Around 15% of near-Earth asteroids are binaries—an asteroid with an orbiting "moon" sometimes nearly as big as itself—but what happens when a binary slams into our planet? The answer, at least some of the time, is the creation of two huge craters, and Spanish researchers believe they have found evidence of one such double impact 458 million years ago, the BBC reports.
They say two large craters in Sweden, dubbed Lockne and Malingen, appear to have been formed at the exact same time by an impact big enough to have caused an "atmospheric blow-out" over the site.
"Double impact craters must be of the same age, otherwise they could just be two craters right next to each other," lead researcher Dr. Jens Ormo explains.
In the case of the Swedish craters, the area would have been sea-covered at the time of impact, and the researchers analyzed plankton-like fossils found there to determine the craters' age—and that they likely resulted from a double impact.
Backing up that theory: The craters are only about 10 miles apart, reports the Independent, which describes the distance as "close enough that the two asteroids would have been pulled together by gravity, but far enough for the craters to not have overlapped." There are a number of other proposed double-impact sites, in Quebec, southern Russia, and southern Germany.
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