77. Iridium: T. Rex And The Terrible, Horrible, No Good, Very Bad Day

Alvarez noticed a great disturbance in the rocks, as if millions of voices suddenly cried out in terror and were suddenly silenced.

Featured above: “Xandar, our reign has gone on long enough.” “Indeed. Summon the meteors.”

Show Notes

First things first, since last episode, Podyssey included The Episodic Table Of Elements on their list of the 13 Best Science Podcasts Of All Time. That is some high praise, to say the least, and some incredible programs to be mentioned alongside! Thanks to them and to you for listening.

I found two separate dates for Kepler’s calculation of the age of the Earth: 3992 BC, and 4977 BC. You can see these in the references for that section of the script. Those are two pretty different dates! I was unable to find an English translation of the primary source (Mysterium Cosmographium) that contained this information, but lots of secondary sources are in agreement that he did do the work.

Speaking of sources, one stood out this episode. Walter Alvarez wrote a book detailing his years-long geological investigation: T. Rex And The Crater Of Doom. It goes into much more detail than I could here (obviously), and honestly it should be required reading in any introduction geology course. Alvarez does a great job laying the groundwork (if you’ll pardon the pun) for a solid geology education.

Luis Alvarez is in a rare club of people who’ve witnessed the first two atomic explosions, but only one person saw the first three. Lawrence Johnston was one of Alvarez’s close colleagues, doing the same work measuring the energy output. Alvarez stayed on the ground for that third flight, though.

More incredible, in my opinion, is the story of Tsutomu Yamaguchi, an engineer for Mitsubishi. He was in Hiroshima on business the morning of August 6, 1945, and bore witness to the first wartime atomic detonation. He returned home to Nagasaki and actually went back to work the next day. His boss was in the middle of calling him crazy for his description of the first atomic bomb… when the second one went off over this city, too.

He not only survived both, but actually lived to the age of 93. And, presumably, got to say “I told you so.”

Sadly, Luis Alvarez didn’t live to see the paper about the Chichxulub crater. He died in 1988, just a couple years before its publication.

Very recently, Harvard astronomers argued that it was probably a comet, not an asteroid, that landed at Chichxulub. That helps for the theatrical aspect of the impact, although I doubt the dinosaurs cared very much about that.

And finally, Krakatoa Frankenstein — a great name for a band. I must apologize, though, because it was actually Tambora, not Krakatoa, that led to Frankenstein’s creation. See, “Year Without A Summer” was so unpleasant that a group of youngsters on Lake Geneva couldn’t play outside. Instead, they stayed indoors and told each other ghost stories — and the rest is history.

Episode Script

Satellites and spacecraft are often powered by solar panels, but when heading to the outer reaches of our solar system, that’s not really an option. It’s simply too dark out there for solar panels to provide enough energy. Mission planners need to provide some other power source — ideally one that’s constant, low-maintenance, and lasts for years.

For these situations, it’s hard to beat the RTG: the Radioisotope Thermoelectric Generator. It’s an impressively simple device. It’s based on a lump of material like plutonium-238, something so wildly radioactive that it actually releases a fair bit of heat. That’s surrounded by a bunch of devices called thermocouples, which convert heat energy into electricity. And… that’s it. It has no moving parts, and as a bonus, it can also keep a probe warm enough to continue operating in the cold vacuum of space.1

One minor issue: Space is just lousy with little rocks flying around as fast as bullets, and they can cause a lot of damage. The RTG is protected from this interplanetary hailstorm with a thick shell of extremely hard, durable material — iridium.

This design is extremely reliable. Since its invention in 1954, RTGs have provided the beating hearts of our most successful robotic explorers, including the Pioneer, Viking, and Voyager probes; New Horizons, Cassini, and the Mars Curiosity rover.2 3 4 There’s also one at the core of the Red Planet’s newest, arrival, Perseverance, which just made planetfall this month.

Quite an impressive accomplishment for element 77. But when it comes to iridium-clad rocks hurtling through space, there’s one we haven’t mentioned yet that blows everything else away.

You’re listening to The Episodic Table Of Elements, and I’m T. R. Appleton. Each episode, we take a look at the fascinating true stories behind one element on the periodic table.

Today, we’re learning about the huge impact made by iridium.

Iridium is the second half of Smithson Tennant’s chemical double-feature, discovered at the same time as osmium.5 While he named that element for its smell, a different trait caught his eye with this one. He wrote,

As it is necessary to give some name to bodies which have not been known before, and most convenient to indicate by it some characteristic property, I should incline to call this metal Iridium, from the striking variety of colours which it gives, while dissolving in marine acid.”6

Along with “iris” and “iridescent,” “iridium” traces its roots to the Greek word iridos, meaning a rainbow or something similarly colorful. While iridium’s oxides are quite vibrant, as a pure metal, it’s a lustrous silvery color that’s rather typical of metals.

Iridium resembles its platinum group neighbors in other ways, too. It’s  extremely dense, highly resistant to corrosion, and difficult to melt. It’s also really rare, accounting for only one part per billion of the Earth’s crust.7

Why, then, does a thin layer of ancient iridium dust cover the entire planet?

James Ussher was a man who took the Bible very seriously. As he should have. He was Church of Ireland archbishop in the early 17th century. He was also something of a scholar, possessing extensive knowledge of not only theology, but also ancient history, astronomy, and numerous languages. He combined these many passions with the 1650 publication of Annales Veteris Testamenti, his calculation of the age of the Earth using the Bible as a timetable.8

He examined portions of the Bible that most people would rather skip — all the bits about who begat whom, and which kings’ reigns lasted how long, and esoteric particulars of the Jewish calendar. Using this information, he determined that God created the Earth at precisely 6:00pm on October 23, 4004 BC.

Knowing everything we know today, it may be hard to suppress a reflexive smirk at the idea that the world sprang into existence at a particular moment in time based on one Bronze Age family’s genealogy. Indeed, it’s not difficult to find people mocking Ussher’s work today, especially when October 23rd rolls around, but Stephen Jay Gould would caution against such chauvinism of the present.

Gould was a widely renowned paleontologist, biologist, and writer, who worked at Harvard, New York University, and the American Museum Of Natural History in the latter half of the twentieth century. By no stretch was he any kind of Christian apologist — he was vocally agnostic, and advocated that science and religion represented “Non-Overlapping Magisteria;” i.e., entirely different realms of thought seeking to answer different kinds of questions. Nonetheless, in 1991, he wrote an article defending Ussher’s competence and intellect. “I shall be defending Ussher’s chronology,” Gould wrote, “as an honorable effort for its time and arguing that our usual ridicule only records a lamentable small-mindedness based on mistaken use of present criteria to judge a distant and different past.”

Contemporaries of that “distant and different past” certainly did not find Ussher’s work laughable. Plenty of others had performed the same intellectual exercise, including Johannes Kepler and Isaac Newton, but Ussher’s work was the most popular.9 10 11 12 13No one needed to come to Ussher’s defense back then, because his work was seen as so authoritative that it was included in printings of the King James Bible.

More than a century would pass before James Hutton put forth the first solid argument for an older Earth. Hutton was a Scottish farmer and a particularly keen observer of the natural world. He witnessed the wind and weather and came to see the world as a place of continuous yet gradual change, often so slow as to be imperceptible. He envisioned the Earth’s geology as a cycle, driven by subterranean heat, in which rocks were driven to the highest mountain peaks and swept beneath the ocean’s waves. In a 1788 book called Theory Of The Earth, Hutton wrote, “The result, therefore, of this physical enquiry is that we find no vestige of a beginning, no prospect of an end.”14

He proposed that these forces have been essentially constant throughout history, and called his theory Uniformitarianism. It was revolutionary in its own right, as influential as the theories of heliocentrism or natural selection. The older idea that sudden events could cause grand, sweeping changes — like, for instance, a cataclysm brought on by worldwide flood — became known as Catastrophism.15 16

As scientists learned more about natural history, Catastrophism was pushed ever further from the realm of acceptable science. The story of the Earth was seen exclusively as one of slow, gradual change.

This was the set of assumptions Walter Alvarez had to work with in the 1970s, while studying a stretch of sedimentary rock that sure seemed to show evidence of a sudden, drastic catastrophe.

Alvarez was a newly minted American geologist working in Gubbio, a village in the Italian countryside. Just outside town, there’s a treasure trove of exposed sedimentary rock — 400 meters of it, which provides about 50 million years worth of geological history.17

He was investigating fossils of single-celled organisms known as foraminifera, or “forams” for short. Forams are extremely prolific little things, and have been for hundreds of millions of years. While vertebrate fossils are pretty rare, the earth outside Gubbio is loaded with forams.

There was something peculiar about these strata, though. Alvarez found a distinct layer of clay one centimeter thick that was completely devoid of fossils. Below that line, the foraminifera fossils were large and came in many forms. In the younger rock above the line, the forams were relatively small and not very diverse at all.

His interest was piqued. What might cause such a change, and how long could it have taken?

Based on the forams, certain varieties of rocks, and the presence of other fossils, this clay must have been deposited about 65 million years ago, precisely when the Cretaceous period came to an end and the Paleogene began. This also happens to be the occasion of one of the largest mass extinctions in history — certainly the most famous. That was when the dinosaurs died.

The disappearance of the dinosaurs was one of the greatest mysteries in all of science, but Alvarez was the first to notice this coincidental geological oddity. He knew he was on to something big.

The first thing he needed to determine was how long it took for this thin layer of clay to form. If it was deposited over a long time, that would suggest a slow, drawn-out extinction event. A short period of time could indicate the opposite.

At no point was Walter Alvarez working alone. The entire time, he was collaborating with scientists all over the world — Jan Smit and Bill Lowrie and Al Fischer and many others. But at this point, he needed to enlist the help of one specific and unusually familiar recruit: His dad.

Luis Alvarez had been a highly accomplished physicist. He had worked with Ernest Lawrence and was involved in the Manhattan Project. He designed the detonators for the atomic bombs, and as one of the scientists measuring the bombs’ energy output, he was a witness to both the Trinity explosion in New Mexico and the detonation over Hiroshima. He later went on to win a Nobel Prize for developing new ways to track charged particles moving through space.

By the late 1970s, he basically considered his very accomplished career to be over, but at his son’s request, he agreed to do one last job. Luis possessed the kind of expertise that Walter thought could help the team learn how quickly the clay was deposited.

Together, they came up with a lot of ideas. Luis first suggested trying to date the rock using beryllium-10, which would work a lot like carbon dating. Beryllium-10 falls upon the Earth at a relatively constant rate when cosmic rays collide with nitrogen and oxygen in the atmosphere. Measuring the amount of remaining radioactive beryllium could reveal how long it had taken the clay layer to form. And wouldn’t you know, Luis just happened to know a guy with a cyclotron — an atom-smashing machine that could perform the task.

It looked pretty promising until some new information came to light: Beryllium-10’s half life was not two and a half million years, as had been previously reported. It was actually more like one and a half million years. For the work they wanted to do, that was more than a mere inconvenience. That meant that any beryllium-10 that fell to Earth 65 million years ago would be all dried up by now. It would be immeasurable.

But the idea wasn’t worthless, for it sparked another: In the same way that Earth is struck by an essentially constant stream of cosmic rays generating beryllium-10, it’s also hit by a steady supply of space rocks. Nearly fifty tons of space debris lands on Earth every day. Some of this takes the form of meteors large enough to burn a trail of light across the sky, but mostly it’s just dust. Lots and lots of space dust.

There are plenty of differences between space rocks and Earth rocks, allowing for easy identification. For instance: Certain heavy elements are rare in the crust, because early in our history, they would have sunk into the molten depths of the Earth. Asteroids, which are basically unchanged since they first formed, are comparatively high in those heavy elements. Iridium is one such element. Walter and Luis reasoned that if the clay bed contained appreciable iridium, it must have taken a long time to form as meteors fell to Earth over thousands of years. If the layer lacked iridium, that might indicate that it had formed much more quickly.

They collected samples and shipped them off to Frank Asaro at Lawrence Berkeley Laboratory. It took him a long time to complete the work. The Alvarezes Elder and Younger waited patiently for nine months, and when they finally heard back, Asaro sounded like a doctor with an unfortunate diagnosis: He wanted Luis and Walter to come into his office so they could discuss the results in person.

They had predicted the soil might contain iridium at a rate around 0.1 parts per billion. Asaro detected iridium levels thirty times higher.

That was anomalous, to say the least. Before jumping to any conclusions, they needed to make sure this wasn’t just something strange about the rocks in Italy. Enlisting help from a worldwide community of scientists, they tested samples from Denmark, Spain, New Zealand — and each time they found the same thing. In fact, one of them contained iridium at 160 times the background level. It was a verified global phenomenon. Something happened 65 million years ago that spread iridium far and wide.

The Alvarezes figured the iridium must have come from outer space somehow. Perhaps a nearby stellar explosion showered Earth with heavy elements, or maybe the solar system had passed through a gas cloud, or maybe it was Jupiter’s fault, somehow. Or maybe… a colossal asteroid had slammed into the Earth?

No, they thought, that didn’t make any sense. Sure, that could devastate the region around the impact, even for thousands of kilometers. But worldwide? Inconceivable.

Well… maybe.

Prodded on by another colleague at Berkeley, Luis remembered the 1883 explosion of Krakatoa, the Indonesian volcano. It was one of the most violent events in recorded history, causing a noise that was heard over 3,000 kilometers away and spewing ash 80 kilometers high. Dust from the explosion spread throughout the atmosphere, causing brilliant sunsets on the opposite side of the Earth and temperatures to drop worldwide for over a year. Luis also recalled that a large-scale atomic war might trigger nuclear winter for the same reasons.

Perhaps the impact of an asteroid or comet could cause the same thing to happen, robbing plants of the sunlight they needed to live and disrupting ecologies across the globe. That could certainly trigger a mass extinction, but the impact would have to be very big.

He estimated that an appropriately sized rock must be around 300 billion metric tons. Probably ten kilometers wide, releasing 100 million times more energy than the largest atomic bomb.

From the foraminifera to the iridium, it all amounted to a hypothesis that made a lot of sense, which the team published in a 1980 issue of the journal Science. Their work was well founded, but they worried about how it might be received. After all, this was exactly the kind of catastrophe that geologists found laughable.

Besides, there was one very conspicuous hole in their theory: Where, exactly, was the big hole in the ground?

Alvarez et. al. had considered that, of course. They had no idea where the crater made by the proposed impact might be found. Heck, maybe the impact happened over the open ocean, in which case no one would ever find a crater. That didn’t make the theory any less plausible.

It had taken years for the team to turn all their disparate ideas into a cohesive theory of armageddon triggered by deep impact. Actually finding the site of that event would have to be an entirely separate endeavor — one that would continue for more than a decade after the Alvarez team published their paper.

Teams all over the world were on the hunt, but much of the research ultimately yielded nothing. It was a long time before a glimmer of hope turned up in a Texas riverbed, where geologists discovered evidence of an ancient, colossal tsunami, as well as little beads of glass called tektites, which form when molten ejecta cool as they fall from the sky.

Further evidence helped narrow the search down to the area around the Caribbean, and another team realized that a Mexican oil company might have actually discovered the crater several years earlier. In 1991, they published a paper detailing a crater on Mexico’s Yucatan Peninsula, buried beneath what’s now the village of Chicxulub.

Direct study of the area’s rocks indicated that they were precisely the right age — 64.98 plus or minus 0.05 million years. And the crater was the right size, too — 150 kilometers wide.

It was enough evidence to tell the story of one very bad day. Possibly the worst day in the history of life. Top three, at least.

If it was an asteroid that fell to Earth, it would have struck without warning. If it was a comet, though, it would have announced its arrival with a tail of volatile gas that lit up the sky for days. Either way, the mass was the size of San Francisco and came crashing down 150 times faster than a passenger jet. Unable to get out of the way fast enough, the air was compressed and heated to a temperature four times hotter than the sun. One second later, the space rock vaporized completely, leaving a flaming wound in the Earth forty kilometers deep. A spreading fireball of limestone sterilized the landscape with light and fire for hundreds of kilometers around the impact site. Chicxulub is and was right on the shore, and the collision would have issued forth a wall of water one kilometer high. Molten fragments of rock flew halfway to the moon before falling back down, covering the Earth for years with a thick layer of ash, dust, and iridium.

A lot has happened since then, so you needn’t go digging through 65 million years of dirt to add element 77 to your collection. There are richer sources more readily available.

Every platinum group metal has been used as the tip of a pen at some point or another, and this is no exception.18 Obviously, it is the other half of the Osmiridium alloy mentioned last episode, but the Centropen 2156, the Hero 329, the Moonman M600S, and many others also include a tiny little bit of today’s metal.

Iridium also provides the tip of another implement: spark plugs for automobiles. Manufacturers claim that this imparts all sorts of advantages, like better fuel economy, faster acceleration, and more engine power.19 Apparently, the 2004 Toyota Sienna minivan will accept nothing less than iridium-tipped spark plugs.20 That said, these claims haven’t been rigorously tested, so take them with a grain of salt.

Of course, that doesn’t matter for our purposes. These things could turn a car engine to dust — so long as you find one that actually is tipped with iridium, it’ll do just fine for your element collection.

Thanks for listening to The Episodic Table of Elements. Music is by Kai Engel. To learn how Frankenstein’s monster emerged from the Krakatoa explosion, visit episodic table dot com slash I r.

Next time, we’ll raise our standards with platinum.

Until then, this is T. R. Appleton, reminding you, if you missed it last week,21 the Eiffel Tower-sized asteroid Apophis will reappear in the sky on Friday the 13th, April, 2029 — hopefully not too close.22


  1. NASA Jet Propulsion Laboratory, Radioisotope Thermoelectric Generators (RTG).
  2. NASA Cassini, Radioisotope Thermal Generators (RTGs). Last updated September 25, 2018.
  3. NASA, Radioisotope Power Systems.
  4. NASA Mission And Spacecraft Library, Pioneer 10, 11.
  5. Platinum Metals Rev., 1961, 5, (4), 146. The Discovery Of Iridium And Osmium. Donald McDonald.
  6. Philosophical Transactions Of The Royal Society Of London p. 414, On Two Metals, Found In The Black Powder Remaining After The Solution Of Platina. Smithson Tennant, 1804.
  7. Encyclopedia Britannica, Iridium. last updated November 6, 2019.
  8. Encyclopedia Britannica, James Ussher. Last updated January 1, 2021.
  9. Isaaci Newtoni Opera Quæ Exstant Omnia, Volume 5, p. 7.. Isaac Newton, 1785.
  10. The Culture Of Astronomy: Origin Of Number, Science, Law, And Religion, p. 268. Thomas Karl Dietrich, 2011.
  11. Chronology And Geography, p. 256-257.. William Hales, 1830.
  12. A Glance At The Universe, this page. Cleofas Uchoa, 2018.
  13. Experimenting On A Small Earth, p. 63. William W. Hay, 2012.
  14. Encyclopedia Britannica, James Hutton. Last updated May 30, 2020.
  15. National Geographic Resource Library, Uniformitarianism. Last updated January 27, 2020.
  16. Encyclopedia.com, Earth Science: Gradualism And Catastrophism.
  17. Nautilus, The Day The Mesozoic Died. Sean B. Carrol, January 21, 2016.
  18. PeriodicTable.com, Iridium. Theodore Gray.
  19. Green Garage, 13 Advantages And Disadvantages Of An Iridium Spark Plug. Brandon Miller, February 14, 2020.
  20. PeriodicTable.com, Iridium. Theodore Gray.
  21. Pix11, Apophis Asteroid Will Be Visible From Earth This Weekend. Aprylete Russell, February 21, 2021.
  22. Space.com, Huge Asteroid Apophis Flies By Earth On Friday The 13th In 2029. A Lucky Day For Scientists. Meghan Bartels, May 1, 2019.

3 Replies to “77. Iridium: T. Rex And The Terrible, Horrible, No Good, Very Bad Day”

  1. I’m curious if you had candidatesfor the other top 2 worst days in the history of life on earth… my best guess would be a eruption of Yellowstone or one of the lesser known super volcanoes.

    1. Nothing officially, but that’s exactly the kind of thing I was leaving wiggle room for! It’s possible that Earth could have been bathed in ozone-destroying gamma rays to kick off the Ordovician extinction, and those phenomena can last for only a few seconds. There have also (probably) been a few other huge impacts like the one at Chicxulub, and like you mentioned, supervolcanoes are no joke! Apparently one of the best examples is at the Siberian Traps.

      Also, I have to shout out an honorable mention to the possible impact of Theia, a Mars-sized body that may have collided with the Earth and led to the formation of the Moon. Such a world-ending event truly exceeds the capacity of my imagination. But that probably happened 4.5 billion years ago, and it’s almost certain that life had not yet arisen by that time.

  2. Well, the earliest days of the Earth are call the Hadean Eon for a reason(though, comparisons to the Christian Hell than the Greek Hades(talking about the underworld, not it’s eponymous Olympian overseer), but yeah, pretty sure the Earth was a few hundred million years into the Archean Eon by the time of abiogenesis and LUCA, but yeah, no telling how many events qualified for Terrible, Horrible, No good, very bad day status for entire ecosystems, biomes, or continents and have had their traces well hidden by millions of years.

    Also, that Yamaguchi guy had some incredible luck… not sure if it was bad or good, but it was certainly incredible.

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