The Earth has been experiencing more frequent asteroid strikes

Enlarge / The craters used for this analysis and their locations. (credit: Dr. A. Parker, Southwest Research Institute)
How often does a big rock drop on our planet from space? As we've gotten a better understanding of the impact that did-in the dinosaurs, that knowledge has compelled people to take a serious look at how we might detect and divert asteroids that pose a similar threat of planetary extinction. But something even a tenth of the size of the dinosaur-killer could cause catastrophic damage, as you could easily determine by placing a 15km circle over your favorite metropolitan center.
So, what's the risk of having a collision of that nature? It's actually hard to tell. The easiest way to tell is to look for past impact craters and try to figure out the frequency of these impacts, but the Earth has a habit of erasing evidence. So, instead, a group of scientists figured out a clever way of looking at the Moon, which should have a similar level of risk. They found that the rate of impacts went up about 300 million years ago.
Erasing history
Some impact craters on Earth are pretty obvious, but erosion and infilling with sediments make others much harder to find. We wouldn't have noticed Chicxulub or the Chesapeake Bay Crater were there if we hadn't stumbled across them for other reasons. As we go back in time, plate tectonics can erase evidence of impacts from the sea floor, as the rock they reside in gets subducted back into the mantle. And then, about 550 million years ago, the Great Unconformity wipes off any evidence of impacts that might have been left on land.
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