When Galileo trained his telescope on the moon in 1609 and discovered perfectly circular craters dominating its topography, astronomers began to wonder how they formed. A few astronomers, like Franz von Gruithuisen, an early-19th-century German, proposed asteroid impacts as the cause. But most rejected this theory based upon one simple, supremely confounding fact: The moon’s craters are almost perfect circles. And, as anyone who has thrown a rock into dirt can tell you, that isn’t what an impact scar should look like. Instead, the mark will be oblong, oval, and messy. (Gruithuisen probably didn’t help his cause by also claiming to have seen cows grazing upon moon grass in these craters.) Further misleading any theorists, astronomers could make out little mountains in the center of each depression. Thus, for 300 years the majority of astronomers and physicists believed that (1) cows did not graze upon moon meadows, and (2) lunar volcanoes, rather than meteors, had pocked its face.
Then, in the early 1900s, astronomers like Russia’s Nikolai Morozov* began observing newly developed high explosives and made a rather startling discovery: Large explosions differ from thrown rocks in a number of ways, but most ominously—at least for our species’ continued existence—they leave circular craters regardless of their angle of impact. As Morozov wrote in 1909 after conducting a series of experiments, asteroid impacts would “discard the surrounding dust in all directions regardless of their translational motion in the same way as artillery grenades do when falling on the loose earth.”
Before Morozov’s discovery, astronomers were aware that asteroids could be devastating. “The fall of a bolide of even ten miles in diameter … would have been sufficient to destroy organic life of the earth,” wrote Nathan Shaler, dean of Harvard’s Lawrence Scientific School and proponent of the volcano theory, in 1903. But most believed this was an entirely theoretical exercise, partly because, as Shaler noted in his defense of the lunar volcanism theory, the very existence of humanity proved this sort of impact could not have occurred.
Morozov’s calculations changed that. Once you know the true origins of the scars on the moon, you don’t have to be an astronomer—or even own a telescope—to arrive at the sobering conclusion that asteroids carry apocalyptic potential and that their impacts are inevitable.
Shaler was, in a way, presciently incorrect. An asteroid of nearly the size he described did impact Earth and did wipe out the planet’s dominant species. Only rather than wiping out humans it cleared the evolutionary path for a shrew-sized placental mammal to eventually crawl, walk, and consider a camping trip to the apocalypse.
You might think the survival of your shrewlike ancestor proves that a larger-brained mammal like yourself would stand a reasonable chance. Unfortunately, the shrew had a number of apocalypse-friendly adaptations humans have since lost. The shrew could survive on insects, burrow away from the heat, and had fur to warm itself during the freezing decade that followed. You could replicate some of the shrew’s survival strategies. You could burrow and expand your diet. But evolution has robbed you of others, and your opposable thumbs might not be enough to save you when that twinkling star enters Earth’s atmosphere at around 12.5 miles per second.
At impacts of that speed, Earth’s atmosphere behaves like water. Smaller rocks—called meteors—hit the atmosphere like pebbles into a pond; they decelerate rapidly at high altitudes, either burning away in their friction with the air or decelerating to their low-altitude terminal velocity. But the mountain-sized Chicxulub asteroid hits our atmosphere like a boulder into a puddle. It maintains its velocity until impact, plunging through the entire 60 miles of atmosphere in around three seconds. The asteroid screeches over Central America, emitting a sonic boom that reverberates across the continents.
It falls so quickly that the air itself cannot escape. Under intense compression, the air heats thousands of degrees almost instantly. Before the asteroid even arrives, compressed and superheated air vaporizes much of the shallow sea that covers the Yucatán in the late Cretaceous. Milliseconds later, the rock plunges through what’s left and slams into bedrock at more than 10 miles per second. In that instant, a few near-simultaneous processes occur.
First, the impacting meteor applies so much pressure to the soil and rock that they neither shatter nor crumble, but instead flow like fluids. This radical effect actually makes it easier to visualize the formation of the crater, because the undulations of the earth almost exactly replicate the double-splash of a cannonballer in a backyard pool. The initial splash in all directions is followed by a delayed, vertical sploosh when the cavity created by the impactor rebounds to the surface.