Scientists have, for decades, tried to understand the rate that asteroids hit the Earth. They have usually done so by studying the craters and dating the rocks around them. However, the problem with doing this is that many experts assumed that the earliest craters have worn away due to erosion and other geological processes.
However, the new study found that we can learn a lot about the impact history on Earth by studying the Moon.
Researchers believe both bodies are hit in the same proportion over time. Furthermore, according to scientists, the Moon is immune to processes like plate tectonics that gradually destroy the Earth's craters.
Speaking about it, William Bottke, an asteroid expert at the Southwest Research Institute in Boulder, Colorado and a co-author of the paper said, "The only obstacle to doing this has been finding an accurate way to date large craters on the Moon."
The research saw the team studying the surface of the Moon using thermal data and images collected by NASA's Lunar Reconnaissance Orbiter (LRO), to determine the ages of the lunar caters. The NASA spacecraft's thermal radiometer showed scientists how heat is being radiated off the Moon's surface, with larger rocks giving off more heat than finer lunar soil.
Paper co-author Rebecca Ghent, calculated the rate at which Moon rocks break down into soil, and revealed a relationship between the amount of large rocks near a crater and the crater's age. Using Ghent's technique, the team compiled the ages of all lunar craters younger than about a billion years.
According to the scientists, younger craters tend to be covered by more boulders and rocks than older craters. This happens because the boulders ejected by an asteroid strike get ground down over hundreds of millions of years by a constant rain of tiny meteorites.
On comparing the ages and numbers of craters on the Moon to those on Earth, they made the remarkable discovery that they are extremely similar, challenging the idea that Earth had lost so many craters. "This means that the Earth has fewer older craters on its most stable regions not because of erosion, but because the impact rate was lower prior to 290 million years ago," said Bottke.
Dr Thomas Gernon, Associate Professor in Earth Science at the University of Southampton, and co-author on the study, further added, "Proving that fewer craters on Earth meant fewer impacts--rather than loss through erosion--posed a formidable challenge".
Gernon added an unlikely line of evidence to piece together the story. Long extinct diamond volcanoes called kimberlite pipes that stretch a couple of kilometres below the surface in stable terrains are largely intact. This indicated that large impact craters formed over the same period and in the same terrains should also be preserved. This explained the similarity of the Earth and Moon's impact crater records, and helped the team establish that the sparsity of craters formed before 290 million years ago is because there were fewer asteroid strikes before then.
The team's work led to the discovery that the rate of crater formation over the last 290 million years has been two to three times higher than in the previous 700 million years.
The team's findings related to Earth, meanwhile, have implications for the history of life--which is punctuated by major extinction events and rapid evolution of new species. Although extinction events could have many causes, the team points out that asteroid impacts are very likely to have played a major role. In particular, the dinosaurs proliferated about 250 million years ago, and "as a species were particularly vulnerable to large impacts from the get-go, more so than earlier animal groups", says Gernon.
"It's perhaps fair to say it was a date with destiny for the dinosaurs--their downfall was somewhat inevitable given the surge of large space rocks colliding with Earth", Gernon concluded.