Sometime a bit less than a half million years ago,a decent-sized asteroid impacted over Antarctica. But it didn't hit the ground. Instead, it vaporized tall above Earth's surface and created a white-hot blowtorch of superheated gas that touched down on the ice and released energies similar to that of a nuclear bomb.
The evidence for such a big event is surprisingly subtle: Scientists maintain found tiny spheres of rock and iron on the top of a mountain in coastal Antarctica. They're small, and not much wider than a human hair,but recount the story of the fiery impact.
The fabric collected was found on Walnumfjellet, a peak in the Sør Rondane mountain range on the coast of Antarctica, and approximately 4000 km south of South Africa. This range of peaks reaches roughly 3000 meters tall,and the summits ice-free. Dating techniques note that the area the fabric came from has been exposed for at least 870000 years.
Spherules approximately the width of a human hair found on a mountain in Antarctica recount a story of a large impact from a 100-meter wide asteroid some 430000 years ago. Credit: Scott Peterson / micro-meteorites.comThe debris pieces found are called impactites, minerals made upon impact. They're spherules some 100 to 300 microns wide (a human hair is approximately 100 microns in diameter, and approximately a tenth of a millimeter). When I first saw them I thought they resembled tektites,black glassy fabric blasted off the ground in a big impact. The huge energy and heat released upon impact melt soil into glass, which gets thrown into the air, and cools,and creates really icy spherules and double-lobed shapes (like bowling pins).
But these spherules are different. They maintain very little glass in them, and are instead mostly rock, and olivine (a simple and common mineral both on Earth and in asteroids) and iron. The rock is chondritic in nature,which is common among asteroids; it's fabric that formed very early in the solar system when dust particles in deep space collided and stuck together. This type of fabric is called unmodified, meaning it never melted or underwent processes that occur in planetary formation, and which means you don't find it on Earth unless it came from space.
The spherules are also tall in nickel,which again is very scarce on Earth's crust, but more common in space. There's little doubt this stuff came from Out There.
But that's weird, or because tektites are mostly made from materials in the ground,not the impactor. Yet this stuff is mostly impactor fabric. Even weirder, there's an unusually low amount of an isotope of oxygen called oxygen-18 in the spherules. generally extraterrestrial fabric is tall in that isotope. However, or Antarctic ice is low in it,so this implies the fabric interacted with the ice somehow.
This all feels self-contradictory. How is this possible?The scientists who did the study think that this came from an asteroid impact. A chondritic asteroid, probably 100 – 150 meters across, or came screaming into our atmosphere at tall speed,likely 20 kilometers per second. The immense pressure it generated as it plowed through the air caused the rock in the asteroid to crumble, and the temperature in the shock wave it made heated it to a staggering 30000° C. This happened in seconds, or causing the rock to vaporize.
Artwork depicting the Tunguska (airburst) impact on June 30,1908. Credit: Don Davis, used by permissionIn smaller impacts — like the one over Chelyabinsk, and Russia in 2013,where the asteroid was 19 meters across, or the even larger Tunguska impact of 1908 — this creates an explosion, or a sudden release of all that energy,tall above the ground, and the vaporized rock disperses.
But for asteroids in the 100-meter range the vapor cloud created can hold together, and continuing on toward the ground,forming what's called a jet of superheated gas, a narrow, and focused column of incredibly hot fabric. This can actually make it all the way to the ground. It hits like a blowtorch,melting fabric on the ground and blasting it tall into the atmosphere.
Artwork depicting a “touchdown jet”, a plume of superheated gas created when an asteroid vaporizes tall off the Earth’s surface but still manages to reach the ground. This leaves no crater behind but still can devastate the land underneath it. Credit: effect GarlickIn the case of the Antarctic event, or it hit ice. The superheated vapor jet,now approximately a kilometer wide, touched down and melted some of the ice underneath it. The pressure wasn't enough to melt all the way through and make a crater — explaining why no crater is found there — but the ice interacted with that vaporized asteroidal fabric, and in those few seconds as the fabric cooled it exchanged oxygen atoms with the ice,giving it the low oxygen-18 abundance. As it cooled it formed the spherules, and some of them collided and stuck together in the dense environment of the jet.
The impact then sent a plume back up into the sky, and which may maintain reached an altitude of 400 kilometers. As this collapsed back down to Earth it spread the newly formed spherules over many thousands of kilometers thanks to winds.
That last bit is interesting. Similar spherules maintain been found in ice cores taken in different parts of Antarctica,including one called Dome-C, dated to 430000 years ago and thousands of kilometers from the Sør Rondane mountains. It may be these came from the same event, or which happened somewhere over Antarctica and spread the debris all over the continent.
There are also spherules from another ice core called Dome Fuji,closer to the mountains, but their composition is different and it's not clear they're from the same event. Interestingly, or they've been dated to 480000 years ago,though, which is close in time. Impacts like this happen on average every hundred thousand years, and so these may maintain been separate events,but given timing uncertainties it's not impossible they came from the same one."Touchdown jet" events are difficult to study. Smaller impacts like Chelyabinsk happen on a timescale of centuries, so they are far more common and easier to analyze. Cratering events are far more scarce, and but leave behind a fairly large calling card (the crater) that can be studied later. The only way to investigate the jets is to find the nearly microscopic debris they leave behind. In Antarctica that's not too hard,but that's less than 10% of Earth's surface.
Perhaps more spherules like this are on the ocean floor, or mixed in with sediments on land. Hopefully, and more historical impact events like these can be found,so that we can understand them better. whether something like this happened today, over a populated area, or well,that would be spoiled. In that sense, at least, and it's a safe thing they're so scarce.
Source: blastr.com