THE TUNGUSKA EVENT, THE LARGEST IMPACT ON EARTH

Tunguska, The X Files, 1996

The terrible heat wave continues and The Grandma doesn't want to go out. She is better at home with her air conditioning.

Today, she has decided to watch an interesting documentary about the Tunguska event, the largest explosion and impact event on Earth in recorded history.

First time The Grandma listened to Tunguska was in November 1996 when she watched an episode of her favourite TV Series, The X Files. The episode talked about this mysterious event and proposed more questions than answers offered. It was a double episode Tunguska-Terma belonged to the fourth season of the series.

Some years later, she listened to Tunguska in a conference about climate change and its consequences. The documentary explained how humans are helping to destroy the Earth and how many menaces from outer space exist.

More information: CNET

On a day like today in 1908, a large explosion occurred near the Podkamennaya Tunguska River in Yeniseysk Governorate, the unsolved mystery about Tunguska started...

The heat wave that Europe is living during these days, the fires that are destroying our forests, the drought that some parts of the continent are suffering, the images of Polar bears without food and outside their habitat are evidences of a climate change that nobody wants to accepts, and the worst, nobody -who has the power to do it- is doing nothing to stop it.

The Tunguska event was a large explosion that occurred near the Podkamennaya Tunguska River in Yeniseysk Governorate, now Krasnoyarsk Krai, Russia, on the morning of 30 June 1908 (NS).

The explosion over the sparsely populated Eastern Siberian Taiga flattened 2,000 square kilometres of forest, yet caused no known human casualties. The explosion is generally attributed to the air burst of a meteor.

It is classified as an impact event, even though no impact crater has been found; the object is thought to have disintegrated at an altitude of 5 to 10 kilometres rather than to have hit the surface of the Earth.

The Tunguska event is the largest impact event on Earth in recorded history. Studies have yielded different estimates of the meteoroid's size, on the order of 60 to 190 metres, depending on whether the body was a comet or a denser asteroid.

Fallen trees from the impact focus, Tunguska

Since the 1908 event, there have been an estimated 1,000 scholarly papers, most in Russian, published on the Tunguska explosion.

In 2013, a team of researchers published analysis results of micro-samples from a peat bog near the center of the affected area showing fragments that may be of meteoritic origin.

Early estimates of the energy of the air burst range from 10–15 megatons of TNT, 42–63 petajoules, to 30 megatons of TNT, 130 PJ, depending on the exact height of burst estimated when the scaling-laws from the effects of nuclear weapons are employed.  However, modern supercomputer calculations that include the effect of the object's momentum find that more of the energy was focused downward than would be the case from a nuclear explosion and estimate that the airburst had an energy range from 3 to 5 megatons of TNT, 13 to 21 PJ. A newer finding suggests the explosive power may have been around 20–30 megatons.

The 15-megaton (Mt) estimate represents an energy about 1,000 times greater than that of the atomic bomb dropped on Hiroshima, Japan -roughly equal to that of the United States' Castle Bravo (15.2 Mt) ground-based thermonuclear detonation on 1 March 1954, and about one-third that of the Soviet Union's Tsar Bomba explosion on 30 October 1961, which at 50 Mt, is the largest nuclear weapon ever detonated.

More information: BBC

It is estimated that the Tunguska explosion knocked down some 80 million trees over an area of 2,150 km2, and that the shock wave from the blast would have measured 5.0 on the Richter magnitude scale.

An explosion of this magnitude would be capable of destroying a large metropolitan area, but, due to the remoteness of the location, no human fatalities were officially documented. Several reports have indicated that two people may have died in the event, but those deaths remain unofficial. The Tunguska event has helped to spark discussion of asteroid impact avoidance.

On 30 June 1908, at around 07:17 local time, Evenki natives and Russian settlers in the hills north-west of Lake Baikal observed a column of bluish light, nearly as bright as the Sun, moving across the sky.

About ten minutes later, there was a flash and a sound similar to artillery fire. Eyewitnesses closer to the explosion reported that the source of the sound moved from the east to the north of them.

The sounds were accompanied by a shock wave that knocked people off their feet and broke windows hundreds of kilometres away.

The size of the Tunguska explosion in Russia, 1908

The majority of witnesses reported only the sounds and tremors, and did not report seeing the explosion. Eyewitness accounts vary regarding the sequence and duration of the events.

The explosion registered at seismic stations across Eurasia, and air waves from the blast were detected in Germany, Denmark, Croatia, the UK, and as far away as Batavia and Washington, D.C. It is estimated that, in some places, the resulting shock wave was equivalent to an earthquake measuring 5.0 on the Richter magnitude scale. Over the next few days night skies in Asia and Europe were aglow, with contemporaneous reports of photographs being successfully taken at midnight in both Sweden and Scotland.

It has been theorized that this effect was due to light passing through high-altitude ice particles that had formed at extremely low temperatures—a phenomenon that many years later would be produced by space shuttles.

In the United States, a Smithsonian Astrophysical Observatory program at the Mount Wilson Observatory observed a months-long decrease in atmospheric transparency consistent with an increase in suspended dust particles.

More information: The Guardian

It was more than a decade after the event before any scientific analysis of the region took place.

In 1921, the Russian mineralogist Leonid Kulik led a team to the Podkamennaya Tunguska River basin to conduct a survey for the Soviet Academy of Sciences.

Although they never visited the central blast area, the many local accounts of the event led Kulik to believe that the explosion had been caused by a giant meteorite impact. Upon returning, he eventually persuaded the Soviet government to fund an expedition to the suspected impact zone, based on the prospect of salvaging meteoric iron.

Kulik was finally able to lead a scientific expedition to the Tunguska blast site in 1927. He hired local Evenki hunters to guide them to the center of the blast area, where they expected to find an impact crater. To their surprise, there was no crater to be found at ground zero. Instead they found a zone, roughly 8 kilometers across, where the trees were scorched and devoid of branches, but still standing upright. The trees farther away had been partly scorched and knocked down in a direction away from the center.

Leonid A. Kulik

Much later, in the 1960s, it was established that the zone of levelled forest occupied an area of 2,150 km2, its shape resembling a gigantic spread-eagled butterfly with a wingspan of 70 km and a body length of 55 km. Upon closer examination, Kulik located holes that he erroneously concluded were meteorite holes; he did not have the means at that time to excavate the holes.

During the next ten years there were three more expeditions to the area. Kulik found several dozens of little pothole"bogs, each some 10 to 50 metres in diameter, that he thought might be meteoric craters.

After a laborious exercise in draining one of these bogs the so-called Suslov's crater, 32 min diameter, he found an old stump on the bottom, ruling out the possibility that it was a meteoric crater.

In 1938, Kulik arranged for an aerial photographic survey of the area covering the central part of the levelled forest, 250 square kilometres. The negatives of these aerial photographs -1,500 negatives, each 18 by 18 centimetres- were burned in 1975 by order of Yevgeny Krinov, then Chairman of the Committee on Meteorites of the USSR Academy of Sciences, as part of an initiative to dispose of hazardous nitrate film. Positive prints were preserved for further study in the Russian city of Tomsk.

More information: The Conversation

Expeditions sent to the area in the 1950s and 1960s found microscopic silicate and magnetite spheres in siftings of the soil. Similar spheres were predicted to exist in the felled trees, although they could not be detected by contemporary means.

Later expeditions did identify such spheres in the resin of the trees. Chemical analysis showed that the spheres contained high proportions of nickel relative to iron, which is also found in meteorites, leading to the conclusion they were of extraterrestrial origin. The concentration of the spheres in different regions of the soil was also found to be consistent with the expected distribution of debris from a meteoroid air burst. Later studies of the spheres found unusual ratios of numerous other metals relative to the surrounding environment, which was taken as further evidence of their extraterrestrial origin.

Chemical analysis of peat bogs from the area also revealed numerous anomalies considered consistent with an impact event.

The isotopic signatures of stable carbon, hydrogen, and nitrogen isotopes at the layer of the bogs corresponding to 1908 were found to be inconsistent with the isotopic ratios measured in the adjacent layers, and this abnormality was not found in bogs located outside the area.

The region of the bogs showing these anomalous signatures also contains an unusually high proportion of iridium, similar to the iridium layer found in the Cretaceous–Paleogene boundary.
 

Flattened trees from the Tunguska Event

These unusual proportions are believed to result from debris from the falling body that deposited in the bogs. The nitrogen is believed to have been deposited as acid rain, a suspected fallout from the explosion. The leading scientific explanation for the explosion is the air burst of an asteroid 6–10 km above Earth's surface.

Meteoroids enter Earth's atmosphere from outer space every day, travelling at a speed of at least 11 km/s. The heat generated by compression of air in front of the body, ram pressure, as it travels through the atmosphere is immense and most meteoroids burn up or explode before they reach the ground.

Since the second half of the 20th century, close monitoring of Earth's atmosphere through infrasound and satellite observation has shown that asteroid air bursts with energies comparable to those of nuclear weapons routinely occur, although Tunguska-sized 5-15 megaton events are much rarer.

Eugene Shoemaker estimated that 20 kiloton events occur annually and that Tunguska sized events occur about once every 300 years. More recent estimates place Tunguska-sized events at about once every thousand years, with 5 kiloton air bursts averaging about once per year. Most of these air bursts are thought to be caused by asteroid impactors as opposed to mechanically weaker cometary materials based on their typical penetration depths into the Earth's atmosphere. The largest asteroid air burst to be observed with modern instrumentation was the 500 kiloton Chelyabinsk meteor of 2013, which shattered windows and produced meteorites.

More information: Atlas Obscura

In 1930, the British astronomer F. J. W. Whipple suggested that the Tunguska body was a small comet. A comet is composed of dust and volatiles, such as water ice and frozen gases, and could have been completely vaporised by the impact with Earth's atmosphere, leaving no obvious traces. The comet hypothesis was further supported by the glowing skies or skyglows or bright nights observed across Europe for several evenings after the impact, possibly explained by dust and ice that had been dispersed from the comet's tail across the upper atmosphere. The cometary hypothesis gained a general acceptance amongst Soviet Tunguska investigators by the 1960s.

In 1978, Slovak astronomer Ľubor Kresák suggested that the body was a fragment of Comet Encke. This is a periodic comet with an extremely short period of 3 years that stays entirely within the orbit of Jupiter. It is also responsible for the Beta Taurids, an annual meteor shower with a maximum activity around 28-29 June. The Tunguska event coincided with the peak activity of that shower, and the approximate trajectory of the Tunguska object is consistent with what would be expected from a fragment of Comet Encke. 

The Tunguska Crater, nowadays

It is now known that bodies of this kind explode at frequent intervals tens to hundreds of kilometres above the ground. Military satellites have been observing these explosions for decades. During 2019 astronomers hope to search for hypothesized asteroids ~100 meters in diameter from the Taurid swarm between July 5-11, and July 21-August 10.

In 1983, astronomer Zdeněk Sekanina published a paper criticising the comet hypothesis. He pointed out that a body composed of cometary material, travelling through the atmosphere along such a shallow trajectory, ought to have disintegrated, whereas the Tunguska body apparently remained intact into the lower atmosphere.

Sekanina argued that the evidence pointed to a dense, rocky object, probably of asteroidal origin. This hypothesis was further boosted in 2001, when Farinella, Foschini, et al. released a study calculating the probabilities based on orbital modelling extracted from the atmospheric trajectories of the Tunguska object. They concluded with a probability of 83% that the object moved on an asteroidal path originating from the asteroid belt, rather than on a cometary one, probability of 17%.

More information: All That Is Interesting

In June 2007, scientists from the University of Bologna identified a lake in the Tunguska region as a possible impact crater from the event. They do not dispute that the Tunguska body exploded in mid-air but believe that a ten-metre fragment survived the explosion and struck the ground.

Lake Cheko is a small, bowl-shaped lake approximately 8 km north-northwest of the hypocentre. The hypothesis has been disputed by other impact crater specialists.

A 1961 investigation had dismissed a modern origin of Lake Cheko, saying that the presence of metres-thick silt deposits at the lake's bed suggests an age of at least 5,000 years, but more recent research suggests that only a metre or so of the sediment layer on the lake bed is normal lacustrine sedimentation, a depth consistent with an age of about 100 years.

Acoustic-echo soundings of the lake floor provide support for the hypothesis that the lake was formed by the Tunguska event. The soundings revealed a conical shape for the lake bed, which is consistent with an impact crater. Magnetic readings indicate a possible metre-sized chunk of rock below the lake's deepest point that may be a fragment of the colliding body.

The Tunguska Event Epicenter

Finally, the lake's long axis points to the hypocentre of the Tunguska explosion, about 7.0 km away. Work is still being done at Lake Cheko to determine its origins.

The scientific consensus is that the explosion was caused by the impact of a small asteroid; however, there are some dissenters.

Astrophysicist Wolfgang Kundt has proposed that the Tunguska event was caused by the release and subsequent explosion of 10 million tons of natural gas from within Earth's crust.

The basic idea is that natural gas leaked out of the crust and then rose to its equal-density height in the atmosphere; from there, it drifted downwind, in a sort of wick, which eventually found an ignition source such as lightning. Once the gas was ignited, the fire streaked along the wick, and then down to the source of the leak in the ground, whereupon there was the explosion.

More information: Sputnik News

The similar verneshot hypothesis has also been proposed as a possible cause of the Tunguska event. Other research has supported a geophysical mechanism for the event.

The Tunguska event is not the only example of a great unobserved explosion event. For example, the 1930 Curuçá River event in Brazil was an explosion of a superbolide that left no clear evidence of an impact crater.

Modern developments in infrasound detection by the Comprehensive Nuclear-Test-Ban Treaty Organization and infrared DSP satellite technology have reduced the likelihood of undetected airbursts.

A smaller air burst occurred over a populated area in Russia on 15 February 2013, at Chelyabinsk in the Ural district of Russia. The exploding meteoroid was an asteroid that measured about 17 to 20 metres across, with an estimated initial mass of 11,000 tonnes, and inflicted over 1,200 injuries, mainly from broken glass falling from windows shattered by its shock wave.

More information: 7News

Tunguska is the only hard evidence

we have of a recent impact on planet Earth. 

So we can look at that and say ...

if that was a city underneath there,

it would be completely obliterated.

Bill McGuire