increased as many as three times, according to a new study published Thursday in the journal Science. And while that sounds like a long timescale, this is a significant increase compared to the previous 700 million years.
“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.”
For decades, scientists have been trying to determine the rate that asteroids impact Earth. They studied impact craters on Earth and the age of the rocks around them. But that method came with its own roadblocks. The biggest issue: The earliest impact craters were missing.
It had been previously assumed that most of Earth’s older craters produced by asteroid impacts have been erased by erosion and other geologic processes. But the new research shows 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.”
Scientists have for decades tried to understand the rate that asteroids hit Earth by using radiometric dating of the rocks around them to determine their ages. But because it was believed erosion caused some craters to disappear, it was difficult to find an accurate impact rate and determine whether it had changed over time.
A way to sidestep 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 lunar craters until NASA’s Lunar Reconnaissance Orbiter (LRO) started circling the Moon a decade ago and studying its surface.
“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.
Using LRO data, the team was able to assemble a list of ages of all lunar craters younger than about a billion years. They did this by using data from LRO’s Diviner instrument, a radiometer that measures the heat radiating from the Moon’s surface, to monitor the rate of degradation of young craters.
During the lunar night, rocks radiate much more heat than fine-grained soil called regolith. This allows scientists to distinguish rocks from fine particles in thermal images. Ghent had previously used this information to calculate the rate at which large rocks around the Moon’s young craters—ejected onto the surface during asteroid impact—break down into soil as a result of a constant rain of tiny meteorites over tens of millions of years. By applying this idea, the team was able to calculate ages for previously un-dated lunar craters.