Earth is constantly being pummeled by asteroids and by space debris. While most of these objects are too small to pose a threat — and safely burn up in the atmosphere — the planet is occasionally bombarded by space rocks large enough to survive the fiery descent through the sky, tumbling down to the planet’s surface and leaving behind craters as testament of their violent collision with Earth.
The same phenomenon occurs on the moon as well — with one major difference: given that Earth is more massive and has a stronger gravitational pull, it attracts a larger number of space rocks.
“About twenty asteroids strike Earth for every one that strikes the moon, though large impacts on either body are rare,” explains NASA.
Many of the craters left behind by big asteroids have endured the test of time, allowing scientists to study their features in a quest to determine the age of the largest and the most intact ones. These efforts revealed that Earth’s craters are relatively young.
According to a new study, published today in the journal Science, Earth seems to have a very small number of asteroid craters older than 300 million years and younger than 650 million years. In addition, almost no older craters appear to have survived from before that time.
The logical conclusion was that Earth’s ancient craters had been lost due to erosion and other geological processes, such as weathering and plate tectonics — or that they were simply hidden under the planet’s ice sheets. The new study, however, challenges these assumptions, revealing the true reason why Earth has so few older craters.
The answer to the puzzle was actually found on the moon. Over the course of time, Earth’s natural satellite has been subjected to the same kind of treatment from wandering asteroids as our planet. The moon’s craters, however, have remained virtually untouched over the last few billions of years.
This has given scientists the idea to study the timeline of asteroid strikes on the moon, and calculate their impact rate in order to get a better understanding of what happened in our planet’s distant past — as well as in the early solar system. After all, NASA describes the moon as “the most complete and accessible chronicle of the asteroid collisions that carved our young solar system.”
Their findings revealed that the apparent absence of older craters on Earth had nothing to do with erosion, but was actually the result of a smaller number of asteroids falling down from the sky during that point in time.
“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 study co-author William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado.
“The answer to Earth’s impact rate was staring everyone right in the face.”
The conclusion came after the team examined extensive data from NASA’s Lunar Reconnaissance Orbiter (LRO). This allowed them to put together the first comprehensive timeline of large craters — at least six miles wide — that formed on the moon in the last billion years. The team studied images and thermal data picked up by LRO’s Diviner instrument — a thermal radiometer that can spot the heat signature of young lunar craters — and came up with a list of ages for all the moon craters younger than a billion years.
This allowed them to calculate the frequency of asteroid impacts on the moon during that time-line and — by using the moon as Earth’s mirror — to determine the asteroid impact rate on our planet. The results unveiled two surprising facts: that the number of asteroids crashing into Earth and the moon has nearly tripled in the last 300 million years, and that it came to an abrupt halt sometime around 650 million years ago.
As Science Daily points out, the data suggests that the rate of asteroid crater formation experienced a surge some 290 million years ago, increasing by a factor of 2.6. At that time, the Earth was nearing the end of the Paleozoic Era — and had about 40 million years to go until the dawn of the dinosaurs in the Mesozoic Era. This means that the number of asteroids that slammed into the Earth and the moon since shortly before the time of the dinosaurs is 2.6 times larger than during 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 study lead author Sara Mazrouei, a researcher in the Department of Earth Sciences at the University of Toronto.
“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.”
The sudden increase in asteroid impact frequency after the 290-million-year marker was likely caused by a large collision in the asteroid belt, which scattered plenty of debris into the inner solar system, sending it flying toward Earth. While the number of craters formed after that time was found to be two to three times larger than in the more distant past, the study also uncovered “a dramatic cut-off” around 650 million years ago — before life blossomed on the planet, notes Gizmodo.
“The almost complete absence of terrestrial craters older than 650 million years may indicate a massive global-scale erosion event near that time,” the authors wrote in their paper.
According to Gizmodo, this conclusion supports the Snowball Earth hypothesis, which states that the planet became almost entirely engulfed in ice sometime earlier than 650 million years ago.
“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,” said study co-author Rebecca Ghent, a planetary scientist at University of Toronto and the Planetary Science Institute in Tucson, Arizona.