Teeny Tiny Rock Fragments Testify That a Meteor Caused the Biggest Impact Event in Recorded History

On June 30, 1908, an enormous explosion in a remote stretch of Siberia flattened and burned nearly 1,000 square miles of forest, totaling around 80 million trees. Called the Tunguska event (named after a nearby river), it’s the largest impact event in Earth’s recorded history. A local testified about his experience during the event to an expedition that came through the area in 1930:

I suddenly saw that directly to the north, over Onkoul’s Tunguska Road, the sky split in two and fire appeared high and wide over the forest . The split in the sky grew larger, and the entire northern side was covered with fire. At that moment I became so hot that I couldn’t bear it, as if my shirt was on fire; from the northern side, where the fire was, came strong heat….When the sky opened up, hot wind raced between the houses, like from cannons, which left traces in the ground like pathways, and it damaged some crops.

The Tunguska blast inspired more than 1,000 scientific publications, many of them seeking to identify the explosion’s cause. For years, researchers speculated that a meteor caused the destruction, Nature reports, and now new evidence making that link has been discovered.

Researchers led by Victor Kvasnytsya at the Institute of Geochemistry, Mineralogy and Ore Formation of the National Academy of Science of Ukraine in Kiev say that they have found a smoking gun. In what Kvasnytsya describes as the most detailed analysis yet of any candidate sample from the Tunguska event, the researchers conclude that their fragments of rock — each less than 1 millimetre wide — came from the iron-rich meteor that caused the blast.

The researchers reexamined rocks collected back in the 1970s from the blast site. Using transmission electron microscopy, a chemistry technique, they found that the rocks—originally labeled as coming from Earth—featured tell-tale meteorite mineral concentrations, such as troilite and schreibersite. The team thinks this evidence shows that the Tunguska blast was caused by an asteroid-turned-meteorite.

This is one of those puzzles, though, that inspires fiercely held pet theories. Other explanations for the blast include a comet, a natural H-bomb, a black hole, antimatter and a sudden release of natural gas from the Earth’s core. Most in the scientific community reject these conjectures, but not everyone’s convinced that the evidence Kvasnytsya’s team turned up will end this debate forever. Nature reports:

 “We get a lot of meteorite material raining down on us all the time,” adds Bland. Without samples of adjacent peat layers for comparison, “it’s hard to be 100% sure that you’re not looking at that background”.

More from Smithsonian.com:

Doomsday in Tunguska 
The Last Massive Exploding Meteor Hit Earth in 1908, Leveling 800 Square Miles of Forest 

Rachel Nuwer | | READ MORE

Rachel Nuwer is a freelance science writer based in Brooklyn.