Tunguska event
The Tunguska event was a large explosion of between 3 and 50 megatons TNT equivalent that occurred near the Podkamennaya Tunguska River in Yeniseysk Governorate, Russia, on the morning of 30 June 1908.
The explosion over the sparsely populated East Siberian taiga felled a large number of trees, over an area of of forest, and eyewitness accounts suggest up to three people may have died. The explosion is attributed to a meteor air burst, the atmospheric explosion of a stony asteroid about wide.
The asteroid approached from the east-south-east, probably with a relatively high speed of about. Though the incident is classified as an impact event, the object is thought to have exploded at an altitude of rather than hitting the Earth's surface, leaving no impact crater.
The Tunguska event is the largest impact event on Earth in recorded history, though much larger impacts are believed to have occurred in prehistoric times, including the Chicxulub impact that ended the Cretaceous period. An explosion of this magnitude would be capable of destroying a large metropolitan area. The event has been depicted in numerous works of fiction. The equivalent Torino scale rating for the impactor is 8: a certain collision with local destruction.
Description
On 30 June 1908 N.S., at around 7:17 a.m. local time, Evenki natives and Russian settlers in the hills northwest of Lake Baikal observed a bluish light, nearly as bright as the Sun, moving across the sky and leaving a thin trail. Closer to the horizon, there was a flash producing a billowing cloud, followed by a pillar of fire that cast a red light on the landscape. The pillar split in two and faded, turning to black. About ten minutes later, there was 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 explosion registered at seismic stations across Eurasia, and air waves from the blast were detected in Germany, Denmark, Croatia, and the United Kingdom - and as far away as Batavia, Dutch East Indies, 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 scale.
Over the next few days, night skies in Asia and Europe were aglow. There are contemporaneous reports of brightly lit photographs being successfully taken at midnight in Sweden and Scotland. It has been theorized that this sustained glowing effect was due to light passing through high-altitude ice particles that had formed at extremely low temperatures as a result of the explosion - a phenomenon that decades later was reproduced on a much smaller scale by Space Shuttles. In the United States, a Smithsonian Astrophysical Observatory program at the Mount Wilson Observatory in California observed a months-long decrease in atmospheric transparency consistent with an increase in suspended dust particles.
Selected eyewitness reports
Though the region of Siberia in which the explosion occurred was very sparsely populated in 1908, there are accounts of the event from eyewitnesses, and regional newspapers reported the event shortly after it occurred.The testimony of S. Semenov, as recorded by Russian mineralogist Leonid Kulik's expedition in 1930:
Testimony of Chuchan of the Shanyagir tribe, as recorded by I. M. Suslov in 1926:
Sibir newspaper, 2 July 1908:
Siberian Life newspaper, 27 July 1908:
Krasnoyaretz newspaper, 13 July 1908:
Scientific investigation
Since the 1908 event, an estimated 1,000 scholarly papers have been published about the Tunguska explosion. Owing to the site's remoteness and the limited instrumentation available at the time of the event, modern scientific interpretations of its cause and magnitude have relied chiefly on damage assessments and geological studies conducted many years after the event. Estimates of its energy have ranged from.Not until more than a decade after the event did any scientific analysis of the region take place, in part due to the area's isolation and significant political upheaval affecting Russia in the 1910s. 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 a giant meteorite impact had caused the event. Upon returning, he persuaded the Soviet government to fund an expedition to the suspected impact zone, based on the prospect of salvaging meteoric iron.
Kulik led a scientific expedition to the Tunguska blast site in 1927. He hired local Evenki hunters to guide his team to the centre of the blast area, where they expected to find an impact crater. To their surprise, there was no crater at ground zero. Instead, they found a zone, roughly across, where the trees were scorched and devoid of branches, but still standing upright. Trees farther from the centre had been partly scorched and knocked down away from the centre, creating a large radial pattern of downed trees.
In the 1960s, it was established that the zone of levelled forest occupied an area of, its shape resembling a gigantic spread-eagled butterfly with a "wingspan" of and a "body length" of. Upon closer examination, Kulik found holes that he erroneously concluded were meteorite holes; he did not have the means at that time to excavate the holes.
During the next 10 years, there were three more expeditions to the area. Kulik found several dozen little "pothole" bogs, each in diameter, that he thought might be meteoric craters. After a laborious exercise in draining one of these bogs, he found an old tree 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 leveled forest. The original negatives of these aerial photographs 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 increasingly flammable nitrate film. Positive prints were preserved for further study in Tomsk.
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 meteor 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 carbon, hydrogen, and nitrogen 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 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. 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.
Other scientists disagree: "Some papers report that hydrogen, carbon and nitrogen isotopic compositions with signatures similar to those of CI and CM carbonaceous chondrites were found in Tunguska peat layers dating from the TE and that iridium anomalies were also observed. Measurements performed in other laboratories have not confirmed these results."
Researcher John Anfinogenov has suggested that a boulder found at the event site, known as John's stone, is a remnant of the meteorite, but oxygen isotope analysis of the quartzite suggests that it is of hydrothermal origin, and probably related to Permian-Triassic Siberian Traps magmatism.
In 2013, a team of researchers published the results of an analysis of micro-samples from a peat bog near the centre of the affected area, which show fragments that may be of extraterrestrial origin.
Although it is widely claimed the impact resulted in the felling of over 80 million trees, this was based on a preliminary guess multiplying tree density by impacted area that overestimated the area of devastation by a factor of four, so is likely to be incorrect. The largest accurately recorded felled trees are around in trunk diameter and not around that has been claimed by some sources.
The observation of decreased atmospheric transparency at the Mount Wilson Observatory in California have been used to calculate that roughly a million tons of dust were released into the stratosphere. Further modelling suggests that shock waves from the falling meteor generated up to 30 million tons of nitric oxide in the stratosphere and mesosphere which resulted in a peak ozone depletion of 35-45% for early 1909 and ones of ~30% in 1910 and ~15% in 1911 and a tripling of UV-B intensity at the ground at latitude 30°N.
Earth impactor model
The leading scientific explanation for the explosion is a meteor air burst by an asteroid above the Earth's surface.Meteoroids enter Earth's atmosphere from outer space every day, travelling at a speed of at least, the escape velocity of the Earth. The heat generated by compression of air in front of the body as it travels through the atmosphere is immense and most meteoroids burn up or explode before they reach the ground. Early estimates of the energy of the Tunguska air burst ranged from to 30 megatons of TNT, depending on the exact height of the burst as estimated when the scaling laws from the effects of nuclear weapons are employed. More recent 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 air burst had an energy range from 3 to 5 megatons of TNT. The 15-megaton estimate represents an energy about 1,000 times greater than that of the Trinity nuclear test, and roughly equal to that of the United States' Castle Bravo nuclear test in 1954 and one third that of the Soviet Union's Tsar Bomba test in 1961. A 2019 paper suggests the explosive power of the Tunguska event may have been around 20–30 megatons.
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 events, on the order of 5–15 megatons, 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 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 observed with modern instrumentation was the 500-kiloton Chelyabinsk meteor in 2013, which shattered windows and produced meteorites.