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1908 Tunguska explosion was caused by a comet
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It has been a debate for more than 100 years: what caused the Tunguska explosion in 1908, an event which devastated 830 square miles of Siberian forests? Perhaps it is not surprising that the mystery has stood for more than a century as it took the exhaust plumes of space shuttles (decidedly modern objects) to discover the answer.
"It's almost like putting together a 100-year-old murder mystery," said Michael Kelley, the James A. Friend Family Distinguished Professor of Engineering at Cornell who led the research team. "The evidence is pretty strong that the Earth was hit by a comet in 1908." [EurekAlert]
Space shuttle plumes is only one part of the story—the final part. The real mystery began after the explosion 100 years ago, when strange clouds appeared all across the European continent. The clouds, of the noctilucent type, did not usually show up in places as warm as England. They are normally found in the Antarctic and Arctic, where the Earth’s “highest clouds, [form] naturally in the mesosphere at about 55 miles over the polar regions during the summer months when the mesosphere is around minus 180 degrees Fahrenheit.” [EurekAlert]
So how to freezing clouds and space shuttles solve a 100-year-old murder mystery? The key is water vapor. Comets, unlike meteors (the other suspect in the explosion), are composed of massive amounts of ice. As they burn up in the atmosphere of a planet, that ice is rapidly converted to violently moving water vapor known as eddies.
Space shuttles also give off considerable water vapor (300 metric tons) when they launch. And what’s more, that vapor has been known “to travel to the Arctic and Antarctic regions, where they form the clouds after settling into the mesosphere.” [EurekAlert] Therefore, the formation of noctilucent clouds in 1908 points to a comet causing the explosion.
There is still a problem with this theory: how did the water vapor travel 3,000 miles instead of simply diffusing as would be expected? Current physics does not explain the phenomenon, so researchers created a new hypothesis, that the travel of the vapor was tied to “counter-rotating eddies with extreme energy. Once the water vapor got caught up in these eddies, the water traveled very quickly -- close to 300 feet per second.” [EurekAlert]
Whether this theory is correct will need to be tested. However, probing the atmosphere at such a high altitude has proven difficult in the past. Perhaps that is why it has taken so long, and needed the help of space shuttles, to finally come to a conclusive decision about the cause of the 1908 Tunguska explosion. "Our observations show that current understanding of the mesosphere…region is quite poor," Seyler said. [EurekAlert] Hopefully, however, this research is a step towards better understanding.
The study was published in Geophysical Research Letters.
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