[For the tl;dr crowd: A scientist claims to have found meteorites from the Tunguska event. I am very skeptical, because of reasons, but willing to be convinced.]
[UPDATE (May 7, 2013): A Russian scientist and impact expert, Natalya Artemyeva, has said the claim that these rocks are from the Tunguska impact is "ridiculous", mirroring much of what I write below, if somewhat less circumspectly. My thanks to Jason Major at Universe Today for the link.]
On June 30, 1908, a chunk of cosmic debris—either a small comet or asteroid—came screaming in over the Russian landscape. As it plowed through the air at hypersonic velocity, it underwent fierce compression. Its tremendous energy of motion was converted into heat, causing it to disintegrate and explode while it was still 5-10 kilometers in altitude, releasing the equivalent energy of a multimegaton nuclear weapon. The fireball set hundreds of square kilometers of forest aflame, and then the tremendous blast wave touched down and knocked over trees like they were toothpicks.
This explosion happened in the Podkamennaya Tunguska River region of the Siberian forest, and is now known as the Tunguska event, the largest impact event in modern history. It dwarfed what happened over Chelyabinsk in February 2013.
A lot is now known about the explosion; in the century since the impact we’ve learned quite a bit about such things (including from weather-related events, weirdly). But a great mystery remains: Why have no fragments, no meteorites, from the explosion ever been found?
Scientists have gone over the area many times and found nothing. But now a Russian scientist claims he may have discovered some actual meteorites from the Tunguska impactor. Andrei Zlobin, of the Vernadsky State Geological Museum, went on an expedition to the site in 1988, and found several stones near a river, noting that over time some of the debris may have collected there.
In a paper online, Zlobin discusses how he found them and reports his initial analysis. Let me be very clear: these are not even confirmed meteorites yet, and even if they are they may not be from the 1908 event. Meteorites fall all over the Earth, and these could be from a completely different event, or—and again, I want to be very clear—they might not be meteorites at all. His analysis did not include the necessary chemical tests that must be performed to confirm their origin (which, I'll note, Zlobin is careful to point out in the paper).
That right there is enough to start ringing my skeptical alarms. The expedition was 25 years ago; why did it take so long to report results? And after all that time, why are the reported results so vague? Why no chemical tests? Moscow boasts one of the world’s premier meteoritics labs; it’s odd the samples weren’t taken there for definitive tests. Given how important (and incredibly valuable) these rocks may be, that’s the very first thing I would’ve done.
However, to be fair, some the rocks shown in the paper do look like they’re consistent with meteoritic origin. That’s not enough to know, of course, but sometimes you can say something isn’t a meteorite just by looking at it. A few look to pass that immediate test. He also makes a decent argument that the fireball was not hot enough to melt rocks on the surface, so any rocks found with molten features could be from the impactor itself.
The one he nicknames the "Whale" is the most interesting; it has what looks like a weak rollover lip, a little ledge of material going over the surface that happens in meteorites sometimes as they pass through the air at high speed, melt, and have the molten material blown over the side. I have a couple of meteorites with this feature myself. Of course, erosion and other processes on Earth can shape terrestrial rocks the same way, so again this is in no way definitive.
Also, I said some of the rocks: Quite a few in the paper don’t look by eye to be meteoritic. I can’t say for sure of course; just that I’m pretty skeptical here. Some are the wrong color or the wrong shape. One of the rocks he shows up close, which he calls the “Boat”; is so concave it’s almost folded over itself like a cup, and I’ve never seen a rocky meteorite like that before. That’s not proof either way; it’s just more weight in the “skeptical” column.
Also, he talks about finding shatter cones, which are common in impacts…but only in big impacts, where a sizeable chunk of rock hits the ground. The enormous pressures generated upon impact create cone-shaped patterns in the rock. The thing is, you need a pressure that’s tens of thousands of times the air pressure at sea level to create shatter cones, and if the Tunguska impactor had done that, the trees underneath would’ve been blasted apart into splinters and a huge crater would’ve formed. Since that didn’t happen, the presence of shatter cones is unlikely.
Again, all of this is speculation until a carefully performed chemical analysis is made. To be clear: This find is interesting, and certainly worth following up, but I’ll wait for a better analysis before getting too excited about it. Until then, color me skeptical.
Still, if these rocks are in fact debris from the Tunguska impact, this is a major scientific find. While we have a pretty good grasp of what happened that summer day over a century ago, there are still big gaps in our knowledge. Was the impactor a comet—an icy body with rock and dust embedded in it—or an asteroid, something more rocky? If the latter, was it a solid body, or was it a rubble pile, a collection of rocks held together weakly by their own gravity?
Actual physical samples would prove invaluable to this most basic of questions. And then we can understand better how the impact happened, how the solid body interacted with our atmosphere, why it exploded at the height it did, and how the explosive events unfolded.
As Chelyabinsk showed us, these things do happen. Rarely, which is good! Well, good if you don’t like random asteroid impacts. But if you’re a scientist trying to figure all this out, the lack of actual impacts makes them hard to study, and having a piece of this historic impact literally in your hands would be a fantastic advance for the field. I do hope that’s what these new finds turn out to be, but until we know for sure what these rocks are—if they’re even from space at all—I’ll be patient.