The Inch-Thick Tombstone of the Dinosaurs

Bad Astronomy
The entire universe in blog form
April 2 2014 7:45 AM

The Man Who Went Up a Hill and Came Down a Mass Extinction Event

K-Pg boundary
I wonder how many vaporized dinosaur molecules are in this.

Photo by Phil Plait. Phil Plait's hand used under Creative Commons.

A little while back I was in southern Colorado filming a science documentary. We were in Trinidad State Park, talking about the mass extinction event that wiped out the dinosaurs. It was a fun shoot, but in the end there were some things I had to leave out of the interview I wish I could’ve covered.

So before we left I decided to grab the opportunity while I was still there to get my phone and make a short video. I did, but decided to wait a while to put it up. So there it sat until I happened to find it again the other day. I think it’s a fun little segment, so here you go.


I’ll note that in the video I call it the “K-T boundary,” for Cretaceous-Tertiary, but that’s rather old-fashioned. It’s now called the Cretaceous-Paleogene boundary, or K-Pg boundary. It’s habit to use the old term, but I’ll try harder in the future. Not only that, but in the video, I said that “K” is used as the abbreviation for Cretaceous because the letter “C” had already been used for something else. A geologist told me that, but it turns out it’s actually from the German word Kreide, which is what they call the Cretaceous. Live and learn. Or in this case, don’t get mass extincted and learn.

Squatting there at the top of that talus slope was transformative. Being able to see that line of gray clay with my own eyes was really quite moving. I can’t stress enough just how big a mystery this was when I was a kid; I read book after book with authors speculating on what killed off the dinosaurs. After all, they were walking around the planet for hundreds of millions of years, far far longer than we humans have!

dinosaur impact
The start of a very bad day.

Drawing by Shutterstock/Esteban De Armas

And now we know. Well, we know a big piece of the puzzle. The devil’s in the details, though. For example, it’s not clear whether the impactor was a rocky asteroid, or an icy comet. That may seem like an esoteric point, but it’s actually crucial. An asteroid 10 kilometer (6 miles) across—the size of the dinosaur killer—would be easy to detect long before impact because asteroids like that tend to have orbits that make them easier to spot way in advance. We’d have decades of warning, in general, if we saw one like that headed our way.

Many comets, though, come from the far depths of the solar system, dropping in from billions of kilometers out. They pick up speed rapidly as they fall toward the Sun and may not give us nearly as much warning. For example, the comet Hale-Bopp, a gorgeous sight that graced our skies in 1997, was only discovered 19 months before it passed the Earth. The solid nucleus of that comet was something like 30 kilometers across, which means it was probably 25 times as massive as the dinosaur killer. Had that been aimed at us, we wouldn’t be here talking about it. Happily, impacts like that are extremely rare (like tens of millions of years apart or more, on average).

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

Still, knowing more about ancient comet and asteroid impacts is rather important. A new paper just came out making the claim that the impact that wiped out the dinosaurs was in fact from a comet, not an asteroid. These claims have been around a while, and while they’re very interesting and even persuasive, they’re not conclusive; I don’t think we can say for sure what’s what just yet. More research will hopefully tip the needle to one side or the other.

In the meantime, I’ll keep reading about this impact (I found a great paper that has all kinds of details on the K-T K-Pg boundary layer) and learning more about it. I love astronomy, of course, and I’m fascinated by giant impacts. Dinosaurs were a love of mine ever since I was a kid (just like a gazillion other kids), too, so this event ties together three of my biggest interests! How can I resist?



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