Bad astronomy | Impact of an ancient asteroid on the Wyoming/Nebraska border

It wasn’t a dinosaur killer, because it happened just before they appeared. But the idea is the same*.

An asteroid about 4 kilometers wide – nearly half the diameter of the Chixulub impactor – slammed into Earth at 70,000 kilometers per hour. The fantastic kinetic energy of the asteroid’s motion has been converted into light and heat – like 5 million one-megaton bombs all detonate at once – as well as being transferred into the ground, gouging vast amounts of the Earth’s surface.

This material flew off, given a huge boost by the impact. Some blocks of earth four meters in diameter were thrown at twice the speed of sound, falling in distant places at a great distance, carving smaller but still respectable secondary craters in the ground.

When it was over, a 50 to 65 kilometer wide crater marked the planet, glowing with residual heat, surrounded by thousands of smaller craters over hundreds of thousands of square kilometres.

This happened around the end of the Pennsylvanian subperiod and the beginning of the Permian period, 280 million years ago, shortly before dinosaurs evolved to roam the region. Yet time and tide…eventually water and mud flooded the area and turned it to stone. The primary crater and its satellite secondaries remained buried for hundreds of millions of years, until the tectonic forces of the Laramide Orogeny pushed the Rocky Mountains up. Erosion has cleaned the softer mudstone from the secondary craters, leaving the harder material as a sort of fossilized remnant of them. The main crater, however, hundreds of kilometers away, remained buried.

The first of the secondary craters was discovered in 2018 by geologists. They vary in size from a few meters to 80 meters in diameter, with an average of around 30 meters in diameter, enough to knock down a fairly large house. The craters were initially thought to have come from a single small asteroid that broke into pieces, with each piece carving out a small crater. However, new research has discovered many other such craters in the area, and they are clearly secondary, and, as a bonus, indicative of the direction and size of the much larger impact (link to article).

With this information, they were able to get an idea of ​​where the big impact was: near the Wyoming/Nebraska border.

Which, I note, is very roughly 100 km from my home. If I had been in my yard that day and looked to the northeast, I would have witnessed the Apocalypse. Blinded by the flash and probably burned by the heat pulse, about a minute later I would have felt a magnitude 9 earthquake due to the shock wave passing through the ground, then a wind hundreds of miles away the hour that would have, at this point, scattered my ashes. Then the sound wave would have swept through, louder than behind a modern commercial jet engine.

I’ll be honest: I’m glad I missed it. 280 million years between me and that sounds like a pretty decent timeline buffer.

But geologists have found proof of the cataclysm. In the new work, they identify 31 confirmed secondary craters, tare the very first confirmed secondary craters ever seen on Earth. Other secondary candidate craters from other impacts have been found but remain unconfirmed. We see secondary craters on airless bodies like the Moon and Mercury, carved out by immense impacts on those worlds, but on Earth our atmosphere and hydrosphere are eroding them, with water and air doing relatively quick work. to clear these features.

But in this case, this force worked to our advantage: in some places, the secondary craters of the Wyoming/Nebraska impact were created in quartz sandstone, probably covered with shallow water. The secondary impacts compressed and heated the quartzite material, which is tough. As the Rocky Mountains pushed up, the craters were reexposed to the elements, and eventually the craters were revealed.

Secondary craters are formed from much less energetic events than the primary crater, but we’re still talking about rocks over a hundred tons crashing into the ground at the speed of sound, about 1,000 km/ h, releasing the energy equivalent of up to a dozen tons of TNT. The lower impact angle can create elongated craters with rim characteristics that distinguish them from primary craters. It also creates shock waves when it hits the ground that alters quartz crystals, which is considered a smoking gun for alien impacts.

The rocks also fall in clusters, with several boulders striking close to each other, sometimes breaking along the way to create craters that fall along a line, called crater chains. All of this is exactly what is seen in several of the Wyoming craters identified by the team. But in addition to showing that they are indeed secondary impact craters, they also reveal the direction of the main impact site: tracing the aligned craters and the elongated craters backwards along the path they came, triangle to the likely location where the main asteroid hit. Geological processes can cause the ground to rotate and move over time, but taking that into account, scientists believe it struck about 100 miles east of Cheyenne, just over the Nebraska border. .

Nothing is there now; the crater would be buried under hundreds of meters of rock. However, this area is full of oil drilling sites and logs of what has been excavated are recorded. These can be examined for anomalies consistent with an impact crater – it’s on the team’s to-do list, but it can’t be done until the US Geological Survey Core Research Center reopens after the pandemic.

Also on this list is confirm the other 60 craters the team discovered that this could also be part of the secondary strewn field. If confirmed, it will be an incredibly rich discovery instead of just being deeply meaningful, so hey, win-win.

Only 208 impact craters are confirmed on Earth, with 31 more now. Adding 60 more is a scientific boon and a very big deal. Adding to this a possible massive primary crater, with good dating information, is extremely important. Such large impacts affect the entire planet, and there might be more evidence that they are already in a geologist’s lab somewhere.

It happened a long time ago, but there’s still a lot it can teach us about our world today, and it’s a lesson in what can happen if we ignore the sky above. above us.


* CORRECTION (February 21, 2022): I originally wrote that it was a dinosaur killer, but dinosaurs didn’t appear until the Mesozoic era, which started after the end of the Permian period/Paleozoic era, which was after this event. My apologies for any confusion.

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