How a Rare CO Chondrite Asteroid Rewrote the Dinosaur Extinction Story

The Chicxulub Impactor Was a Rare CO Chondrite Asteroid

Estimated reading time: 10 minutes

Key Takeaways

• Researchers used high-precision nickel isotope measurements to identify the Chicxulub impactor as a rare type of asteroid called a CO chondrite, specifically an Ornans-type carbonaceous chondrite.
• This shifts the blame for the mass extinction away from the asteroid’s own chemical makeup and toward what happened when it slammed into the Earth’s crust.
• Because CO chondrites are dry and volatile-poor, the new identification implies that the Chicxulub impactor delivered roughly half the sulfur that earlier models had assumed.
• The answer, according to the research, is the target rock itself — the Yucatán Peninsula’s crust.
• The convergence of two independent isotopic systems — ruthenium and nickel — pointing to the same origin region and composition class is scientifically powerful.

Sixty-six million years ago, something hit Earth so hard it ended the reign of the dinosaurs. Scientists have known about the Chicxulub impactor for decades. They knew it was big. They knew it was fast. They even had a rough idea of what kind of rock it was.

But a new study published in July 2026 just changed the details in a big way — and those details matter more than you might think.

Researchers used high-precision nickel isotope measurements to identify the Chicxulub impactor as a rare type of asteroid called a CO chondrite, specifically an Ornans-type carbonaceous chondrite. This isn’t a minor tweak to the story. It shifts the blame for the mass extinction away from the asteroid’s own chemical makeup and toward what happened when it slammed into the Earth’s crust.

Here’s what the science says, why it rewrites key parts of the extinction model, and what it tells us about how statistically unlucky our planet was on that day.

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What Is the Chicxulub Impactor and Why Does Its Composition Matter?

The Chicxulub impactor is the asteroid (or comet, though that theory has largely been ruled out) that struck the Yucatán Peninsula in present-day Mexico roughly 66 million years ago. The impact left behind a crater approximately 180 kilometres (112 miles) wide, and the energy released is estimated at somewhere between 10²³ and 10²⁴ joules — more than a billion times the energy of the largest nuclear weapon ever tested.

The impact triggered what scientists call the end-Cretaceous mass extinction, wiping out roughly 75% of all species on Earth, including all non-avian dinosaurs. It is one of the five major mass extinctions in Earth’s history.

For years, scientists debated exactly what kind of asteroid caused all this. The reason that question matters is straightforward: the asteroid’s composition controls what gases and particles got injected into the atmosphere, which then determined how bad the climate catastrophe was.

Different types of asteroids carry different amounts of sulfur, water, and other volatile elements. Sulfur in the atmosphere reflects sunlight and cools the planet dramatically. So if the impactor was rich in sulfur, the climate story looks one way. If it was dry and sulfur-poor, the story looks very different.

The new 2026 study, titled “The origin of Cretaceous-Palaeogene impactor revealed by nickel isotopes” by Makhatadze et al., published in Science Advances (Vol. 12, Issue 29, DOI: 10.1126/sciadv.aef4858), gives us the clearest chemical fingerprint yet for what that asteroid actually was.

EurekAlert news release

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What Is a CO Chondrite Asteroid — and How Rare Is It?

To understand why this identification is significant, you need to know a little about the types of meteorites that fall to Earth.

Chondrites are the most primitive class of stony meteorites. They formed in the early solar system and haven’t been significantly melted or differentiated since. Within chondrites, there are several subgroups based on their chemistry and physical structure.

Carbonaceous chondrites are a special class that contain carbon-bearing compounds, water-bearing minerals, and relatively high abundances of volatile elements. They are thought to make up roughly 5% of all meteorite falls on Earth.

Within carbonaceous chondrites, there are several types: CM, CR, CV, CI, and CO, among others. These classifications are named after specific “type specimens” — landmark meteorites found in specific locations.

CO chondrites are named after the Ornans meteorite, which fell in France in 1868. They are classified as Ornans-type carbonaceous chondrites, and they are among the driest of the carbonaceous chondrite group. Compared to CM and CR types, CO chondrites carry:

• Markedly lower amounts of water
• Lower abundances of sulfur
• Fewer volatile elements overall

This makes CO chondrites fundamentally different from the wetter, more volatile-rich carbonaceous chondrites that scientists had previously assumed were the best match for the Chicxulub impactor.

And here’s the critical point: CO chondrites represent one of the rarest categories of meteorites recovered on Earth. NASA’s own modeling suggests that roughly 40–50% of near-Earth asteroids have carbonaceous-chondrite-like compositions, and that more than 20% of asteroids striking Earth could broadly match the Chicxulub geochemical signature — but that’s the broad category. The specific CO subtype is a much smaller slice of that picture.

Earth didn’t just get hit by a big asteroid. It got hit by a rare, specific type of asteroid that very few scientists had predicted.

Sci.News coverage

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How Nickel Isotope Analysis Identified the Impactor

The new study’s method is worth understanding, because it’s what makes this finding more precise than anything that came before.

Isotopes are versions of the same element that have different numbers of neutrons in their nucleus. Different classes of meteorites formed in slightly different regions of the early solar system, under different conditions — and as a result, they have subtly different isotopic ratios. These ratios are like chemical barcodes that survive for billions of years and can be measured even in tiny amounts of extraterrestrial material mixed into Earth’s rock layers.

At the Cretaceous-Paleogene (K–Pg) boundary — the thin layer of rock that marks the exact moment of the mass extinction — there is a globally distributed enrichment of rare metals like iridium, platinum, and nickel that came from the vaporized asteroid. Scientists can analyse the nickel isotope ratios in that boundary clay and compare them to known meteorite classes.

Here’s what the 2026 study found:

• The K–Pg marine clay layers showed nickel isotope ratios that match CO chondrites and some ungrouped carbonaceous chondrites.
• The data excluded several previously proposed meteorite classes, including CV types.
• Previous studies had favoured CM or CR carbonaceous chondrites based on chromium-54 anomalies, platinum-group elements, amino acids, and fossil meteorite fragments — but the new, more precise nickel measurements show that CO-type material can reproduce those observed signatures without violating other constraints.

This is a significant step forward. Earlier geochemical evidence had been somewhat ambiguous about whether CO types could even be in the running. The new nickel isotope data reopens that compositional space and makes CO the best-fit candidate.

EurekAlert news release

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How This Shifts the Blame for the Dinosaur Extinction

This is where the science gets really interesting — and where the CO chondrite identification has direct consequences for how we model the extinction event itself.

The classic story of the end-Cretaceous mass extinction goes roughly like this: the asteroid hit, vaporized, and injected a massive amount of sulfur dioxide and other gases into the stratosphere. This sulfur aerosol layer reflected sunlight, causing a global “impact winter” that collapsed photosynthesis, destroyed food chains, and killed off most life on Earth.

The problem with that story is that it depends heavily on how much sulfur the asteroid itself contributed.

A CO chondrite impactor changes the math.

Because CO chondrites are dry and volatile-poor, the new identification implies that the Chicxulub impactor delivered roughly half the sulfur that earlier models had assumed, compared to estimates based on more volatile-rich carbonaceous chondrites like CM or CR types.

So where did the climate-killing sulfur actually come from?

The answer, according to the research, is the target rock itself — the Yucatán Peninsula’s crust. That crust is rich in sulfur-bearing evaporite rocks like anhydrite. When the asteroid slammed into them, it vaporized enormous quantities of sulfur from the ground, not from the asteroid.

Beyond sulfur, the updated model gives even more weight to fine particulate ejecta — the microscopic dust that was blasted into the upper atmosphere. This dust loading, combined with the target-rock sulfur, is now thought to be the primary driver of the post-impact radiative forcing that caused the impact winter.

In simpler terms: the rock that killed the dinosaurs was drier than we thought, but the ground it hit was sulfur-rich enough to do catastrophic damage anyway — and the sheer amount of dust it kicked up may have been just as important.

Karmako post | Sci.News coverage

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The Link to the Outer Solar System — and the 2024 Ruthenium Findings

The 2026 CO chondrite paper doesn’t stand alone. It builds on a major 2024 study that was already shifting the conversation about where the Chicxulub impactor came from.

In August 2024, Fischer-Gödde et al. published findings in Science (DOI: 10.1126/science.adk4868) using ruthenium isotope analysis at the K–Pg boundary. Their conclusion: the Chicxulub impactor was a carbonaceous-type C-class asteroid that formed in the outer solar system, beyond Jupiter.

This was important because it distinguished the impactor from the more common inner-belt asteroids that typically threaten Earth. The outer solar system is where carbonaceous, volatile-rich, and sometimes dry carbonaceous bodies originate.

The 2026 nickel study dovetails with this finding by narrowing that outer-solar-system object down to a CO chondrite-like composition. The two studies together build a coherent picture:

• The Chicxulub impactor originated beyond Jupiter (ruthenium isotopes, 2024)
• It was specifically a rare CO chondrite type asteroid (nickel isotopes, 2026)
• This explains its dry, volatile-poor chemistry
• And this, in turn, refines every climate model built on the assumption that the asteroid itself was the primary sulfur source

The convergence of two independent isotopic systems — ruthenium and nickel — pointing to the same origin region and composition class is scientifically powerful. It’s not one measurement saying this. It’s two completely different chemical signals telling the same story.

Sci.News coverage

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Why Earlier Studies Got It Wrong — and What Changed

Scientists didn’t just guess at the Chicxulub impactor’s composition. They had good reasons for earlier conclusions. And it’s worth being clear about what changed.

Earlier lines of evidence included:

• Chromium-54 anomalies in boundary layers
• Platinum-group element (PGE) abundances
• Amino acid signatures
• Fossil meteorite fragments embedded in sediment

All of these had pointed toward CM or CR carbonaceous chondrites as the best match. Several earlier studies had explicitly ruled out CO and CV types based on these datasets.

So what changed?

The 2026 study used higher-precision nickel isotope measurements than were previously available. The nickel isotope system is particularly sensitive to the specific subtype of carbonaceous chondrite material. When the K–Pg clay is analysed with these more precise tools, the signature matches CO chondrites — and doesn’t actually violate the constraints set by the older chromium, PGE, and amino acid data.

In other words, the older data wasn’t wrong. The new data is just more precise, and it resolves an ambiguity that the earlier measurements couldn’t.

This is how science is supposed to work: each new analytical technique adds resolution to the picture. The picture doesn’t get thrown out; it gets sharpened.

EurekAlert news release

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The Comet vs. Asteroid Debate — and Where It Stands Now

One persistent alternative theory holds that the Chicxulub impactor wasn’t an asteroid at all, but a fragment from a long-period comet. This idea has been proposed and revisited several times.

NASA addressed this directly in a 2022 technical report (NTRS 20220010797) titled “The breakup of a long-period comet is not a likely source of the Chicxulub impactor.” Their conclusions, based on dynamical and geochemical arguments, are clear:

• An asteroid remains “highly favored over a comet” as the Chicxulub impactor
• Chromium-54 and other isotopic evidence better match carbonaceous asteroidal material than cometary material
• Dynamical probability calculations also disfavour a long-period comet breakup scenario

The 2026 CO chondrite identification refines this consensus rather than challenging it. It’s still an asteroid. It’s just now identified as a specific, rare type of asteroid — one from the outer solar system with a dry, volatile-poor composition.

NASA technical report (PDF)

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How Rare Was This Impact — and What Are the Odds?

One of the more striking takeaways from the 2026 study is the sheer statistical bad luck involved.

CO chondrites are one of the rarest subclasses of an already-rare meteorite group. Carbonaceous chondrites themselves represent only about 5% of all meteorite falls. Within that 5%, CO chondrites are a small minority. Ornans-type CO meteorites specifically are described in recent scientific coverage as belonging to “one of the rarest categories of meteorites recovered on Earth.”

NASA models suggest that around 40–50% of near-Earth asteroids broadly have carbonaceous-chondrite-like compositions, and perhaps more than 20% of all asteroids striking Earth could match the general Chicxulub geochemical signature. But the specific CO chondrite identification narrows this down to a much rarer subset.

The fact that the object which wiped out the dinosaurs was not a common, inner-belt, easily explained rock — but a rare, outer-solar-system CO chondrite — says something profound about the role of chance in Earth’s history.

If Earth had been struck by a more common, volatile-rich carbonaceous chondrite, the sulfur loading might have been heavier, the impact winter more intense. If it had been struck by an ordinary chondrite, the chemistry would have been completely different. The specific nature of the CO impactor shaped the specific nature of the extinction.

The dinosaurs weren’t just unlucky. They were unlucky in a very particular way.

Sci.News coverage | Chicxulub crater

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What This Means for Planetary Science and Future Asteroid Research

Beyond the history of the dinosaurs, the CO chondrite identification has practical implications for how we think about asteroid impacts going forward.

For climate modelers, it means existing simulations of impact winter need to be recalibrated. The sulfur contribution from a CO chondrite impactor is lower than previously assumed. Fine dust and target-rock sulfur need to be weighted more heavily in those models.

For planetary defence researchers, it reinforces the importance of knowing not just where a threatening asteroid is, but what it’s made of. Composition determines the scale and character of the damage. An asteroid’s type affects how it breaks apart in the atmosphere, how much material vaporises on impact, and what gases it injects into the atmosphere.

For solar system scientists, the convergence of nickel and ruthenium isotope data pointing to an outer-solar-system origin raises interesting questions about orbital dynamics. How did a rare CO chondrite from beyond Jupiter end up on a collision course with Earth? What does that tell us about the mechanisms that scatter large objects from the outer solar system into the inner solar system?

These questions are now firmly back on the research agenda.

EurekAlert news release | Sci.News coverage

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