Research from existing asteroid craters could help mitigate further extra-terrestrial events on Earth.

Tons of extra-terrestrial solid material collide with Earth daily. Most of this material is small enough that it burns up in the atmosphere, but some fragments are large enough to cause significant issues. In 2013, a 20m in diameter body exploded over Chelyabinsk, Russia, injuring more than 1,500 people.

The most impressive, relatively recent, extra-terrestrial event happened in 1908, when a body exploded over Siberia; millions of trees were flattened in what has become known as the Tunguska event.

An international research team comprising scientists from two continents and four countries, dug out trenches in the rims of four craters (Kaali Main and Kaali 2/8 in Estonia, Morasko in Poland, and Whitecourt in Canada) located on two different continents that formed thousands of years apart.

Drs Jüri Plado and Argo Jõeleht from the Institute of Ecology and Earth Sciences of the University of Tartu, Estonian, noticed: “Surprisingly, in all those places we found the same thing: millimetre- to centimetre-sized pieces of charcoal intermixed within material ejected during its formation, and located at the same place in respect to the crater.”

Extra-terrestrial situation

Dr Ania Losiak, the lead author of this study from the Institute of Geological Sciences, Polish Academy of Sciences and the University of Exeter said: “At first, we thought those charcoals were formed by wildfires that occurred shortly before the impact, and charcoals just got tangled in this extra-terrestrial situation.

“But something was not right with this hypothesis, there were too many coincidences; why would there be large wildfires shortly before formation of four different small impact craters divided by thousands of kilometres and years?

“Why would it be found only in a very specific location within the proximal ejecta blanket? It made no sense, so we decided to investigate further and analyse properties of charcoal pieces found intermixed within material ejected from craters and compare it with wildfire charcoals.”

Like bodies studied in a criminal investigation, the properties of organic remnants turned into charcoal reflect the conditions in which they were killed. Based on their properties it was possible to recognise charcoals formed as a result of a wildfire and those found within proximal ejecta of impact craters.

Professor Claire Belcher from the University of Exeter explained: “Impact charcoals are really weird: they look as if they were all formed in much lower temperatures than wildfire charcoals; they lack sections that were formed while directly touching the flame; and they are all very similar to each other, while in a fire it is common to find strongly charred wood just next to barely affected branches.

“This is definitely not what we expected when we started this study: we think that impact charcoals were formed when fragments of trees shattered by the impact were intermixed with local material ejected from the crater,” added Losiak.

Evacuation zone

“This study improves our understanding of environmental effects of small impact crater formation so that in the future, when we discover an asteroid a few meters across or more coming our way only a couple of weeks before the impact, we will be able to more precisely determine the size and type of evacuation zone necessary,” said Professor Chris Herd from University of Alberta, Canada.

The research appears in the journal Geology.

Image: Kaali Main crater, Estonia. Credit: Wwikgren.