Sixty-six million years ago Earth had a very bad day.
A mountain-sized impactor fell out of the sky at kilometers-per-second speed, slamming into the shallow sea off what is now Mexico’s portion of the Yucatán Peninsula. The impact released as much energy as 100 million nuclear bombs, gouging a 200-kilometer-wide, 20-kilometer-deep scar in Earth’s crust and unleashing monstrous earthquakes, tsunamis and firestorms. Global temperatures plunged, and food chains collapsed as planet-smothering plumes of soot and vaporized rock blotted out the sun, driving more than half of then extant species—including the dinosaurs—to extinction. The scattered survivors that arose from the ashes included our mammalian ancestors, setting the stage for a new era of life on Earth.
This planetary cataclysm remained shrouded in mystery until one of those survivors’ distant descendants, physicist Walter Alvarez, pieced together its outline in the 1970s and 1980s. Alvarez and his colleagues discovered a layer of debris laid down in 66-million-year-old rocks around the world that was curiously enriched with elements such as iridium, which is vanishingly rare in Earth’s crust but abundant in asteroids and comets. They eventually linked the layer’s origin to the dinosaur-killing impact and its now submerged giant scar: a site called Chicxulub, which Alvarez dubbed the “Crater of Doom.” But for decades, scientists have debated finer details: Was the impactor an asteroid instead of a comet—and if so, what type? Where in the vastness of space had it come from? And could the telltale iridium and global mass extinction alike have been somehow home-brewed, arising not from the impact but rather from immense volcanic eruptions that spewed rare-element-laced magma from reservoirs in Earth’s mantle?
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A study published in Science on August 15 now offers the most definitive answers yet, charting this epochal event’s deep origins through extremely precise measurements of isotopes of ruthenium found in its debris. The work conclusively shows that, like the iridium and other rare elements in Alvarez’s layer, this ruthenium did not come from volcanism but rather originated from a clearly extraterrestrial source. And subtle variations in abundance between the isotopes strongly suggest the Chicxulub impactor that delivered it wasn’t a comet or a run-of-the-mill giant space rock—it was a conspicuously “carbonaceous” asteroid, rich in carbon and organic compounds.
“I find these results very convincing,” says Steve Desch, an astrophysicist at Arizona State University, who was not involved with the work. “They dovetail nicely with lots of other evidence.” That evidence includes earlier measurements of other isotopes and minerals in Alvarez’s layer, as well as geochemical studies of a handful of pristine shards of the shattered impactor that scientists have managed to recover intact from ancient sediments and fossils. On balance, Desch says, the available evidence suggests the impactor “probably wasn’t a comet.” But absolute certainty would require more detailed isotopic studies of sizable amounts of pristine cometary material (which researchers have not yet obtained).
The new paper’s “interpretation is not new,” says geochemist Richard J. Walker of the University of Maryland, who is unaffiliated with the work, chiefly referring to a 1998 study that reached similar conclusions based on analyzing an isotope of chromium. “But this study presents a much more ironclad determination that the Chicxulub impactor was a carbonaceous asteroid.”
Such asteroids are relatively rare. Thought to have formed in the outer solar system beyond Jupiter before our world itself coalesced, they only reached the inner solar system’s asteroid belt after being hurled there en masse by orbital interactions among the giant planets more than 4.5 billion years ago, just a few million years after the sun began to shine. That bulk import of organic material from the icy outer solar system, scientists say, may have provided the newborn Earth with the crucial chemical building blocks of biology, as well as much of the water that now fills its oceans. In a grand, almost poetic sense, the doom of the dinosaurs and the dawn of mammals were set eons ago by the very same process that helped kick-start life on our planet in the first place.
“This impact totally changed the picture of our planet and caused the emergence of mammalian life,” says the new study’s lead author Mario Fischer-Gödde, a geochemist at the University of Cologne in Germany. “And it follows from a sequence of events that began in the very early days of the solar system so that, more than four billion years later, you and I are able to sit here having this conversation.”
Ruthenium is a silvery metal that, like iridium and other “platinum group” elements, is scarcely found at all on or near Earth’s surface. This scarcity is because these are siderophile, or iron-loving, elements, and when our fresh-formed world was little more than a partially molten ball of slag, they glommed on to iron and other dense metals in Earth’s interior, sinking to form its inaccessible core. That means almost all the platinum group elements that are now in the crust were delivered there later, from meteorites, asteroids and comets that struck our planet after it had cooled. This makes the elements excellent tracers of impact events throughout most of Earth’s history. For the Chicxulub event, Fischer-Gödde says, “we can basically assume that 100 percent of the ruthenium found in the global boundary layer is from the impactor itself.”
Ruthenium is especially useful, Fischer-Gödde says, because it offers more isotopes to examine than most of its platinum-group kin. In turn, these isotopes are associated with different astrophysical production routes—some, for instance, form rapidly via supernova explosions of short-lived massive stars while others assemble more gradually in the slow-simmering innards of midsize stars. Researchers can calculate the ratios of isotopes that should emerge from these processes, allowing them to cross-check whether any observed variations in abundance are linked to such cosmic effects.
All these isotopes can find their way into giant molecular clouds on flecks of stardust, ultimately incorporating into the planets, asteroids and comets that are born when such clouds collapse to form new stellar systems. And most importantly, about 15 years ago scientists discovered that asteroids display a curious isotopic dichotomy: the more stony asteroids that formed inward of Jupiter bear one arrangement of isotopic ratios, while the carbonaceous ones that formed farther out exhibit another. Altogether, this is a recipe for using abundance variations in minuscule amounts of isotopic material from any given impactor to determine whether it was carbonaceous—and thus establish the general vicinity where it formed.
“These stellar nucleosynthetic variations in isotopes help trace how different parts of the solar system evolved during its earliest formation,” says James Day, a geochemist at the University of California, San Diego’s Scripps Institution of Oceanography, who was a reviewer for the paper. “What’s so exciting is that Mario and his team have used these ruthenium isotopes as a fingerprint to identify where the Chicxulub impactor came from.”
For their study, Fischer-Gödde and his colleagues assayed seven ruthenium isotopes in all using an advanced technique called multicollector inductively coupled plasma mass spectrometry. They sampled from the infamous dinosaur-dooming layer at three different sites around the globe, as well as from two carbonaceous meteorites and five other craters from different impacts that occurred across the past half-billion years. Additionally, they sought ruthenium from far more ancient rocks, circa 3.5 billion years old, which contained debris associated with a flurry of massive impacts from that far-distant time.
“By measuring all seven ruthenium isotopes and checking if their ratios match the patterns expected from astrophysical processes, we can distinguish and rule out terrestrial effects,” Fischer-Gödde says. “That’s why this is like a fingerprint—these ratios are set by things like thermonuclear fusion inside stars that no process on Earth can replicate. We measured, we checked, and it all lines up.... So, for the Chicxulub event, our result isn’t just showing it was a carbonaceous asteroid—it’s also the nail in the coffin for the idea that these platinum-group elements came from volcanism or any other terrestrial origin.”
The work was grueling, requiring the painstaking separation of mere nanograms of ruthenium from much larger masses of rock where the element existed in very low parts-per-billion concentrations.
“For many of these samples, we’re talking about taking a fist-sized piece of material, 20 to 30 grams of rock, and extracting a little speck you probably can’t see without a microscope,” Walker says. “That’s what you have to do to reach this ridiculously high precision of isotopic measurement.”
“It’s hard work,” Fischer-Gödde acknowledges, noting that he has spent the past decade honing his technique. “I’m German, and so I’m normally humble, but I’m comfortable saying I’m the world’s leading expert in this—because it’s so tedious, there are only a few people on the planet doing it.”
All that tedium has paid off. Of the studied impacts from the past half-billion years, only Chicxulub showed a distinctly carbonaceous, outer-solar-system mix of ruthenium isotopes; the other five bore signatures of stony impactors that originate closer to the sun. The ruthenium ratios from the most ancient impacts suggest carbonaceous impactors, too—a finding that is consistent with a wealth of other evidence that the Earth was bombarded with material from the outer solar system during the first billion years or so of its history. This, most experts say, arose from a bizarre dynamical instability that rearranged the orbits of the giant planets shortly after the solar system formed, sending showers of impactors toward the sun. Future work, Fischer-Gödde and others say, could involve studying ruthenium and additional isotopes from a variety of sources—including comets and lunar craters—to further clarify key impact events in Earth’s long history and to pinpoint the exact subtype of carbonaceous asteroid that was responsible for the dinosaurs’ demise.
Two obvious questions that remain are how the Chicxulub impactor found its way to Earth billions of years after it was ejected from the outer solar system—and when the next similarly sized doomsday rock might strike. Bill Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colo., who was not part of this study, believes that he and his colleagues already found both answers via dynamical modeling that was presented in a paper published in 2021. “We came up with a good dynamical rationale for where the impactor came from—the central to outer main asteroid belt,” Bottke says. “I would say our work is still the state of the art.”
That 2021 paper also estimated that Chicxulub-class objects should hit Earth just once every 250 million to 500 million years—suggesting that we have good odds for not facing another cataclysmic asteroid impact any time soon.