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About 430,000 years ago, a meteorite exploded over Antarctica.
The only reason we know it now is that scientists just found tiny particles of once melted space rock that have been hidden in the ice ever since.
Based on an analysis of these particles, the event was unusual – not strong enough to produce an impact crater, but it was also not light weight. The stream of molten, vaporized material that exploded in the aerial explosion would have been more dangerous than the Tunguska event that razed a Siberian forest in 1908.
While crater-producing impacts are quite rare, rocks that enter and explode in Earth’s atmosphere are not. They are called bolides and NASA has recorded 861 of them since 1988 at the time of writing. Superbolides – like the Chelyabinsk meteor in 2013 or the Kamchatka meteor in 2018 – occur several times a century.
Aerial events more powerful than Tunguska are even rarer, but they are still believed to have been more common than impact crater events throughout Earth’s history. This is sobering, according to a team led by cosmochemist Matthias van Ginneken from the University of Kent, UK.
“Although landing events do not threaten human activity if they occur over Antarctica,” he said, “if they were to occur over an area densely populated, it would cause millions of casualties and serious damage over distances of up to hundreds of kilometers. “
The problem is, because these events don’t tend to leave a crater, we have trouble identifying them (heck, sometimes we even have trouble identifying craters), so it’s hard to know how common they are.
Cue 17 tiny particles, found on the summit of Walnumfjellet in the Sør Rondane Mountains of Queen Maud’s Land in East Antarctica. The largest of these bits is no more than half a millimeter in diameter.
Some of the spherules. (Scott Peterson / micro-meteorites.com)
They may look like fairly ordinary grains of dirt to the naked eye, but van Ginneken and his team subjected them to a scanning electron microscopy, revealing that they were some kind of bolide product called condensation spherules. .
The team’s study found that blobby spherules are composed primarily of iron and olivine, with a high nickel content – exactly consistent with a rare kind of meteorite known as pallasite, confirming that the globlets are from abroad.
They were also poor in oxygen-18, an isotope of oxygen that is found at lower concentrations in colder conditions, such as polar ice and water. Compared to condensation spherules found in other parts of Antarctica, previously dated to 480,000 and 430,000 years ago, the researchers found them surprisingly similar.
This similar chemical profile suggests that the spherules were all created from the same meteoritic event 430,000 years ago. It also contains clues to the explosion itself.
Condensation spherules from a simple air explosion are rare, according to the researchers; they are usually associated with crater events. But the team couldn’t find any craters associated with their grains. In addition to the large area they were scattered over, this suggests that the event that produced the spherules must have been an air blast – but much more powerful than what we usually see.
We don’t 100% know how meteors explode in the air, but scientists believe that the high-pressure air in front of the falling meteor seeps through cracks in the rock, increasing internal pressure and causing it to rupture. rock.
The heat associated with this process would have vaporized material in the meteorite; the explosion threw him to the ground. There, the vaporized material condensed again, mixing with the Antarctic ice sheet: hence the oxygen-18 profile of the spherules.
Because this event would have been even more powerful than Tunguska – an explosion of at least 3 megatons – the results suggest that such “intermediate” meteor events have the potential to be devastating; Even without impact, the Tunguska event razed 2,150 square kilometers (830 square miles) of forest, chopping down around 80 million trees.
It is therefore incumbent upon us, according to the researchers, to try to fill the gaps in the superbolid registry.
“To complete the assessment of the impact of asteroids on Earth, we recommend that future studies focus on identifying similar events at different targets, such as rocky or shallow oceanic subsoils, as the ice cap Antarctic glacial only covers 9% of the Earth’s land surface, ”van Ginneken told me.
“Our research may also prove useful in identifying these events in deep seabed sediment cores and, if the plume expansion reaches landmasses, the sediment recording.”
As if we had nothing to worry about …
The team’s research has been published in Scientific progress.
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