1980

Iridium Impact Hypothesis

Luis Walter Alvarez (1911–1988), Frank Asaro (1927–2014), Helen Vaughn Michel (b. 1932), Walter Alvarez (b. 1940)

Iridium is not a common element near the surface of Earth (it dissolves readily in iron, so most of it is down in the core with all that molten iron). There’s a lot more of it in several sorts of asteroids, though, along with other precious metals, which has led to some interesting schemes to bring one of these huge metal-rich space rocks close enough to Earth to be mined. But a strange layer of sediment found around the globe wherever Cretaceous-era rock layers are found strongly suggests that such an asteroid once came a lot closer to Earth than these would-be miners would want, and in a significantly less satisfying way for everything that lived here at the time.

The idea was that analyzing for iridium might help determine how many years the end-of-the-Cretaceous boundary layer represented, but the iridium numbers were so large that they looked like mistakes. Frank Asaro and Helen Vaughn Michel, American chemists specializing in radioisotope decay, had assigned dates and origins to many artifacts and geological samples before, but this was something new. In 1980, the Alvarezes, American geologist–son Walter and physicist-father Luis, advanced the bold hypothesis that this layer marked the worldwide debris from a gigantic asteroid impact, and that this caused the mass extinctions (dinosaurs and more) seen in the fossil record at this exact point.

It’s hard to remember the doubt with which this idea was first received. Geologists prefer not to invoke gigantic one-time-only special events as explanations. If you allow too much of that sort of thing, you can hand-wave a justification for whatever theory you like. But the evidence has only strengthened over the years. Chromium isotope ratios, among other lines of evidence, also point to exactly the same theory. The amount of iridium in the global layer suggests the impact of an asteroid about the size of Manhattan, likely at the site of Chicxulub crater in southern Mexico, which would have catastrophically altered the entire planet’s climate. We should not be surprised that the dinosaurs died—we should be surprised that anything else survived.

SEE ALSO Isotopes (1913)

Radar imaging of the Yucatán peninsula has helped to pin down the traces of an enormous impact crater whose age and size make it the leading candidate for the Cretaceous-ending extinction event.