Asteroid locations are identified using the crime-scene approach

Location, age, and diameter of confirmed impact craters: (A) Kaali Main and Kaali 2/8, both in Estonia; (B) Morasko in Poland; and (C) Whitecourt in Canada, along with field images of in situ charcoals (at thin white arrows) found within their proximal ejecta blankets. Charcoals are present throughout the ejecta (panels A1, B1, and B2) but are most abundant close to the base (panels A2, C1, and C2). Thick white arrows in panels A and B point out the locations of trenches where charcoals visible in A1, A2, B1, and B2 were collected. Numbers in C and in the field photos refer to sample numbers. Credit: Geology (2022). DOI: 10.1130/G50056.1

Every day, tens of thousands of tons of alien solid debris collide with Earth. The majority of this debris burns away in the atmosphere, but some particles are large enough to present quite a problem. A 20-meter-diameter body erupted above Chelyabinsk in 2013, badly wounding nearly 1,500 people. The most recent impact crater on Earth was made in 2007 when an asteroid collided with a tiny Peruvian town. Sikhote-Alin is the world's youngest crater-strewn field, formed by an asteroid impact in far-eastern Russia in 1947. The most spectacular and recent extraterrestrial incident occurred in 1908, when a body burst above Siberia, leveling 2000 km2 of woodland.

We can only prepare for this natural danger if we understand how frequently these types of minor effects occurred in the past and how they affected the ecosystem. A recent paper in the journal Geology of the Geological Society of America demonstrates that investigating the carcasses of animals destroyed by asteroids might educate us exactly how much damage happens at the site of such a cosmic encounter. The researchers excavated trenches on the rims of four craters (Kaali Main and Kaali 2/8 in Estonia, Morasko in Poland, and Whitecourt in Alberta, Canada), which developed thousands of years apart on two separate continents.

Dr. Jüri Plado and Dr. Argo Jeleht discovered the same thing in all of those locations: millimeter- to centimeter-sized particles of charcoal intermixed amid debris ejected during its development and positioned at the same location in relation to the crater.

Dr. Ania Losiak of the Institute of Geological Sciences, the Polish Academy of Sciences, and the University of Exeter, the study's lead author, stated, "At first, we thought those charcoals were formed by wildfires that occurred shortly before the impact, and charcoals just got tangled in this extraterrestrial situation." But there were too many coincidences with this scenario; why would there be big wildfires immediately before the emergence of four different minor impact craters separated by thousands of kilometers and years? Why would it only be detected in a single spot inside the proximal ejecta blanket? It made little sense, so we decided to dig further and compare the qualities of charcoal fragments discovered intermixed inside debris ejected from craters to wildfire charcoals.

The qualities of organic residues transformed into charcoal, like the properties of bodies analyzed in a criminal investigation, indicate the conditions under which they were murdered. We can distinguish between charcoals created as a consequence of a wildfire and those discovered inside the proximate ejecta of impact craters based on their characteristics.

Impact charcoals are very strange, according to Professor Claire Belcher of the University of Exeter: they were all formed at 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, whereas in a fire it is common to find strongly charred wood just next to barely affected branches.

According to Losiak, "this is definitely not what we expected when we started this study: we believe impact charcoals were formed when fragments of trees shattered by the impact were mixed with local material ejected from the crater."

According to University of Alberta Professor Chris Herd, "this study improves our understanding of the 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."

According to Professor Witek Szczuciski of Adam Mickiewicz University in Poznan, "our research may also help to find new impact craters on Earth; we expect that we are missing more than ten craters formed within the last ten thousand years from our records." We need to find them before their family come to see us.

Journal Information: A. Losiak et al, Small impact cratering processes produce distinctive charcoal assemblages, Geology (2022). DOI: 10.1130/G50056.1