Nearly 15% of all asteroids travel in pairs, yet very few impact craters on Earth have been identified as "doublets" — dual craters arising from sudden back-to-back impacts. But a new analysis has identified a potential candidate in Sweden, the product of a binary pair striking the Earth some 458 million years ago.
Top image: Clearwater Lakes in Quebec, Canada — a potential doublet. Other candidates include Kamensk and Gusev in southern Russia; and Ries and Stenheim in southern Germany.
The fact that binaries are as common as they are — nearly one in every six near Earth asteroids (NEAs) measuring over 650 feet (200 meters) across — is troublesome. Should we ever identify an NEA on a collision course with Earth, there's a 15% chance that it'll be accompanied by a partner. Assuming we'll eventually develop the technology required to steer these objects off course, there's a decent chance that we'll have to nudge not just one, but two objects. It's important, therefore, to acknowledge the risk posted by binaries, and a good place to start is to find historic evidence that double whammies do in fact occur.
Uncommon And Elusive
Of the 184 known craters on Earth, very few have been proposed as potential doublets. The problem, of course, is confirmation. When two asteroids strike in tandem, the resulting carnage often gives the appearance of one massive impact. Additionally, only 15% of binaries that actually strike the Earth's surface are far enough from each other to produce true doublet craters with no overlap; thus, only 3% of all impact craters are likely to be doublets.
Making things even more difficult, geologists, when assessing these sites, have had to deal with poorly preserved, inadequate, and insufficient materials to make accurate radiometric datings; in order to be confirmed as doublets, core samples have to be shown as being the same age. Indeed, some of these apparent double-impacts may not even be the result of binaries, appearing a few weeks, months, or years after the initial strike.
The Best Evidence Yet For A Double Impact
Recently, Jens Ormo and his team from the Centre for Astrobiology in Madrid, Spain, decided to analyze core samples taken from a potential doublet in Sweden known as Lockne and Malingen. These two craters, which appeared about 458 million years ago during the Ordovician Period, are located 10 miles (16 km) away from each other. Lockne measures about 4.6 miles (7.5 km wide) and Malingen is about 10 times smaller. Simulations indicate that the largest asteroid was about 1,968 feet (600 meters wide), and the smaller one about 850 feet (250 meters).
The relatively short distance between the two craters suggests a possible double impact. Indeed, had they been further apart, they would have likely not qualified as doublets because they would have exceeded the maximum distance at which an asteroid and its moon can stay bound by gravitational forces.
After drilling at the site and analyzing the samples (they drilled 485 feet (148.8 meters) through the sedimentary infill), Ormo found tiny plankton-like fossils in the neighboring craters, suggesting a marine impact. These fossils, called chitinozoans, were trapped in sedimentary rocks, which were aged using a method called biostratigraphy. The age of the chitinozoans were the same for each crater, leading the researchers to conclude that they were in fact dealing with a likely doublet.
The researchers recently presented this evidence at the 45th Lunar and Planetary Science Conference. In their accompanying report, they write: "[We] here propose the Lockne-Malingen doublet structure to be the first unequivocal example of an impact binary on Earth."
Their results are set to appear in an upcoming edition of Meteoritics and Planetary Science.