By comparing the age of impact craters on the Moon to those on Earth, researchers say they have discovered a surge in the rate of asteroid strikes starting around 290 million years ago — a finding that runs contrary to scientific convention. And indeed, not everyone is convinced by the evidence.
Scientists who study ancient impact craters on Earth have a problem: erosion. Weathering, volcanism, tectonics, and other geological processes have swept away the traces of our planet’s oldest impact craters, making it difficult to chronicle the history of asteroid impacts on Earth.
New research published today in Science suggests the Moon, with its extensive collection of impact craters, can help.
As neighbours, the Earth and Moon have experienced similar rates of asteroid strikes over the course of the Solar System’s history. But the Moon, unlike Earth, isn’t subject to the same erosional processes. Consequently, the Moon’s surface has retained a long-term record of its impact history. If these lunar craters can be dated, then it should be possible to recreate the historical bombardment rates on Earth.
By using a new technique to date lunar craters from a distance, a research team led by Sara Mazrouei from the Department of Earth Sciences at the University of Toronto were able to accomplish exactly this. The technique allowed them to date lunar craters with diameters greater than 10 kilometers (6 miles) and younger than 1 billion years.
This exercise yielded two important findings, both of which challenge conventional notions of Earth’s impact record.
The first is that the impact rate on Earth increased by a factor of 2.6, or between two and three times, about 290 million years ago, at the end of the Paleozoic Era and just slightly before the appearance of dinosaurs. The second big finding, in the words of the researchers, is the “almost complete absence of terrestrial craters older than 650 million years.”
This absence, according to the researchers, suggests a “massive global-scale erosion event near that time.” The event they’re talking about is the theorised Snowball Earth — a time when most if not all of our planet was coated in ice.
The idea that the frequency of impacts on the Moon and Earth has jumped since 290 million years ago goes against conventional thinking. Theoretically, the rate of asteroid strikes should be decreasing — and not increasing — as the debris left over from the formation of the Solar System gradually dissipates. The authors of the new study are presenting a big claim — one requiring the requisite big evidence.
And indeed, some of the experts we spoke to weren’t entirely convinced by the new paper, saying it relied on too many assumptions, the methods weren’t persuasive enough, and that more evidence is needed to back the claims.
To date the lunar craters, Mazrouei’s team used thermal data and images collected by NASA’s Lunar Reconnaissance Orbiter (LRO). A device on the LRO called the Diviner provided this data by measuring the heat radiating from the Moon’s surface. At night, large rocks give off more heat than fine soil, allowing the Diviner to do its work. And it just so happens that young craters on the Moon are surrounded by large rocks.
“When a crater is formed after an impact, it excavates a lot of rocks,” Mazrouei explained to Gizmodo. “The rocks around the crater eventually turn into regolith, or lunar sand, due to further micrometeorite bombardments. This process takes about 1 billion years. As craters become older, they become less rocky. We used this relationship to get ages for lunar craters.”
With this technique, the researchers were able to compile the ages of all lunar craters younger than around a billion years — a rather time-consuming process, to be sure.
“It took a few months to map all of the young lunar craters. However, I started this project when I started my PhD five years ago,” said Mazrouei. “We had to check our data and methods to make sure they are robust. After about two years, when we started comparing lunar craters to terrestrial ones, we saw some very interesting results. We had to ensure and prove to multiple reviewers, when we submitted our paper, that we had accounted for all terrestrial craters on stable terrains on Earth.”
As noted, Mazrouei and her colleagues correlated the age of lunar impact craters with those on Earth. The common belief among scientists is that many of Earth’s craters have been wiped away by geological processes, but by comparing the lunar craters to terrestrial ones, Mazrouei and her colleagues showed that, for very large craters — those bigger than 20 km in diameter and younger than 650 million years — a complete record of terrestrial impact craters does in fact exist. To be clear, however, this only applied to craters found on stable terrains, excluding areas like the oceans, the Amazon, and so on.
Analysis of this data exposed the frequency spike in impacts starting around 290 million years ago, but it yielded other interesting findings.
By re-assessing the terrestrial impact record, the researchers discovered a dearth of craters on Earth between 300 million and 650 million years ago during the Precambrian. This surprising deficit of craters, according to the study, is not the result of erosion, but rather an unexpected decline in asteroid impacts during this historical period.
The data also showed a dramatic cut-off in the number of terrestrial craters around 650 million years ago during the Precambrian. This is also a surprise, but as the authors wrote in the study, there’s a possible explanation:
Given erosion rates on stable continental terrains after 650 [million years ago], similar conditions further back in time would have allowed most craters of Precambrian age...to survive. Instead, the paucity of Precambrian craters is coincident with major episodes of globally extensive “Snowball Earth” glaciation. Pervasive subglacial erosion at approximately 650 to 720 [million years ago] is thought to have removed kilometers of material from the continents, enough to erase most existing...impact craters.
It’s a fascinating possibility, and further evidence in favour of the burgeoning Snowball Earth hypothesis.
That crater formation over the past 290 million years is two to three times higher than it was before the time of dinosaurs is surprising. But Mazrouei offered a plausible explanation:
“The increase in the impact rate might be due to a large breakup event in the main asteroid belt,” she said. “When two large asteroids collide, they produce a swarm of fragments close to the impact site. These eventually move on to Earth - crossing orbits and a few end up hitting the Moon and Earth.”
The timing of this impact spike is quite interesting, and not just from an astronomical perspective. The apparent increase in impacts may have influenced the history of our planet. One of the largest extinction events happened around 250 million years ago — the Permian-Triassic mass extinction — which occurred shortly after the observed increase in the rate of asteroid impacts.
“However, we cannot say whether there is any connection to this event,” clarified Mazrouei. “Our work sets the stage for other scientists to explore this issue and see if there were any correlations.”
As to whether or not Earth is more vulnerable to asteroid strikes in 2019 than it was 300 million years ago is something that could not be gleaned from the data. For now, “with our data we can say that the impact rate has increased in the past 290 million years,” said Mazrouei, but her team has not been able to “distinguish if there was just one spike, multiple spikes, or if this rate is still increasing.” For our sake, let’s hope it’s not the latter.
“I think their analysis is sound, and they make a convincing case for an increase in the rate of asteroid impacts,” Simone Marchi, principal scientist at the Southwest Research Institute in Boulder, Colorado, told Gizmodo. “The precise increase—a factor of two or three — may be less well-determined given uncertainties, but the proposed 2.6 seems reasonable based on available data.” Marchi said the observed increase is not “inconceivable,” and he’s in agreement that it “could be the response to a break up of an asteroid in the main belt.”
Speaking to Gizmodo, Aaron J. Cavosie, a senior research fellow in the Faculty of Science and Engineering at Curtin University, said the paper’s conclusion about the rate of post-Paleozoic cratering “may be accurate,” but, being model-driven, the study failed to present sufficient geological evidence. What’s more, he said the researchers worked under the somewhat unfounded assumption that the Moon is “a reliable record for cratering on Earth.”
“This may be, but it’s hard to verify,” Cavosie said. “The best way to evaluate their model is by acquiring better ages and size estimates for terrestrial craters. This is the only record where we can conduct field studies and apply high-precision methods—but obviously, field work on the Moon is a bit of a trick.”
Cavosie said recent discoveries of post-Paleozoic impact craters with diameters greater than 10 kilometers, including Yallalie crater in Australia and Pantasma crater in Nicaragua, “appear consistent with their model,” adding that “future discoveries of older large craters would question it.”
“Overall while the idea of a big increase in the number of asteroid impacts on Earth is intriguing, I do not think that the data cited by the authors strongly supports the paper’s conclusion,” Jay Melosh, university distinguished professor of Earth, atmospheric and planetary sciences at Purdue University, told Gizmodo.
“The authors rely on an apparent lack of craters on the Earth before about 300 million years ago. However, there are not that many craters on the Earth of any age — about 180 confirmed craters at the present time — and the area of rocks that are older than 300 million years is relatively small, so they are dealing with statistics of small numbers — a notoriously uncertain procedure.”
Another issue of contention for Melosh was how the researchers measured the rate at which boulders degrade around lunar craters over time. He said they didn’t measure boulder size directly, instead inferring the degradation rate from the speed at which the area around the crater cools after the Sun sets or warms when it rises.
Melosh said the assumption that boulders break down at a constant rate via tiny meteorites is exactly that — an assumption. It’s completely plausible, he said, that the degradation rate increases as the boulders get smaller.
“After all, smaller rocks take less energy to break up — and this could explain the lack of boulders around craters before 300 million years ago, which the authors interpret as meaning that these craters are much older,” said Melosh. “[O]verall, I do not think that the case is proven, nor is it disproven — the data is simply not good enough to strongly support their contention, in my opinion.”
As for the idea that ice removed most Precambrian craters, Cavosie said it was an “intriguing” possibility. The researchers demonstrated “temporal coincidence and a possible mechanism,” he said, but this “does not prove causality.” What’s more, Cavosie said more than two dozen Precambrian craters are known to exist, instead of the two described in the new study.
The destruction of craters by erosion, he said, creates a sedimentary record of shocked minerals (meaning minerals deformed by intense heat and pressure), which scientists have documented at Vredefort and Sudbury, the largest Precambrian craters. If ancient glaciers “vacuumed up Earth’s craters,” Cavosie said, than sediments dating back to this period “should be bursting at the seams” with shocked minerals. So “this is a testable hypothesis - go grab your rock hammer!”
Mazrouei and her colleagues have presented an intriguing new study, but it’s clear more work is required to solidify their case. Recreating the record of asteroid impacts on Earth is important, both for understanding our planet’s geological history and to better appreciate the influence of these cataclysmic impacts on the trajectory of life on this planet.