Over the weekend I came across an excitedly-headlined BBC report suggesting that not one but two impacts in quick succession doomed the dinosaurs 65 million years ago.
One was the giant, now-famous pounding called Chicxulub that gouged out a crater 110 miles (180 km) wide in what's now the Yucatán Peninsula. The other was a smaller crater named Boltysh, only 16 miles (24 km) across, in central Ukraine.
I thought, "Hmm . . . this sounds familiar" — and I was right.
Loyal followers of S&T.com will remember reading about Boltysh in 2002. That's when Simon P. Kelley (Open University, United Kingdom) and Eugene Gurov (National Academy of Ukraine) used isotopic dating to show that the Boltysh impact occurred 65.2 million years ago, give or take several hundred thousand. Chicxulub is a little older, 65.5 million years, but the dating uncertainties overlapped enough that the two hits might have been nearly simultaneous.
Geologists realize that Boltysh is too small to have played a role in the near-total destruction of life on Earth that did in the dinosaurs, a biological hiatus known as the Cretaceous-Paleogene extinction. (You thought it was "Cretaceous-Tertiary," didn't you? The naming convention got changed in 2004.) Statistically, an asteroid or comet creates a Boltysh-size hole on Earth roughly every million years. Sure, it would have been ugly for anything foraging along the Dnieper River that day, but life elsewhere would not have been affected in a serious way.
So why the renewed fuss? You can dismiss the BBC's headline — Boltysh wasn't found to be riddled with plant-munching alien spores or anything else that would change its status as a secondary blip in the dinosaurs' demise.
Instead, in the August issue of Geology, David Jolley (King's College, Scotland) and four colleagues describe what they found after drilling deep into the sediments that now fill the crater's floor. It turns out that ferns and flowering plants had just begun to flourish in the region's recovering ecosystem — only 2,000 to 5,000 years after the impact — when they were wiped out by the aftermath of Chicxulub's much-bigger blast.
Cosmically speaking, the chance of Earth getting nailed twice in such quick succession by impacts of this size is only about 1 in 1,000, so this was no coincidence. Nor were Boltysh and Chicxulub simultaneous — it wasn't a binary asteroid.
Instead, Jolley and his team (which includes Gurov and Kelley) suggest that something must have stirred up the asteroid belt enough to redirect multiple large bodies Earth's way. Perhaps two large asteroids collided about that time, spraying a shower of fragments throughout the inner solar system, or maybe a clutch of asteroids were yanked into Earth-crossing orbits due to a gravitational resonance with Jupiter.
Whatever the cause, Chicxulub likely resulted from an asteroid of some kind, likely a carbonaceous chondrite, based on the tiny piece of one that UCLA researcher Frank Kyte found a decade ago in a deep-sea drill-core from the central Pacific Ocean. So, just like T. rex and its buddies, the idea that a rogue comet caused the Cretaceous-Paleogene extinction is probably dead too.