The last page in this survey of space-related Earth science is about the big bang—the cosmic impact of a large meteor or comet that civilization will witness, sooner or later.
Craters Are Everywhere
Other planets are carpeted with impact crater, but not planet Earth. Earth heals its wounds so quickly, thanks to the vigor with which it recycles its surface, that relatively few craters are documented, and most of those are buried. So before the space program, geologists didn't think about impacts as an agent of geologic change. They do now.
We could have figured this out in 1610, when Galileo turned his telescope toward the Moon and saw, to his astonishment, that it was not a heavenly body at all but a pockmarked rocky wasteland. Ancient legends describe what must be impacts. Nevertheless, not until 1965, when Mariner 4 sent us images of craters on Mars, was it widely recognized that the whole solar system—including Earth—was subject to enormous collisions with cosmic debris.
The Cretaceous Catastrophe
Then in 1979 it was first surmised that just such a collision was the mysterious event (the "K-T event") that ended the Cretaceous Period and ushered in the Tertiary Period some 66 million years ago. When the buried ruins of the K-T crater were mapped in the Yucatán and named for the obscure locality of Chicxulub ("CHEE-shu-loob"), this argument triumphed.
Consider what the researchers found. All over the world, precisely on the K-T boundary, is a thin layer of clay and soot. Evidently the majority of the world's vegetation caught fire at the same time, and a long rain of fine dust fell on all places at once.
Simulations have clarified what probably happened. A rocky asteroid some 5 kilometers across burned through the atmosphere, with a terrible flash and a roar heard round the world. As it struck the ground, most of its mass and an equal amount of the Earth turned into melt and vapor. This mass swiftly shouldered its way out of the atmosphere and condensed into grains of dust, flying in ballistic trajectories and low orbits that carried the grains to the whole planet.
When they came back down, every one of those quadrillions of particles became a shooting star, and the sky lit up at an effective temperature of over 1000 K (about 1500°F), from horizon to horizon. The brightness stabbed downward for a quarter-hour, and when it ended whatever could burn was in flames. The resulting smoke, and the fine dust from the impactor, together plunged the world into complete darkness for a period of months. The numerical models suggest that by the end of that time, most of the world's land surface was near freezing. Some paleontologists argue that whatever numerical models say, the rocks don't fully support it. Geologists have learned that the rocks usually win these arguments.
The Hazard We Never Suspected
Whatever happened exactly, the dinosaurs, the ammonoids and many other groups nevertheless went extinct, and certainly we would have too. Such an impact seems to occur every 100 million years on average, so maybe we shouldn't lay odds on something that bad happening to us. But a very much smaller impact, involving a 500-meter object, would release a few hundred megatons of energy. It would burn up an area some 300 kilometers across if it struck on land. If it hit the Pacific Ocean instead, it would raise a tsunami of up to 100 meters height at the shore. Of course, nothing would burn in that case . . . take your pick. It wouldn't kill all of us, but it would be as bad as any of the worst natural events ever to affect us in history. One of those is likely every 10,000 years or so on average.
When this knowledge sank in, some people began to talk about watching the skies rather more carefully. There's a whole population of asteroids whose orbits cross Earth's, a couple thousand probably. For this part of the subject—possible impacts and their impact on humanity—NASA is a rich resource, including the Spaceguard Survey, an excellent detailed study of the size of the problem and especially the types of hazards that impacts of all sizes impose. Read it and weep.
Since that time, scientists have drawn up a numerical scale of cosmic impacts, along the lines of the Mercalli scale of earthquake intensity. This scale, the Torino scale, is actually being used today to assess the threat of orbiting Near-Earth Objects found by telescopes.
Are All Extinctions Impact-Related?
But enough of human affairs—what about the geologic past? I already mentioned the K-T boundary. There are lots of other boundaries in the geologic time scale, and some of them seem to be impact-related.
On the whole, though, the really big changes that mark geologic time boundaries—mass extinctions, mostly—seem to be due to earthly causes like volcanism or climate changes. For instance the Permian-Triassic or P-Tr extinction, the greatest of them all, was probably not caused by impacts.
The Tsunami Threat
Another less-mentioned aspect of cosmic impacts is their ability to create tsunamis. In today's textbooks, tsunamis appear in the earthquake chapter, but in fact two-thirds of all impacts should create tsunamis as they strike the sea. They would create waves many times greater than those triggered by the largest earthquakes.
For the last 200 years of field mapping, geologists have not been looking for traces of tsunamis in the rocks. It seems clear, though, that impact mega-tsunamis must have wiped every kilometer of the whole world's coastline clean, probably more than once, in any given million years. And a million years is practically a geologic blink of the eye. The signs ought to be there, along with those of ancient hurricanes and earthquakes, and we have begun to go back to the outcrops and interrogate the rocks again.
PS: A famous impact happened in the 20th century, in Siberia over the forests of the Tunguska River region. It looked and acted just like a huge nuclear bomb, only without the radiation, but the heavenly body that caused it was no bigger than an apartment building.