An asteroid exploded over Antarctica 2.5 million years ago, rocky evidence suggests

An asteroid about to hit Earth might have exploded over Antarctica more than 2.5 million years ago, said researchers who analysed bits of rocks found in the continent's ice.

According to a report published inEarth and Planetary Science Letters, the asteroid's explosion is the oldest known midair detonation documented in the geologic record. Apart from this, there have been only two ancient recorded airbursts, dating 480,000 and 430,000 years ago.

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Asteroids or comets that have hit the surface of Earth or other rocky bodies have caused conspicuous craters. However, some objects coming towards Earth have left no crater behind, as they might have disintegrated in midair before reaching the ground. But the evidence of such airbursts is scant.

Asteroid explosions can cause massive destruction. Generally, airbursts occur at altitudes ranging from a few to several tens of kilometres. The tremendous kinetic energy of the incoming object transfers into an impact plume, a whirlpool of pressure disturbances and heat.

Matthias van Ginneken, a cosmochemist at the University of Kent in England, told Science News, "All the energy is released in the atmosphere in the form of shock waves and thermal radiation."

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In 2013, a roughly 20-meter asteroid broke up high over Chelyabinsk, Russia. The shockwave reached the ground, shattering thousands of windows. Another catastrophic airburst occurred in Tunguska, Siberia, in 1908. During this, a boulder about thrice as large as the Chelyabinsk body flattened a forest of over 2,000 square kilometres.

Matthias and his colleagues analysed 116 tiny bits of rock found within Antarctica's ice, each about the width of a human hair and many spheroidal in shape. The researchers concluded that olivine and spinel are the minerals dominating the rock composition, and they are chemically consistent with ordinary chondrite, a kind of asteroid. They also deduced from the rocks that the midair explosion might've happened 2.3 million to 2.7 million years ago.

The precise ratio of different forms of oxygen found in the rocks confirmed that they were produced in an airburst whose impact plume interacted with ice. Therefore, they reached the surface. "It's like a huge torch touching the ground and vapourising everything," said van Ginneken.

Jason Pearl, a physicist at Lawrence Livermore National Laboratory in California who was not involved in the research, said that sizeable chunks of extraterrestrial material bombard Earth frequently. He said these types of events occur roughly every 50 to 500 years. "It's totally believable that events would have occurred in that period," he added.

However, the team is researching more airbursts in the geologic record. "I'm convinced there are more examples," said van Ginneken.

(With inputs from agencies)