At Stevns Klint, the end of the Cretaceous period shows up as a layer of
clay that’s jet black and smells like dead fish. When the stinky Danish
samples were analyzed, they, too, revealed astronomical levels of iridium.
A third set of samples, from the South Island of New Zealand, also showed
an iridium “spike” right at the end of the Cretaceous.
Luis, according to a colleague, reacted to the news “like a shark
smelling blood”; he sensed the opportunity for a great discovery. The
Alvarezes batted around theories. But all the ones they could think of
either didn’t fit the available data or were ruled out by further tests. Then,
finally, after almost a year’s worth of dead ends, they arrived at the
impact hypothesis. On an otherwise ordinary day sixty-five million years
ago, an asteroid six miles wide collided with the earth. Exploding on
contact, it released energy on the order of a hundred million megatons of
TNT, or more than a million of the most powerful H-bombs ever tested.
Debris, including iridium from the pulverized asteroid, spread around the
globe. Day turned to night, and temperatures plunged. A mass extinction
ensued.
The Alvarezes wrote up the results from Gubbio and Stevns Klint and
sent them, along with their proposed explanation, to Science. “I can
remember working very hard to make that paper just as solid as it could
possibly be,” Walter told me.
THE Alvarezes’ paper, “Extraterrestrial Cause for the Cretaceous-
Tertiary Extinction,” was published in June 1980. It generated lots of
excitement, much of it beyond the bounds of paleontology. Journals in
disciplines ranging from clinical psychology to herpetology reported on
the Alvarezes’ findings, and soon the idea of an end-Cretaceous asteroid
was picked up by magazines like Time and Newsweek. One commentator
observed that “to connect the dinosaurs, creatures of interest to but the
veriest dullards, with a spectacular extraterrestrial event” seemed “like
one of those plots a clever publisher might concoct to guarantee sales.”
Inspired by the impact hypothesis, a group of astrophysicists led by Carl