Jun 302018

On this date in 1908, at around 07:17 local time, Evenki people and Russian settlers in the hills north-west of Lake Baikal in Siberia 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 kilometers away. 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. It is estimated that, in some places, the resulting shock wave was equivalent to an earthquake measuring 5.0 on the Richter magnitude scale. It also produced fluctuations in atmospheric pressure strong enough to be detected in Great Britain. Over the next few days, night skies in Asia and Europe were aglow. It has been theorized that this was due to light passing through high-altitude ice particles that had formed at extremely low temperatures—a phenomenon that would be produced by space shuttles. In the United States, the Smithsonian Astrophysical Observatory and the Mount Wilson Observatory observed a months-long decrease in atmospheric transparency due to an increase in suspended dust particles. Unfortunately, investigation of the area by scientists did not commence until 10 years after the event, which contributed to confusion in the testimony of eye witnesses because they did not have any photographs or videos to supplement their memories. Here is the testimony of S. Semenov, recorded by a Russian expedition in 1930:

At breakfast time I was sitting by the house at Vanavara Trading Post [65 kilometres/40 miles south of the explosion], facing north. […] I suddenly saw that directly to the north, over Onkoul’s Tunguska Road, the sky split in two and fire appeared high and wide over the forest [as Semenov showed, about 50 degrees up—expedition note]. The split in the sky grew larger, and the entire northern side was covered with fire. At that moment I became so hot that I couldn’t bear it as if my shirt was on fire; from the northern side, where the fire was, came strong heat. I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few meters. I lost my senses for a moment, but then my wife ran out and led me to the house. After that such noise came, as if rocks were falling or cannons were firing, the Earth shook, and when I was on the ground, I pressed my head down, fearing rocks would smash it. When the sky opened up, hot wind raced between the houses, like from cannons, which left traces in the ground like pathways, and it damaged some crops. Later we saw that many windows were shattered, and in the barn, a part of the iron lock snapped.

The first scientific expedition arrived at the scene more than a decade after the event. In 1921, the Russian mineralogist Leonid Kulik, visiting the Podkamennaya Tunguska River basin as part of a survey for the Soviet Academy of Sciences, deduced from local accounts that the explosion had been caused by a giant meteorite impact. He persuaded the Soviet government to fund an expedition to the Tunguska region, based on the prospect of meteoric iron that could be salvaged to aid Soviet industry. Kulik’s party eventually undertook an expedition in 1927.

Upon arrival, Kulik made arrangements with the local Evenki hunters to guide his party to the impact site. Reaching the explosion site was an extremely arduous task. Upon reaching an area just south of the site, Evenki hunters would go no farther, fearing what they called the Valleymen. Kulik had to return to the nearby village, and his party was delayed for several days while they sought new guides. The spectacle that confronted Kulik as he stood on a ridge overlooking the devastated area was overwhelming. To the explorers’ surprise, they found no crater. There was instead around ground zero a zone 8 kilometers (5.0 miles) across of trees scorched and devoid of branches, but standing upright. The trees farther away had been partly scorched and knocked down in a direction away from the center. Much later, in the 1960s, it was established that the zone of leveled forest occupied an area of 2,150 km2 (830 sq mi), its shape resembling a gigantic spread-eagled butterfly with a “wingspan” of 70 km (43 mi) and a “body length” of 55 km (34 mi). 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 dozen little “pothole” bogs, each some 10 to 50 meters (33 to 164 feet) in diameter, that he thought might be meteoric craters. After a major effort to drain one of these bogs (the so-called “Suslov’s crater”, 32 m (105 ft) in 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 leveled forest (250 km2 (97 sq mi)). The negatives of these aerial photographs (1,500 negatives, each 18 by 18 cm (7.1 by 7.1 in)) 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.

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. 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 (4–6 mi) above Earth’s surface. The explosion’s effect on the trees near the hypocentre of the explosion was replicated during atmospheric nuclear tests in the 1950s and 1960s, and was similar to the effects of the conventional of the conventional explosives test Operation Blowdown, conducted to simulate the effects of high explosives in jungles. These effects are caused by the blast wave produced by large explosions. The trees directly below the explosion are stripped as the blast wave moves vertically downward, while trees farther away are knocked over because the blast wave is traveling closer to horizontal when it reaches them. Soviet experiments performed in the mid-1960s, with model forests (made of matches on wire stakes) and small explosive charges slid downward on wires, produced butterfly-shaped blast patterns strikingly similar to the pattern found at the Tunguska site. The experiments suggested that the object had approached at an angle of roughly 30 degrees from the ground and 115 degrees from north and had exploded in mid-air.

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 vaporized 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. It is now known that bodies of this kind explode at frequent intervals tens to hundreds of kilometers above the ground. Military satellites have been observing these explosions for decades.

The indigenous Evenki have lived in this part of Siberia since the Neolithic. When the Tunguska event occurred, there were ethnic Russians living there alongside the Evenki, and their cuisines had, of course, intertwined. The Evenki are nomadic foragers and reindeer herders, so the principal proteins in their diet are reindeer meat, bear, rabbit, and fish. I hope to meet some Evenki when I attend the World Nomad Games in September this year, and possibly take a side trip to Irkutsk before returning home. That way I can wallow in local foods for a few days. I am especially interested in the indigenous fish called omul, or Baikal omul, which is endemic to Lake Baikal. It is a whitefish in the salmon family that is used locally in all manner of dishes. It is smoked, grilled, baked, pickled, and eaten raw. The two dishes I am most interested to try are are zagutai and raskolotka. Zagutai is made out of small cuts of omul that are first marinated in a special salt solution, then canned with layers of onions. These layers are later soaked in vegetable oil. Zagutai can be eaten with flavored mayonnaise, sour cream, or mustard.

Raskolotka, also known as stroganina, originated around Lake Baikal but is now popular throughout Russia, served in special restaurants. The omul is fished through ice holes in the lake in winter, and freezes quickly because the outside temperatures where the fishing takes place hover around -25˚C (-13˚F). The fish is then cut into thin strips with a special knife, and eaten raw with a half and half mix of salt and black pepper.

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