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You may not think that giant impact craters are particularly subtle or in any way capable of hiding us. In this case, you will be surprised by the discovery, announced this week, of a crater 31 km wide that we did not know existed.
Let's leave aside the important questions – no, the crater is not home to a Godzilla or a Lovecraftian horror. It is filled with ice. And that's how it escaped our attention for so long.
The crater is under the Hiawatha Glacier in northwestern Greenland. Airborne radar surveys are one of the tools used by researchers to monitor the contraction of the Greenland icecap. The high-resolution data obtained shows the shape of the surface of the ice cap, some of its inner layers and even the rock below. In this case, he revealed a suspiciously circular depression in the ice near the edge of the glacier.
After further radar investigation, a team led by Kurt Kjær of the University of Copenhagen discovered that this form was not a mere coincidence. The bedrock data had a screaming hole 320 meters deep, supplemented by a slightly elevated rim and a central peak. But to confirm that it was a crater, researchers had to put their boots on the ground (or on the ice).
Buried under 900 m of ice, the hole could not be drilled and pushed directly. But beyond the edge of the glacier, the sediments of the glacier base are deposited along the drainage channels: fragments of the crater have been brought to the researchers.
In these sediment grains, the researchers found what they were looking for. There were small pieces of quartz bearing the revealing shock patterns seen in the rocks shattered by the impacts. And there were also tiny pieces of glass, molten rock cooled almost instantly. Even the exposed bedrock had tiny faults parallel to the edge of the crater, which corresponds to the explanation of the impact crater.
The sediment grain chemistry suggests which type of object created the crater. Compared to the bedrock of the region, the grains contain an additional amount of iron, nickel and other elements associated with ferrous meteorites.
With this in mind, the researchers calculated an estimate of the size of the impactor. Assuming a reasonable speed of 20 kilometers per second, an iron meteorite with a diameter of about 1.5 km (a little less than a mile) would create a crater of this size. This would obviously be a violent event (as if the huge crater had not told you).
When?
And that brings us to the million dollar question: when did it happen? This is not the easiest question to answer, and as we will see later, any answer promises to be controversial. The discussion of the age study is therefore fairly measured.
One clue is that the crater cuts the drainage channels in the bedrock. Two channels reach the glacier side of the crater, meeting at the edge. A channel can be seen on the opposite side of the crater, with nothing on the floor of the crater itself. This means that the channels are probably pre-impact, just as a street is earlier than a pothole.
Similar drainage channels under other glaciers in the area would have been at the origin of rivers before the Greenland icecap became a thing, there are about 2.6 million of them. years. But without a better idea of the age of the chains, knowing that the crater is younger than the chains does not tell you so much.
The researchers also tried to analyze the shape of the crater to obtain a rough estimate of its age according to its "freshness". Plausible rates of glacial erosion ranged from 5,000 to 50 million years ago, which does not help to reduce it exactly. .
The best hypothesis of the researchers is based on the appearance of the ice overlay in the radar data. Smooth, luminous layers are visible almost all the way down. According to similar work done elsewhere, this ice cream dates back to around 12,000 years ago. Below is a dark layer and a chaotic mixture at the bottom.
The seemingly dark layer on the radar instrument marks the ice filled with rocks and sediments that formed during a brief period of regional cooling known as Young Dryas, between 11,700 and 12 years ago. 800 years old. The chaos behind it is strange and does not appear in the radar images. on the outside the crater. You will find layers visually identifiable from older periods.
Researchers interpret this to mean that chaotic ice was present at the moment of impact, perhaps even when the crater was cool. This would set the date of the event between 2.6 million years and 12,000 years.
Here we go again
This last stage is the one that is the subject of debate and which provokes controversy. To understand why, you need some background on the Younger Dryas period.
Most research on the cause of this interesting climate event focused on mechanisms likely to disrupt the circulation of the Atlantic Ocean and stop its heat transport – roughly the plot of Two days later. Unveiling a large amount of fresh water in the North Atlantic is basically the best way to block this circulation, and many scientists believe that the sudden release of accumulated glacial meltwater from Canada is probably the cause.
But this is not a certainty, and over the past decade, a small group of scientists has put forward a very different assumption: an impact event. The scientists' thesis focuses on the discovery of alleged fallout materials on archaeological sites in North America. The problem is that none of these discoveries has withstood the scrutiny of other scientists who have explored the same sites. And, of course, there was no impact crater that could fit.
Predictably, this small group of scientists is terribly interested in this new crater. And as most people in this field consider that the impact hypothesis is totally unsupported – and are additionally really tired of hearing about it – the authors of the crater study are in an uncomfortable position. The paper would have were subjected to a particularly rigorous peer review process, with examiners making sure that he had no casual connection to Young Dryas' fight. Now that it has been published, others are already weighing with skepticism.
So, is the impact crater properly formed when the young Dryas are cooling down? Or is it a few million years older than that? Or is it even older, researchers misinterpreting the importance of the chaotic ice it contains? Only additional searches will fix this problem. For example, there does not appear to be any evidence of material ejected from the crater in Greenland ice cores, but modeling studies may specify the signals to be searched for. An ice nucleus inside the crater would obviously be helpful, as would a more in-depth examination of the ice at the edge of the glacier just past the crater.
Until then, all we know is that a new impact crater – among the 25 largest in the world – has been added to the map. This is an interesting story in itself, but the story of the crater remains to be clarified.
Progress of science, 2018. DOI: 10.1126 / sciadv.aar8173 (About DOIs).
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