By Around 2.5 million years ago, an asteroid may have exploded over Antarctica.
The evidence comes from a chemical analysis of more than 100 tiny pieces of rock entrained within the White Continent’s ice, researchers report in the Feb. 1 Earth and Planetary Science Letters. The timing makes the midair detonation the oldest known airburst, the team says. Only two other ancient airburst events are known in the geologic record, dating to 480,000 and 430,000 years ago (SN: 3/31/21).
The surfaces of Earth and other rocky bodies are littered with conspicuous craters caused by direct hits from asteroids or comets (SN: 7/23/15). But sometimes an incoming object disintegrates in midair before reaching the surface. Since there’s no crater left behind, there’s scant evidence for airbursts in the geologic record.
However, such events can still be enormously destructive: When an airburst occurs — typically at altitudes ranging from several to a few tens of kilometers — the enormous kinetic energy of the incoming body is transferred into an impact plume, a maelstrom of pressure disturbances and heat. “All the energy is released in the atmosphere in the form of shock waves and thermal radiation,” says Matthias van Ginneken, a cosmochemist at the University of Kent in England.
That’s what happened in 2013 after a roughly 20-meter asteroid broke up high over the Russian city of Chelyabinsk (SN: 2/15/13). When the resulting shock wave reached the ground, it shattered thousands of windows. In 1908, an even more catastrophic airburst took place over Tunguska, Siberia, when a body roughly three times as large as the Chelyabinsk boulder disintegrated in the atmosphere and flattened over 2,000 square kilometers of forest.
Now, van Ginneken and his colleagues believe they’ve uncovered evidence for an airburst that occurred between 2.3 million and 2.7 million years ago over Antarctica.
The team analyzed 116 tiny bits of rock, each about the width of a human hair and many of them spheroidal, found within the continent’s ice. The rocks are dominated by the minerals olivine and spinel, which makes them chemically consistent with a type of asteroid known as an ordinary chondrite, the researchers conclude. Furthermore, the precise ratio of different forms of oxygen in the rocks suggests that they had been produced in an airburst whose impact plume interacted with ice and had therefore reached all the way down to the ground.
Such “touchdown” airbursts are particularly destructive, van Ginneken says. “It’s like a huge torch touching the ground and vaporizing everything.”
Earth is bombarded by sizeable chunks of extraterrestrial material pretty frequently, says Jason Pearl, a physicist at Lawrence Livermore National Laboratory in California who was not involved in the research. Chelyabinsk- and Tunguska-type events are predicted to occur roughly every 50 and 500 years, respectively, he says. It therefore makes sense to undercover evidence of an airburst a couple million years ago. “It’s totally believable that events would have occurred in that time period.”
Van Ginneken is keen to look for more airbursts in the geologic record. There’s got to be others to be found, he says. “I’m convinced there are more examples.”
