Tunguska explosion caused by meteor

There has been a major development in the investigation into what has become known as the Tunguska Event. For the uninitiated, here is a little background from Wikipedia:

The Tunguska event was an enormously powerful explosion that occurred near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia, at about 07:14 KRAT (00:14 UT) on June 30 [O.S. June 17], 1908. [citations omitted] The explosion, having the epicentre (60.886°N, 101.894°E), is believed to have been caused by the air burst of a small asteroid or comet at an altitude of 5–10 kilometres (3–6 mi) above Earth's surface. Different studies have yielded widely varying estimates of the object's size, on the order of 60 m (200 ft) to 190 m (620 ft).[citation omitted] It is the largest impact event on or near Earth in recorded history.[citation omitted] [...]

At around 07:17 local time, Evenks natives and Russian settlers in the hills northwest of Lake Baikal observed a column of bluish light, nearly as bright as the Sun, moving across the sky. About 10 minutes later, there was a flash and a sound similar to artillery fire. Eyewitnesses closer to the explosion reported the sound source moving east to north. The sounds were accompanied by a shock wave that knocked people off their feet and broke windows hundreds of kilometres away. The majority of witnesses reported only the sounds and the tremors, not the sighting of the explosion. Eyewitness accounts differ as to the sequence of events and their overall duration.

The explosion registered on seismic stations across Eurasia. In some places the shock wave would have been equivalent to an earthquake of 5.0 on the Richter scale.[citation omitted] 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;[citation omitted] it has been theorized that this was due to light passing through high-altitude ice particles formed at extremely low temperatures, a phenomenon that occurred again when space shuttles re-entered Earth's atmosphere.[citations omitted] In the United States, the Smithsonian Astrophysical Observatory and the Mount Wilson Observatory observed a decrease in atmospheric transparency that lasted for several months, from suspended dust.

Because of the remoteness of the Tunguska region and the chaos caused by World War I, the Communist Revolution, and the Russian Civil War, it was not until 1927, when an expedition from the Soviet Academy of Sciences led by Leonid Kulik finally reached the area. Here is how Wikipedia describes what they found:

The spectacle that confronted Kulik as he stood on a ridge overlooking the devastated area was overwhelming. To the explorers' surprise, no crater was to be found. There was instead around ground zero a vast zone (8 kilometres [5.0 mi] across) of trees scorched and devoid of branches, but standing upright. Those farther away had been partly scorched and knocked down in a direction away from the centre. Much later, in the 1960s, it was established that the zone of leveled forest occupied an area of some 2,150 square kilometres (830 sq mi), its shape resembling a gigantic spread-eagled butterfly with a "wingspan" of 70 kilometres (43 mi) and a "body length" of 55 kilometres (34 mi).[citation omitted]

How bad did it look? This bad:

Picture from Leonid Kulik's 1927 expedition to area of Tunguska Event. (From Ars Technica.)

Note that this was in 1927 -- 19 years after the event.

As Nature magazine points out:

At an estimated 3 to 5 megatonnes of TNT equivalent, it was the biggest impact event in recorded history. By comparison, the meteor that struck the Russian region of Chelyabinsk earlier this year 'merely' packed 460 kilotonnes of TNT equivalent.

What is really scary is, what if this had happened today? In an era of intercontinental ballistic missiles with nuclear warheads? Wouldn't we understandably assume this was a nuclear attack? Because, how would we know?

The Tunguska Event has remained a mystery for over a century because of an almost complete lack of evidence as to a cause. As Nature explains:

Numerous scientific expeditions failed to recover any fragments that could be attributed conclusively to the object. Hundreds of microscopic magnetic spheres have been found in the 1950s and 1960s in Tunguska soil samples, but there is continuing debate about whether they are the remnants of a vaporized meteor. “There’s really not much out there, and nothing that’s definitively Tunguska,” says Phil Bland, a meteorite expert at Curtin University in Perth, Australia.

The lack of samples has allowed wild speculation about the cause of the event, with some of the more esoteric explanations invoking antimatter and black holes. But most geoscientists think that part of an asteroid, or perhaps a comet, broke away and fell to Earth as a meteor.

Now, we appear to have found some definitive proof. From Ars Technica:

[A] study in the journal Planetary and Space Science provides, for the first time, evidence that the impact was not caused by a comet. Researchers collected microscopic fragments recovered from a layer of partially decayed vegetation (peat) that dates from that extraordinary summer.
Victor Kvasnytsya from the National Academy of Sciences of Ukraine and his colleagues used the latest imaging and spectroscopy techniques to identify aggregates of carbon minerals—diamond, lonsdaleite, and graphite. Lonsdaleite in particular is known to form when carbon-rich material is suddenly exposed to a shock wave created by an explosion, such as that of a meteorite hitting Earth. The lonsdaleite fragments contain even smaller inclusions of iron sulphides and iron-nickel alloys, troilite and taenite, which are characteristic minerals found in space-based objects such as meteorites. The precise combination of minerals in these fragments point to a meteorite source. It is near-identical to similar minerals found in an Arizona impact.
The samples point to one thing: the Tunguska impact is the largest meteorite impact in recorded history. US researchers have estimated that the Tunguska blast could have been as much as the equivalent of a five megaton TNT explosion—hundreds of times more powerful than the Hiroshima blast. The meteorite tore apart as it entered the atmosphere at an angle, so that little of it reached the ground intact. That is why all that remains are such small specks that have been fossilised in the Siberian peat.

Nature gives more details:

Kvasnytsya says that Ukrainian scientist Mykola Kovalyukh, who died last year, collected the fragments in 1978 from a peat bog close to the epicentre of the blast. Research on the fragments in the years following their discovery found that they contained a form of carbon called lonsdaleite, which has a crystal structure somewhere between graphite and diamond, and forms under extreme heat and pressure. But the grains also contained less of the dense metal iridium than is typically found in meteorites — the meteor fragments that are actually recovered on the ground — so researchers had concluded that they were terrestrial rocks altered by the impact. The findings, published in the 1980s in Russian, went largely unnoticed by Western scientists at the time.
Kvasnytsya and his colleagues decided to take a closer look at the fragments using a battery of modern analytical techniques. Transmission electron microscopy showed that the carbon grains were finely veined with iron-based minerals including troilite, schreibersite and the iron–nickel alloy taenite. This patterning and combination of minerals is very similar to that in other iron-rich meteorites. “The samples have almost the entire set of characteristic minerals of diamond-bearing meteorites,” says Kvasnytsya.
“An iron-rich, stony asteroid fits with our understanding of Tunguska,” says Collins [Gareth Collins, Earth-impact researcher at Imperial College London]. Over the past 20 years, several modelling efforts have concluded that a stony asteroid was the only culprit that could have produced the effects reported on the ground [citation omitted] . However, a small but significant minority of scientists still backs the comet hypothesis, he adds.

But, of course, there are doubts:

“They’ve got some interesting stuff here,” but the team does not yet have conclusive proof, says Bland. The low levels of iridium and osmium in the samples are “a red flag” that raises doubts that the fragments originated in an asteroid, he says, and the peat sediment in which the samples were found has not been convincingly dated to 1908. “We get a lot of meteorite material raining down on us all the time,” adds Bland. Without samples of adjacent peat layers for comparison, “it’s hard to be 100% sure that you’re not looking at that background”.

However, in mysteries like this, with the technology we have now you will never get proof beyond a reasonable doubt. In my opinion, absent new evidence to the contrary, the Tunguska mystery is solved.