Crater former: An artist's impression of an asteroid approaching Earth.

Credit: NASA

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SYDNEY: Evidence of a half-kilometre-wide meteorite, which struck Earth 1.2 billion years ago, has been discovered near the town of Ullapool in Scotland.

Analysis of rock layers surrounding the site has revealed that they were formed in the aftermath of a collision, and not from volcanic ash flow as previously thought.

Estimated to be at least six kilometres wide, the massive impact crater is thought to have been created by a meteorite at least 500 metres across, crashing into Earth at a dizzying speed of 61,200 km/h.

Spectacular evidence

A team of geologists from the University of Oxford and Aberdeen University, both in the U.K., have described the discovery in the current edition of the journal Geology.

"This is the most spectacular evidence for a meteorite impact within the British Isles found to date," said co-author Kenneth Amor of the University of Oxford. "What we have discovered about this meteorite strike could help us to understand the ancient impacts that shaped the surface of other planets, such as Mars."

Although there are around 175 known craters or their remnants on Earth, only two other impact sites have been as comprehensively described as Ullapool: the Ries crater in southeastern Germany, and at the Chicxulub crater in Mexico. Chicxulub is one of the largest impact structures in the world, measuring 180 kilometres across, and resulting from an asteroid at least 10 kilometres wide, which is credited with killing off the dinosaurs.

Chance discovery

The reason that so few impact craters have survived compared to the Moon is that just a third of meteorites that hit Earth strike land, and of these, most are weathered away or lost completely through plate tectonics. The Scottish crater would have been rapidly buried by sandstone after impact, said the experts, preserving it and thus offering a rare insight into the effects it may have had on our planet.

Amor and his team discovered the crater by chance while analysing rock samples from the site. Careful scrutiny revealed 'shocked quartz' which is created under conditions of high pressure and extreme heat. The experts also found high levels the element iridium and the isotope chromium-53; both of which are commonly found in meteorites.

The Earth was a very different place 1.2 billion years ago when this crater was formed, said Amor. Experts believe that Scotland would have been around 15º south of the equator and attached to what is now the North American landmass. It would have been a semi-arid environment and devoid of vegetation as terrestrial plants only evolved 425 million years ago. Microbial life alone would have been around to witness the devastation

"An observer would experience an earthquake, violent winds travelling at several hundred miles per hour and a very hot expanding fireball rising above the crater," explained Amor. "Dust and rock fragments thrown high up into the atmosphere may travel long distances and take from hours to days to settle out."

Devastating but infrequent

"The type of ejected deposit discovered in north west Scotland is only observed on planets and satellites that possess a volatile rich subsurface, for example, Venus, Mars and Earth," added co-author Scott Thackrey of the University of Aberdeen. "Due to the rare nature of these deposits, each new discovery provides revelations in terms of the atmospheric and surface processes that occur round craters just after impact."

The frequency of meteorite impacts on Earth today is significantly lower than it was 1.2 billion years ago - we can expect the Earth to be struck by a 500 meter-wide asteroid just once every 50,000 to 250,000 years. The biggest impacts, caused by asteroids larger than 10 km across, occur only once every 100 million years, but can have catastrophic results.