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About 66 million years ago the non-avian dinosaurs disappeared, along with a large share of life on Earth. For a long time nobody could say why. The answer that most scientists now accept began with a father and son asking a much smaller question. This post covers who the Alvarezes were, how a plan to time a clay layer turned into an asteroid hypothesis, a worked estimate of the asteroid's mass and energy, and what a student can learn from how they reasoned.
Who were Luis and Walter Alvarez?
Luis Alvarez was a physicist at the University of California, Berkeley. He won the 1968 Nobel Prize in Physics for contributions to particle physics, including the discovery of many resonance states using the hydrogen bubble chamber. His son Walter Alvarez was an associate professor of geology at Berkeley, as the author list of their 1980 paper records.
A 1979 Berkeley Lab news article explains how they came together. Walter had spent several years dating limestone layers near Gubbio, Italy, using magnetic stratigraphy: reversals of Earth's magnetic field are recorded in rocks and can be used to date them. Between the Cretaceous and Tertiary limestones lies a thin band of clay, laid down at the time of the extinctions. Fossils of tiny marine organisms are common below it and absent from it.
How did a question about time lead to an asteroid?
Luis wanted to know how long the clay layer had taken to form. He proposed iridium as a clock. Iridium is rare in Earth's crust, but it arrives steadily from space in meteoritic dust. If the dust falls at a known rate, the amount of iridium in a layer tells you how long that layer took to collect.
Frank Asaro and Helen Michel, nuclear chemists at the lab, measured iridium and many other elements by neutron activation analysis. The 1979 article reports that the clay held far more iridium than expected, which was not what a steady clock should show.
The team then tested the alternatives. Could the clay simply have built up very slowly, letting ordinary dust accumulate? Walter's magnetic dating showed no significant slowdown in sedimentation. Could the iridium have come from Earth? Iridium is one of the least abundant elements in the crust, so an extraterrestrial source fitted better.
Their paper, Extraterrestrial Cause for the Cretaceous-Tertiary Extinction, appeared in Science on 6 June 1980. Its abstract reports:
Deep-sea limestones in Italy, Denmark and New Zealand show iridium increases of about 30, 160 and 20 times background, at exactly the boundary.
The iridium is extraterrestrial but did not come from a nearby supernova.
An earth-crossing asteroid would throw about 60 times its own mass into the atmosphere as pulverised rock. Some would stay in the stratosphere for years, darkening the sky and suppressing photosynthesis.
Four independent estimates put the asteroid's diameter at 10 ± 4 km.
Worked example: how big was the impact?
The paper gives the diameter; you can estimate the rest. Assume a rocky density of 3,000 kg/m³ and an impact speed of 20 km/s. These two values are assumptions for the exercise, not figures from the paper.
Radius. Diameter 10 km, so r = 5 km = 5 × 10³ m.
Volume. V = (4/3)πr³ = (4/3) × 3.1416 × 1.25 × 10¹¹ m³ ≈ 5.24 × 10¹¹ m³.
Mass. m = ρV ≈ 3,000 × 5.24 × 10¹¹ ≈ 1.57 × 10¹⁵ kg.
Dust. The paper's factor of about 60 gives 60 × 1.57 × 10¹⁵ ≈ 9.4 × 10¹⁶ kg of rock thrown up.
Kinetic energy. KE = ½mv² = 0.5 × 1.57 × 10¹⁵ × (2 × 10⁴)² ≈ 3.1 × 10²³ J.
Common mistakes:
Using the diameter as the radius. That makes the volume, and every later number, 8 times too large.
Converting units after cubing. Change km to m first; 5 km cubed is 125 km³, which is 1.25 × 10¹¹ m³, not 1.25 × 10⁵.
Forgetting the half in ½mv².
Where is the crater?
If an asteroid that size hit, it should have left a crater. In 1991 Hildebrand and colleagues proposed Chicxulub, on Mexico's Yucatán Peninsula, as a possible boundary impact crater. A 2010 review by 41 researchers, Schulte and colleagues in Science, concluded that the global iridium-rich layer and impact debris support a single asteroid impact at Chicxulub as the trigger of the mass extinction.
The dates have also sharpened. The 1980 paper used 65 million years. Renne and colleagues (2013) dated the layers around 66 million years ago and showed the impact occurred within 33,000 years of the extinction. For how mass extinctions fit into the bigger picture, see our post on evolution for NEET.
What can a student take from the Alvarezes' way of working?
The story is a model of reasoning from evidence. A physicist brought a measuring tool; a geologist brought the rocks and the dating. When the result was odd, they listed the other explanations and ruled each out with data before proposing the asteroid. And their hypothesis made a prediction, a crater, that others later found.
You can use the same structure on a tricky question:
Ask a measurable question. "How long did this take?" is easier to answer than "Why did this happen?"
List alternatives. Before you accept an answer, write two other explanations and what would rule them out.
Estimate. Put numbers on the idea, as in the impact example, to see if it is even possible.
For order-of-magnitude problems like the impact estimate, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
Redo the impact estimate tonight for a 6 km asteroid, and compare the energy with the 10 km case.
