An international research team of planetary scientists, led by Dr Matthias van Ginneken of the School of Physical Sciences at the University of Kent, a former researcher at the Vrije Universiteit Brussel, the Universitรฉ libre de Bruxelles and the Royal Belgian Institute of Natural Sciences, has found new evidence for a meteorite airburst just above the Antarctic ice cap 430,000 years ago.

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Extra-terrestrial particles, known as condensation spherules, were recovered during the 2017-2018 BELAM (Belgian Antarctic Meteorites) expedition funded by the Belgian Federal Science Policy Office and organised from the Belgian Princess Elisabeth Station in Antarctica. The particles, found near the top of Walnumfjellet in the Sรธr Rondane Mountains of East Antarctica, indicate an unusual touchdown event where a jet of melted and vaporised meteoritic material resulting from the atmospheric entry of an asteroid at least 100 m in size reached the surface at high velocity.

After passing through the atmosphere, meteoroids larger than 100 metres can form an impact crater in the Earthโ€™s crust, but they will more often explode in the air and cause a powerful and destructive shockwave. The best-known example of such an event is the much smaller Chelyabinsk impact over Russia in 2013, which destroyed a huge number of windows. The Tunguska event, also in Russia, in 1908, produced an even larger shockwave that toppled trees over an area of 20km2ย and caused major damage up to 100 km from the site. โ€œImpacts like this occur every 1,000 to 5,000 years. They are crucial to understand how such events occurred throughout Earthโ€™s history and what their effects on the environment may have been,โ€ย says Flore Van Maldeghem, PhD student at the Vrije Universiteit Brussel and co-author of the study. โ€œIf such an explosion were to take place over a region with a large population, it would be absolutely catastrophic, with a huge number of casualties,โ€ adds VUB professor Philippe Claeys.

Out of this world

โ€œThe chondritic chemical composition and the high nickel content of the debris indicate that the recovered particles are not terrestrial. Their unique oxygen isotope ratios indicate that they reacted with oxygen from Antarctic ice during their formation in the impact cloud,โ€ says Professor Steven Goderis. This is only possible if the airburst occurred close enough to the ice surface and the shockwave melted and vaporised the ice, mixing it with meteoritic particles in the impact cloud. Such an event can only result from the hypervelocity entry into the atmosphere of an asteroid at least 100m in size. This type of explosion relatively close to the Earthโ€™s surface has less destructive power than the formation of an impact crater but is still significantly larger than when it occurs at high altitude (for example, between 30 and 50km above sea level in the case of Chelyabinsk).

The study, published in the journal Science Advances, is an important discovery in the field of geology, where evidence for such events is scarce. Such impact particles are difficult to identify and characterise. โ€œBut in this case, the atypical shape of the condensation particles (see photo) made it relatively easy to distinguish them from other extra-terrestrial dust particles found in Antarctica, such as micrometeorites and microtektites. The results of this research can therefore help to identify similar events in the geological past,โ€ says VUB PhD student Bastien Soens, co-author of the study.

The study underlines the importance of mapping the threat posed by medium-sized asteroids as accurately as possible, since future objects of similar size are likely to explode in the atmosphere and generate a shockwave. If this explosion occurs too close to the Earthโ€™s surface, the damage could be severe, especially in densely populated areas. It is therefore important to try to identify these types of events over time in other geological contexts, such as sediment cores, in order to assess their frequency and better identify potentially dangerous asteroids in terms of size and speed.

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