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Astronomie - Scientists close in on hidden Scottish meteorite crater

10.06.2019

 

Scientists think the time has come for a full geophysical survey of The Minch, to see if the Scottish strait is hiding an ancient meteorite crater.

The idea that such a structure lies between the Western Isles and mainland Scotland was first raised back in 2008.

They found evidence on the Highlands coast for the rocky debris that would have been produced by a giant impact.

Now, the team from Oxford and Exeter universities believes it can pinpoint where the space object fell to Earth.

Writing in the Journal of the Geological Society, Dr Ken Amor and colleagues say this location is centred about 15-20km west-northwest of Enard Bay - part way across The Minch towards Stornoway in the Outer Hebrides.

The feature would be buried deep under the seafloor, they add.

It's an intriguing prospect. The evidence gathered so far suggests the event occurred about 1.2 billion years ago when the continents were arranged very differently from how they are now, and life on our planet would have existed almost exclusively in the oceans.

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The way the rocks are laid out allows the team to trace back to an origin

The key supporting evidence is a group of reddish-coloured rocks on the eastern side of The Minch known as the Stac Fada deposit.

These are determined to be ejecta from the impact - the material hurled outwards when a 1-2km-wide object slammed into what was probably then some kind of rift valley.

The rocks are fragmented and contain melt particles, and also what geologists term shocked quartz - a type of mineral that has at some point been subjected to enormous pressures.

Shocked quartz is very often associated with meteorite events.

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The latest examinations of the Stac Fada deposit have now given the researchers - from Oxford and the Camborne School of Mines - some directional information that allows them to be more precise about where the ejecta came from.

"If you imagine debris flowing out in a big cloud across the landscape, hugging the ground, eventually that material slows down and comes to rest. But it's the stuff out in front that stops first while the stuff behind is still pushing forward and it overlaps what's in front," explained Dr Amor.

"That's what we see and it gives us a strong directional indicator that we can trace backwards.

"Also, we've examined the orientation of magnetic particles within the fabric of the rock at several locations, and this too allows us to triangulate back to an origin," the Oxford researcher told BBC News.

The lines converge out in The Minch.

The team is examining some seismic surveys that were done in the 1970s as part of an oil prospecting programme, but they are of poor quality.

Likewise, they are investigating gravity data. This indicates something anomalous in the strait, but again it is all somewhat uncertain.

"What we really need is a new high-resolution geophysical survey - a 3D seismic survey," said Dr Amor.

"Unfortunately, being offshore that would cost a lot of money. I shall be putting in a grant proposal to do some seismic work. That would be a first step and would greatly assist the definition of any impact structure."

Quelle: BBC

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Ancient asteroid crater located off coast of Scotland

Space object about a mile wide believed to have crashed into Earth around 1.2bn years ago

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A field photo taken at Stoer showing the laminar beds of sandstone, in the middle of which is the impact deposit left by the asteroid. Photograph: Handout/PR No credit

The location of an ancient impact crater made by the biggest asteroid ever to hit Britain has been traced to a spot under the sea between mainland Scotland and the Outer Hebrides.

Researchers at Oxford and Aberdeen universities found signs of the violent collision in Scotland on a field trip in 2008, but have only now pinpointed where the asteroid came down.

Tests on rocks near Ullapool in north-west Scotland revealed that an object about a mile wide had crashed into a spot in the Minch, a strait that separates the mainland and northern Inner Hebrides from Lewis and Harris, six miles west of the village of Lochinver.

The 38,000 mph collision, which thumped a 12-mile-wide crater into the ground, happened 1.2bn years ago, when most life on Earth was still in the oceans and plants had yet to take root on land. At the time, what is now Scotland was a semi-arid land that lay close to the equator.

“The impact would have sent huge roiling clouds of dust and gas at several hundred degrees in all directions from the impact site,” said Ken Amor, an Oxford researcher who led the latest study. What is left of the crater is submerged in 200-metre-deep water and covered in sediment.

The first hints of the impact came more than a decade ago when Amor was helping undergraduates on a geology field trip in the Scottish Highlands. On the last day, the scientists stopped in Stoer, a small village, to inspect an unusual rock formation known as the Stac Fada member (SFM).

Previous researchers speculated that the distinctive red sandstone had come from a volcano, but Amor realised that “strange green blobs” in the rock resembled features of an impact crater that underlies the town of Nördlingen near the Danube in western Bavaria. Amor took samples of the SFM back to Oxford and found strong evidence of an asteroid strike: quartz crystals that had been deformed by the shock of an impact.

He also found high levels of platinum and palladium, metals that are enriched in meteorites, the name given to space rocks that survive their fiery passage through the atmosphere to reach Earth’s surface.

Having confirmed that an asteroid had come down near Stoer, Amor’s team set about finding the impact crater. Using three independent techniques that drew on the scatter of dust and rocks thrown up by the impact, and the orientation of tiny magnetic grains blasted into the sky, the researchers traced backwards to the point of impact. Details of the work appear in the Journal of the Geological Society.

Assuming the crater has not been eroded away, it should be possible to detect with a seismic reflection survey of the kind used by prospecting oil companies, Amor said. Further studies of the rock might enable scientists to trace the meteorite back to a family of asteroids still orbiting the sun.

It is unclear how common such impacts are because craters are wiped from Earth’s surface by erosion, burial and plate tectonics. However, asteroids of the size of the one that hit the Minch are thought to strike between once every 100,000 years and once every 1m years. While the impact would have been dramatic, sending a mushroom cloud and fireball high into the sky, it was minor compared with the spectacular strike in Mexico’s Yucatán peninsula 66m years ago, when a space rock estimated at six to 50 miles wide put an end to the reign of the dinosaurs.

Quelle: The Guardian

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