"Antarctica's Tunguska Event"

by Michael Reilly

"Giant, extinction-sized asteroids hurtling into Earth may grab headlines and star in Hollywood blockbusters, but airbursts from smaller asteroids and comets are thought to occur once every 500-1,000 years, making them one of the most pressing threats to humanity from space. That makes the discovery of an Antarctic airburst - a sort of austral Tunguska Event 481,000 years ago - a really big deal. Researchers working on the frozen continent discovered extraterrestrial debris layered in ice cores and strewn across the Transantarctic Mountains. In their presentation this week at the Lunar and Planetary Science Conference in Texas, the researchers said the dust samples were scattered across 2,900 kilometers of Antarctica. The wide distribution of the material points strongly to an airburst origin.

Space typically rocks enter the atmosphere doing a crisp 40 to 60 times the speed of sound. Even when they don't hit the ground they can explode as they become superheated in the atmosphere. When that happens, the heat and shockwave keeps traveling toward Earth's surface. In the desert, such firestorms can leave behind huge swaths of glassy, melted sand behind. In Siberia in 1908, the Tunguska explosion tossed thousands of acres of full-grown trees to the ground like matchsticks.

But because they don't leave behind craters, airbursts can quickly fade into the geological background, making ancient events hard to detect. This latest discovery, led by Luigi Folco and Matthias van Ginneken at the University of Siena, Italy, hints that it may be possible to look for widespread aprons of dust and debris to as markers to of past airbursts. From the BBC: "These events are tricky to spot after they happen. If you go to Tunguska now, you've really got your work cut out trying to find any trace of that event - and that was 1908," Philip Bland of Imperial College, London told BBC News. "What makes Luigi and Matthias' work so exciting is that it may give us a way of spotting these events in the geological record. If these spherules are the signature, we know what to look for in future."

Such a tool could prove vital to confirming just how often these potentially deadly explosions rock the planet. Though the objects behind the blasts are thought to be small - just 20-30 meters (66-98 feet) across is plenty to generate a multi-megaton detonation - if one hit a major population center, it could kill a million people."

- http://news.discovery.com/earth/antarcticas-tunguska-event.html