When we look at the surface of other rocky planets, we see them pitted with the scars of violent impacts. Our own planet is not immune to these; Earth just heals better. Plate tectonics and an active atmosphere smooths over the inevitable nicks and dings. An academic side-effect of this resurfacing is that it is hard to study things like large impact craters here on the Earth.

Even though the primary force was downward, large impact craters actually sport a peak in the middle of the bowl. Really large craters get a raised peak ring instead. The basics here are understood: under the extreme conditions of a large impact, some of the rock in the center rebounds upward like the sploosh that follows the entry of a rock into a pond. But why does that singular peak turn into a raised ring in the biggest craters? That has been hard to figure out, and without examples of these craters to work with on the Earth, explanations have been difficult to test.

There is one huge impact crater with a peak ring here, though: the Chicxulub crater at the edge of the Yucatán Peninsula, left by the dinosaur-killing impact at the end of the Cretaceous Period. The Chicxulub crater is now mostly buried beneath the seafloor and hundreds of meters of limestone. But it was targeted for a major international drilling effort earlier this year, and the first study from that effort (written by a large team led by Imperial College London’s Joanna Morgan) tests explanations for the crater’s peak ring.

One leading hypothesis for generating a crater rim involved the melting of a large volume of rock at the dead center of the impact, which allows it to drop downwards again. Another proposal explains it through a much larger central peak that collapses outward in first few violent minutes of the crater’s formation. It was this “dynamic collapse” hypothesis that the researchers were focused on testing.

With a numerical simulation, the researchers outline how this would work. Over a period of about 10 minutes, the crater changes wildly, seeming more like the world’s largest bowl of Jell-O than a 200-kilometer-wide region of solid rock. The rock that ends up in the peak ring actually starts out several kilometers below the surface, just off the center of impact. Initially, it is heaved away from the center and upward—many kilometers upward—as the bottom of the impact holes reaches a depth of 20 kilometers. For lack of any other term we can relate to, it then sloshes towards the central peak region as that is thrown as much as 20 kilometers above the original surface. By the 10 minute mark, that peak has collapsed and flattened out, with the rock we’ve been following ending up draped on top of surface rock away from the center.

(Wikipedia has an excellent animated GIF of this process if you're finding it hard to visualize that explanation.)

Existing surveys of the Chicxulub crater held an enigma for proponents of this hypothesis. If Chicxulub’s peak ring rock had started out below the shallow rock, it should be made of denser stuff. But measurements showed it was actually low density material. Many other observations lined up, but this one did not. So is the hypothesis wrong, or were the rocks in the peak ring so beat up that their density changed?

The expedition’s rock cores through the peak ring reveal quite a bit. There's an upper layer of smashed-up fragments of rock, including bits of what were once molten rock flung up during the impact. Below that, they saw that the bulk of the peak ring was made up of granite “basement” rock. To find this type of rock outside the crater, you have to drill down more than 3 kilometers. This is exactly what the dynamic collapse hypothesis predicts.

The granite is about as beat up as you would expect given what it has been through. To the naked eye and under a microscope, the violence is apparent. The rock is fractured, smeared, and cut through by layers of once molten rock generated during the impact chaos. Density measurements confirmed that the granite is unusually low-density as a result of all that damage.

The researchers argue that this confirms the dynamic collapse hypothesis, which has some pretty interesting implications. This could help pin down lots of other details about the Chicxulub event, like the energy of the impact, the size of the initial hole, and the amount of dust and pollutants kicked up into the atmosphere.

It also informs research into how the damage beneath impact craters affects the habitat of underground microorganisms, which is relevant to the early Earth and questions about the potential habitability of other planets. And knowing what depth peak ring rocks come from allows planetary geologists to glean clues about the subsurface of other worlds using what we can see in craters.

All it takes to learn this is a 66-million-year-old scar in the Earth’s surface as large as the Mexican state it straddles.

Science, 2016. DOI: 10.1126/science.aah6561 (About DOIs).