The deadly legacy of a crater
Premium- Meteorites that are about 1 kilometre across hit us about every one million years
On a sunny Tuesday morning in March about 66 million years ago, this planet’s dinosaurs were wiped out. In fact, about 75% of the planet’s plant and animal species were wiped out. Well, maybe not all on that Tuesday afternoon, but they were gone within a relatively short geological span: maybe a few years, maybe a few tens of thousands of years.
What set off this so-called Cretaceous–Paleogene extinction event? A huge asteroid—possibly up to 15km wide—smashed into the Earth. This collision was hypothesized for years—until 1980, when a team of scientists announced that they had discovered the scar the asteroid left on the Earth’s surface. This is the Chicxulub crater, 180km in diameter, half on and half off Mexico’s Yucatan Peninsula.
The Chicxulub collision was an earthshaking catastrophe in every sense. It set off enormous tsunamis. It sent debris shooting so high that some of it actually escaped the Earth’s gravity and has roamed through the solar system ever since—one asteroid, you might say, giving birth to plenty more. The rest of the debris fell back to Earth, getting so heated as it scythed through the atmosphere that on touchdown, it sparked vast forest fires.
All right, it may not have been on a Tuesday afternoon. But the collision is thought to have happened in the northern hemisphere’s spring, so March is a safe guess.
Shift focus to a cloudy Tuesday morning in 1908, when an enormous explosion flattened some 80 million (!) trees in a 2000 sq. km area near the Tunguska River in central Russia. This catastrophe was caused by an asteroid thought to be about 50-60m wide. Unlike Chicxulub, it left no impact crater. The huge rock had become so hot as it flew through the atmosphere at nearly 100,000 kmph that it exploded when it was still a few km above the surface.
That definitely was a Tuesday morning. It was June the 30th and everybody was feeling fine, at least until the blast. It’s a largely uninhabited forest area, but there were reports that three people were killed.
There’s plenty more to say about these spectacular, calamitous events. But leave all of it aside and speculate with me. Take a look at the Moon sometime through binoculars or a telescope—or just look at any photograph of our faithful satellite. You’ll note that its surface is pockmarked with craters. That’s only to be expected, really. After all, there’s plenty of debris floating about out there, and collisions are inevitable. So why, then, is there only scattered evidence of impacts, like Chicxulub, on Earth? Why is the Earth not pockmarked like the Moon? Is something protecting our planet?
Well, yes. It’s called an atmosphere. Certainly, bits of space debris constantly collide with the Earth. What saves us is that the great majority of them are destroyed as they descend through the atmosphere. That’s why we sometimes see a “shooting star"—a meteorite—on a dark night. That’s yet another rock from space, burning up with the friction from our atmosphere. So yes, we bump into space rocks all the time. But it’s the rare one that’s big enough to make it all the way through the atmosphere and hit the ground. How rare?
Consider some numbers. Even if the Tunguska meteorite didn’t reach the ground in 1908, the reality is that boulders of that size collide with the Earth about every 900 years, and we should expect that some of them will touch down. Meteorites about 1km across hit us about every one million years. A 15km monster like the one that left the Chicxulub crater, about every 65 million years. So, we may be due one of those gigantic extinction events, not that I want to alarm you.
Given those numbers, and taking into account the Earth’s age of about 4.5 billion years, you’d expect that we’d have at least a few thousand craters left by all these collisions, maybe about 60 of them as large as Chicxulub. True, we may not be able to see them all. After all, more than 70% of the planet is under water. Even so, it’s surprising that we know of only about 200 impact craters on Earth. For various reasons, only about 20 of those are “marine impact craters", and most of those 20 are in relatively shallow waters—like Chicxulub.
There’s something else that’s surprising about these craters. You will remember that last year, we dispatched the DART mission to crash into the asteroid Dimorphos in an attempt to deflect its path. (See my column ). Dimorphos has a twin called Didymos; the two orbit each other. Of the asteroids that surround the Earth—the pool from where we get the great majority of our meteor strikes—about 15% are “binaries" like Didymos and Dimorphos. Yet, of the 200 impact craters on Earth, you can just about count on one hand—far less than 15%, anyway—those that were caused by binary strikes.
All of which leads up to an unusual discovery some 400km off the coast of Guinea, in West Africa, in 2020. Using a technique called “seismic reflection", similar to ultrasound, scientists came across “a ≥8.5-km-wide structure buried below ~300 to 400 m of Paleogene sediment with characteristics consistent with a complex impact crater." (The Nadir Crater offshore West Africa: A candidate Cretaceous-Paleogene impact structure, 17 August 2022).
What’s more, this crater is estimated to be ... 66 million years old. The same age as Chicxulub. This is why it’s set off a buzz in paleogeological circles. Was this a binary strike? Could the Chicxulub rock and whatever caused the smaller Nadir Crater have both come from a “common parent asteroid" that broke up, maybe on entry into our atmosphere?
Were the dinosaurs hit by more than one asteroid, not that they would have really cared?
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.
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