[ad_1]
Centuries ago, an asteroid the length of a football field passed through the solar system on a collision course with Earth. He rushed towards the south pole of the planet, aiming directly at an icy, unpopulated expanse: Antarctica.
This was 430,000 years ago, in the middle of the Pleistocene era. Elsewhere, some of the early Neanderthals spread across Europe, mammoths roamed the northern hemisphere, and Earth’s ice caps grew thicker and thicker.
Space rock crashed into the thick atmosphere of the planet. Friction tore it apart, and as the decaying meteor tumbled toward the Antarctic Plateau, it left a flaming and incandescent trail in its wake. As it neared the ice, the meteor exploded in the sky, launching a jet of superheated gas and vaporized cosmic debris directly to the ground.
These types of mid-air explosions can cause immense damage, but they don’t dig craters into the earth’s crust – which means finding their remaining fingerprints and figuring out how often they occur has been a bit of a game of play. riddles.
Now, scientists studying tiny particles found in Antarctica have discovered evidence of this ancient meteorite aerial explosion, and they’ve used chemical clues locked in the particles to piece together what happened hundreds of thousands of years ago. ‘years.
“We know asteroids are dangerous, and recent studies suggest that aerial explosions are more dangerous than large asteroids, because larger asteroids are very rare,” says Matthias van Ginneken, planet scientist at the University of Kent and lead author of a new study describing the old explosion in the journal Scientific progress.
In 2013, a house-sized asteroid exploded over the Russian city of Chelyabinsk, shattering glass and injuring more than 1,600 people. If a city had been in the sights of the largest Antarctic meteor 430,000 years ago, it would have been destroyed. The explosive force was four times stronger than the 1908 meteor explosion that razed forests near Tunguska, Russia, and thousands of times stronger than the nuclear bomb that exploded in Hiroshima, Japan.
Aerial explosions such as the one over Chelyabinsk – and one that exploded over the Bering Sea in 2018 – often happen unexpectedly because smaller asteroids are hard to find, even with the best ones. Earth telescopes. “We now have a way to find traces and remains of such impacts in the geological records, which could be important in re-evaluating the impact history of our planet,” says van Ginneken.
Frozen detectives
In February 2018, van Ginneken visited Antarctica – a dream trip for him – in search of cosmic breadcrumbs. As a Ph.D. student, he had studied tiny grains collected from other Antarctic sites, but had yet to see the frozen continent with his own eyes. When he arrived with the Belgian Antarctic Meteorite Expedition, it was the end of the field season and they only had two weeks to scour the landscape for microscopic alien confetti.
The team studied two dozen sites, and one of them – a high, flat area of barren rock bordering the Antarctic Plateau in the Sør Rondane Mountains – was a treasure. Cleaned by glaciers over 800,000 years ago, the summit site has perfectly preserved cosmic debris.
“In Antarctica, there isn’t much else that falls on top of the mountains – it’s very clean, there is no human activity, there is no vegetation,” said van Ginneken. “So all material that falls from space is kept for very long periods of time.”
He and his colleagues collected over 12 pounds of sediment from the summit and returned it to the lab. Ultimately, they selected 17 spherules, tiny round grains of molten meteorite forged during impacts, for detailed examination. Immediately, says van Ginneken, he could tell that the black grains were of extraterrestrial origin and that something was wrong: instead of being single spheres like most micrometeorites, some of them were glued together.
When he and his team probed the oxygen composition of spherules, the grains turned out to be even stranger, containing oxygen isotope ratios that are incompatible with known asteroids. These reports suggest that the spherules formed in direct contact with Antarctic ice, which is unusual for an aerial explosion.
The spherules closely resembled alien dust that van Ginneken had studied before – grains encrusted in huge ice cores recovered from the nearby Japanese Antarctic station at Dome Fuji and the Franco-Italian station at Dome Concordia on the other. side of the mainland. These grains are about 430,000 years old, an age that scientists can calculate based on their position in the ice cores – buried 1.5 miles below the surface.
Due to the similarities between the samples, the team estimated that the grains all formed during the same event. Given the lack of craters in Antarctica, as well as spherules scattered across the continent, they suspected that some sort of mega-Chelyabinsk-type aerial explosion had occurred in the distant past.
Chemical indices
Tracing the history of spherules from there was not straightforward, however, in part because of the weird oxygen isotopes. Normally, the spherules that form from a molten meteorite in a mid-air explosion do not interact with a planet’s surface before they solidify and fall to Earth. Natalia Artemieva, of the Planetary Science Institute, therefore used computer simulations to test whether a more complex type of aerial explosion could have occurred.
“We already knew things like this happen, we just need a slightly larger body to allow the plume to reach the surface (but not too big to crater – just ‘kissing’ ice would be. perfect), ”Artemieva wrote in an email. . “After a few attempts, we found a possible scenario.”
In the Antarctic impact model, vaporized debris from an exploding asteroid is hurled toward the ground in an extremely hot plume of gas, which hits the planet’s surface like an interplanetary tsunami. It’s sort of a hybrid between a Chelyabinsk-type aerial explosion, which does not produce a downward plume, and a normal crash creating a crater.
The team called this a “touchdown” impact, and it’s very similar to other explosions that Mark Boslough, a physicist at the University of New Mexico, has modeled. Boslough suspects that one of these events is the culprit behind a mysterious 30-million-year-old glass strewn across the Eastern Sahara – smooth, yellow shards resembling sea glass that have confused scientists due to their otherwise inexplicable presence in the middle of the desert. .
Boslough says the simulations in the new document are solid and that it wouldn’t be surprising if a touchdown exploded over prehistoric Antarctica. And there are a large number of space rocks near Earth that are the right size – between 300 and 500 feet in diameter – to produce touchdown impacts, so understanding how often these violent collisions are critical. with our planet are happening.
“It’s pretty scary when you think about it,” van Ginneken says. The new research, however, could provide a way to detect more landings in the geological records, allowing scientists to better understand the threat these events pose to Earth.
Considering other possibilities
Christian Koeberl of the University of Vienna finds the team’s interpretation reasonable, but is a bit skeptical. The problem begins, he says, with pinning an age on the spherules, which is extremely difficult to do. While the team identified a resemblance to dust from other sites, it’s not a foolproof association – a point van Ginneken agrees on.
“It’s not necessarily their fault, it’s just hard to do,” says Koeberl. “It’s a common problem.”
Instead, Koeberl says it’s possible the spherules were as old as the cleaned surface they were found in – relics of a much older impact formation event. If so, perhaps the lack of a crater isn’t all that surprising: a small impact gouge could have been erased by the displacement of the ice caps.
Koeberl says that while these types of impacts are common, there should be plenty of evidence for their existence in the geological record – but touchdown impacts have not been definitively discovered before. He is also not convinced that the isotopic ratio of oxygen indicates mixing with ice. It’s possible the team recovered fragments of a rare type of asteroid that scientists have not previously characterized, but van Ginneken thinks that’s unlikely.
“I think the data is good and the measurements are correct, and interpretations are not impossible, but not as limited by the data as the paper seems to say,” Koeberl says. “There are more possibilities, but it’s an interesting story to come out.”
Scientists who hope to understand how often aerial explosions occur are also looking up to the sky and making a detailed census of objects that could explode above their heads. As of now, we still don’t have a way to deflect cosmic dangers – but a mission launched later this year to drop a spaceship into an asteroid and veer it off course could provide a way to protect our planet.
In the meantime, it will be crucial to better understand the magnitude of the explosion that an air blast could produce to help those in its path to get out of the way in time.
[ad_2]
Source link