By Deborah Van Hoewyk

A long time ago, in an asteroid belt – or maybe a comet cloud – but definitely far far away … a “space rock” escaped its orbit and slammed into Earth. It was the end of an era.

An Astronomical Event
A long time would be 66 million years ago (mya), apparently on a day in the northern-hemisphere spring. The impact marked the end of a geological period called the Cretaceous, which ran from 145 to 66 mya). The Cretaceous followed the Triassic (237- 201 mya) and Jurassic (201-145 mya); these three periods comprise the Mesozoic Era, or the age of dinosaurs. The dinosaurs, except for those that could fly, were pretty much instantaneously over as well.

There is not yet agreement on whether the “space rock,” 9.6 km (±6 miles) in diameter, was an asteroid, basically a giant rock, or a comet, made of ice, rock, and dust. Whether it came from the asteroid belt, a donut-shaped collection of debris left over from when the planets of our solar system were formed, or from one or another of the more distant debris clouds that generate comets, doesn’t really seem to matter. Either way, Jupiter got into the act and set the space rock on course for a place named Chicxulub (chicks-oo-loob) on the Yucatán Peninsula of Mexico.

What did Jupiter do? Exerted gravity. Jupiter is about 318 times the mass of Earth, and its gravitational pull is about 2.4 times that of Earth. As astrophysicist Amir Siraj, a student at Harvard when he did his research on the “Chicxulub Impactor,” explains, “the solar system acts as a kind of pinball machine,” and “Jupiter, the most massive planet, kicks incoming long-period comets into orbits that bring them very close to the Sun.” The complicated interaction of these “sun grazer” comets with solar tidal forces makes them smaller and faster, and increases the chance they’re headed straight for earth.

What Happened the Day the Dinosaurs Died?
About a thousand species of dinosaurs inhabited the earth. From ancestors to extinction, they lasted about 230 million years, with the age of recognizable dinosaurs running for about 165 million years. They originated when the earth had a single land mass, called Pangaea; as Pangaea split into areas we now know as the seven continents, the dinosaurs went along for the ride on most of them.

Mexico hosted a range of dinosaurs, with most fossilized remains coming from northern Baja California and Coahuila. There were horned dinosaurs like the Cohuilaceratops, 4 meters (±13 feet) long and weighing just over a ton – but its horns were 1.2 meters (±4 feet) long. There were some of the largest duckbilled hadrosaurs – remains of Tlatolophus galorum were just unearthed in Coahuila, as well as smaller duck-bills like Velafrons and Latirhinus. Tlatolophus was 8-12 meters (26-39 feet) long, weighing in at about 3.6 tons. There were the “apex predators,” the tyrranosaurs, who preyed on the hadrosaurs; Mexico’s best-known tyrranosaur was the medium-sized Labocania, only 7 meters (23 feet) long and weighing in at a ton and half.

Mexico’s dinosaurs were the first to go when the Chicxulub Impactor hit the tip of the Yucatán peninsula with the equivalent of a 100-million megaton blast – 60,000 times the energy of all the nuclear weapons now in existence – hollowing out a crater ±150 km (90 miles) wide.

The impact generated a core of super-heated – over 10,000 degrees – plasma, i.e., matter that has reached a fourth state, beyond solid, liquid, and gas, basically a gas “soup” of charged ions (positive) and electrons (negative).

This thermal pulse lasted for only a few minutes, but it sent out an air blast, a shockwave of air pressure that created winds well over 1,000 kilometers (600 miles) per hour. The air blast radiated across the seas, sending raging wildfires through ancient forests and bringing tsunamis with waves 100-300 meters (±330-990 feet) over the coastline of the Gulf of Mexico.

The impact also produced a seismic pulse equal to a magnitude 10 earthquake, causing landslides on the sea floor, which was already deeply eroded by the backwash from the tsunamis. The seismic pulse radiated far from Chicxulub, with tremors gathering surface water and pushing it up the Western Interior Seaway, an inland sea that split what is now the United States in two.

All terrestrial flora and fauna, along with marine life, within a 1,500-1,800 km (900-1008 miles) radius was roasted or buried alive. And all of this occurred before any debris ejected from the crater could fall back to earth. Within minutes, however, the debris – mostly rocky rubble and “ejecta spherules” (tiny glass beads called tektites, composed of melted rock and dust) – began falling over the Gulf of Mexico and the Caribbean, covering the end of the Yucatán Peninsula and the adjacent sea floor several hundred meters (±800 feet) deep; 350-600 kilometers (210-360 miles) away in Campeche, at the base of the Yucatán, the ejected debris was 50-300 meters (165 to 980 feet) thick.

Chicxulub and Mass Extinction
The gas, dust, and “geologic shrapnel” flung up by the Chicxulub impact did far more than fall to earth in Mexico. It followed global air currents to create an “impact winter,” in which the entire earth was plunged into darkness and freezing cold as the ejected material circled the globe and blocked out the sun. It was a winter that lasted three years. Hot fragments set off wildfires around the world. Acid rain poured down. Plants died. Animals that ate plants died. Animals that ate the animals that ate the plants died. In what is now called the “Cretaceous-Paleogene Extinction Event (K-Pg),” three-quarters of all species then on earth – including and especially the dinosaurs – went extinct.

Not everyone agrees that Chicxulub is the whole story. After studying over a thousand dinosaur-egg fragments, Chinese scientists have argued that in the two million years before K-Pg, dinosaur diversity was dropping. They conclude that the “end-Cretaceous catastrophic events [saliently massive volcanic eruptions in India that would also have caused climate change] probably acted on an already vulnerable ecosystem and led to nonavian dinosaur extinction.”

There were survivors. Some of the avian dinosaurs, ancestors of today’s birds (not the pterosaurs, though, pterodactyls were actually flying reptiles); early small mammals that lived in burrows; small amphibians like frogs and salamanders; and reptiles – snakes, lizards, alligators, crocodiles, turtles – made it through.

Researchers in evolutionary biology at Cornell University have been studying the ancestors of primates (that’s us!) and marsupials (kangaroos and possums). Before the K-Pg extinction, these mammals had been arboreal, living their entire lives in the trees, not a good place to be as incredibly high temperatures sent wildfires tearing through forests. Right around the time of the impact, however, these mammals were making an evolutionary transition out of the trees and a few survived.

It should be noted that we are now in the “Holocene Extinction,” in which human activity – increasing population, overconsumption, and pollution – threatens the extinction of over a million species in the next ten years; the World Wildlife Fund calls it the “largest mass extinction event since the end of the dinosaur age.”

How Do We Know All This?
You could spend time struggling through academic analyses to gather all this information, but there’s an easier way. David Attenborough, the British dean of natural history documentaries, has just released his latest work, a 90-minute film titled Dinosaurs: The Final Day with David Attenborough (premiered April 15, 2022, on the BBC; in the U.S., May 11, 2022, as a two-part special, Dinosaur Apocalypse, on the PBS show Nova).

The documentary shows Attenborough walking with dinosaurs as the Chicxulub impactor is gathering speed in space (the dinosaurs were created by the animators of The Lion King and The Jungle Book). Surprisingly, the documentary starts out not in Mexico but in Hell Creek, North Dakota, about 3000 km (1800 miles) from Chicxulub. Attenborough spent three years with the researchers at the Tanis dig, and devotes most of the documentary to exploring their findings.

The key? The “K-Pg boundary,” a sharp demarcation between life before, during, and after the impact. At the actual boundary there is a concentration of iridium, a hard, iridescent mineral rare on earth but known to occur in meteorites (meteors are “space rocks” once they have entered Earth’s atmosphere, and meteorites are the post-impact remains of the meteors). In 2016, a group of international marine scientists drilled a thousand feet into the edge of the Chicxulub crater about 30 km (18 miles) northwest of Progreso, Yucatán, extracting a core sample that showed iridium at the point of impact.

The Tanis dig shows this even more clearly. British paleontologist Robert DePalma, working with an international team, found large, astonishingly well-preserved fossils below and in a crumbly layer of rock full of the ejecta spherules. Fossils from the Tanis dig are so well preserved because the post-impact inundation of watery mud engulfed living creatures, preserving them much as volcanic lava preserved the victims at Pompeii in the year 79 CE.

Among these fossils are saltwater fish, obviously not native to North Dakota, with gills full of spherules chemically identical to ones found at Chicxulub, indicating that the fish came with the water that rushed from the Gulf of Mexico up the Western Interior Seaway. It is thought that the spherules may have taken as little as 13 minutes after impact in Mexico to fall in Hell Creek.

Chicxulub and the Future
Will Chicxulub happen again? Probably. But definitely not in our lifetime. Amir Siraj, who identified the sun-grazer comets, has determined that, even though the chances of a similar impact are 10 times greater than previously thought, it’s only projected to occur every 250-730 million years.

In the meantime, Chicxulub has lessons for the future. The crater was open on its northeast side to the Gulf of Mexico, allowing nutrient-rich water to circulate in the crater. Life started up very quickly in the form of microscopic marine life, perhaps within 200 years. A consortium of scientists who study the return of life to Chicxulub say it offers lessons for restoring the oceans, threatened today by oxygen deletion, ocean acidification, and rising temperatures. According to Christopher M. Lowery, of the Institute for Geophysics at the University of Texas (Austin), Chicxulub “might be an important analog for the recovery of biodiversity after we finally curtail carbon dioxide emissions and pollution.”