A team of scientists led by Ken Amor from the Department of Earth Sciences at Oxford University has uncovered evidence of the largest meteor ever to strike the British Isles. Based on scattered debris material first discovered near Ullapool, northwest Scotland in 2008, the researchers from Oxford and Aberdeen Universities have concluded that a meteorite hit the Earth 1.2 billion years ago 15 to 20 km (9 to 12 mi) off the coast of Scotland.

Because meteorite, asteroid, and comet strikes have had such a major impact on the history of the Earth, they are of major interest to scientists. Thanks to such impacts, the dinosaurs went extinct and made way for the mammals. They've also been held responsible as possible causes of other mass extinctions, major geological events, and even the presence of water and various elements in the Earth's crust.

The trouble is that, unlike other worlds like the Moon, Mercury, and Mars, the active volatile nature of the Earth quickly obliterates such evidence unless circumstances are just right, such as in the Minch Basin, which is the area around a strait in northwest Scotland that separates the northwest Highlands and the northern Inner Hebrides from Lewis and Harris in the Outer Hebrides. Here the water and younger rocks preserved clues to the event.

"The material excavated during a giant meteorite impact is rarely preserved on Earth, because it is rapidly eroded, so this is a really exciting discovery," says Amor. "It was purely by chance this one landed in an ancient rift valley where fresh sediment quickly covered the debris to preserve it. The next step will be a detailed geophysical survey in our target area of the Minch Basin."

Since the Minch is underwater, the team relied on field observations involving the mapping of the scattering of broken basement cast rock fragments that were blown apart when the continental margin sediments were hit by the meteorite. Along with measuring the alignment of magnetic particles, which act as a sort of geological clock, the scientists were able to plot the trajectory of the fragments and trail them back to the impact site and the time when Scotland was still part of a giant continent near the equator that would have resembled ancient, watery Mars.

From the geological data, the team estimates the impact crater was up to 14 km (8.7 mi) in diameter. How big the meteorite was isn't certain, but with a width like that, it was either very large, going very fast, or both.

"It would have been quite a spectacle when this large meteorite struck a barren landscape, spreading dust and rock debris over a wide area," says Amor.

The research was published in the Journal of the Geological Society.