Did the Tunguska impact leave behind any meteorites?

This article was originally published in the Bad Astronomy Newsletter by Phil Plait, an astronomer and science communicator, as well as a friend of Asteroid Day.
He writes his Bad Astronomy Newsletter three times a week at badastronomy.substack.com.

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Did the Tunguska impact leave any meteorites?

That’s a very interesting question. On June 30, 1908, an object from space — either an asteroid or a piece of a comet, likely 50 or so meters in diameter — exploded over a Siberian forest near the Podkamennaya Tunguska River region of the Siberian forest near the Podkamennaya Tunguska River in Russia. Now called the Tunguska Impact, it exploded with the yield of a 15-megaton bomb, flattening the forest for hundreds of square kilometers. The explosion was roughly 8.5 kilometers above the ground; like the Chelyabinsk impact in 2013 the incredible pressure of ramming through Earth’s atmosphere at hypersonic speed crushed the impactor and caused so much energy to be released so suddenly it exploded.

However, unlike Chelyabinsk, no meteorites from Tunguska have ever been definitively found. There was a paper that came out some years ago claiming maybe some had, but I had my strong doubts even then, and the lack of follow-up all these years later makes me comfortable in saying that never amounted to anything.

So where are the fragments of the original rock? I’ve wondered if it completely vaporized, and some planetary scientists have concluded this as well. But that may not be the case.

Artwork depicting the Siberian Tunguska Impact fireball of 1908. Credit: Don Davis, DonaldEDavis.com

An immense glowing cauliflower-shaped cloud appears over a wooded area, seen from a height

The Chelyabinsk asteroid was found to be somewhat fragile; the rock didn’t have high tensile strength (likely it had lots of fractures running through it from previous impacts by other asteroids in space long ago) and fell apart as it rammed through the air. Yet meteorites were found everywhere downrange form the explosion, including one that was a meter wide and weighed roughly a ton. It fell into a small lake, and there’s a very cool video of it.

So maybe that’s true for Tunguska as well. A new paper published in the planetary science journal Icarus discusses this possibility. Looking at the possible trajectory of the impactor (from eyewitness accounts) and given what’s known about it (where it exploded, how high up, and so on), they plot a couple of possible areas where meteorites may have fallen. The strewn fields — the technical term for the area where meteorites fall — would be ovals roughly 10 km wide, about 10 km NNW of the explosion.

Mounting an expedition to go look would be difficult. The area is not easily accessible, and the weather there ain’t great. It’s Siberia, plus it’s marshy. The first expedition sent took nearly 20 years after the explosion. Even then the weather had covered up a lot of the small-scale evidence of the impact (thought he fallen trees must have been amazing to see). It’s now been well over a century, so it’s unclear if anything would be left. Anything really big, like a meter across, could’ve plunged into the ground far enough to be completely covered up, too.

Interestingly, there’s a lake close to the explosion site called Cheko Lake. There’s been speculation for a long time that it formed due to the impact of a big chunk, and the astronomers in the paper discuss this. They used what are called Monte Carlo simulations to figure out the likely strewn fields. Basically, you take all the known variables (trajectory, speed, descent angle, and so on) and run a model to see where rocks might fall. Then you change the variables a little bit and run it again. You do this over and over, thousands of times, to determine how spread out the fall was; the center of the distribution of simulations has the highest probability of being correct, while those near the edge are unlikely.

What they found is that Cheko Lake is far enough from the predicted strewn field, about 3.5 km, that they rule it out as an impact site at a confidence of about 95%. Not impossible, but pretty unlikely.

I also learned there is a weird rock found not far from the explosion site called John’s Stone. It’s 2 meters long and those authors claim there’s evidence it’s from an impact. I’m a little skeptical (as have others been) and the authors of the strewn field paper feel it’s likely to be terrestrial. I’d be curious to see more work done on that by other scientists.

Anyway, this work is important, because it tells us more about what happens during an impact — the importance of that is obvious enough — plus, if it pans out (haha! “pans out” because they’re looking for small nuggets) it helps scientists figure out where to look for meteorites after an impact, which isn’t necessarily obvious. Strewn fields can be huge, and searching them time consuming. Anything to narrow that down is helpful.