Cosmic forensics: signs of a Tunguska meteorite?
“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… I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few metres… 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.”
This eyewitness account doesn’t describe the detonation of a nuclear bomb. The events occurred in 1908, long before the Manhattan Project. The account describes the Tunguska Event, an enigmatic explosion in central Siberia thought to be caused by a comet or meteoroid. It must have exploded before reaching the ground, unleashing the energy of a thousand Hiroshima bombs.
The blast flattened over 2,000 square kilometers of Siberian forest, though the trees at ground zero were left standing, stripped of their branches and charred. Bright night skies and strange sunsets were reported across central Asia and northern Europe for several days. Due to its extremely secluded location, it would be nearly two decades before an expedition examined the destruction.
What they found was baffling—all the signs of a major impact, but no crater and not a single piece of an impactor. L.A. Kulick, the Russian mineralogist who led the first expedition, undertook massive efforts to dig for fragments, but his effort was never rewarded. Consequently, it was thought that the impactor, possibly an icy comet, had utterly vaporized in the explosion, leaving no trace.
But the comet hypothesis has some issues. Researchers examining the peaty sediment have found slight anomalies of iridium and isotopes of hydrogen, carbon, and nitrogen suggestive of chondritic (stony) asteroids. Others have also found meteorite-like microparticles in the remains of trees there. Still, there’s no smoking gun.
This week in the journal Geochemistry, Geophysics, Geosystems (or G3), a team of Italian researchers reports that they’ve found tantalizing evidence of what could be a stony chunk of meteorite beneath a small lake eight kilometers from the epicenter of the blast.