Russian Mystery Blast Caused by Comet, Acid Rain Hints

James Owen
for National Geographic News
July 22, 2008
The theory that a comet-based meteorite devastated a vast area of Siberia when it blew apart in Earth's atmosphere a hundred years ago is supported by growing evidence of acid rain triggered by the event, scientists announced.

The Tunguska explosion occurred on June 30, 1908, when some 80 million trees covering 770 square miles (2,000 square kilometers) were flattened by an immense blast. The explosion emitted a force equivalent to about 200 Hiroshima-size atomic bombs.

Few people witnessed the spectacle in the remote eastern Russia region, and no confirmed impact crater or meteorite fragments have ever been found.

(Related: "Crater From 1908 Russian Space Impact Found, Team Says" [November 7, 2007].)

Yet the widely held belief that a meteorite caused the explosion is supported by analyses of peat deposits in the disaster zone north of Lake Baikal, researchers reported at a recent scientific conference in Moscow to mark the event's centennial.

Ongoing research led by geologist Evgeniy Kolesnikov of Moscow State University has revealed a significant spike in uncommon types of nitrogen and carbon isotopes in peat layers dating to the time of the catastrophe.

Definite Evidence

These chemical traces, researchers say, suggest that high levels of acid rain fell in response to a cometary meteorite burning up in the atmosphere.

Around 200,000 tons of nitrogen likely showered the region, they say.

Extreme temperatures of 3,992 to 4,892 degrees Fahrenheit (2,200 to 2,700 degrees Celsius) produced by the meteorite may have caused oxygen to react with nitrogen in the atmosphere to form nitrogen oxides, team member Natalya Kolesnikova of Moscow State University said by email.

Nitrogen oxides react in the atmosphere to create nitric acid, which collects in clouds and water droplets and falls as acid rain.

The 1908 Tunguska peat layer also yielded unusually high amounts of iridium, an element abundant in space but rare on Earth.

"That something from outside Earth exploded there, I think is without doubt," said atmospheric physicist Gabriele Curci of the University of L'Aquila, Italy.

"There is definitely evidence of acid rain," he added.

"An explosion at many thousands of degrees in the atmosphere is very efficient in producing nitrogen oxides. It's the same mechanism that creates hydrogen oxides after lightning."

ET Source

The evidence for heavy acid rain that covered a wide area supports the notion that the Tunguska meteorite blasted apart in the atmosphere and so left no obvious impact crater, Curci said.

"I don't think the meteorite hit the ground—it probably exploded at 9 or 10 kilometers [5.6 to 6.2 miles] of altitude," she said.

Meanwhile, the iridium traces detected by Kolesnikov's team were around 50 times higher than normal, Curci noted.

"It was definitely an extraterrestrial source," he added. "There is no other explanation."

Kolesnikova, of Moscow State University, said the study findings also suggest the meteorite involved was a comet—a body made up mainly of frozen water and gas—and not an asteroid, which is made of rocky material.

"The Tunguska cosmic body was rich in nitrogen, together with carbon and hydrogen," she said. "That means the body was a comet."

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