The asteroid that struck what is now Mexico’s Yucatan Peninsula 66 million years ago has been identified as a CO chondrite, a rare subtype of carbonaceous meteorite from the Ornans class that carries markedly less sulfur than the meteorite types previously proposed as the impactor. The identification comes from a paper published on 18 July 2026 in Science Advances, led by Georgy V. Makhatadze, a postdoctoral researcher at the Institut de Physique du Globe de Paris who carried out the high precision nickel isotope measurements at the centre of the study.
I will admit some hometown pride in flagging that byline: Makhatadze is a fellow Georgian, and it is not every week this desk gets to cover a paper settling a thirty year argument about the extinction of the dinosaurs with one of our own leading it.
The finding itself arrives thirty years into that debate about exactly what in the Chicxulub impact did the actual killing, and it does not sit comfortably with the sulfur heavy version of that story that has circulated in some form since the 1990s.
How researchers worked out what hit Earth
Identifying an impactor that vaporised on contact 66 million years ago is not a matter of finding a fragment of it lying around. Makhatadze’s team analysed nickel isotopes preserved in clay from the Cretaceous-Paleogene boundary layer, the thin, globally distributed band of sediment that marks the exact moment of the impact, collected at five sites across Europe. Nickel has five stable isotopes, which gave the team more independent constraints to work with than earlier single element analyses of the same boundary clay, and they compared the resulting signature against eleven known subgroups of carbonaceous meteorite.
The match pointed to CO chondrites of the Ornans type, a subgroup that is rare even among carbonaceous chondrites, which themselves make up only around 5 per cent of meteorites recovered on Earth. That rarity is part of why this identification has drawn attention on its own terms, independent of what it means for the sulfur question. It also carries a specific chemical signature: CO chondrites contain markedly less of several volatile elements, sulfur prominent among them, than the CM and CR chondrite types that earlier work had floated as plausible impactor candidates.
The object itself, on the team’s reconstruction, was somewhere between 10 and 15 kilometres across and struck at around 64,000 kilometres an hour, energy figures that were never really in dispute. What has been in dispute, on and off for three decades, is what that energy actually did to the atmosphere once it arrived, and the composition of the thing doing the striking turns out to matter a great deal to that second question.
Why sulfur mattered so much to begin with
The idea that sulfur released by the Chicxulub impact drove a sudden, severe global cooling, the so called impact winter, goes back to modelling work from the 1990s that estimated the impactor could have injected hundreds of gigatons of sulfur into the atmosphere as aerosols, blocking sunlight and collapsing photosynthesis worldwide within months. That figure, and the extinction mechanism built on top of it, has been repeated across decades of popular and scientific accounts of the extinction, often as though it were a settled physical measurement rather than a model estimate built on assumptions about the impactor’s composition that nobody could directly check at the time.
Those assumptions have been getting harder to sustain for a couple of years now, not just because of this one paper. A separate study led by Katerina Rodiouchkina, who works across Ghent University, Vrije Universiteit Brussel, and Luleå University of Technology in Sweden, published in Nature Communications in January 2025, used empirical measurements rather than modelling to put the actual sulfur release at around 67 gigatons, roughly five times lower than the older estimates of 325 gigatons that much of the impact winter narrative had been built on. The CO chondrite identification adds a second, independent line of evidence pointing the same direction: not only was less sulfur released than assumed, but the specific rock that hit Earth was a type unusually poor in sulfur to begin with.
What this does and does not overturn
It would be a stronger claim than the evidence supports to say this disproves an impact caused the extinction, or even that it settles the mechanism. Philippe Claeys of the Vrije Universiteit Brussel, a co-author on the chondrite paper, was direct about the limits of what the finding changes: “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 asteroid still hit, the extinction still happened, and the two are still connected. What is shifting is the specific physical mechanism connecting the impact to the kill, away from sulfate aerosols cooling the planet and toward fine silicate dust thrown into the upper atmosphere, which independent modelling work published around the same time suggests could have amplified atmospheric heating and intensified wildfires in the hours after impact.
This is two studies and a modelling paper, not a settled consensus. The sulfur estimate from the Rodiouchkina paper and the impactor identification from the Makhatadze paper both point in the same direction, and that convergence is a stronger basis for revising the older sulfur heavy narrative than either result would be alone. But identifying the impactor’s class is not the same as fully reconstructing the kill mechanism. The dust based alternative currently gaining ground is itself an active area of modelling, not a finished picture.
A brief personal note here, since I doubt I am the only reader who will notice it: Makhatadze is a Georgian surname, carrying the “-dze” ending common to many Georgian family names, the same pattern found in names like Shevardnadze. As a Georgian myself, I did not expect to see it attached to a paper resolving a thirty year argument over what killed the dinosaurs, and it was a nice surprise to spot it while reading through the author list.
A rock this rare, hitting exactly here
There is a separate oddity sitting underneath all of this that the papers do not fully resolve: CO chondrites are an unusual thing to find at all, let alone as the specific object responsible for the most consequential impact in the planet’s recent geological history.
Carbonaceous chondrites as a whole are thought to originate from the outer reaches of the asteroid belt or further out still, and the CO subtype within that group is a small fraction of an already small fraction.
Whether that rarity is coincidence, or says something about the population of objects capable of reaching an Earth crossing orbit from that far out, is not a question this paper set out to answer, and it is not one the current evidence resolves either way.
