As Space mysteries go things don’t get any bigger than Tunguska! In 1908 something exploded over a remote region of Siberia devastating over 2,000 km² of forest. Unfortunately, an organised scientific expedition didn’t visit the area until 1927 and were unable to locate any material from the object. We will probably never know for sure what it was that penetrated the Earth’s atmosphere at cosmic velocity and then exploded with an energy that is estimated to have been between 10 and 30 megatons of TNT.

A photo taken by the 1927 expedition showing the devastation caused by the 1908 Tunguska explosion Image: Wikipedia 

So you can understand the scientific world’s surprise when the normally restrained planetary sciences journal Icarus published a paper by a Russian team claiming to have located material from the 1908 Tunguska meteorite. Anfinogenov et al. (2014) presented evidence which they claimed pointed to a large sandstone boulder called “John’s Stone” as being debris from the cataclysmic Tunguska explosion. But sandstones are also very common rocks on Earth and unknown amongst the meteorite population so far studied. Anfinogenov and team suggested that John’s stone was a new type of meteorite, possibly from Mars.

Following publication of these claims I was contacted by Henning Haack of the Natural History Museum of Denmark, who has studied the Tunguska event in some detail. He naturally felt that these extraordinary claims needed to be followed up and verified, or otherwise, as soon as possible. If John’s Stone was from Mars, oxygen isotope analysis would be the most powerful way to test this. That’s where I came in. Rocks from the earth and Mars have distinct oxygen isotope composition: A previous post “The Mysterious Martian” gives further details.

Dr Yana Anfinogenova, a member of the Russian team was extremely helpful and supportive of our new study providing us with samples of John’s Stone. Henning also contacted Henner Busemann of ETH Zurich, a specialist in noble gas analysis of extraterrestrial samples. Our combined oxygen isotope (see diagram below) and noble gas analyses were conclusive; John’s Stone was not a space rock but a relatively normal terrestrial sandstone. The results of our study have just been published by Icarus as a comment on the original paper and, as is normal in these matters. the authors of the original paper have responded to our comment.

Perhaps it is disappointing that John’s Stone is not debris from the Tunguska explosion, on the other hand the hunt for the real culprit goes on. Is this just of academic interest? Well, no! Had Tunguska taken place over a population centre the results would have been appalling. At Chelyabinsk we got close to this sort of catastrophe. These events need to be studied and understood. The Russian team that published the 2014 paper may not have correctly identified the Tunguska meteorite, but at least they are trying to locate it and one day may succeed. And that would be really exciting.

In terms of its oxygen isotope composition John’s Stone plots on the Terrestrial Fractionation Line (TFL) within the range of terrestrial quartz-rich rocks. It plots well away from any known Martian sample (SNCs or NWA 7034). The oxygen isotope data clearly indicates that John’s Stone is a rock from Earth and not a fragment from the Tunguska meteorite.