The largest impact event in recorded history may have been caused by the shock waves generated by an iron asteroid that entered the Earth's atmosphere at a shallow angle before heading back into space, a team of Russian scientists have proposed.
The researchers say that the massive explosion associated with the event may not not have been caused by a space object that fell towards the ground, as is generally believed, potentially explaining the almost total absence of meteorite fragments on the Earth's surface, according to a study published in the journal Monthly Notices of the Royal Astronomical Society.
On the morning of June 30, 1908, a huge explosion occurred in the skies above the Podkamennaya Tunguska River in a remote region of Siberia, which released hundreds of times more energy than the nuclear bomb dropped on Hiroshima while flattening more than 80 million trees across some 500,000 acres of forest and killing hundreds of reindeer, according to the Encyclopaedia Britannica.
But despite the widespread destruction on the ground, only a few tiny fragments that could potentially have come from the offending object have ever been found. While it is classified as an impact event, to date, no impact crater has been uncovered even though the damage on the ground had a clear epicenter.
The explosion—which came to be known as the "Tunguska event"—has long puzzled experts, with dozens of different hypotheses proposed to explain the phenomenon, ranging from a comet entering the atmosphere to a massive release of natural gas from the Earth's crust.
However, the general consensus among scientists is that the explosion was caused by an asteroid which entered the Earth's atmosphere at incredibly high speed and blew up as a fireball due to the combination of pressure and heat generated by air friction.
"A century later some still debate the cause and come up with different scenarios that could have caused the explosion," Don Yeomans, a former NASA planetary scientist, wrote. "But the generally agreed upon theory is that on the morning of June 30, 1908, a large space rock, about 120 feet across, entered the atmosphere of Siberia and then detonated in the sky."
"That is why there is no impact crater," said Yeomans. "The great majority of the asteroid is consumed in the explosion."
But despite the asteroid hypothesis, the Russian researchers say that the event has not received "reasonable and comprehensive interpretations to date." And in their latest paper, the Russian researchers propose a new explanation of the Tunguska event based on mathematical modelling of the behavior of space objects that pass through the Earth's atmosphere and continue again into outer space without blowing up or striking the ground.
The objects they examined in their computer modelling had diameters of 50, 100 and 200 meters (164, 328 and 656 feet,) and were made from three types of materials—iron, stone and water ice—all of which have previously been proposed as possible materials for the object that caused the event.
According to the team's modelling, the explosion could have been caused by an iron asteroid—with a probable size between 100 and 200 meters (320-650 feet)—passing through the atmosphere at a minimum altitude of 10-15 kilometers (6.2-9.3 miles,) while travelling at a speed of around 20 kilometers per second (around 44,700 miles per hour.)
The asteroid may have shed around half its initial mass—which the team estimated to be around three million tons—during its 1,800-mile journey through the atmosphere; its high speed increased the temperature of the object to several thousands of degrees Celsius at the surface.
According to the scientists, any of the iron material lost during this process would have sublimated—or turned directly from solid to gas—meaning they would leave little detectable trace, as they appear exactly like iron oxides found on Earth. This rapid loss in mass would have created a destructive shockwave capable of flattening trees over wide areas without leaving any traces of iron droplets on the Earth's surface.
According to the scientists, their results rule out an object made of stone or ice—for example, a comet.
"We calculated the trajectory characteristics for space objects with a diameter of 200 to 50 meters, consisting of iron, ice or rock, such as quartz and lunar soil," Sergei Karpov, an author of the study, told Science in Siberia. "The model shows that the Tunguska body could not consist of stone or ice, since the low strength of these internally heterogeneous materials, unlike iron, leads to the rapid destruction of such bodies in the atmosphere under tremendous aerodynamic pressure."
Furthermore, this version of events can help to explain "optical effects associated with a strong dustiness in high layers of the atmosphere over Europe, which caused a bright glow of the night sky," the authors wrote in the study.
The researchers note that their study has some limitations, thus, further research is needed to support their model. However, they say that comparisons to the Chelyabinsk meteor that exploded over another part of Russia in 2013 indicate that it's feasible an iron asteroid like the one detailed in the study could cause such a large, destructive shockwave.
While the Tunguska event was the largest impact event in Earth's recorded history—despite the lack of a crater—much larger impacts have occurred in prehistoric times, for example, the asteroid that struck our planet around 66 million years ago, famously leading to the extinction of the dinosaurs.
Although the Tunguska event may have led to the deaths of only three people—these fatalities are unconfirmed—its destructive impact should not be underestimated.
Eyewitnesses reported seeing a fireball light up the horizon, before the ground started trembling and hot winds strong enough to throw people over and shake buildings surged through the area. Furthermore, seismic instruments as far away as western Europe picked up seismic activity that was generated by the blast.