Researchers Narrow Down the Type of Meteorite that Killed the Dinosaurs
Approximately 66 million years ago, one of the most catastrophic events in Earth's geological history unfolded in a matter of hours. During the boundary between the Cretaceous and Paleogene periods — an interval geologists designate as the K–Pg boundary — a massive extraterrestrial impactor slammed into what is now the Yucatán Peninsula of southern Mexico, triggering an extinction-level event (ELE) that wiped out approximately three-quarters of all plant and animal species on Earth, including the non-avian dinosaurs. This cataclysm reset the evolutionary clock of life on our planet, ultimately paving the way for the rise of mammals — and eventually, humans.
Scientists have long named the impactor the Chicxulub meteorite, after the nearby town of Chicxulub Pueblo. The evidence of its arrival is unmistakable: a 180-kilometer-wide (112-mile) crater lies buried beneath the surface of the Gulf of Mexico and the Yucatán, and thousands of water-filled limestone sinkholes — known as cenotes — trace a near-perfect arc above the crater's rim. Now, an international team of researchers has taken a significant step forward in answering a question that has tantalized scientists for decades: exactly what kind of space rock caused all of this?
A Rare and Primitive Type of Space Rock
According to the latest findings, published in the peer-reviewed journal Science Advances, the Chicxulub impactor may have been a rare variety of space rock known as a carbonaceous chondrite of the Ornans class — or a CO chondrite. The study was led by postdoctoral researcher Georgy V. Makhatadze and colleagues at the Institut de Physique du Globe de Paris (IPGP), in collaboration with researchers from the Vrije Universiteit Brussel (VUB), the Pacific Center for Isotope and Geochemical Research (PCIGR) at the University of British Columbia (UBC), and the Department of Lithospheric Research at the University of Vienna.
Asteroids and other "space rocks" are essentially primordial material left over from the formation of the Solar System, roughly 4.5 billion years ago. These objects range from tiny grains of dust to bodies hundreds of kilometers across. They regularly enter Earth's atmosphere, with most burning up as meteors and others exploding in mid-air in events known as airbursts. However, larger impactors — like the Chicxulub object, estimated at 10 to 15 kilometers (6 to 9 miles) in diameter — occasionally reach the surface. Such impacts generate enormous explosions that eject colossal quantities of material into the stratosphere, blocking sunlight, halting photosynthesis, and triggering a prolonged period of global cooling often described as a "nuclear winter."
The Power of Nickel Isotope Analysis
The key to the team's discovery lies in an elegant geochemical technique: high-precision nickel isotope analysis. Different classes of meteorites carry subtly distinct isotopic "fingerprints" — variations in the ratios of isotopes such as ⁶²Ni and ⁵⁸Ni — that reflect the unique conditions under which they formed in the early Solar System. By measuring these isotopic signatures in clay samples deposited at the K–Pg boundary layer, researchers can compare them against the known isotopic profiles of meteorite types catalogued over many decades.
Makhatadze and his team gathered K–Pg boundary clay samples from multiple sites around the world over several years. These thin clay layers, often just a centimeter or two thick, are a global geological calling card of the Chicxulub impact — they are enriched in iridium and other platinum-group elements that are rare in Earth's crust but common in certain types of asteroids. This iridium anomaly, first identified by physicist Luis Alvarez and his geologist son Walter Alvarez in 1980, was the original evidence that led scientists to propose an extraterrestrial cause for the K–Pg extinction. The new nickel isotope study builds on this foundation with far greater precision.
By comparing the nickel isotopic composition of the boundary clay to a comprehensive library of meteorite samples, the researchers concluded that the data best matched a CO carbonaceous chondrite. This points toward an origin in the outer region of the Main Asteroid Belt, or potentially more distant reservoirs such as the Kuiper Belt or other debris-rich zones of the outer Solar System.
Why a CO Chondrite Is Such a Surprising Finding
Carbonaceous chondrites are among the most scientifically valuable meteorites known to science. They are ancient, largely unaltered remnants from the Solar System's infancy, preserving chemical and mineralogical signatures that predate the formation of the planets. Yet they are rare: carbonaceous chondrites account for only about 5% of all meteorites recovered on Earth. Within that already-small group, CO chondrites (Ornans-class) are rarer still — they represent just a tiny fraction of the carbonaceous chondrite category and are considered among the most primitive and least-studied objects available to planetary scientists.
What makes the CO chondrite identification particularly significant is its chemical composition. CO chondrites are notably depleted in volatile elements — substances with low boiling points that tend to escape under heat, including carbon, zinc, water, and crucially, sulphur. This has important implications for long-standing theories about the precise kill mechanisms of the K–Pg extinction event.
"Carbonaceous chondrites of the Ornans class are definitely not like the typical meteors you find in museum collections. A CO contains much less volatile elements — like carbon, zinc, water and particularly sulphur — than other classes of meteorites we've discovered so far on Earth. It doesn't alter our theory of what caused the extinction event — but it makes it less likely that sulphur contained in the impactor was the smoking gun. The fine debris thrown into the atmosphere would have been the primary factor."
Rethinking the Kill Mechanisms of the K–Pg Extinction
The question of how the Chicxulub impact killed so many species so quickly has been the subject of intense scientific debate. Several extinction mechanisms have been proposed and studied, including:
- Impact winter: Vast quantities of dust, soot, and fine debris ejected into the stratosphere blocked sunlight for months to years, collapsing food chains that depended on photosynthesis.
- Sulphur aerosols: The impactor striking sulphur-rich evaporite rocks in the Yucatán region was thought to have vaporized enormous quantities of sulphur, creating highly reflective aerosols that amplified and prolonged global cooling.
- Wildfires: The thermal radiation from the impact and re-entering ejecta may have ignited widespread wildfires across the globe.
- Acid rain: The vaporization of sulphur and nitrogen compounds from the target rock created sulphuric and nitric acids, potentially devastating terrestrial and marine ecosystems.
- Tsunamis and seismic activity: The sheer energy of the impact triggered massive seismic waves and mega-tsunamis across the proto-Gulf of Mexico.
Previous research had suggested that sulphur released from the impactor itself may have contributed significantly to the aerosol loading and subsequent cooling. However, if the Chicxulub impactor was indeed a sulphur-poor CO chondrite, this scenario becomes far less tenable. Instead, Dr. Claeys and colleagues suggest that the physical debris — the vast cloud of pulverized rock and silicate dust ejected into the upper atmosphere — was the primary driver of the impact winter and the ensuing extinction cascade. This finding shifts the emphasis back toward the target rock (the sulphur-rich Yucatán sediments) rather than the impactor itself as the source of any volatile sulphur loading.
Implications for Planetary Defense
Beyond rewriting the story of Earth's most famous mass extinction, this research carries important implications for modern planetary defense efforts. Understanding the composition of potential impactors is critical for developing effective mitigation strategies. NASA's Planetary Defense Coordination Office (PDCO) and the European Space Agency's Planetary Defence programme both emphasize the importance of characterizing near-Earth objects (NEOs) not only in terms of their size and trajectory, but also their mineralogical and chemical composition.
Knowing whether a threatening asteroid is a dense iron meteorite, a loosely bound rubble pile, or a volatile-rich carbonaceous chondrite fundamentally changes how a deflection mission would be designed and executed — as demonstrated by NASA's DART (Double Asteroid Redirection Test) mission, which successfully altered the orbit of the asteroid Dimorphos in 2022. The new data on CO chondrite composition helps populate the scientific knowledge base that informs these critical decisions.
Furthermore, the team's advanced isotopic methodology — using nickel isotopes as a geochemical tracer — provides a powerful new tool that can be applied to study other impact events throughout Earth's geological history, potentially unlocking secrets about ancient extinctions and the delivery of materials to our planet over billions of years.
Looking Ahead
While many questions remain — including the precise orbital history that sent this rare CO chondrite on a collision course with Earth 66 million years ago — this study represents a major refinement in our understanding of one of the pivotal moments in the history of life. The research underscores the value of international scientific collaboration and the power of cutting-edge isotope geochemistry to extract extraordinary information from layers of ancient clay. For more information on the Chicxulub impact and ongoing research, readers can explore resources from NASA's Solar System Exploration.
As researchers continue to refine their models and analyze additional boundary samples from around the globe, the story of the Chicxulub impactor will only become richer and more detailed — a testament to the enduring scientific fascination with the event that forever changed the trajectory of life on Earth.
