For decades, astronomers and planetary scientists have wondered exactly what it was that hit Earth 66 million years ago and caused the extinction of 75%! of all species of life, including the non-avian dinosaurs. Was it a comet or an asteroid?

We know it was big, 10 kilometers across or so, and hit in the Gulf of Mexico just off the Yucatan coast, a site called Chicxulub (CHICK-shoo-lube). Many of its aftereffects are well understood, though some are still a mystery. However, all that we’ve seen can’t distinguish between an asteroid or comet, and the direct evidence (the remnants of the impacting mass) is buried under kilometers of crust and water. 

However, using a clever technique, scientists have now found strong evidence that the dinosaur killer was in fact an asteroid, and a specific type called a carbonaceous chondrite [link to journal paper].

The impact was so large — it dwarfed the combined explosive yield of every nuke on the planet, equaling about a hundred million megatons^* — it ended the Cretaceous geological period and introduced the new Paleogene period. There’s a layer of clay in the geological stratigraphic record at that time — called the K-Pg boundary — and it had a far higher amount of the element iridium in it than usual. Iridium is rare on Earth but more abundant in comets and asteroids, which is how the idea of an impact ruining the dinosaurs’ day was suggested in the first place.

The scientists examined debris from this layer in samples from around the world. They looked specifically at the element ruthenium, which again is elevated in the K-Pg layer. Even more specifically, they looked at ruthenium isotope ratios.

Elements are defined by the number of protons they have in the nuclei of their atoms. Hydrogen has one, helium 2, and so on. Ruthenium has 44. That number defines the chemistry of the atom. However, there are also neutrons in an atomic nucleus, and that number can vary even in a single element, and we call those variations isotopes. Ruthenium has many isotopes, including seven that are stable (as opposed to ones that radioactively decay). Two of these are ruthenium-100 (which has 56 neutrons) and ruthenium-102 (58 neutrons). Some isotopes are more common and some more rare; for ruthenium about 13% is in the form of Ru-100 and 32% Ru-102.

But that ratio isn’t universally true. Different kinds of asteroids, for example, have slightly different amounts of Ru-100 versus Ru-102. By carefully measuring that ratio, scientists can identify where the material came from. 

In this case, the ratio closely matches not just that of asteroids, but carbonaceous chondrite asteroids. Chondrites are asteroids that don’t show any modification by thermal processes (like melting), and carbonaceous means they have more carbon in them than average. Because they were never heated, these types of asteroids are thought to have formed in the outer solar system when the Sun and planets were born, and have remained largely unmodified by large impacts ever since. Because of this, they’re called primitive asteroids.

We have lots of samples of different kinds of asteroids, because sometimes they fall to Earth, though usually in a way not quite so devastating as the Chicxulub impactor. We can study these meteorites and learn about asteroids.

This graph is what got to me. They plotted the abundance of ruthenium-102 versus that of ruthenium 100 to get the ratio of the two in various kinds of meteorites. Iron meteorites (open red squares) tend to fall in the middle to upper right of the line, for example, while ordinary (non-carbonaceous) ones (black diamonds) tend to fall more in that clump to the upper right. The samples from the K-Pg boundary (blue filled circles) all cluster to the lower left, which is where carbonaceous chondrites (black filled circle) are. That’s pretty clear. Remember too these samples came from all around the planet, so it can’t be local geology affecting these results.

This isn’t the first time this method has been used to look at Chicxulub. Previous studies examined chromium and platinum group elements (iridium, osmium, and a few others) and also pointed toward the asteroid being a carbonaceous chondrite. So this adds to the pile, and is pretty convincing.

If this asteroid formed in the outer solar system, how did it get here? Almost certainly that was due to the gravitational influence of the outer planets, especially Neptune. Objects orbiting the Sun that far out get tugged by the planets’ gravity, and over time their orbits can change. If they get dropped down close enough to the Sun, an encounter with Jupiter can then dramatically warp the asteroid’s trajectory, sending it on a path that eventually intersects ours.

Unfortunate for the dinosaurs, but lucky for us. We wouldn’t be here if that rock hadn’t hit Earth.

Knowing where it came from is handy, too. The effects of a big impact depend on a lot of things — for example, there’s a lot of chlorine and bromine in the seabed in the Gulf of Mexico, and the dinosaur-killer impact vaporized that stuff and flung it into the upper atmosphere. Ozone is very reactive with them, so this likely erased the ozone layer, allowing more dangerous ultraviolet sunlight down to Earth, making the impact effects even worse.

The composition of the impactor makes a difference too, so knowing what this one was helps scientists nail down what those effects might be. Seeing as how there are still a lot of asteroids out there, I kinda feel like the more we know the better, especially if we find one headed our way and need to send up a space probe to knock it out of the way à la DART. The asteroid’s composition and tensile strength will make a huge difference in how well that sort of thing would succeed. 

We’re a step closer now. Lots more steps to go, but each one, I hope, is in the right direction.

^* Correction (Aug. 26, 2024): I had originally written “a billion megatons”, but astronomer Ned Wright pointed out that was too high. He’s right; I checked my math and got 120 million megatons! Oops. I grabbed the number from an old article and didn’t obey my own rule to always check the math on a number before repeating it. My apologies.

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