Researchers have estimated the ages of craters on the lunar surface, finding that the rate of large impacts nearly tripled 290 million years ago — an increase that might be ongoing.
We know that the solar system can be a chaotic place, where things crash into each other every now and then. The proof stares at us every night: The pockmarked surface of the Moon has been littered by hundreds of thousands of craters, created in asteroid impacts of all sizes.
Earth must also have been bombarded; however, our planet’s active geology, including weather and plate tectonics, is extremely efficient in removing the marks left behind by ancient impacts. Only 190 impact craters are known in Earth’s surface, and only one of them is more than 2 billion years old. But the Moon is expected to receive the same amount of impacts as Earth, and since the mechanisms that erode craters on Earth are not present there, the craters are preserved indefinitely. A group of researchers has now found a new way to estimate the ages of lunar craters, offering insights on the impact history of the Moon and Earth.
Dating Lunar Craters
On Earth, crater ages can be determined thanks to radiometric dating of the rocks around them. But only two lunar craters have been dated using this method, thanks to samples returned by the Apollo missions. An alternative method is counting superimposed craters — the more that are superimposed, the older the underlying crater.
This time scientists, though, focused on heat. Planetary scientist Rebecca Ghent (University of Toronto), who pioneered the new method, realized that younger craters would have more rocks on their surface than older craters — over time, micrometeorite impacts grind down rocks flung out during impacts — and larger rocks would take more time to cool during lunar night. So, using the Diviner thermal radiometer onboard NASA’s Lunar Reconnaissance Orbiter (LRO), Ghent and colleagues measured craters’ temperatures to estimate the size of rocks within them, and thus their ages.
The team focused on 111 craters larger than 10 kilometers across to make sure that the impacts excavated enough material from the lunar bedrock. The new method can only date craters up to 1 billion years old; after that time, any excavated rocks have been ground down too much for the instruments onboard the LRO to detect any thermal difference between craters. The team decided to look at these younger craters and see if there was any trend in the flux of asteroids hitting the Moon. “I expected that we might not find any change in the flux — the assumption has always been that the flux was constant for 3 billion years,” Ghent says.
To the team’s surprise, the analysis revealed a sudden increase in asteroid impacts starting 290 million years ago. Almost half of the craters they measured are younger than 290 million years, so the impact rate increased by a factor of 2.6 at that time. Thanks to the mathematical analysis of their data, they are confident that the change was not gradual but sudden. However, they haven’t been able to determine yet if the observations are consistent with a transient phenomenon, or if the increased rate persists today. This question is something they expect to answer in future work.
The team points to the asteroid belt as a possible origin for the recent impacting material. “Occasionally one of the [asteroids] will break up, two asteroids may collide, or an asteroid could become gravitationally unstable,” Ghent says. “When that happens, you get a whole collection of particles in a spectrum of sizes in the place of one big asteroid, and those can slowly make their way into crossing orbits.”
What Does This Mean for Earth?
Another surprise came when the team compared their results with Earth’s impact record. Among the scant evidence of impact craters on our planet’s surface, there is an overabundance of craters younger than 290 million years. Scientists had known this for a long time, but they had blamed it on erosion wiping out some of the oldest craters, something they called “preservation bias.” Now, this new study points to a different explanation: There are less old craters because there were fewer impacts in the first place.
In order to confirm that deduction, the team tried to determine which parts of Earth have been stable enough in the last 650 million years to preserve the impact craters. They found a valuable marker in kimberlite pipes – extinct volcanoes with a carrot shape that are buried several kilometers below the surface. These features have been well studied because they are mined for diamonds. If the kimberlite pipes are preserved in a certain region, it’s safe to assume that the area is stable, and any crater large enough should have been preserved as well. “That was the last supporting fact that we needed to get this paper through,” says Sara Mazrouei (University of Toronto), first author of the study.
The team’s search for kimberlite pipes yielded preserved regions covering 11% of Earth’s surface. In these regions the impact rates between Earth and the Moon seem to match, dispelling the reasoning behind preservation bias.
“Whatever happened on Earth, to me there are too many ways of dodging the bullet scientifically. But there is no way to dodge that bullet on the Moon, because that is the record — it is our mirror,” says Peter Schultz (Brown University), an expert on planetary surface impacts who was not involved in the study. Schultz thinks that trying to find a matching pattern on Earth’s surface might not even be necessary, since the trend appears so clearly on the lunar surface. “This is a fascinating paper,” Schultz says. “Especially because the Diviner data are like a brand new set of eyes, they give us a different way to assess age.”