Possible Asteroid Explosion Detected in Antarctica Approximately 2.5 Million Years Ago

Approximately 2.5 million years prior, an asteroid is thought to have detonated over Antarctica. This is supported by a chemical examination of over 100 small rock fragments embedded in the ice of the White Continent, as reported by scientists in the February 1 issue of Earth and Planetary Science Letters. This midair explosion is purportedly the oldest known airburst. The geological record indicates only two other ancient airbursts, one 480,000 years ago and the other 430,000 years ago.

The surfaces of the Earth and other rocky planets are often marked by noticeable craters from direct asteroid or comet impacts. Occasionally, however, an incoming object disintegrates in midair before hitting the surface, leaving little evidence of such airbursts in the geologic record due to the lack of resultant craters.

Despite the lack of physical evidence, these midair disintegrations can be remarkably destructive. Typical airbursts occur at altitudes from several to tens of kilometers, where the enormous kinetic energy of the incoming object is converted into an impact plume, generating powerful pressure disturbances and heat. "The whole energy is dispersed in the atmosphere as shock waves and thermal radiation," explains Matthias van Ginneken, a cosmochemist at the University of Kent in England.

Such an event took place in 2013 when an asteroid about 20 meters in diameter broke up in midair over Chelyabinsk, Russia. The resulting shockwave shattered thousands of windows upon reaching the ground. An even more disastrous airburst happened in 1908 over Tunguska, Siberia, when an object about three times the size of the Chelyabinsk asteroid disintegrated in the atmosphere, flattening more than 2,000 square kilometers of forest.

Van Ginneken and his team now posit that an airburst took place between 2.3 to 2.7 million years ago over Antarctica. The team studied 116 small rock fragments, each about as wide as a human hair and many of them spheroidal, found in the continental ice. The majority of the rocks contain minerals known as olivine and spinel, making them chemically similar to ordinary chondrite asteroids, concludes the research team. Furthermore, the specific ration of oxygen forms in the rocks implies they were formed in an airburst that interacted with ice and therefore reached ground level.

Such "touchdown" airbursts can be particularly devastating, says Van Ginneken. "It's akin to a giant torch making contact with the ground and vaporizing everything."

Jason Pearl, a physicist at Lawrence Livermore National Laboratory in California who wasn't part of the research, notes that Earth is frequently hit by sizeable extraterrestrial materials. He suggests that Chelyabinsk and Tunguska events likely occur every 50 and 500 years respectively. Therefore, finding evidence of an airburst a few million years ago seems plausible. "It is entirely believable that such events would have happened in that time frame," he says.

Van Ginneken is eager to search for more historical airbursts. He's certain that there are more examples awaiting discovery. "I'm sure that there are more examples out there," he affirms.