An asteroid a kilometer across slammed into the island at a speed that was at least a dozen times faster than a rifle bullet. Releasing as much energy as 15,000 times the largest nuclear weapon ever exploded on Earth, it carved out a huge crater and is over 30 kilometers across.
This is known. When did this happen?
Initial evidence suggested that it was not all that long ago, and possibly as recently as 12,000 years ago. The idea that this impact may have caused the collapse of some Paleoamerican people's civilization was controversial.
New research shows that is not the case. 58 million years ago, the scientists get the impact date, which is less than 10 million years after the larger impact that wiped out the non-avian dinosaurs.
The crater was discovered using airborne radar mapping, which bounces radio waves off Earth's surface and back up to a receiver. This kind of radar can be used to map the terrain of the rock below. The huge depression in the ground under the ice was confirmed by follow-up observations, as well as a raised rim and central peaks, which were created in the middle of a crater after the initial immense pressure of the impact subsided.
The age of the crater is a different question. The research used indirect evidence to get an upper limit to the age by looking at the structure of ice around the crater. The evidence of the age was circumstantial and there was evidence against a young age as well. I was not sure.
Direct evidence is what is needed. The results of the new research were different from the original work.
There are a few places where there is clear evidence of melting glaciers. The crater is still under ice, but just to the northeast the glacier ends, and water from melting has carried the debris out into the open. Mineral samples were collected 10 kilometers away from the edge of the crater.
Some of the grains of zircon that they found were intact, but others showed signs of the kind of crystal damage that can happen under the immense shock-wave pressure of an impact. They used radioactive dating to get the ages of the samples. The crystals that did not show the shock pattern are from the local bedrock.
The zircon was 58 million years old. They looked at dozens of sand grains that had shocked features and used a similar dating technique called 40Ar/39Ar or argon-argon dating to get a consistent age of around 60 million years.
The impact date was long before the ice sheet. I had a big issue with the original, much younger date, because there was no evidence of an impact in the ice itself, no dust or any other sort of material you would expect from such a huge event. That is a serious problem for a young impact, but not if the impact predated the start of the glaciers.
There are some geological events that happened around 60 million years ago, such as a global temperature minimum, when carbon from the atmosphere was sequestered, and there was an increase in atmospheric carbon that started a warming trend. There is no evidence that the impact was related to that. The crater is large on a human scale but small on a global one, and the impact probably wasn't enough to have a planet-wide effect.
I noted in my original article that there was a claim that the North American people were wiped out by an event like an impact around 12,900 years ago. The evidence for this is very shaky and most mainstream scientists are very skeptical of it. The original minimum impact age of 13,000 years breathed some new life into the idea, but now I think it is safe to put it down again.
That is science. It is a method for testing hypotheses and learning from the results, and no claim is so solid that it shouldn't be tested. It looks like the initial evidence of youth has been replaced by better evidence of great age. Either way, this adds to the small number of impact craters known on Earth and helps us understand the threat of impacts and how Earth responds to them. That is all good.
All Rights Reserved © 2024
