"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."
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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