In 1908, 830 square miles of Siberian forest were leveled when a comet entered Earth's atmosphere and broke apart. For many years after the explosion, scientists were unsure just what type of space object caused the massive destruction and desolation. But a comparison of events immediately after what has been named the Tunguska explosion and the events following a space shuttle launch has helped scientists come to a conclusion.
Noctilucent Clouds Key to Comet Identification in Tunguska Explosion
Various objects could have caused the Tunguska explosion, ranging from comets to meteors. Cornell University researchers have come to the conclusion that a comet was the most likely culprit for the event. Following the 1908 explosion, the night skies shone brightly for several days across Europe, something that was particularly noted in Great Britain. Noctilucent clouds were a newer phenomenon, only first described after the eruption of Krakatoa in the late 1800s. The description of the skies over Europe after Tunguska indicate that noctilucent clouds were present.
Noctilucent clouds are the highest clouds in Earth's atmosphere, occurring in the mesosphere some 55 miles up, generally in polar regions. Noctilucent clouds occur most often during summer months, when the temperature near the ground is warm, which causes the temperature higher up to be colder, around -180 degrees Fahrenheit.
A comet, which is a wandering ball of ice and dust, would have injected massive amounts of water vapor into the atmosphere at this level as it broke up over Siberia, seeding the environment to create noctilucent clouds.
How the Space Shuttle Led to This Discovery
"It's almost like putting together a 100-year-old murder mystery," says 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."
After a space shuttle launch on August 8, 2007, Kelley and colleagues saw the noctilucent cloud phenomenon in the days that followed. Noctilucent clouds had also been observed after previous shuttle launches in 1997 and 2003.
The space shuttle exhaust plume resembles the comet's action. A single space shuttle flight injects 300 metric tons of water vapor into Earth's upper atmosphere, with the particles traveling to Arctic and Antarctic regions, forming noctilucent clouds.
New Physics of the Upper Atmosphere
Another mystery of the Tunguska explosion would then be how the water vapor traveled so far without scattering and diffusing, as conventional physics would predict.
"There is a mean transport of this material for tens of thousands of kilometers in a very short time, and there is no model that predicts that," Kelley says. "It's totally new and unexpected physics."
Scientists are explaining this new physics with counter-rotating eddies with extreme energy. When water vapor enters one of these eddies, it is rushed along very quickly, at nearly 300 feet per second, allowing it to travel great distances and stay intact.
It is difficult to study the wind structure in the upper regions of Earth's atmosphere, and these observations show scientists that they still have much to learn about the mesosphere.
Source: Cornell University
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