New research on Tunguska finds such events happen less often than we thought

Enlarge / The famed Tunguska event of 1908 scorched a five-mile swath of trees and caused many to fall away from the center of the blast in a distinct radial pattern. (credit: Photo12/Universal Images Group via Getty Image)

Last month marked the 111th anniversary of the Tunguska event, a blast that flattened trees across half a million acres of Siberian forest on June 20, 1908. Scientists have been puzzling over the details ever since. We now have fresh evidence about what transpired back then, in the form of new data gleaned from a well-documented rare meteor burst near Chelyabinsk, Russia, in February 2013. That data shores up the hypothesis that the Tunguska event was most likely due to an asteroid impact. The findings are described in a series of scholarly papers commemorating the event, published in a special July 15 issue of the journal Icarus.

Seismometers all over the world recorded the Tunguska impact, which hit 5.0 on the Richter scale in some locations. But there weren’t many human eyewitnesses, given its remote location—first-hand observations came mostly from a few Russian settlers and Evenki natives. They described a streak of light across the sky, followed by another flash of light and a loud sound with accompanying shock wave. “Suddenly the sky appeared like it was split in two, high above the forest, the whole northern sky appeared to be completely covered with blazing fire,” a farmer named Sergei Semenov recalled; he’d been having breakfast just 40 miles (64km) from the impact. “At that moment, I felt a great wave of heat as if my shirt had caught fire.” The shock wave was strong enough to knock him off his chair.

Sky fall

Still, the impact site was so remote that nobody investigated for more than a decade. It wasn’t until 1927 that Russian mineralogist Leonid Kulik led a scientific expedition to the area. His Evenki guides believed the blast had been a punishment from their god of thunder, Agda. Kulik, on the other hand, believed it had been a meteor and was surprised to find no impact crater. But trees had been scorched over a five-mile radius, with all their branches blown off. Kulik made three more expeditions, during which he discovered small bogs resembling potholes. He thought those might be impact craters but found an old stump at the bottom of one when he drained it, effectively ruling out that hypothesis.

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