Given enough time, a major impact becomes a certainty. The asteroids are patient and operate on time scales different from mere mortals.
“It’s not the type of thing you panic over,” says David Kring, a geologist at the Lunar Planetary Institute in Houston. “But it’s a real threat, and we should take prudent steps to understand the hazard better. What are those prudent steps? First, a geological one — you study the geological past, measure the consequences of past impact events.”
For a long time, geologists overlooked impact craters on Earth because they were intellectually reluctant to embrace anything that smacked of catastrophism. The theory of “uniformitarianism” saw the world in terms of slow, gradual processes — a view that helped Charles Darwin develop his theory of evolution.
Catastrophism made a comeback after the father-son team of Luis and Walter Alvarez announced in 1980 their discovery of iridium in a narrow band of sediment marking the end of the Cretaceous Era, when dinosaurs and half the species on the planet vanished in a mass extinction. Iridium is rare on Earth but relatively common in asteroids. A decade later, scientists said they had matched the end-Cretaceous impact to a massive crater beneath the tip of the Yucatan peninsula near the town of Chicxulub.
“Part of that revolution was to realize that just a relatively small impact, from a 10-mile object, was able to redirect the course of evolution,” said David Morrison, a NASA scientist and a major force in the mapping of NEOs.
“In other words, the leverage was huge,” he said. “This object, geologically, was quite minor, and had a tremendous impact on biology.”
Lurking somewhere on Earth, eroded and buried and scrubbed by time, are mega-craters, remnants of what scientists refer to as the Late Heavy Bombardment of about 4 billion years ago. We don’t know if life had yet appeared when 70-mile-wide rocks slammed into the planet, but it wouldn’t have been a good time to be alive.
“The oceans boil,” said Jay Melosh, a planetary scientist at Purdue University and an expert on impact craters. “Most of the oceans boil away, the Earth has a steam atmosphere that lasts for thousands or tens of thousands of years, and the surface, of course, is very, very hostile.”
There is in the study of ancient craters a glimpse of our blessings as a planet. Such craters are hard to find because this is not a static world but a living, dynamic place.
Every time we look at the cold, gray, cratered moon, we see what might have been.