A massive explosion flattened entire forests in a remote region of Eastern Siberia along the Tunguska River in the early morning of June 30, 1908. Surprisingly, the explosion left no crater, posing a mystery that has puzzled scientists ever since: what could have caused such a massive explosion without leaving any trace of itself?
Daniil Khrennikov of Russia’s Siberian Federal University and colleagues have now published a new model of the impact, which may finally solve the mystery. According to Khrennikov and colleagues, the explosion was caused by an asteroid that grazed the Earth, entered the atmosphere at a shallow angle, and then passed back out into space.
“We argue that the Tunguska event was caused by an iron asteroid body, which passed through the Earth’s atmosphere and continued to the near-solar orbit,” they say. If they are correct, the theory suggests Earth escaped an even larger disaster by a hair’s breadth.
First, some background. Scientists have long conjectured about the cause of the Tunguska impact. The most popular theory is that the explosion was caused by an icy body, such as a comet, entering the atmosphere. The ice then rapidly heated up and evaporated explosively in mid-air, never touching down.
Such an explosion could have been powerful enough to flatten trees without leaving a crater. And it would have left only vapor in the atmosphere as evidence.
However, some of the other evidence contradicts this theory. There were only a few eyewitness reports of the incident. These describe how “the sky split in two,” as well as a massive explosion and widespread fire. They do, however, show that the impactor traveled 435 miles (700 km) through the atmosphere before exploding that morning.
So Khrennikov and colleagues simulated the effect of meteorites made of rock, metal, or ice traveling at a speed of 12 miles per second through the atmosphere (20 kilometers per second). (Meteorites travel at a minimum speed of 11 kilometers per second when they enter the atmosphere.)
These objects are immediately heated by friction with the atmosphere. However, while iron vaporizes at approximately 5,432 degrees Fahrenheit (3000 degrees Celsius), water vaporizes at only 212 degrees Fahrenheit (100 degrees C). As a result, icy meteorites do not last long.
Indeed, Khrennikov and colleagues calculate that an icy body large enough to cause such a large explosion would have traveled only 186 miles (300 kilometers) through the atmosphere before completely vaporizing. This suggests that the Tunguska meteorite was not made of ice.
Instead, Khrennikov and colleagues say that a different scenario fits the facts. They believe the explosion was caused by an iron meteorite the size of a football stadium. This must have passed through the upper atmosphere, heated up fast, and then passed out into the Solar System again. The shock wave generated by this trajectory was responsible for the flattening of trees.
The shock wave would have caused a small explosion, and any vaporized iron would have condensed into dust that would have been indistinguishable on the ground. Importantly, no visible asteroid remnants would have been left behind in this scenario.
It could also explain reports of dust in Europe’s upper atmosphere following the impact.
If Khrennikov and his colleagues are correct, Earth had a lucky near miss that morning. A direct impact with a 656-foot-wide (200-meter-wide) asteroid would have devastated Siberia, leaving a crater 2 miles (3 kilometers) wide. It would also have had catastrophic effects on the biosphere, possibly bringing modern civilization to an end.
Because of the remoteness of the region, the Tunguska impact is thought to have killed only three people. It was clear that things could have gone much worse.