We’ve talked before about how advances are made in science one step at a time, with the work of a lot of different people. And sometimes, advances are made by asking “But couldn’t it be…” and waiting to see if you are right or wrong. This means that someone is always checking out weird alternatives, just to make sure the weird stuff, isn’t reality.

Such is the case with the object that ended the reign of the dinosaurs nearly 66 million years ago. What was it? We’ve been calling that seven-mile wide object an asteroid much of this time, but was it really? Could it have been a comet? That’s the hypothesis proposed in a paper published last week in Scientific Reports with lead author Amir Siraj, an undergraduate at Harvard University.

Siraj and his adviser used statistical analysis and gravitational simulations to calculate what fraction of long-period comets, those that originate from the far distant Oort cloud, are shifted off their original orbits by Jupiter’s gravitational field. This shift would make them swing into what we call a “sungrazing” orbit, passing very close to the sun. Close enough in fact to experience a tidal disruption event that breaks the comet into lots of smaller pieces.

And perhaps, some of the larger pieces could come back around and hit Earth on their new trajectories. The new calculations increased the chances of these impacts by a factor of ten from previous calculations, and show that about twenty percent of these long-period comets become sungrazers. This new rate of impact aligns with the 66 million-year age of the Chicxulub crater, which is the impact crater of our dinosaur-killer.

Additionally, there is evidence that the object was a carbonaceous chondrite in composition, which is rare for a main-belt asteroid but possibly common for a comet. The paper also cites several other large impact craters here on Earth that also match with the expected impact rate of sungrazing comets. More samples, particularly from those other craters and even comets themselves, are needed to confirm this hypothesis.

In the meantime, the other thing science is good at doing is rejecting alternative claims on the basis of stronger evidence. In a paper released just yesterday in Science Advances, a team of scientists published the results of their analysis of nearly 3,000 feet of rock core from the Chicxulub crater. They found conclusive evidence that the object that impacted Earth and wiped out the dinosaurs was an asteroid, based on the chemical fingerprint of the dust within the crater.

Per the press release: The telltale sign of asteroid dust is the element iridium – which is rare in the Earth’s crust, but present at elevated levels in certain types of asteroids. An iridium spike in the geologic layer found all over the world is how the asteroid hypothesis was born. In the new study, researchers found a similar spike in a section of rock pulled from the crater. In the crater, the sediment layer deposited in the days to years after the strike is so thick that scientists were able to precisely date the dust to a mere two decades after impact.

A layer of iridium dust is actually what kicked off the quest to understand what happened 66 million years ago. The layer is called the KT-boundary, and it can be found all over the world. In the 1980s, scientists analyzed the layer and found the iridium, and then the evidence began to pile up, culminating in the discovery of the Chicxulub crater in the 1990s. Chicxulub is a 125-mile-wide crater at the northern edge of the Yucatan peninsula. And now we know definitively that its age matches the time of the disappearance of dinosaurs in the global fossil record.

The team found that the concentrations of iridium were highest in a five-centimeter section of a core pulled from the top of the crater’s peak ring. This is where the rocks rebounded and then collapsed after the impact. You can search online for some amazing videos of simulations of the impact.

Also per the press release: In addition to iridium, the crater section showed elevated levels of other elements associated with asteroid material. The concentration and composition of these “asteroid elements” resembled measurements taken from the geologic layer at 52 sites around the world.

To sum up, a seven-mile-wide asteroid, not a comet, hit the Earth 66 million years ago in what is now known as the Chicxulub crater. The dust from the vaporized asteroid and rocks in the crater ended up in the atmosphere, circled the planet, blocked the sun, brought about a nuclear winter, and wiped out the dinosaurs along with about 75% of the rest of the plant and animal life on the planet. And mammals became the dominant species, so uh, thank you, asteroid?

More Information

Center for Astrophysics press release

“Breakup of a long-period comet as the origin of the dinosaur extinction,” Amir Siraj and Abraham Loeb, 2021 February 15, Scientific Reports

The University of Texas at Austin press release

“Globally distributed iridium layer preserved within the Chicxulub impact structure,” Steven Goderis et al., 2021 February 24, Science Advances