Thirty minutes. That’s all it took, 66 million years ago, to put the future on track for you to be reading this. Had the Manhattan-Island-size asteroid that crashed into the Gulf of Mexico arrived 30 minutes earlier or later, we wouldn’t be around to unravel the consequences of that mighty collision. We know very little about that huge rock, other than it was rich in the element iridium. This was the telltale clue which established that the catastrophe visited upon our planet eons ago was caused by something from space, not from home-grown volcanoes as had been thought prior to about 50 years ago.
Starting in the late 1970s, a father-and-son team, physicist Luis and geologist Walter Alvarez, pioneered studies that gave us the “asteroid impact” theory. Together with a team of researchers, they found iridium in concentrations hundreds of times greater than normal in thin layers of 66-million-year-old sedimentary rock, first in Italy, then in numerous sites worldwide. Iridium is rare on Earth but common in asteroids, so they speculated that the iridium they were finding originated in a massive extraterrestrial object. Around the same time the Alvarez team was publishing their theory, a Pemex (Mexican petroleum) geologist identified a huge crater — 120 miles in diameter and 6/10 of a mile deep—in the sea north of Chicxulub pueblo in Mexico’s Yucatan Peninsula. Many other lines of evidence have since led the majority of geologists to accept the impact scenario.
The theory in a nutshell: A 6-mile-wide asteroid collided with our planet with a velocity of 12 miles per second 66 million years ago, at what geologists call the Cretaceous–Paleogene (K-Pg, formerly K-T) boundary. The immediate result was to send vast clouds of vaporized rock high into the atmosphere, followed by plumes of acidic gases and dust, which together blocked out the sun for perhaps 20 years worldwide. Photosynthesis ceased, plants died, followed by herbivores and, at the top of the food chain, carnivores. In just a few years, over 75 percent of all species, terrestrial and marine, became extinct, including all non-avian dinosaurs.
On land, mammals, which until then had co-existed with predatory dinosaurs by staying small and (often) nocturnal, emerged into a land of opportunity where, in the absence of predators, they flourished and multiplied. The secret to their survival was their size (small animals need less food, and reproduce faster than larger ones), their fur (to make it through those cold, sunless years) and the fact that many were burrowing, living out those desperate times underground. Most species perished, so it wasn’t that wonderful for mammals, but enough survived to evolve — eventually — into the likes of you and me.
Here’s the kicker: That asteroid had been traveling through space for a long, long time — perhaps a million or more years. We have no way of knowing. Yet it hit Earth, with the energy a billion times that of the Hiroshima atomic bomb, at just the right place (from our point of view) to do the maximum damage. That is, the impact point happened to be one of the few places on Earth with an abundance of hydrocarbons and sulfur compounds, just the right stuff to result in enormous quantities of sunlight-reflecting aerosols that made for a decades-long global winter.
At the latitude of the Chicxulub crater, Earth rotates at about 1,000 mph. Had the asteroid hit half an hour earlier, it would have made landfall 500 miles east, in the Atlantic; half an hour later, 500 miles west, in the Pacific. Either way, the resulting quantities of dust and aerosols would have been much, much less: Dinosaurs might well have made it through while mammals would have remained small and inconspicuous.
What a difference 30 minutes made!
Barry Evans (he/him, barryevans9@yahoo.com) is reminded that the odds against his birth were trillions to one, while the odds of his death are one to one.
This article appears in There’s a Fish Market Splashing into Eureka.