Sixty-six million years ago, Earth was having a very bad day.
A mountain the size of the impact fell from the sky at kilometers per second, smashing into a shallow sea off what is now Mexico's Yucatan Peninsula. The impact released as much energy as 100 million nuclear bombs, ripping a scar in the Earth's crust 200 kilometers wide and 20 kilometers deep and triggering monstrous earthquakes, tsunamis and firestorms. Global temperatures plummeted and food chains collapsed as plumes of soot and vaporized rock choked the planet blocked the sunleading to the extinction of more than half of the species then in existence, including the dinosaurs. Among the scattered survivors who rose from the ashes were our mammal 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 outlines in the 1970s and 1980s. Alvarez and his colleagues discovered a layer of debris, deposited in 66-million-year-old rocks around the world, that was curiously enriched in elements like iridium, which is vanishingly rare in the Earth’s crust but abundant in asteroids and comets. They eventually linked the layer’s origin to the impact that killed the dinosaurs and its now-submerged giant scar: a place called Chicxulub, which Alvarez dubbed “Crater of Destiny.” But for decades, scientists have debated the finer details: Was the impactor an asteroid rather than a comet, and if so, what kind? Where in the vastness of space did it come from? And could the telltale iridium and the global mass extinction have somehow been home-grown, caused not by an impact but by huge volcanic eruptions that erupted magma containing rare elements from reservoirs in the Earth's mantle?
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Study published V Science August 15 now offers the most definitive answers to date, mapping the deep origins of this epochal event through extremely precise measurements of ruthenium isotopes found in its debris. The work convincingly shows that, like the iridium and other rare elements in the Alvarez layer, this ruthenium did not come from volcanism, but rather came from a clearly extraterrestrial source. And subtle differences in abundance between the isotopes strongly suggest that the Chicxulub impactor that delivered it was not comet or an ordinary giant space rock – it was clearly a “carbonaceous” asteroid, rich in carbon and organic compounds.
“I find these results very compelling,” says Steve Desch, an astrophysicist at Arizona State University who was not involved in the work. “They fit very well with a lot of other evidence.” That evidence includes earlier measurements of other isotopes and minerals in the Alvarez layer, as well as geochemical research from a handful intact fragments of a broken striker that scientists have been able to reconstruct intact from ancient sediments and fossils. Ultimately, Desch says, available evidence suggests that the impact object “was probably not a comet.” However, more detailed isotopic studies of large volumes of intact cometary material (which the researchers have not yet obtained) would be needed to be absolutely certain.
“The interpretation of the new paper is not new,” says geochemist Richard J. Walker of the University of Maryland, who was not involved in the work, citing mainly a 1998 study that said: came to similar conclusions based on chromium isotope analysis. “But this study represents a much more solid determination that the Chicxulub impactor was a carbonaceous asteroid.”
Such asteroids are relatively rare. They are thought to have formed in the outer solar system beyond Jupiter before our world formed, but they only reached the inner solar system’s asteroid belt after being flung there en masse by orbital interactions between giant planets more than 4.5 billion years ago, just a few million years after the sun began to shine. This massive import of organic material from the icy outer solar system, scientists say, may have provided the infant Earth with the essential chemical building blocks of biology, as well as much of the water that now fills its oceans. In a grand, almost poetic sense, the demise of the dinosaurs and the rise of mammals were preordained eons ago by the same process that helped kickstart life on our planet in the first place.
“This collision completely changed the landscape of our planet and led to the emergence of mammals,” says lead author of the new study Mario Fischer-Goedde, a geochemist at the University of Cologne in Germany. “And this 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 can sit here and have this conversation.”
Ruthenium is a silvery metal that, like iridium and other platinum group elements, is almost never found on or near the Earth's surface. This rarity is explained by the fact that they are siderophile, or iron-loving, elements, and when our newly formed world was just partially melted slag ballthey stuck to iron and other dense metals deep inside the Earth, sinking to form its inaccessible core. That means that almost all of the platinum-group elements now in the crust were delivered there later, from meteorites, asteroids, and comets that struck our planet after it cooled. That makes the elements excellent tracers of impact events throughout much of Earth’s history. For the Chicxulub event, Fischer-Goedde says, “we can in principle assume that 100 percent of the ruthenium found in the global boundary layer came from the impactor itself.”
Ruthenium is particularly useful, Fischer-Gödde says, because it offers more isotopes to study than most of its platinum-group relatives. These isotopes, in turn, are linked to different astrophysical production pathways — some, for example, are quickly formed through the supernova explosions of short-lived massive stars, while others are assembled more gradually in the simmering interiors of mid-sized stars. Researchers can calculate the ratios of isotopes that should result from these processes, allowing them to double-check whether any observed changes in abundance are due to such cosmic effects.
All of these isotopes can end up in giant molecular clouds on stardust grains, eventually becoming embedded in the planets, asteroids, and comets that are born when such clouds collapse. form new star systems. And most importantly, about 15 years ago, scientists discovered that asteroids exhibit a curious isotopic dichotomy: The rockier asteroids that formed inside Jupiter have one arrangement of isotopic ratios, while the carbonaceous ones that formed further away show another. In general, this is a recipe for using variations in the abundance of tiny amounts of isotopic material from any given impactor to determine whether it was carbonaceous, and thus to establish the general neighborhood where it formed.
“These stellar nucleosynthetic isotope variations help trace how different parts of the solar system evolved during its early formation,” says James Day, a geochemist at the University of California, Scripps Institution of Oceanography in San Diego, who reviewed the paper. “What’s so exciting is that Mario and his team used these ruthenium isotopes as a fingerprint to determine where the Chicxulub impactor came from.”
For their study, Fischer-Goedde and his colleagues analyzed seven isotopes of ruthenium using a cutting-edge technique called multicollector inductively coupled plasma mass spectrometry. They took samples from the infamous dinosaur-dooming layer at three different sites around the world, as well as from two carbonaceous meteorites and five other impact craters that occurred over the past half-billion years. They also looked for ruthenium in much older rocks, some 3.5 billion years old, that contained debris associated with a string of powerful impacts from that distant time.
“By measuring all seven ruthenium isotopes and checking whether their ratios match the patterns expected from astrophysical processes, we can distinguish and rule out terrestrial effects,” Fischer-Goedde says. “That’s why it’s like a fingerprint — these ratios are set by things like nuclear fusion inside stars, which no process on Earth can reproduce. We measured it, we checked it, and it all lined up… So for the Chicxulub event, our result doesn’t just show that it was a carbonaceous asteroid — it’s also the nail in the coffin of the idea that these platinum group elements came from volcanism or any other terrestrial origin.”
The work was grueling, requiring the painstaking separation of just nanograms of ruthenium from much larger masses of rock where the element existed in very low concentrations, measured in parts per billion.
“For a lot of these samples, it’s about taking a fist-sized piece of material, 20 to 30 grams of rock, and extracting a tiny speck that you probably wouldn’t see without a microscope,” Walker says. “That’s what you have to do to achieve this ridiculously high precision of isotopic measurements.”
“It’s hard work,” admits Fischer-Goedde, noting that he has spent the last decade honing his technique. “I’m German, so I’m usually modest, but I feel comfortable saying I’m the world’s leading expert on this stuff — because it’s so tedious that there are only a few people on the planet doing it.”
All that tedium paid off. Of the impacts studied over the past half-billion years, only Chicxulub showed a distinctly carbonaceous mix of ruthenium isotopes from the outer solar system; the other five showed signs of rocky impactors that originated closer to the sun. The ruthenium ratios from the oldest impacts also suggest carbonaceous impactors, a finding that fits with a wealth of other evidence that Earth was bombarded by material from the outer solar system during the first billion years or so of its history. This, most experts say, arose from a strange dynamic instability that rebuilt the orbits of the giant planets shortly after the solar system formed, sending a stream of impactors toward the sun. Future work, Fischer-Goedde and others say, could include studying ruthenium and additional isotopes from a variety of sources, including comets and lunar craters, to further elucidate key impact events in Earth’s long history and pinpoint the subtype of carbonaceous asteroid that was responsible for the dinosaurs’ demise.
Two obvious questions remain: How did the Chicxulub impactor find its way to Earth billions of years after it was ejected from the outer solar system, and when might the next similarly sized doomsday rock strike? Bill Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, who was not involved in the study, believes he and his colleagues have already found both answers with dynamical modeling presented in paper published in 2021. “We came up with a good dynamic rationale for where the impactor came from — from the central and outer main asteroid belt,” Bottke says. “I would say our work is still at the cutting edge of the state of the art.”
The 2021 paper also calculated that Chicxulub-class objects would only hit Earth once every 250 to 500 million years, suggesting we have a good chance of not experiencing another catastrophic asteroid impact anytime soon.