Double impact craters created at the same time are very rare and one such instance has been found at the Swedish county of Jämtland, which researchers say were created some 460 million years ago.
Researchers at the University of Gothenburg believe that some 470 million years ago, two large meteorites collided in the asteroid belt between Mars and Jupiter and as a result several fragments were thrown off into new orbits some of which crashed on Earth, such as the two in Jämtland 458 million years ago.
One of the craters measuring 7.5 kilometres was found 20 kilometres south of Östersund in Brunsflo while the other, which has a diameter of 700 metres was found 16 kilometres from there. Researchers say that the two meteor strikes were not the only ones that happened during the timeframe and many more would have smashed into Earth, but they are yet to be found.
Erik Sturkell, Professor of Geophysics at the University of Gothenburg, and colleagues explain that when the two meteors smashed into Earth, Jämtland was under the sea with a water depth of 500 metres at the points where two meteorites simultaneously struck. They add that double impacts of these nature are very unusual and is the first double impact on Earth that has been conclusively proved.
Back in 1940s, workers working at a quarry found an unusual-looking red limestone slab and a few years later when researchers analysed it, they understood that there was a meteorite in the slab. The meteor deposits in the limestone are a result of the leftovers of meteors that do not disintegrate completely, explains Sturkell.
Small meteorites survive the fall, while large ones explode and disintegrate. In Jämtland we have only found minerals from the meteorites, small grains of chromite.
At the time when the two meteors struck in Jämtland simultaneously, researchers say, water was forced away during the impact, and for a hundred seconds these enormous pits were completely dry. Then as the water rushed back in , it brought with it fragments from the meteorites mixed with material that had been ejected during the explosion and with the gigantic wave that tore away parts of the sea bed.
With the discovery first made as early as 1940, those working in the quarry knew exactly what to look for and this has led to discovery of quite a few meteorites with around 90 meteorites discovered from meteorite impacts on Kinnekulle over the past fifteen years.