By Thomas

It was an early June morning in 1908, somewhere in Siberia, as without a warning “the sky split in two and fire appeared high and wide over the forest”. That is at least how eyewitness S. Semenov recalls the event. (Wiki)

He goes on: “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 metres. 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 Tunguska Event – a massive explosion that knocked down some 80 million trees over an area of 2,150 square kilometres (Figure 1) and luckily didn`t cause any (reported) fatalities. But what caused it? The most probable cause is an asteroid (or maybe a comet) entering Earth`s atmosphere and then exploding while still in the air, like the Chelyabinsk air burst earlier this year, just bigger. Other explanations that make for better Hollywood B-movie scripts are a comet that turned into a “natural H-bomb” when entering Earth`s atmosphere, a black hole passing through the Earth, Antimatter and last, but most definitely not least, a “Death Ray” build by Nikola Tesla.

Trees after the 1908 Tunguska event. Source
Figure 1: Trees after the 1908 Tunguska event.

While these explanations are – how should I put this – “less likely”, they reflect that the Tunguska event remains a bit enigmatic to us. The air burst explanation is a good one, but it is nearly impossible to finally proof it. The event happened in a remote location, it happened at a point in human history when the dominant species on the planet was not homo cellphonecameras and fragments of the potential meteor where never found – and it is very, very unlikely that this will happen in the future.

A new study (Vannucchi et al. 2015) throws yet another explanation into the ring – an explanation which, if it would be true, would have a profound impact on the study of terrestrial impact craters, especially the way we identify them. Unlike on many of the other solid bodies in the solar system, impact craters on Earth are often not very well-preserved due to the constant erosion by wind and weather. To test whether an old structure that might be an impact crater is indeed one, scientists often have to rely on “fingerprints” of the impact event left behind in the target rock. Some of the most reliable of these fingerprints are the so-called PDFs, which stands for Planar Deformation Features. These features are left by very high shock pressures in mineral grains and can be studied under the microscope. What makes them such good indicators for impact events is that there is no other mechanism on Earth that can create the shock pressures as high as needed to create PDFs.

Of course, the Earth can build up quite some pressure from within and release this pressure in single events such as volcanic eruptions (Figure 2), but the shock waves generated by those events are not as powerful as those created by impact events. The reason for this is that the pressure that builds up under the Earth’s surface builds up slowly and will be released before it reaches the strength needed to create PDFs. In the case of an impact however, the pressure that is generated depends to a great deal on the speed of the impactor1 – and this speed can be very high.

Eruption of the Sarychev Volcano as seen from SpaceSource
Figure 2: Eruption of the Sarychev Volcano as seen from Space

To illustrate the difference, let us suppose you want to destroy an oven, using only a metal pot filled with water and some tape. You can tape the lid tight on top of the pot and then put your pot on your oven and heat it up. You hope that the produced water vapour inside the pot creates enough pressure so that your whole pot explodes and destroys the oven.2 I don`t really know how much pressure you could create by this method, but if you get it high enough, what will happen is that your tape will fail and the worst thing that will (probably) happen is that the lid flies off and you stand in a cloud of very hot water vapour, which should be quite hurtful – but your oven should remain intact. Like the rocks in the Earth’s crust, the tape on your pot couldn`t hold the pressure over a certain point. If you however take your taped water pot and quite skilfully throw it out of a helicopter cruising 1 mile high and let it “land” on top of your oven … well you can see what happens to “metal things dropped from 1 mile high” in this video. And if there happens to be another “metal thing” on the landing spot, that one wouldn`t look much better either.

There are some speculations that the Earth itself, under special conditions, can indeed create explosive events strong enough to produce shock waves in a pressure range at which PDFs are formed. And here we come back to the study mentioned above and the Tunguska event:

About 3 km away from the epicentre of the Tunguska event is Mount Stojkovic, and there is a circular structure in which the scientist conducting the study identified samples with PDFs3 . Their favoured interpretation for the nature of this about 250 million year old circular structure is that it was created by a so-called verneshot – a “hyperexplosive volcanic gas eruption” which would be able to create shock features, such as PDFs. The 1908 event then might have been a smaller verneshot event.

The problem is that a verneshot is so far a not-observed, hypothetical event. Sure, the structure at Mount Stojkovic lies in the right geological setting that was envisioned when the verneshot idea was first proposed (Morgan et al. 2004). But apart from this there seems to be little to support the idea. The main argument in the study for the verneshot is actually not something supporting the verneshot idea, but the consideration of how unlikely the rival hypothesis is that instantly springs to mind: The structure at Mount Stojkovic might “just” be a 250 million year old impact crater that happens to lie close to the 1908 event. The argument made in the study is that an impact event and a later air burst event happening in the same location is so improbable that there must be another explanation – and therefore it must be a verneshot. And that part of the study is implausible to me.4

First of all, how can an event happening about 100 years ago make a 250 million year old event improbable? If tomorrow there is an air burst over Meteor crater in Arizona (Figure 3), which most definitely is an impact crater, will that suddenly make it not an impact crater? No!

Meteor Crater, ArizonaSource
Figure 3: Meteor Crater, Arizona

Of course that is an unlikely event. In the study it is actually calculated how unlikely it is that a single air burst is happening over one of the craters that were formed on Earth within the last 250 million years: It is about 1 in 17000.5

That indeed seems like a low chance. But meteor air bursts are quite frequent. Yes, Tunguska was the biggest observed so far, so let us assume, an event like this only happens every 15000 years. That would still mean that within the last 250 million years about one of those Tunguska-like events is likely to have happened over an impact crater. And even if the probability would be much lower – it wouldn`t matter. We could probably find something unusual that has happened within 3 km of every meteor crater of this world. They will stay meteor craters regardless.

So we are left with two possibilities: We have impact cratering and air bursts, well documented and understood processes which happened to happen locally (!) close to each other, or the verneshot, a hypothetical event that, if really a thing, would mean there is more destructive power under our feet than we already know from “normal” volcanic and supervolcanic eruptions.

The second one might be more exciting, but the first one seems (so far) more plausible to me. But I`m fascinated by impacts, so I`m biased.

What do you think?

1 Other contributing factors are the size and nature of the impacting body and the nature of the target material.

2 Please don`t try that at home.

3 Actually, the study is a bit confusing for me: They sometimes use the term PDF and sometimes refer to the features as lamella, which as far as I understand it, is not necessarily a high shock pressure feature (If you want to know more about those features, see for example here: Reimold et al.). However, I`m not qualified at all to identify or judge this kind of features! So my confusion stems probably from a lack of understanding of the terminology.

4 Keep in mind: That does not necessarily mean that it really is implausible. I might just have misunderstood the argument.

5 1 in 17000 is the part of the Earths surface covered with impact craters that formed between now and 250 million years ago, thus an event happening at a random location (such as an air burst) has a chance of 1 in 17000 to happen “over” one of these impact sites. I also don`t quite get why, if you make a probability argument like this, you cut it off at 250 million years. I know it is the supposed age of the structure at Mount Stojkovic, but that shouldn`t matter if you want to know how likely it is that an air burst happens over an impact site. If an air bursts happens over a 300 million year old impact site, could we just shrug it off and say “Those things happen” but when the structure is 200 million years old the air burst suddenly becomes something special? For what reason?