On June 30, 1908, an asteroid destroyed 80 million trees in a remote, sparsely populated area of approximately 2,150 square kilometers near the Podkamennaya Tunguska River in Siberia, known as the Tunguska Event. The blast, roughly equivalent to 10-15 million tons of TNT, was similar in scale to the 1954 Castle Bravo nuclear bomb test, the largest asteroid impact event in history, yet no one has found asteroid fragments or the impact site.
Eyewitness reports: The sky was split in two, and the sky in the northern part of the forest seemed to be completely covered by fire.
The prevailing theory is that the asteroid formed Lake Cheko, a freshwater lake about 8 kilometers from the epicenter of the explosion, about 500 meters wide and 54 meters deep. Luca Gasperini, director of research at the Italian National Research Council, said: Lake Checo’s cone shape and depth are similar to impact craters. The research team’s research results were published in the journal “Geochemistry, Geophysics, Geosystems” in 2012. It is estimated that the sediments at the bottom of the lake have accumulated for 100 years, and the evidence of trees at the bottom of the lake indicates that the lake area was originally a forest.
However, in 2017, researchers led by Denis Rogozin of the Institute of Biophysics of the Siberian Branch of the Russian Academy of Sciences analyzed that the lake sediments were at least 280 to 390 years old, earlier than the Tunguska event.
In a new study in the journal Doklady Earth Sciences, Rogozin’s research team provides more evidence to refute the idea that Lake Cheko was the site of the Tunguska asteroid impact. Lake Checo’s unusual conical shape was previously considered unique by many researchers, adding weight to the hypothesis that an asteroid formed Lake Checo. But Rogozin’s research team analyzed two nearby lakes, Zapovednoye and Peyungda, 50 kilometers and 60 kilometers away from the suspected impact site, both of which are conical.
But Gasperini says the lake’s conical shape isn’t the only evidence that Lake Checo was formed by the Tunguska event. In the 2018 arxiv paper, Gasperini and his team hypothesized that the Tunguska event was caused by a “rubble pile” asteroid with a fragile structure. The asteroid split into two pieces, one with a width of about 60 meters and the other with a width of about 6 ~10 meters, smaller pieces hit the Earth, forming Lake Checo.
The team found a 10-meter-wide anomaly at the bottom of the lake, possibly the leftover fragments of an asteroid. Drilling cores could test the anomalous composition to confirm the hypothesis, but Gasperini’s team was unable to revisit the site because of the Russo-Ukraine war.
What could have happened to this asteroid?
If Lake Checo wasn’t formed by the Tunguska crater, what about the asteroid that sparked the fire more than a century ago? The 2020 “Monthly Notices of the Royal Astronomical Society” journal paper stated that a large iron asteroid passed through the Earth’s atmosphere and then left the Earth without breaking up. According to the research team, this could explain why no asteroid fragments were found.
Another paper on arxiv in April proposed another hypothesis: that the asteroid broke up and scattered. While many pieces burn up as they pass through the atmosphere, some may survive and scatter. The paper shows that the asteroid debris may be located about 16 to 19 kilometers northwest of the epicentre, and that dirt and vegetation may make the trace disappear.
(This article is reproduced with the authorization of Taipei Planetarium; the first picture is a schematic diagram, source: pixabay)
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