Looking at the moon, it’s easy to see what happens when a planetary body is pummeled by ancient space rubble. There are thousands of visible craters on the moon, pockmarks left over from asteroid strikes. On our planet, by contrast, there are just 190 confirmed craters. But while the visible evidence doesn’t match up, the Earth and the moon have suffered similar instances of injury over the past 4.5 billion years. A new study explains the seeming disparity.
In a study published Thursday in Science, scientists determined that from about 1 billion years ago to 290 million years ago, the rate at which asteroids struck the Earth and the moon was relatively constant. Then, at that 290 million years mark, the rate of asteroid impact became about 2.6 times higher. That level persists today. In the video above, which is probably the most relaxing video chronicling destruction ever made, you can see how this rate of lunar impacts changed over time.
Study co-author Rebecca Ghent, Ph.D., an associate professor at the University of Toronto, tells Inverse that the most likely explanation for why we’re in this period of relatively frequent asteroid impacts is because there was probably an ancient collision of large objects in the asteroid belt, which lies between Mars and Jupiter, causing that asteroid to break up into smaller pieces.
“Over time, some of those fragments can be ‘kicked out’ of the asteroid belt and into orbits that allow them to hit the Earth or Mars,” Ghent explains. “If one of these breakup events occurred sometime before 290 million years ago, that could account for the current increased flux.”
But craters on the Earth and the moon still don’t match up.
Meteor Crater in Arizona.
On Earth, impact craters are harder to find because of weathering and erosion on the planet’s surface. Scientists previously thought that a large number of Earth’s oldest craters couldn’t be found because of these processes — while we can see, for example, very very old craters like the massive, 2-billion-year-old Vredefort divot in South Africa, or the relatively recent 50,000-year-old Meteor Crater in Arizona, geologists thought that others had been lost due to a natural, geological process. Now, this team of scientists argues that we couldn’t find craters older than 290 million years and younger than 650 million years for a reason — they simply aren’t there.
They came to this conclusion by studying the Earth’s neighbor, the moon. The moon, Ghent says, is a “good witness” for the solar system events that have affected Earth because the moon is very close to Earth but doesn’t have the geological processes that erase the records of phenomena like impact craters.
“Relative to the scale of the solar system and the position of the asteroid belt, which is the source of meteors that hit us, Earth and moon are basically in the same place,” Ghent says. “Therefore, we can study the Moon to find out what probably happened to Earth in earlier times, for which there is no longer a direct record.”
How the scientists dated the moon's impact craters.
Ghent and her colleagues used data collected by NASA’s Lunar Reconnaissance Orbiter (LRO) to assemble a list of all the lunar craters younger than a billion years, including their ages. A radiometer on the LRO’s Diviner instrument measures the heat that radiates from the moon’s surface, and thermal data helps scientists figure out the rate at which rocks break down into soil — larger rocks, for instance, give off more heat. By gathering these data, they were able to calculate the ages of previously undated lunar craters.
After examining 111 moon craters that were less than a billion years old, the data — which revealed there were fewer older craters — suggested that the rate of asteroid impacts increased 290 million years ago. And because the moon is a well-established analog for Earth when it comes to solar system effects, it was fair to say the same would hold true here. It’s an idea that not only offers a compelling argument for Earth’s missing craters but also gives us a better understanding of geological processes as a whole.
“As humans, we are driven to explore and to understand our natural environment, and it’s important to study this process just like we study others,” Ghent says. “Changes in the rate can signal interesting processes in our solar system neighborhood — the objects that hit us and form craters come from the asteroid belt — so ultimately, it gives us insight to what’s happening there.”
