View Full Version : Tunguska Impact Mystery Solved-- it was a Meteorite

Ross 54
2013-Jun-27, 07:05 PM
An international team of scientists has found meteoric grains in a peat deposit from the time of the Tunguska impact, 1908. This removes the uncertainty about what sort of object was responsible for the impact. http://arstechnica.com/science/2013/06/mystery-solved-meteorite-caused-tunguska-devastation/

2013-Jun-27, 07:21 PM
found meteoric grains
Also known as an oxymoron in common language.

It's amazing how they can make that distinction between an asteroid, and dirt from a comet in small grains like that.

Ross 54
2013-Jun-27, 09:24 PM
Several minerals found in the sample are named. The article emphasizes that-- "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." -- They mean the famous Arizona Meteor Crater, at Canyon Diablo, which is most of a mile across.

2013-Jun-27, 10:27 PM
If it's similar to Canyon Diablo, then it (might) have been an iron, and those are fairly rare as a percentage of falls (3-5%). An interesting parallel, though, to Canyon Diablo is that the number of known and calculated recoverable fragments is a tiny fraction of the total estimated mass of the impactor, which says that most of this enormous block of iron and nickel was well and truly vaporized.

2013-Jun-27, 11:54 PM
The actual paper says: (http://www.sciencedirect.com/science/article/pii/S0032063313001116)
Carbon isotope studies from the two analyses of the Tunguska foil showed δ13C=−16.01.9‰ and δ13C=−15.22.1‰, suggesting δ13C=−15.62‰ as an average characteristic of the carbon reservoir. That value is close to δ13C of some extraterrestrial samples.Man .. terrestrial carbon typically ranges in values from -6 to -40‰!

Arguing that a δ13C=-15.6‰ 'is close to some extraterrestrial samples', is an extremely weak line of argument. This figure is closer to Earth's mantle and carbonates, than it is to something extraterrestrial.

For instance, terrestrial C4 photosynthetic plants have an average δ13C=−13‰.
The organic amino acid, Glycine, in the Stardust return sample, was determined to be extraterrestrial on the basis of it having a whopping δ13C= +296‰!

The other characteristics might swing the determination in the direction of the sample being meteoric, but the δ13C, in isolation, certainly doesn't.