Asteroid Day this year, 30th June 2019, is exactly 111 years after the Tunguska impact event in Siberia, which destroyed an area of pristine forest the size of Tokyo. With blasted and burnt tree trunks leveled and stripped bare over such a vast area, it is as though a large atomic bomb had been dropped on the forest.

The debate still goes on in the research literature, but a popular theory is that this impact was caused by a small comet fragment, in the region of 328 feet (100 meters) in diameter, that exploded at an altitude of around 5 miles (8 kilometers). Its immense downward momentum caused the resulting fireball to target the forest below, like a great blast of dragon breath. No crater was formed, and due to its remote location, it’s thought nobody was actually killed.

The date of the impact and its apparent direction of travel suggest it was likely caused by a chunk of comet originating from the Taurid meteor stream, the largest meteor stream seen from Earth. The impact energy has been inferred from seismic records at the time of the explosive event as well as computer simulations of the air burst and resulting damage on the ground. At around 5 to 15 megatons of TNT, the magnitude of the Tunguska explosion is often compared to 1000 Hiroshima bombs.

Collision with a larger comet, say 0.6 miles (1 kilometer) in diameter, would have generated an explosion around 1000 times larger again, i.e. 1 million Hiroshima bombs. It’s hard to imagine the scale of such an event, but this is the kind of energy scale that has been proposed for the Younger Dryas impact event.

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When Did the Younger Dryas Event Occur?

The Younger Dryas impact is thought to have occurred between 10,800 to 10,900 BC. Again, it might have been caused by the Taurid meteor stream , although this time the general view is that we collided with a swarm of comet debris, rather than a single object. The impact was so devastating and covered such a wide area, that it is thought to be responsible for an entire geological age (a mini ice-age lasting 1,300 years) known as the Younger Dryas period when temperatures in the northern hemisphere were 59 degrees Fahrenheit (15 degrees Celsius) lower than they are today.

The event caused the extinction of many large animals, especially in the Americas, such as the mastodon and sabre-tooth tiger. Quite possibly, many of today’s well-known myths and religions, including Ragnarok and Christianity, derive originally from a comet-cult inspired by this event. It could hardly be a more important topic.

The Debate on Younger Dryas

The research debate surrounding this impact event has been very contentious. Supporters of pre-existing theories for the demise of the American megafauna and the occurrence of the Younger Dryas mini ice age have, naturally, been highly critical of this recent interloper into their long-standing stalemate.

But ultimately, it is not evidence of large animal extinctions or rapid climate change that determine whether a cosmic impact event occurred at this time, as these effects are not diagnostic for such events. That is, these effects can also be generated by other mechanisms. To discover whether a cosmic impact occurred at the beginning of the Younger Dryas period, or the YD ‘boundary’ as it is known, we should instead consider the geochemical evidence, which consists of unusual chemicals or materials generated by the impact remaining in the ground, as this is diagnostic.

Normally, a large asteroidal impact will leave a crater, even if the impact occurs into an ocean – the crater will be in the sea bed in this case. No crater has yet been confirmed for the YD event, although one, and perhaps two, geologically young but very large impact craters under the Greenland ice sheet, discovered in only the last year, appear to be good candidates.

Nevertheless, because the ages of these craters are not yet established, we should instead consider the kind of geochemical signals left behind by Tunguska-like air-bursts at the YD boundary. In fact, many types of exotic material, potentially of impact origin, have been found at this boundary, which appears as a conspicuous band of discolored sediment, or ‘black mat’, at many sites across North America and beyond. Arguably, the three most convincing types of evidence are an abundance of iridium-enriched magnetic grains, nanodiamonds, and high levels of platinum group metals, such as platinum itself or iridium, at the base of the YD black mat.

Magnetic grains, often too small to see with the naked eye, can be formed from iron, common in asteroids and comets, vaporizing at very high temperature, in a massive explosion for example, and then condensing into microscopic magnetic iron droplets, like water condensing as rain. If formed within a cometary air-bust they would be flung through the atmosphere for great distances, creating a carpet or layer of fine magnetic grains over a large expanse of Earth’s surface. Therefore, a sudden large peak, or abundance, in a layer of sediment of iridium-enriched magnetic grains is a good indicator of a cosmic impact, since other sources should be relatively constant over time.

Likewise, nanodiamonds are thought to form within carbon-rich droplets condensing from vaporized carbon generated by a high-temperature and pressure explosion. Asteroids and comets can be rich in carbon, and a cosmic impact might create the conditions required for the formation of carbon droplets containing nanodiamonds. Alternatively, nanodiamonds are common within some asteroids and comets, which means an impact event would simply distribute them over a large patch of Earth’s surface. Therefore, a sudden large peak in nanodiamonds within sediments also very likely indicates a comet or asteroid strike.

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