Mon. Sep 26th, 2022
How to quickly make a big hole
enlarge How to quickly make a big hole

You know that scene in every forensic crime drama where someone finds out what angle the bullet was fired from and points back to the source? In the case of the Chicxulub asteroid impact and the late Cretaceous mass extinction 66 million years ago, there is no mystery about the shooter. (Space did.) But the trajectory is interesting for other reasons, and researchers have long tried to recover the trail from the crater off the coast of Yucatán.

Unsurprisingly, 66 million years have taken their toll on the crater, so researchers have given several very different answers. Did the asteroid impact from the southeast at a very low angle? Did it come at a moderate angle from the southwest? Many studies that were supposed to model the impact simply defaulted to a 90-degree strike and avoided the entire argument. The details matter, though, and exactly which rocks vaporize — and what climate-changing gases they release — depends on that angle of impact.

Much new research on the crater has recently been published thanks to a major expedition that drilled a rock core through the crater’s summit ring. (Impact with this violent impact leaves a raised ring in the center instead of a single peak.) A new paper led by Gareth Collins of Imperial College London makes the latest contribution using model simulations to see which angle of impact is the best corresponds to the observed characteristics of the crater.

Not aligned

To uncover the geometry of what’s left of the crater, which is part land and part seafloor, scientists are turning to sonar-like seismic imaging and extremely accurate gravity measurements. Seismic imaging highlights features such as sediment layers and bedrock, as well as faults. Variations in gravity tell scientists about the density of the rocks below. In this case, that also happens to include pinpointing the location where the Earth’s mantle comes closest to the surface.

All this makes it possible to plot three critical points on the map: the geometric center from the crater rim, the center of the peak ring and the highest point of the mantle. At a perfect 90 degree turn, these three points would overlap. The peak ring would form a rose in the center of the crater, and the high spot of the mantle would be right below it – the product of a gelatinous vertical recoil after the tremendous force of the impact.

The colors here show differences in accurate measurements of gravity.  The three main points are labeled in the middle.
enlarge The colors here show differences in accurate measurements of gravity. The three main points are labeled in the middle.

Because they don’t line up, the question is which combination of impact angle and direction puts the center of the peak ring just southwest of the center of the crater, with the high point of the mantle’s rebound just northeast of the center. from the crater. To test this, the researchers used a 3D simulation of the extreme physics of an asteroid impact. They repeated the simulation for impacts at angles of 90, 60, 45 and 30 degrees and at speeds of 12 kilometers per second (the minimum plausible) and 20 kilometers per second.

Within just a few minutes, the impact creates a “transient crater” about 20 kilometers (12 mi) deep as a temporary mountain range is thrown up, after which things bounce back to form a shallower final crater.

Impacts at 60 and 45 degrees cause the top of the mantle and the center of the peak ring to be shifted on either side of the center of the crater — just like we see in the real crater. But at 30 degrees, the highest point of the mantle meets the center of the peak ring on the other side, so they’re both shifted in the same direction.

Oblique impact

There is also a visible difference in the peak ring itself. The 60- and 45-degree impacts show that some of the outermost sedimentary rocks near the surface are “sucked” down under deep rocks pushing up from the center. Outside the crater, sedimentary rocks lie on top of deeper granites, but this relationship is reversed in the peak ring. That’s exactly what the rock core from the recent expedition showed. But in the 30-degree impact simulation, the deep-on-top pattern is not materialized on the downside of the crater.

A simulated impact at 60 degrees, from right to left.  Note that the subtle clumps on either side of the crater floor (the peak ring) are not centered (0).  The dark gray layer at the bottom represents the mantle, and the highest point in the last frame is shifted in the other direction.
enlarge A simulated impact at 60 degrees, from right to left. Note that the subtle clumps on either side of the crater floor (the peak ring) are not centered (0). The dark gray layer at the bottom represents the mantle, and the highest point in the last frame is shifted in the other direction.

Taken together, these results point to an impact angle closer to 60 degrees, ruling out some previous ideas of low-angle impact. But the simulations also disputes the claims that the impact came from the southwest. They show that the highest point of the mantle marks the upward direction and the center of the peak ring marks the downward side. In the case of the Chicxulub crater, this means the impact came from the northeast.

This intermediate angle of impact would maximize the climate impact of the event. Given the depth of the relevant rock layers that have evaporated, this impact would release approximately 2 to 3 times as much sulfur (causing short-term cooling) and CO2 (causes prolonged warming) as a 90 degree attack. And compared to an impact at a super low angle of 15 degrees, it would release about 10 times more. Looking at how the mass extinction event played out, it seems that release was sufficiently effective.

Nature Communications, 2020. DOI: 10.1038/s41467-020-15269-x (About DOIs).

By akfire1

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