Ancient Earth hammered by double space impact
Using tiny, plankton-like fossils, they established that neighbouring craters in Sweden are the same age - 458 million years old. Details of the work were presented at the 45th Lunar and Planetary Science Conference in The Woodlands, Texas, and the findings are to be published in the Meteoritics and Planetary Science journal.
However, other scientists cautioned that seemingly contemporary craters could have landed weeks, months or even years apart. A handful of possible double impacts (or doublets) are already known on Earth, but Dr Jens Ormo says there are disputes over the precision of dates assigned to these craters.
"Double impact craters must be of the same age, otherwise they could just be two craters right next to each other," the researcher from the Centre for Astrobiology in Madrid, Spain, told BBC News.
Dr Ormo and his colleagues studied two craters called Lockne and Malingen, which lie about 16km apart in northern Sweden. Measuring about 7.5km wide, Lockne is the bigger of the two structures; Malingen, which lies to the south-west, is about 10 times smaller.