MOST huge asteroids would not wipe out swathes of life on Earth, if two large impacts are anything to go by.
While the dinosaurs were wiped out by a devastating asteroid impact 65 million years ago, two other massive asteroids which struck the Earth millions of years later had no discernible effect on the mammals living in North America at the time.
The two asteroids struck in the late Eocene, about 35.5 million years ago. While each delivered only about a quarter as much energy as the deadly Chicxulub asteroid that gouged a 180-kilometre crater out of the Yucatan coast off Mexico, they were still huge impacts. One left a 90-kilometre crater at Chesapeake Bay in Maryland and the other a 100-kilometre crater at Popigai in Russia.
Advertisement
That suggests asteroids do not gradually increase their killing power with size. Instead, only the largest asteroids, whose size exceeds a critical threshold, are capable of causing widespread extinctions, says Linda Ivany of Syracuse University in New York state.
The discovery came out of a study by John Alroy of the University of California at Santa Barbara. Alroy collected data on the origin and extinction of North American mammals during the late Eocene. He broke the data down into million-year intervals, and tried to match them to asteroid impacts during the same time frames. On average, asteroids that left craters larger than 1 kilometre wide struck during every other interval. Yet he found that none of the large extinctions of mammals coincided with any of the biggest impacts.
While the Chicxulub asteroid is known to have wiped out the dinosaurs, sparking the rapid evolution and geographical spread of mammal species, the next largest events, the Chesapeake Bay and Popigai craters, had no such effect. “You’d really expect a crater that big to pulverise the poor little mammals,” he told the Society of Vertebrate Paleontology meeting in Norman, Oklahoma, earlier this month. He says that the changes in mammal populations over the period probably came from competition and migration between continents.