How We Figured Out an Asteroid Killed the Dinosaurs

Thank you to Brilliant for supporting PBS Quick, name an event from natural history…I’ll wait.

Okay, was it that time 66 million years ago when a giant space rock crashed into our planet and killed the dinosaurs?

If you were born after 1980, you probably grew up with that story of the dino-slaying meteor.

But before 1980? Dinosaur extinction was kind of a mess of wild hypotheses – everything from very hungry caterpillars, to eggshells that were too thick or too thin, to poisonous plants had been proposed.

So how did we get from…all that, to our current understanding of what might be the most famous event in natural history, featuring the most famous rock of all time?...

….with the Rosetta stone and Dwayne Johnson tied for second place, obviously.

The story of how this hypothesis went from a radical fringe idea to maybe the most well-known event in all of natural history is one that gets to the root of how science works… It begins with a strange dusting of rare metal found buried in the earth – the first in a long trail of clues that, now, is leading us beyond the earth entirely… …not just into deep time, but also into deep space, to trace the extraterrestrial origin story of the asteroid that rocked our world.

In the span of just four decades, we’ve gone from not knowing there was a space rock at all to knowing exactly where that planet-killer came from.

Throughout the 19th and 20th centuries, scientists of all kinds took up the challenge of explaining why the non-avian dinosaurs disappeared at the end of the Cretaceous Period.

After all, the end of the Age of Reptiles - which allowed the Age of Mammals to begin - was a pivotal evolutionary turning point that directly led to the modern world we know today… So not knowing what caused it was one of the greatest mysteries of the time, and of all time.

Dinosaurs had reigned for almost 200 million years and then, in the blink of a geologic eye, they were gone – their fossil record just…stops.

Clearly, something sudden and dramatic had happened - but what?

Some of the hypotheses were fairly reasonable – like massive volcanic eruptions or rapid shifts in the planet’s climate, atmosphere, and sea levels – things that we know can and have caused mass extinctions before.

Others were…a little more out there.

Like, the explosion of a nearby supernova, or a deadly dinosaur pandemic.

And a few were downright bizarre… Including the idea that small mammals ate all the dinosaur eggs… ….or that an explosion of caterpillars munched their way through all the plant life that the dinosaurs depended on… ….or that the evolution of toxic flowering plants led to mass dinosaur poisoning.

One guy even suggested that dinosaurs just stopped mating, which was probably the most tragic idea of the bunch.

Among suggestions like these, the possibility of a big space rock being to blame seemed like just one more outlandish idea.

But this idea, which became known as the impact hypothesis, came with one pretty important thing that most of those other ideas lacked: actual evidence.

See, in 1980, two papers were published that provided the first clues for what would later become perhaps the most iconic event in natural history.

One paper, led by the father and son team of Luis and Walter Alvarez, emerged from a happy accident – if you can bring yourself to be happy about an apocalypse, that is.

While studying tectonics and paleomagnetism in the mountains of northern Italy, Walter became interested in a sequence of rock layers that dated right to the mass extinction event at the end of the Cretaceous, the K-Pg boundary.

After roping in Luis, his Nobel Prize-winning physicist father, to help study the layers in greater detail, the duo discovered something strange… The boundary contained a spike of the element iridium – a metal that’s exceedingly rare on Earth, but common in meteorites that land here.

And in these layers, from Italy to Denmark to New Zealand, iridium was present at over 20 times the expected concentration.

The second paper, led by the Dutch geologist Jan Smit, reported a similar iridium spike in a different K-Pg boundary layer in Spain.

The collection of sites revealed that the iridium wasn’t just a local, one-off anomaly – this spike was global.

Both teams argued that the most likely culprit was an extraterrestrial body impacting Earth, coating the planet in a fine dusting of the metal.

And while the explosion of a supernova could also have deposited iridium at that scale, it would have left other elements too, like plutonium, which was missing from those layers.

As the 1980s progressed, more tell-tale signs of an impact were found buried alongside the iridium in K-Pg layers around the world… Like shocked quartz, reported in 1984 from a boundary layer in Eastern Montana, that seemed to have been exposed to sudden, extreme levels of pressure and temperature.

The debate had only just begun though, and, to many scientists, the asteroid impact hypothesis still seemed too fringe to be true.

It just felt too /out there/.

After all, if a giant space rock collided with the Earth, wouldn’t there be more evidence of such a massive impact? Like, a crater, or some other scar on the planet’s surface?

It would take more than a spike of iridium and some deformed quartz to convince the world of a dinosaur-slaying asteroid impact.

Especially because, according to some scientists, massive volcanism could also potentially explain those anomalies.

Plus, there was even a plausible candidate: a huge igneous province called the Deccan Traps in India, and it seemed to have erupted pretty close in time to the extinction event.

In 1991 though, a much more in-your-face geological clue came to light that gave a boost to the impact hypothesis.

A massive submerged crater was reported just off the coast of the Yucatan Peninsula, around 180 kilometers across.

Clearly, a gigantic space rock had indeed hit the earth, and, seeing as it was dated to the end of the Cretaceous, it seemed to be the asteroid we were looking for.

With a wealth of clues now converging in its favor, the impact hypothesis moved from the fringes to the center of the debate, and its support in the scientific community grew stronger and stronger.

In 2010, a panel of 41 experts from multiple disciplines and nations published an in-depth review of the evidence for the impact hypothesis.

They agreed that an impact was the single most likely cause of the extinction.

This was seen as something of a landmark moment – a consensus many thought brought the mainstream scientific debate to a formal close.

But that didn’t mean that new details about the impact stopped emerging.

In 2019, for example, a team of researchers revealed a site in North Dakota that captured the immediate aftermath of the impact. The site, named Tanis, seems to record the very day that the asteroid hit.

The researchers had already spent years studying the Tanis site in almost total secrecy, and the exact location is still closely guarded.

But the trickle of information that has been emerging from excavations there paints a sensational picture of chaos and death, now frozen in time… …Like a fossilized turtle impaled on a tree branch, probably as a seismic wave that came shortly after the impact triggered a surge of water forceful enough to skewer the poor reptile.

As well as potentially the first discovery of an individual dinosaur that died on the day of the impact itself – a small herbivore called Thescelosaurus Its fossil leg was found with signs that it may have been quickly ripped off the rest of its body, without any other signs of disease, injury, or bite marks from scavengers.

But perhaps the most interesting fossils found at the Tanis site are the fish.

Many of them have debris from the impact lodged in their gills, as they breathed in particles of molten rock that rained down from the sky.

And based on analysis of some of the fish bones’ growth patterns, they seemed to have died not long after spawning season, which would put the timing of the impact around spring in the northern hemisphere.

It’s pretty amazing that in the span of just forty-odd years, we went from not even being confident there was an asteroid impact to knowing, that it hit, where it hit, and even what time of year it hit.

But, until just recently, there was still at least one big outstanding question: where did the asteroid come from?

Because space is a…pretty big place. That’s, like, one of the main things about it.

And in 2024, researchers announced that they’d figured out the extraterrestrial origin story of the asteroid itself.

Much like the discovery of iridium that birthed the hypothesis to begin with, this most recent insight came from once again studying the chemical secrets of the K-Pg boundary layers… Specifically, a different rare element left behind by the impact called ruthenium, which comes in different forms depending on the type of space rock that it was once part of.

By comparing the ruthenium signature at the K-Pg boundary to ruthenium samples from five other smaller-scale impact sites across deep time, they were able to narrow down the type of asteroid that hit.

And it turns out that the dino-slayer was no ordinary space rock.

See, the other five impact sites had ruthenium signatures typical of asteroids that formed in the inner solar system, close to the Sun, in the same region that Earth and the other rocky planets formed.

But the K-Pg asteroid belonged to a different, much rarer class.

Its ruthenium signature was more characteristic of what’s known as a carbonaceous chondrite – a carbon-rich asteroid from the outer solar system, beyond the gas giant Jupiter.

Which means that this latest piece of the impact hypothesis puzzle that finally rounded out the most famous story in natural history, also added a surprising twist to it, too… Carbonaceous chondrites represent less than 5% of the rocks that fall to Earth, and they date back to the dawn of our solar system itself.

66 million years after it hit, and four decades after we first picked up its trail, we now know what it actually was:...

A rare, distant, and ancient visitor from the far reaches of our stellar neighborhood that found itself on a collision course with Earth… A chance meeting that would change the trajectory of life, and the way that we think about the history of our world, forever.

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Also thanks to this month’s impactful Eontologists!

Addie, Annie & Eric Higgins, Carl Woelfel, Jackie Scott-Ralston, Jake Hart, John Davison Ng, JuanM, Melanie Lam Carnevale, Nico Robin, Raphael Haase, Tony Dai, and S.T.E.V.E.

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