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.
Binary mismatch
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 Miljkovi? 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?
Miljkovi? 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 (see "Do twin asteroids pose twice the risk?", below).
Not quite symmetrical
The simulations also suggest that it is possible to identify which of Earth's single craters had binary origins. These craters should be subtly asymmetrical, and that makes the crater near Chicxulub in Mexico ? thought to be the result of an asteroid impact 65.5 million years ago that wiped out the dinosaurs ? a strong candidate.
"The Chicxulub crater shows some important asymmetries," says Miljkovi?. "It is worth considering that it was formed by a binary asteroid."
Petr Pravec at the Academy of Sciences of the Czech Republic in Ondrejov agrees with Miljkovi? that the crater's features make it a particularly likely contender.
Studying the gravity anomalies created by an impact is a powerful way to find out more about a crater ? particularly one that is now buried, like the Chicxulub crater. Pravec says recent gravity surveys taken at the Chicxulub impact area support Miljkovi?'s conjecture. "The signatures also suggested that the Chicxulub crater might have been formed by a binary asteroid impact," he says.
What might the binary Chicxulub asteroid have looked like? Miljkovi?'s simulations, coupled with the Chicxulub crater's diameter of about 180 km, and its shape, suggests it may have been two rocks with a combined diameter of 7 to 10 km ? the same diameter as the single rock previously imagined to be the culprit. The twin impactors could have been up to 80 km apart, she says, "but these numbers are just guidelines".
It has long been suspected that binary asteroids can generate single craters, says Jean-Luc Margot at the University of California, Los Angeles. "The new study puts this conjecture on solid analytical footing."
Journal reference: Earth and Planetary Science Letters, doi.org/kcx
Do twin asteroids pose twice the risk?
If binary asteroids can form single craters, then Earth is more likely to hit by a binary impact in future than our planet's crater record would suggest. Could these double whammies be harder to spot or deflect than single asteroid hits?
The existential threat posed by asteroids has gained attention in recent years ? underlining the risk, one is due to skim Earth in just two weeks. There are several new efforts to scan the skies for asteroids, and a plethora of suggestions for how they might be deflected.
"I am not sure if any of the proposed asteroid deflection techniques could deflect both binary components with a single weapon," says Katarina Miljkovi? at the Institute of Earth Physics in Paris, France, who led the new study.
Alan Harris, a retired asteroid researcher formerly at NASA's Jet Propulsion Lab in Pasadena, California, has one idea: "A nuclear explosion might be directed at the smaller body, and by blowing it away, the recoil on the main asteroid might effectively deflect it from a collision course."
Being able to deploy the appropriate defence would depend on our ability to spot whether or not an object heading our way is a binary. Don Yeomans of NASA's Near Earth Object Program thinks that won't be a problem for a future asteroid-deflecting spacecraft. "There is a slim chance that the autonomous navigation camera might be confused with two images in its field of view, but I should think these issues would be easily overcome," he says.
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