When Galileo aimed his telescope at the moon in 1609 and discovered perfectly round craters that dominated the topography, astronomers began to wonder how they formed. Some astronomers, such as Franz von Gruithuisen, a German from the early 19th century, suggested asteroid impacts as the cause. But most rejected this theory on the basis of one simple, extremely confusing fact: the moon’s craters are near-perfect circles. And, as anyone who has thrown a rock into the earth can tell you, an impact scar shouldn’t look like this. Instead, the mark is elongated, oval, and messy. (Gruithuisen probably did not help its case by also claiming to have seen cows grazing on moon grass in these craters.) Further misleading theorists, astronomers were able to discern small mountains in the center of each depression. So for 300 years, most astronomers and physicists believed that (1) cows didn’t graze on lunar pastures, and (2) lunar volcanoes, instead of meteors, poked his face.
Then, in the early 1900s, astronomers liked the Russian Nikolai Morozov began observing newly evolved explosives and made a rather surprising discovery: large explosions differ from thrown rocks in a number of ways, but most ominously – at least for the survival of our species – they leave circular craters regardless of their angle of attack . As Morozov wrote in 1909 after conducting a series of experiments, asteroid impacts would “throw the surrounding dust in all directions, regardless of their translational motion, in the same way that artillery shells do when they hit loose Earth.”
Before Morozov was discovered, astronomers were aware that asteroids could be devastating. “The fall of a car as much as ten miles in diameter … would have been enough to destroy organic life on Earth,” Nathan Shaler, dean of Harvard’s Lawrence Scientific School and advocate of the volcano theory, wrote in 1903. But most believed this was an entirely theoretical exercise, in part because, as Shaler noted in his defense of the lunar volcanism theory, the existence of mankind proved that this kind of impact could not have happened.
Morozov’s calculations changed that. Once you know the true origin of the scars on the moon, you don’t have to be an astronomer – or even own a telescope – to come to the sobering conclusion that asteroids have apocalyptic potential and their impact is inevitable.
In a sense, Shaler was presciently incorrect. An asteroid almost the size he described did affect the earth and did destroy the dominant species of the planet. Only instead of exterminating humans, it paved the evolutionary path for a shrew-sized placenta mammal to eventually crawl, walk, and contemplate a camping trip to the apocalypse.
You would think the survival of your shrew-like ancestor proves that a larger-brained mammal like you would have a fair chance. Unfortunately, the shrew had some apocalypse-friendly adaptations that humans have since lost. The shrew was able to survive on insects, burrow away from the heat, and had fur to warm itself up during the frigid decade that followed. You could replicate some of the shrew’s survival strategies. You could dig in and expand your diet. But evolution has robbed you of others, and your opposable thumbs may not be enough to save you when that twinkling star enters Earth’s atmosphere at 12 miles per second.
At impacts of that speed, the Earth’s atmosphere behaves like water. Smaller rocks – called meteors – hit the atmosphere like pebbles in a pond; they slow down quickly at high altitudes, either burning away in their friction with the air or slowing to their final speed at low altitudes of 164 mph. But the Chicxulub asteroid the size of a mountain hits our atmosphere like a boulder in a puddle. It maintains its speed until impact, dipping through the entire 60 miles of atmosphere in less than three seconds. The asteroid screeches over Central America, emitting a sonic boom that echoes across the continents.
It falls so quickly that the air itself cannot escape. Under intense compression, the air heats up thousands of degrees almost instantly. Before the asteroid even arrives, compressed and superheated air evaporates much of the shallow sea that covers the Yucatán in the late Cretaceous. Milliseconds later, the rock plunges through what is left, smashing against the rock at more than 10 miles per second. At that point, a few nearly simultaneous processes take place.
First, the impacting meteor exerts so much pressure on the ground and rock that they don’t shatter or crumble, but instead flow like liquids. This radical effect actually makes it easier to visualize the crater’s formation because the Earth’s undulations almost exactly mimic the double splash of a cannonball in a backyard pool. The initial splash in all directions is followed by a delayed vertical splint as the cavity created by the impactor rebounds to the surface.