Asteroids 2, dinosaurs 0. The infamous space rock that slammed into Earth and helped wipe it clean of large dinosaurs may have been a binary – two asteroids orbiting each other.
The dino-killing asteroid is usually thought of as a single rock with a diameter of 7 to 10 kilometres, but it may really have been two widely separated rocks with that combined diameter.
The surprise conclusion comes from a re-evaluation of the proportion of asteroid craters on Earth that were formed from binary impacts. It could also spell bad news for those hoping to protect our world from catastrophic collisions in future.
Earth bears the scars of twin-asteroid impacts: the Clearwater Lakes near Hudson Bay in Canada, for instance, are really twin craters that formed about 290 million years ago. Examples like Clearwater are rare, though. Just 1 in 50 of craters on Earth come in such pairs.
That is a puzzle because counts of the rocks zooming around in the vicinity of Earth suggest binaries are far more common. "It's been known for 15 years that about 15 per cent of near-Earth asteroids are binary," says Katarina Miljković at the Institute of Earth Physics in Paris, France. All else being equal, 15 per cent of Earth's impact craters should be the result of twin impacts. Why does the real figure appear so much lower?
Miljković and her colleagues have found an explanation. They ran computer simulations of binary asteroids hitting Earth and found that they often form a single crater.
This makes sense, given that a crater can be 10 times the diameter of the asteroid that made it. The team found that only unusual cases involving two small, widely separated asteroids are guaranteed to form a pair of distinct craters. The researchers' simulations confirmed that such binary asteroids are rare enough to explain why paired craters account for only 2 per cent of all Earth's craters.
An obvious implication is that binary asteroids hit Earth more often than the crater record appears to suggest – with ramifications for efforts to prevent future impacts.