When Galileo trained his telescope to the moon in 1609 and discovered perfectly circular craters dominating its topography, astronomers began to wonder how they formed. Some astronomers, such as Franz von Gruithuisen, an early 19th century German, have proposed asteroid impacts as the cause. But most rejected this theory on the basis of a simple and extremely baffling fact: the craters of the moon are almost perfect circles. And, as anyone who’s thrown a rock in the dirt can tell you, that’s not what an impact scar should look like. Instead, the mark will be oblong, oval, and messy. (Gruithuisen probably did not help his cause by also claiming to have seen cows grazing on lunar grass in these craters.) Further fooling theorists, astronomers could distinguish small mountains at the center of each depression. So, for 300 years, the majority of astronomers and physicists believed that (1) cows did not graze on lunar grasslands, and (2) lunar volcanoes, rather than meteors, had stung his face.
Then, in the early 1900s, astronomers like the Russian Nikolai Morozov* began to observe newly developed powerful explosives and made a rather surprising discovery: Large explosions differ from rocks thrown in several ways, but most disturbing – at least for the continued existence of our species – they leave circular craters which whatever their angle of impact. As Morozov wrote in 1909 after conducting a series of experiments, asteroid impacts “would throw out surrounding dust in all directions, regardless of their translational motion, as artillery grenades do when they strike. fall on loose earth ”.
Before Morozov’s discovery, astronomers knew asteroids could be devastating. “The fall of a fireball even ten miles in diameter … would have been enough to destroy the organic life of the earth,” Nathan Shaler, dean of Lawrence Scientific School at Harvard and proponent of the volcano theory, wrote in 1903. But most believed this to be an entirely academic exercise, in part because, as Shaler noted in his defense of the theory of lunar volcanism, the very existence of mankind proved that this kind of impact would not have could not have happened.
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 come to the sobering conclusion that asteroids have doomsday potential and that their impacts are inevitable.
Shaler was, in a way, dead wrong. An asteroid almost the size he described did impact on Earth and did wipe out the dominant species on the planet. It was only instead of wiping out humans that it paved the way for evolution for a placental mammal the size of a shrew to end up crawling, walking, and considering a camping trip to the apocalypse.
You might think the survival of your shrew ancestor proves that a larger-brained mammal like you would have a reasonable chance. Unfortunately, the shrew has had a number of apocalypse-friendly adaptations that humans have since lost. The shrew could survive on insects, dig burrows in the heat, and have fur to keep warm during the frigid decade that followed. You can 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 might not be enough to save you when this twinkling star enters Earth’s atmosphere at 12.5 miles per second.
At the impacts of this speed, the Earth’s atmosphere behaves like water. Smaller rocks – called meteors – hit the atmosphere like pebbles in a pond; they decelerate rapidly at high altitudes, either burning in their friction with the air or decelerating to their low altitude terminal speed of 164 mph. But the mountain-sized asteroid Chicxulub hits our atmosphere like a boulder in a puddle. It maintains speed until impact, plunging through 60 miles of the atmosphere in less than three seconds. The asteroid howls over Central America, emitting a sound boom that reverberates across the continents.
It falls so fast that the air itself cannot escape. Under intense compression, the air heats up thousands of degrees almost instantly. Even before the asteroid arrived, compressed and superheated air vaporized much of the shallow sea that covered the Yucatán at the end of the Cretaceous. Milliseconds later, the rock plunges through what is left and crashes into the bedrock at over 10 miles per second. At this moment, a few almost simultaneous processes are occurring.
First, the hitting meteor applies so much pressure to the ground and rock that it doesn’t shatter or crumble, but rather flow like fluids. This drastic effect actually makes it easier to visualize the formation of the crater, as the ripples of the earth 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 projection as the cavity created by the impactor bounces back to the surface.
