Earth suffered double hit, say scientists

By James Vincent

An artist's impression of a double asteroid.
An artist's impression of a double asteroid.

Imagine Bruce Willis at the end of Armageddon, riding an Earth-bound asteroid wired with dynamite and preparing to blow the charges and save the planet. He turns around, only to see - another asteroid. Also headed for Earth. Without explosives.

It sounds improbable even by Hollywood standards, but an investigation by scientists shows such a double impact rocked the Earth 458 million years ago.

Dr Jens Ormo and his team from the Centre for Astrobiology in Madrid, Spain, analysed tiny, plankton-like fossils in two neighbouring craters in Sweden, concluding the pair were formed by a binary asteroid strike in the Ordovician Period.

The site in Sweden is one of several proposed double impact craters, but other scientists have warned they could have been created years apart and that our methods of dating impacts is not precise enough.

Despite this, astronomers say binary asteroids are quite common and that nearly 15 per cent of near-Earth asteroids have twins.

These pairs are thought to form when a type of asteroid known as a "rubble pile" (consisting of numerous lumps of rock drawn together by gravity) begin to spin quickly as they're warmed by the Sun, eventually throwing off a piece of rock to form a tiny moon.

However, not every asteroid pair forms a double crater as it hits the Earth. Ormo has suggested the pair of Swedish craters, known as Lockne and Malingen, are a possible double impact, after drilling down into the formations to retrieve tiny fossilised sea creatures known as chitinozoans and dating them to the same general time period.

The space between the two craters also suggests they were the result of a double impact.

The research by Ormo was presented at the 45th Lunar and Planetary Science Conference in Texas, and the findings will be published in the Meteoritics and Planetary Science journal.

- Independent

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