planetary craters:
what made all those holes? Part 2

By David Bryant

In 1908 a massive explosion laid waste to thirty square miles of Siberian pine forest near the Tungus River. Thousands of trees were felled around an obvious epicentre and many animals (and quite probably local tribesmen) were incinerated or killed by the blast. Strange night-glows in the sky persisted for several days, while witnesses reported a blinding light and shattering concussion. However, despite numerous expeditions to the region, no satisfactory explanation has been forthcoming.
A 2014 TV documentary followed the adventures of a group of international scientists as they investigated the Tunguska impact site in Siberia: each one had a pet theory:

rockngem magazine issue 65
rockngem magazine issue 65
rockngem magazine issue 65

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• An asteroid or meteoric impact
• An encounter with a chunk of anti-matter
• The explosive release of trapped gases from the mantle
• Aliens sacrificing their spacecraft to save humanity (No: really!)

One might reflect on the strange fact that these highly-qualified science professionals held such a diverse range of theories to explain a single event! The explanation for this is surprisingly straight forward: there is absolutely no concrete evidence at the site to prove conclusively what caused the explosion!
Apart from the felled trees, the only tangible ‘evidence’ was found by a geophysicist who located eventually a small outcrop of sandstone: samples from this appeared to contain shocked quartz grains which, bizarrely, he claimed to be proof that an explosive release of mantle gases from beneath the Siberian Traps basalt layer had been responsible for the devastation! As many of you will know, shocked quartz is frequently encountered at the site of an extraterrestrial impact: in fact (along with shatter cones) such shocked grains are considered to be the ‘smoking gun’ of an impact crater.
Little need be said about the anti-matter and UFO theories: there is, as yet, no proof that anti-matter exists, except as necessary extra mass to provide the gravity needed to hold the Universe together! And no fragments of exotic or alien technology have been located in the region of the explosion.
The generally-held ‘establishment’ belief is that Tunguska – and all large crater-forming events – were the result of an asteroidal impact. Unfortunately, no proven meteoric material was – or has ever – been found at the site: no nickel-iron, nor chondrite fragments. Neither have they been found at any other major impact craters such as Tenoumer, Manicouagan and Roter Kamm. This is strange, since many small to medium-sized craters are sources of stony or iron meteorites: Wolf Creek, Barringer, Gebel Kamil and Henbury being good examples. Even the giant Chicxulub event left behind only micro-tektites and raised iridium levels around the globe.
So what did explode above Siberia in 1908? And what made all the really large craters on Earth (and many of the small ones too!)?
In February, 2013 (thanks to phone and dashboard cameras) we were all treated to grandstand views of an event that could easily have been a ‘second Tunguska’: a large object entered the atmosphere above Siberia before exploding over the city of Chelyabinsk.As soon as the images of the meteor trail appeared online and on TV, I was struck by the fact that it was pure white, rather than the more frequently observed smoky grey-brown. Soon, reports from Siberia revealed that, despite intensive searching, only small, rounded fully-crusted wholestones were being recovered. This was in sharp contrast to the estimated entry mass of over 10,000 tonnes. Furthermore, nothing extraterrestrial was discovered beneath the much-photographed circular hole in Lake Cherbarkul, merely an ancient algae-covered rock.
So what was the object that did all this damage? There could be only one explanation: a small cometary fragment.
Despite popular impressions, comets are neither rare nor harmless cosmic snowballs: out at the very edge of the Solar System, the Oort Cloud and Kuyper Belt hold trillions of them.
For reasons that are not fully understood, one will occasionally tumble out of its orbit, plunging down the gravitational slope towards the Sun. Some swing round the Sun on a parabolic path and return to the depths of space: others are captured and remain in elliptical orbits, gradually sublimating as they pass by the Sun.
Although the greater part of a comet’s mass is water ice, it should be remembered that a cubic metre of ice has a mass of nearly a tonne: even a smallish comet 500m across would build up a devastating amount of kinetic energy as it accelerated into the inner Solar System, with a mass of around 1.04 million tonnes and a velocity approaching 70km/sec. Their surfaces are coated with a regolith of the material they pass through on their passage: this often becomes the raw material for periodic meteor showers when a comet that has lost its ice crosses the Earth’s orbit.
So it is entirely possible that collisions with small comets are responsible for those problematic large craters with little or no solid evidence of the impactor: their regoliths would be vaporised, producing a ‘spike’ of Iridium in the rocks, but no significant debris field would remain, as would be the case with an asteroidal impact.
So why has my hypothesis not generally found favour? Could it be that the money and resources spent searching for potentially Earth-crossing asteroids would be better spent elsewhere: remember, aperiodic comets (such as ISON) can appear at any time with insufficient warning to mount any form of ‘Bruce Willis’ style mission!

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